Non-Destructive Evaluation Methods

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Non-Destructive Evaluation Methods

Thomas Lüthi

Cover image: Anax imperator, micro X-ray tomographic view, Iwan Jerjen, Empa Version 2013 Copyrights: The presented images are under copyright of the sources indicated in the respective captions, they may be used for educational purposes only. Most of them are downloaded from the internet or scanned from company flyers, some are the due to personnel communications. Text copyright © 2003-2013 Thomas Lüthi The author would like to express his thank to colleagues from Empa's Centre for Non-Destructive Testing for their support.

PREFACE Non-destructive evaluation (NDE) or non-destructive testing (NDT) has been defined as comprising those test methods, used to examine or inspect a part, a material or a system without impairing its future usefulness. The term is generally applied to investigations of material integ rity, excluding medical diagnosis and homeland security, even if for those several methods are used equally. As the approaches are not exactly the same, it is sometimes helpful to study those applications and to learn new implementations. Since 1920, the art of testing without destroying the test object has developed from a laborat ory curiosity to an indispensable tool of production, safety and structural integrity, maintenance and life-time prediction. Certain types of flaws are inherently coupled with the manufacturing process and each manufacturing process has its own special flaw types. Furthermore each manufacturing technique may or may not have its very own flaws. For non-destructive testing of new products a pro found knowledge of the manufacturing process and its connected flaws, therefore, is indispensable. A second type of characteristic that can be influenced by the manufacturing process is the material itself, especially due to heat treatment of all kind. A third type, finally, is connected to the geometry of the component or dimensions of certain parts (wall or coating thickness) or the correct positioning of internal structures. The usual flaw type introduced during operation has something to do with mechanical load (static or dynamic) and/or chemical processes the product was undergoing. The main flaw types looked upon at in-service inspections, therefore, are cracks or corrosion. Mechanical damage can also be cavitations, impacts, buckles, etc. Some mechanical, thermal or chemical induced alterations of the material can be found by nondestructive techniques. The measuring of dimensions at in-service inspections is also very common, especially in combination with corrosion detection (wall-thinning, etc.). The detection of the position of internal structures is quite common for aircraft components and it is a unique aid for damage assessment.

Fatigue cracks caused damage to the fuselage of this Boeing 737 (Aloha Airlines flight 243), it lost one third of its roof while cruising in 24'000 ft, the accident caused the death of a flight attendant and injury to many passengers (April 1988)

CONTENTS 1

INTRODUCTION..............................................................................1 1.1

1.2

1.3

1.4

1.5

1.6

2

Methods Overview.....................................................................................1 1.1.1

Surface, Near-Surface and Volume Inspection.................................................1

1.1.2

Imaging Methods........................................................................................2

1.1.3

Classification Criteria...................................................................................3

Statistical Aspects.....................................................................................3 1.2.1

Flaw Response and Discrimination Criterion....................................................3

1.2.2

Probability of Detection (POD).......................................................................4

1.2.3

Flaw Response and Noise.............................................................................5

1.2.4

Receiver Operation Characteristics (ROC).......................................................6

1.2.1

Risk Considerations.....................................................................................6

Basic Concepts on Data Acquisition and Interpretation.............................7 1.3.1

Signal and Noise.........................................................................................7

1.3.2

Modulation Transfer Function (MTF)...............................................................8

1.3.3

Edge Spread Function (ESF) and Line Spread Function (LSF).............................9

1.3.4

Modulation.................................................................................................9

1.3.5

Nyquist-Shannon Sampling Theorem............................................................10

1.3.6

Fourier Transform......................................................................................11

Training, Qualification and Certification of Personnel..............................12 1.4.1

Introduction.............................................................................................12

1.4.2

NDT Levels...............................................................................................12

1.4.3

Experience, Training and Qualification..........................................................13

1.4.4

Vision Requirements..................................................................................14

1.4.5

Certification..............................................................................................14

Human Aspects.......................................................................................15 1.5.1

Influences on Human Reliability...................................................................15

1.5.2

Colour Sensibility of the Human Eye.............................................................16

1.5.3

Colour Vision Deficiency (CVD)....................................................................17

1.5.4

Contrast Sensitivity of the Human Eye..........................................................17

1.5.5

Dark Adaptation........................................................................................18

1.5.6

Visual Perception.......................................................................................19

Standardisation.......................................................................................20 1.6.1

National Standards (Switzerland).................................................................20

1.6.2

European Standards..................................................................................20

1.6.3

International Standards..............................................................................20

1.6.4

US-American Standards.............................................................................20

1.6.5

Japanese Standards...................................................................................20

1.6.6

Standards for Aviation................................................................................21

1.7

Laboratory Accreditation.........................................................................21

1.8

Examination Procedure and Report – General Requirements...................21 1.8.1

Examination Procedure...............................................................................21

1.8.2

Examination Report...................................................................................22

1.8.3

Marking...................................................................................................24

VISUAL AND OPTICAL INSPECTION..............................................25 i

2.1

Introduction............................................................................................25

2.2

Equipment...............................................................................................25

2.3

2.4

2.5

2.6

2.7

3

Measuring Equipment and Reference Parts for Comparisons.............................25

2.2.2

Equipment for Direct Visual Techniques........................................................25

2.2.3

Equipment for Remote Visual Techniques......................................................27

2.2.4

Illumination..............................................................................................28

2.2.5

Equipment for Optical Methods....................................................................29

2.2.6

Forensic Equipment...................................................................................30

Testing Procedures for Visual Testing.....................................................30 2.3.1

Standard Procedures..................................................................................30

2.3.2

Automated Optical Mass Inspection (AOI).....................................................31

2.3.3

Confocal Laser Scanning Microscopy (CLSM)..................................................32

2.3.4

Optical Coherence Tomography (OCT)..........................................................33

2.3.5

Scanning Near Field Optical Microscopy (SNOM).............................................34

Optical Measuring Methods.....................................................................34 2.4.1

General....................................................................................................34

2.4.2

Fringe Projection.......................................................................................34

2.4.3

Electronic Speckle Pattern Interferometry (ESPI)...........................................35

2.4.4

Laser Shearography...................................................................................36

2.4.5

Moiré Interferometry..................................................................................37

Aspects of Quality Assurance..................................................................38 2.5.1

Resolution Targets.....................................................................................38

2.5.2

Viewing Conditions....................................................................................39

2.5.3

Special Vision Requirements.......................................................................39

Procedure and Record.............................................................................39 2.6.1

Procedure.................................................................................................39

2.6.2

Record.....................................................................................................40

Trends, Summary and Conclusions..........................................................40

INFRARED AND THERMAL INSPECTION........................................41 3.1

Introduction............................................................................................41

3.2

Heat Transfer..........................................................................................41

3.3

3.4

ii

2.2.1

3.2.1

Heat Transfer Mechanisms..........................................................................41

3.2.2

Thermal Properties....................................................................................43

3.2.3

Influence of the Atmosphere.......................................................................43

Equipment...............................................................................................43 3.3.1

Heating Equipment....................................................................................43

3.3.2

Performance Parameters of Infrared Instruments...........................................44

3.3.3

Non-Contact Temperature Sensors...............................................................45

3.3.4

Contact Temperature Sensors.....................................................................49

3.3.5

Optical Components...................................................................................49

Thermographic Techniques.....................................................................51 3.4.1

Overview..................................................................................................51

3.4.2

Selective Heating......................................................................................51

3.4.3

Transmission Thermography.......................................................................52

3.4.4

Pulse or Transient Thermography.................................................................53

3.4.5

Flying Spot and Flying Line Technique..........................................................56

3.4.6

Lock-In Thermography...............................................................................56

3.5

3.6

3.7

4

3.4.7

Pulse Phase Thermography.........................................................................58

3.4.8

Step Heating (Long Pulse)..........................................................................58

3.4.9

Vibro-Thermography..................................................................................59

3.4.10

EddyTherm...............................................................................................59

3.4.11

Thermoelastic Stress Analysis (TSA).............................................................59

Aspects of Quality Assurance..................................................................60 3.5.1

Laboratory Blackbody................................................................................60

3.5.2

Targets....................................................................................................60

3.5.3

Thermal Resolution....................................................................................61

3.5.4

Imaging Spatial Resolution.........................................................................61

3.5.5

Measurement Spatial Resolution..................................................................62

Procedure and Record.............................................................................62 3.6.1

Procedure.................................................................................................62

3.6.2

Record.....................................................................................................62

Summary and Conclusions......................................................................62

LIQUID PENETRANT INSPECTION.................................................63 4.1

Introduction............................................................................................63

4.2

Equipment...............................................................................................64

4.3

4.4

4.5

4.6

4.2.1

Classification of Testing Products.................................................................64

4.2.2

Chemical Compatibility and Thermal Restrictions............................................64

4.2.3

Penetrants................................................................................................65

4.2.4

Excess Penetrant Removers........................................................................65

4.2.5

Developers...............................................................................................65

4.2.6

Electrostatic Spray Guns.............................................................................66

4.2.7

Black Light Lamps.....................................................................................66

4.2.8

Blue Light Systems....................................................................................66

4.2.9

Flaw Inspection Systems............................................................................67

Testing Procedure...................................................................................69 4.3.1

Precleaning..............................................................................................69

4.3.2

Penetration...............................................................................................69

4.3.3

Excess Penetrant Removal..........................................................................70

4.3.4

Drying.....................................................................................................70

4.3.5

Development............................................................................................70

4.3.6

Inspection................................................................................................70

4.3.7

Retesting.................................................................................................71

4.3.8

Bleed back...............................................................................................71

Aspects of Quality Assurance..................................................................71 4.4.1

Chemical Agents.......................................................................................71

4.4.2

Viewing Conditions....................................................................................72

4.4.3

Safety Aspects..........................................................................................72

Procedure and Record.............................................................................73 4.5.1

Procedure.................................................................................................73

4.5.2

Record.....................................................................................................73

Special Techniques and Trends...............................................................73 4.6.1

High Temperature Penetrant Testing............................................................73

4.6.2

Low Temperature Penetrant Testing.............................................................74

4.6.3

Filtered Particle Testing..............................................................................74 iii

4.7

5

Automated Signal Detection........................................................................75

Summary and Conclusions......................................................................75

5.1

Introduction............................................................................................77

5.2

Leak Testing Techniques.........................................................................77

5.4

5.5

5.6

5.7

5.2.1

Tightness Requirements.............................................................................77

5.2.2

Restrictions and Safety Factors....................................................................78

Tracer Gas Methods.................................................................................79 5.3.1

Principles of Detection................................................................................79

5.3.2

Pirani Gauges...........................................................................................79

5.3.3

Ionisation Gauges......................................................................................80

5.3.4

Techniques where Tracer Gas in Flowing into the Object..................................80

5.3.5

Techniques where Tracer Gas is Flowing out of the Object...............................81

Pressure Change Methods.......................................................................82 5.4.1

Test Instruments.......................................................................................82

5.4.2

Pressure Decay Technique..........................................................................83

5.4.3

Pressure Rise Technique.............................................................................83

5.4.4

Bell Pressure Change Technique..................................................................83

5.4.5

Flow Measurement Technique......................................................................84

5.4.6

Bubble Emission Technique.........................................................................84

Various Techniques.................................................................................85 5.5.1

Tightness of Piping Systems........................................................................85

5.5.2

Acoustic Techniques...................................................................................86

5.5.3

Air Tightness of Buildings............................................................................86

5.5.4

Radionuclide Leakage Test..........................................................................87

Procedure and Record.............................................................................88 5.6.1

Procedure.................................................................................................88

5.6.2

Record.....................................................................................................88

Summary and Conclusions......................................................................88

MAGNETIC INSPECTION...............................................................89 6.1

Introduction............................................................................................89

6.2

Diverted Magnetic Flux............................................................................89

6.3

6.4 iv

Process Acceleration..................................................................................75

4.6.5

LEAK TIGHTNESS INSPECTION.....................................................77

5.3

6

4.6.4

6.2.1

Magnetic Field Distribution..........................................................................89

6.2.2

Magnetic Properties...................................................................................89

6.2.3

Models for the Diversion by a Slot................................................................90

6.2.4

Model for the Diversion by a Subsurface Cylindrical Hole.................................92

Magnetising Equipment...........................................................................93 6.3.1

Magnetic Field of an Electric Conductor.........................................................93

6.3.2

Magnetic Field of a Coil..............................................................................94

6.3.3

Hand Yokes..............................................................................................95

6.3.4

Prods.......................................................................................................95

6.3.5

Magnetising Mandrels.................................................................................95

6.3.6

Magnetic Benches......................................................................................96

Magnetisation.........................................................................................97

6.5

6.6

6.7

6.8

6.9

6.10

6.11

7

6.4.1

Overview..................................................................................................97

6.4.2

Axial Current Flow.....................................................................................97

6.4.3

Prods; Current Flow...................................................................................98

6.4.4

Induced Current Flow.................................................................................98

6.4.5

Threaded Conductor..................................................................................98

6.4.6

Adjacent Conductor or Cable.......................................................................99

6.4.7

Fixed Installation.......................................................................................99

6.4.8

Portable Electromagnet (Yoke)....................................................................99

6.4.9

Rigid Coil.................................................................................................99

6.4.10

Flexible Coil..............................................................................................99

Detection of the Magnetic Flux..............................................................100 6.5.1

Dry Magnetic Particles..............................................................................100

6.5.2

Wet Magnetic Particles.............................................................................100

6.5.3

Peelable Detection Media..........................................................................101

6.5.4

Induction Coils........................................................................................102

6.5.5

Hall Effect Sensors...................................................................................102

6.5.6

Magneto-Optical Sensors..........................................................................103

Demagnetisation...................................................................................104 6.6.1

Basic Principle.........................................................................................104

6.6.2

Reversing DC Demagnetisation..................................................................104

6.6.3

Demagnetising Tunnels............................................................................104

6.6.4

Yoke Lift Off Technique.............................................................................105

6.6.5

Maurer Demagnetising.............................................................................105

Aspects of Quality Assurance................................................................105 6.7.1

Field Direction and Strength......................................................................105

6.7.2

Quality of the Magnetic Detection Media......................................................106

6.7.3

Viewing Conditions...................................................................................107

6.7.4

Safety Aspects........................................................................................107

Procedure and Record...........................................................................108 6.8.1

Procedure...............................................................................................108

6.8.2

Record...................................................................................................108

Special Applications..............................................................................109 6.9.1

Underfloor Corrosion................................................................................109

6.9.2

Wire Cable and Stay Cable Inspection.........................................................110

6.9.1

Tube and Pipeline Inspection.....................................................................112

6.9.2

Underwater Application.............................................................................113

6.9.3

Evaluation of Barkhausen Noise.................................................................113

High Resolution Magnetometry.............................................................115 6.10.1

TFI, AMR and GMR Sensors.......................................................................115

6.10.2

Fluxgate Magnetometers...........................................................................117

6.10.3

SQUIDs..................................................................................................117

Summary and Conclusions....................................................................118

EDDY CURRENT INSPECTION......................................................119 7.1

Introduction..........................................................................................119

7.2

Physical Background.............................................................................119 7.2.1

Current Flow and Ohm Law.......................................................................119

7.2.2

Magnetic Behaviour..................................................................................120 v

7.3

7.4

7.5

7.6

7.7

7.8

vi

7.2.3

Induction and Inductance.........................................................................120

7.2.4

Self-Inductance and Inductive Reactance....................................................121

7.2.5

Mutual Inductance...................................................................................121

7.2.6

Impedance.............................................................................................122

7.2.7

Depth of Penetration and Current Density...................................................122

7.2.8

Phase Lag...............................................................................................123

Testing Arrangements...........................................................................124 7.3.1

Characteristic Frequency and Effective Permeability......................................124

7.3.2

Complex Impedance Plane and Operating Point............................................124

7.3.3

Encircling Probes.....................................................................................125

7.3.4

Tube Testing with Encircling Probes and Internal Coaxial Probes.....................127

7.3.5

Probe and Through-Transmission Testing of Sheets and Foils.........................128

7.3.6

Rotating Probes.......................................................................................130

Equipment.............................................................................................131 7.4.1

Examination System................................................................................131

7.4.2

Eddy Current Instruments.........................................................................131

7.4.3

High Frequency (HF) Signal Processing.......................................................132

7.4.4

Demodulated Signal Processing.................................................................133

7.4.5

Display...................................................................................................133

7.4.6

Digitisation.............................................................................................133

7.4.7

Probes...................................................................................................133

7.4.8

Mechanised Units.....................................................................................133

7.4.9

Eddy Current Imaging..............................................................................133

7.4.10

Reference Test Pieces...............................................................................134

7.4.11

Instrument and Probe Setting....................................................................134

7.4.12

Display and Output..................................................................................135

Receiving Media....................................................................................135 7.5.1

Coils......................................................................................................135

7.5.2

T-Probes................................................................................................136

7.5.3

GMR Probes............................................................................................137

7.5.4

Probe Arrays...........................................................................................137

7.5.5

Flexible Probe Arrays................................................................................139

Aspects of Quality Assurance................................................................140 7.6.1

Typical Sources of Errors in Measurement...................................................140

7.6.2

Verification of Eddy Current Equipment.......................................................140

7.6.3

Reference Blocks.....................................................................................141

Procedure and Record...........................................................................143 7.7.1

Procedure...............................................................................................143

7.7.2

Record...................................................................................................143

Special and Advanced Techniques.........................................................143 7.8.1

Sorting...................................................................................................143

7.8.2

Measurements in Magnetic Saturation.........................................................144

7.8.3

Multi Frequency Technique........................................................................144

7.8.4

Swept Frequency Technique......................................................................145

7.8.5

Remote Field Eddy Current (RFEC).............................................................146

7.8.6

Pulsed Eddy Current (PEC)........................................................................146

7.8.7

Detection of reinforcement bars.................................................................148

7.8.8

Magneto-Optical Imaging (MOI).................................................................149

7.8.9

Alternating Current Field Measurement (ACFM)............................................149

7.9

8

MICROWAVE AND TERAHERTZ INSPECTION...............................151 8.1

Introduction..........................................................................................151

8.2

Physical Principles of Microwaves.........................................................151

8.3

8.4

8.5

8.6

8.7

9

Summary and Conclusions....................................................................150

8.2.1

General..................................................................................................151

8.2.2

Reflection and Refraction..........................................................................153

8.2.3

Absorption and Dissipation........................................................................155

8.2.4

Standing Waves......................................................................................155

8.2.5

Scattering..............................................................................................155

8.2.6

Homodyne and Heterodyne Demodulation...................................................156

Techniques of Microwave Inspection.....................................................156 8.3.1

Transmission Techniques..........................................................................156

8.3.2

Reflection Techniques...............................................................................157

8.3.3

Standing Wave Techniques.......................................................................159

8.3.4

Scattering Techniques..............................................................................159

Equipment.............................................................................................160 8.4.1

Microwave Circuit Components..................................................................160

8.4.2

Ground Penetrating Radar (GPR)................................................................161

8.4.3

Terahertz Equipment................................................................................163

Applications..........................................................................................165 8.5.1

Applications of Ground Penetrating Radar....................................................165

8.5.2

Applications of Materials Characterisation and Research................................166

Special and Advanced Methods..............................................................167 8.6.1

Synthetic Aperture Focusing Technique (SAFT).............................................167

8.6.2

Near Field Terahertz Scanning...................................................................168

8.6.3

Homeland Security...................................................................................169

8.6.4

Metamaterials.........................................................................................169

Summary and Conclusions....................................................................170

MAGNETIC RESONANCE INSPECTION.........................................171 9.1

Introduction..........................................................................................171

9.2

Principles of Magnetic Resonance.........................................................171

9.3

9.4

9.2.1

Application of Magnetic Fields....................................................................171

9.2.2

Magnetic Resonance.................................................................................171

9.2.3

Magnetic Resonance Signal.......................................................................172

9.2.4

Molecular Mobility....................................................................................172

Instruments and Applications...............................................................172 9.3.1

Nuclear Magnetic Resonance.....................................................................172

9.3.2

Magnetic Resonance Imaging....................................................................173

Spatial Encoding and Image Contrast....................................................175 9.4.1

Slice Selection.........................................................................................175

9.4.2

Frequency and Phase Encoding..................................................................176

9.4.3

Image Construction.................................................................................176

9.4.4

Other Spatial Location Methods.................................................................177

9.5

Electron Paramagnetic Resonance.........................................................177

9.6

Summary and Conclusions....................................................................177 vii

10 ULTRASONIC INSPECTION..........................................................179 10.1

Introduction..........................................................................................179

10.2

Physics of Ultrasound............................................................................180

10.3

10.2.1

Wave Propagation....................................................................................180

10.2.2

Ultrasonic Velocity...................................................................................181

10.2.3

Attenuation of Sound Waves.....................................................................182

10.2.4

Acoustic Waves at Interfaces.....................................................................183

10.2.5

Acoustic Waves at Interlayers....................................................................186

10.2.6

Properties of the Sound Beam...................................................................187

Equipment.............................................................................................190 10.3.1

Instruments............................................................................................190

10.3.2

Cables and Connectors.............................................................................191

10.3.3

Properties of the ultrasonic pulse...............................................................192

10.3.4

Piezoelectric Transducers..........................................................................192

10.3.5

Probes for Contact Testing Technique.........................................................195

10.3.6

Focusing Probes......................................................................................198

10.3.7

Wheel Probes..........................................................................................198

10.3.8

Equipment and Probes for Bubblers and Squirters.........................................199

10.3.9

Equipment and Probes for Immersion Testing..............................................200

10.3.10 Equipment and Probes for Air Coupling.......................................................202 10.3.11 Electromagnetic-acoustic Transducers (EMAT)..............................................202 10.3.12 Magnetostrictive Transducers....................................................................203 10.3.13 Phased Array Transducers.........................................................................204 10.3.14 Capacitive Micro-Machined Ultrasonic Transducers (CMUT).............................207 10.3.15 Laser Ultrasonics.....................................................................................207 10.3.16 Tone Burst Generators.............................................................................208 10.3.17 Impact-Echo Equipment...........................................................................208 10.3.18 Dry-Point-Contact Probes..........................................................................208

10.4

10.5

10.6

10.7

Ultrasonic Techniques...........................................................................209 10.4.1

Pulse Echo Technique...............................................................................209

10.4.2

Tandem and LLT-Examination....................................................................210

10.4.3

Transmission Technique............................................................................210

10.4.4

Pitch and Catch Technique (Double Probe Technique)....................................211

10.4.5

Guided Waves.........................................................................................212

10.4.6

Image Presentations................................................................................213

Characterisation and Sizing of Discontinuities.......................................216 10.5.1

Techniques for the Classification of Discontinuity Shape................................216

10.5.2

Echo Height Evaluation.............................................................................217

10.5.3

Probe Movement Sizing Techniques............................................................219

10.5.4

Time-of-Flight Diffraction (TOFD) Technique................................................219

10.5.5

Synthetic Aperture Focusing Technique (SAFT).............................................221

Aspects of Quality Assurance................................................................221 10.6.1

Calibration Blocks and Calibration..............................................................221

10.6.2

Characterisation and Verification of Ultrasonic Examination Equipment............222

10.6.3

Probe and Sound Field Characterisation......................................................222

10.6.4

Sound Field Simulation.............................................................................223

Procedure and Record...........................................................................223 10.7.1

viii

Procedure...............................................................................................223

10.7.2

10.8

10.9

Record...................................................................................................223

Special and Advanced Techniques.........................................................225 10.8.1

Special Mode Conversion Applications.........................................................225

10.8.2

Doppler Applications................................................................................226

10.8.3

Acoustic Microscopy.................................................................................227

10.8.4

Advanced Thickness Measurement.............................................................228

Summary and Conclusions....................................................................229

11 ACOUSTIC EMISSION INSPECTION.............................................230 11.1

Introduction..........................................................................................230

11.2

Features and Limits...............................................................................230

11.3

11.2.1

General..................................................................................................230

11.2.2

Definitions..............................................................................................231

11.2.3

Kaiser Effect...........................................................................................231

11.2.4

Felicity Effect..........................................................................................231

Instrumentation....................................................................................232 11.3.1

Acoustic Emission Sensors........................................................................232

11.3.2

Signal Conditioning and Processing............................................................233

11.4

Data Analysis........................................................................................233

11.5

Applications and Derived Techniques....................................................234

11.6

11.5.1

Power Transformer Partial Discharge Localisation.........................................234

11.5.2

Applications for Pressure Vessels...............................................................234

11.5.3

Applications for Structural Integrity Assessment and Health Monitoring...........234

11.5.4

Applications for Wood...............................................................................235

11.5.5

Micro Acoustics in Geophysics....................................................................236

11.5.6

Applications for Process Monitoring............................................................236

Procedure and Record...........................................................................237 11.6.1

Procedure...............................................................................................237

11.6.2

Record...................................................................................................237

11.7

Acousto-Ultrasonics..............................................................................237

11.8

Acoustic Resonance Analysis and Non-Linear Acoustics........................238

11.9

11.8.1

Acoustic Resonance Analysis.....................................................................238

11.8.2

Tap Testing.............................................................................................239

11.8.3

Non-Linear Acoustics................................................................................240

Summary and Conclusions....................................................................241

12 RADIOGRAPHIC INSPECTION.....................................................242 12.1

Introduction..........................................................................................242

12.2

Physics of Radiography.........................................................................243

12.3

12.2.1

Attenuation of X-Rays..............................................................................243

12.2.2

Thomson Scattering.................................................................................244

12.2.3

Compton Scattering.................................................................................245

12.2.4

Unsharpness and Spatial Resolution...........................................................246

12.2.5

Contrast.................................................................................................247

12.2.6

Attenuation of Neutrons............................................................................248

Radiation Sources.................................................................................249

ix

12.4

12.5

12.6

12.7

12.8

12.9

12.3.1

Bremsstrahlung and K-Shell Radiation........................................................249

12.3.2

Conventional X-Ray Tubes........................................................................250

12.3.3

Microfocus X-Ray Tubes............................................................................252

12.3.4

High Energy X-Ray Sources.......................................................................253

12.3.5

Flash X-Ray Machines...............................................................................253

12.3.6

Gamma Ray Sources................................................................................254

12.3.7

Neutron Sources......................................................................................256

12.3.8

Synchrotrons..........................................................................................258

Radiation Detection...............................................................................260 12.4.1

X-Ray Film, Film Processing, Film Illuminator...............................................260

12.4.2

Intensifying Screens.................................................................................263

12.4.3

Film Digitisation......................................................................................264

12.4.4

Computed Radiography (CR) with Imaging Plates (IP)...................................265

12.4.5

Scintillators............................................................................................266

12.4.6

Image Intensifying Systems......................................................................268

12.4.7

Charge-Coupled Device (CCD) Concepts......................................................269

12.4.8

Detector Panels.......................................................................................270

12.4.9

Neutron Detectors...................................................................................272

Radiographic Techniques......................................................................273 12.5.1

Standard Radiographic Techniques.............................................................273

12.5.2

Special Radiographic Techniques without Signal Processing............................273

12.5.3

Film Laminography and Film Tomography...................................................274

Tomographic Techniques.......................................................................275 12.6.1

Computed Laminography..........................................................................275

12.6.2

Computed Tomography (CT).....................................................................276

12.6.3

Beam Hardening Correction.......................................................................278

12.6.4

First Article Inspection, Rapid Prototyping and Reverse Engineering................279

12.6.5

Local Tomography...................................................................................280

Aspects of Quality Assurance................................................................280 12.7.1

Determination of the Focal Spot Size..........................................................280

12.7.2

Controls Related to Film Radiography.........................................................280

12.7.3

Controls Related to Film-Less Radiography..................................................281

12.7.4

Image Quality of Radiographs....................................................................281

12.7.5

Secondary Radiation and Undercut.............................................................281

Procedure and Record...........................................................................282 12.8.1

Procedure...............................................................................................282

12.8.2

Record...................................................................................................282

Special and Advanced Techniques.........................................................284 12.9.1

Compton Backscatter Technique................................................................284

12.9.2

Absorption Edge Scanning.........................................................................285

12.9.3

Phase Contrast Radiography......................................................................286

12.9.4

Dual-Energy X-Ray Absorptiometry............................................................287

12.9.5

Electron Radiography and Autoradiography.................................................288

12.9.6

Emission Techniques................................................................................289

12.9.7

X-Ray Flash Technique.............................................................................290

12.9.8

X-Ray Refractometry................................................................................291

12.10 Radiation Protection..............................................................................291 12.10.1 Fundamentals.........................................................................................291 12.10.2 Dose Rates and Limitations.......................................................................292 x

12.10.3 Possibilities of Restrictions against Radiation...............................................294 12.10.4 Shielding of X-Rays..................................................................................295 12.10.5 Shielding of Gamma Rays.........................................................................296 12.10.6 Shielding of Neutrons...............................................................................297

12.11 Summary and Conclusions....................................................................297

13 APPENDIX...................................................................................300 13.1

13.2

13.3

Literature..............................................................................................300 13.1.1

Selected Books........................................................................................300

13.1.2

Selected Journals.....................................................................................301

Web Sites..............................................................................................303 13.2.1

NDT Societies.........................................................................................303

13.2.2

Producers, Service and Trade (extract).......................................................304

13.2.3

Selected Research and Information Sites.....................................................312

EN Standards (related to NDT)..............................................................313

14 DICTIONARY...............................................................................324 INDEX.........................................................................................345

xi

xii

1

INTRODUCTION

1.1

Methods Overview Basically the non-destructive testing methods can be classified into six major categories: ▪

Visual-optical;



Penetrating radiation;



Magnetic-electrical;



Mechanical vibration;



Thermal;



Penetrating gas or liquid.

The objective of each method is to provide information about one or several of the following parameters: ▪

Discontinuities and separations (cracks, voids, inclusions, delaminations etc.) at the surface, connected to the surface or internally;



Structure or malstructure (microstructure, grain size, segregation, misalignment, orientation etc.) dependent on size and location;



Dimensions and metrology (thickness, diameter, gap size, discontinuity size, etc.);



Physical (electrical, magnetic, thermal), mechanical, and surface properties (reflectivity, conductivity, elastic modulus, sonic velocity, etc.);



Composition and chemical analysis (alloy identification, impurities, elemental distributions, etc.);



Stress and dynamic response (residual stress, crack growth, wear, vibration, etc.);



Signature analysis (image content, frequency spectrum, field configuration, etc.).

The used abbreviations mean AT: acoustic emission testing, ET: eddy current testing, LT: leak testing, MT: magnetic testing, NMR: nuclear magnetic resonance, PT: penetrant testing, RT: radiography (includes here techniques with neutron radiation as well), TT: infrared thermography testing, UT: ultrasonic testing, ST: strain testing, VT: visual testing (includes here also optical methods), WT: microwave testing (non-official abbreviation). 1.1.1

Surface, Near-Surface and Volume Inspection The different methods may be divided into methods that inspect surfaces only, a surface near zone, and the volume. The meaning of near-surface is that the method is able to inspect a volume partially or in full for thin components, but that the penetration depth is limited. Details have to be taken from the relevant chapters.

1

Table 1-1: Ranges of detection Method

Surface

Near-Surface

Volume

AT

X

ET

X

LT

not applicable

MT

X

NMR

X

X

backscatter techniques

X

PT

X

RT

1.1.2

Remarks

TT

X

X

UT

surface reflection

surface waves

VT

X

optical coherence tomography

WT

surface reflection

X

X

Imaging Methods Several methods are imaging inherently, either directly visible for the human eye or detectable with a camera or camera-like system. If the camera itself is a scanning type or an array type is, if of interest, discussed in the relevant chapters. Other methods can be made imaging using scanning techniques. If the latter argument is valid, techniques that use more than one image to get the final information (e.g. computed tomography) have to be considered as scanning techniques. Table 1-2: Imaging possibilities of different methods (examples) Method

Inherently imaging

AT ET

magneto-optical imaging

LT MT

Remarks

acousto-ultrasonics

generally not applicable

X X

magnetic particles

NMR

2

Scanning possible

mostly not applicable

magnetic field sensors X

tomography possible

single sensor

tomography possible

PT

X

RT

film, imaging plate, flat panel

TT

infrared camera, contact temperature sensor

UT

laser vibrometer

X

VT

X

SNOM

WT

X

X

tomography possible (?)

1.1.3

Classification Criteria As for most non-destructive methods exact sizing of flaws is often not easy or impossible at all, the term indication is widely used. The detection threshold of the method and/or the technique is a theoretical value and relies on the technique, the object, and the surrounding conditions; it is the level where no difference can be found between signal and noise. Above the reporting or recording level all indications have to be recorded. This level is not always given, but it is useful to compare the results with later investigations (e.g. crack growth) or if there is the need to increase the load on the component and therefore new fracture mechanic calculations are necessary. The acceptance level describes the limit whether a component is acceptable or not. As it is often not such easy to reject a component if the acceptance level is exceeded, in prac tice also an upper threshold or a “discussion limit” exists. This can be especially the case if repairing could induce other unwanted phenomenon (residual stress etc.) or if the time delay for a substitution is too long. If the acceptance level is exceed at an in-service inspection potential measures can be reducing the load or reducing the time interval to the next in-service inspection. The definition of the criteria is generally not made by the institution that executes the non-de structive evaluation. In practice it can happen that the investigation has to be performed without any criteria at all, and that the discussion on acceptance begins when the results are presented.

1.2

Statistical Aspects

1.2.1

Flaw Response and Discrimination Criterion The response from a NDE system or process may take the form of a signal output (or outputs) or a direct or indirect image. Acceptable conditions can be differentiated from unacceptable conditions in the case of an electronic output by: ▪

Threshold discrimination (amplitude, time, etc.);

and in the case of image analysis by: ▪

Pattern recognition (number, frequency, etc.);



Threshold discrimination.

The discrimination can be automated or performed by a human operator. The definition of recording and acceptance criteria can be different for the same part depending on the production step and/or the position on the part (depending on the different damage mechanisms) and/or the detectability of the signal e.g. due to geometrical reasons. A direct or indirect relationship between an NDE response and a system performance characteristic may be functional under laboratory conditions, but may be impractical in applications under production or in-service conditions. Non-destructive evaluation involves the measurement of complex parameters with inherent variations in both the measurement process and part under test. The output from such a measurement / decision process can be analysed as a problem in conditional probability. The out come can be classified into the four following results: ▪

True positive (TP): a defect exists and is detected;



False positive (FP): no defect exists but one is identified (type II error or false call);



False negative (FN): a defect exists but is not detected (type I error);



True negative (TN): no defect exists and none is detected.

TP and FN outcomes giving the total opportunities for positive calls (= effective number of defects), FP and TN outcomes giving the total opportunities for false alarms (= effective number of no defects). The probability of detection (POD), therefore, is defined as POD=

TP TP+FN

(1.1)

3

TP

FN

Change criterion setting

Change signal characteristics Figure 1-1: For a given signal the POD is influenced by the setting of the criterion and the characteristics of the signal Similarly, the probability of false alarms (POFA) can be expressed as FP (1.2) FP+TN The desired results of the application of NDE procedures are defect detection (signal present) or signal non-detection (signal absent). The basis for detection is that of sensing a signal response and determining if the signal response is above a predetermined threshold. Both, sensing and interpretation are relying on the signal and the noise that are subjected to the discrimination media (machine or human operator). POFA=

1.2.2

Probability of Detection (POD) The characteristic performance level of a given NDE procedure can be established by subjecting a number of test samples containing defects (often cracks with different lengths) in a range of sizes to the procedure. It is only valid for one NDE problem, one material (one material com bination), one NDE technique, and usually one type of NDE equipment with one setting. The inspectors or the machine can be ordered to classify the readings into given classes. If a defect is classified too low it is assessed as not detected (FN). If it classified too high, consequently it is assessed as detected. For single POD determination parts without defects are not necessary, but the distribution of defects should cover the whole range. The real defect size has to be measured using other methods.

4

Figure 1-2: POD plots for four different non-destructive evaluation methods on the same set of test specimen with cracks (NASA) 1.2.3

Flaw Response and Noise For a given distribution of the flaw response and the noise, a simple decrease of the discrimination threshold will indeed increase the POD; however, at the same time it will increase the POFA as well, as the number of falls calls is also increased. A lower discrimination threshold will reduce the number of accepted parts with a flaw and, at the same time, will increase the number of rejected parts due to noise. Discrimination threshold

Flaw response

Noise

Type I error (FN) Accept

Type II error (FP)

Reject

5

Figure 1-3: Probability distribution of a noise signal and flaw indication, the chosen distribution for the explanation is normal, noise and flaw response, however, can show any other distribution type as well 1.2.4

Receiver Operation Characteristics (ROC) As the POFA rate is not reflected by the POD curve, it has to be recorded and used as a secondary characteristic in proficiency assessment. The receiver operation characteristics method was developed during World War II to qualify the proficiency of radar operators. Later it was applied to human perception, medical diagnosis and since the 1980s also in NDE. The method assumes conditional probability in detection / discrimination and utilises both POD and POFA perform ance as the process performance level and discrimination proficiency of the operator / equip ment are varied. Data required in the use of the ROC curve method for subset data analysis are obtained from the specimen set used for POD analysis. Now, however, it is important that parts without any defects at all are included as well. A wrong classification of a defect now results in FN if classi fied too low and FP if classified too high. Parts without defect classified as defective are also assessed as FP. Different effective defect sizes have to be displayed as different curves in the ROC plot, as well as different instrumentations (material, equipment, sensitivity etc.). The more cruel part of the game is to alter the working conditions (noise, vibration, temperature etc.) or to let inspectors do the job when they are less concentrated (tired or immediately after a shift change). On the other hand it can be shown that specific training and experience of the inspectors will result in a tendency in the direction of the upper left corner. The same can be done with machines if they are designed to do automated signal processing and pattern recognition and are able to be taught. Sensitivity 1

1.0

0.8

0.8

3

2 1

Noise 0.6 POD

0.6

0.4

0.5 0

0.4

Reliability 0.2

0.2

0 -4 -3 -2 -1

0

1

2

3

4

5

6

7

0.0 0.0

0.2

0.4

0.6

0.8

1.0

POFA

Figure 1-4: Distributions of noise and flaw signals (standard deviations for all curves = 1) and corresponding ROC curves (right) for various differences of the mean values of signal and noise, higher differences increase the reliability of the test, increasing method sensitivity increases POD as well as POFA The above given relations do only deal with the probability of finding defects, however, they do not state the possibility of remaining defects in tested parts. For some applications very low defect rates are required, key words are ppm-quality or six sigma (6 σ = 3.4 ppm). In practice such low defect rates can only be reached by controlling the whole process in combination with 100% testing and statistically they can only become proofed for large numbers. 1.2.1

Risk Considerations Terms related to risk can be defined as follows: risk ≈ frequency x consequences

(1.3)

risk sensation ≈ risk / acceptance

(1.4)

where acceptance = f(benefit, attitude, experience, willingness ...) 6

(1.5)

and on the other hand: (1.6)

safety sensation ≈ safety x acceptance

(1.7)

Consequences

safety ≈ risk-1

Acceptance

Frequency Figure 1-5: Different risk levels and direction of growing acceptance In reality the real load onto and the real strength of structures are statistically distributed. The overlap of the two curves is called the remaining risk; the distance of the two peaks is related to the safety factor. The remaining risk can be reduced be enhancing the safety factor or by reducing the width of the deviations of the probabilities, the latter by: ▪

A change of the design;



A better control of the load situation (e.g. by the use of sensors, etc.);



A better control of the strength situation (e.g. by non-destructive evaluation of the materials and structures, etc.).

Load

Strength

Remaining risk

Figure 1-6: Example of the remaining risk as overlap of the load and the strength distribution; the principle is not restricted to mechanical load

1.3

Basic Concepts on Data Acquisition and Interpretation

1.3.1

Signal and Noise Noise is in space or time statistically distributed and independent on the origin of the signal. The relation of signal to noise is called signal-to-noise ratio and abbreviated SNR or S/N SNR=

P Signal PNoise

(1.8)

As the power values are often not easily available, the usual way to get the SNR is to determ ine the values of mean and standard deviation. The result is the same. SNR=

Mean StdDev

(1.9)

7

The minimum necessary SNR for a correct evaluation is highly dependent on the application. The usual way to measure the SNR is to determine the mean value and the standard deviation over a certain time or a certain area. Multiple measurements, however can differ up to 10%.

Figure 1-7: Artificial images with a nominal signal height of 174 and 76 (8 bit) and a nominal noise of 0, 5, 10 and 20, no edge effect Pure increasing the gain, in general, will increase the signal as well as the noise and therefore, showing no effect onto the SNR. Independent signal streams or images can be integrated, reducing the noise by the factor of √n.

Figure 1-8: Example of the ISO 21550 target (upper row) with 40% noise and 4 and 16 times integrated (16 images with individual noise distributions are necessary) 1.3.2

Modulation Transfer Function (MTF) As no system is perfect, there will always be differences in the modulation of the real object and the measurement; the modulation transfer function is the ratio between those two. For a lot of systems the MTF can be approximated by n

MTF=(sinc π (x))

(1.10)

where sinc corresponds to the sine cardinal function. Two definitions thereof are in use (and it is not always clearly stated which one is taken), the one used here is sincπ (x)≡

sin(πx) πx

;

sincπ(0) ≡ 1

(1.11)

x must contain the (spatial) frequency and be dimensionless; in the general form x is the ratio between the effective signal frequency and the sampling frequency.

8

1

(sincπ (x))n

0.75 1 2

0.5

3 n=4

0.25

0 0

0.25

0.5

0.75

1

fSignal / fSampling Figure 1-9: sincπ curves for different ratios fSignal / fSampling 1.3.3

Edge Spread Function (ESF) and Line Spread Function (LSF) The edge spread function is the resulting profile across a step function after digitisation; the line spread function is its derivative. The modulation transfer function is the normalised magnitude of the Fourier transform of the latter. LSF(x)=

d ESF(x) dx

ESF

(1.12)

LSF

Figure 1-10: Edge spread function (left) and corresponding line spread function In addition the term point spread function (PSF) exists. 1.3.4

Modulation The modulation M describes the unsharpness of a signal. For imaging techniques a usual way to determine the value is to image line pair targets. Those are periodic patterns with identical widths of the two lines. For technical applications the pattern is usually formed by bars, for optical applications, sine patterns are also common. The modulation is given by M=

V max−V min V max+V min

(1.13)

The minimum discernible modulation is about 2 to 5%. It depends on the observer, the illumin ation, and the background noise. The value can depend on the type of screen or the print qual ity as well.

9

Vmin

Vmax

Figure 1-11: Definition of the modulation M For electronic signals, the principle remains the same. In all cases the noise level has to be considered; the remaining modulation can be substantially influenced by the noise level.

Figure 1-12: Modulation of sine patterns with contrast levels of 50% ± 50% down to 50% ± 1% (Norman Koren) 1.3.5

Nyquist-Shannon Sampling Theorem In order to fully reconstruct a band-limited signal, i.e.,one with a zero power spectrum for fre quencies f > B (B = band limit), it must be sampled at a rate f ≥ 2B. A signal sampled at fN = 2B is said to be Nyquist sampled and fN is the Nyquist frequency (Harry Nyquist, 1889-1976). No information is lost if a signal is sampled at this frequency and none additional information is gained by sampling at a higher frequency. When digitising pulsed signals (e.g. ultrasound, microwaves or eddy currents etc.), one has to take into account, that the power spectrum is extended to higher frequencies. A doubled sampling rate of the nominal or main frequency will not suffice.

10

Δφ = 0

Δφ = 0.25

Δφ = 0.5

Signal fN fN / 2 fN / 3 Signal fN fN / 2 fN / 3 Figure 1-13: Sampling theorem and aliasing: a signal consisting of black and white is scanned with an increasing detector size (upper row top: very small detector, bottom large detector). Depending on the shift Δφ (in signal unit lengths) between the centre of the mid pixel and the centre of the mid detector the outcome is different. At f N the response always show a modulation of 100% (pure black and pure white are present); at f N / 2 the response depends highly on the relative detector position; at fN / 3 the response cycle corresponds to three signal cycles – the wrong frequency, it is aliased If the sampling frequency is too low, aliasing may occur; artificial low frequency signals in repetitive patterns appear, typically visible as moiré patterns. In non-repetitive patterns, aliasing appears as jagged diagonal lines - “the jaggies”. Many digital camera sensors have antialiasing or low-pass filters (the same thing) to reduce response above the Nyquist frequency. Anti-aliasing filters blur the image slightly; they reduce resolution. Smoothing filters have the same effect concerning anti-aliasing and blurring. If a value of fSignal / fSampling = ½ is applied to the sincπ function, the result is ~ 0.64. The apparent discrepancy to the above figure comes from the fact, that the sinc π function is based on sine patterns, not on rectangular ones. 1.3.6

Fourier Transform For continuous signals the generalised Fourier transform F(ω) (Jean Baptiste Joseph Fourier, 1768-1830) describes the spectrum of the signal f(t) and vice versa for the inverse Fourier transform. ∞

F(ω)= ∫ f (t)e−iωt dt

(1.14)

−∞ ∞

f (t)=

1 ∫ F(ω)eiω t dω 2π −∞

(1.15)

The fast Fourier transform (FFT) and the inverse fast Fourier transform (IFFT) do the same for discrete signals, however they are restricted to a length of 2n (2, 4, 8, ...). As the calculation time needed for such operations is much smaller, they are mostly used. N−1

X(n)= ∑ x(k)e

−2 π in k N

n = 0, 1, ... N-1

(1.16)

k=0

N−1

x(k)=

1 ∑ X(n)e N n=0

2 π in k N

k = 0, 1, ... N-1

(1.17)

FFT operations can be done in more than one dimension. Commercial mathematical programs like Matlab have such routines included; ImageJ (http://rsb.info.nih.gov/ij/) includes a 2D FFT. Sometimes signal corrections can be done more effectively in the frequency domain.

11

10 mm

1.6 mm/cycle

Figure 1-14: Computed tomography image of a SiC reinforced CVD SiC sample: spatial domain image (left) and frequency domain image showing a) the parameters of the reinforcement lattice (30°, 60°, 90°, square, 1.6 mm size and b) a slight disorientation (double indications) of some reinforcement layers in one of the directions (Empa)

1.4

Training, Qualification and Certification of Personnel

1.4.1

Introduction It is generally agreed, that NDT personnel needs special training, qualification and certification. Furthermore it is agreed, that this should be subdivided into three levels with respective know ledge, tasks and responsibilities. To reach qualification, candidates have to pass a theoretical and practical examination in the respective field. For certification in addition it is necessary to prove experience and certain visual requirements. The certification is limited in time a can be renewed only when candidates fulfil certain conditions. There are basic differences between the European and the US-American concept to reach this. The American concept (SNT TC-1A) foresees that the employer alone is responsible for the whole certification process (with some exceptions at level III). Training can be done by the em ployer itself or by a third party of free choice of the employer. The responsibility for the quality of the qualification stays with the employer in full. The European concept leaves certification to an independent and non-commercial certification body, which has to be accredited by the national accreditation body (in Switzerland: SAS – Swiss Accreditation Service, http://www.sas.ch). Training and qualification sites must be accepted by the certification body according to a written procedure. The employer has to assure basic training in the specific situation of the company (materials, products, equipment). In Switzerland training, qualification and certification of NDT personnel is performed under the authority of the Swiss Society for Nondestructive Testing (SGZP), http://www.sgzp.ch. In Europe the national societies are organised in the European Federation for Nondestructive Testing (EFNDT), http://www.efndt.org. The corresponding worldwide organisation is the International Committee for Nondestructive Testing (ICNDT), http://www.icndt.org.

1.4.2

NDT Levels European levels are indicated by Arabic, US-American by Roman numbers. The levels 1 and 2 are NDT inspector levels; level 3 is for NDT engineers. The demands and tasks of the different levels are as follows: Level 1: ▪

Set up NDT equipment;



Perform the tests;



Record and classify the results of the test in terms of written criteria;



Report the results.

Level 1 certified personnel shall not be responsible for the choice of test method or technique to be used, nor for the assessment of the results. Level 2:

12



Select the NDT technique for the test method to be used;



Define the limitations of application of the testing method;



Translate NDT standards and specifications to NDT instructions;



Set up and verify equipment settings;



Perform and supervise tests;



Interpret and evaluate results according to applicable standards, codes or specifications;



Prepare written NDT instructions;



Carry out and supervise all tasks at or below level 2;



Provide guidance for personnel at or below level 2;



Report the results of non-destructive tests.

Level 3: ▪

Assume full responsibility for a test facility or examination centre and staff;



Establish and validate NDT instructions and procedures;



Interpret standards, codes, specifications and procedures;



Designate the particular test methods, procedures and NDT instructions to be used;



Carry out and supervise all tasks at all levels;



Provide guidance for NDT personnel at all levels.

Level 3 personnel have demonstrated the competence to evaluate and interpret results in terms of existing standards, codes and specifications. The have sufficient practical knowledge of applicable materials, fabrication and product technology to select NDT methods, establish NDT techniques and assist in establishing acceptance criteria where none are otherwise available. They demonstrated a general familiarity with other NDT methods. Level 3 personnel may, if authorised by the certification body, manage and supervise qualification examinations on its behalf. 1.4.3

Experience, Training and Qualification The necessary industrial experience for a level 1 or a level 2 certification is one to nine months , dependent on the method and the level. For level 3 it ranges between twelve and eighteen months, dependent on general education and way to access the level. The minimum training requirements are given for all levels. For candidates with a technical dip loma of a college or a university the training hours can be reduced. The typical training syllabus contains terminology, physical principles, product knowledge and related capability of the method and derived techniques, equipment, information prior to testing, testing, evaluation and reporting, assessment, quality aspects, environmental and safety conditions, and developments. Table 1-3: Minimum training requirements [hours] (EN ISO 9712) NDT method

Level 1

Level 2

Level 3

AT (acoustic emission testing)

40

64

48

ET (eddy current testing)

40

48

48

Pressure method

24

32

32

Tracer gas method

24

40

40

MT (magnetic testing)

16

24

32

PT (penetrant testing)

16

24

24

RT (radiographic testing)

40

80

40

ST (strain gauge testing)

16

24

20

TT (infrared thermographic testing)

40

80

40

UT (ultrasonic testing)

40

80

40

VT (visual testing)

16

24

24

LT (leak testing)

The training ends with a qualification examination, consisting of three parts: general, specific, and practical. Level 3 personnel has to pass a basic examination that includes the knowledge of materials science and process technology, the knowledge of the certification system and a gen eral knowledge in at least four methods (level 2 complexity). Furthermore the practical part of the level 2 examination of the respective method has to be passed. 13

1.4.4

Vision Requirements Near vision acuity has to be checked once a year. The minimum requirement is the ability of reading of Jaeger number 1 or Times Roman N 4.5 with at least one eye, either corrected or uncorrected. Furthermore a sufficient colour vision has to be proved, to allow the candidate to distinguish and differentiate contrasts between the colours used in the NDT method concerned as specific by the employer. Typically this can be checked with pseudo isochromatic plates of Velhagen or Ishihara. They are designed in four ways: ▪

Transformation plates: where a person with normal colour vision sees one figure and a CVD (colour vision deficiency) person sees another;



Vanishing plates: where a person with normal colour vision sees the figure while a CVD person will not;



Hidden-digit plates: where a person with normal colour vision does not see a figure while a CVD person will see the figure;



Diagnostic plates: designed to be seen by normal subjects while CVD person seeing one number more easily than another.

Figure 1-15: Ishihara test plates (coloured and grey scale processed) Test like the Farnsworth-Munsell test, where coloured bottoms have to be laid in a certain order can give more detailed information about the colour vision ability of a person.

Figure 1-16: Farnsworth-Munsell colour vision test D-15 (left) and 100 Hue Test (TU Darmstadt, X-Rite) 1.4.5

Certification If a candidate has proved all above mentioned requirements, a certification is possible for the method and certain product or industrial sectors: Product sectors are castings (c), forgings (f), welded products (w), tubes and pipes, including flat products for the manufacturing of welded pipes (t), wrought products (wp), and composite materials (p). Industrial sectors combine a number of product sectors including all or some products or defined materials. Such sectors are metal manufacturing (M), pre- and in-service testing of equipment, plant and structures (S), railway maintenance (R) and aerospace (A). The certificate is valid for five years. After this period a renewal is necessary, that will be granted upon the proof of ongoing practice in the respective field. After another five years a recerti fication, based upon an additional examination, is necessary. For level 3 personnel another possibility is the fulfilling of a structured credit system.

14

1.5

Human Aspects

1.5.1

Influences on Human Reliability Human reliability in the context of the above discussed is influenced by various factors: ▪

Working conditions like illumination, noise, vibration etc.;



Physiology of the working environment;



Working atmosphere, personal orientation;



Vigilance (permanent watchfulness, caution);



Monotony – habituation;



Education, training, experience, information.

Missed critical signals

Identified critical signals

A lot of research was done in this field, not especially in NDE. Some of the results are expected, others less.

Time

Signal frequency (number per 30 minutes)

Figure 1-17: For a permanent watchfulness the number of missed signal increases with time (Mackworth, left), the relation between signal frequency and identified signals shows a maximum, a too small frequency lead to reduced caution (Schmidtke, right), many non-relevant signals, however, will reduce the attention as well (Jerison & Pickett, without figure)

Outward fare

Return fare

Figure 1-18: Monotony and vigilance of engine drivers in the Shinkansen express train TokyoOsaka (high Hz value means high concentration): with the older train 1965, the caution drops permanently during the journey, after the change to faster trains 1966, the caution keeps stable along the whole journey, five years later the engine drivers are used to high speed and the former situation is re-established (Endo-Kogi) An increase of concentration, and therefore of the results can be supported by:

15

1.5.2



Adapted signal frequency;



Adapted illumination and room climate, background music can have positive effects;



Adapted changes of the personnel;



Short breaks (some minutes every hour);



Double occupation of positions if necessary (four eyes principle);



Education, training, experience;



Information on results (if possible, e.g. if defected samples are added by purpose).

Colour Sensibility of the Human Eye Three types of cone photoreceptor and a single type of rod photoreceptor are present in the human retina. The rods are photo receptors that contain the visual pigment rhodopsin. They are highly sensitive to blue-green light with peak sensitivity around 500 nm wavelength. Cones contain cone opsins as their visual pigments and, depending on the structure of the opsin molecule, having a maximum sensitivity to either long wavelengths (red or L-cones), medium wavelengths (green or M-cones) or short wavelengths (blue or S-cones). Cones of different wavelength sensitivity are the basis of colour perception in our visual image. Blue cone

Rod

Green Red cone cone

Figure 1-19: Spectral sensitivity of cones and rods (after Dowling 1987) Under normal conditions, the maximum sensitivity lies at 555 nm a greenish yellow colour (photopic), while in darkness, it follows the sensitivity of the rods (scotopic). Under dark room conditions (typically below 20 lx) the maximum is slightly reduced to about 550 nm, but in general it is similar to the photopic behaviour.

16

Figure 1-20: Normalised response of an average human eye to various amounts of ambient light (1 ftc (foot-candle) = 10.6 lx) As under normal conditions a typical signal colour must be distinctive against its surroundings (like red or orange), under dark room conditions it must be chosen according to the sensitivity of the human eye, namely greenish yellow. 1.5.3

Colour Vision Deficiency (CVD) Usually all three types of cones in the retina can become perceived, we speak about trichro mats. Dependent on which one cannot or only reduced become perceived (due to missing photo pigment), there are different possibilities of colour weakness (anomalous trichromasy) or colour blindness (dichromasy). Most of the cases are due to an abnormal recessive gene on the X chromosome. The Y chromosome does not contain any colour vision gene at all. Therefore, if boys inherit the colour-blind gene, then it cannot be overcomed with a second normal colour vision gene. About 8% of the males and 0.3% of the females are born with a colour deficiency. The way a male inherits most forms of colour-blindness is from his mother, who inherits the colour-blind gene from her father or mother. Protanomaly (1% males, 0.01% females) is referred to as red-weakness (L-cones affected). Any redness seen in a colour by a normal observer is seen more weakly by the protanomalous viewer, both in terms of its saturation and its brightness. Red, orange, yellow, yellow-green and green, appear somewhat shifted in hue towards green, and all appear paler than they do to the normal observer. Under poor viewing conditions, such as when driving in dazzling sunlight or in rainy or foggy weather, it is easily possible for protanomalous individuals to mistake a blinking red traffic light from a blinking yellow or amber one, or to fail to distinguish a green traffic light from the various "white" lights in store fronts, signs and street lights that line the streets. Protanopia (1% males, 0.01% females) is the full red-blindness. The deuteranomalous person (5% males, 0.25% females) is considered green-weak (M-cones affected). Similar to the protanomalous person, it is poor at discriminating small differences in hues in the red, orange, yellow, green region of the spectrum. It makes errors in the naming of hues in this region because they appear somewhat shifted towards red for it. One very import ant difference between the two is that deuteranomalous individuals do not have the "loss of brightness" problem. Deuteranopia (1% males, 0.01% females) is the full green-blindness. The last form, tritanomaly and tritanopia (S-cones affected), happens with equal frequency in males and females (about 0.01%, abnormal gene on the chromosome 7). Other forms of monochromasy (cone or rod) are rare. Colour deficiency can also be the result of a cerebral apoplexy. For more detailed information see http://colorvisiontesting.com.

1.5.4

Contrast Sensitivity of the Human Eye The information of the rods and cones are finally collected in the ganglion cells. The response function of the ganglion cells on repetitive contrast patterns can be measured using a contrast image having the spatial frequency on one axis and the contrast on the other one. In this

17

receptive field the contrast is reduced until the ganglion cell just barely responds to the intro duction of such a stimulus into a featureless field.

Figure 1-21: Photopic contrast sensitivity function (Kalloniatis & Luu) To use such a pattern in a general way, the spatial frequency has to be given in cycles/degree. It can be demonstrated that in ganglion cells there is an optimal spatial frequency of stimula tion. Certain diseases/disorders like multiple sclerosis, cataracts, refractive error or amblyopia (“lazy eye”) are reducing the contrast sensitivity function. 1.5.5

Dark Adaptation The eye operates over a large range of light levels. The sensitivity of our eye can be measured by determining the absolute intensity threshold, that is, the minimum luminance of a test spot required to generate a visual sensation. This can be measured by placing a subject in a dark room and increasing the luminance of the test spot until the subject reports its presence. Con sequently, dark adaptation refers to how the eye recovers its sensitivity in the dark following exposure to bright lights. In 1865 Aubert was the first to estimate the threshold stimulus of the eye in the dark by measuring the electrical current required to render the glow on a platinum wire just visible. He found that the sensitivity had increased 35 times after time in the dark and introduced the term "adaptation". Dark adaptation forms the basis of the duplicity theory which states that above a certain luminance level, the cone mechanism is involved in mediating vision; photopic vision. Below this level, the rod mechanism alone provide scotopic (night) vision. The range where two mechanisms are working together is called the mesopic range, as there is not an abrupt transition between the two mechanisms. The dark adaptation curve depicts this duplex nature. The first curve reflects the cone mechanism. The sensitivity of the rod pathway improves considerably after 5-10 minutes in the dark and is reflected by the second part of the dark adaptation curve. One way to demonstrate that the rod mechanism takes over at low luminance level is to observe the colour of the stimuli. When the rod mechanism takes over, coloured test spots appear colourless. This duplex nature of vision will affect the dark adaptation curve in different ways. Dark adaptation is reached by chemical processes in the retina cells; the process and effect in cones and rods are different.

18

Figure 1-22: Dark adaptation curve (Hecht & Mandelbaum) Further information on vision in general may be found under http://webvision.med.utah.edu (medical and very detailed) or http://www.perret-optic.ch. 1.5.6

Visual Perception Eye movements are integral to visual perception; to understand vision, it is necessary to understand the role that eye movements play. The work of Alfred Yarbus was very important in expanding the view of the role of eye movements as externally visible reflections of cognitive events. In one of Yarbus' classic experiments on eye movements during perception of complex objects, he monitored the eye movements as people viewed Ilja Repin's painting "The Unexpected Visitor" (1884-88, original: Tretjakov gallery, Moscow).

Figure 1-23: Ilja Repin's “The Unexpected Visitor” and scan paths for a single observer viewing the painting under each of the seven instructions (see text) (A. Yarbus: Eye Movements and Vision, New York, 1967) Before viewing, the subjects were instructed to perform one of seven tasks: 1) Free examination of the picture, 19

2) Estimate the material circumstances of the family in the picture, 3) Give the ages of the people, 4) Surmise what the family had been doing before the arrival of the "unexpected visitor," 5) Remember the clothes worn by the people, 6) Remember the position of the people and objects in the room, and 7) Estimate how long the "unexpected visitor" had been away from the family. The pattern of eye movements and fixations varied dramatically with different instructions to the subjects. Yarbus concluded that "... the distribution of the points of fixation on an object, the order in which the observer's attention moves from one point of fixation to another, the duration of the fixations, the distinctive cyclic pattern of examination, and so on are determined by the nature of the object and the problem facing the observer at the moment of perception." The fact that eye movement patterns are not determined by the stimulus alone, but are dependent on the task being performed, suggests that eye movements are an integral part of perception and not simply a mechanism evolved to deal with the 'foveal compromise' (the uneven distribution of photo receptors across the retina that allows both high resolution and a wide field of view).

1.6

Standardisation

1.6.1

National Standards (Switzerland) According to the CEN/CENELEC Internal Regulations Switzerland has to overtake European Standards to its own national standardisation scheme. The Swiss national standards body is the Swiss Association for Standardisation (SNV), http://www.snv.ch, among others member of CEN and ISO. These standards are published in German and French as SN EN. For standards concerning non-destructive testing the technical committee INB/TK 180 is responsible. Other national standards concerning pressure equipment devices are published by the Swiss Association for Technical Inspections (SVTI), http://www.svti.ch and those concerning civil engineering by the Swiss Society of Engineers and Architects (SIA), http://www.sia.ch.

1.6.2

European Standards In Europe the national standards bodies of the EU and EFTA countries are organised in the European Committee for Standardization (CEN), http://www.cenorm.be. Standards are published in English, French and German, national standards bodies have the right to do their own translations. Preliminary standards are denominated as prEN, current standards as EN. For standards concerning non-destructive testing the technical committee TC 138 is responsible. EN standards are metric. A list of currently valid EN standards concerning NDT is given in the appendix.

1.6.3

International Standards Internationally, the national standards bodies are organised in the International Organization for Standardization (ISO), http://www.iso.org. Common standards of ISO and CEN are denominated EN ISO (in CEN countries).

1.6.4

US-American Standards For NDT purposes there are three main sources of US standards. A general one is ASTM International, formerly known as the American Society for Testing and Materials, http://www.astm.org, with its Section 03 – Metals Test Methods and Analytical Procedures, Volume 03.03 – Nondestructive Testing. The one concerning pressure equipment devices is the ASME International Boiler and Pressure and Vessel Code, especially section V (Nondestructive Examination), section VIII (Pressure Vessels) and section XI (Rules for Inservice Inspection of Nuclear Power Plant Components). ASME is The American Society of Mechanical Engineers, http://www.asme.org. The third one are the MIL-standards, http://www.mil-standards.com. In general US standards are in English only and non-metric. With some exceptions, US standards are used in Europe for products designed for the export to Asia and the American continents only.

1.6.5

Japanese Standards The Japanese standards are published by the Japanese Industrial Standards Committee (JISC), http://www.jisc.go.jp.

20

1.6.6

Standards for Aviation The EASA Certification Specificationsare published by the European Aviation Safety Agency (EASA), an agency of the EU, http://www.easa.eu.int/.

1.7

Laboratory Accreditation Laboratories can be accredited as testing laboratories according to EN ISO/IEC 17025. The responsibility for this in Switzerland has the SAS – the Swiss Accreditation Service, http://www.sas.ch, within the State Secretariat of Economic Affairs (SECO). Downloads are often available in Swiss governmental languages only. Accreditation means the formal recognition of authority to execute a specific service as described in the scope of accreditation. Competence is the key to transparency, confidence and comparability. Competence can be separated in three fields: ▪

Personnel: technical know how, practical experience in the relevant technical field and continuous education;



Technical infrastructure: decisive criteria, equipment, procedures;



Organisational structure: independence, impartiality, quality management.

Especially the first two points make a substantial difference of such an accreditation in comparison to ISO 9000. On the European level the national accreditation bodies are organised in the European Co-operation for Accreditation (EA), http://www.european-accreditation.org. They publish several guidelines among them the EA-4/15: Accreditation for Bodies Performing Non-Destructive Test ing and several others concerning measuring uncertainty, calibration etc. Only laboratories with a valid accreditation are allowed to use the corresponding logo in their reports etc. (federal ordinance SR 946.512).

1.8

Examination Procedure and Report – General Requirements

1.8.1

Examination Procedure Examination procedures often are part of an order or of contracts. In complex cases they can be the result of extensive negotiations between the ordering and the executing party. A product standard, or a standard describing a specific technique, can serve as a product examination procedure if it is self-sufficient with respect to the examination. Beneath technical aspects that are dependent on the examination method, an examination procedure should cover the following general aspects: ▪

Reference documents (standards, specifications, etc.);



Description of the product to be examined (material, geometry, surface conditions, heat treatment, manufacturing state, etc.);



Goal and extent of the inspection;



Location of examination and corresponding environmental criteria;



Available support, utilities (scaffold etc.) and resources (electricity, water, pressurised air etc.);



Details concerning environment (disposal etc.) and safety (electrical, radiation protec tion, etc.);



Description of examination method, technique, sensitivity and sampling;



Required reference blocks, calibration intervals;



Corrective actions in case the intermediate or final check does not conform to the requirements;



Required signal processing and requested documentation of intermediate data;



Registration and acceptance criteria (evaluation and recording levels), extension of test;



Method of recording or marking;



Required cleaning and conservation;



Requested qualification and certification of examination personnel;



Extent of examination report, supplements and corresponding details (language, etc.).

The following details concerning software should be clarified in advance:

21

1.8.2



Software (including version) used;



Deliverable output (original, intermediate and final data), soft or hard;



Formats;



Responsibility for storage and later possibility of play-back.

Examination Report An examination report should allow a third party to reproduce the tests carried out in full. Therefore, it shall contain at least the following information; the technical details are further spedified in the respective chapters: ▪

Unambiguous identification of the examining institution;



Reference to contractual documents and standards;



Location(s) and date(s) of examination;



Clear identification when sub-contracted sampling or performance of tests are included;



Reference to supplements to the report;



Recommendations, if agreed



Complete identification of the object examined, including state;



If sampling was involved, clear indication of its base;



Identification of the methods, techniques and examination equipment used, including ref erence blocks;



Any deviation from the procedure (e.g. restricted access, inadequate surface finish, surface temperature, etc.);



Results of examination and evaluation;



Type of labelling and marking of the test objects;



Name, qualification and signature of the examiner or any other responsible for the examination.

If the organisation is accredited to perform NDT under ISO/IEC 17025 several further require ments may apply.

22

Figure 1-24: Example of the cover sheet of an examination report (Empa) For NDT reports the operator usually has to sign the respective record.

23

Figure : 1-25: Example of the assessment and the signature part of a record (Empa) 1.8.3

Marking Sometimes it is necessary to mark the objects permanently. For this reason metallic indentators exist; they are referenced on the examination record with a corresponding rubber mark.

Figure 1-26: Indentator and rubber mark (Empa)

24

2

VISUAL AND OPTICAL INSPECTION

2.1

Introduction Visual inspection is used to characterise the surface optically of the parts under test. Principally some sort of visual inspection has to be performed prior to all other NDT methods; the “real” visual testing, however, is a method with its own characterisation. The inspection of the object in full is usually called general visual testing. In this case the distance between the observer and the object exceeds 600 mm. Typically this includes the surface finish or coating, distortion or damage, general fit or alignment and identification of missing parts of the component. For a local visual testing the distance is usually not exceeding an arm's length. One can differentiate between direct visual testing, where there is an uninterrupted optical path from the observer's eye to the object. In opposite, the remote visual testing is characterised by an interrupted optical path, where the signal is transmitted electronically or the image is stored in one way or another. Replica techniques and roughness measurements can be regarded as being a part of this method as well. Deduced from the expressions radar and sonar, lidar, light detection and ranging, is used for corresponding measurements. A basic knowledge of the product and the involved materials and processes often is necessary to find also indications, which give only hints for possible flaws. This can be geometrical de formations, changes of the colour the surface quality etc.

2.2

Equipment

2.2.1

Measuring Equipment and Reference Parts for Comparisons Often used measuring devices are check gauges, templates, calliper gauges, and similar. The surface roughness has not always to be measured, but can become determined be comparing it to reference patters. For machined surfaces this can be the roughness comparator Rugotype. For the definition of the surface quality of steel castings and the surface texture of preci sion steel castings sets of comparators by SCRATA (Steel Castings Research and Trade Association) are available. Similar comparators exist according to BNIF 359 (Bureau de Normalisation des Industries de la Fonderie).

Figure 2-1: Visual tactile roughness comparators Rugo-Test (left) and SCRATA (Studotech, SCTI) 2.2.2

Equipment for Direct Visual Techniques No equipment is necessary for unaided testing. Possible equipment for aided testing are mirrors, lenses, endoscopes or fibre optics. Endoscopes can be differentiated into borescopes, which contain a rigid part and fibrescopes with a flexible device using fibre optics to transmit images from the interior or generally inaccessible parts of the object. Typical optics at the tip look forward (0°), lateral (45° or 90°), or backwards (110°). This is usually done by a prism (not a mirror). The field of view is dependant on the lens. The resolution of fibrescopes are limited to the number of single fibres, typical values are 2'000 to 70'000. 25

For long distances or small radii the articulation cables in fibrescopes and videoscopes have to be replaced by a pressure system.

Figure 2-2: Sketch of a rigid borescope, a fibrescope and a videoscope (from top, Olympus)

Figure 2-3: Borescopes (left) and fibrescope (Volpi)

Figure 2-4: Endoscope's tip with 90° prism (UXR)

26

Figure 2-5: Sketch of an insertion tube (not drawn to scale) (EverestVIT) 2.2.3

Equipment for Remote Visual Techniques For the acquisition of images all types of cameras can be used, including photo, video or thermal, either directly or with the aid of videoscopes. The images are viewed on the usual screens and the common image / film processing software and archiving technique is used. Special cameras are necessary for underwater use or within other liquids. An automated evaluation only makes sense together with remote equipment.

Figure 2-6: Equipment for remote visual inspection: pipeline crawler (left) and underwater rover (Inuit, Rovtech) Remote inspection is also done with UAV (unmanned aerial vehicle), also called drones, using photo cameras (2D or 3D), video cameras or thermal cameras. Technically the application is for places where a direct access is complicated or expensive like for power lines, bridges, photovol taic panels and similar.

Figure 2-7: UAV, here equipped with a 3D photo camera (Dercopter) Wire cables of cableways or mines have periodically to be inspected visually. To be able to overview the whole circumference mirrors are necessary. The concentration of the inspector, however, will decrease after short time, so the probability of detection is reduced. Systems were designed to inspect such cables using several cameras with automated or off-line data interpretation.

27

Figure 2-8: Equipment for remote visual wire cable inspection with four cameras (Stuttgart University, IFT) 2.2.4

Illumination The illumination can be done directly with a lamp at the top of the scope (warm light) or in combination with a light guide (cold light). The spectral characteristics of a light guide can be dependant on its length. Elongations are possible, however, for every interface a certain loss is to accept. To tune spectral or intensity characteristics, filters are used. Spectroscopic or ultraviolet light sources are available as well. For certain applications like the interpretation of tempering colours it is of great importance to chose the correct colour temperature of the light source. For medical application a halogen lamp may be sufficient, while for a lot of technical application a xenon lamp is necessary. Table 2-1: Typical colour temperatures Light Source

Colour Temperature [K]

Candle

1'500

Sodium discharge lamp

2'000

Halogen lamp

2'700 - 3'200

Soft white

3'000

Cold white

4'000

Moon light

4'100

Xenon lamp

4'500 - 5'000

Horizon daylight

5'000

Daylight noon

5'500

Daylight overcast

6'500 - 7'500

Clear blue poleward sky

15'000 - 27'000

The colour temperature is the Temperature of an ideal black body radiator that radiates light of comparable hue to that of a light source. It can be shown as Planckian locus in the CIE 1931 chromaticity diagram.

28

Figure 2-9: Colour temperatures on the Planckian locus in the CIE 1931 chromacity diagramm 2.2.5

Equipment for Optical Methods The usual equipment used for most optical methods consists of illumination sources (usually lasers), cameras and the necessary tools for the beam treatment (mirrors etc.).

Figure 2-10: Measuring equipment for fringe projection; for reverse engineering (left) and the famous Kinect motion sensing input device for the Xbox video game (GOM, public domain)

Figure 2-11: Measuring equipment for fringe projection (left), moiré interferometer (GOM, Photomechanics)

29

Figure 2-12: Equipment for electronic speckle pattern interferometry: for 3D pulsed ESPI with three cameras (left) and for small parts (Dantec, Optonor)

Figure 2-13: Equipment for shearography: multi-wavelength sensor with three peripheral laser diodes (810 to 850 nm) and camera housing for three synchronised cameras allowing to access all six displacement derivatives simultaneously (left), stationary and mobile applications using vacuum for contour changing (Empa, Dantec) 2.2.6

Forensic Equipment For forensic applications special combinations of filters and goggles are in use. They are neces sary to avoid disturbance of the human eye by unwanted reflections. Similar applications and in combinations with camera systems for NDT use are at least possible.

Figure 2-14: Equipment and application for forensic use (Labino)

2.3

Testing Procedures for Visual Testing

2.3.1

Standard Procedures For the process evaluation a demonstration test piece has to be used. The indications on it should be similar to those to be detected on the component to be tested. Surface conditions,

30

reflectivity, contrast conditions, and accessibility should also be near to the component itself. Usually the technique has to be proved at a location with poor optical accessibility. For general visual testing a minimum light intensity of 160 lx is necessary, for detailed local in spections, this has to be increased to at least 500 lx. The success of visual testing is influenced by the light and its colour temperature, dazzling, vibrations and factors that may reduce the concentration of the observer, as these are climate, noise, duration (fatigue) and ergonomic position. 2.3.2

Automated Optical Mass Inspection (AOI) For certain applications, mainly the optical inspection in circuit board manufacturing, automated systems are in use. They can be included in-line of the manufacturing process chain after the different steps (e.g. paste printing, assembling, soldering) or off-line. The images are usually gathered with several camera directions and the illumination is achieved with a series of LEDs that allow the most efficient incidence of light. The camera resolution can be as high as 10 μm. Dependent on the size of the board and the number of parts, such an inspection can last up to several minutes. A combination with X-ray systems is possible. The evaluation is done automatically using neuronal networks; the programming of such a process is very demanding. At first use a lot of false calls will occur and an on-going enhancement of the procedure is necessary, therefore it is best suited for mass inspection. For certain applic ation fields like telecommunication, automotive or air & space, such inspection systems are required.

Figure 2-15: AOI unit and some typical defects: missing BGA paste (top left), chip thombstoning (top right), solder bridge (bottom left) and wrong diode polarity (Viscom) Similar equipment for timber where knots, resin pockets, checks, wanes, blue stains and dimensional errors can be found automatically are used as well. Various other applications in mass production exist.

31

Figure 2-16: Automatically detected flaws in timber (Woodeye) 2.3.3

Confocal Laser Scanning Microscopy (CLSM) Confocal laser scanning microscopy (CLSM) is focusing a laser beam to a certain depth and reading the reflected photons using a photo detector. Scattered light from other regions is eliminated by a confocal pinhole. It can be applied to larger structures. Typical lateral and depth resolutions are below 1 μm, depending on the scan area. The investigation depth is limited by the optical penetration. With image processing the acquired scans can be used to con struct a 3D image. Confocal microscopy is often used in biology, various technical adoptions exist.

(G) (F) (B) (A) (C)

(D) (E) Figure 2-17: Principle of CLSM with laser source (A), source pinhole (illumination aperture, B), dichroic mirror (beam splitter, C), objective lens (D), focal plane (E), detector pinhole (confocal aperture, F) and photo detector (G), open designed CLSM (Biomedical Photometrics)

32

Figure 2-18: CLSM Scan of a CD surface, 12 x 12 μm (Stuttgart University, IKP) 2.3.4

Optical Coherence Tomography (OCT) Optical coherence tomography (OCT) permits non-destructive / non-invasive imaging and it combines, to a large extent, the advantages from confocal microscopy (in terms of spatial res olution) and high-frequency ultrasound imaging (in terms of penetration depth). OCT fills a technological niche for probing optically semi-transparent tissue or material. OCT retains depth information via propagation time of light within the medium. High spatial resolution in lateral and depth direction is obtained from the coherent properties of optical waves. Signal contrast can be retrieved from some of the optical parameters. Light from a low coherence light source ("white light") is split by a beam splitter into two optical arms. One is terminated by the turbid object and the other by a reference mirror. After reflection from any point within the object and from the reference mirror, both light portions are recombined in the same beam splitter and relayed to a detector. An interference signal is only detectable when the optical paths of both arms match within the coherence length of the light source. Thus by moving the reference mirror, the stationary object can be scanned in depth direction. OCT is not restricted to visible light only, it can be applied within the infrared spectrum as well. Due to the penetration depth infrared radiation is often more appropriate. In medical applica tions it is used for tissue characterisation, often with an endoscope / catheter. OCT can be used in combination of spectroscopy and/or polarisation imaging. Using a 2D sensor array instead of a single sensor parallel optical low-coherence tomography (pOCT) enables real-time 3D imaging for topographic and tomographic applications.

Figure 2-19: OCT principle (ISIS)

33

Figure 2-20: OCT image of several layers of a roll of adhesive tape (left), comparison of an OCT image (below) with the one from histology (ISIS, LightLab Imaging) 2.3.5

Scanning Near Field Optical Microscopy (SNOM) Basically scanning near field optical microscopy (SNOM) means scanning a surface through a hole smaller than the wavelength of the light (typically < 100 nm). This allows a higher resolu tion. As only a very limited amount of intensity is penetrating through the hole, the method is slow. Similar methods exist for microwaves and others.

2.4

Optical Measuring Methods

2.4.1

General Optical measuring methods determine the two or three dimensional surface contour non-invasively, contact free and over larger areas. Dependent on the questions the pure shape or differ ences in the shape, e.g. due to mechanical or thermal stress, are determined. In comparison to tactile methods, optical methods have several advantages: ▪

Contact-free and therefore also usable for very sensitive surfaces;



Massively parallel as areas can be measured at the same time;



Highly accurate, up to 1% of the wavelength (typically 5 nm);



Flexible, the same or similar equipment can be used to answer several questions;



The methods can be automated for certain applications.

ESPI, also called TV holography or digital holography, and shearography are laser optical full field measuring techniques. They are based on the laser speckle effect which occurs, if a rough surface is illuminated by laser light. Several sources have slightly different definitions of the techniques. 2.4.2

Fringe Projection Fringe projection is a non-destructive and contact-free method of surface contour determina tion. The surface is illuminated with a fringe pattern which becomes deformed by the contour of the object. The projected fringes do not need to be in the visible domain, infrared is common as well. A multi-camera system reads the pattern and a full 3D model of the surface can be extracted. Fringe projection is a very fast tool for outer surfaces that can be illuminated com pletely. Applications are in quality assurance, reverse engineering (e.g. for design models with free-form surfaces), rapid prototyping and 3D visualisation. It is also common for medical, dental and forensic inspections. A special application is the triangulation by light sectioning method.

34

Figure 2-21: Application of fringe pattern technique at a wind tunnel model of the X38 reusable launch vehicle (RLV), data acquisition and point cloud model (GOM) 2.4.3

Electronic Speckle Pattern Interferometry (ESPI) In ESPI a laser beam is expanded by a lens and illuminates the surface to be measured. The reflected light is combined with a reference beam which is directly coupled from the laser to the camera. The camera records a series of speckle images. The comparison of the images shows the changes in the speckle structure and generates correlation fringes. They result from displacement and deformation of the surface between the recorded images. Software automatically analyses these fringes and calculates quantitative displacement values. Advanced ESPI systems use several laser illumination directions or cameras to generate 3D information about displacement and deformation as well as contour information (3D-ESPI System).

Figure 2-22: Application of 3D ESPI: vibration of a brake disc excited at 5046 Hz, top: in plane deformation Vx and Vy, below: out of plane deformation Vz and 3D view; the images of the three cameras are distorted due to perspective and have to be corrected before calculating the phase images (Dantec) From this data, strains, stresses, vibration modes, and many more values can be derived. Material industry uses such a technique to measure Young modulus, Poisson ratio, crack growth, true strain / true stress functions. High speed measuring systems can also deliver dynamic material values which are used for crash tests and crash simulation.

35

all other components, which are highly stressed and critical for automotive safety. But also noise vibration harshness (NVH) questions are solved with pulsed ESPI techniques. A pulsed laser fires two laser pulses with variable time delay and one to three high speed ESPI cameras record the images. The measuring result shows the operational deflection which is used to eliminate sound sources, optimise damping systems, remove brake squeal or judder etc. While the typical application in NVH is the reduction of noise, ESPI can also be used to optimise the sound, e.g. in door slam tests. Another advantage of the pulsed ESPI technique is that shock events can be analysed as well, e.g. to show the propagation and reflection of Rayleigh waves. Concerning vibrational analysis similar results can be obtained using laser Doppler vibrometers. 2.4.4

Laser Shearography Laser shearography is an other speckle interferometric technique that is widely used in non-de structive evaluation. However, the optical set-up is slightly modified. Instead of a reference beam, the image of the object is doubled, laterally sheared and superimposed in the camera. A speckle pattern is generated which now shows the deformation gradient of the surface being tested or analysed. Modern phase shifting techniques and fringe unwrapping techniques also allow the automatic analysis of this information. Because laser shearography measures only the gradient of the deformation, it is relatively insensitive to rigid body movements. Therefore, this technique is typically used for defect recognition in a production line or in maintenance.

(A) (E)

(B) (D)

(F) (C) (G) Figure 2-23: Schema of laser shearography: (A) laser, (B) beam spreading, (C) object, (D) beam splitter (here shown a Michelson interferometer (Albert Michelson, 1852-1931, NP 1907), (E) and (F) mirrors, and (G) camera During the production process of modern composite materials, many different components are bonded together. Often, parts of this assembly process are carried out manually. Therefore, it is of utmost importance for product reliability and quality control to carry out a non-destructive test at certain steps of the production line. Shearography has proven to be a very capable tool for all kinds of NDT applications. Aerospace industries use shearography systems to test com posite materials of glass fibre reinforced plastics (GFRP), carbon fibre reinforced plastics (CFRP), GLARE (aluminium-fibreglass sandwich), foam and aluminium composites, etc. Fully automatic inspection systems have been installed for ARIANE 5 inspection as well as for heli copter rotor blade inspection. For maintenance inspection portable shearography inspection systems have been developed using vacuum or heat load to reveal the defects. In automotive industries tire testing and dash board inspection are well known applications.

36

Figure 2-24: Applications of laser shearography: delamination in a CFR laminated plate (left), internal fibre structure with several crossed fibre layers visible (Optonor) 2.4.5

Moiré Interferometry Moiré interferometry is a technique to study strains and deformations of structural elements with very high accuracy. It requires highly stable environment and therefore it is mainly a laboratory tool. Moiré is the generic term for full field measurement techniques, which utilise the interference effect between some form of specimen grating and reference grating to magnify the surface deformations and create a contour map which is related to surface displacement – a moiré fringe pattern. The term was first used by weavers of mohair and is not the named after the French physicist. For relatively large displacements, mechanical moiré uses the interference of lined gratings to achieve sensitivity of measurement of up to 25 µm. Optical moiré, or more commonly known as moiré interferometry, in which a diffraction grating is illuminated by laser light, increases the sensitivity to sub-micrometre level, enabling measurement of elastic strains in engineering materials. The moiré pattern is a full field representation of the relative displacement between the gratings. This property of moiré makes it an excellent tool for observing and quantifying the gradients in localised deformation. In practice, a grating is attached on the surface of the test piece. The grating deforms together with the test piece and when an undeformed (reference) grating superimposed onto it, a moiré pattern depicting the nature and the magnitude of the deformation field is obtained. Each moiré fringe represents a line of constant displacement in the direc tion perpendicular to the direction of the reference grating. The displacement value U of a fringe is U=Np

(2.1)

where N is the fringe number relative to a known zero displacement and p is the pitch of the reference grating, that is the perpendicular distance between the lines. The moiré effect described above is termed mechanical moiré because the fringes are formed from the mechanical crossing of the two gratings. The mechanical moiré effect is limited by the frequency (inverse of pitch) of the grating employed. The typical upper limit for these types of gratings is 40 lines/mm; above diffraction effects dominate. This means a minimum deformation of 25 µm would be required to generate one fringe. This is quite adequate for large deformations, but in order to measure small elastic strains much higher grating densities, which would give higher deformation sensitivities, are required. Moiré interferometry provides higher sensitivity by employing the principles of light interference and diffraction. The diffraction effect split the incoming light beams into multiple preferred rays on the grating. When the grating is undeformed, the -1 and +1 diffraction order from the beams emerge perpendicular to the grating without any interference. When the specimen is deformed, the -1 and +1 orders will exit the specimen grating warped and interfere with each other. The result is a moiré fringe pattern of the in-plane displacements. The interpretation of the pattern is identical to that of mechanical moiré.

37

Figure 2-25: Moiré interferometry shows the deformation of a silicon humidity sensor between 0% and 90% relative humidity (Empa)

2.5

Aspects of Quality Assurance

2.5.1

Resolution Targets To quantify or qualify the remote equipment, the camera alone or in connection with the other devices, resolution targets are used. Usually these are optical test charts or line charts. The resolution is influenced by several properties: horizontal / vertical / angular, field of view, resolution in the centre or at the edge, contrast conditions, involved colours, light intensity and light colour, static or dynamic observation. One of the most common used resolution targets is USAF 1951 (positive or negative version). The positive pattern is recommended for quality control of microscopes and magnifiers. The negative pattern is ideal for collimators and other illuminated test equipment. Targets with different contrast patterns are available. The ISO PIMA resolution and contrast targets are two more out of a lot of possibilities. The PIMA resolution pattern (pattern-induced multi-sequence alignment) offers a simply method in obtaining MTF data. The target principles can be used for all imaging NDT applications, with or without optical cameras.

Figure 2-26: USAF 1951 resolution target (Edmund) For contrast and colour resolution, adequate targets, similar to printing, are used.

38

Figure 2-27: Colour target (Kodak) 2.5.2

Viewing Conditions The local illumination can be measured with an illuminance meter or a luxmeter. Lux is a derived unit based on lumen (1 lx = 1 lm m-2). This unit takes into account the intensity as perceived by the human eye (380 – 780 nm). The colour temperature of the light source and the reflection conditions of the surface have to be taken into consideration. Certain colours (higher blue content) cannot be interpreted correctly if the colour temperature is too low. For general visual testing an illuminance of at least 160 lx must be achieved, for local visual testing one of at least 500 lx.

Figure 2-28: Illuminance meter or luxmeter (Rotronic) 2.5.3

Special Vision Requirements In addition to the general vision requirements, personnel doing visual testing should fulfil a far vision acuity of 0.63 according to EN ISO 8596, with at least one eye, either corrected or uncorrected.

2.6

Procedure and Record

2.6.1

Procedure The NDT procedure usually has additionally to define the following special technical aspects: ▪

Special surface preparation;



Illumination (type, strength, direction);



Equipment;



Requested resolution;



Necessary cleaning and conservation.

39

2.6.2

Record The record should contain the following information:

2.7



Description of the equipment used, including serial numbers and firmware version;



Viewing conditions;



Reading of resolution targets;



Applied conservation.

Trends, Summary and Conclusions Visual testing is a rather simple method. With the necessary knowledge about material, manu facturing process and load, the surface information can lead to the detection of inner defects or to weak points of a structure. For some materials a near surface inspection is possible. Optical coherence tomography in the near infrared range is a common methods for medical dia gnostics. It can be assumed that related techniques like spectroscopy and polarised light could find their way for applications in the field of non-destructive evaluation as well. A possible trend is the application of 3D methods for imaging. For consumer electronics devices exist that allow the acquisition and the display of 3D images and films, even without special goggles. Such devices could also become a trend for visual inspection.

Figure 2-29: 3D lens (Panasonic) Another future possibility is the use of a light field camera, also called plenoptic camera. The picture is not taken by a usual sensor, but the light field detector captures the colour, intensity and vector direction of the incoming rays of light. This allows to refocus the final image and possibly to change the direction of the projection as well. Since 2012 consumer products are on the market.

Figure 2-30: Light field camera (Lytro) Optical methods in general acquire the outer shape. An applied load results in a change of the surface and the differences can be used to conclude on inner structures. Optical methods are non-contact and can become designed to be very accurately. Multi-wavelength technique with its possibilities of the simultaneous access to the different directions is a prospective tool for the application to dynamic processes. 40

3

INFRARED AND THERMAL INSPECTION

3.1

Introduction Sir William Herschel (1738-1822), an astronomer, discovered infrared (IR) in 1800. He built his own telescopes and was therefore very familiar with lenses and mirrors. Knowing that sunlight was made up of all the colours of the spectrum, and that it was a source of heat as well, Herschel wanted to find out which colour(s) were responsible for heating objects. He devised an experiment using a prism, paperboard and thermometers with blackened bulbs where he measured the temperatures of the different colours. Herschel observed an increase in temperature as he moved the thermometer from violet to red in the rainbow created by sunlight passing through the prism. He found that the hottest temperature was actually beyond red light. The radiation causing this heating was not visible; Herschel termed this invisible radiation "calorific rays". Today, we know it as IR. In 1880 the bolometer – a thermal detector whose electric conductivity changes when heated by an impinging radiation – was invented by Samuel Langley (1834-1906) and later perfected by Charles Abbot (1872-1973), who used it to sense the thermal radiation of a cow some 400 m away. IR thermography is a relatively young inspection method. Even if the theoretical basics have been known for a longer time, the instrumentation became not affordable until the interest for such systems for security reasons has grown. For the moment only a small number of stand ards dealing with this and every producer defines the specification of its products a bit different. A comparison between the various products , therefore, is not allways easy. Thermography is widely used in civil engineering; for other fields, however, it also shows prospective possibilities.

3.2

Heat Transfer

3.2.1

Heat Transfer Mechanisms Heat may be transferred by different mechanisms, in solids mainly by conduction, in liquids and gases mainly by convection. Conduction is the propagation of heat energy whenever a temperature difference exists between two solid bodies in contact or among parts of a body. Convec tion involves the mass movement of gas or liquid molecules over large distances. Two solid bodies will exchange energy by convection if they are in contact with a fluid. Radiation is a process of heat transfer and is characteristic of all matter at temperatures higher than absolute zero. Radiated energy may be transported over large distances through gases or a vacuum with no conduction or convection at all. For conduction the relevant relation is formally corresponding to the second Fick law of diffusion (Adolf Fick, 1829-1901) ∂T =α ∇ T ∂t

(3.1)

where α is the thermal diffusivity. The spectral emission of a blackbody is given by Planck law, (Max Planck, 1858-1947, NP 1918) where h is the Planck constant (6.63 · 10-34 J s), k the Boltzmann constant (1.38 · 10-23 J K-1, Ludwig Boltzmann, 1844-1906), c the velocity of light and λ the wavelength. K B λ, T =

2h c 2 λ5

1 e

hc kT λ

(3.2)

−1

The maximum wavelength is simply related to the temperature by the Wien law (Wilhelm Wien, 1864-1928, NP 1911) λmax T=2.898⋅10−3 K m

(3.3)

Components begin to glow in the visible range for temperatures above about 600 °C. Most of the processes that are interesting for non-destructive testing, therefore, happen in a range with IR radiation only. Aside from a shift towards shorter wavelengths, it should not be forgotten that a warmer blackbody always emits more than a colder one, at any wavelength.

41

Spectral em ission

200 °C

100 °C

RT

0

10

20

30

40

Wavelength [µm]

Figure 3-1: Spectral emission of a blackbody at different temperatures; the maximum wavelengths are 9.7 µm at room temperature, 7.8 µm at 100 °C and 6.1 µm at 200 °C The total radiation emitted by a blackbody is given by the Stefan-Boltzmann law (Joseph Stefan, 1835-1893) K B T =σ T 4

(3.4)

where σ is the Stefan-Boltzmann constant (5.67 · 10 W m K ). -8

-2

-4

Kirchhoff law (Gustav Kirchhoff, 1824-1887) gives the relation between a blackbody and a real component. ελ =

K λ, T  K B λ , T

(3.5)

Figure 3-2: Different thermal emission of metals (top row), ceramics and polymers (bottom row) in a water stabilisation bath of 45 °C with visible convection, range 25 °C (blue) to 45 °C (white) (Empa) A frequently used simplification of real part supposes their surface to be grey, meaning that its spectral radiation properties do not depend on the wavelength. The second usual simplification supposes the surface to be diffuse, meaning that its directional radiation properties do not depend on the emission angle. For the most common materials, emissivity tables can be found in literature. In general, such tables present total emissivities, rarely spectral emissivities, and almost never directional emissivities. The tables often give a good idea of the approximate emissivity values but in actual temperature measurement conditions the only knowledge of the total emissivity may not be sufficient.

42

3.2.2

Thermal Properties The basic thermal properties of materials are the thermal conductivity k, the specific heat capacity C and the density ρ. Thermal conductivity is a material heat transport characteristic responsible for the attenuation of heat flux – especially in the steady state case. The two dynamic characteristics are the thermal diffusivity α and the thermal effusivity e k ρC

(3.6)

e=  k ρ C

(3.7)

α=

Thermal diffusivity is a measure of the rate at which heat is diffusing through a material. Generally, materials that have high thermal conductivity have high thermal diffusivity as well and respond more quickly to changes in their thermal environment. Especially for comparison reasons to microwave and ultrasonic inspection, it is useful also to define the thermal impedance Z=

ΔT 1 ∝ Q e

(3.8)

Thermal effusivity is a measure of the ability of a material to increase its temperature as a response to a given energy input. It is often referred to as thermal inertia and is used in calcu lating a thermal mismatch factor Γ that characterises a thermal contact between two bodies, where index i corresponds to the incident thermal change to higher temperature, index t to the transferred one Γ=

e i−et e iet

=

Z t−Zi Z iZt

(3.9)

Γ = 0 means no thermal mismatch, that is, the interface of two materials is not detected on the surface; Γ = -1 specifies the case when the second material is a perfect conductor; conversely Γ = 1 is realised when the second material is a perfect insulator. The thermal mismatch factor corresponds to the reflection coefficient for microwaves and ultrasonic waves. 3.2.3

Influence of the Atmosphere As most measurements are done under normal atmospheric conditions, the transmittance of the atmosphere is of great importance. Most systems are designed to operate in a short wavelength range of 3 to 5 µm or in the long wavelength range of 8 to 13 µm. The main reason is the absorption of carbon dioxide and water.

3.3

Equipment

3.3.1

Heating Equipment In most cases the object under test must not be in thermal equilibrium, so external heating (or cooling) is necessary. Dependent on the mechanisms of heat transfer several possibilities exist: ▪

Conduction: by heating or cooling elements in close contact;



Convection: by air or water with a different temperature;



Radiation: by heating elements, photo flashes or lasers.

Under certain circumstances other possibilities, like ultrasonic or inductive heating, are pos sible. Sometimes such techniques have the advantage, that the heating source can not directly be detected by the thermographic instrument.

43

Table 3-1: Thermal properties at room temperature (values vary with manufacturing process) Specific heat capacity [J kg-1 K-1]

Thermal conductivity [W m-1 K-1]

1'005

0.07

1'000

4'193

0.59

0.14

1'570

1'900

1'200

0.30

0.13

832

0.38

0.17

922

1'600

1'200

0.64

0.52

888

1.28

1.04

1'260

Zirconia

5'100

582

0.65

0.22

1'390

Aluminium oxide

3'970

765

46

Silicon carbide

3'160

675

490

Brick (red)

1'700

879

0.76

0.51

1'060

Material Air (thin gaps) Water GFRP  GFRP II CFRP  CFRP II

3.3.2

Density [kg m-3] 1.2

Diffusivity [10-6 m2 s-1] 58

15.2 230

Effusivity [W s½ m-2 K-1] 9.19

11'800 32'300

Glass

2'440

837

0.88

0.43

1'340

Concrete

2'400

837

1.51

0.75

1'740

Aluminium

2'770

875

Steel

7'830

434

Cooper

9'000

406

365

100

36'500

Diamond

3'516

502

660

374

34'100

177 63.9

73

20'700

18.8

14'700

Performance Parameters of Infrared Instruments For scanners and imagers, one distinction based on instrument performance limitations is that between qualitative and quantitative thermography. A qualitative thermogram displays the distribution of infrared radiance over the target surface, uncorrected for target, instrument and media characteristics. A quantitative thermogram displays the distribution of infrared radiosity over the surface of the target, corrected for target, instrument and media characteristics. Generally, instruments that include the capability to produce quantitative thermograms are more costly and require periodic recalibration. Many applications can be solved with qualitative instruments. Instruments can be designed point sensing or thermal imaging. Until mid-1990s opto-mechanical scanning with point sensors was common. Today, focal plane arrays have replaced scanning imagers in most applications. The main performance characteristics are:

44



Temperature range;



Temperature accuracy (quantitative only);



Temperature repeatability (quantitative only);



Speed of response: the time it takes the instrument output to respond to a step change in temperature at the target surface; the instrument speed of response is on the order of μs for photo detectors and ms for thermal detectors; there is always a trade-off between speed of response and temperature sensitivity;



Spectral range;



Frame repetition rate;



Minimum resolvable temperature difference (MRTD): the thermal resolution is usually improving with higher temperature and should therefore be related to the it;



Total field of view (FOV): horizontal and vertical image size for any given lens in degrees;



Instantaneous field of view (IFOV): the imaging spatial resolution, the size of the smallest picture element that can be imaged; the value is usually given in mrad and should be related to the corresponding value of the modulation transfer function (MTF);



Instantaneous measurement field of view (IFOVmeas) (quantitative only): the measurement spatial resolution for the minimum target spot size on which an accurate measurement can be made; the value is usually given in mrad and should be related to the corresponding value of the slit response function (SRF);



The noise equivalent temperature difference (NETD), i.e. the amount of noise due to thermal effects; this value is highly dependent on the detector material and especially on its cooling (to typically 80 K).

Apparent temperature [%]

100 80 60 40 20 0 0

2

4

6

8

10

Slit width [a.u.] Figure 3-3: Slit response function 3.3.3

Non-Contact Temperature Sensors Due to additional costs and weight and possible reliability problems, it is attractive to build detector systems that do not use separate cooling. Such cameras can be classified into two categories, namely: those based on capacitance detections of changes of temperature dependent dielectric constant of pyroelectric material; and those based on detecting resistance changes in materials having large temperature coefficients of resistivity, such as vanadium oxide (VO x). Pyroelectric detectors generate a state of electric polarity in response to a change in temperat ure. A change in detector temperature generates a transient change in the surface charges thus causing a transient current available for pick-up by the readout unit. Pyroelectric detectors are sensitive to temperature variations only; their response is independent on the wavelength, so that an interference filter is added in the optic window to limit spectral sensitivity. This can be overcome, but the added complexity is a severe drawback. Since mid-1980s it became possible to design pyroelectric detectors as array detectors made of ferroelectric ceramic thin film such as lead scandium tantalate or barium strontium titanate (BST). They offer high sensitivity without need to be cooled.

Figure 3-4: Uncooled microbolometer IR sensors with a typical pitch of 40 µm (Ulis) Recent developments made it possible to manufacture focal plane arrays (FPA) of a microbolometer type that consist purely from silicon and can therefore be produced with methods wellknown from classical microelectronics, so they are low cost. Such detectors are effective in the 45

lower temperature range, but the image and detection quality decreases for higher temperature. To maintain constant temperature of the detector device, they are usually thermoelectric ally stabilised.

Figure 3-5: Comparison of the image of a microbolometer camera (left) and one with a BST chip for fire fighting purposes (Erhatec) In photonic detectors the signal is obtained by measuring directly the excitation generated by the incident photons. Heating of the sensitive surface is unnecessary. Photonic detectors are of two types: photoemissive and quantum. For photoemissive detectors, the signal observed expresses the measured electron flow pulled away from the photo cathode under the effect of both the incident photons and a static polar isation. The signal is amplified in photomultiplier tubes. The spectral sensitivity depends on the properties of the detector material itself and of the IR transmittance of the envelope. These detector are point detectors, it is possible to make image converter tubes. The main applications are night vision image intensifiers. Quantum detectors are solid state detectors in which photon interactions either change con ductivity (photoconductive detectors) or generate voltage (photoelectric or photovoltaic detectors). As photoconductive detectors need an external low noise current, they are less attractive. Photoelectric detectors act as power generators supplying a signal without need for polarisation. Such detectors have to be cooled to reduce self-activation of electrons and therefore to reduce noise. The temperature of background limitation performance (BLIP) is dependent on the detector material and the wavelength. For this three techniques may be used:

46



Liquid nitrogen (77 K): relatively cheap, if the camera should be movable into all directions, the dewar vessel must be adequately designed and a refilling tank is often necessary;



Thermoelectric cooling with a Peltier element or a cascade of them (down to about 170 K): relatively cheap, limited efficiency (Jean Peltier, 1785-1845);



Gas expansion by the Joule-Thomson effect James Prescott Joule, 1818-1889, Sir William Thomson, later Lord Kelvin, 1824-1907), using a Stirling cooler (Robert Stirling, 17901878) based on nitrogen or argon (70 K): relatively expensive, limited live of the cooling element (where the detector is in close contact, usually glued on it).

Figure 3-6: Stirling cooler (EBS) For a long time taking images with such detectors was only possible using an electromechanical scanning device together with a single detector, thus resulting in small image formats or slow frequencies. In the 1970s it became possible to build such detectors as focal plane array (FPA) as well, obtaining the video signal directly by an on-chip electronics drive. A variety of technologies have emerged. Schottky-barrier detectors are available in 512 x 512 arrays or larger or as line detectors where the second direction is acquired by electromechanical scanning. Some of the possible detector materials have to be cooled to very low temperatures (below 60 K), thus producing potential problems for silicon technology, as charge storage is difficult at low temperatures.

Detectivity [cm W -1 Hz½]

In super lattice detector arrays, alternating layers of different semiconductors of different thickness makes it possible for the wavelength of absorbed radiation to be tuned: photoconduction occurs in a narrow range of wavelengths. The potential of this technology has been evolving with the possibility of growing the detecting super lattice layers directly on the silicon containing the readout circuits. Some material combinations can operate at room temperature.

Wavelength [µm] Figure 3-7: Spectral detectivity curves for common photoelectric detector materials (EBS) Z plane technology has suggested ways to improve IR camera systems by adding processing to the detection function of the array. Super lattice and Z plane technology with operating temperatures close to 77 K detect in the long wavelength band.

47

Figure 3-8: IR camera systems (FLIR, NEC, Pyroline, Thermoteknix) Intrinsic photon detectors are arrays of photoconductive or photoelectric detectors. As well as for super lattice detectors hybrid technology is used because silicon process is a well estab lished fabricating process. Often the basis is a mercury cadmium telluride (MCT) detector. It can be directly used as hybrid together with silicon or with an interlayer buffer of gallium arsenide. Extensions are SPRITE detectors (signal processing in the element) which are widely used. Such detectors operate at both atmospheric bands at temperatures of 77 K. Especially for higher speed detectors the temporal response of the detector material is of great importance. Table 3-2: Typical properties of often used IR detector materials Material

Temporal response

Spectral response [µm]

Microbolometer

e.g. V2O5

4 – 20 ms

8 – 14

Pyrolelectric

e.g. BST

4 – 20 ms

8 – 14

InSb

1 µs

2.0 – 5.5

PbS

30 µs – 6 ms

1.0 – 4.5

PbSe

20 – 80 µs

– 7.5

Si

50 ps – 400 ns

0.2 – 1.0

InSb

50 ns

2.0 – 5.5

InGaAs

5 ps

0.9 – 1.7

MCT

3.5 ns

3 – 14

PbSnSe

3.5 ns

3 – 12

Type

Photoconductive

Photovoltaic

New developments use quantum well infrared photo detectors (QWIP), typically based on GaAs/AlxGa1-xAs. QWIPs operate by photo excitation of electrons between ground and first excited state sub-bands of multi-quantum wells (MQWs) which are artificially fabricated by placing thin layers of two different, high-band gap semiconductor materials alternately. The band gap discontinuity of two materials creates quantised sub-bands in the potential wells associated with conduction bands or valence bands. The structure parameters are designed so that the photo-excited carriers can escape from the potential wells and be collected as photocurrent. They show the unique property that the band gap can be varied by continuously changing x. This makes it possible to build multi band detectors that can detect several wavelengths simultaneously. The usual classes are:

48



SWIR short wavelength infrared: 1 – 3 μm;



MWIR medium wavelength infrared: 3 – 5 μm;



LWIR long wavelength infrared: 8 – 12 μm;



VLWIR very long wavelength infrared: > 12 μm (typically up to 20 μm).

Figure 3-9: Twelve 640 x 512 QWIP focal plane arrays on a 3 inch GaAs wafer (JPL NASA) 3.3.4

Contact Temperature Sensors Contact temperature sensors include material coatings and thermoelectric devices. Material coatings are relatively low in cost and simply to apply, but some may have the disadvantage of providing only qualitative temperature measurements. Thermochromic liquid crystals, TLC, (cholesteric, chiral nematic or a combination of both) are substances that can be blended to produce compounds having colour transition ranges at typ ical temperatures of -20 to 250 °C. Liquid crystals can be selected to respond in a temperature range for a particular test and can have a colour response from temperature of 1 to 50 °C. When illuminated with white light while in their colour response range, liquid crystals will scatter the light into its component colours, outside they are generally colourless. The response time varies from 30 to 100 ms. They are available as coatings (in micro encapsulated form to protect the chemicals from contamination) or as sheets.

Figure 3-10: Thermochromic liquid crystals, left: colour change after touching the sheet with the hand, right: reversible temperature display used for various applications (Edmund, Hallcrest) Thermally quenched phosphors are organic compounds that emit visible light when excited to ultraviolet radiation. The brightness of a phosphor is inversely proportional with temperature over a range from room temperature to about 400 °C. Other coatings are heat sensitive paints, thermochromic compounds and heat-sensitive papers. Fusible frosts and waxes can also be used to indicate surface temperatures. Thermoelectric devices are widely used for measuring temperature. Typical devices are thermocouples, thermopiles and thermistors. 3.3.5

Optical Components Only a small number of materials are suited for the use for optical components designed for IR radiation. Normal glass or quartz glass is not useful, as its transparency for wavelengths over 2.5 µm and 3.5 µm, respectively, is only very limited. Such materials are used for lenses as well as for IR windows like for vacuum applications.

49

Table 3-3: Properties of IR materials Sapphire (Al2O3)

CaF2

BaF2

Ge

ZnSe

0.17 - 6.5

0.13 - 13

0.15 - 15

1.8 - 23

0.58 - 22

Index of refraction

1.77 (1 µm)

1.40 (4.6 µm)

1.42 (9.7 µm)

4.1 (10 µm)

2.40 (10 µm)

Melting Point [°C]

2'040

1'360

1'280

942

1'700

Thermal expansion [10-6 K-1]

5.0 - 6.7

2.4

-

5.5

9.1

1'400

158

82

692

100

Material Range [µm]

Knoop hardness [kp mm-1]

With its low index of refraction and high hardness, the colourless sapphire would be a well suited material for short wavelengths, however, it is not completely water resistant and it is very expensive. Calcium fluoride is colourless as well. It can be used only up to about 150 °C, as for higher temperatures it can break very easily and above 500 °C it begins to oxidise (CaO). The costs are about the same as for sapphire. Barium fluoride is a low prize material that can resist temperatures up to 600 °C. Due to its poor mechanical properties, it is not suited for situations with substantial pressure differences. Germanium has a high index of refraction, so it will only transmit about 50% of the incident radiation. Therefore anti reflective coatings (typically 4.7 to 10.7 µm) are often used, reducing the index of refraction by a factor of 1.5 and increasing the transmission to about 90%. In this case the angle of incidence onto the lens has to be limited to less than 35°. Zinc selenite is a yellowish crystal that is also transparent for visual light to a certain degree. Due to its high index of refraction it is mainly used with coatings. A disadvantage is the relat ively high prize.

Figure 3-11: Various lenses for IR cameras, easy to recognise, as they are usually not transparent for visual light (Infratec) Filters can be internally or externally and be designed as cut on or cut off filter, spectral nar rowband IR filter or filters against reflections. The temperature of thin polymer films can only be determined reliably if the measurement takes place in a very narrow spectral band. Some systems can be equipped with motor driven filters.

50

Figure 3-12: IR filters (Infratec)

3.4

Thermographic Techniques

3.4.1

Overview Passive thermography investigates simply the thermal flux of the object under usual conditions. This can bee the produced temperature by the object (heating, ovens, electric components, etc.) or the temperature input due to natural sources (sun, wind cooling, etc.). Active thermography uses additional heating or cooling sources, they can be pulsed, periodic or continuous. Qualitative thermography simply uses the signature of the thermal image. Examples are the detection of thermal leaks or delaminations. Comparing thermography compares the thermal images (temperature or phase) of two different situations of the same object. The temporal stability of the camera and the surroundings is important. Quantitative thermography determines temperature and/or phase absolutely.

3.4.2

Selective Heating In some cases it is possible to heat parts of components selectively and to determine their pos ition. Examples for that are bad solders or other weak contacts in electric or electronic components, the search for the position of reinforcement bars by inductive heating, the search for heating tubes, or the determination of zones with higher energy absorption, e.g. while fatigue testing.

Figure 3-13: Strongly heated electronic contact at arc and electronic devices (viZaar, EBS)

51

Figure 3-14: Thermal images of building investigation applications: source of heat leakages in shutter housings (left), position of floor heating tube (FLIR)

Figure 3-15: Outlets of cooling channels of a turbine blade, the warm air flow (denoted by arrows) was simply induced with a hair-dryer (Empa) 3.4.3

Transmission Thermography If the object can be heated from one side and the resulting surface temperature is measured at the other side, we can speak about transmission thermography. If the temperature distribution becomes more or less stable, the images can be averaged and the noise reduced. With such a procedure no information on the depths of potential differences of heat transfer are gained.

5 cm Figure 3-16: Thermographic view of a CFRP pressure vessel filled with hot water, left: tap side, right: bottom side (Empa)

52

25 mm Figure 3-17: Transmission thermogram of a CFRP plate with artificial delaminations (Empa) Pulse or Transient Thermography Pulse thermography is one of the most popular thermal stimulations methods in IR thermo graphy. One reason for this popularity is the quickness of the test relying on a short thermal stimulation pulse, which duration going from 0.3 ms for high conductivity material (such as metal parts) to about 4 s for low conductivity specimens (such as plastic and graphite epoxy laminates). As long as the thermal gradient into the heat flow direction is substantially larger than into perpendicular directions, the dimension of a flaw is pretty well imaged onto the sur face.

(A)

(B)

Time

3.4.4

(C)

(D) Figure 3-18: Principle of pulse thermography: (A) sample with thermal barrier, (B) homogeneous pulsed heating of the surface, (C) thermal diffusion, (D) effect of the thermal diffusion (thermal tailback, example for a thermal mismatch < 0, lateral thermal flow not accounted for) After a Dirac pulse (Paul Dirac, 1902-1984. NP 1933) – a theoretical pulse of a duration zero with energy input Q – the development of the temperature difference dependent on time and the depth z is −z

2

Q ΔT z, t = e 4 αt ρC k π t

(3.10)

As such a pulse is not possible in reality and the temperature can only be observed at the surface the situation for a semi-indefinite solid and a pulse duration of t p is ΔT t =

2Q t tp  ρC k π

ΔT t =

2Q   t −  t−t p  tp  ρC k π

for

t ≤ tp for

(3.11) t > tp

(3.12)

53

Heating pulse from back side Begin of visible temperature difference at front side 2 sec

Temperature difference [°C]

Figure 3-19: Temporal temperature development after a heating pulse from the back side of a CFRP plate with an artificial delamination; the image corresponds to a view from below in the precedent figure; the zone behind the delamination is changing temperature as well but the contrast is kept for a while (Empa) 100

PVC

GFRP

10

Concrete

1 Copper 0.1 0.01 0.1

SiC

Steel

1

Al2O3 10

100

Time [s] Figure 3-20: Temperature development on the surface of various materials after a net energy impact of 20 kJ m-2 within 0.1 s; the surface temperature of materials with high thermal effusivity is little influenced and due to the high reflectivity, such an impact on metals is very ineffective without coatings

Figure: 3-21: Pulse thermogram of a glued carbon fibre reinforcement strip with an interface flaw, image height: 50 mm (Empa)

54

Temperature difference [°C]

100

Defect 10

Coating

Base material

Base material Resulting curve

1 0.1

1

Time [s]

10

100

0.1

1

Time [s]

10

100

Figure 3-22: Principle of detection of flaws (left) and of coatings (right); after about 3 s the temperature contrast of a flaw with lower thermal constants is maximum (left), shortly after 1 s the influence of the base material with higher thermal constants becomes visible, after about 4 s the surface temperature follows the line of the base material (right) For real objects the above equations are not correct, especially at the end of the pulse. An undisturbed temperature development in a double logarithmic presentation has a slope of -0.5; the height of the curve is a function of the energy input and the thermal properties of the material. A local change of the heat flux will result in a temporal or enduring deviation of the predicted behaviour. Such changes can e.g. be flaws or the presence of a coating. Local delaminations of the coating are detectable as well. The advantage of this technique is that the time dependence offers information about the depth of such irregularities. In a first approximation, the observation time t is a function of square of the depth z and the loss of contrast C is proportional to the cube of the depth. t~

z2 α

(3.13)

1 z3

(3.14)

C~

The detection limits for transient thermography lie at a diameter to depth ratio of about two to three.

Figure 3-23: Transient thermograms of an artificial delamination with a diameter to depth ratio of three (left) and eight (Empa)

55

Time

II



Figure 3-24: Development of the maximum radial gradient for carbon fibre bundle parallel to fibre direction (left), perpendicular to it, the resulting image of a circular delamination behind a unidirectional carbon fibre reinforcement (Empa) If the thermal properties are not isotropic, the indications may be distorted. The amount of dis tortion is dependent on time, thus the shape of a resulting image is dependent on depth. To reduce noise the so called box car technique can be used. Here the same point or the same area is measured several times under the same conditions and the results are averaged. Heating of the object during this process should be suppressed. 3.4.5

Flying Spot and Flying Line Technique If the depth of potential flaws is known in advance, e.g. for debonding of coatings, heating and measurement can be done on the move. The distance between the continuous heating source and the detecting sensor is dependent on the thermal properties of the material and the velocity of the equipment.

Heating

Detection

Figure 3-25: Principle of flying spot and flying line technique

Figure 3-26: Flying line thermography: artificial delaminations with diameters between 2 and 11 mm above a 30 µm zirconia layer on steel, heating with a weld laser, the pulse frequency of the laser (not a strictly continuous source) results in some artefacts (Empa) 3.4.6

Lock-In Thermography In opposite to the transient thermography, lock-in thermography uses a modulated heating. The heat input is form of a sine wave and temperatures are measured in time delays of T/4 of the exciting period; the absolute positions of these four measurement times are not relevant. The source of excitation can be optical (light source), acoustical (ultrasonic sonotrode) or inductive (RF coil).

56

Heat input [a.u.]

S1

S2

S3

S4

Time

Figure 3-27: Principle of lock-in thermography Between exciting and resulting wave a phase difference exists. To reduce noise usually several measurements are necessary. The relationship between signal amplitude A and signal phase φ is given by −1

φ=tan 

S 1−S3 S 2−S4

(3.15)





A=½ (S1−S3)2+(S2−S 4)2

(3.16)

½ From the structure of these equations it is obvious that the technique responds only to signal modulations and that the phase image is independent of local variations of illumination intensity, surface absorption or thermal emission coefficients because such factors are cancelled in the ratio. The relationship between stimulation frequency and thermal diffusion length μ, an estimation of the detection depth, is given by μ=



2α ω

(3.17)

For materials that show a high diffusivity, therefore, short thermal diffusion lengths (typically below 1 mm) are hard to reach as they need as they need an excitation in the range of hundreds of Hz. On the other hand to reach diffusion lengths in the centimetre range of materials with low diffusivity needs minutes to hours as averaging is necessary. The diffusion length in an average soil (assumed diffusivity of 0.35 · 10 -6 m2 s-1) for a day's cycle is roughly 25 cm, the one for a year's cycle roughly 4.7 m. The day cycle is technically used for the inspection of pavements etc. The thermal contrast C is a complex function C f=CA f i Cφ f 

(3.18)

The values for the amplitude function, as well as for the phase function may be negative or very small for certain frequencies. According to Seidel et al. (J. Appl. Phys. 75 (9) 1 May 1994 p. 4396) for a point source and a point defect the contrast function can be given as 2

π 1i Δk 1 Δ ρC −2z    1  e 2z µ k ρC z 1i µ and with the replacement of Y = z/μ follows Gµ =

1i µ

(3.19)

2

π Y i Δk 1 Δ ρC −2Y 1i  1  e (3.20) 3 k Y 1i ρC z The amplitude contrast is proportional to the real part, the phase contrast to the imaginary one. Easy solutions can be found for Δk = 0 or Δ(ρC) = 0. Gµ=

57

0.3 0.2

0.4

Phase

0.1 0.0 -0.1 -0.2

Amplitude

-0.3

Phase contrast Cφ [a.u.]

Thermal contrast C [a.u.]

0.4

0.3 0.2 0.1 0.0

Y -0.1

-0.4 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Y=z/µ

-0.2 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

Amplitude contrast CA [a.u.]

Figure 3-28: Example of amplitude and phase contrast for a point source and a point defect with the condition Δ(ρC) = 0, in the right part Y is given in steps of 0.05

Figure 3-29: Lock-in thermography of an artificial delamination of a diameter to depth ratio of 4, amplitude image (left) showing no signal and phase image (Empa) 3.4.7

Pulse Phase Thermography Pulse phase thermography combines pulse and lock-in thermography. Like in pulse thermography the component is heated by a pulse. An ideal pulse would generate thermal waves of all frequencies, real pulses reduce the upper limit. Typical values are 25 Hz for a pulse duration of 10 ms, 2.5 Hz for one of 100 ms. The surface temperature data are recorded during the whole time of the cooling (or heating). This data contains the information as if the component would have been heated in a modulated way with the frequencies zero to the upper limit given by the pulse duration and the camera speed (with respect to the Nyquist criterion). This information can get extracted by a pointwise Fourier transform. As in the pulse thermography, noise can be reduced by several passes of the heating.

3.4.8

Step Heating (Long Pulse) In most IR radiometric techniques the sample cools after pulse heating. In contrast, the technique of time-resolved infrared radiometry (TRIR) step heating follows the surface temperature rise as a function of time during the heating pulse. This approach allows identification of subsurface features and determination of thermal properties with the same speed as other thermal techniques but keeps the required heating power and resulting surface temperature small. This permits heat source such as microwaves and radio frequency induction heating where high peak power often is not available. While thermal non-destructive testing techniques generally concentrate on discontinuity detection and imaging, time resolved IR radiometry with step heating has also been used success fully to determine material parameters such as thermal diffusivity and thickness. This has allowed obtaining information about material structure such as corrosion, porosity or voids.

58

3.4.9

Vibro-Thermography The vibro-thermographic non-destructive testing (also called sonic infrared or thermosonic) entails the mapping of a structure's surface temperature while the structure is subjected to forced mechanical oscillations. Regions of imperfections convert energy to heat through visco-elastic dissipation, collisions of internal free surfaces in cracks or other mechanisms. Discon tinuities may appear hot when the surface temperature is mapped. The energy input can be applied continuously or as a burst. Good results are obtained, if the signal is processed lock-in.

Figure 3-30: Application of burst phase thermography on a turbine blade, ultrasonic energy input 100 ms, 2000 W, thermal image (left) two seconds after burst, phase image at 0.24 Hz (www.ndt.net/article/dgzfp01/papers/v27/v27.htm) 3.4.10

EddyTherm Heating by induction can be used for metallic objects. A small coil is put around the object and a short AC pulse of typically 100 ms induces eddy currents within the object. Due to the skin effect the chosen frequency will determine the penetration depth and therefore the volume that is primarily heated (details see chapter on eddy currents); typical frequencies are between 100 and 500 kHz. Cracks oriented perpendicular to the direction of the induced current will cause cold spots along its flanks and hot spots around its tips. However, these spots are situated within the sound material and this can lead to difficulties for the exact sizing. Cracks of a length of 0.4 mm and a depth of 0.1 mm were reportedly found.

Figure 3-31: EddyTherm equipment with IR camera, transformer with coil and object (turbine blade) and thermal image of a crack with cold spots at the flanks and hot spots at the tips (MTU) A special advantage of this method is, that cracks under coatings, even under thin conductive coatings can be found, which is not always possible with eddy currents. It is also possible to find cracks after grinding the surface that usually closes the crack by smearing it up; with liquid penentrant testing such cracks are usually not detectable. 3.4.11

Thermoelastic Stress Analysis (TSA) Thermoelastic stress analysis, also known as stress pattern analysis by thermal emission (SPATE) can be used usually to monitor applied elastic stress. Under certain conditions the determination of plastic stress, as well as residual stress is possible. Under the assumption of adiabatic conditions an applied load will result in a temperature change of the object; the direction of the temperature change (warming or cooling) is dependent on the applied load. ΔT=−N T T Σ Δσ i

(3.21)

59

where NT is known as the thermoelastic constant of the material with αT as the coefficient of thermal expansion, Cp is the specific heat capacity at constant pressure. Due to the different thermal behaviour, zones of comprehensive and tensile stress can be separated. NT =

αT ρ Cp

(3.22)

As there is an immediate heat exchange with other parts of the object and the surroundings, such measurements are usually performed with cyclically applied loads.

Figure 3-32: Thermoelastic stress analysis: stress concentration at crack tip (left) and stress distributions around holes in a torsion shaft, showing positive (red-yellow) and negative (bluegreen) stress components (Cedip)

3.5

Aspects of Quality Assurance

3.5.1

Laboratory Blackbody This emission of a blackbody, for which physical laws are thoroughly settled, exists inside an isothermal closed cavity whatever the nature of its walls. The emission of this completely closed isothermal cavity is impossible to observe, indeed, a small aperture must be made through a cavity wall. The resulting device is called a laboratory blackbody. Geometrical calculations show that its current emission is very close to that of the ideal blackbody which repres ents both the perfect absorber and the best emitter at local thermal equilibrium. Laboratory blackbodies are used as calibration source. Blackbodies are often capable to maintain a tem perature difference to a reference surface (differential blackbody).

Figure 3-33: Laboratory blackbody for a typical operation range from 100 °C to 1000 °C (Omega) 3.5.2

Targets In combination with a laboratory blackbody various targets are available. The most common are the four-bar target and the slit target. They are used to determine the thermal resolution, the imaging spatial resolution and the measurement spatial resolution. Often they are mounted on a target wheel.

60

Figure 3-34: Four-bar target (front (source direction) / back (sensor direction)) and target wheel mounted on a laboratory blackbody (Inframet) 3.5.3

Thermal Resolution The thermal resolution can be measured using a four-bar target in conjunction with a differential blackbody that can establish one blackbody isothermal temperature for the set of bars and another blackbody isothermal temperature for the set of conjugate bars. The test should usually be repeated three to four times at different distances or with different target sizes.

W S

T1

T2

Figure 3-35: Four-bar target, ΔT = T2 – T1 The procedure can be as follows: ▪

Set up the test pattern such that ΔT exceeds the manufacturers specification for the MRTD;



Determine the spatial frequency of the target (mrad per the sum of W and S);



Determine the temperature difference and the temperature where the structure can now longer be dissolved.

Note that the determined value for the minimum resolvable temperature difference (MRTD) is rather dependent on the operator; statistical methods to determine the probability of resolution are common. 3.5.4

Imaging Spatial Resolution The imaging spatial resolution can be determined using a four-bar target. For any system, the modulation transfer function (MTF) will vary with scan angle and background and will almost always be dependent upon the direction of the target orientation. The procedure can be as follows: ▪

Set ΔT to at least 10 times the manufacturers specified MRTD;



Select a distance d to simulate the manufacturers specified imaging spatial resolution calculating the instantaneous field of view (IFOV) by IFOV=



W d

(3.23)

Calculate the MTF from the resulting image either in voltage output or in grey scales.

Alternatively the MTF can be determined using an edge target, measuring the edge spread function (ESF) and calculating the Fourier transform of its derivative. In such a way the target is much simpler to manufacture and only one distance is needed. 61

3.5.5

Measurement Spatial Resolution The measurement spatial resolution (instantaneous measurement field of view (IFOVmeas)) can be measured using a variable slit target and a procedure that measures the slit response function (SRF) of the imaging system. Because there are other errors in the optics and the 100% level of the slit response function is rather slowly, the slit width at which the slit response function reaches 90% is usually accepted as the measurement spatial resolution. The procedure is as follows: ▪

Set ΔT to at least 10 times the manufacturer's specified MRTD;



Select a distance and slit width to simulate the manufacturer's specified measurement spatial resolution;



Open the slit until 90% of Vmax is reached.

3.6

Procedure and Record

3.6.1

Procedure The NDT procedure usually has to define the following special technical aspects:

3.6.2



Thermal environment, operating status, etc.;



Surface conditions concerning thermal properties and reflections;



Inspection technique (active, passive, qualitative, comparing, quantitative);



Geometric, thermal and temporal resolution;



Type and arrangement of the examination system;



Thermal range;



Thermal calibration and verification intervals for the camera;



Requirements for signal processing and signal evaluation;



Necessary cleaning.

Record The record should contain the following information:

3.7



Surface and peripheral conditions and special surface treatment (cover paint, etc.);



Description of the equipment used, including serial numbers and firmware version of instrument and heating devices, etc.;



Identification of the calibration used;



Instrument settings (thermal range, etc.), lenses and filters used.

Summary and Conclusions In its general application thermography is well suited for the investigation of flat objects with a relatively simple geometry. Delaminations and debonds can be determined relatively easily, fast and area-wide. Information on the depth can be gained with time resolving methods. This information can include the thickness of coatings or, under certain conditions, the thickness of the object in full as well. Thermography is less suited for complex geometries. It is not possible to provide full three dimensional information of flaws and features behind a material composition. Polymers, wood, concrete and some ceramic materials show slow thermal reactions and are, therefore, easier to deal with. Metals and some other ceramic materials have high thermal diffusivities and demand for fast detection systems. The surfaces of electric conductors, further more, are usually highly reflective and have to be covered with an adequate layer. The quantitative determination of materials properties is possible, but in general the method uses a comparison between different areas on the surface or to given reference samples. A lot of statements made using thermographic methods do not adequately account for such facts. The developments made in the last years concerning the detectors are very promising. Future systems become faster, have larger arrays and need less complicated scanning mechanisms and cooling. Thus offers more possibilities in spatial and time resolution or increases the signal/noise ratio.

62

4

LIQUID PENETRANT INSPECTION

4.1

Introduction Liquid penetrant testing is a method that is used to reveal surface breaking flaws by bleed out of a coloured or fluorescent dye from the flaw. A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later it became the practice in railway workshops to examine iron and steel components by the "oil and whiting" method. In this method, heavy oil commonly available in railway workshops was diluted with kerosene in large tanks so that locomotive parts such as wheels could be sub merged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century through to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for ferromagnetic iron and steels. Many of the early developments were carried out by Magnaflux in Chicago in association with the Switzer Bros., Cleveland. More affective penetrating oils containing highly visible (usually red) dyes were developed to enhance flaw detection capability. This method, known as the vis ible or colour contrast penetrant method, is still used quite extensively today. In 1942, a sys tem of penetrant inspection where fluorescent dyes were added to the liquid penetrant was introduced. Liquid penetrant testing can be used for nearly all materials, if they met with the applied chemicals and if there surface is not too porous. It cannot give any result on the depth and only inaccurate information on the volume of a flaw. Generally, the minimum flaw dimensions detectable with this method are a width of 0.5 µm and a depth of 50 µm. Painting and galvanic layers have to be removed chemically or mechanically. An advantage of this method is its simplicity; indications can be shown to third parties without a lot of explanations. On the other hand, an automation of the assessment is elaborate and only used for mass production. The general proceeding is as follows: ▪

Careful mechanical and chemical precleaning and degreasing of the surface, followed by drying;



Application of the penetrant;



Excess penetrant removal and drying of the surface (if necessary);



Application of the developer;



Inspection;



Post-cleaning and conservation of the surface (if necessary).

Liquid penetrant testing can be summarised being a very simple method, where nearly everything can be done the wrong way. (A)

(B)

(C)

(D)

Figure 4-1: Principle of liquid penetrant testing: (A) surface breaking crack, (B) application of the penetrant, (C) excess penetrant removed, (D) application of the developer

63

4.2

Equipment

4.2.1

Classification of Testing Products Table 4-1: Classification of testing products (EN ISO 3452-2) Penetrant Type

Denomination

Excess penetrant remover Type

Denomination

Developer Type Denomination

I

fluorescent penetrant

A

water

a

dry

II

colour contrast penetrant

B

lipophilic emulsifier 1 oil-based emulsifier 2 rinsing with running water

b

water-soluble

III

dual purpose (fluorescent colour contrast penetrant)

C

solvent (liquid) class 1: halogenated class 2: non-halogenated class 3: special application

c

water-suspendable

D

hydrophilic emulsifier 1 optional pre-rinse (water) 2 emulsifier (water-diluted) 3 final rinse (water)

d

solvent-based (nonaqueous for type I)

e

solvent-based (nonaqueous for types II and III)

f

special application

E

water and solvent

Sensitivity levels shall be defined separately for penetrant, excess penetrant remover and developer and for product families. Sensitivity levels for fluorescent product family are: ▪

½ (ultra-low);



1 (low);



2 (medium);



3 (high);



4 (ultra-high).

Sensitivity levels for colour contrast family are: ▪

1 (normal);



2 (high).

No sensitivity levels are defined for the dual purpose product family. 4.2.2

Chemical Compatibility and Thermal Restrictions Chemicals that contain sulphur or halogens are not allowed for austenitic and nickel-based ma terials, neither for titanium alloys. Under service conditions even minor rests may disintegrate and result in strongly corrosive products. Chemical and physical properties of some non-metallic material can be adversely affected when applying not matching products. Finally, a potential contamination with deleterious effects of lubricants, fuels or hydraulic liquids may be taken into consideration. For all parts associated with peroxide rocket fuel, explosive stores, oxygen equipment or nuclear application, the compatibility of penetrant testing agents shall require special consideration. The above specified chemical agents can only be used within a certain temperature range, for temperatures below 10 °C and above 50 °C designated product families exist. Too low temperatures lead to freezing or heavily reduced viscosity of the agents, too high temperatures to their disintegration or rapid volatilisation. Especially in the low temperature range, there is a danger of water condensing in the discontinuities and on the surface and this water will prevent the penetrant from entering the discontinuities.

64

4.2.3

Penetrants Penetrants are either coloured (mostly red, sometimes blue) or fluorescent under black light or both at the same time. They are oil-based and soluble in water or solvent. Between the three involved states of aggregation: solid (s), liquid (l) and gaseous (g), there are three surface tensions that define the contact angle θ: cos θ=

σ sg−σ sl

θ > 90°

σgl σsg

(4.1)

σ gl

(A)

θ < 90°

(B)

(C)

σsl

Figure 4-2: Surface tensions (A) and wetting: wetting (B), non-wetting (C) In addition, the value of the contact angle is dependent on the surface conditions. The wetting capability of the penetrant to the surface to be investigated, as well as especially also to the developer must be high. The driving force to fill a void is given by the capillary pressure. The liquid penetrant will continue to fill the void until an opposing force balances the capillary pressure. This force is usually the pressure of trapped gas in the void as most flaws are opened only to one surface of the part. Viscosity has little effect on the ability of a penetrant material to enter a defect, but it does have an effect on speed at which the penetrant fills a defect. For ceramic materials a further possibility is the use of fuchsine as penetrant (fuchsine dye test). The penetration process may be done the usual way or under low pressure, to allow air bubbles to leave the flaws. 4.2.4

Excess Penetrant Removers To remove the penetrant an emulsifier is necessary. It can be directly added to penetrant liquid itself or applied in an extra step. These post-emulsifiable penetrants are very sensitive. Oilbased lipophilic emulsifiers quickly mix with the oil-based penetrant, so the emulsifying time becomes critical. In addition, contamination, ageing of the emulsifier and surface conditions may influence the correct emulsifying time. Water-dilutable hydrophilic emulsifiers, however, cannot mix and act in layers. Therefore there application time is much less critical. For excess penetrant removers that are delivered as a concentrate and can be mixed with wa ter by the user, the concentration may be critical to prevent “over-washing”.

Figure 4-3: Water-air-gun (Ely Chemical) 4.2.5

Developers The developer is a white contrast medium. It can be applied as dry powder or with a quick drying liquid (spray, dip or wash). After drying the indications become visible in the absorbent de veloper. Flaws containing a larger volume bleed out and lead to enlarged indications. Specialised developers are peelable in order to archive the assessment.

65

4.2.6

Electrostatic Spray Guns The application of the penetrant, but especially of the developer can be done using an electro static spray gun. Dry as well as wet developers can be applied. Even otherwise not reachable regions can be covered relatively uniform. Another advantage of this application is that the developer particles tend to attach the part and do not build a fog cloud. In the tip of the gun, the particles are charged negatively, the part under test is grounded. Usually the gun is fed with low voltage direct current that is converted with a built in high voltage generator to 20 kV to 100 kV. To prevent sparkling, the high voltage should be reduced automatically if the gun tip is moved too near to the part. Sometimes the part can build Faraday's cages; in this case the operator should have the possibility to turn off the voltage to allow these regions being reached as wel.

Figure 4-4: Electrostatic spray gun (Kremlin-Lips) 4.2.7

Black Light Lamps Dependent on the penetrant the inspection is done at normal (white) light (minimum 500 lx) or under ultraviolet radiation (minimum 10 W m -2, maximum 20 lx residual illumination). Typical outputs of the bulb of black light lamps lie at 100 W, so the lamps usually have an active or passive cooling. They can be portable, stationary (integrated in inspection systems) or designed as rod or magnifying light. Up to date lamps are using gas discharge bulbs on micro power xenon light (MPXL) technology or LEDs.

Figure 4-5: Black light lamps: (from left) portable, for the integration in systems and as endoscope (Labino, Tiede, Helling) The suitable range of the wavelength of the ultraviolet radiation lies between 315 and 400 nm, with a maximum at 365 nm. Portions with shorter wavelength (below 330 nm) may result in health impairs and portions with longer wavelength have a visual component and increase the amount of residual light. Therefore, it is necessary to pay special attention to the quality of the filter glasses. The same black light lamps are used for magnetic particle inspection. The requirement of a maximum residual light intensity when using fluorescent media is due to contrast reasons. But the human eye in darkness is also more sensitive to slightly lower wavelengths, and the yellowish-green colour of the fluorescent media hits the maximum sensitivity of about 500 nm. 4.2.8

Blue Light Systems Instead of ultraviolet lamps blue LEDs (typically 450 nm) can be used. Their power consumption is only about 10 % and they can be made mobile in form of a torch as well. Not to be irrit ated by blue reflections, the operator wears dichroic glasses.

66

Figure 4-6: Blue light equipment (Rilem) 4.2.9

Flaw Inspection Systems For smaller parts the whole process can be done in manual, semi-automatic or full-automatic penetration inspection systems. Automatic assessment may be done with camera systems; re producible part positioning and stable illumination is necessary. For a full assessment, usually several, up to twenty cameras are necessary.

Figure 4-7: Manual flaw inspection system (Deutsch)

67

Figure 4-8: Computer controlled full automatic inspection system for a fluorescent, post-emulsifiable (hydrophilic) family with large parts in transportation baskets on their way to the entrance. Typical throughput is 12 to 20 parts per hour (excluding non-automated inspection time and post-cleaning), depending on size and weight of parts (Pratt & Whitney)

Figure 4-9: Full automated system for a fluorescent penetration system with the ability of lowering and lifting the parts (Puchold)

Figure 4-10: Inspection station for automated camera-based assessment (Tiede)

68

4.3

Testing Procedure Preparation and precleaning Drying Water-solvent penetrant

Water and solvent

Post-emulsifiable penetrant Prerinse

Water

Hydrophilic emulsifier

Solvent-based penetrant

Lipophilic emulsifier

Solvent

Rinse Control of cleaning

Drying

Wet developer solvent-based

Dry developer

Wet developer water-based Drying

Inspection Post cleaning and conservation Figure 4-11: Testing procedure for different techniques and agents 4.3.1

Precleaning The surfaces to be inspected must be dry and free from any dust, grease, rust, oil, painting, weld spatter or any other pollution. For the mechanical cleaning of the surface, sand-blasting is not an adequate solution, as small surface cracks may be closed by the impact of the grains. Blasting techniques, if at all, with less impact forces are more suited. If necessary a chemical etching has to follow the blasting process. The chemical cleaning is often done using solvents that allow a quick drying.

4.3.2

Penetration

Figure 4-12: Beneath submerging and painting, the penetrant can also be applied by spraying (with or without aerosol) or by a pen (Tiede, MR Chemie, Magnaflux) Small pieces can be submerged into penetrant bathes, for larger parts the penetrant is applied locally by spraying or painting. Typical penetration times are between five and sixty minutes. The penetrant must not run dry (e.g. due to higher part temperature) during this process. For bathes, where the penetrant agent is used several times, special attention is to be paid to prevent contamination. 69

4.3.3

Excess Penetrant Removal In the next step, the excess penetrant has to be removed. This is a critical point, as too little leads to insufficient contrast or marring bleed out, too much removes the penetrant in the cracks also, at least that much that they are not properly detectable afterwards. If fluorescent penetrants are used, this step has to be done under black light, at least at a radiation of 3 W m-2. For coloured penetrants after developing still a weak red background should be visible. For reasons of economy, most flaw inspection systems use water for this step. Water-soluble penetrants and emulsified mixtures can be easily removed with a water-air-gun. With adequate pressure of the compressed air, there is no danger of over washing. Water temperature and air pressure have to be controlled. The water has to be specially treated before handed over to the normal waste water canalisation. The reaction of hydrophilic emulsifiers and the penetrant occurs in layers , while lipophilic emulsifiers very quickly mix with the penetrant, so their application can be time critical.

4.3.4

Drying Especially if water-soluble penetrants are used, drying is necessary. It has to be made quickly after excess penetrant removal using ▪

Wiping with a dry, clean, lint-free cloth;



Evaporation at ambient temperature after hot water dip;



Evaporation at elevated temperature (air < 80 °C, part < 50 °C);



Forced air circulation (if compressed air is used, particular care shall be taken to ensure that it is free of water and oil).

No drying is necessary if water-based developers are used. 4.3.5

Development This is again a critical step as the development layer must be uniform but thin, usually less than 100 µm.

4.3.6

Inspection The inspection has to be done at one or several certain dwell times after development, for wet developers the time count begins after drying. Typical times are between ten and thirty minutes. For recording purposes, pictures can be taken, if necessary, using an ultraviolet filter. Special developers can be peeled; however, they represent only one moment of the development and do not necessarily give the overall picture. If not automated, the usual way to perform the inspection is to do it by eye; the use of low magnifying glasses (up to three) is al lowed. The developer shows indications in an enlarged way, especially if the flaw has a larger volume. Exact sizing therefore is not always possible. Usual differentiations are into number, appearance (single, cumulated), frequency, shape (round, linear or arranged in a line) and size. If siz ing is necessary, the use of a transparent ruler is recommended. For castings reference tables exist.

70

Figure 4-13: Examples of a crack indications: fluorescent penetrant (left) and coloured penetrant showing clear bleed out (Ardrox, Sofranel)

Figure 4-14: Linear indications in full or intermittent (above) and round (ASNT) 4.3.7

Retesting If retesting is necessary, the whole process, including the precleaning, has to be redone. Without a solid cleaning of the surface, that excludes rests of the former penetrant being present, a change of the penetrant family or even the product is not allowed.

4.3.8

Bleed back A special inspection technique called bleed back is sometimes applied to parts from aerospace or medical implants. It is used to get more information about the severity of indications. The developer is slightly wiped off with a soft brush or a Q-tip dampened with solvent; the solvent must not flood the surface. After the solvent has evaporated, the surface can be re-inspected immediately; the brightness of the indication, however, is often too low. If a relevant indication does not reappear, the evaluation is carried on after a redevelopment. It is to note, that such techniques do not conform to usual standards.

4.4

Aspects of Quality Assurance

4.4.1

Chemical Agents For the producers of chemical agents there are a lot of physical and chemical properties regularly to be checked and to be reported on the product. They can have a relation to reproducibil ity, to sensitivity, or to environmental or safety aspects. Most of them cannot be easily supervised by the user. For the verification of the sensitivity the reference test block No. 1 EN ISO 3452-3 is used. It consists of a set of four plates with nickel-chromium layers of different thickness. Cracks are induced by stretching the plates. The sensitivity is defined by the amount of cracks that are found depending on the thickness of the layer and the product family. This reference test block is used by producers only.

71

The user has only to control those agents that are in potential re-use or could be contaminated otherwise. Especially penetrants decompose when heated too much (usually over 70 °C) and the fluorescent brightness is decreasing with enhanced ultraviolet radiation. On one side there are single properties that have to be controlled like colour contrast or fluorescence of the pen etrant, the concentration of emulgators or wet developers, or a possible fluorescence of the developer due to contamination. On the other side the whole system can be controlled using the reference test block No. 2 EN ISO 3452-3. It contains from a thin stainless steel plate that is coated with nickel on one side. With hardness impacts from the other side, cracks fields are brought in with dimensions between 3 mm and 5.5 mm. Every block, therefore, is unique and an absolute comparison to others is not possible. The part with the cracks is used to compare the sensitivity. The block contains additionally four fields with roughness between Ra 2.5 mm to 15 mm. This part helps to compare the excess penetrant removal dependence on the surface conditions.

Figure 4-15: Reference test block No. 2 EN ISO 3452-3 (Deutsch) Other reference test blocks for similar purpose exist (ASME, JIS, Navy). Refractometers are used for to measure the concentration of hydrophilic penetrant removers. 4.4.2

Viewing Conditions Illuminance and residual illuminance can be measured with a illuminance meter. As the influence of the black light lamp onto this instrument in general is not known, residual illuminance measurement should be performed outside of the direct ultraviolet radiation. The output of the bulbs of the black light lamps may decrease rapidly, the radiation level have to be tested at regular intervals (once per months) using an UV-A radiometer.

Figure 4-16: Refractometer (left) and UV-A radiometer (Ely Chemical , Magwerks) 4.4.3

Safety Aspects As mentioned above, to avoid hazardous ultraviolet radiation, filters of black light lamps should be visually inspected before each use. People that work under ultraviolet radiation for longer time should ware glows. Photochromatic spectacles, however, are not allowed while working at the inspection site. Liquid penetrant testing may require the use of toxic, flammable and/or volatile materials. In such cases, working areas therefore should be adequately ventilated and far from sources of heat or flames. Extended or repeated contact of the agents with the skin or mucous membranes shall be avoided.

72

4.5

Procedure and Record

4.5.1

Procedure The NDT procedure usually has additionally to define the following special technical aspects:

4.5.2



Special surface preparation;



Type and sensitivity of product family;



Special requirements on application temperature and dwell time;



Necessary cleaning and conservation.

Record The record should contain the following information: ▪

Designation of the product family (name; batch);



Type of application (submerge, spray, etc.), temperature and dwell times;



Viewing conditions, type and serial number of the black light lamp;



Applied conservation.

Figure 4-17: Example of a PT examination record (Empa)

4.6

Special Techniques and Trends

4.6.1

High Temperature Penetrant Testing For special applications penetrant testing can be applied at temperatures > 50 °C. Common temperatures are 50 (± 5) °C or 100 (± 5) °C in the medium temperature range or 100 (± 5) °C, 150 (± 5) °C and 200 (± 5) °C in the high temperature range. Such applications are carried out for the in-service inspection of hot running machine parts or tubing. Beneath adequate agents, such testing usually requires a thermostatic cell, gloves and brushes suitable for the applied temperature and a surface thermometer (contact type) with a ± 5 °C accuracy.

73

The procedure for qualification can be made with a usual test block or special aluminium panels containing thermal cracks. The results have to be compared with the ones found at room tem perature (with the same agents). If the several steps cannot be carried out within the thermostatic cell, strict time limitations have to be applied. Special attention has to be paid to a possible drying out of the penetrant. 4.6.2

Low Temperature Penetrant Testing Similar to high temperature applications low temperature applications are possible as well. Such applications are carried out for the in-service inspection under low temperature environment conditions or for low temperature machine parts or tubing (chemical plants, etc.). Within the range of +10 °C to -5 °C (dependent on the surrounding temperature and humidity), the main trouble comes from water, either as liquid (moisture), hair-frost or even ice. To get rid of the water: ▪

Gently warm the surface for several minutes, so as to make water evaporate from dis continuities, and/or;



Use a volatile, water-soluble solvent, such as acetone or isopropyl alcohol, as current de greasers used are hydrocarbon-based and won't help in any way to remove water;



Allow some minutes for evaporation, be sure evaporation does not cool the part's surface enough to have water condensing on it again .

As the water is generally not completely removed from discontinuities, the penetration time has to be increased. As the capillary effect is the driving factor, higher viscosity due to low temper atures is not a drawback to penetrant testing; even jellied penetrants may lead to very good crack detection. The usually applied developers are sprayed solvent-based (non aqueous wet). The spray cans shall be kept at 10 °C at least to get a good spray, giving a thin, even layer. To prevent dim in dications the solvent shall evaporate within short time. This process may be speeded up with a gentle flow of warm air (not by an infrared heater). 4.6.3

Filtered Particle Testing The method itself is old, corresponding patents are dated of the 1950s. Nowadays it could find a renaissance when testing carbon matrix components for thermal barrier coatings and similar applications. Coloured or fluorescent liquid penetrant test techniques can be used on most solid materials. However, if the materials are porous, the background haze or colouration of the entire surface by the penetrant reduces the contrast of indications. The surfaces of such materials trap so much penetrant fluid, that delineation of a discontinuity is no longer possible. For this sort of test condition the filtered particle test technique is effective. The technique depends on selective motion of liquid and solid matter. The test fluid consists of a liquid vehicle that carries solid dyed particle tracers of adequate size and shape in suspension. The fluid is sprayed on a porous material. At a site of a discontinuity, more liquid is absorbed than anywhere else, while the suspended particles are filtered out and deposit on the part surface. A highly visible indication will appear almost at once. (B)

(C) (A)

Figure 4-18: Filtered particles testing on a component with a surface crack: (A) colourless penetrating fluid, (B) random particles at surface, (C) accumulated particles over the crack The particles can be coloured or fluorescent, the liquid usually is colourless. Special attention has to be paid to the fact that a part of the liquid remains in the component and could lead to direct damage or to related reactions (including stress corrosion cracking or induced stress release by cracking).

74

4.6.4

Process Acceleration The process can become accelerated by bringing the penetrant quicker into flaws and out of them. On one side this is treated using ultrasonic vibrations. On the other side tests are carried out to use ferromagnetic penetrants, or penetrants with ferromagnetic particles combined with the application of magnetic fields.

4.6.5

Automated Signal Detection Optical signal detection is possible; it makes, however, only sense for large lots of identical parts. Possible future developments tend to measure other properties, e.g. the electron paramagnetic resonance (EPR) of penetrants and therefore being able to further automate the recognition. One possible candidate as such a penetrant is nitroxyl.

4.7

Summary and Conclusions Liquid penetrant testing is widely accepted to be a very simple method. Never the less, for nonstandard situations it needs experience and manual skills of the NDT operator. The result is vis ible for a limited time only, and as the process is time consuming, a repetition is rarely taken into account. So the classification of indications by the operator is often final. If there are no indications, neither above nor below the registration level, there is no direct proof that the process was carried out properly. The problem with automated detection and recognition systems is somehow similar. It is relatively easy to sort out suspicious specimen and lead them to a second, man-operated inspection station for final decision. The much more complicated part is to proof, that the other specimen really have no flaws (type I error; false negative), without setting the discriminating level such small that the false calls increase to large numbers. Liquid penetrant testing can be widely used; overhead application is possible, as well as endo scopic remote operations. Certain restrictions may be related to weather or climate conditions; underwater applications or similar tasks are not possible.

75

76

5

LEAK TIGHTNESS INSPECTION

5.1

Introduction Leak testing is a form of non-destructive testing used in either pressurised or evacuated systems and components for detection and location of leaks and for measurement of fluid leakage. The word leak does not refer to the quantity of fluid passing through the hole. A leak may be a crack, a crevice, a fissure hole, a porosity, a permeable element or any other passageway that, contrary to what is intended, admits water, air or other fluids or lets fluids escape. If the diameter of the leak is much greater than the leakage path length, it is called an aperture leak, otherwise a capillary leak. The term leakage rate refers to the flow of fluid through a leak without regard to physical size of the hole through which flow occurs. Fluid denotes any gas or liquid that can flow. Visual leak testing has been performed since mankind had begun to produce object that ought to be tight. Devices similar to the ones used today came up first in mid-19 th century, when the spark coil leak tester was invented and 1906, when the Pirani gauge and the thermocouple gauge were presented. The mass spectrometer was invented 1910, but it was not used for leak testing until World War II, when the engineers from the Manhattan Project had need for such testing. The model at this time had a sensitivity of 10 -7 Pa m3 s-1, today's laboratory equipment is five decades more sensitive. Coarse leaks may be found by searching for the noise of a gas flow that pours in or out of the component or using one or more acoustic (ultrasonic, typically 40 kHz) emitters that are enclosed in the object. A further simple method is the pressure dye test, that is the use of liquid penetrant equipment, the developer is applied at the opposite wall side, however. For quantitative information more sensitive techniques must be used. The leakage rate can be expressed as qL =

Δ p V Δt

(5.1)

The usual units are Pa m3 s-1 or mbar l s -1 (1 : 10). The leakage rate relies on the temperature and on the type of gas flow (molecular or viscous laminar). Furthermore, leaks can show a very different behaviour for reverse flow, particularly if elastic or plastic components (springs, diaphragms, gaskets) are part of the boundary wall. In general it is not possible to measure the total leakage rate of a component or a system and to determine the location of one or several leaks at the same time. Usually two different techniques have to be used.

5.2

Leak Testing Techniques

5.2.1

Tightness Requirements Zero leakage rates shall not be specified. The required leak tightness shall be related to the function of the object under consideration. Leakage rates in the order of 10-4 Pa m3 s-1 can be acceptable for compressed air cylinders. This corresponds to a pressure variation of 5'000 Pa in a 10 l volume in 24 hours or 0.5 l loss meas ured at atmospheric pressure. A leakage rate of 10-10 Pa m3 s-1 is typical for cardiac pacemakers. This corresponds to a loss of 1 cm3 every 30 years, approximately. The total tightness of a system can be considered in terms of tightness of all components of that system. To meet requirements the sum of the leakage rate for each component plus the sum of the leakage rates at each connecting point shall be less than the overall allowable leakage rate of the system. To take into account factors that are not quantifiable, it may be advised to adopt leak tightness values lower than this by a factor from three to ten.

77

Table 5-1: Minimum detectable leakage rates under usual industrial conditions of different leak testing techniques, 1) tracer gas methods, 2) pressure change methods, M: measurement, L: localisation; under laboratory conditions the respective sensitivity can be 100 to 1'000 times better Leakage rate Pa m3 s-1 Radionuclides Vacuum test Bombing

1)

1)

1)

Vacuum box

1)

NH3 detection

1)

Gauge pressure Sniffing test

1)

1)

Pressure change

2)

Penetration Flow

2)

Bubble test

2)

Acoustic

10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

-----

-----

-----

-----

-----

-----

-----

-----

-----

---M

-----

-----

-----

-----

-----

-----

----L

······

······

····M

-----

-----

-----

-----

-----

---M

······

······

····M

-----

-----

-----

-----

-----

-----

----L

······

····M

-----

-----

-----

-----

-----

-----

----L

-----

-----

-----

-----

-----

-----

---M

-----

-----

-----

-----

-----

-----

----L

-----

-----

-----

-----

---M

····M

-----

-----

-----

-----

----L

-----

-----

-----

---M

-----

-----

----L

·····L

----L

······

·····L

p < po

p > po LD

(A)

LD

(C)

(B)

p > po

p > po He 1 - 100 h

p < po

(D1)

(D2)

LD

(D3)

Figure 5-1: Some leak testing techniques: bubble test (A), immersion and liquid application, local vacuum technique (B), sniffing test (C), bombing test (D1 to D3), LD = leak detector 5.2.2

Restrictions and Safety Factors The the test fluid shall be compatible with the object materials. Vacuum tests can be affected by the presence of materials such as porous materials or organic compounds (plastics, rubber, lubricants, etc.) Certain tracer gases are not compatible with some materials and problems due to corrosion, sorption or permeation may occur. For example:

78



Halogen gases (except SF6) are not usable for the testing of nickel alloys and stainless steels;



Ammonia is not compatible with cooper and cooper alloys;



Helium or hydrogen may present problems with some elastomers / polymers since permeation can be significant.

Possible hazardous properties of test gases shall be taken into account, such as: ▪

Ammonia is a toxic, flammable gas which can be corrosive in the presence of moisture; it needs an absorption treatment and a subsequent neutralization;



Halogen-containing gases cause significant damage to the upper atmosphere;



Most gases, including inert gases, e.g. helium and nitrogen are asphyxiant.

5.3

Tracer Gas Methods

5.3.1

Principles of Detection A partial difference of tracer gas is created across the boundary of the object to be tested. The tracer gas having passed through the leak is revealed by its physical or chemical properties. Chemical detection is generally based on reactions that cause a local colour change; the object surface shall be therefore visible in this case. Detection based on physical properties usually involves a sensor that gives an electrical signal which varies with the tracer gas pressure. Such detectors can be: ▪

Mass spectrometer, tuned for the specific tracer gas used (generally He-4);



Pirani gauge, for tracer gas with thermal conductivity different from that of the ambient atmosphere;



Alkali-ion diode (ionisation gauge), for halogen gas and electron capture equipment (e.g. for SF6);



Photometer with band-pass filter in the frequency range of the tracer gas absorption or emission.

Figure 5-2: Complete residual gas analyser station (including vacuum pump, left), Pirani gauge (Inficon) 5.3.2

Pirani Gauges The Pirani gauge head (Marcello Pirani, 1880-1968) is based around a heated wire placed in a vacuum system, the electrical resistance of the wire being proportional to its temperature. At atmospheric pressure, gas molecules collide with the wire and remove heat energy from it (effectively cooling the wire). As gas molecules are removed (i.e. the system is pumped down) there are less molecules and therefore less collisions and so the wire heats up and its electrical resistance increases. A simple circuit utilising the wire detects the change in resistance and once calibrated can directly correlate the relationship between pressure and resistance. This effect only works in the pressure region from atmosphere to around 10 -3 mbar. Therefore other forms of gauge have to be used to measure pressures lower than this. The range of Pirani gauges is divided into two types: constant current and constant resistance. The names refer to how the electrical measurement of the wire is controlled. The constant cur rent type has a power supply that gives a constant current all the time to the filament. There fore the filament resistance changes are measured. The constant resistance type has a power supply that changes the current supplied to keep the resistance of the filament the same. The constant resistance type has a slightly larger pressure range but requires more complicated electronics.

79

5.3.3

Ionisation Gauges Below the operating range of the Pirani gauge, an ionisation gauge is used to measure pres sure. There are a range of gauge heads and filament materials to cover specific pressure ranges and vacuum requirements in this region. The ionisation gauge consists of three distinct parts, the filament, the grid and the collector. The filament is used for the production of electrons by thermionic emission. A positive charge on the grid attracts the electrons away from the filament; they circulate around the grid passing through the fine structure many times until eventually they collide with the grid. Gas molecules inside the grid may collide with circulating electrons. The collision can result in the gas molecule being ionised. The collector inside the grid is negative charged and attracts these positive charged ions. Likewise they are repelled away from the positive grid at the same time. The number of ions collected by the collector is directly proportional to the number of molecules inside the vacuum system. By this method, measuring the collected ion current gives a direct reading of the pressure. The design of the gauge head effects how efficiently electrons are produced, how long they sur vive, and how likely they are to collide with a molecule. Combining these factors together gives the gauge sensitivity; a high sensitivity means a more efficient operation of the gauge. There are other factors which determine the lowest pressure that a gauge head can measure. One of these limiting factors is the X-ray limit. When an electron collides with the grid, there is a probability of a photo electron being produced. Once generated, there is also a chance that the photo electron will hit the collector and produce an electron. For the collector, however, there is no electrical difference between collecting a positive charge or loosing a negative one. This means that every time an electron is knocked off the collector, the electronics measure it as receiving a positive ion instead. This effect is very small and depends on the design of the gauge head. It normally generates a current measured in the pA range. At 10 -10 to 10 -11 mbar, however, this is the current produced by the gauge head itself. If pressure is plotted against current, the graph can be seen to tail off as this X-ray current becomes the dominant effect. The X-ray current therefore limits the lowest pressure that the ionisation gauge can measure.

5.3.4

Techniques where Tracer Gas in Flowing into the Object Three slightly different vacuum techniques may be used. Vacuum technique (total): The object, placed in an enclosure (bag or chamber), is evacuated and connected to the detector. The enclosure is then filled with the tracer gas or a gas mixture containing it. This technique allows the evaluation of the leakage rate but does not permit precise location of the leak(s).

Figure 5-3: Tire testing (Von der Heyde) Vacuum technique (partial): Only suspect areas are covered with enclosures filled with tracer gas. Vacuum technique (local): The object is evacuated and connected to the detector. Suspect areas on the external surface are sprayed with tracer gas. Leaks can be localised with this tech nique, but it is not possible to measure the total leakage rate.

80

5.3.5

Techniques where Tracer Gas is Flowing out of the Object Chemical detection with ammonia: The object is filled with anhydrous ammonia or an ammonia-nitrogen mixture to the specified overpressure. A colour-change developer (generally a mixture containing a pH indicator, e.g. bromophenol blue), applied to the outside surface, will reveal and locate leakages. Vacuum box, using internal tracer gas: Large objects, containing a gas or a gas mixture suit able to be used as tracer gas, are tested by a vacuum box evacuated and connected to a leak detector, applied on the outer side. Vacuum box applying the gas on opposite side: Open objects can be tested using partial enclosures, capable of being evacuated, which are tightly applied to the wall (vacuum box, suc tion cup). Tracer gas is supplied on the opposite surface of the wall by a spray gun (probe jet) or by cups, filled by the tracer gas. Pressure technique by accumulation: The object, pressurised with the tracer gas, is placed in an enclosure. After a specified time, the accumulated tracer gas is measured using a leak detector connected to the enclosure. The leakage size can then be estimated (or determined if the enclosure volume and pressure are known). Sniffing test: The object is pressurised with tracer gas (or a mixture). Leak searching is per formed on the atmospheric side of the object wall, using a sampling probe connected to a leak detector. This technique detects leakage and locates the leaks (direct probing).

Figure 5-4: Halogen sniffing test equipment (Inficon) Pressurisation-evacuation test (bombing test, back pressurising test): The sealed object is subjected to a high pressure of tracer gas (bombing), usually helium, in order to force it into the object, if leakage exists. After the bombing and a flushing to remove absorbed tracer gas from the outer surfaces, the object is placed in a vacuum chamber, connected to a leak detector for the detecting of the escaping tracer gas.

Figure 5-5: Sniffing test equipment with one or several probes (Delta)

81

Figure 5-6: Bombing test equipments (VIC, Varian) Vacuum chamber technique: Small objects, containing a gas suitable to be used as tracer gas are placed in a chamber. This is subsequently evacuated to a pressure lower than the internal pressure of the object. The leak detector is connected to the vacuum chamber. The total flow of tracer gas from the object is measured.

Figure 5-7: Example for the vacuum chamber technique (Inficon) Instead of a tracer gas, specific radioactive isotopes (typically Kr-85) may be used for special applications.

5.4

Pressure Change Methods

5.4.1

Test Instruments The following types of gauges are used:

82



Vacuum gauges suitable for the test pressure range;



Pressure gauges: absolute or differential pressure gauges may be used; when differential pressure gauges are used, reference pressure variations shall be monitored and taken into account;



Thermometers: resistance thermometers are generally used for temperature measurements, but other devices may be employed;



Hygrometer: for the measurement of the humidity or the dew point of test gas, capacitive or resistive type instruments may be used;



Flowmeters may of mass or volumetric types, with an integrator device.

Figure 5-8: Flowmeters (VIC) 5.4.2

Pressure Decay Technique This technique is applicable to test objects or systems that can withstand an internal overpressure without deformation or significant variation of the volume. The internal free volume shall be known, unless the leakage rate is specified as a rate of pressure fall within the pressure involved. If the test is carried out under conditions different from those characteristics of nor mal operation (fluids, pressure), this shall be taken into account in the test procedure and in the test acceptance limits. The object under test is subjected to a positive differential pressure (either by pressurisation or by placing it in a vacuum chamber). The pressure source is then isolated and, after a suitable temperature stabilisation time, the pressure and the temperature reading (dew point readings if required) is recorded at regular intervals. Two techniques may be used: Absolute technique: The indicated pressure is the absolute pressure. Reference vessel technique: The pressure within the test object is compared to the pressure inside a reference object. The latter may be a sealed volume located within the test object, of such geometry that it can assume the temperature of the atmosphere of the object. The reference object shall have a pressure slightly lower than the object and shall be tested for tight ness before and after the test; its temperature shall be recorded.

5.4.3

Pressure Rise Technique This technique is applicable to components or systems that can withstand vacuum or an external pressure. It is usually applied to objects that have to be evacuated during normal operations. A pressure difference (lower pressure inside the object) is created across the object boundary. The object is either connected to a vacuum pump system or placed in a pressurised chamber. After the specified pressure difference is attained, the object is isolated and the internal pres sure shall be recorded at regular time intervals. The free internal volume of the object shall be accurately assessed. If the volume is unknown, several measurements of rapid pressure rise, brought about by a measured leakage through a valve, can be made. The volume can be estimated from the average from the pressure increase and the total amount of gas fluid admitted. If the leakage rate is measured as the pressure increase per unit or time, then knowledge of the enclosed volume is not required.

5.4.4

Bell Pressure Change Technique This technique is applicable for test objects or systems that can withstand either pressure or vacuum. It shall be taken into account in the test acceptance limits if the test is carried out under conditions different from those characteristics of normal operation (operating fluids, pressure, temperature). The complete test objects or the section under test is enclosed by a rigid test chamber (bell chamber). Knowledge of the internal free volume of the test objects is not required; however, the free volume of the chamber is required to be known. A pressure tracer is connected to the sealed volume of the chamber. A pressure difference is created between the test object and the

83

chamber, preferably by pressurising / evacuating the volume of the test object. Sufficient time is allowed for any expansion / contraction of the boundary of the test object to cease. A special application is the Torrent permeability measurement technique applied to the quality determination of concrete or possibly to other open-porous materials as well. A small bell is put onto the zone of interest, it is evacuated and the time to refill the volume in full or to a certain degree is measured. The air flow is due to open porosity in the surface zone of the material. An additional evacuated circular zone is avoiding too large influence due to surface roughness of the inspected region. Water content and ageing of the concrete are influencing the result and have to be respected.

Figure 5-9: Torrent permeability measurement technique: application to concrete (left), vacuum system and two chamber vacuum bell (EPFL, Proceq) 5.4.5

Flow Measurement Technique This technique is applicable to components or systems that can withstand a slight overpressure or vacuum. Knowledge of the internal free volume is not required. Constant pressure tech niques can be applied to objects that deform under pressure differences, but the volume has to be maintained constant during the test. The technique consists of determining the extent of leakage by measuring the flow rate of gas into or out of the test object. The test can also be carried out measuring the volume of gas necessary to restore the pressure existing at the start of the test. If gas is flowing into the object: ▪

The object is connected to a vacuum pump through a line equipped with a flowmeter and a pressure control system with a precision valve to maintain a constant internal pressure in the object;



The object is evacuated and placed in a chamber connected with a constant source of pressure through a flowmeter.

If gas is flowing out of the object:

5.4.6



The object is connected to a constant source of pressure through a flowmeter;



The object is pressurised and then placed in a chamber connected to the open atmo sphere through a flowmeter, atmospheric pressure changes shall be taken into account (long term tests);



The object is pressurised and then isolated; at the end of the test period a measured volume of gas is introduced into the object until the pressure existing at the beginning of the test is restored.

Bubble Emission Technique This technique involves the establishment of a pressure difference across the object wall and the observation of bubble formation in a liquid medium located on the low pressure side. The minimum detectable leakage rate depends on the pressure difference, the gas and the liquid used for testing.

84

The surface temperature shall not below 5 °C and not above 50 °C. Direct or indirect visual examination is possible. The immersion technique is applicable for the examination of objects that can be completely immersed in a container of detection liquid, including sealed or temporary sealed ones. A stream of bubbles originating from any isolated point shall be interpreted as a leakage. Three variations exist: ▪

Direct pressurisation of the object: the object is pressurised and placed in the selected detection liquid, the surface after the stabilisation (soak) time is observed for a minimum period which depends on the test specification;



Use of detection liquid at elevated temperature: the object is sealed close to an atmospheric pressure and placed into a detection liquid at elevated temperature;



Use of vacuum: the sealed object is completely immersed in the detection liquid in a vacuum chamber with a viewing port.

The liquid application technique is applicable to any object in which a pressure differential can be created across the boundary to be examined. Instead of an immersion the detection a suit able liquid surfactant is applied by brush, spray or other methods locally. Growing foam from any isolated point shall be interpreted as a leakage. Again a direct pressurisation of the object or the use of a vacuum chamber is possible.

5.5

Various Techniques

5.5.1

Tightness of Piping Systems The tightness of piping systems can have three issues: ▪

Tightness of fresh water provision mainly by public suppliers due to loss, sometimes due to external contamination as well;



Tightness of waste water piping of private owners with the obligation of repeated inspection;



Tightness of piping systems in context with chemical plants.

The characteristics of the water supply of Zurich and some surrounding communities are an annual production of 55 mio. m 3 with a daily consumption roughly between 100'000 and 200'000 m3. The length of the piping system is about 1'500 km. Due to different elevations the pressure is not stable. Most of the tubing is metallic, near the end-user often made of plastic and storage basins and corresponding channels are tiled. Waste water pipes are made of plastic, metals, concrete or masonry. To a certain amount they are designed to withstand other things than water. This can be chemicals from households causing corrosion or submerged parts mainly after storms. The main issue for problems is blocking and the consequential damages. Inspection and maintenance is mainly done visually or with the use of optical systems. For cer tain applications local, partial or global under- or overpressure with water or air is applied.

85

Figure 5-10: Equipment for optical and pressure change inspection for piping systems (Rausch) In Switzerland the formulation of guides and the training of personnel is arranged by the Swiss Water Pollution Control Association (SVA). 5.5.2

Acoustic Techniques Sometimes leaks can be located with acoustic methods. This can be done by contact, non con tact or even remote. It usually needs some experience for a proper reconnaissance and loca tion, especially if the leak has to be detected trough a shell wall. In some cases it is possible to introduce temporarily an acoustic emitter into a container. To avoid noise, the chosen frequency usually is within the ultrasonic range.

Figure 5-11: Equipment for acoustic leak detection (Sonotec) 5.5.3

Air Tightness of Buildings To find leaks in buildings a system called ”Blower Door” is often used. The room is set to a depression of usually 50 Pa and the amount of air change per hour is measured. A typical value that should be reached is 3.5. To locate leaks infrared systems can be used if the temperature of the inflowing air is significantly colder than the room temperature.

86

Power outlet

Hidden hole

Figure 5-12: Application of “Blower Door” (left), thermographic image of a room in its normal state (top) and after applying the depression; the brick stones show different air tightness and cold air is flowing through the power outlet and a hidden hole (Infitec, Empa) 5.5.4

Radionuclide Leakage Test Radionuclide leakage tests are only performed if a very low leakage rate is demanded (hermetically sealed devices). A usual way is to perform a bombing test using krypton-85 with a half live of 10.76 y, typically diluted in gaseous nitrogen as tracer gas. After the bombing the emitted radiation is measured. The fraction due to external adsorbed gas is typically beta-radiation (electrons) that can be stopped by thin layers of absorbers. The more energetic gamma radiation produced by disintegration of Kr-85 gas molecules contained can then be detected and counted with suitable scintillation counters. To keep the Kr-85 gas within the device for some time the charcoal gettering technique is used. A small amount (mg range) of charcoal is placed in the inside of such devices and adsorbs large quantities of the radioactive gas for sufficient time to ensure detection after bombing. This is used for electronic devices or critical safety equipment like air bag initiators, etc.

Figure 5-13: Pressurisation unit (left) and counting unit for radionuclide leakage test (Isovac) The special advantage and the reason for its very high efficiency of this technique is that the gas has only to enter the device; its detection is possible through the wall (in opposite to the normal bombing test). The disadvantage is the necessity of radiation protection measures.

87

5.6

Procedure and Record

5.6.1

Procedure The NDT procedure usually has to define the following technical aspects:

5.6.2



Type of leak testing technique;



Special requirements on application, pressure, temperature and time.

Record For reproducible testing the following is necessary, if applicable:

5.7



Technique used;



Description of apparatus employed (leak detector type, type and concentration or pressure of tracer gas, liquid type, type, volume and tightness of auxiliary enclosures) including statements of measuring range, uncertainty and resolution;



Calibration operations with the parameters and relevant signals, reference conditions;



Test conditions (pressure, temperature, duration);



Descriptions of calculations made.

Summary and Conclusions For leak testing various types of techniques exist, varying on the purpose (leakage rate meas urement or leak localisation), on the sensitivity needed and on the possible arrangement. The term leak testing, therefore, is not denoting one specific technique, but one goal, namely to get information on leaks. The applications can vary very widely. For obvious reasons it is mostly used in connection with pressurised or vacuum systems.

88

6

MAGNETIC INSPECTION

6.1

Introduction The earliest known use of magnetism to inspect an object took place as early as 1868. Cannon barrels were checked for defects by magnetising the barrel and sliding a magnetic compass along the barrel's length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. In the early 1920’s, William Hoke realised that magnetic particles (coloured metal shavings) could be used with magnetism as a means of locating defects. Hoke discovered that a surface or subsurface flaw in a magnetised material caused the magnetic field to distort and extend beyond the part. This discovery was brought to his attention in the machine shop. He noticed that the metallic grindings from hard steel parts, which were being held by a magnetic chuck while being ground, formed patterns on the face of the parts which corresponded to the cracks in the surface. As the detection medium often are magnetic particles, this method is generally known as magnetic particle testing. The physical principle, however, is diverted magnetic flux, and so this expression is clearer and covers all the situations as well, were the detection is done otherwise than with magnetic particles. The expression magnetic flux leakage (MFL) is usually used only if the effect is measured with sensors. Thin-film inductive (TFI), anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR) sensors are or were used in computer hard drive construction. However, the basic principles can also be used to detect diverted magnetic flux. Giant magnetoresistance was patented at the end of the 1980s by Peter Grünberg (*1939). The same effect was found independently by Albert Fert (*1938). For their invention the two were awarded the Nobel Prize in physics in 2007. The method can be used for ferromagnetic materials; as rule of thumb, the material must have a relative permeability µr (dimensionless) of at least 100. In opposite to liquid penetrant testing, it can be applied as well if the part is covered with painting or other solid coatings up to a thickness of about 50 µm. Furthermore, the source of diverted flux needs not to pass the sur face; larger sources can be detected as well if there position is much deeper. With the use of adequate probes, this method can be automated completely.

6.2

Diverted Magnetic Flux

6.2.1

Magnetic Field Distribution For all magnetisation techniques it is important if the field is applied with a permanent magnet or with direct current or, on the other hand, with alternating current. For the two application types, the formation of the magnetic field within the object can be differ very much. The main effect to be addressed is the skin effect, i.e. the distribution of the flow of the electric current, and therefore of the generated magnetic field. For alternating current it is higher near the surface of the electric and/or magnetic conductor. This effect becomes stronger with increased frequency of the applied electric field (more details see chapter on eddy current testing).

Figure 6-1: Current and magnetic field distribution for DC (left) and AC (very simplified) 6.2.2

Magnetic Properties The magnetic induction must reach a sufficient value (minimum 1 T) and, to generate a flux diversion, the magnetic field must cut the possible flaw. In general, flaws can be found if there direction has an angle of up to 60° to the optimum field direction. Therefore, a magnetisation in two perpendicular directions is sufficient to find all possible flaw directions.

89

Figure 6-2: Detectability of flaws The minimum flaw dimensions for this method are a width of 0.5 µm and a depth of 10 µm. The ratio of width to depth to length should be 1 : 10 : 50. The relation between the magnetic field strength H and the magnetic induction B is given by B=µ H

with

µ=µo µr

(6.1)

with the permeability µ and the magnetic field constant µo (4 π 10 V s A m ). Pure iron and some iron-nickel alloys show the highest value for the relative permeability µ r of up to 106. However, this number is not a constant, but depends on the magnetic history. Therefore, the measurement of the induction is not simple, while the measurement of the magnetic field strength is relatively simple. Therefore, for ferromagnetic steels a value of 2 to 5 kA m -1, in extreme cases (some cobalt-base alloys) up to 9 kA m -1, are said to be sufficient to reach the magnetic saturation. -7

-1

-1

If magnetisation is too high, spurious background indications may appear, which could mask relevant indications.

Induction B [T]

2

Saturation

1 Virgin curve

Remanence

0

Coercivity -1

-2 -4

-2

0

2

4

Magnetic field strength H [kA m ] -1

Figure 6-3: Example of the magnetic hysteresis loop and virgin curve with the coercivity and the remanence (also called retentivity) For the description of the diversion of the magnetic flux only a few models exist. 6.2.3

Models for the Diversion by a Slot The equations of Zatsepin and Shcherbinin (Defektoskopiya, 2 (1966) pp. 50-65), modified by Friedrich Förster (1908-1999) give an approximation for the normal component H z and the transverse component Hx of the diverted flux of a surface breaking slot with a width w and a depth d. Hg is the total of the diverted flux generated by the slot, x the transverse distance (slot at x = 0) and s the lift-off distance (surface at s = 0).

90

Hz

Hx

Lift off s d w Hg Figure 6-4: Coordinate axes for the equations of Zatsepin and Shcherbinin  x

Hx

1 −1 = [tan Hg π

 x

w d 2

 x− −1

2

w  s sd 2

−tan

2

w d 2

2

x−

w  s sd 2

(6.2)

]

2

w w 2 2  sd  x−  s 1 2 2 = ln[  ] 2 2 Hg 2 π w 2 w 2 x−  sd  x  s 2 2 x

Hz

(6.3)

The detailed shape of the transverse component is fairly dependent on the lift off and the slot depth. For zero opening width, no magnetic flux is diverted. Note that the signal measured by coils is the first derivative of the respective field component.

Relative field strength H/Hg [%]

If this model is applied to cracks that show a small ratio between width and depth, it becomes clear that the detectability is governed by the lift off. The detected signal is rapidly decreasing with increasing distance to the surface and it is soon at noise level. Cracks that are completely closed show no diverted magnetic flux and are, therefore, not detectable with this method. 120

Hx

80

Hz s = 0.025

s = 0.025

s = 0.1

40 s = 0.5 0 -40

s = 0.5 s = 0.1

-80 -120 -2

-1

0 Position

1

2 -2

-1

0

1

2

Position

Figure 6-5: Transverse (left) and normal component of the magnetic field diversion due to a slot of a width of one unit (-0.5 to 0.5) and a depth of one unit as well, the three curves denotes the lifts off of 0.025, 0.1 and 0.5 units; very near to the surface the edge effect is maximum

91

Relative transverse field strength over crack centre Hx/Hg [%]

100 80 s = 1/4 60

s=1

40 s=5

20 0 0

0.2

0.4

0.6

0.8

1

Crack width (crack depth: one unit) Figure 6-6: Diverted tangential field strength dependence on crack dimensions and lift off 6.2.4

Model for the Diversion by a Subsurface Cylindrical Hole A sometimes used approximation for the determination of the resulting field in magnetic flux leakage is adapted from Bray & Stanley (see appendix). The formulas are given in a slightly dif ferent way. The magnetic field from a subsurface side drilled hole of permeability μ i and radius r in a material of permeability μm and magnetised with the applied field Ha is given as Hx

x 2 −hs2

2

 2r −m

(6.4)

2 xhs 2 = 2r −m  Ha  x2 hs2 2

(6.5)

Ha

=

2

2 2

x hs 

Hz

where x is the position of the measuring probe and s its lift off, the distance to the surface, and with m=

2 μm μm μi

[1−

μ m−μ i μ mμ i

2

 

r 2 μ m−μi 2  ] r 2h μ mμi −1

(6.6)

Assuming that in most cases μm >> μi, this term may be simplified to m≃2r 2 [1−

r 2 −1  ] 2h

The model fails for a surface groove (h=0).

92

(6.7)

Relative field strength H/Ha [%]

Hz

Lift off s

Hx h

Radius r Ha

0.4 Hx 0.2

0.0

-0.2

Hz

-0.4 -20

-10

0 Position

10

20

Figure 6-7: Diverted field of a side drilled hole with radius one, at a depth of three and measured at a lift off of one unit

6.3

Magnetising Equipment In most cases, the necessary magnetic fields are generated electrically. The use of permanent magnets is generally limited to special applications where electricity is not easily applicable (ropes for cable cars, pipelines, etc.). When magnetisation is generated from time-varying currents, the rms-value is the required quantity. The use of pulsed or phase-cut currents requires specific measurements. Table 6-1: Relationship between wave form, mean and rms-value for a peak current I and alternating current (AC) and various forms of direct current (DC) Wave form

Symbol

rms I

Alternating current

0

Half wave direct current (HWDC)

I π

I 2

Full wave direct current (FWDC)

2I π

I 2

0.826 I

0.840 I

Three phase, halfwave rectified

6.3.1

Mean

2

Magnetic Field of an Electric Conductor According to the Biot-Savart law (Jean-Baptiste Biot, 1774-1862, Félix Savart, 1791-1841) an electric current I in a straight conductor generates in the distance r a circular magnetic field of H=

I 2πr

(6.8)

93

Figure 6-8: Magnetic field of an electric conductor (non-magnetic material, DC) 6.3.2

Magnetic Field of a Coil In a coil the field portions of the single conductors add and the resulting field of a coil with n windings and the length ℓ is approximately H≈

nI ℓ

(6.9)

Specially for short coils (compared to the diameter) and at the ends of all coils this formula leads to considerably too high values, as the field depends additionally on the diameter D of the coil and the axial distance x from the centre of the coil. On the central axis of the coil, the magnetic field strength therefore is ℓ ℓ x −x nI 2 2 H=    ℓ   ℓ2 x2 D2   ℓ−2 x2 D2

(6.10)

The values at the centre and at the end of a coil can be found with x = 0 and x = ℓ/2, respect ively: HC=

nI

(6.11)

 ℓ D  2

2

and HE =

nI

4 ℓ

2

(6.12)

D 2 

Relative field strength [%]

100 80 Long coil

60 40 20

Short coil

0 -3

-2

-1

0

1

2

3

Position [D] Figure 6-9: Relation of the real field strength to the one obtained with the simple formula for a long coil (ℓ/D = 5) and a short coil (ℓ/D = 1)

94

Figure 6-10: Closed (left) and hinged magnetising coil, flexible coil (MR Chemie, Tiede, Empa) 6.3.3

Hand Yokes A magnetic yoke consists of one or two coils, positioned in the legs or in the cross brace. The magnetic field is lead over two, often adjustable poles. As there is no closed magnetic reflux, the air gap is not negligible any more. In Europe DC yokes are seldom in use and in accordance with the applicant only. In this case the yoke is made of massive low carbon steel. AC yokes consist of laminated soft steel sheets as used for transformers. Cross yokes generating a magnetic field in two directions (longitud inal and transverse) are available as well.

Figure 6-11: Magnetising yokes with adjustable or fix poles, cross yoke (Tiede) The force needed to pull off of a yoke with the area of one pole A is given by F=HB A

(6.13)

A minimum of 44 N is recommended. 6.3.4

Prods If the part itself is used as electric conductor, electrodes are used as prods. The necessary current is typically between 1 kA and 10 kA.

Figure 6-12: Prods and high current generator (MR Chemie, Magnaflux) 6.3.5

Magnetising Mandrels Mandrels may be used for a magnetisation in two directions at the same time. They consist of an electric conductor (usually copper) and DC pole made of laminated soft steel sheets. Mandrels are suitable for the inspection of annular or tubular components.

95

Figure 6-13: Magnetising mandrels (Tiede) 6.3.6

Magnetic Benches The equipment may be integrated in a complete unit, suitable for different magnetisation tech niques at the same time or sequential.

Figure 6-14: Simple magnetic bench (Magwerks)

Figure 6-15: Automatic installation for coaxial parts (2 kg, length 200 mm, Ø 50 mm), two magnetising directions (threaded conductor 700 A, yoke 10'000 A turns), clock cycle 13.5 seconds, evaluation with eight cameras (Tiede)

96

6.4

Magnetisation

6.4.1

Overview More than one technique may be necessary to find discontinuities on all test surfaces and in all directions. Different magnetisation may be applied one after another, also called sequential magnetisation, or at the same time, called multidirectional magnetisation. In this case at least one has to be a technique using AC to allow the magnetic field to turn. In general the techniques can be differentiated into current flow techniques and magnetic flow techniques. Magnetising techniques Current flow techniques

Magnetic flow techniques Threading conductor

Axial Prods

Conductor

Adjacent

Induced Yoke

Cable (coiled) Fixed installation Hand yoke Rigid

Coil

Flexible

Figure 6-16: Magnetising techniques (A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

Figure 6-17: Magnetising techniques by sketch: current flow: (A) axial, (B) prods, (C) induced; magnetic flow: (D) threaded conductor, (E) adjacent conductor, (F) adjacent cable, (G) yoke, (H) fixed installation, (I) rigid or flexible coil 6.4.2

Axial Current Flow Current passes through the component, which shall be in good electrical contact with the pads. Care shall be taken to avoid damage to the component at the point of electrical contacts. Pos97

sible hazards include excessive heat, burning and arcing. Certain contact materials, such as copper or zinc may cause metallurgical damage to the component if arching occurs. Lead contact pads may be used, but only in well ventilated conditions, because they may generate harmful vapours. Contact areas should be as clean and as large as practicable.

Figure 6-18: Magnetic field of axial current flow with DC (left) and AC, ferromagnetic material; in the interior of a hollow conductor there is no magnetic field (independently on AC or DC) The required current I for a component with the perimeter p is given by I=H p

(6.14)

With items of varying cross section, a single value of current shall be used only when the cur rent values required to magnetise the largest and smallest sections are in a ratio of less than 1.5 : 1. When a single value of current is used, the largest section shall govern the current value. The inner surface of hollow pipes or similar objects cannot be tested with this technique as the magnetic field strength inside is zero. 6.4.3

Prods; Current Flow Current is passed between hand-held or clamped contact prods, providing an inspection of a small area of a larger surface. The prods are then moved in a prescribed pattern to cover the required total area, overlap is necessary. Particular care shall be taken to avoid surface damage due to burning or contamination of the component by prods. Zinc plated or galvanised prods shall not be used. Arching or excessive heating shall be regarded as a defect requiring a verdict on acceptability. For a prod distance d of up to 200 mm and a not too much curved surface, the rms value is given by Irms =2.5H d

to

Irms=3H d

(6.15)

Usually the area with a radius of 25 mm around the prods cannot be tested using this tech nique. 6.4.4

Induced Current Flow Current is induced in a ring shaped component by making it, in effect the secondary of a trans former. This technique only works with AC and the secondary loop must be closed. The required induced current Iind for a component with the perimeter p is given by Iind=Hp

(6.16)

The induced current cannot be easily calculated from the primary current. With items of varying cross section, a single value of current shall be used only when the current values required to magnetise the largest and smallest sections are in a ratio of less than 1.5 : 1. When a single value of current is used, the largest section shall govern the current value. 6.4.5

Threaded Conductor Current is passed through an insulate bar or flexible cable, placed within the bore of a compon ent or through an aperture. The required current I for a central conductor and for a component with the perimeter p (inner or outer surface) is given by I=Hp

98

(6.17)

Figure 6-19: Magnetic field for a threaded conductor (DC) 6.4.6

Adjacent Conductor or Cable One or more insulated current-carrying cables or bars are lead parallel to the surface of the component, adjacent to the area to be tested and supported a distance d above it. The return cable for the electric current shall be arranged to as far removed from the testing zone as possible and, in all cases, this distance shall be greater than 10 d, where 2 d is the width of the tested area. The cable shall be moved over the component at intervals of less than 2 d to ensure that the inspection areas overlap. The rms value of the current flowing in the cable is required to be Irms =4 πd H

(6.18)

When testing radiused corners on cylindrical components or branch joints, the cable may be wrapped around the surface of the component or the branch and several turns may be bunched in the form of a closely wrapped coil. In this case d is the distance to the nearest winding and the necessary current can be divided by the number of turns. 6.4.7

Fixed Installation The component, or a portion of it, is placed in contact with the poles of an electromagnet of a magnetic bench.

6.4.8

Portable Electromagnet (Yoke) The poles of an AC yoke are placed in contact with the component surface. The testing area shall not be greater than that defined by a circle inscribed between the pole pieces and shall exclude the area immediately adjacent to the poles. DC electromagnets and permanent magnets may only be used by agreement at the time of enquiry and order.

6.4.9

Rigid Coil The component is placed within a current-carrying coil. When using rigid coils of a helical form, the pitch of the helix shall be less than 25% of the coil diameter. The necessary current depends on the length of the component. For short components, where the length to diameter ratio is less than 5, it is recommended that magnetic extenders are used. The current required to achieve the necessary magnetisation is thus reduced.

6.4.10

Flexible Coil A coil is formed by winding a current-carrying cable tightly around the component. The area to be tested shall lie between the turns of the coil (in opposite to an adjacent cable coil). To achieve the required magnetisation using DC or rectified current, the rms value the current flowing in the cable shall have a minimum value of Irms=3H T

Y2  4T

(6.19)

Where T is the wall thickness of the component, or its radius if it is in form of a solid bar of cir cular section, and Y is the space between adjacent windings in the coil. In case of AC, T shall be set to 10 mm. 99

6.5

Detection of the Magnetic Flux If magnetic particles are used, it is usual to apply the magnetic field for a while, the so called post-magnetisation time after the particles are applied, allowing a correct orientation of them.

6.5.1

Dry Magnetic Particles Dry magnetic particles are usually coloured (red, black, grey, yellow or others), sometimes fluorescent. To achieve good contrast for coloured media, it may be necessary that a thin layer of contrast aid paint of usually white colour is applied prior to testing. The thickness of this layer should not exceed 50 µm. Size and shape of the particles are important. Dry magnetic particle products are produced to include a range of particle sizes from 20 µm to about 300 µm. Small particles are more sensit ive to the diverted fields from very small discontinuities. However, dry testing particles cannot be made exclusively of the fine particles. Coarser particles are needed to bridge large discon tinuities and to reduce the powders dusty nature. Additionally, small particles easily adhere to surface contamination, such as dirt or moisture and get trapped in surface roughness features producing a high background level.

Figure 6-20: Coloured dry magnetic particles indicate large crack, note the particles at the holes (left) (NDT RC) Long, slender particles tend to align themselves along the lines of magnetic force. However, they lack the ability to flow freely. Therefore, globular particles are added. The mix of globular and elongated particles results in a dry powder that flows well and maintains acceptable sensitivity for larger flaws. The use, therefore, makes only sense for rough surfaces and/or larger dis continuities, e.g. for castings. Dry magnetic particle are necessary for hight temperature applications where a carrier fluid would evaporate or dissolve. 6.5.2

Wet Magnetic Particles

Figure 6-21: Wet fluorescent magnetic particles (left) and wet coloured particles, the component is partly covered with contrast aid paint (NDT RC, Ely Chemical) The usual detection media is a suspension of coloured (black or brown) or fluorescent particles in a carrier fluid. Fluorescent particles have a ferromagnetic core and are coated with pigments that fluoresce when exposed to ultraviolet light. Particles that fluoresce green-yellow are most common; other fluorescent colours are available as well. Fluorescent media usually give the highest sens100

itivity provided there is an appropriate surface finish, good drainage to maximum indication contrast and well controlled viewing conditions (as in liquid penetrant testing). Similar to liquid penetrant testing, a dual use version (coloured and fluorescent) is possible as well, usually hav ing an orange colour.

Figure 6-22: Comparison of different magnetic particle colours; black, daylight fluorescent, fluorescent (Deutsch) The particles used in the wet method are smaller in size than those used in the dry method. The particles have a typical size of 10 µm and smaller and the synthetic iron oxides have particle diameters around 0.1 µm. This very small size is a result of the process used to form the particles and is not particularly desirable, as the particles are almost too fine to settle out of suspension. However, due to their slight residual magnetism, the oxide particles are present mostly in clusters that settle out of suspension much faster than the individual particles. This makes it possible to see and measure the concentration of the particles for process control purposes. Wet particles are a mix of long slender and globular particles as well.

Figure 6-23: Various possibilities of delivery forms of magnetic particle: powder, concentrate, paste and spray (Tiede) The carrier may contain oil-based or water-based components. Water-based carriers form quicker indications, are generally less expensive, present little or no fire hazard, give off no petrochemical fumes and are easier to clean from the part. Water-based solutions are usually formulated with a corrosion inhibitor and wetting agents. However, oil-based carriers offer superior corrosion and hydrogen embrittlement protection to those materials that are prone to be attacked by these mechanisms. Wet magnetic particles are delivered as final suspension (in spray cans, etc.) or as liquid or paste-like concentrate. 6.5.3

Peelable Detection Media The easiest way to apply peelable detection media is to take a suspension of dry powder in a solvent that quickly evaporates. After its volatilisation the residual powder pattern can get picked off using a transparent adhesive tape or similar media.

Figure 6-24: Black iron powder on adhesive tape show crack, machining traces and noise due to clumped powder, height 20 mm (Empa) The magnetic rubber technique can be used to examine areas difficult to reach, such as the threads on the inside diameter of holes, where the moulded plugs can be removed and 101

examined under ideal conditions and magnification if desired. The trade-off, of course, is that inspection times are much longer. The technique uses a liquid rubber containing suspended magnetic particles. The rubber compound is applied to the area to be inspected on a magnetised component. Inspections can be performed using either an applied magnetic field, which is maintained whilst the rubber sets (active field), or the residual field from magnetisation of the component prior to pouring the compound. A dam of modelling clay is often used to contain the compound in the region of interest. The rubber is allowed to set, which takes from 10 to 30 minutes and then the rubber cast is removed from the part. It conforms to the surface contours and provides a reverse rep lica of the surface. The rubber cast is examined for evidence of discontinuities, which appear as dark lines on the surface of the moulding. It can be retained as a permanent record of the inspection. In its most sensitive application, this detection method is capable of revealing finer cracks than other magnetic techniques. Even Luders bands, ferritic or austenitic precipitations or heat affected zones of welds can be replicated. Usually this method is used under laboratory conditions only. 6.5.4

Induction Coils For the detection of diverted magnetic flux induction coils can be used. They are, however, only capable to pick up changes of the leaking field. This is of importance if the magnetisation is done with DC or permanent magnets or the component is investigated while remanent. With an appropriate bending they may be adapted to different shapes. There exist two types of coils: a U-bended coil measuring the radial component of the diverted field, called LD-coil (local default) and a full coil around the part, measuring the axial compon ent, called LMA-coil (loss of metallic area). While a LMA-coil has to be wound for every application, a LD-coil can be opened and closed. Induction coils are widely used in rope and cable testing. However, dependent on their positioning, they give only partial or none information on the circumferential position of a flaw, they cannot always differentiate between one or several flaws, and they are not working, if the device is not or slowly moving (too low or no signal).

(A)

(B)

Figure 6-25: Sensing head for wire rope inspection (A) Hall effect sensors (before sealing), (B) induction half coil (LD-type, sealed) (Empa) As soon as sensors are used, and this is valid for all types of them, the user has to decide what part of the signal he wants to get. 6.5.5

Hall Effect Sensors An electric current I is passed through the conductor or semiconductor. In a magnetic field, the Lorentz force on the moving electrons tends to push them to one side of the conductor. A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage UH between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall effect (Edwin Hall, 1855-1938). UH=

102

R H IBn b

(6.20)

b

Bn I

UH

Figure 6-26: Functionality and orientation of Hall effect sensors (NDT RC) RH is the Hall coefficient of the material and depends on the electric conductivity and therefore on the temperature. Bn is the component of the magnetic field that is perpendicular to the direct current in the Hall element. It is therefore important, how the element is oriented. Hall elements cannot only be used within measuring equipment, but designed as arrays for testing also. The well-known applications are rope and cable testing, pipeline inspection and testing for underfloor corrosion. Their range is between 10 -6 to 102 T. 6.5.6

Magneto-Optical Sensors Magneto-optical (MO) sensors are using the Faraday rotation. This effect describes that the polarisation plane of an electromagnetic wave is turned when transmitted through a transparent medium in a magnetic field. The angle of the rotation θ depends on the induction B, the transmitted distance d and the Verdet constant V (Émile Verdet, 1824-1866). Typical values are several 10 rad T-1 m-1. θ=V Bd

(6.21)

The Verdet constant itself depends on the media and the wavelength; shorter wavelengths usually lead to increased rotation. A common substance showing this effect is epitaxial grown bis muth and gallium dotted yttrium iron garnet (YIG, Y3Fe5O12) typically in the form of Y2.5Bi0.5Fe3.8Ga1.2O12 or instead of yttrium with lutetium as Lu 2Bi1Fe4.1Ga0.9O12. Magneto-optical sensors are useful above 10-4 T.

Figure 6-27: Faraday rotation Commercially already implemented is the principle in a technique called magneto-optic imaging for eddy currents. The area that can be inspected may have a length of several cm. It is, however, also possible to read out a transparent layer positioned within an exciting coil and to get an image of the distribution of the magnetic field, and therefore more accurate information than with the averaged value of the whole area by a receiving coil. Principally it is possible to analyse the amplitude and/or the phase information of the image.

103

6.6

Demagnetisation

6.6.1

Basic Principle After the performance of the test the parts remain in remanent magnetisation. This is often unwanted and a demagnetisation may be necessary ▪

Not to affect further machining of the part by cuttings clinging to the component;



Not to influence welding processes (especially DC) by arc blow (may causes the weld arc to wander or filler metal to be repelled from the weld) and by electron beam welding;



To prevent rotating parts from producing inductive fields and interfering with electronic devices (airspace);



To prevent abrasive particles to cling to bearing or fraying surfaces and increase wear;



To prevent uneven coating (chromium plating).

Figure 6-28: Shearing-blades before and after demagnetising (Maurer) The simplest thing would be, to heat the component above Curie temperature, this, however, is rarely practicable. The Curie temperature or Curie point (Pierre Curie, 1859-1906), is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. The three effectively used methods are based at the same tech nique of reducing the remanent fraction.

Figure 6-29: Principle of demagnetisation (NDT RC) 6.6.2

Reversing DC Demagnetisation Most benches have a built-in in automatic counter pole magnetisation. In a first step the part is magnetised in one direction with a field higher than the previous one. In the following steps, the direction of the field changes continuously, while the strength is decreasing.

6.6.3

Demagnetising Tunnels Small parts can be pushed through a high frequency AC coil; the increasing distance, and therefore the reduced field that constantly change its direction demagnetise the part.

104

Figure 6-30: Hand-held demagnetising device and demagnetising tunnel (Magwerks, Tiede) 6.6.4

Yoke Lift Off Technique Lifting off an AC yoke from the part has a similar effect as a demagnetising tunnel.

6.6.5

Maurer Demagnetising The usually used low frequency demagnetising often is not effective as magnetic fields in certain directions are not affected enough. Furthermore the component has to be brought to full saturation before the cycle can start. Often usual equipment is not build to allow the necessary high magnetic fields, so pulsed coils are in use.

Figure 6-31: High frequency demagnetisation with antecedent saturation (Maurer)

6.7

Aspects of Quality Assurance

6.7.1

Field Direction and Strength An easy way to find the directions of the magnetic field is done with the Berthold penetrameter. It consists of a soft iron cylinder that is separated into four 90° segments by non-magnetic foils (brass). These simulate a crack. The device can be easily rotated and allows the verification of the field direction; especially it can be used to verify, if angular cracks would be found as well. For a quantitative measurement of the field strength it is not suited. Similar magnetic quality indicators exist, partly as crosses, partly as rings or double rings.

105

Figure 6-32: Berthold penetrameter (left), magnetic quality indicators, magnetic field meter (Tiede, Western Instruments) Simple field indicators are small mechanical devices that utilise a soft iron vane that will be deflected by a magnetic field. They show the amount and the direction of the magnetic field, however, due to the mechanics of the device their range is limited. They are well suited for measuring the residual magnetic field after demagnetisation.

Figure 6-33: Magnetic field indicators (Western Instruments) The equipment measuring the field strength usually uses Hall effect sensors. The equipment has to be calibrated using an appropriate reference magnet each time. 6.7.2

Quality of the Magnetic Detection Media Producers of magnetic detection media have to test their products concerning manifold characteristics like performance, colour, particle size, temperature resistance, fluorescent coefficient and fluorescent stability, flash point, corrosion induced by detecting media, viscosity of the car rier fluid, mechanical stability (for use in benches with centrifugal pump), foaming, pH value and storage stability. For the end user the necessary controls can be performed very pragmatically. It consists from a visual colour test of the detection media and contrast aid paint and a regular performance test of magnetic detection media used repeatedly or mixed form a concentrate. This can be done using reference test blocks like EN ISO 9934-2 No. 1 (earlier known as MTU No. 3) made from a permanent magnetic material with a crack containing surface. The cracks are induced by grinding and stress corrosion. As for liquid penetrant testing, each block is unique and can only be compared to itself. A second semi-quantitative method is the use of the reference test block EN ISO 9934-2 No. 2 (earlier known as AFNOR C). This one contains of two steel blocks, separated by an aluminium foil. Two permanent magnets at each end generate a magnetic field with decreasing strength of -100 A/m (at -4 cm) to 100 A/m (at 4 cm), indicated by an engraving along the aluminium foil. For similar purpose the Fluka test block was used, still widely spread. Another possibility is the ANSI KETOS steel tool ring. It is used together with a threaded conductor and contains up to twelve holes at different distances to the centre. For each combination of current and product (wet or dry, coloured or fluorescent) the indication of a certain hole number is required.

106

Figure 6-34: EN ISO 9934 -2 reference test blocks No. 1 (left) and No. 2 (Tiede, Srem) A further check, not foreseen in the European standardisation, can be used to determine the concentration of the magnetic suspension. The standard process used to perform the check requires agitating the carrier for a minimum of thirty minutes to ensure even particle distribu tion. A sample is then taken in a pear-shaped 100 ml sedimentation tube having a stem graduated to 1.0 ml in 0.05 ml increments for fluorescent particles and graduated to 1.5 ml in 0.1 ml increments for visible particles. The sample is then demagnetised so that the particles do not clump together while settling. The sample must then remain undisturbed for a minimum of 60 minutes for a petroleum-based carrier or 30 minutes for a water-based carrier, unless shorter times have been documented to produce results similar to the longer settling times. The volume of settled particles is then read. If the particle concentration is out of the acceptable range, particles or carrier fluid must be added to bring the solution back in compliance with the requirement.

Figure 6-35: Sedimentation tube (Deutsch) If the suspension is already used, the amount of that what is seen as concentration can consist of three parts: contamination, primarily coming from inspected components, unaffected magnetic particles and fluorescent material dislodged from the particles by the mixing pump. The latter only happens with fluorescent particles. Differences in colour, layering or banding within the settled particles would indicate contamination. Some contamination is to be expected but if the foreign matter exceeds 30% of the settled solids, the solution should be replaced. While not technically contamination, this condition should be further evaluated by allowing the collected sample bath to set for 10 to 12 hours and viewing under ultraviolet light. If a band that fluoresces brighter than the bulk of particles is evident on top of the settled solids, the bath contains excessive unattached fluorescent pigments and should be discarded. For water-based suspensions the pH-value should be measured regularly as too high values (> 9) may lead to skin irritation and for too low values the corrosion inhibition is no longer guaranteed. 6.7.3

Viewing Conditions See visual testing and liquid penetrant testing, respectively.

6.7.4

Safety Aspects Safety aspects concerning black light lamps see liquid penetrant testing.

107

Magnetic particle testing may require the use of toxic, flammable and/or volatile materials. In such cases, working areas therefore should be adequately ventilated and far from sources of heat or flames. Extended or repeated contact of detecting media and contrast paints with the skin or mucous membranes shall be avoided.

Figure 6-36: High current generator showing warning signs (Tiede) Magnetic testing often creates high magnetic fields close to the object under test and the magnetisation equipment. Items sensitive to these fields should be excluded from such areas. Especially people wearing pacemakers are to be warned, corresponding signs shall be attached to magnetic benches, etc.

6.8

Procedure and Record

6.8.1

Procedure The NDT procedure usually has to define the following special technical aspects:

6.8.2



Special surface preparation (e.g. degreasing);



Type of magnetic media;



For scanning applications: direction of scanning, size and type of probe, surface speed, coverage of the probe;



Requirements for magnetisation;



Necessary demagnetisation and conservation.

Record The record should contain the following information:

108



Description of the equipment used, including serial numbers, etc.;



Magnetisation technique, including indicated current values, waveform, tangential field strengths, contact or pole spacing, coil dimensions, etc.;



Detection media and contrast aid paint if used, including batch numbers;



Viewing conditions;



Demagnetisation and maximum residual field strength after test;



Applied conservation.

Figure 6-37: Example of a MT examination record (Empa)

6.9

Special Applications

6.9.1

Underfloor Corrosion Because the magnetic flux leakage method responds to both far side and near side corrosion, it is necessary to introduce a strong magnetic field into the component wall. The closer this field becomes to saturation for the component, the more sensitive and repeatable the method becomes. For typical steels used in bulk liquid storage tank construction this value is generally between 1.6 and 2 T. In this range any residual magnetism from previous scans or operations will be eliminated during subsequent scans so that the resulting flux leakage signals remains relatively constant and repeatable.

Figure 6-38: Testing and test equipment for underfloor and tank corrosion (Silverwing, MFE) For a given magnetising system the flux density achieved in the component will depend on the thickness and permeability of the material. So the factor controlling flux density becomes one 109

of plate thickness. There will be an upper thickness limit for each given magnetising system above which flux density will be too low to give adequate sensitivity to pitting. One advantage of using an electro-magnet is that the magnetising current can be increased to cater for a wider range of plate thickness than can be achieved with a given permanent magnet. However, this will be at the expense of size, weight and often the use of an independent (battery) power sup ply. The magnetic field is recorded by a set of Hall effect sensors usually arranged perpendicular to the scanning direction. The two possibilities for the orientation of the sensing elements pick up either the normal (vertical) or the transverse (horizontal) component. There are advantages and disadvantages with both these alternatives. There are two types of noise that will be encountered by the systems. The first is the large eddy current signal generated by the movement of the magnet over a conducting surface. The second type of noise is that generated by surface roughness and permeability variations in the plate material. Eddy current signals are a function of the velocity of the magnet carriage and in a typical scan have three stages - a rising signal during acceleration - a steady state (DC) value at scanning speed - and finally a decreasing value during braking. Eddy current effects tend not to show up when the normal component of the magnetic field is being used and this means that those sys tems using coils or vertically mounted Hall effect sensors do not need to worry about this noise. However, horizontally mounted Hall effect sensors do respond well to the eddy current noise so it needs to be eliminated. Since the deceleration during braking is sharper than the accelera tion during start-up and the signals generated are of opposite polarity, it is convenient to elim inate the stopping signal by rectification of all signals and only measuring positive components. Passing all signals through a high pass filter with a suitable cut-off frequency eliminates the start up and steady state signals. The remaining noise will be represented by relatively slow changes of permeability and much sharper (higher frequency) signals from vibrations in the system due to surface roughness. Arranging the sensors in differential pairs reduces the first of these, and the second by passing all signals through a low pass filter with a suitable cut-off frequency. On the face of it may seem sensible to choose to measure the transverse component of the field so that eddy current effects are no longer a problem. But it is possible to get the sensor closer to the surface when mounted horizontally and this gives a sensitivity advantage, especially on coated floors. Some of the above given aspects on noise and noise reduction are valid for the following applications as well. 6.9.2

Wire Cable and Stay Cable Inspection Cables for cable cars and civil engineering structures (mainly bridges) are inspected using per manent magnets or coils. Only the latter are suitable for large cable diameters (up to 200 mm or more) but have to be wounded with typically up to 150 windings for every single rope and inspection segment. Detecting coils are used for the general testing, Hall effect sensors if a closer view is necessary or the coils are not suitable due to slow motion. The possible motion with Hall effect sensors, however, is much lower. Hall effect sensors are arranged circumferential and give additional information about the position of a wire crack. This method can be used for imaging. Such techniques are also called magnetic rope testing (MRT). According to the given formulae, it must be clear, that closed cracks cannot be found by magnetic methods, however, MFL is an excellent technique to find gaping ones.

110

Figure 6-39: Inspection equipment for cable car ropes (left) and bridge tension cables (Empa)

Figure 6-40: Rama IX bridge in Bangkok where all the 68 stay cables were investigated using MFL (Empa)

111

Figure 6-41: Imaging magnetic flux leak testing: signal of the induction coil (top), signal of the Hall effect sensors (mid) and enlarged sector (400 – 800 mm) (Empa) 6.9.1

Tube and Pipeline Inspection Ferritic pipelines are inspected using permanent magnets, typically of the NdFeB type. The device inserted into the tube is called pipeline inspection gauge (PIG) and can have a length of several metres. For gas pipelines the only other possible technique is ultrasonic electromagnetic-acoustic transducer (EMAT) technique. Relevant indications are the wall thickness, buckles or general flaws. The transfer of the magnetic field onto the tube may be done by either flexible bristles or firm brushes. For the first possibility, the magnets are farer away from the tube and the field is lower, for the second one resistance against a displacement becomes stronger. The pig may be centred by wheels. (E)

(C)

(D)

(A) (B) Figure 6-42: Sketch of a pipeline pig, left: bristle type with (A) cross brace with attached magnets, (B) flexible bristles, right: brush type with (C) magnet, (D) brushes, for both types: (E) circumferential sensors The leaking flux is detected by Hall effect sensors, arranged along the circumference. Usually a second row is used to fill the gaps of the first one. The autonomy of such devices guaranties an inspection distance of up to 1'000 km. For ferromagnetic tubes the probes are simpler, but usually the pigs are not self-governed. Magnetic flux leakage technique is often combined with remote field eddy currents and/or ultrasonic techniques. Magnetic flux leakage is suitable for wall loss measurements and detection of sharp defects such as pitting and grooving. It is effective for aluminium finned carbon steel tubes as the field is unaffected by the presence of such fins.

112

Figure 6-43: Pipeline pig (Pipetronix)

Figure 6-44: Magnetic flux leakage probe for air coolers (above) together with ultrasonic probe, internal rotary inspection type (R/D Tech) 6.9.2

Underwater Application In opposite to liquid penetrant testing, the detection of diverted magnetic flux is possible underwater (salt or sweet) or similar surroundings. Of course all equipment has to be suitable for this usage (water tightness and pressure). Specially adapted wet suspensions exist.

Figure 6-45: Underwater magnetising and inspection equipment and daylight fluorescent indication (Asams, Circle) 6.9.3

Evaluation of Barkhausen Noise Ferromagnetic materials consist of small magnetic regions resembling individual bar magnets called domains. Each domain is magnetised along a certain crystallographic easy direction of magnetisation. Domains are separated from one another by boundaries known as Bloch walls (Felix Bloch, 1905-1983). AC magnetic fields will cause Bloch walls to move back and forth. In

113

order for a domain wall to move, the domain on one side of the wall has to increase in size while the one on the opposite side shrinks. The result is a change in the overall magnetisation of the sample. If an inductive sensor is placed near the sample while the domain wall moves, the resulting change in magnetisation will induce an electrical pulse. The first electrical observations of domain wall motion was made by Heinrich Georg Barkhausen (1881-1956) in 1919. He proved that the magnetisation process, which is characterised by the hysteresis curve, in fact is not continuous, but is made up of small, abrupt steps caused when the magnetic domains move under an applied magnetic field. When the electrical pulses produced by all domain movements are added together, a noise-like signal called Barkhausen noise is generated. Barkhausen noise has a power spectrum starting from the magnetising frequency and extending beyond 2 MHz in most materials. It is exponentially damped as a function of distance it has travelled inside the material. This is primarily due to the eddy current damping experienced by the propagating electromagnetic fields that Bloch wall movements create. The extent of damping determines the depth from which information can be obtained (measurement depth). The main factors affecting this depth are: ▪

Frequency range of the Barkhausen noise signal analysed and



Conductivity and permeability of the test material.

Measurement depths for practical applications vary between 0.01 and 1.5 mm. Two important material characteristics will affect the intensity of the Barkhausen noise signal. One is the presence and distribution of elastic stresses which will influence the way domains choose and lock into their easy direction of magnetisation. This phenomenon of elastic properties interacting with domain structure and magnetic properties of material is called a magnetoelastic interaction. As a result of magnetoelastic interaction, in materials with positive magnetic anisotropy (iron, most steels and cobalt), compressive stresses will decrease the intensity of Barkhausen noise while tensile stresses increase it this fact can be exploited so that by measuring the intensity of Barkhausen noise the amount of residual stress can be determined. For materials with negative magnetic anisotropy the effect is contrary. The measurement defines the direction of principal stresses as well. The other important material characteristic affecting Barkhausen noise is the micro-structure of the sample. This effect can be broadly described in terms of hardness: the noise intensity con tinuously decreases in microstructures characterised by increasing hardness. In this way, Barkhausen noise measurements provide information on the microstructural condition of the material. Many common surface treatments such as grinding, shot peening, carburising and induction hardening involve some modification of both stress and microstructure and can be readily detected using the method. Various dynamic processes such as creep and fatigue similarly involve changes in stress and microstructure and can be monitored with Barkhausen noise as well. Practical applications of the magnetoelastic Barkhausen noise method can be broadly divided into three categories:

114



Evaluation of residual stresses;



Evaluation of microstructural changes;



Testing of surface defects, processes and surface treatments that may involve changes in both stresses and microstructure.

Figure 6-46: Inspection equipment and Barkhausen noise scan of layers with flaws (Stresstech, IZFP)

6.10

High Resolution Magnetometry Some applications for the measurement of small magnetic fields are to be found in medical applications, NDT, physics, especially geophysics and computer hardware (hard drives and MRAM, magnetic random access memories). Most of such detectors make use of the magnetoresistance effect. The magnetic remanence method (MRM) is used to find very small ferromagnetic particles (like 150 µg iron) in non-ferromagnetic surroundings like such particles in the critical part of aircraft turbine blades. Low frequency eddy current (LFEC) is used for larger penetration than it is usual with normal eddy current instruments. Thermoelectric method with magnetic readout (TEM) to find segregations, zones with local overheating due to damaged machining tools or zones with a higher fatigue rate (before the effective crack initialisation) can be done with high resolution magnetometry. Determination of the magnetic permeability for austenitic steels (> 1) using a small permanent magnet and analysing the resulting field.

Figure 6-47: Device for the determination of the magnetic permeability (Stefan Mayer Instruments) Common for such techniques is that they are also influenced by other sources of local changes of the magnetic surroundings like earth magnetism or the noise signals given by electric conductors (50 Hz and 16 2/3 Hz from train lines). If compensation is not possible such experiments have to be realised in magnetically shielded rooms. Normal conducting devices can be used down to the pT range, for the fT range superconducting devices (SQUIDs) working either at 4 K or up to 77 K are necessary. 6.10.1

TFI, AMR and GMR Sensors Between 1990 and 1995, disc drives used a read/write technology called thin-film inductive (TFI). These heads were made of wire-wrapped magnetic cores. Voltage generated from the disc (a permanent magnet) moved past the wired core. TFI read heads reached their limit because when magnetic sensitivity was increased, their ability to write decreased.

115

By the mid-1990s, hard drive producers began introducing drives that used AMR heads. An AMR head uses a TFI head to write. However, the read element is composed of a thin strip of magnetic material. The essence of AMR heads is their resistance-transducing capability. A strong signal is generated when the stripe's resistance changes in the presence of a magnetic field. The disc drive senses and decodes the resistance changes caused by reversing magnetic polarities. This heightened sensitivity provides a higher signal output per unit of recording track width on the disc surface. The results: more information can be placed on each disc and fewer components are necessary. The effect is most usually seen in magnetic multi-layered structures, where two magnetic layers are closely separated by a thin spacer layer a few nm thick. It is analogous to a polarisation experiment, where aligned polarisers allow light to pass through, but crossed polarisers do not. The first magnetic layer allows electrons in only one spin state to pass through easily – if the second magnetic layer is aligned then that spin channel can easily pass through the structure, and the resistance is low. If the second magnetic layer is misaligned then neither spin channel can get through the structure easily and the electrical resistance is high. A GMR read film structure is composed of free, pinned and antiferromagnetic layers: ▪

Pinned layer – its magnetisation is fixed by the antiferromagnetic layer;



Antiferromagnetic layer; fixes the magnetisation direction behind the pinned layer;



Free layer – its magnetisation is free to rotate when it senses the magnetic field.

The operation principle is like the detection of polarised light. spin up

spin down

spin up

spin down

p

f

Figure 6-48: Example for a spin-valve design, various possibilities exist (Leeds University) The change in magnetisation angle (spin) of the free layer translates into a resistance change and voltage output - hence the name spin-valve. For reliable and stable device performance, these thin layers must be of high crystal quality and should have few physical and magnetic defects. Such devices are used as eddy current sensors as well. Magnetoresistive and GMR sensors can be applied in the range of 10 -10 to 10-2 T. Tumanski and Strabowski (Meas. Sc. Technol., 9 (1998) pp. 488-495) developed an automatic measuring system for mapping of the magnetic field measured by means of magnetoresistive sensor. Although this system was primarily designed for electrical steel testing, it proved to be a versatile for magnetic field imaging, e.g. for non-destructive testing of various materials or for analysing of magnetic field distribution. Because of some similarities with the thermovision method, the whole system was called magnetovision. The usual used sensors have a probe size of about one mm2, smaller areas are possible, but the sensitivity decreases. The usefulness of such a system has been demonstrated for measuring the Villari effect (details see ultrasonic testing), other applications are possible.

116

Figure 6-49: Magnetovision camera and distribution of the magnetic field of a nickel sample with a hole in a fatigue test (Wroclaw University) 6.10.2

Fluxgate Magnetometers For the time being fluxgate magnetometers are used for geophysical prospect ion, to find buried ferromagnetic parts or in aeronautic and space applications with high sensitivity at low frequency. A fluxgate consists from a magnetically soft core (ferritic glasses) that is magnetised with sinusoidal AC and a sensing coil. If no external field is present the measured signal shows only odd harmonics. If an external field is present also even harmonics are present that can be demodulated using a lock-in technique and analysed. Using three such devices, the resulting field direction can be analysed. If necessary the gradient of the field can be calculated using several elements one after the other.

core magnetising coil sensing coil

Figure 6-50: Schematic principle of a fluxgate (left), three-axis fluxgate magnetometer with a diameter of 25 mm and a length of 200 mm (Bartington) The usual sensors can be used between 20 to 200 Hz and show rather large dimensions; new developments allow a bandwidth up to 20 kHz with diameters of typically 6 mm and a height of 15 to 20 mm. Those devices can be interesting for certain eddy current applications, especially if they can be built small enough. Their range is from 10-11 to 10-2 T. 6.10.3

SQUIDs Superconductive quantum interference devices (SQUIDs) are the most sensitive detectors that exist. In a superconductive loop one or two very small breaks, so called Josephson junctions (Brian David Josephson, *1940, NP 1973), are introduced. The small breaks act as barriers the electrons have to tunnel across. A SQUID acts like a transistor and is able to amplify small changes in magnetic field into measurable electric current. A SQUID can even do this in the presence of a large external field (up to 7 T). The changes of the magnetic field may be measured within the superconducting loop itself, or, sometimes more practical, by the aid of an additional loop, coupled to the superconductive one.

117

If high temperature superconducting materials are used, the cooling may be done by liquid nitrogen. The SQUID itself often is cooled via a sapphire cold-finger and is not directly put into the cooling liquid.

Figure 6-51: Unshielded SQUID with sapphire cold-finger (left), Magnetocardiograph (Fino, Cardiomag Imaging) The main disadvantages of SQUIDs are their costs and their extreme sensitivity on vibration and radio frequency noise. They are used for medical applications, e.g. for magnetocardiography (MCG), complementing electrocardiography (ECG). Their range is from 10-14 to 10-6 T.

6.11

Summary and Conclusions Due to its nature of interaction, magnetic testing is restricted to ferromagnetic materials. The direction of the magnetic field must cut the anomaly. This can be surface-braking or sur face-near, the detection threw thin layers is possible. Magnetic particle testing is reliable, relatively easy to reproduce, fast and fairly common. Sizing is possible at the surface only; no information is acquainted about the depth of issues diverting the magnetic flux. Sharp changes in geometry, e.g. threads, edges or weld seams can also influence the behaviour of magnetic particles and lead to artefacts. Off-shore applications are possible. For components tested in benches the clamping positions cannot be tested during the same pass, often magnetic particle testing there is not possible at all. Improperly applied prod magnetising can lead to damages. The common way to detect the diverted magnetic flux are magnetic particles, where the use of dry powders is seldom in Europe (exception hight temperature applications). Techniques using sensors are used for specific applications; those are pipes, rods, ropes, floors, and others. Using such equipment, it is to a certain degree possible to size flaws within the material; a priori information, however, is necessary in most cases. Automated evaluation of the results is possible for systems giving an image of the magnetic particles and especially also for the ones using sensors. For small series, however, the arran ging and debugging of the sensor arrays is to elaborate. So even for automated test equipment the inspection is still often done manually, that is by the human eye. For materials characterisation magnetic imaging systems exist. For testing purposes, however, the resolution is limited, so the method, at least for the time being, has no real high-end applications. Due to several reasons a demagnetisation after the process is necessary. A full demagnetisation of geometrically complicated parts, however, can be very demanding.

118

7

EDDY CURRENT INSPECTION

7.1

Introduction Eddy current testing has its origins with Michael Faraday's (1791-1867) discovery of electromagnetic induction in 1831 and the experiments of Dominique Arago (1768-1853). 1851 Léon Foucault (1819-1868) discovered that the force required for the rotation of a cooper disc becomes greater when it is made with its rim between the poles of a magnet, the disc at the same time becoming heated by the eddy currents induced in the metal. In French eddy current has the name of its inventor “courant de Foucault”. However, it was not until World War II that these effects were put to practical use for testing materials. By 1950, Friedrich Förster (1908-1999) had developed a precise theory for many basic types of eddy current arrangements, including both absolute and differential or comparator test systems and probe or fork coil systems used with thin sheets and extended surfaces. Each test was confirmed by precise solutions of the Maxwell differential equations for the various boundary condi tions involved with coils and test objects, at least for symmetrical cases such as cylindrical bars, tubes and flat sheets where such mathematical integrations were feasible. The introduction of sophisticated, stable quantitative test equipment and of practical methods for analysis of quantitative test signals on the complex impedance plane were by far the most important factors contributing to the rapid development and acceptance of electromagnetic induction and eddy current tests during 1950 to 1965. In the early 1970s Hugo Libby promoted the use of Lissajous figures (Jules Lissajous, 18221880) for eddy current testing of tubes. This became the standard representation form for eddy current instruments. In thin materials such as tubing and sheet stock, eddy current can be used to measure the thickness of the material. This makes eddy current a useful tool for detecting corrosion damage and other damage that causes a thinning of the material. The technique is used to make corrosion thinning measurements on aircraft skins and in the walls of tubing used in assemblies such as heat exchangers. Eddy current testing is also used to measure the thickness of paints and other coatings. Eddy current inspection is used in a variety of industries to find defects and make measurements. One of the primary uses of eddy current testing is for defect detection when the nature of the defect is well understood. In general the technique is used to inspect a relatively small area and the probe design and test parameters must be established with a good understanding of the flaw that has to be detected. Since eddy currents tend to concentrate at the surface of a material, they can only be used to detect surface and near surface defects. Eddy currents are affected by the electric conductivity and magnetic permeability of materials as well. Therefore, eddy current measurements can be used to sort materials and to indicate if a component has seen high temperatures or been heat treated, as the resulting microstructure changes the conductivity of some materials.

7.2

Physical Background

7.2.1

Current Flow and Ohm Law The Ohm law (Georg Simon Ohm, 1789-1854) is one of the most important basic laws of electricity. It defines the relationship between the three fundamental electrical quantities voltage U, current I and resistance R. The reciprocal value of the resistance is the conductance G (unit Siemens 1 S = 1 Ω-1). U=R I

or

I=GU

(7.1)

In this form, the values of the resistance and the conductance refer to the overall system. The resistivity ρ and the electric conductivity σ are related according to R=

ρℓ A

and

G=

σA ℓ

(7.2)

with ℓ the length and A the cross section of the conductor.

119

Table 7-1: Electric and magnetic properties of some materials, TT: transition temperature, index C: Curie, index N: Néel Material Aluminium

ρ [µΩ m]

σ [MS m-1]

µr [-]

0.03 - 0.07

14 - 35

1

Cementite (Fe3C)

TT [°C]

para 215

Chromium

Type (RT) ferri

C

0.03

37

1

35

Cobalt

0.9

10

240

1120

Carbon fibre / Graphite

50

0.02

1

dia

Copper (IACS)

0.02

58

1

dia

Lead

0.21

4.8

1

1.9

0.5

1

Manganese Manganese-zinc ferrite MnxZn(1-x)O ∙ Fe2O3

0.2 - 5 · 106

Monel 400 NiCu30Fe

0.2 - 5 · 10-6 600 - 10'000

(antiferro)

N

ferro

C

dia - 173

antiferro

N

120 - 200

0.55

1.8

Mumetal Ni81Fe14Mo5

0.6

1.7

100'000 > 250'000

400

C

ferro

Nickel (pure)

0.08

13

2400

355

C

ferro

Silver

0.02

63

1

Steel (ferritic)

0.11

8-9

200 - 2'000

769

C

Steel (austenitic)

0.7 - 0.9

1.1 - 1.4

1 - 1.3

para

Titanium

0.8 - 1.7

0.6 - 1.3

1

para

0.11

8.7

1

para

Tin

20 – 50

ferri

C

(ferro)

C

dia ferro

The Néel temperature (Louis Néel, 1904-2000) is the analogue to the Curie temperature for antiferromagnetic materials. Some sources give the electric conductivity compared to the International Annealed Copper Standard (IACS), 100% IACS corresponds to 58 MS m -1. 7.2.2

Magnetic Behaviour The possibilities of eddy current testing depend among others on the permeability. Diamagnetic materials are repelled by a magnetic field, the relative permeability is slightly below one and the magnetic susceptibility is below zero, respectively. Their magnetic behaviour is not dependent on the temperature. Bismuth shows a very strong diamagnetic behaviour. Paramagnetic materials are slightly attracted by a magnetic field, the relative permeability is above one and the magnetic susceptibility is above zero, respectively. Their magnetic behaviour depends on the temperature, higher temperatures lead to reduced permeability. Ferromagnetic and antiferromagnetic materials show such behaviour only above a certain transition temperature; ferromagnetic materials become paramagnetic above the Curie temperature, antiferromagnetic materials become paramagnetic above the Néel temperature. The relative permeability of ferromagnetic materials is much higher than 1. Ferrimagnetic materials have no conduction electrons and therefore they cannot show any loss caused by eddy currents. Ferrites are used for the construction of eddy current probes.

7.2.3

Induction and Inductance The process of generating electrical current in a conductor by placing the conductor in a chan ging magnetic field is called electromagnetic induction or just induction. The Faraday law for a wire loop is UL=

120

dΦ m dt

(7.3)

where UL is the induced voltage and dΦ m/dt the rate of change in magnetic flux. Induction is measured in unit of Henry [H] (Joseph Henry, 1797-1978) which reflects this dependence on the rate of change of the magnetic field. One Henry is the amount of inductance that is required to generate one volt of induced voltage when the current is changing at the rate of one ampere per second. Note that current is used in the definition rather than magnetic field. This is because current can be used to generate the magnetic field and is easier to measure and control than magnetic flux. When induction occurs in an electrical circuit and affects the flow of electricity it is called inductance, L. An electrical transformer uses inductance to change the voltage of electricity into a more useful level. In non-destructive testing, inductance is used to generate eddy currents in the test piece. It should be noted that since it is the changing magnetic field that is responsible for inductance, it is only present in alternating current (AC) circuits and that high frequency AC will result in greater inductive reactance since the magnetic field is changing more rapidly. 7.2.4

Self-Inductance and Inductive Reactance The property of self-inductance is ance is defined as the induction of wire itself is changing. In the case current in the circuit itself induces induced.

a particular form of electromagnetic induction. Self-inducta voltage in a current-carrying wire when the current in the of self-inductance, the magnetic field created by a changing a voltage in the same circuit. Therefore, the voltage is self-

The term inductor is used to describe a circuit element possessing the property of inductance; a coil of wire is a very common inductor. In circuit diagrams, a coil or wire is usually used to indicate an inductive component. Increasing the number of turns in a coil n or the rate of change of magnetic flux increases the amount of induced voltage. Therefore, the Faraday law must be modified for a coil of wire and becomes the following: UC=n

dΦm

(7.4)

dt

In a circuit, it is much easier to measure current than it is to measure magnetic flux. For that the inductance L is used, it is for a coil with the cross section A C, the length ℓC and a core with the permeability µr L=

µcore n2 A C

(7.5)

ℓC

The induced voltage now becomes UC=L

dI dt

(7.6)

Soon after Faraday proposed his law of induction, Heinrich Lenz (1804-1865) developed a rule for determining the direction of the induced current in a loop. Basically, Lenz law states that an induced current has a direction such that its magnetic field opposes the change in magnetic field that induced the current. Some sources therefore include a negative sign in the above formula to indicate the polarity of the induced current. The reduction of current flow in a circuit due to induction is called inductive reactance X L.

XL =ω L

with

ω=2π f

(7.7)

Since inductive reactance reduces the flow of current in a circuit, it appears as an energy loss just like resistance. However, it is possible to distinguish between resistance and inductive reactance in a circuit by looking at the timing between the sine waves of the voltage and cur rent. In an AC circuit that contains only resistive components, the voltage and the current will be in-phase. If inductive reactance is present in the circuit, the phase of the current will be shifted by π/2. 7.2.5

Mutual Inductance When the current in an inductor changes, the varying flux can across any other inductor nearby, producing induced voltage in both inductors. In eddy current testing, AC is supplied to the exciting coil. This AC generates an alternating magnetic field which, when placed near a metal or another electric conductive material, causes current to flow in the metal conductor by mutual inductance. The current in the conductor will generate a secondary magnetic field, which induces a current in the receiving coil. This mutual inductance causes a change in the impedance of the coil. The impedance signals sensed by the receiving coil are the measurements of the test specimen. Hence, the eddy current technique uses the effect of electromagnetic fields and induction to characterise physical properties of the conductive material. 121

Primary magnetic field Induced magnetic field Primary coil Measuring coil

Induced (eddy) current Figure 7-1: Principle of eddy current testing Everything that influences the induced current in the material will lead to a changed impedance signal and therefore to a change of the measured signal. Such influences can be due to mater ial changes, geometrical changes, flaws or inclusions, etc. Flaws like closed delaminations that have only a minor influence on the induced current will result in only small changes of the sig nal and therefore they are hardly or not at all detectable. The fraction of total flux from one coil linking another coil is the coefficient of coupling. This coefficient increases by placing the receiving coil close to the conductive specimen. When surface probes are used, the spacing between the probe and conductor is called lift-off, when encircling probes are used, then the coupling is called coil fill factor. 7.2.6

Impedance Electrical impedance Z, is the opposition that a circuit presents to alternating current. Impedance is measured in ohms and may include resistance R, inductive reactance XL and capacitive reactance XC. The capacitive reactance can be dropped as most of the eddy current probes have little or no capacitive reactance. The impedance represents the length of the relevant vector in the impedance plane with the axes R and XL



2

2

(7.8)

Z= R X L

with the phase angle −1

θ=tan 

XL R



(7.9)

For a coil with no mutual inductance, the resistance is typically very small in comparison to its reactance and may be neglected and so the impedance becomes

Zo =ωL o

(7.10)

Often the impedance is normalised to this value and becomes, written in its complex form Z R−R o ωL = i Zo ω Lo ω Lo

(7.11)

For the interpretation of the results, only the resistive effect caused by the test object is of importance. R – Ro is also called the effective resistance. The values for R and L are depending on the material that generates the mutual inductance. So beneath the frequency, which usually may be changed, the permeability and electric conductiv ity of the material influences the measured result. 7.2.7

Depth of Penetration and Current Density Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the winding of the coil and flow is limited to the area of the inducing magnetic field. Eddy currents concentrate near the surface adjacent to an excitation coil and their strength decreases with distance from the coil. Eddy current density decreases exponentially with depth. This phenomenon is known as the skin effect. The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electric conductivity and magnetic permeability of the specimen. The penetration depth decreases with increasing frequency, increasing conductivity or increasing magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about

122

37% of the surface density, is called the standard depth of penetration, δ. The expression “standard” denotes plane wave electromagnetic field excitation within the test sample; conditions that are rarely achieved in practice. Although eddy currents penetrate deeper than one standard depth of penetration they decrease rapidly with depth. At two standard depths of penetration, eddy current density has decreased to 13.5% of the surface density, at three depths to 5% of the surface density. δ=

1

(7.12)

π σ µ f

Figure 7-2: Eddy current depth of penetration (NDT RC)

Standard penetration depth [mm]

1000 Graphite

100

Steel (austenitic)

10 1 0.1

Steel (ferritic) Copper

0.01 0.001 0 0.1

Aluminium

1

10

100

1000

10000

Frequency [kHz] Figure 7-3: Standard penetration depths for some materials 7.2.8

Phase Lag Phase lag is a parameter of the eddy current signal that makes it possible to obtain information about the depth of a defect within a material. Phase lag is the shift in time between the eddy current response from a disruption on the surface and a disruption at some distance below the surface. The generation of eddy currents can be thought of as a diffusion process meaning that the eddy currents below the surface take a little longer to form than those at the surface. Therefore, subsurface defects will be detected by the eddy current instrument a little later in time than surface defects. Both, the signal voltage and current will have this phase shift or 123

phase lag with depth, which is different from the phase angle θ, the phase angle, the current is shifted with respect to the voltage. Phase lag is an important parameter in eddy current testing because it makes it possible to estimate the depth of a defect and with proper reference specimens, determine the rough size of a defect. The signal generated by a flaw depends on both amplitude and phase of the eddy currents being disrupted. A small surface defect and large internal defect can have a similar effect on the magnitude of test coil impedance. However, because of the increasing phase lag with depth, there will be a characteristic difference in the test coil impedance vector. At one standard depth of penetration, the phase lag is one rad (57°). This means that the eddy currents flowing at one standard depth of penetration below the surface, lag the surface currents by 57°. At two standard depths of penetration they lag the surface currents by two rads. Therefore, by measuring the phase lag of a signal, the depth of a defect can be estimated.

7.3

Testing Arrangements

7.3.1

Characteristic Frequency and Effective Permeability To calculate the final situation, the appropriate Maxwell differential equations have to be solved. To make the results generally valid, Förster introduced a characteristic frequency f c, it is defined by the argument of the Bessel-function becoming zero. The determination of the characteristic frequency depends on the test arrangement and given for simple cases only. The frequency is normalised to this value in the form of f/fc. The concept of effective permeability permits considerable simplification of analysis of eddy current tests. However, this simplification results from use of several assumptions which fit the ideal mathematical analysis, but which are not generally true for many practical eddy current test systems.

7.3.2

Complex Impedance Plane and Operating Point In practice, the operator will know the used frequency and the screen of his instrument will show a part of the complex impedance plane defined by the resistance and the reactance. The instrument has to be zeroed and the operating point is initially set to the centre of the screen. To perform the inspection the knowledge of the correlation of the complex impedance plane and the operating point is not necessary, however, to understand the correlation, it is.

Normalised reactance

1.0

0.8

0.6

0.4

0.2

0.0 0.0

0.1

0.2

0.3

0.4

Normalised resistance Figure 7-4: Two possible positions of the screen within the complex impedance plane; the two curves indicate two different geometries, the operating point moves along one of those lines with changing frequency and changing material and jumps to another if material is missing In a first step the orientation of the screen within the complex impedance plane has to be set using the phase shifter of the instrument. This is usually done lifting of the probe from the sur -

124

1.0

1.0

0.8

0.8

Normalised reactance

Normalised reactance

face or with the aid of artificial dents for tubes. For the lift off effect the operating point moving to the left is the usual convention.

0.6

0.4

0.2

0.0 0.0

0.1

0.2

0.3

Normalised resistance

0.4

0.6

0.4

0.2

0.0 0.0

0.1

0.2

0.3

0.4

Normalised resistance

Figure 7-5: Orienting and scaling of the screen In a next step the scale of the screen is adjusted using samples with defined characteristics like grooves or holes. If the frequency is now changed, the operating point will follow the respective line on the screen. The advantage of such a procedure is that a change of the position of the operating point can give information on the nature of its origin – change of the geometry or foreign material involved. 7.3.3

Encircling Probes Wires, rods or bars can be tested by using encircling probes. Usually the test equipment stays stationary and the components under test are moving.

Figure 7-6: Fine (left) and hot wire testing with an encircling probe (Prüftechnik) The values of the components of the effective permeability for solid non ferromagnetic bars are given in tables. With d as diameter the characteristic frequency is given by f c=

2 π σ µ d2

(7.13)

The Bessel-function to be solved to get the effective permeability μ eff in this case would be

125

 

µeff =−

2 fc f

J1 

if  fc

if Jo   fc

(7.14)

1−i

In reality, the bar with diameter d will not fill the coil with a diameter D completely; the coil fill factor η then is given by η=

d2 D2

(7.15)

1.0

1.0 Normalised reactance

0.8

ℜ (μeff)

f/fc

f/fc

0.6 0.4 0.2 0.0 0.0

0.1

0.2

0.3

0.4

ℑ (μeff)

0.8 0.6

η = 1/3

η = 2/3

0.4 0.2 0.0 0.0

η=1

0.1

0.2

0.3

0.4

Normalised resistance

Figure 7-7: Complex plane of effective permeability for an encircling probe, position (0,1): f/f c = 0, position (0,0) f/fc = ∞ (left) and complex impedance plane for different probe fill factors (non-ferromagnetic material) The complex effective permeability, finally, can be used to calculate the resistance (real axis) and the reactance (imaginary axis) of the complex impedance plane. Uimag Uo Ureal Uo

=

=

ωL =1−ηη µr ℜ µeff  ω Lo

R−R o ω Lo

=η µr ℑµeff 

(7.16) (7.17)

In the case of ferromagnetic material, the influence of the relative permeability is dominant.

126

Normalised reactance

100

f/fc μr = 200

80 60

μr = 100

40 μr = 50

20 0 0

10

20

30

40

Normalised resistance Figure 7-8: Complex impedance plane for different ferromagnetic materials in an encircling probe with a probe fill factor of 0.5 7.3.4

Tube Testing with Encircling Probes and Internal Coaxial Probes Tubes can be tested by using encircling probes or internal coaxial probes. Easy solutions exist only for thin-walled tubes. In this case the effective permeability can be calculated from ℜ µeff =

1 1

ℑ µeff =

f 2  fc

f fc 1

f  fc

2

(7.18)

(7.19)

Figure 7-9: Testing of boiler and heat exchanger tubes with an internal coaxial probe (R/D Tech)

127

1.0

1.0

f/fc Normalised reactactance

f/fc

ℜ (μeff)

0.8 0.6 0.4 0.2

0.8 η = 1/3

0.6 0.4

η = 2/3

0.2 η=1

0.0

0.0 0

0.1

0.2

0.3

ℑ (μeff)

0.4

0.5

0

0.1

0.2

0.3

0.4

0.5

Normalised resistance

Figure 7-10: Complex plane of effective permeability for a thin-walled tube, position (0,1): f/f c = 0, position (0,0): f/fc = ∞ (left) and complex impedance plane for different probe fill factors (non-ferromagnetic material) With the wall thickness w and the inner diameter di the characteristic frequency is given by f c=

2 π σ µ w di

(7.20)

In this case the maximum value for the imaginary part of the effective permeability and therefore also for the normalised resistance is at f = fc. Again the probe fill factor has to be taken into account with do the outer and di the inner diameter for encircling probes η=

d 2o

(7.21)

D2

and for internal coaxial probes η=

D2 d 2i

(7.22)

The diagrams for different probe fill factors are looking similar to above. For an increasing wall thickness and encircling probes, the situation becomes more and more similar to the situation of bar testing. The relationship between probe fill factor, effective per meability and the axes in the impedance plane are the same as above.

Figure 7-11: Internal coaxial probes (Zetec) 7.3.5

Probe and Through-Transmission Testing of Sheets and Foils Sheets and foils can be tested using one single probe (reflection mode) or an exciting and a receiving probe with the component in between (transmission mode). The characteristic fre quency with the thickness of the flat conductor D and the distance of the two probes A is here

128

f c=

1 πσµD A

(7.23) E

A

D

s

R Figure 7-12: Arrangement for transmission (left) and probe coil arrangement Similar to the above mentioned arrangements an efficiency factor for the probe-coil arrangement exists. Furthermore for a probe arrangement (single or dual probe) the term A (distance between the two coils) makes no sense and has to be replaced by A eff. These two important factors can be determined experimentally for any existing probe coil arrangement. Finally the probe lift-off s has to be taken into consideration and the characteristic frequency is given by f c=

1 π σ µ D A eff s

(7.24)

Figure 7-13: Inspection of a brake disc and a weld with a probe coil arrangement (BAM, PND) The design as well as the structure of the probe itself (shape and configuration) can be optim ised for a specific application.

Figure 7-14: Set of different probes (Hocking, Rohmann)

129

Figure 7-15: Different probe structures (Olympus) 7.3.6

Rotating Probes Externally rotating probes are used for rods or bolts on continuous basis. The advantage compared to coil probes is the smaller probe volume and therefore the higher sensitivity. The dis advantage is the mechanics with moving parts, combined with the necessity to carry the signals from this rotating part to a fixed one without producing too much noise. As soon as the shape is no longer circular, the signal and the sensitivity depend on the angular position of the probe.

Figure 7-16: External rotating probes at hexagonal bolt (Rohmann) Another rotating probe for use with the reflection methods detects inner surface discontinuities effectively. The arrangement is e.g. used for inspection of reactor U-shaped heat exchanger tubes for corrosion and cracks. Such probes are common for the inspection of bore holes of all types. In combination with a probe pusher puller unit the signal can be displayed as a waterfall display where one horizontal signal line corresponds to one turn and the vertical deflection to the position of the probe. The usual rotation speed corresponds to the frequency of the power supply.

130

Figure 7-17: Rotating probes (Förster, Rohmann) The probe direction, finally, can be arranged parallel to the rotation axis, thus to increase the area of simultaneous testing.

7.4

Equipment

7.4.1

Examination System The instrumentation employs an eddy current instrument, one or several probes and interconnecting cable or cables. This combination, or together with any mechanised equipment and peripheral units for data storage, etc. form an examination system. Factors to be considered include:

7.4.2



Type and metallurgical condition of the material;



Shape, dimensions and surface conditions of the component;



Purpose of measurement e.g. detection of cracks or evaluation of thickness, etc.;



Type, position and orientation of the discontinuities to be revealed;



Environment conditions in which the measurement is performed.

Eddy Current Instruments The choice of the eddy current instrument depends on the purpose of examination. Of particular importance are the adjustable parameters of the instrument, the range of such parameters and the form of the signal display. In general the user shall be able to vary the value of frequency, gain, balance (unless automatic balance), phase, filters and zero of the display. An instrument is of specific application when the relationship of the measured quantity and the display is explicitly defined in the range of application. In this case the probe is specific to this instrument and only some of the above stated parameters may be varied.

Figure 7-18: Eddy current instruments, portable single hand held (left), standard portable type, version typically used for automated systems for integration into racks (Rohmann, Hocking, Förster) The general characteristics are power supply (battery, AC power supply), safety and environmental effects (including warm up time), physical presentation (portable, cased or rack mounted), number and types of plugs and sockets. The technology can be described as:

131



Digitalisation (wholly analogue, part analogue part digital);



Excitation (single frequency, multi frequency, sweep frequency, pulse);



Channels (single, multi channel);



Settings (manual, remote controlled, stored, pre-set);



Display and output (if any).

Electric characteristics concern the generator unit (source of excitation), input stage character istics, balance, high frequency signal processing, demodulated signal processing, signal display and digitisation. Balance

Generator

Probe

HF signal & demodulation

Input

X

LF signal

Y Display Accessories

Digitisation Figure 7-19: Functional block diagram of an eddy current instrument For limited applications as measurement of coating thickness or conductivity various instruments exist, usually they have a numeric output and the set-up possibilities, if any, are very limited.

Figure 7-20: Eddy current instruments for measurement of coating thickness and conductivity (Automation, Förster) 7.4.3

High Frequency (HF) Signal Processing HF filters reduce the signal frequency content which can have an undesirable effect on the result. The filters used before demodulation are referred to as carrier frequency filters (HF filters). These are usually band path filters which suppress any signal frequencies which do not correspond to the excitation frequency. Synchronous demodulation extracts the vector component from the HF signal. The polarity of the demodulation shall be positive which means that a delay of the signal will cause the signal vector to rotate clockwise. The use of synchronous demodulation to extract the reactive component is called quadrature demodulation, while the one for the extraction of the active (resistive) component is called in phase demodulation. For one single probe position the demodulation will deliver one vector, if the probe is moved over a defect, that vector will continuously change its characteristics. Amplitude demodulation extracts the low frequency amplitude from the HF signal.

132

7.4.4

Demodulated Signal Processing Vector amplification generally consists of two transmission channels of identical design. These channels amplify the vector components generated by synchronous demodulation. In some instruments, the components can be amplified with different gains. The filters used after demodulation are referred to as low frequency filters (LF filters). The bandwidth and cut-off frequency is chosen to suit the application, e.g. wobble, surface speed, etc. Usual filters are low-pass, high-pass and band pass filters. Phase setting permits rotation of the demodulated signal vector on the complex plane display.

7.4.5

Display Several types of presentation exist. The complex plane display have as axes the resistive and the reactive component of the demodulated signal. With the elliptical display method the evaluation is based on interpretation of the Lissajous patterns obtained by applying a signal repres entative of the excitation current to one and the signal from the probe to the other axis. The time synchronous display uses the time as one and one part of the demodulated signal as the other axis. Further methods are frequency spectrum and the several possibilities of imaging.

7.4.6

Digitisation Digitisation may be performed either before or after demodulation using an internal clock or an external encoder. The A/D resolution is the nominal value of the converter input voltage corresponding to one digitisation bit. The number of digitisation bits is an equally useful information even so it can be directly accessed through the maximum input voltage and the resolution. The sampling rate is the frequency at which the A/D conversion is made.

7.4.7

Probes The choice of probe depends on the purpose of examination, details see separate chapter on receiving media.

7.4.8

Mechanised Units The units used for mechanised scanning may contain one single axis, e.g. for the testing of bars or tubes with coils, up to a multi axial robotic manipulator for testing of complex parts. Often the same or similar equipment can be used for other methods as magnetic flux leakage (MFL) or ultrasonics.

Figure 7-21: Examples for mechanised equipment (Empa, FAA) 7.4.9

Eddy Current Imaging As eddy current inspection for various applications is a scanning method, the results can be dis played as images using the image axes as geometrical axes and coding the result (amplitude, phase, vector length, etc.) by grey scales or colours.

133

Figure 7-22: Carbon fibres reinforced ceramic; visible are the two top layers (left) and artificial delaminations under a metal coating (Empa, BAM) If imaging is done with differential probes, one side of a feature will be shown in one (e.g. the positive) direction, the other side in the opposite one.

Figure 7-23: Patch, stringers, and resin concentration in a CFC panel, left side the uncorrected image (positive and negative directions of the signals), right side the corrected version (Magdeburg University) 7.4.10

Reference Test Pieces The application of eddy current examination requires the use of reference test pieces. They contain known features which can be used to set up the examination system, to make func tional checks, to ensure the capability of the examination and to provide calibration curves. Normally the reference test piece shall be of the same material and the same finished state as the product to be tested. The features can take the form of: ▪

Holes or notches with specified dimensions;



Natural or induced defects of known characteristics;



A range of known coating thickness;



A range of known material properties.

The measurable characteristics of the features and the reference test pieces shall not change significantly with time. 7.4.11

Instrument and Probe Setting Instrument settings are derived from knowledge of the purpose of the examination and the product to be tested. Some settings e.g. filtering, phase and sensitivity can be derived from the use of reference test pieces. The fixing and guiding of the probe influence the effectiveness of the examination. Changes in the probe clearance influence the sensitivity. A signal due to the lift-off, or in case of tubes due

134

to a buckle, can be used to dynamically control the sensitivity. Where the examination is mechanised, the speed of the probe over the surface being examined and the scanning path (overlap) shall be maintained throughout the examination. 7.4.12

Display and Output The usual display corresponds to the complex impedance plane. The orientation of the phase signal, the 0° direction and the orientation of the rotation system are usually due to conventions. Such is e.g. the lift-off signal goes to left (0° direction) and the orientation is clockwise. Usual instruments allow a different gain in x- and y-direction, clearly given that the above dis cussed convention is clear. They have usually only one alarm level that can be set, but the zones where alarm is given can be chosen very differently. Dependent on the task the alarm level is set to the registration level or to the acceptance level. Advanced settings make only sense for automated inspection.

Figure 7-24: Examples of possible settings of the alarm regions (red): square, box, circle, sectorial The output can be in form of a protocol (with or without screen dump of the signal image) or with failure markings directly onto the sample.

7.5

Receiving Media

7.5.1

Coils The most common way is the use of an induction coil, usually placed in the centre of the excit ing coil. It is, however, also possible to use one single coil for exciting and receiving. Absolute probes alone do not compare with the immediate surroundings or with a reference sample, especially they do not compensate for lift-off effects. They may have an internal or external possibility to compensate for temperature differences or other sources of noise. Their main application is the measurement of thickness (wall or layer), permeability and conductivity. Within a comparative arrangement they can perform comparative measurements with an external reference. This arrangement is often used for sorting (geometry, heat treatment), comparing with a sound reference sample. The dimension of the probes must be adequate to the task. Differential probes make a direct comparison on the object itself, using two or more coils. Due to their arrangement, they can measure the difference in given directions only. When moved over an indication, their signal will be seen as two symmetrical parts. They compensate for a lot of noise and for lift-off, but they are not able to measure the wall thickness and, therefore, also not slight changes in it. Dependent on the probe assembly, it is possible to measure the absolute signal of each part separately and to make the difference electronically. Double differential and multi differential probes also exist. These techniques allow a high-pass filtering independently of the speed of the probes relative to the component.

Figure 7-25: Possible arrangement of a differential probe (left, seen from components side) and arrangement of a double differential arrangement (example for encircling probes)

135

(A)

(B)



V

(C)

(D)



V

Figure 7-26: Probe arrangements: (A) absolute, (B) separated coils for exciting and receiving used for reflection technique (with phase measurement), (C) differential; both coils over component or one over reference sample, (D) impedance bridge (with phase measurement) For a lot of applications it makes sense, if the user can produce the probes and the coils him self. This is especially true, if a lot of different geometries have to be tested.

Figure 7-27: Coil winding device with counter for the coil turns, ferrite core, copper wire and binocular (Alstom) 7.5.2

T-Probes For T-probes the exciting and the receiving coil are oriented with perpendicular axes. This results in a much less sensitivity on lift-off and input or end effects for tube testing. Such probes are used for testing of welds and tubes. For weld inspection such probes are oriented with both coil axes parallel to the surface. It is possible to find cracks with a length of 5 mm and a depth of 1 mm. The crack must not be filled with electric conductive material. It can also be found under relatively thick non-conductive or thin conductive but not ferromagnetic coatings. Furthermore it is possible to inspect welds between different materials.

136

Figure 7-28: Example of the mechanism of a T-probe in differential mode (www.ndt.net/article/wcndt00/papers/idn037/idn037.htm)

crack or debonding

scan direction

absolute

differential

Figure 7-29: Possible arrangements of T-probes for weld inspection 7.5.3

GMR Probes GMR probes (details see diverted magnetic flux testing) are suited as sensors for eddy current signals. As mass products they are relatively cheap and can be used as arrays with small pitch (200 µm) and therefore good spatial resolution.

Figure 7-30: GMR crack detecting equipment and ability demonstration of GMR sensors with a surface scan of a US one cent coin (NASA, Sensors) 7.5.4

Probe Arrays The roots of probe arrays go back to the mid-1980s when the former BBC (Brown Bovery Ltd., Baden) patented an early version of this technique. At this time the idea did not succeed due to too weak computing power.

137

Figure 7-31: “Probe mat” about 1985 with 162 sensors (BBC, now Alstom) The idea has some very important advantages in comparison to conventional systems: ▪

No or less moving parts (in comparison e.g. with surface probes);



Faster and more reproducible scanning, especially for complex shapes;



Better sensitivity due to small examination volume (e.g. in comparison with encircling or coaxial internal probes);



Excellent adaptability to many geometrical shapes (not all, but most of) with fixed or flexible arrangement;



Miscellaneous possibilities of combinations (absolute or differential in several directions, etc.).

Often the coils are arranged in two of more rows and are alternately used as exciters and receivers. The distance between two pairs must be large enough to prevent magnetic coupling. Such a technique allows furthermore multiplexing. t=1 E R A1

E

A2

R B1

B3

C1

C3

t=4 E R E R Figure 7-32: Schema of array probes; possible cycle left with dark as exciting (E) and grey as receiving (R); possible arranged with three rows right, B1 to B3 and C1 to C3 giving two absolute signals in horizontal direction, A1 to C1 and A2 to C1 two absolute signals in vertical direction, differences can be calculated from both pairs In order to get a uniform signal output (amplitude and phase) of all the channels, they usually must be normalised using a sample with an adequate calibration defect like a notch.

138

Figure 7-33: Array probes for bars, rods or tubes (left) and plane objects (R/D Tech)

Figure 7-34: Array probes adapted for complex shapes (R/D Tech) 7.5.5

Flexible Probe Arrays Arrays can also be flexible to be adapted to the surface. If necessary a suitable counter part has to be used.

Figure 7-35: Flexible array for weld inspection and corresponding C scan (Zetec)

139

Figure 7-36: Flexible array and customisation for complex geometries (Olympus)

7.6

Aspects of Quality Assurance

7.6.1

Typical Sources of Errors in Measurement There is a variety of typical sources of errors in measurement common for most eddy current techniques. Some of them can be overcome using accessories for adequate probe guidance (templates, etc.). (C) (A)

(F)

(D)

(B)

(G)

(H)

(E)

(I) (J)

Figure 7-37: Possible sources of errors in measurement: (A) lift-off, (B) tilt effect, (C) influence of convex or concave surface, (D) edge effect, (E) geometric effect, (F) input effect, (G) end effect, (H) wobble, (I) coating too thick, (J) base material not thick enough and in addition the material effect if the electromagnetic properties of the product are not constant 7.6.2

Verification of Eddy Current Equipment For a consistent and effective eddy current examination it is necessary to verify that the performance of the component parts of the eddy current test system is maintained within acceptance limits. The physical conditions of the reference blocks and reference test pieces shall be verified to be within acceptable limits before being used to verify the systems or probes. The measuring system used for verification shall be in a known state of calibration. There are three levels of verification. Each of it defines the time intervals between verification and the complexity of the verification. They are: the global functional check, the detailed func tional check and the characterisation. When the performance of the global functional check is not within the specified limits, then a decision shall be made concerning the product examined since the previous successful verification. That can be: ▪

If the present registration and acceptance levels were lower than specified: repeat the inspection for all parts with registered indications (to reduce falls calls);



If the present registration and acceptance levels were higher than specified, repeat all inspections;



Tolerances and exceptions have to be defined.

Therefore the global check must be easily to perform at high frequency and with little effort.

140

Table 7-2: Verification levels Level Global function check

Object

Stability of sysFrequently tem performance (hourly, daily)

Stability of selecDetailed function- ted characteristal check and cal- ics of the instruibration ment, probes and accessories

Characterisation

7.6.3

Typical time period

Less frequently but at least annually and after repair

Instruments

Responsible entity

Reference test pieces

User

Calibrated measuring instruments, reference test pieces

User

Calibrated laborAll characteristics Once (on release) atory measuring of the instruand when reinstruments and ment, probes and quired reference test accessories pieces

Manufacturer, user

Reference Blocks Reference blocks are used for the characterisation and comparison of probes. The user has to fabricate its own sets, in order to satisfy the specific needs, especially the geometrical dimen sions. Standard materials are low carbon steel (ferromagnetic), aluminium (high conductivity) and austenitic steel (low conductivity). The details are given in EN ISO 15548-2. For surface probes the following characteristics can be determined: ▪

Angular sensitivity (block A1);



Probe position mark (block A1), this mark indicates the position of a flaw relative to the probe housing;



Edge effect (block A1);



Response to a hole (block A2);



Response to a slot (block A1);



Length of coverage (block A1), this is derived from a map of the probe response to the slot by taking the maximum width (distance perpendicular to the slot), usually a -6 dB value for the envelope is taken;



Width of coverage (block A1), this is derived the same way, the measured distance is parallel to the slot Wenv, with Ls as length of the slot the width of coverage is defined as Wcov=Wenv −L s

(7.25)



Minimum slot length for constant probe response (block A5);



Minimum depth of surface breaking slot for constant probe response (block A4);



Lift-off effect (block A1);



Effect of probe clearance on slot response (block A1), the probe has to be balanced before each scan with a different clearance;



Effective depth of penetration (blocks A3);



Effective depth of detection of sub-surface slot (blocks A1 and A3).

To compare signals from different probes the reference block A6 is used. This block looks sim ilar to A1 but it is standardised in dimensions and material, high conductivity, low conductivity and non-ferromagnetic, low conductivity and ferromagnetic. The block shall nevertheless be of the same class as those used for A1 to A5. The transfer signal is the maximum value of the sig nal during a scan perpendicular to the slot. A ferrite can be used to get a reproducible phase transfer signal. To obtain a signal it is neces sary to have a relative movement probe / ferrite in the preferred orientation of the probe.

141

A1

A3

A2

A4

A5

Figure 7-38: Reference blocks for surface probes For coaxial probes the following characteristics can be determined: ▪

Probe position mark (block B1 and C1, respectively);



End effect (block B1 and C1, respectively);



Axial symmetry (block B2 and C2, respectively);



Response to a hole (block B1 and C1, respectively);



Minimum slot length for constant probe response (block B3 and C3, respectively);



Eccentricity effect (block B4 and C4, respectively);



Fill effect (block B5 and C5, respectively);



Effective depth of penetration (block B6 and C6, respectively);



Effective depth of detection under ligament (block B7). B1

C1

B7

B2

C2

360°

Figure 7-39: Reference blocks for coaxial probes, in addition: B3 and C3: tube or rod with longitudinal groove, B4 like B2 with increased inner diameter, C4 like C2 with decreased diameter, B5 like B1, set with increasing inner diameters and constant wall thickness, C5 like C1, set with decreasing diameters, B6 like B1, set with constant inner diameter and increasing outer diameters, C6 like C1, set with central bores with increasing diameters For the conductivity measurement special calibration blocks traceable to national standards are available. Their values are usually given for a frequency of 60 kHz at room temperature.

142

Figure 7-40: Calibration block for conductivity measurement and calibration plates for sheet thickness gauging (GE Inspection Technologies, ABB)

7.7

Procedure and Record

7.7.1

Procedure The NDT procedure usually has to define the following special technical aspects:

7.7.2



Scanning plan;



Inspection and characterisation technique and class;



Special surface preparation;



Direction of scanning, size and type of the probe, surface speed, width of coverage of the probe;



Reference test pieces and verification intervals for the instrument and the probes;



Type and arrangement of the examination system;



Requirements for signal processing and signal evaluation;



Special requirements on application (measurement in magnetic saturation or at elevated temperature, demagnetisation, etc.).

Record The record should contain the following information: ▪

Description of the equipment used, including serial numbers of instrument and probes, etc.;



Identification of the reference test pieces used;



Instrument settings (frequency, phase, filters, etc.);



A technical sheet (or equivalent) in cases where the examination procedure allows for a variation of the method of examination, equipment or set-up.

For important investigations the used reference test pieces have to be stored in conditions that they may not alter.

7.8

Special and Advanced Techniques

7.8.1

Sorting Sorting is a process where the eddy current response of two absolute probes put onto two objects is compared. All characteristics that influence the eddy current response, i.e. electric conductivity, magnetic permeability and the geometry have to be taken into consideration. To set appropriate confirmation levels, a determination of parameter settings – type of sensor (coil or probe) and frequency – have to be determined. For ferromagnetic materials an antecedent demagnetisation may be indispensable.

143

Figure 7-41: Sorting equipment with adequate coils can be used for various parts (IBG) For some applications the choice of the frequency can be crucial. 50 kHz Al

ℜ (μeff)

0.8

Cu Au Ag

0.6

0.6 0.4

0.2

0.2

0 0.1 0.2 0.3 0.4 0.5

5'000 kHz

1

Pb

0.8

0.4

0

500 kHz

1

1

C

0.8 Sn Ni

0.6

Ti

0.4

γ-Fe

0.2

0

0 0

0.1 0.2 0.3 0.4 0.5

0

0.1 0.2 0.3 0.4 0.5

ℑ (μeff) Figure 7-42: Position of the operating point for different frequencies and various materials 7.8.2

Measurements in Magnetic Saturation If ferromagnetic materials have to be tested with eddy current it can be of a great advantage, if the component or parts of it are brought into magnetic saturation with an DC electromagnet. In this case the relative permeability becomes near to one and the behaviour of the material is comparable to a paramagnetic material. Demagnetisation may be necessary.

Figure 7-43: Testing with magnetisation unit, SLOFEC system (Prüftechnik, TOA) A non-stationary application of measuring in magnetic saturation is called SLOFEC, saturation low frequency eddy current, and is used for large areas, like tanks, vessels or pipes, specially also for duplex steels. 7.8.3

Multi Frequency Technique Multiple frequency eddy current techniques simply involve collecting data at several different frequencies and then comparing the data or mixing the data in some way. It is not even neces -

144

sary to use several coils, as one coil can be operated at several frequencies at the same time. The instrument, however, must be adequate to the technique. The impedance of an eddy current probe may be affected by the following factors: ▪

Variations in operating frequency;



Variations in electric conductivity and the magnetic permeability of an object or structure, caused by structural changes such as grain structure, work hardening, heat treat ment, etc.;



Changes in lift-off or coil fill factor resulting from probe wobble, uneven surfaces and eccentricity of tubes caused by faulty manufacture or denting;



The presence of surface defects such as cracks and subsurface defects such as voids and non-metallic inclusions; Dimensional changes, for example, thinning of tube walls due to corrosion or deposition of metal or sludge;



The presence of supports, walls and brackets or edges.

Several of these factors are often present simultaneously. In the simple case where interest is confined to detecting defects or other abrupt changes in geometry, a differential probe can be used to eliminate unwanted factors, providing they vary in a gradual manner. For example, variations in electric conductivity and tube thinning affect both coils of a differential probe sim ultaneously. However, if unwanted parameters that occur abruptly are affecting the measurements, they can sometimes be negated by mixing signals collected at several frequencies. An example of where a multi frequency eddy current inspection is used is in heat exchanger tube inspections. Heat exchanger assemblies are often a collection of tubing that has support brackets on the outside. When attempting to inspect the full wall thickness of the tubing, the signal from the mounting bracket is often troublesome. By collected a signal at the frequency necessary to inspect the full thickness of the tube and subtracting a second signal collected at a lower frequency (which will be more sensitive to the bracket but less sensitive to features in the tubing), the affects of the bracket can be reduced.

Figure 7-44: Example of signal responses with changing frequency, 13 – 88 kHz, the colours are for clarification reasons only (ECT) 7.8.4

Swept Frequency Technique Swept frequency eddy current techniques involve collecting eddy current data at a wide range of frequencies. This usually involves the use of a specialised piece of equipment such as an impedance analyser, which can be configured to automatically make measurements over a range of frequencies. The swept frequency technique can be implemented with commercial equipment but it is a difficult and time-consuming measurement. The advantage of a swept frequency measurement is that depth information can be obtained since eddy current depth of penetration varies as a function of frequency. Swept frequency measurements are useful in applications such as measuring the thickness of conductive coatings on conductive base metal, differentiating between flaws in surface coatings and flaws in the base metal, differentiating between flaws in various layers of built-up struc ture. An example application is the lap spice of a commercial aircraft. Swept frequency measurements make it possible to tell if cracking was occurring on the outer skin, the inner skin or a double layer. A commercial system using this principle is called frequency scanning eddy current technique (FSECT) and is used to measure the thickness of (electric conductive) MCrAlY coatings on new and serviced high temperature gas turbine blades, possibly even coated with an additional thermal barrier coating (TBC). New coatings are homogeneous, due to diffusion and corrosion, however, they separate in layers with different properties. The system measures the materials response at about 30 frequencies and compares the data with a layer model.

145

Figure 7-45: FSECT equipment used for coating characterisation on turbine blades (CESI) Swept frequency technique is also used to find the most effective frequency for sorting pro cesses. 7.8.5

Remote Field Eddy Current (RFEC) Eddy current testing for external defects in tubes when external access is not possible, e.g. with buried pipelines, is conducted using internal probes. When testing thick-walled ferromag netic metal pipes with conventional internal probes, very low frequencies (e.g. 30 Hz for a steel pipe 10 mm thick) are necessary to achieve the through-penetration of the eddy currents. This situation produces a very low sensitivity of flaw detection. The degree of penetration can be increased by the application of a saturation magnetic field. However, because of the large volume of metal present, a large saturation unit carrying a heavy direct current may be required to generate an adequate saturating field. The difficulties encountered in the internal testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field eddy current method, which allows measurable through penetration of the walls at three times the maximum frequency possible with the conventional direct field method. In its basic form, the probe arrangement consists of an exciting coil and a receiver coil kept at a rigidly fixed separation along the axial direction. The separation between exciting coil and receiver coil should be about two and a half times the inner diameter of the tube. The moving magnetic field of the exciter coil induces strong circumferential eddy currents. They, in turn, produce their own magnetic field which opposes the magnetic field from the exciter coil. Due to the difference of the two fields, near the exciter coil the resulting field is dominated by the inner surface while after a certain distance the outer surface is relevant. (C) (B) (A) Direct coupling

Transition zone

Remote field

Figure 7-46: Schematic showing location of remote field zone in relation to exciter coil (A), multiple sector receiver coils (B) and pipe wall (C) So there is a direct coupling zone, a transition zone and a remote field zone. 7.8.6

Pulsed Eddy Current (PEC) Conventional eddy current inspection techniques use sinusoidal alternating electrical current of a particular frequency to excite the probe. The pulsed eddy current technique uses a step func tion voltage to excite the probe, forming a continuum of frequencies. As a result, the electro magnetic response to several different frequencies can be measured with just a single step. Since the depth of penetration depends on the frequency of excitation, information from a range of depths can be obtained all at once. If measurements are made in the time domain

146

(that is by interpreting the signal strength as a function of time), indications generated by flaws or other features near the inspection coil will appear first and more distant features will appear later in time. The eddy current response is influenced by the distance probe-electrical leading material, by the electrical resistance in the material and by its wall thickness. VE

Broadband excitation

t tP VD Distance and possibly frequency dependent signal

t

Figure 7-47: Principle of pulsed eddy current inspection; the excitation voltage is rectangular, with a peak duration of typically 0.5 – 1 seconds While the driving coil has to be of a certain size, the receiving coil can be small, often it is even replaced by other sensor types like Hall or GMR sensor. Those can be placed much nearer to the surface and are therefore more sensitive. Furthermore often not a single sensor but a full array is used. To improve the strength and ease interpretation of the signal, a reference signal is usually collected to which all other signals are compared. Flaws, conductivity and dimensional changes generate a variation of the signal. Displayed is the difference between the reference signal and the measurement one. The distance of the flaw and other features relative to the probe will cause the signal to shift in time. Therefore, time gating techniques (like in ultrasonic inspec tion) can be used to gain information about the depth of a feature of interest. For such a technique one has to take into consideration that the propagation velocity of the electromagnetic field through the conductive material depends also on the frequency. Dispersion takes place and the pulse change in shape as it progresses through the metal. For the visualisation of such effects the Wigner-Ville distribution may be used. This giving an image with the time as one dimension and the frequency as the other, calculating the Fourier transform of the signal at several times. The value of the transform is represented by colours or grey scales.

147

Figure 7-48: Application of pulsed eddy current for the examination of corrosion of ferritic tubes and vessels under thermal insulation, called INCOTEST, Insulation Component Test and for aircraft inspection (Applus RTD, GE MCS)

Figure 7-49: Imaging pulsed eddy current of a laminated sheet package used in aircraft industry; the two red areas in time gate 2 show areas of corrosion while time gates 3 and 4 show the internal structure. (GE Inspection Technologies)

Figure 7-50: Equipment for sheet thickness and positioning measurement (left) and for thickness measurement of storage tank base with PEC (ABB, Oceaneering) 7.8.7

Detection of reinforcement bars The technique is also used to detect and analyse reinforcement bars and other metal building elements in concrete. The information that can be taken off is the location of bars, their concrete coverage and their diameter. The techniques used are inductive or pulsed inductive. Alternatively also microwaves can applied.

148

Figure 7-51: Eddy current used for analysing reinforcement bars in concrete (Proceq, Hilti) 7.8.8

Magneto-Optical Imaging (MOI) Magneto-optical imaging uses the effect of Faraday rotation. The eddy currents are not excited with a coil but with two transformers oriented in orthogonal directions parallel to the surface and the transparent foil. The second transformer follows the first one with a phase-shift of 90°. Thus resulting in a rotating electric field. It needs some experience to detect and interpret flaws in the resulting image. (G) (E) (F) (D)

(C)

(B) (A)

Figure 7-52: Principle of a magnet-optic imaging device: (A) induction foil with light reflector, (B) multi directional excitation unit, (C) magnet-optical sensor, (D) polariser, (E) light source, (F) analyser, (G) camera

Figure 7-53: MOI application at an air plane, the operator is seeing the image in a head-up display (left), crack indications at a rivet, the serpentine like pattern is due to magnetic domains (PRI) 7.8.9

Alternating Current Field Measurement (ACFM) Alternating current field measurement does not use real eddy currents and it is also related to the AC potential drop method; AC flows through the parts in one direction only. The presence of flaws (mainly cracks) will result in a changed current flow direction and therefore in a change of the resulting magnetic field. With this method the latter is measured. ACFM is of importance

149

in off-shore testing and can be specifically used even if the parts are covered with shells. The result is the width and the depth of a crack.

Figure 7-54: ACFM probe heads for on-shore and off-shore use (TSC)

7.9

Summary and Conclusions Eddy current testing can be used for all electrically conductive materials, however, applications to non-metals are seldom. For testing purposes it is widely used for aerospace applications and in automated processes. Furthermore the technique is common for sorting and measurement of materials and components properties. The method is used to detect surface and near surface issues, thin layer, therefore, are possible. The result is primarily and integral statement on the geometry, the local electrical conductivity and the local magnetic permeability. For testing purposes, therefore, the result is often a comparison of the signal from a reference test piece with known characteristics and the one from the part under test. As the reference test piece should closely correspond to the sample under test, the variety of tasks in practice is limited and the method is suited for repeated applications. Probes can be operated with various frequencies; however, they are very much affected by the geometry, so users with varying applications will have the tendency to produce their own ones. New probe types will allow increasing the depth to be inspected with eddy current. The advantages of probe arrays is obvious, better resolution, less moving parts, so they are more and more applied to automated and semi-automated processes. Full magnetisation (and demagnetisation) of ferromagnetic parts and structures may be necessary; applications at elevated temperature or off-shore are possible. Several new approaches deal with new generations of eddy current receivers. For high-end application a deepened knowledge of the relevance of the complex impedance plane is necessary.

150

8

MICROWAVE AND TERAHERTZ INSPECTION

8.1

Introduction Microwaves or radar (radio detection and ranging) waves are a form of electromagnetic radiation located in the spectrum at frequencies roughly between 300 MHz and 300 GHz, corres ponding to wavelengths in vacuum between 1 m and 1 mm. Major subintervals of the microwave frequency band are designated with various letters; different sources use different letters. Although the general nature of microwaves has been known since the time of James Maxwell (1831-1879), not until World War II did microwave generators and receivers useful for the inspection of materials become available. Heinrich Hertz (1857-1894) conducted experiments that proved Maxwell's theories were correct. He began testing these theories by using a highvoltage spark discharge to excite a half-wave dipole antenna and was able to demonstrate propagation of electromagnetic waves. Guglielmo Marconi (1874-1937) built on Hertz's work and began experiments in Italy sending a signal using Morse code. 1909 he was awarded the Nobel Prize in physics. His announcement of the invention of radio was, however, only a replic ation of the invention of Nikola Tesla (1856-1943), who had it patented as early as 1897. Walter Schottky (1886-1976) was embedded in solid-state physics. His most important contribution to microwaves is the investigation of metal-semiconductor rectifying junctions, 1938, which is the basis for the gate contact of all MESFETs (metal-semiconductor field effect transistor). By the end of the 1930s Philip Smith (1905-1987) developed a circular chart that shows the entire universe of complex impedances in one convenient circle. The first successful radio range-finding experiment occurred in 1924, when the British physicist Sir Edward Appleton (1892-1965, NP 1947) used radio echoes to determine the height of the ionosphere. The first practical radar system was produced in 1935 by another British physicist, Sir Robert Watson-Watt (1892-1973), and by 1939 England had established a chain of radar stations along its south and east coasts to detect aggressors in the air or on the sea. World War II saw rapid developments and refinements in the naval and military radar by researchers in the United States. The first use of microwaves was for components such as wave guides, attenuators, cavities, antennas and antenna covers (radomes). The use of microwaves for evaluating material prop erties and discontinuities in other materials began with the evaluation of the concentration of moisture in dielectric materials, as microwaves of an appropriate wavelength are strongly absorbed and scattered by water molecules. Next, the thickness of thin metallic coatings on non-metallic substrates and of dielectric slabs was measured. This was followed by the determination of voids, delaminations, macro porosity, inclusions and others in plastic or ceramic materials. Success in these measurements also indicated that microwave technique could give information related to changes in chemical or molecular structure that affect the dielectric constant and dissipation of energy at microwave frequencies. Some of the properties measured include polymerisation, oxidation, esterification, distillation and vulcanisation. Above 320 GHz the spectral transparency of air is often interrupted, so for radar applications this range is not interesting and little research was done in this field until recently. For milli metre-wave energies from 100 GHz to 3 THz (3'000 – 100 µm) the common term is terahertz. Beyond 3 THz and out to 30 µm (10 THz) is more or less unclaimed territory, as few if any components exist. Various techniques nowadays also used for ultrasonics like phased array technology or synthetic aperture focusing technique (SAFT) have their origins in radar technology.

8.2

Physical Principles of Microwaves

8.2.1

General In free space an electromagnetic wave is transverse; that is, the oscillating electric and magnetic field that constitute it are transverse to the propagation direction of the wave. A particu larly simple propagating electromagnetic wave is the linearly polarised, sinusoidal varying, plane electromagnetic wave.

151

In microwave inspection a homogeneous material can be characterised in term of magnetic permeability µ, dielectric permittivity ε and electric conductivity σ. In general these quantities are themselves functions of the frequency. Moreover, the magnetic permeability and the dielec tric permittivity must usually be treated as complex quantities, rather than purely real ones, to account for certain dissipative effects. However, a wide variety of applications occur in which both terms can be regarded as mainly real and constant in value. In this case the relations can be given as ε=ε o ε r

with

εo =

1 µo c2

(8.1)

where εo is the relative dielectric permittivity (also called relative dielectric constant). For an electromagnetic wave incident upon a material, one part of the incident wave is transmitted through the surface and into the material and the other part is reflected. If the reflected wave is subtracted, in both amplitude and phase from the incident wave, the transmitted wave can be determined. When the reflected wave is compared, in both amplitude and phase with the incident wave, information about the surface impedance of the material can be obtained. Plane electromagnetic waves propagating through a conductive medium (σ >> ω ε) show the same skin effect as discussed for eddy current testing. As the standard depth of penetration depends on the frequency, the penetration of electromagnetic waves of the given range into conductive materials is only small. The phase velocity of a plane harmonic conductive electro magnetic wave in a conductive medium can be given as

v=δ ω=



2ω μσ

(8.2)

with ω as the angular frequency. Even if the magnetic permeability and the electric conductivity would not rely on the frequency, the velocity depends highly on it. As a result, a conductive medium is said to be highly dispersive, because a wave packet, which comprises sinusoidal components of many different frequencies disperse as it propagates. The velocity of an electromagnetic wave in a non-conductor is given by the relation 1 (8.3) µ ε This velocity can be expressed relative to the velocity of electromagnetic waves in vacuum, the ratio being the index of refraction η v=



c µε η= = =  µr εr v µo εo

(8.4)

The intrinsic impedances of free space Zo and of a material Z are given by

 

Zo = and Z=

µo εo

=µo c

µr µo

(8.5)

(8.6)

εr εo

The skin depth of an electromagnetic wave propagating in a weakly conductive medium (σ << ω ε) is given approximately by the relation

δ≈



2 ε σ µ

(8.7)

In this case, the wavelength is approximately given by λ≈

λo

1 σ 2 1−    η 8 εω

(8.8)

where λo is the wavelength in vacuum. This equation is sufficiently accurate for most materials having electric conductivity low enough for practical microwave inspection involving transmission. The criterion for its validity is that the non-attenuating wavelength, λo/η, be short compared to the skin depth. The intrinsic impedance becomes complex and can be approximated as η≈ ηn  1i

σ  εω

(8.9)

where ηn is the no-loss intrinsic impedance for the same material. As σ << ω ε, the addi tional terms are negligible and in most practical, low loss cases not needed. 152

Table 8-1: Dielectric constants of some materials Material

σ [S m-1]

εr [-]

Vacuum

1

Seawater

4

Water (distilled)

1 · 10-4

Water

1 · 10

Concrete

81.6 81

-3

1 · 10 – 1 · 10 -3

Ferrite

-1

1 · 10-3

13 – 16

Calcite

8.2.2

6 – 12 8–9

Wood

1 · 10 – 1 · 10

Glass

-9

-13

2.5 – 7

1 · 10

-12

6

Rubber

1 · 10

-13

3

Mica

1 · 10

-15

5–8

Quartz

1 · 10

-17

3.8 – 5

Reflection and Refraction The laws for reflection and refraction of microwaves at interfaces between media of differing electromagnetic properties are essentially the same as those for the reflection and refraction of visible light. The angle of refraction θt is determined by the Snell law (Willebrord Snell, 15801626) sinθi sinθ t

=

ηt ηi

=

vi vt

(8.10)

For linear polarised plane waves incident perpendicularly on an interface separating two dielec tric media, the ratio for reflection, the reflection coefficient R, and transmission coefficient T in terms of signal amplitude are given by R= T=

η i−η t η iη t 2η i ηiη t

(8.11) (8.12)

If the ratios instead of the amplitude are given in terms of power, the relations are R P= TP=

η i−η t2 η iη t2 4 ηi ηt η iηt 2

(8.13) (8.14)

Analogue relations exist for the amplitudes of the magnetic fields. These relations are equival ent to those given for the transmission and reflection factor for ultrasonics. For angles of incident other than zero, the corresponding relations are more complicated are known as Fresnel equations (Augustin Fresnel, 1788-1827). These relationships are different for microwaves linearly polarised with the electric vector in the plane of incidence (II) and for microwaves linearly polarised with the electric vector perpendicular to the plane of incidence (T). The plane of incidence is determined by the normal to the surface and the incident beam propagation direction.

153

Angle of incidence

1

Amplitude coefficients

0.75

TII

0.5

T⊥

0.25

Brewster angle

0

RII

-0.25 -0.5

R⊥

-0.75 -1 0

15

30 45 60 Angle of incidence

75

90

Figure 8-1: Amplitude coefficients R and T of reflection and transmission for η t / ηi = 2 R II= RT= T II= T T=

tan θt −θ i

(8.15)

tan θt θ i sinθ t−θ i

(8.16)

sinθ tθ i 4 sinθ t cosθi

(8.17)

sin 2θ tsin 2θ i 2 sinθ t cosθi

(8.18)

sin θtθ i 1

Fraction of incident power reflection

0.8 0.6

εr = 1

εr = 2

εr = 2

εr = 1

0.4

Brewster angle

Brewster angle

0.2

R⊥

RII

R⊥

RII

0 0

15

30

45

60

75

90 105 0 10 Angle of incidence

20

30

Figure 8-2: Microwave power reflection coefficients as a function of incident angle and polarisation plane for a reflection from air to a material with ε r = 2 and back, the critical angle in the second case becomes 30° (sin-1 ½) For parallel polarisation of the reflected wave there is angle, known as Brewster angle θB, (Sir David Brewster, 1781-1868) where the reflected amplitude is zero. tan θB=

ηt ηi

(8.19)

A second angle, the critical angle θc, appears only for a situation where the ratio ηt / ηi is < 1.

154

sinθ c= 8.2.3

ηt

(8.20)

ηi

Absorption and Dissipation Microwaves are affected during their propagation primarily by the interaction of the electric field with the dielectric properties of the non-metallic material. Polarisation and conduction are the cumulative results of the molecular-charge-carrier movement in the non-metallic material. Polarisation involves the action of bound charges in the form of permanent or induced dipoles. Conduction refers to whatever small free charge carriers are present. As a microwave passes through the material, the dipoles oscillate because of the cyclic nature of the force on them from the electric field. The dipole oscillation alternately stores and dissipate the electric field energy. Conduction currents only dissipates the energy, that is, convert it to heat.

8.2.4

Standing Waves Interference situations usually prevail when microwaves are used for non-destructive inspection because of the wavelengths and velocities involved, the coherent nature of the microwaves used, the high transparency of most non-metallic materials and the fact that the thickness of the non-metallic materials are within several wavelengths. The standing wave is generated when two waves of the same frequency are propagating in opposite directions and interfere with each other. The result is a formation of a total field with maximum and minimum points remaining in a fixed or standing position. A simple way to form a standing wave is to transmit a coherent wave normal to the surface. Incident

Reflected

Standing wave

Figure 8-3: Pattern of a standing wave formed by an incident wave and a reflected wave 8.2.5

Scattering Microwaves reflect from inhomogeneities by a process known as scattering. The scattering is generally used to describe wave interactions with small particles or inhomogeneities. The term reflection is generally used to describe wave interactions with surfaces that are large compared to wavelength. If the surface is not smooth compared to the wavelength of the microwave used, the reflected wave is not a simple single wave, but is essentially a composite of many such waves of various relative amplitudes, phases and directions of propagation. This effect is greatest when the wavelength is comparable to the dimensions of the irregularities. The Rayleigh approximation is used whenever the diameter of irregularities is roughly one fifth of the wavelength, the optical approximation is valid for diameters larger than ten times the wavelength. 4σ πd 4 =9   Rayleigh region 2 λ πd 4σ =1 optical region π d2

(8.21) (8.22)

155

Figure 8-4: Microwave scattering by metal spheres of various sizes (W.L. Rollwitz in: Metals Handbook, Vol. 17) 8.2.6

Homodyne and Heterodyne Demodulation The basic components is a microwave generator that feeds both a transmitting antenna and a phase-sensitive detector (or comparator). The received signal can be compared in amplitude and phase with the reference signal taken directly from the microwave generator. The reference signal can be taken to be of the form V ref =Vo cosω t

(8.23)

The received signal is them of the form

V rec=V cosω t Φ= VcosΦcosω t – V sinΦ sinω t

(8.24)

Because it is the coefficient of the term that varies in phase with the reference signal, the quantity V cos Φ is referred to as the in phase component and the term V sin Φ is termed the quadrature component. Standard electronic phase-sensitive detectors are available that can detect each of these components separately. In this homodyne demodulation the frequency of the reference and recorded signal are identical. The resulting detector current can also be writ ten in form of 1 I~ VV o 2 V Vo cosΦ 2

(8.25)

In case of a heterodyne detection, the frequency used for demodulation ω o is lower than the carrier frequency of the signal. In this case the resulting detector current is given by I~V V o cos ω−ωo  tΦ

8.3

Techniques of Microwave Inspection

8.3.1

Transmission Techniques

(8.26)

In transmission technique the incident wave is split into a reflected and a refracted or transmitted part at each surface. Only the part that is transmitted twice is measured. In fixed-frequency continuous-wave transmission technique the frequency of the microwave generator is constant. It is used either when the band of frequencies required for the desired interaction is very narrow or when the band of frequencies is so broad that the changes of material properties with frequency are very small and therefore not especially sensitive to it. With this technique both signal components can be detected with little mutual interference and it is therefore used, when the separation of both components is important. The swept frequency continuous-wave transmission technique provides for a transmission measurement over a selected range of frequencies. It is used if the resonant frequency shifts with changes in material properties or the response as a function of frequency over a substan tial bandwidth must be used. Specially designed broadband generators and amplifiers may be necessary. When a measurement of the time of transmission is required the simple measurement of the phase-shift is usually not helpful. In the pulse-modulated transmission technique the microwave generator is gated on and off and the receiver is usually a peak-value detector. Thus the receiver output consists of pulses that are delayed at finite time relative to the trans mitted pulse. 156

If transmission technique is used to form lines or images, computed tomography is possible. By making suitable assumptions, similar algorithms to those used in X-ray CT may be used. One important difference is that the reconstruction may be performed at multiple frequencies to yield additional information.

T

R

(C)

(A)

(B)

Figure 8-5: Arrangement for transmission technique: (A) microwave generator, (B) phase detector with reference signal as additional input, in phase and quadrature as output, (C) test sample 8.3.2

Reflection Techniques The reflection techniques are of two types: single antenna and dual antenna. For the single-antenna system the incident and reflected waves are both transmitted down the wave guide between the generator and the antenna. The phase detector is set so that it compares the phase of the reflected wave with that of the incident wave. This gives two output signals that are proportional to the in phase and quadrature component of the reflected wave, respectively. Such a system works well only for normal or near-normal incidence. The dual-antenna reflec tion system operates at any angle of incidence for which there is appreciable reflection. (A) (C) T/R (B)

(C)

T

R

(A)

(B)

Figure 8-6: Arrangement for reflection techniques: single (above) and dual antenna system with (A) microwave generator, (B) phase detector with reference signal as additional input, in phase and quadrature as output, (C) test sample The reflected wave from the surface, in principle has the same information about the bulk microwave properties of the material as the refracted wave. However it does not contain any information about the inhomogeneous properties of the material within the sample. There are further reflections from any internal discontinuities or boundaries, which ultimately add to the surface-reflected wave when refracted at the surface. In this manner properties beneath the

157

surface are sensed. To get a high reflection from the back wall this may be covered with a layer of conductive material. The simplest microwave reflectometer is based on the fixed-frequency continuous-wave reflection technique. Both the in phase and quadrature component of the reflected wave can be determined, in practice; however, most such techniques have used only the amplitude of the reflected signal. This technique has two limitations. First, the depth of a flaw and second, the frequency response of the material cannot be determined. The usual way of using the reflection technique is the time of flight method with a pulsed system. Every change of the index of refraction along the path of the wave will lead to reflected signal with an either positive or negative phase component depending on the reflection coeffi cient. The simple time signal is called an A-scan. If imaging systems are used, the gained information can be plotted with one spatial and one time of flight axis called B-scan or radargram, or with two spatial axes where either the amplitude (C-scan) or the time of flight (Dscan) can be used for colour coding. For a determination of depth information the knowledge of the wave velocity within the respective materials is necessary. C-scanning in the terahertz range is also called terahertz pulse imaging (TPI). When C-scans are generated, the user has the choice of selecting a time gate where the amplitude information is plotted, so the reflection images of several depths can be acquired. However, the depth / time ratio, as well as the absolute amplitude are relying of the path the wave has already covered. The limitation of pulse modulation is that the pulses required are very narrow if shallow depths are to be determined.

t

t A

B Figure 8-7: A-, B-, C- and D-scan presentation

158

C/D

Figure 8-8: 3.5” floppy disk: C-scan (left) and B-scan (along the dashed line) showing clearly the depth positions and the thickness of front and back cover; a closer look to the left part of the B-scan shows the flange on the front cover, as the wave velocity in the cover material is lower than in air the signal of the diskette and the back cover are slightly shifted to later times, the given term “optical depth”, therefore, is not absolutely correct, it seems to be the time of flight multiplied with an averaged velocity or the light speed (Rice University) When the interaction between a material and microwaves is frequency sensitive, a display of reflection as function of frequency using the swept frequency continuous-wave technique may be valuable. Because phase-sensitive detection over a wide range of frequencies is difficult, only the amplitude of the reflected signal is usually used as output. With this technique the depth of reflectors can be determined. A special application is the identification of specific layers of several closely spaced layers of material. The reflection at even multiplies of one-fourth wavelength is larger than at odd multiplies. The reflected signal identifies specific frequencies for which the layer is even or odd integral multiples of the quarter-wavelength. In both frequency and time domain modulation, the nature of the reflector is determined by the strength of the reflected signal.

Figure 8-9: TPI images of different layers at different depths in an integrated circuit chip (TeraView) 8.3.3

Standing Wave Techniques A standing wave is obtained from the constructive interference of two waves of the same fre quency travelling in opposite directions. Moving the receiving antenna in a dual-antenna system the standing wave can be determined.

8.3.4

Scattering Techniques A dual-antenna system must be used for scattering measurements. To measure all of the scattered radiation, the entire sphere around the irradiated object or material should be scanned and the detected signal graphed as a function of position.

159

8.4

Equipment

8.4.1

Microwave Circuit Components Solid-state sources, such as gallium arsenide gunn diodes or FETs (field effect transistor) provide coherent microwave radiation at the lower power levels desired for non-destructive testing applications. Microwave can be effectively accomplished by rectification (envelope or video detection) with certain semiconductors using a Schottky barrier diode. Sophisticated detection schemes involve the use of homodyne or heterodyne down-conversion techniques, where a modulated microwave signal is mixed with either a microwave of the same frequency (homodyne) or with a lower frequency local oscillator (heterodyne).

Figure 8-10: Mechanically tuned gunn oscillator (left) and self-biased detector (Millitech, Cernex) Attenuators are purely resistive elements that do the opposite of amplifiers, they reduce gain. There are five common types of attenuators used in microwave circuits, the tee, the pi, the bridged tee (pretty unusual), the reflection attenuator and the balanced attenuator. The tee, pi and bridged tee each require two different resistor values, while the reflection and balances attenuators need only a matched pair of resistors. This allows both the reflection and balanced topologies to be used as variable attenuators with a single control voltage or control current. Phase shifters are used to change the transmission phase angle. Phase shifters can be analogue or digital. Analogue phase shifters provide a continuously variable phase, perhaps controlled by a voltage. Electrically controlled analogue phase shifters can be realised with varactor diodes that change capacitance with voltage, or non-linear dielectrics such as barium strontium titan ate, or ferro-electric materials such as yttrium iron garnet. A mechanically-controlled analogue phase shifter is really just a mechanically lengthened transmission line, often called a trombone line.

Figure 8-11: Continuously variable attenuator (left), phase shifter with direct reading (ATM) Filters are in-line components that screen out unwanted frequencies or noise. PIN diodes can be used as high speed microwave switches, which either prevent or permit the passage of microwave radiation. Circulators are non-reciprocal ferrite devices with usually three ports. Energy into port one predominantly exits port two, energy into port two exits port three and energy into port three exits port one. They are used as antenna interfaces. Energy can be made to flow from the transmitter (port one) to the antenna (port two) during transmit and from the antenna (port two) to the receiver (port three) during receive. Circulators have low electrical losses and can be made to handle huge powers, well into kilowatts. By terminating one port, a circulator becomes an isolator, which has the property that energy flows in one direction only.

160

Power dividers and couplers are passive microwave components used for distributing or combining microwave signals. A splitter can be used as either a power combiner or a power divider. A coupler can be used to inject a second signal into a circuit, or as a means to sample a signal within a circuit. Couplers and splitters are usually three or four-port devices. Polarisers are inline devices that convert linearly polarised microwaves to circularly polarised ones or vice versa. Wave guides are metallic transmission lines that are used at microwave frequencies, typically to interconnect transmitters and receivers (transceivers) with antennas. Bended wave guides are divided in E-plane and H-plane bends. An easy rule of thumb is that an E-plane bend is bent the “easy way”, while an H-plane bend is bent the “hard way”. Shorts are either variable or fixed devices that completely reflect the microwave beam at some selected circuit location, while terminators are devices that absorb microwave radiation partially or totally.

Figure 8-12: Dividers, flexible wave guides (E-plane) and magic tee (ATM, MWE) The antennas typically used for non-destructive testing consist of small metal horns, with either circular or rectangular aperture. The gain – expressed in dB compared to an isotropic radiator – and operational bandwidth of these horns are determined by the geometry and electrical size of the horn. Generally an increased frequency results in a smaller horn size to achieve a given gain. Conical horns offer the advantage of transmitting / receiving circular polarised microwave beams, as well as linearly polarised beams of any polar orientation. Spiral antennas transmit / receive circularly polarised beams only. Dielectric lenses can be added to horns to either focus or collimate microwave beams.

Figure 8-13: Horns and Gaussian optics lens antenna (ATM, Millitech) A rather good explanation of single elements and relevant formulas can be found in www.microwaves101.com/encyclopedia (MWE). 8.4.2

Ground Penetrating Radar (GPR) Most known applications of microwave NDT deal with ground penetrating radar, sometimes also called surface penetrating radar. However this equipment cannot only be used for ground investigations, but also for concrete, masonry and similar civil engineering components. GPR systems usually act in reflection technique with perpendicular or near perpendicular incidence. Imaging is possible moving the antenna in some pattern and correlating the result with the position.

161

Table 8-2: Typical values for GPR antennas Antenna [MHz]

Penetration depth [m]

Weight [kg]

Size [mm]

250

3-4

6

560 x 560

500

1.5 - 2

4

400 x 400

1'000

0.5 – 0.75

2

200 x 200

2'000

< 0.5

1

110 x 110

GPR antennas are usually used as shielded, but for certain application also as unshielded devices. Due to their usage, GPR antennas are built as rugged systems. If small penetration is necessary, antennas can be hand held; larger types are mounted on hand wagons or on cars. New developments allow antenna arrays that can cover a certain width and allow easier scanning. In principle phased-array radar systems (details on phased-array see ultrasonic testing), as they are used for defence and weather survey would also be possible.

Figure 8-14: Examples of portable GPR antennas, unshielded dual antenna system (left), GPR equipment used for wall inspection (Gisco, Terraplus)

Figure 8-15: Examples of medium sized GPR equipment (left) and GPR antennas used for road inspection (Era Technology, Empa)

162

Figure 8-16: Rail road application for an array radar antenna (3d Radar) 8.4.3

Terahertz Equipment The interesting frequency band of a 0.1 to 3 THz is not easily accessible. Electronic sources like Gunn or Schottky diodes with subsequent frequency multipliers, provide high output levels (mW range) up to some 100 GHz, yet become inefficient in the sub-millimetre range. Direct optical sources, like quantum cascade lasers, are usually limited to frequencies > 5 THz, even when operated at cryogenic temperatures. Therefore the generation of terahertz radiation is typically a femtosecond laser pulse. The light pulse is focused onto a photoconductive switch (e.g. low temperature GaAs or radiation-damaged silicon-on-sapphire substrate) containing the dipole antenna. The radiated THz wave corresponds to the first derivative of the photo-induced current; a broadband, narrow electromagnetic pulse therefore needs a short optical pulse. Both pulsed and continuous wave designs have been realised. The set-ups of scanning and real-time systems are different.

Figure 8-17: Set-up of a terahertz scanning system (Nikon)

163

Figure 8-18: Set-up of a terahertz real-time system (Nikon) As the whole technology shows a very dynamic development a lot of companies are not too eager to present exactly their equipments.

Figure 8-19: All-electronic emitter and detector for millimetre-wave imaging (left) and commercial microwave camera (SynView, Agiltron)

Figure 8-20: Equipment for IC inspection (TeraView)

164

Figure 8-21: Terahertz dual-antenna system (Fraunhofer-Allianz Vision) A potential method for the generation of electromagnetic waves in the desired frequency range could in future also use the inverse Doppler effect. A frequency shift inverse to the normal Doppler effect happens for waves that show anomal dispersion, the impact, however, is about 100'000 times stronger than for the normal effect.

8.5

Applications

8.5.1

Applications of Ground Penetrating Radar Various applications of GPR are reported, among them: ▪

Utility detection and mapping;



Highway, bridge, rail road bed, airport inspection;



Architectural cladding inspection;



Snow/ice thickness measurement;



Detecting minerals, mining;



Detecting unexploded ordnance and mine detection;



Archaeology, fossils, forensic applications.

The usual way to present the results is in form of images. Several modes exist; the designa tions chosen here are originally used in ultrasonic testing. The original unprocessed B-scan data usually need a lot of experience for interpretation, as all features are mapped in with a hyperbolic distortion.

Concrete surface 1st rebar layer Bottom side Figure 8-22: GPR B-scans: unprocessed image of underground tanks, bridge structure of the Ponte Cebia 3 bridge at the San Bernardino route, length 85 m (Geosearches, Empa) If the surface is scanned as area (typically in a meander shape), the date can be processed to represent the lateral position and the reflection amplitude of certain features (C-scan) or to map thickness in form of times of flight if the velocity within the material is known (D-scan). If calibration data are present, materials properties of the penetrated layer can also be determined, if such properties influence the propagation of electromagnetic waves. It lies in the nature of the subject that probability of detection calculation for mine detection is of high importance.

165

Figure 8-23: GPR C-scan of reinforcement bars in concrete with respective vertical cuts (left), GPR D-scan of pavement thickness of an airport runway (Geosearches, Geophysical)

Figure 8-24: Conductivity map of a field to determine suitability as a vineyard (left), POD chart for land mines with diameters of 5 to 50 cm versus range (Geophysical, Era Technology) 8.5.2

Applications of Materials Characterisation and Research For the time being three main application fields are targeted by microwave and terahertz techniques: composite materials, electronic devices and biomedical applications. The results have often still preliminary image quality or the applications deal with demonstration parts.

Figure 8-25: Applications of microwaves to composite materials: fibre distribution of injection moulded short fibre reinforced laminates (left), kissing disbond in a composite panel (Stuttgart University, NSWC) Polarised microwaves, e.g., can be used to determine the anisotropy in short fibre reinforced injection moulded laminates (Diener, L., in Thompson, D.O. and Chimenti D.E. (Edts.): Review of Progress in QNDE, 14, New York, Plenum Press, 1995, pp. 615).

166

Figure 8-26: Photograph of a kevlar vest hit by a projectile (left) and THz image (230 -320 GHz) of the vest's inside revealing damages in all parts of the sample (SynView)

Figure 8-27: Performance demonstration for the usage of terahertz imaging for the investigation of processes: leaf of a living plant before and after watering (Rice University)

8.6

Special and Advanced Methods

8.6.1

Synthetic Aperture Focusing Technique (SAFT) To increase the reliability and with respect to quality assurance, automatic scanning systems are used together with a high frequency data acquisition system for the complete high-frequency signal. The applied imaging scheme is based on the back propagation of the signals probe back into the component. The original SAFT radar experience dates from the early 1970es and the first digital implementation of one-dimensional SAFT was demonstrated in 1976. The SAFT is a method of simulating a large focusing probe by digitally processing all the range / amplitude information obtained from a scan over a discontinuity with an ordinary unfocussed beam. It has the advantage over a conventionally focusing probe of having a variable focal length. As the beam is scanned over a discontinuity, a signal is first observed when the discontinuity intercepts the leading edge of the beam. For a depth d the radius of the footprint r of the signal is roughly λ d r≈  4  ε r 1

(8.27)

On continuing the scan the signal increases in amplitude along the beam axis and then falls to zero at the trailing edge of the beam. In addition to these amplitude variations the signal path range also varies; in the case of a normal beam the signal path will be minimum when it lies along the beam axis. By electronically correcting for these variations in range across the width of the beam it is possible to mutually superimpose a large number of separate signals from the discontinuity. This process, which is carried out on the digitised signals using suitable computer power, greatly improve the signal/noise ratio, since the noise signals will be randomly superimposed, whilst the discontinuity signals will be preferentially superimposed.

167

Time of flight (B-Scan) [a.u.] Signal strength [a.u.]

Signal strength

Time of flight

Lateral distance [a.u.] Figure 8-28: SAFT principle, at the effective position the amplitude is largest, whilst the time of flight is minimal (time of flight signal given as in B-scans), simplified

Figure 8-29: GPR B-scan image of a concrete sample containing a tube and a flaw; unprocessed image (left) and after 3D FFT SAFT (BAM) Optimum superimposition of the signals will only occur if the corrections for signal path range are on the true position of the discontinuity. Corrections based upon a false position will result in the sum of the superimposed signal being very much reduced. The technique is very sensitive to this effect and, therefore, by carrying out a number of summations, based on different assumed positions of the discontinuity and by recording that at which the maximum superimposed signal amplitude is observed, a very accurate measurement of the position of the discon tinuity is possible. This technique is particularly valuable when used to locate the sources of the diffraction signals at the tips of a planar discontinuity, since the signals are often very small and may be lost in the general noise level when using conventional sizing techniques. 8.6.2

Near Field Terahertz Scanning Some very exciting possibilities are opened if the terahertz scanning is performed in the near field. The standard procedure is using an aperture or placing the sample in close proximity of the detector. Spatial resolutions below 10 µm are reported, partially using some more advanced near field techniques.

168

Figure 8-30: Performance demonstration of the gain in spatial resolution for near field scanning applied to a USAF 1951 resolution target (Rice University) 8.6.3

Homeland Security Terahertz radiation is also used for homeland security to detect items that cannot be found by induction coils. For good reason sometimes they are called naked scanners.

Figure 8-31: Terahertz inspection booth and camera (L3, ThruVision) 8.6.4

Metamaterials Metamaterials were first published by Victor Veselago (*1929) in 1967. In certain frequency bands metamaterials show properties that do not exist in nature. They have a negative magnetic permeability and at the same time a negative dielectric permittivity, resulting in significant effects like a negative index of refraction. They have to be synthesised artificially. They are composed of sub-wavelength metallic resonators mounted periodically on dielectrics. In metamaterials the unit cells can be manipulated and constructed to resonate at various frequencies obtaining unique electromagnetic responses differently from electromagnetic responses of natural substances that are of a limited range as derived from atoms and molecules in unit cells. The most popular unit cell is split ring resonator (SRR), which has two interleaved metallic rings with two opposite gaps. When the incident magnetic field is perpendicular to the SRR plane, electric currents are induced along the rings. Simply, each unit cell acts as an LC oscillator and responses to the external magnetic field at the resonance frequencies.

169

Figure 8-32: Scheme of a split ring resonator and metamaterial (Imperial College) Metamaterials can be used for terahertz applications like antennas and also in ultrasonics, where the wavelength is similar.

8.7

Summary and Conclusions For the time being the only widely used application of microwaves in non-destructive evaluation is ground penetrating radar. Applications using higher frequency ranges are just beginning to leave the laboratory status. The use of microwaves is restricted to non-conductive material, conductive materials, however, can be detected, but not be penetrated or transmitted. Ground penetrating radar is used for various applications in soil but also in pavements, concrete and masonry. The resolution relies on the frequency and is in a range of millimetres to centimetres. For the terahertz technology it will be of an advantage that this technology is also of interest for medical diagnosis, as this will push the developments on the source and detector side; usually non-destructive evaluation can profit from such tendencies. Two developments can be of special interest for non-destructive evaluation. One is the possibil ity of spectroscopy, where the wavelength can be tune to values that result in optimum contrast; the second one is the use of the near field imaging increasing the resolution. The main potential application of metamaterials is the production of focusing lenses. Compared to optical applications, they wavelength for microwaves is rather large, so that the production is less complicated.

170

9

MAGNETIC RESONANCE INSPECTION

9.1

Introduction Nuclear magnetic resonance (NMR) and the tomographic version of it, magnetic resonance imaging (MRI), is mainly a domain of chemical and physical molecular analysis and for medical applications. But also some problems in non-destructive evaluation can be solved excellently using NMR or MRI. Felix Bloch (1905-1983) and Edward Purcell (1912-1997), both of whom were awarded the Nobel Prize in physics in 1952, discovered the magnetic resonance phenomenon independently in 1946. In 1971 Raymond Damadian showed that the nuclear magnetic relaxation times of tissues and tumours differed, thus motivating scientist to consider magnetic resonance for the detection of disease. He also demonstrated in 1977 MRI of the whole body. In 1973 MRI was first demonstrated on small test tube samples by Paul Lauterbur (19292007). In 1975 Richard Ernst (*1933) proposed MRI using phase and frequency encoding and the Fourier transform. This is the basic of current MRI technique. For his achievements in NMR and MRI he was awarded the Nobel Prize in chemistry in 1991. In 1977 Sir Peter Mansfield (*1933) developed the echo planar imaging (EPI) technique, that years later was able to generate images at video rate. In 1993 functional MRI (fMRI) was developed. This technique allows the function of various regions of the human brain. Lauterbur and Mansfield were awarded the Nobel Prize in medicine 2003. In 1999 the first truly portable MRI technology was developed. This makes MRI technology available for non-destructive applications outside of a designated laboratory. Kurt Wüthrich (*1938) was awarded the Nobel Prize in chemistry 2002 for his development for NMR spectroscopy for the determination the 3D structure of biological macromolecules.

9.2

Principles of Magnetic Resonance

9.2.1

Application of Magnetic Fields Hydrogen atoms (protons) present in water and organic compounds possess properties which make them behave like tiny magnets. A sample containing protons, original randomly arranged, will become aligned when placed in a strongly applied magnetic field. The protons align themselves parallel or anti-parallel to the magnetic field. According to the Boltzmann statistics, the population of parallel spinning protons is slightly higher than the one of anti-parallel spinning protons. There is a difference in energy levels between protons that point parallel or anti-parallel and this results in a net magnetic charge in one direction. The magnitude of this magnetic charge depends on the strength of the external magnetic field and the amount of protons in the sample. This accounts for the need to use very strong magnetic fields and the inherent insensitivity of magnetic resonance imaging - the sample has to contain more than about 5% water or respective substances to get a signal. Other nuclei also possess magnetic properties (carbon 13, sodium and phosphorus) and can also be measured with magnetic resonance methods. However, compared to hydrogen protons, the difference in energy levels is much less, which means that the amount of magnetic resonance signal is also reduced. Magnetic resonance imaging of nuclei other than hydrogen is very difficult.

9.2.2

Magnetic Resonance In addition to behaving like tiny compasses that align with an externally applied magnetic field, protons also spin about their axes. This creates a miniature magnetic field around each proton that creates an associated magnetic moment. When a magnetic field is applied, it causes these magnetic moments to precess around the direction of the external magnetic field. This movement can be compared to a spinning top as it starts to slow down. The rate, or frequency, at which the protons precess, is given by the Larmor equation (Joseph Larmor, 1857-1942): ωL=γ Bo

(9.1)

where ωL is the frequency of precession or Larmor frequency, γ is the gyromagnetic ratio and Bo is the strength of the external magnetic field. The gyromagnetic ratio is a constant that is fixed for the nucleus under investigation. For hydrogen protons, γ is 42.6 MHz/T. Therefore, at 2 T, protons precess at 85.2 MHz. The frequency at which the proton will resonate is directly pro 171

portional to the strength of the magnetic field. The Larmor equation is the basis for each NMR experiment. 9.2.3

Magnetic Resonance Signal In order to receive the magnetic resonance signal, a pulse of radio-frequency (RF) energy (several 100 to several 1'000 volts) is passed to the sample at the same frequency as the precess ing protons. The RF energy is transmitted to the sample via a coil around or on the sample under investigation. The RF energy causes the protons at a lower energy level (those parallel to the magnetic field) to have the same energy level as the anti-parallel protons. The change in energy levels gives a net magnetisation vector at 90° to the direction of the magnetic field. The pulse is turned off and a small electrical signal (a few mV) is received via a coil oriented the same way as the one used to transmit the RF energy. The coil detects an oscillating magnetic field as an induced electric current.

9.2.4

Molecular Mobility It is also possible to quantify how long it takes for the magnetisation to return to its original position in the magnet (equilibrium) after the application of a radio-frequency pulse. The time taken to reach equilibrium (called longitudinal, spin-lattice or T1 relaxation) is affected by the molecular mobility of the protons in the sample. Protons in fats are relatively 'fixed' in place and take a short time (about 0.25 seconds) to return to equilibrium, whereas protons in water molecules are a lot more mobile and take about 2 seconds to recover their magnetisation. When a sample receives a radio-frequency pulse, the magnetisation is re-directed to 90° from its original position and the protons precess in phase - they rotate at the same speed relative to each other. Due to various molecular interactions, the protons quickly start to go out of phase and this results in a loss of signal. The rate taken for the protons to lose their phase can be measured with NMR and is termed transverse, spin-spin, or T2 relaxation. Again due to the different molecular mobility of protons, fat has a short T 2 relaxation rate (about 20 ms) and water has a long T2 relaxation rate (about 60 ms). There are many processes which affect the molecular mobility of the protons in a sample, e.g. freezing and ripening. By measuring the mobility of protons, additional information is gained which can be used to characterise a sample. In addition, NMR can also non-invasively measure flow, temperature, diffusion and perfusion either at one time point, or dynamically to monitor a process.

9.3

Instruments and Applications

9.3.1

Nuclear Magnetic Resonance NMR, especially when used with an open coil, called one-sided access NMR (OSA-NMR) or single sided access NMR can be successfully applied for:

172



Diagnosis of moisture contents, moisture distribution and porosity in construction materials and geological core samples;



Measurement of density, moisture and bonding agent in wood and chipboard;



Characterisation of polymer and elastomer materials.

(A)

(C)

(B)

(D)

Figure 9-1: OSA-NMR instrument with open coil sensor (left) and applications at rubber samples (T1 images): (A) the not fully vulcanised sample shows clear heterogeneity, (B) fully vulcanised sample; (C) effects of ageing in oxidising atmosphere, (D) not aged sample (IZFP) The usual generated frequencies are in a range of 0.1 to 20 MHz.

Figure 9-2: Single sided access NMR probe for mobile universal surface exploring (NMRMOUSE) (aixNMR)

Figure 9-3: Portable Halbach magnet for measurements of porosity and pore size distributions of water-saturated drilled geological cores (aixNMR) The probes can be made small, typically 5 cm by 5 cm with an active field of typically 1 cm 2. Combining the magnetic field of a single permanent magnet with a dedicated gradient coil system. The depth selection is achieved by retuning the frequency, while lateral resolution is achieved implementing a pure phase encoding imaging method. The 3D imaging effect is similar to ultrasonics and eddy currents but without moving the sensor, however, this is not yet MRI. 9.3.2

Magnetic Resonance Imaging Due to its costs, magnetic resonance imaging is not very much used in non-destructive testing. However, for both, liquid and solid materials applications, it seems being possible. It has e.g. been successfully applied to the study of polymer blends and composites, ceramic processing, catalysts and filters fouling and fuels research. Other fields that can be very well investigated with MRI deal with flow, viscosity and rheometry. Certain applications can also be done with OSA-instruments.

173

Figure 9-4: Medical MRI systems: closed type with typically 3 T (left), open type with typically 1 T (Siemens) For medical applications open and closed systems are in use. Open systems usually have a smaller field, but for applications other than persons they probably often would fit better. Similar to X-ray tomographic systems, medical systems should be faster and the field exposure to patients and personnel should be as low as possible. For non-destructive testing both aspects have much less weight, in contrast, high fields would be preferred as the give a better resolu tion.

Figure 9-5: MRI Instrument used for preclinical research (like in vivo studies on animals) with up to 15 T (left), open coil with sample inside, portable open MRI instrument for medical applications (Bruker, Cambridge University, MagnaVu) Only few instruments are commercially available that could be of good use for specified ques tions related to materials, some of them reach a spatial resolution of 30 μm.

174

Figure 9-6: Investigations on polar ice structures (left), lower row left: optical thin section, right: MRI of the same section, comparison of clean and fouled water filters (Bruker BioSpin, Cambridge University)

9.4

Spatial Encoding and Image Contrast

9.4.1

Slice Selection The received signals are from the entire sample, they do not have any spatial information. Therefore gradients of magnetic field are temporarily created within the external magnetic field. These gradients are created using gradient coils placed around the sample so that the magnetic field is manipulated in the three axes (x, y and z). Depending on their function, the gradient coils are called: ▪

Slice select gradient;



Frequency encode gradient;



Phase encode gradient.

The slice select gradient is used to acquire a magnetic resonance signal from a particular location in the sample and with a particular thickness. The frequency and phase encode gradients are used to manipulate the in-slice spatial encoding. The slice selection is done by a linear magnetic field gradient, e.g. 2.0 to 2.2 T, corresponding to 85.2 to 93.7 MHz (for hydrogen). If a RF pulse of a single frequency would be transmitted to the sample, the received signal would correspond to a single frequency. However, the signal received would result in an infinitely thin line. If the RF pulse contains not a single frequency but a certain bandwidth, the respective slice thickness increases. There are two ways in which the slice thickness Δx can be varied: ▪

alter the bandwidth of RF frequencies Δω - the narrower the bandwidth, the thinner the slice of excited protons;



increase the slope of the magnetic field gradient B'. Δx=

Δω γ B'

(9.2)

If the gradient is increased and the frequency bandwidth is kept the same, the thinner the slice will become and consequently the less signal received. There is an electronic limitation on how much the bandwidth can be decreased. There is also a hardware limitation on the size of the gradient that can be generated. These combined factors set a limit on how thin a slice can be. The selection of a particular slice thickness for imaging depends on a number of inter-related factors: The pixel resolution within the slice - the smaller the pixels (the higher the image resolution), the less signal received, so slice thickness may be increased to compensate. The structural heterogeneity of the sample: real samples have a high level of structural heterogeneity in all three axes. Due to signal averaging within a particular voxel in a slice, detail will be lost as the slice thickness increases. The molecular mobility of the protons - a high molecular mobility increases the signal, so the slice thickness can be decreased. The use of a slice gradient on its own will not result in a magnetic resonance image. In order to achieve an image, frequency- and phase-encoding gradients have to be used within the slice. 175

9.4.2

Frequency and Phase Encoding The magnetic field within a slice is increased across the sample. The stronger the magnetic field, the faster the precession of the protons. In an imaging experiment, the received signal contains different amplitudes (due to varying water contents in the sample) and different frequencies (due to the magnetic gradients used). The signal containing all this information can be decomposed into its amplitude and frequency components with a Fourier transform. As the strength of the applied gradient field is known, the frequency can be related to position and the final result is an image showing the spatial distribution of water within the sample in one-dimension (1D). In order to acquire a two-dimensional (2D) image, frequency encoding has to be used in com bination with phase encoding. An additional gradient, at 90° to the frequency encode gradient, is applied for a short period of time. The stronger the applied phase encode gradient, the greater the difference in phase between precessing protons. The magnetic resonance signal received will therefore contain phase and frequency information that can be analysed by the FT algorithm. By combining phase and frequency encoding gradients, the protons are spatially labelled within the sample in two dimensions. However, one single phase encode gradient of a particular magnitude will still only enable the acquisition of a 1D image. In order to acquire a 2D image, the phase encode gradient has to be applied in increments of increasing gradient strength. Each increment corresponds to a 1D pro file across the sample in the second dimension. By combining all these one-dimensional profiles, a two-dimensional image is generated. In order to acquire 3D MRI scans, phase encoding in the 3 rd dimension to label the protons is used. As with 2D MRI, each phase encode step in the 3 rd dimension corresponds to one image. The time to acquire 3D MRI scans, therefore, becomes longer, corresponding to the number of slices in the 3rd dimension. Unlike X-ray computed tomography, there is no short cut using a spatial multidimensional detector.

9.4.3

Image Construction A typical time for one phase encode step for an image is 0.5 s, so a 256 x 256 slice will take 128 s. However the time to acquire an image also depends on the number of signal averages. It is rare that an adequate signal can be acquired in one scan, so it is normal for several images to be acquired from the same sample and the images are then averaged.

Figure 9-7: Example of a lemon scanned with MRI, the mid image is the image acquired before analysis with the FT and it contains the same information as the image to the right (Cambridge University) Therefore, when determining the optimum variables for acquiring an MRI image, we are always making a compromise between the following factors that affect the amount of signal we can receive:

176



Amount of water in sample – the more protons, the higher the signal;



Mobility of protons – the more mobile the protons the higher the signal (e.g. water vs. ice);



Resolution (number of phase encode steps and frequency encode steps) – the higher the resolution the better the image detail, but the longer it takes to acquire an image;



Number of signal averages – the more signal averages, the higher the signal/noise ratio, but the longer the image acquisition time;



The thickness of the selected slice – the thicker the slice, the more signal but the image may lose clarity due to volume averaging.

These are the reasons for why MRI is slow, has limited resolution (typically no less than 100 μm per pixel) and cannot easily be used to image dry materials and/or materials which have a low molecular mobility. 9.4.4

Other Spatial Location Methods The raw data in the time domain is also referred to as k-space. The order in which the time domain data is acquired does not matter, as long as the k-space is scanned so that it provides sufficient information to become an image after the Fourier transform. There are more rapid ways of scanning k-space than the frequency / phase encode method described above.

Figure 9-8: Schemas of spiral scanning (left), back projection (mid) and echo-planar imaging (Cambridge University) For example, spiral scanning uses a spiral trajectory through k-space to acquire the MRI signal in about 50 ms. Back projection acquires radial lines across k-space in a similar time to spiral scanning. However, the disadvantage of spiral scanning and back projection methods is that they do not adequately sample the points at the edge of k-space. The points at the very edge of k-space contribute to the fine detail and clarity of the image, so images acquired by spiral scanning and back projection methods can be blurred compared to the conventional approach. Echo planar imaging (EPI) is a method commonly used in functional brain studies (fMRI). It samples k-space in about the same time to acquire one-phase encode step. While EPI is very fast, it is very demanding on the MRI hardware, particularly the gradient amplifiers that control the gradient coils. There is a tendency with EPI for the gradient coils to not precisely cover kspace, so again the images can be blurred.

9.5

Electron Paramagnetic Resonance Electron paramagnetic resonance (EPR), often also called electron spin resonance (ESR), is a spectroscopic technique which detects species that have unpaired electrons. EPR is a magnetic resonance technique very similar to NMR. However, instead of measuring the nuclear transitions, the transitions of unpaired electrons in an applied magnetic field is detected. Like a proton, the electron has spin, which gives it a magnetic property known as a magnetic moment. When an external magnetic field is applied, the paramagnetic electrons can either orient in a direction parallel or anti-parallel to the direction of the magnetic field. This creates two distinct energy levels for the unpaired electrons allowing to measure them as they are driven between the two levels.

9.6

Summary and Conclusions NMR and MRI are investigation methods not yet commonly used for non-destructive evaluation. The applications are reduced to hydrogen atoms – concentration or mobility. For simple magnets, the penetration depth is limited. For MRI for the full use of such a technology a broad knowledge of the physics behind it is necessary. Most commercial equipment is relying upon medical applications, for others they have to be translated to the respective technological situation. Fast systems can be used to control processes where hydrogen is involved. For the moment is unclear what the influence of the upcoming terahertz technology on the possibilities of MRI will be.

177

178

10

ULTRASONIC INSPECTION

10.1

Introduction Ultrasonic testing is a method to control materials with low or moderate acoustical attenuation as metals, ceramics or polymers. Inner and outer inhomogeneities may be found. If the time of flight can be measured, it is also possible to determine velocities or the thickness of objects or coatings. The usual frequency range lies between 0.5 MHz to 20 MHz. In 1793 Lazzaro Spallanzani (1729-1799) assumed that bats orient themselves with sound that is not audible, this was proofed 1938 by George Pierce (1872-1956) and Donald Griffin (19152003). As early as 1826 Jean-Daniel Colladon (1802-1893) and Charles-François Sturm (1803-1855) measured the sonic velocity of water in the lake of Geneva using a submerged bell excited by gunpowder. Christian Doppler (1803-1853) published 1841 an article “Über das farbige Licht der Doppel sterne und einiger anderen Gestirne des Himmels”; his predictions, however, were partly wrong. 1845 the acoustic Doppler Effect was proofed by Christoph Buys-Ballot (1817-1890) in an unorthodox experiment with trumpeters positioned on a moving train. The technically fundamental steps were the following two: Lord Rayleigh (1842-1919, NP 1904) established 1877 to 1878 with the two volumes of his textbook “The Theory of Sound” the the ory of wave propagation in solids, while Jacques (1856-1941) and Pierre Curie (1859-1906, NP 1903) found 1880 the reversible piezoelectric effect, approved mathematically 1881 by Gabriel Lippmann (1845-1921, NP 1908). One of the main reasons for the development of sonar (sound navigation and ranging) was the doom of the Titanic in 1912. In the same year Alexander Belm (1880-1952) described the technique of sending sound waves through water and observing the returning echoes to characterise submerged objects and Lewis Richardson filed the first patent in the UK. The first working system was built by Reginald Fessenden (1866-1932) in the USA in 1914. Paul Langevin (1872-1946) and Constantin Chilowsky patented 1918 an advanced device called hydrophone. This inspired early ultrasound investigators to explore ways to apply the concept to medical diagnosis. In 1929 and 1935, Sergej Sokolov (1897-1957) studied the use of ultrasonic waves for analysing metallic objects. Industrial use of ultrasonic testing apparently started quite simultaneously in four countries: United States, Great Britain, Germany and Japan. In the western world, the key-persons, Floyd Firestone (1898-1986), Donald Sproule (*1903) and Adolf Trost (*1911) had no knowledge of each other as they worked strictly in secret. Not even their patent-applications were published. Sproule and Trost used transmission-technique with separate transmitter- and receiver-probes. Trost invented the so-called "Trost-Tongs". The two probes were contacted on opposite sides of a plate, held in same axis by a mechanical device - the tongs - and coupled to both surfaces by continuously flowing water. Sproule placed the two probes on the same side of the sample. So he invented double-crystal probes. But it has to be mentioned that he used this combination also with varying distance from each other. Firestone was the first to realise the reflection-technique. He modified a radar instrument and developed a transmitter with short pulses and an amplifier with short dead zone. Shortly after the end of World War II, researchers in Japan began to explore medical diagnostic capabilities of ultrasound. The first ultrasonic instruments used an A-mode presentation with blips on an oscilloscope screen. That was followed by a B-mode presentation with a two dimensional, grey scale imaging. Japan's work in ultrasound was relatively unknown in the United States and Europe until the 1950s. Then researchers presented their findings on the use of ultrasound to detect gallstones, breast masses and tumours to the international medical community. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardi ovascular investigation. In the early 1970s, two events occurred which caused a major change. The continued improvement of the technology, in particular its ability to detect small flaws, led to the unsatisfactory situation that more and more parts had to be rejected, even though the probability of failure had not changed. However, the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size would fail under a particular load if a material property, the fracture toughness, was known. Other laws were developed to predict the rate of growth of 179

cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for new philosophy of "fail safe" or "damage tolerant" design. Components having known defects could continue in service as long as it could be established that those defects would not grow to a critical, failure producing size.

10.2

Physics of Ultrasound

10.2.1

Wave Propagation In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, transverse or shear waves, surface waves and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilatational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in gases and liquids, as well as solids because the energy travels through the atomic structure by a series of compression and expansion movements. In the transverse or shear wave, the particles oscillate transverse to the direction of propaga tion. Shear waves require an acoustically solid material for effective propagation and, therefore, are not effectively propagated in materials such as gases or most liquids. Shear waves are usually generated in materials using the mode conversion from longitudinal waves.

(A) propagation, movement (B)

(C)

propagation

movement

Figure 10-1: Longitudinal (A) and shear (C) wave, (B) unaffected material Pure longitudinal and shear waves do only exist, if the dimension of the material perpendicular to the propagation direction is much larger than the wavelength. However, at surfaces and interfaces this is not the case, so various types of elliptical or complex vibrations of the particles make other waves possible. Some of these wave modes such as Rayleigh and Lamb waves are also useful for ultrasonic inspection. Rayleigh waves travel along the surface of a relative thick solid material penetrating to a depth of one wavelength. Rayleigh waves are useful because they are very sensitive to surface defects. Since they will follow the surface around curves, they can also be used to inspect areas where other wave types might have difficulty reaching. Stoneley waves or leaky Rayleigh waves have their origin from a surface wave that leaks into the surroundings. For materials testing the expression leaky Rayleigh wave is common, for geophysics (fluid filled boreholes, seismic profiles, etc.) it is the expression Stoneley wave. Lamb waves, also known as plate waves, can be propagated only in very thin materials (Horace Lamb, 1849-1934). They are complex vibrational waves that travel through the entire thickness of a material. Lamb waves provide a means for inspection of very thin materials. Propagation of Lamb waves depends on density, elastic and material properties of a component, and they are influenced by a great deal by selected frequency and material thickness. Lamb waves show symmetric and antisymmetric modes. 180

Table 10-1: Ultrasonic modes (not complete) Wave types in and on solids

Particle vibration

Longitudinal

Parallel to wave propagation direction

Transverse (shear)

Perpendicular to wave propagation direction

Surface – creeping wave

Parallel to wave propagation direction

Surface – Rayleigh wave

Elliptical orbit - symmetrical mode

Surface – leaky Rayleigh wave

Wave guided along an interface

Plate – Lamb wave

Component perpendicular to surface (extensional wave)

Plate – Love wave

Parallel to plane layer, perpendicular to wave direction

Sezawa

Asymmetric mode

Figure 10-2: Antisymmetric (top) and symmetric Lamb wave 10.2.2

Ultrasonic Velocity For isotropic materials ultrasonic velocities c L (longitudinal) and cT (transverse) have a relation to the elastic constants Young modulus E, shear modulus G and Poisson ratio ν. 1−ν c2L = E ρ 1ν 1−2ν  c2T =

(10.1)

E 1 G = ρ 2 1ν ρ

(10.2)

For Rayleigh waves only approximations exist cR ≈c T

0.861.14 ν 1ν

(10.3)

Table 10-2: Acoustic constants (room temperature) Material

cL [m s-1]

cT [m s-1]

Z [106 kg m-2 s-1]

Aluminium

6'320

3'130

17

Copper

4'730

2'300

42

Nickel

5'630

2'960

52

Steel

5'920

3'230

45

PMMA

2'730

1'430

Aluminium oxide

9'000

5'500

Concrete

4'400

2'600

9.2

Water

1'480

-

1.5

Oil

1'740

-

1.5

Air

330

-

0.00033

3.2 32

181

The acoustic impedance Z is defined as Z=ρ c

(10.4)

Figure 10-3: Dispersion curves of symmetric (S) and antisymmetric (A) Lamb waves in a traction-free aluminium-plate, phase velocities (top) and group velocities (J.L. Rose: Ultrasonic Waves in Solid Media) Plate waves exist in many modes and all show dispersion; the velocity depends on the product of the frequency and the thickness of the plate (f d). The symmetric and the antisymmetric zero-order modes deserve special attention. They exist at all frequencies and can usually carry more energy than the higher order ones. They are also called extensional (S0) and flexural mode (A0). Phase velocities can only be calculated numerically, the relationship between phase and group velocities is cg =

c 2p c p−f d

10.2.3

dcp

(10.5)

d f d

Attenuation of Sound Waves When sound travels through a medium, its intensity diminishes with distance. In idealised materials, sound pressure (signal amplitude) is only reduced by the spreading of the wave (divergence). All natural materials, however, show an effect which further weakens the sound. This further weakening results from two basic causes, which are scattering (microscopic reflec tions in direction other than its original direction of propagation) and absorption (conversion of sound to other forms of energy). The combined effect of scattering and absorption is called attenuation. Attenuation of sound within a material itself is often not of intrinsic interest. However, natural properties and loading conditions can be related to attenuation. Attenuation often serves as a measurement tool that leads to the formation of theories to explain physical or chemical phenomenon, which decreases the ultrasonic intensity.

182

For scattering three domains are of interest (D dimension of the scattering particle): ▪

Rayleigh domain λ >> D, attenuation ~ f4;



Stochastic domain λ ≈ D (also called phase scattering), attenuation ~ f2;



Geometric domain λ << D (also called diffuse scattering), attenuation independent on frequency.

A decaying plane wave is expressed as A=A o e

−αx

(10.6)

where Ao is the amplitude of the propagating wave at an initial location and A the amplitude after the wave has travelled a distance x from that initial location. The quantity α is the attenu ation coefficient of the wave travelling in the x direction. The common unit is decibels per metre (dB m-1) and the correspondence to the wave pressure α=

p 20 log o x p

(10.7)

Quoted values of attenuation are often given for a single frequency, or an attenuation value averaged over many frequencies may be given. Also, the value of the attenuation coefficient for a given material is highly dependent on the way in which the material has been manufactured. Thus, quoted values of attenuation only give a rough indication of the attenuation and should not be automatically trusted. Generally, a reliable value of attenuation can only be obtained by determining the attenuation experimentally for the particular material being used. The determination of the attenuation dependence on frequency can be of used for a global characterisation of materials or compounds (grain size, matrix characterisation of composites, sedimentation, etc.). 10.2.4

Acoustic Waves at Interfaces If an acoustic wave meets an interface of two materials with different impedances (impedance mismatch) a part of the energy is reflected while the other part is transmitted. For perpendicu lar incidence the reflection coefficient R and transmission coefficient T in terms of pressure are defined as R= T=

Zt −Z i

(10.8)

ZiZt 2 Zt

(10.9)

ZiZt

Zi and Zt are the acoustic impedances for the incident and the transmitting material, respect ively. Clearly, the transmission coefficient is always positive, the reflection coefficient, however, can be positive or negative. A change of sign corresponds to a phase change of the reflected wave.

Water

Steel

Steel

Water

I T

R

R

T I

Figure 10-4: Incident (I), reflected (R), and transmitted (T) waves between water and steel and vice versa If the direction of incidence is not perpendicular to the interface, additional mode changes may appear if transverse waves exist. The corresponding angles are due to Snell law. sinαi sinα t

=

ci ct

(10.10)

183

Assuming that the velocity of longitudinal waves is higher than for shear waves for all materials (the corresponding angle wider), the following combinations can happen: ▪

An incident longitudinal wave generates a reflected longitudinal wave, a reflected shear wave, a refracted longitudinal wave and a refracted shear wave;



An incident shear wave generates a reflected longitudinal wave, a reflected shear wave, a refracted longitudinal wave and a refracted shear wave;



The refracted and/or reflected shear waves may not exist in the respective material;



the incident angle may be large enough that the reflected longitudinal wave, the refracted longitudinal wave, or the refracted shear wave do not exist, or any combination thereof;



Additional modes (Rayleigh waves) may also occur. T L

βL

βT

αL

L

Medium i (Zi) Medium t (Zt) L

γL T γT

Figure 10-5: Reflected and refracted waves for an incident longitudinal wave, α L = βL, general case (all modes exist), Zi > Zt

1

Angle for transverse waves 10 20 30

33.2

RLT

Reflection factor

0 RLL = RTT

-1 RTL

-2 -3 -4 -5 0

15

30

45

60

75

90

Angle for longitudinal waves Figure 10-6: Reflection coefficients of steel with a free surface, the respective angle for all longitudinal waves is given below, for shear waves above the graph; transverse waves of an incident angle of 33.2° are totally reflected with a coefficient of -1; incident longitudinal waves of nearly 90° result in series of transverse waves, as the sound beam will hit the surface over a long distance The reflection and the transmission coefficient also depend on the incident angle. For the simplest case of a free surface (no transmission), the corresponding coefficients can be calculated as follows (first index: incident mode, second index: reflected mode):

184

 R TT =R L L = 

cT cL cT cL

2 2

 sin2 αL sin2 α T −cos 2 α T

(10.11)

2 2

 sin2 αL sin2 α T cos 2 α T −4 sinα T

R TL = 

cT cL

2

2 RL T= 

cT cL

(10.12)

 sin2 α L sin2 α T cos2 2α T cT cL

2

 sin2 αL cos2 αT

(10.13)

2 2

 sin2 α L sin2 α T cos 2α T

For a free surface of steel, the total reflectance angle is 33.2°; the presence of air is not changing the situation very much. Especially for corner reflection situations where the portion that with a mode change is much stronger than the reflection without can result in misinterpretations. Such a situation is for steel an incident transverse angle of about 60°. At the first reflection, transverse waves are in total reflectance; at the second, the longitudinal signal is much larger than the transverse one. If the signal interpretation is made by amplitude comparison, the sizing will be much too small. Some techniques, like the LLT technique, use such an effect exclusively.

strong L

weak T T 60°

T Figure 10-7: 60° corner reflection for steel (total reflection for incidence angle 60°) If the reflection and transmission ratio is given in terms of power, then the relations are R P= TP=

Z i−Zt 2 Z iZt 2 4 Zi Zt 2

ZiZt 

(10.14) (10.15)

If the incident wave has such an angle that total reflection for transverse waves is fulfilled, Rayleigh waves are generated. If the angle fulfils the total reflection criterion for longitudinal waves the situation is more complicated. A strict interpretation of the Snell law would prohibit the existence of a longitudinal wave, but as it is the acoustic field consists of the main beam and side lobes, it can exist anyway. A creeping wave probe generates: ▪

Direct transverse wave;



Direct longitudinal wave, where the refraction angle relies on the relation between transducer size and wavelength (d/λ);



Creeping wave along the surface with the longitudinal wave velocity and an intensity that depends also on d/λ;



Further transverse wave, called a head wave, which probe index is shifted forward again relying on d/λ.

For slightly curved surfaces Rayleigh waves will follow the surface, while creeping waves will reenter the full material and form a common ultrasonic beam. A key characteristic of a creeping wave probe is the large d/λ ratio of about 15. It can reach the longitudinal angle of incidence up to approximately 85° with a high intensity of the creeping wave. Creeping waves are leaking into the base material; both waves will also leak into air or liquids, dependent on the base material. 185

liquid

L L

C

R

solid

T (sv)

L

H

T

Figure 10-8: Behaviour of Rayleigh and creeping waves (C .. creeping, H .. head, L .. longitudinal, R .. Rayleigh, T .. transverse) 10.2.5

Acoustic Waves at Interlayers If the thickness of intermediate layers is small enough the reflected wave at the top is interfering with the one from the bottom for respective wavelengths. The resulting signal can be used to calculate the interlayer thickness D using R=



2

K 1K

with

1 Z1 Z2 2 2πD f K=  −  sin   4 Z2 Z1 v2

(10.16)

Where Z1 corresponds to the surrounding material and Z 2 and v2 to the interlayer material. The formula is valid for an interlayer between two parts of the same material.

0.3 30

0.2

R

0.1 20

0 0 10

0.1

D[ mm ]

f [MHz]

0.2

Figure 10-9: Relation between interlayer thickness, frequency and reflection coefficient for steel vs. 82Au18Ni (Empa)

186

10 MHz

20 MHz

Figure 10-10: C-scan of a gold nickel solder to steel of a thickness of 120 μm (SEM, right) with 10 MHz (left) and 20 MHz, flaws are presented in a rather different way due to interferences of the echoes of the two interfaces (Empa) 10.2.6

Properties of the Sound Beam An acoustic source is not only emitting sound in one direction, but over a certain range. Due to the divergence, the diameter of the beam increases with the distance and the pressure per unit area decreases. The angle of the beam depends on the dimensions of the source and the materials properties. For sizing reasons, it is important to know the situation within the sound beam. It can be subdivided in the near field or Fresnel-zone and the far field or the Fraunhofer-zone (Joseph von Fraunhofer, 1787-1826). The length of the near field N can be calculated for circular sources with a diameter D by N=

D2 4λ

(10.17)

or square sources with a side length b by N=

1.37 b2 4λ

(10.18)

The width of the beam d where the pressure is not lower than 10% of the centre value at a dis tance s from the source in the far field and for a circular source is given by d=2.16

λs D

(10.19)

pr

D

N ps  s-1 Figure 10-11: Sound beam of an acoustic source (simplified, without side lobes) Due to interferences the pressure distribution in the near field is strongly dependent on the position, hence sizing is complicated. The pressure p s on the beam axis at a distance s from the source ps and with pN as the pressure at N is given by ps=

pN



π D2 2 2 sin  s −s 2 λ 4

(10.20)

Where pN denotes the pressure at exactly s = N – λ/4 (the last maximum). For reasons of simplification the pressure at near field length can be taken.

187

Normalised pressure [pN = 2]

2

4

1

3

0

2

-1

1

-2 0.0

0.5

1.0

Normalised distance [N]

1.5

0

Spherical wave

0

1

2

3

4

5

Normalised distance [N]

Figure 10-12: Acoustic pressure on the central axis In the far field (after about three N) the pressure can be approximated with a spherical wave ps=

p N π D2 2 4 λs

=

p N πN 2 s

(10.21)

In reality beside the pressure fluctuation along the axis there are also such fluctuations perpen dicular to it, resulting in side lobes.

0.1 N xp zp

N

0

Figure 10-13: Acoustic pressure distribution in the near field of a transducer, plane perpendicular to probe orientation, the fluctuations in depth are as visible as the side lobes of the probe (Empa) In practice the pressures are not known, but the echo heights H (corresponding to the maximum amplitude) on the screen of standard ultrasonic equipment can be determined and compared. p o Ho = p H

(10.22)

Echo heights are compared in the unit decibel [dB]. A gain V is needed to bring a second echo to the same height as a first one

188

V=20log

Ho

(10.23)

H

The letter V stands for the German word “Verstärkung” and is part of a system called AVG (Abstand – Verstärkung – Fehlergrösse) introduced by Krautkrämer. Besides the abbreviation DGS (distance – gain – size) AVG is sometimes also used in English (and French). The following explanations are valid for the far field only. If the distance to the back wall is doubled, a gain VB of 6 dB is necessary. Back wall in this context means, a reflector that is larger than the diameter of the sound beam. If this is not the case the sound beam is reflected only partially and the received sound pressure becomes smaller. For a disc shaped reflector (DSR) perpendicular to the sound path with a diameter DDSR follows pDSR =p FBH =p s

π D2DSR 4λ s

=

p N π DD DSR 2   2 4λ s

(10.24)

In this case the necessary gain for a double distance is 12 dB V DSR =V FBH =20 log

s2 s1

2



(10.25)

In practice such reflectors are simulated by flat bottom holes (FBH). The manufacturing of them, however, is not easy and the production of such reference blocks therefore costly. Sometimes also side-drilled holes (SDH) with a diameter D SDH are used. In this case the necessary gain for a doubled distance is 9 dB and V SDH=20 log

s2 32  s1

(10.26)

The conversion between the reflected pressure of a SDH and a FBH depends on frequency: DFBH =

2 λ2 DSDH s π2

1

4

(10.27)

or DSDH=

D 4FBH π2 2

2λ s

(10.28)

The perpendicular incidence onto a FBH is defined by one direction, while for a SDH the possible perpendicular incidents are defined by a plane. For a calibration on a SDH, therefore, the possible error is smaller. Spherical shaped reflectors show the same distance dependence as circular ones; however their relation on frequency is different p~

π DD k 16λ s2

(10.29)

where Dk is the diameter of the sphere. Such reflectors can be simulated by holes with spher ical bottom.

189

0

Gain [dB]

-10

Back wall

-20

SDH FBH

-30

-40 0

2

4

6

8

10

Normalised distance [N] Figure 10-14: Necessary gain for different reflectors For a given beam direction certain regions of an object may be located in the acoustic shadow due to the geometry of the object or a discontinuity in it.

10.3

Equipment

10.3.1

Instruments Ultrasonic instruments only contain mainly the electronic parts; frequency and most characteristics of the sound beam are solely determined by the transducer. Instruments include the ana logue-to-digital conversion and are mostly menu driven. Some differences can be seen at the screens; very bright versions are available, as well as transreflective ones that are especially suited it the instrument is often used outside in bright sunlight. For new instruments the screens are usually colour ones, as others are no longer available. Some instruments have a special design to be water proof or for the use in explosive surround ings (mining, chemical industry, etc.). The output of the instrument is a series of voltage pulses in the range of several 100 volts with a pulse repetition frequency that is usually in the kHz range. The maximum allowable voltage depends on the frequency of the transducer; for high frequency transducers the excitation voltage has to be reduced. Furthermore the spike direction should match the design of the transducer. Some instruments have a built in dialogue function to determine the connected probe automatically (if the probe is also equipped in such a way).

Figure 10-15: Ultrasonic instruments (GE IT, Staveley)

190

Figure 10-16: Equipments for the measurement of the wall thickness (GE IT, Panametrics-NDT)

Figure 10-17: High frequency (200 MHz) pulser receiver (Panametrics-NDT) 10.3.2

Cables and Connectors The inside of a cable is made of three major components: the conductor, the dielectric and the shield / braid. These components are then surrounded by an outer protective jacket. The conductor acts as the positive connection of the cable while the shield acts as the ground. The dielectric isolates the conductor from the shield. Most cables have one shielded / braided layer. However, to better prevent electrical interference from the environment, double shielded cables have an additional shielding / braided layer in contact with the other. The most common values for coaxial cables are 50 Ω, 75 Ω and 95 Ω. Note that the input impedance at a particular frequency may be quite different from the characteristic impedance of the cable due to the impedance of the source and load. In ultrasonics, on transmit the source is the pulser and the load is the transducer; on receive the source is the transducer and the load is the receiver. The complex impedance of the pulser and the transducer will reflect some of the electrical energy at each end of the cable. The amount of reflection is determined by the length of the cable, the frequency of the RF signal and the electric impedance of the cable and its termination. The effect of the cable is most practically determined by experimenting with shorter and longer cables, with cables of different impedance and by placing a 50 Ω feedthrough attenuator at the pulser / receiver jack. Such a feed-through attenuator suppresses the generation of echoes within the cable due to an impedance mismatch between cable and plug.

191

Figure 10-18: Influence of a change of the cable on frequency distribution (below) and R.F. representation; note the situations at the arrows (Empa) As connectors several different types are used: BNC, Microdot, TNC, UHF or Lemo, dependent on the company; adaptors are available. 10.3.3

Properties of the ultrasonic pulse The ultrasonic pulse is excited by a short electric pulse and vibrates with its eigenfrequency. Damping is necessary to reach a good spatial resolution and to allow a high pulse repetition frequency. Weakly damped transducers have a long pulse with a narrowband spectrum, while heavily damped transducers have a short pulse with a broadband spectrum, their energy out put, however, is smaller. Their main nominal frequency is given by the manufacturer, the frequency where the amplitude is 3 dB lower than the peak frequency is called cut-off frequency or frequency limit. The arithmetic mean of upper and lower cut-off frequency is called centre frequency. The bandwidth is the difference of the upper and the lower cut-off frequency and the relative bandwidth is its relation to the centre frequency. The wave front is the continuous virtual surface joining all points of a wave that have the same phase. The pulse shape represents the amplitude of a pulse as a function of time, where the pulse envelope including all the peaks of amplitude and time.

Figure 10-19: Long, narrowband pulse (left) and short, broadband pulse, the peak maximum may be positive or negative 10.3.4

Piezoelectric Transducers Most transducers use a piezoelectric element. When piezoelectric ceramics were introduced, they soon became the dominant material for transducers due to their properties and their ease of manufacture into a variety of shapes and sizes. The first piezo-ceramic in general use was barium titanate and that was followed during the 1960s by lead zirconate titanate (Pb(Zr,Ti)O3, PZT) compositions, which is now the most commonly employed ceramic for making transducers. For ferroelectric materials the piezoelectric effect takes place only below the Curie temperature, for barium titanate it is 110 °C, for PZT 320 °C. To compare different piezoelectric materials, it is popular to compare everything to quartz. A basic conflict is given through the fact that for the maximisation of the transmission other crystal properties are relevant than for the maximisation of the receiving properties. Sometimes it can be of use to choose one material for the transmitter and another material for the receiver.

192

The choice of the transducer material has also to include a consideration of the load (immersion medium, wedge or delay line material). The damping is done by a backing, usually a highly attenuating dense material that is used to control the vibration of the transducer by absorbing the energy radiation from the back face of the active element. When the acoustic impedance of the backing matches the acoustic imped ance of the active element, the result will be a heavily damped transducer that displays good range resolution. If there is a mismatch in acoustic impedance more sound energy will be reflected forward into the test material. The eigenfrequency f depends on the thickness d of the transducer crystal and the sound velo city v of the transducer material. Transducers with higher frequency are more complicated to produce and they are therefore more expensive. d=

λ 2

with

λ=

v f

(10.30)

Transducers can be used as transmitter or as sensor alternatively or as both of them intermittently. Probes with only one transducer are also called transceivers.

Figure 10-20: Various piezoelectric crystals (Staveley) The basic purpose of the transducer wear plate is to protect the transducer element from the testing environment. For immersion, angle beam and delay line transducers the wear plate has the additional purpose of an acoustic transformer between the high acoustic impedance of the active element and the water, the wedge or the delay line all of which are of lower acoustic impedance. This is accomplished by selecting a matching layer that is ¼ wavelengths thick and of the desired acoustic impedance. The choice of the wear surface thickness is based upon the idea of superposition that allows waves generated by the active element to be in phase with the wave reverberating in the matching layer. Not properly designed wear plates result in dis ruptions in the wave front. Piezo-composite materials usually have a structure called 1-3, where the piezoelectric rods are embedded in a polymer matrix by a dice-and-fill technique. The ceramic and the resin are chosen according to the characteristics required for the composite material. The geometry of the microstructure itself can be adapted. One of the characteristics of a 1-3 structure is that the percentage of the ceramic can be varied by modifying the size of the rods and their spa cing. The designation simply codes the type of composite: piezoelectric material hits the surface in one direction; the resin does so in three. The height of the ceramic rods – long compared to their lateral dimensions – favour their vibra tion according to the thickness mode to the detriment of the radial mode. This results in improved electro-acoustic efficiency that gives the sensor a high level of sensitivity and a high signal/noise ratio. In addition, the natural damping of composite materials allows a relative bandwidth of 60% to 90% to be obtained while retaining a very good level of sensitivity. The optimum size of the rods, not only their length, depends on frequency. The mechanical properties of the polymers are used to enable the piezo-composite materials to be shaped for focused transducers or such that are adapted to the surface.

193

Table 10-3: Various transducer materials, the values for transmission and receiving include piezoelectric coupling coefficients (approximate values) Material

Quartz

15.2

Rel. transm. efficiency 1.0

Rel. receiv. efficiency

1.0

Pulseecho efficiency

Remarks

1.0

inexpensive, rugged, not much used (any more)

1.5

excellent bandwidth, rugged, high temperatures possible, expensive

Lithium niobate LiNbO3

-

2.8

HLS, Hydrated lithium sulphate (Li2SO4) · H2O

11.2

6.9

PZT-4, Lead zirconate titanate

~ 33

65

0.24

15.3

narrow bandwidth, inexpensive

PZT-5A, Lead zirconate titanate

~ 33

70

0.21

14.7

narrow bandwidth, inexpensive

Cadmium sulphide CdS

-

2.3

Zinc oxide ZnO

-

3.3

Barium titanate BaTiO3

31.2

8.4

PMN, Lead metaniobate PbNb2O6

20.5

PVDF, polyvinylidene fluoride

32

0.54

-

-

1.42

-

dissolves in water

-

4.7

good bandwidth, 20 MHz to GHz, expensive

-

-

very narrow bandwidth, inexpensive

-

-

good bandwidth and sensitivity, expensive

9.3

excellent bandwidth, poor transmission, inexpensive

4.1

6.9

2.8 – 3.7

-

-

-

backing

Casting resin

5.2

-

-

-

backing

Tungsten/epoxy (200:100)

9.4

-

-

-

backing

Araldite casting resin

194

Z [106 kg m-2 s-1]

1.35

λ/2

Figure 10-21: Schematic representation of 1-3 piezo-composite structure (Imasonic) 10.3.5

Probes for Contact Testing Technique Normal or straight beam probes either have a hard face cover or are covered with a replaceable polymer membrane to protect the probes surface. The gap between this membrane and the transducer is filled with oil. A hard face cover is a wear plate made from a durable and corro sion resistant material in order to withstand the wear caused by use on materials such as steel. Typical frequencies are between 0.5 MHz and 10 MHz.

Figure 10-22: Ultrasonic fingertip probes with ceramic contact face (left) and normal probes with replaceable polymer membrane (Empa) Double transducer probes also exist, where one half is acting as pulser, the other half as receiver; they are easy to recognise, as they have a dual connector. Both transducers are oriented at a certain roof angle, so they are most effective at a certain depth. Their advantage is that failure echoes show no interferences with the main bang, therefore echoes near the surface can be sized more exactly and the thickness can be measured more exactly. On consequence of the dual element design is a sharply defined distance amplitude curve. In general a decrease in the roof angle or an increase in the transducer element size will result in a longer pseudo-focal distance and increase in useful range.

195

(A)

(B) Acoustic separation

(C)

Transducer Backing

Transducer R

T

Roof angle

Backing

Transducer λ/2 Wear plate

Time delay

Wedge

Figure 10-23: Ultrasonic transducers schematically: (A) normal beam probe, (B) double transducer probe, (C) angle probe Delay line transducers are single element longitudinal wave transducers used in conjunction with a replaceable delay line. The delay allows the element to stop vibrating before a return signal form a reflector can be received. When using a delay line transducer there will be multiple echoes from the end of the delay line and it is important to take this into account. The delay line may be made of a solid material or designed as a captive water column.

Figure 10-24: Double transducer probe (left) and delay line probe (Empa) Delay line transducers are especially useful if the attenuation has to be measured or if the sample under test is at elevated temperature. Some high temperature delay line options are not intended for continuous contact, they are meant for intermittent contact only. For special applications the transducers can be designed and excited in a way, that they emit transverse waves directly (Y-cut crystals). Because shear waves do not propagate in liquids, it is necessary to use a very viscous couplant when making measurements with these; the simplest couplant is honey.

Figure 10-25: Ultrasonic manual scanning (RTD) Angle probes usually consist of a transducer glued onto a PMMA wedge and a damping element. Often the value for the refractive angle for longitudinal waves is higher than the total reflection, so only a transverse wave exists in the object. In general the given number on the probe is valid for steel, for other materials it has to be recalculated. Available from stock are in general 38° (denomination 35), 45°, 60°, 70° and 77° (denomination 80). Transverse waves originated 196

like that are vertical polarised, as the longitudinal particle vibration direction within the wedge does not contain a horizontal component. Angle double transducer probes are mainly used for angular longitudinal waves and for creep waves.

Figure 10-26: Angle probes: wedge and probes separately (left), variable angle beam transducer, angle double transducer (from below) (Panametrics-NDT, IntelligeNDT) Contact probes need a couplant for the transfer of the waves from the probe into the object and back. Different agents can be used, but their type shall be compatible with the materials to be examined. Examples for liquid couplants are: ▪

Water, possibly containing an agent, i.e. wetting, anti-freeze, corrosion inhibitor;



Contact paste;



Oil;



Grease;



Cellulose paste containing water, etc.

Couplants for transverse waves are high viscosity liquids like honey, molasses or heavy fuel oil. The characteristic of the coupling media shall remain constant throughout the verification, calibration operations and the examination. It shall be suitable for the temperature range in which it will be used.

Figure 10-27: Couplants (Sonatest) After the examination is completed, the couplant shall be removed if its presence is liable to hinder subsequent operations, inspections, or use of the object. For some special applications, the couplant can be solid, like metal foils, rubber or low melting crystals. Especially in the last case, the probe is firmly attached to the sample.

197

Figure 10-28: Equipment for contact testing including a positioning sensor allowing imaging, here combined with a bubbler (GE IT) 10.3.6

Focusing Probes For special applications contact probes may also be focused on a point in the material. This is especially useful for cylindrical or toric interfaces; it may also be used for flat surfaces. Of spe cial importance are the aspherical focusing transducers following the Fermat concept (Pierre de Fermat, 1601-1665). The definition of a transducer is made by a calculation that defines a surface from which each point is at the same time of flight at the defect. The transducer is dedic ated to one particular set-up taking into account: ▪

Material to inspect;



Expected refracted angle;



Inspection depth or sound path;



Interface geometry.

Often the resulting shape is quite large in comparison to ordinary probes. For some applications they are also equipped with a soft material wedge to correct for irregularities of the interface.

Figure 10-29: Fermat transducer and its active area (Imasonic) 10.3.7

Wheel Probes A wheel probe – also called roller probe – is a device that couples ultrasonic energy to a test object through the rolling contact area of an oil-filled wire containing one or more radially emit ting transducers. Those are attached to the axle of the wheel. They are applied in pulse echo and through transmission technique. Since the tire material shows high attenuation they are mostly used in low frequency applications. Because the construction cannot prevent some spurious echoes, the achievable signal to noise ratio is worse than that of regular transducers. The implementation of phased array probes is also possible.

198

Figure 10-30: Wheel probes with single transducer (left) and with phased array transducer (Unicorn, NDT Solutions) 10.3.8

Equipment and Probes for Bubblers and Squirters For a lot of applications, mainly if they are automated, the application of the couplant is com bined with the ultrasonic investigation. In this case the couplant is usually water. Bubblers form a local puddle; therefore the technique is called flow gap coupling. Often the bubbler equipment can be combined with immersion transducers. For long distances between the probes and the object, squirters are used. As the sound path goes along the water jet, the flow has to be laminar, in order to keep the noise as low as possible. Squirters can be used in transmission, reflection and pitch and catch technique.

Figure 10-31: Examples for bubblers: equipments for the test of large horizontal areas (left), for testing of welds in all positions; and for manual scanning, in both cases several different probes can be built-in (GE IT, Panametrics-NDT)

Figure 10-32: Examples for squirters without and with water jets (USL, Vogt) The jet technique uses systems that couple the sound by a waterfall like path by nozzles close to the test object. A typical application is the inspection of electronic parts. 199

Figure 10-33: Jet transducer for the inspection of electronic parts (Sonoscan) 10.3.9

Equipment and Probes for Immersion Testing Immersion testing is used if the object can become immersed partly or in full, usually into water (or sometimes other liquids). The equipment, as far as it is immersed also has to be water tight. Typical frequencies are in the range between 1 MHz and 100 MHz. Immersion transducer offer three major advantages over contact transducers: ▪

Uniform coupling reduces sensitivity variations;



Reduction of scan time due to automated scanning;



Focusing of immersion transducers increase sensitivity to small reflectors.

Immersion transducers are available in three different configurations: unfocussed (“flat”), spherically (”point”) focussed and cylindrically (“line”) focussed. Focusing is achieved by either adding a lens or by curving the element itself. The latter usually have as active device a metallised foil of polyvinylidene fluoride (PVDF). The number on the probes designate the focal dis tance in water F (measured from the face of the transducer); for other materials it has to be recalculated with dW the water path, dM the material path, cW the sound velocity in water and c M the sound velocity in material dW =F−dM

cM cW

(10.31)

Figure 10-34: Immersion technique for round objects with a horizontal transducer (Empa) Usually the focal distance is normalised to the near field as FN =

F N

(10.32)

For unfocussed probes the last maximum occurs at a distance equivalent to the near field (F N = 1). By definition therefore, a transducer cannot be acoustically focused at a distance greater than its near field. Therefore for distant focus points, probes with (very) large diameters have to be used.

200

The starting and ending point of the focal zone are located where the on axis pulse-echo signal amplitude drops to -6 dB relative to the amplitude at the focal point. The length of the focal zone ℓF is given by 2

ℓ F=NF N

2 F 1+ N 2

(10.33)

Figure 10-35: Various focussed immersion probes, mid: line focus, right: polymer probes (Harisonic, Panametrics-NDT, Empa) Some immersion transducers can be combined with acoustic mirrors, typically 90° to make them side-looking, e.g. for the inspection of tubes or other limited access applications. A web based focus calculator can be found under: www.imasonic.com/Industry/IMDesign.php.

Figure 10-36: Acoustic mirrors (Panametrics-NDT)

201

Figure 10-37: Immersion scanner (left) and system for large components, usually operated with squirters (Panametrics-NDT, USL) 10.3.10 Equipment and Probes for Air Coupling If the sample under test should or cannot come into contact with any couplant, there is also the possibility for coupling the ultrasound over air. The coupling efficiency, however, in general is pretty low, due to low sound velocity and low density of air. Therefore probes have to be built massively to withstand the necessary large energies. To allow continuous waves usually dual probe systems are used; for pulse-echo technique burst signals are used. Frequencies generally are in the range of 20 to 200 kHz for high penetration for foam, rubber, tires, wood, concrete and natural stone, and from 0.5 to 1 MHz for high resolution for composites and honeycombs.

Figure 10-38: Air coupled arrangements for pitch and catch technique (left) and trough transmission technique (QMI) The arrangement of the probes can be one- or two-sided using techniques like through transmission, shear waves, plate waves or pseudo pulse echo technique with sound barrier between pulser and receiver. A possibility to enhance the coupling is to put the coupling path under higher pressure. An increase of the pressure by a factor p will increase the back wall echo by a factor p2, as at each interface, transducer - air and air - specimen, the transmission coefficient is higher. Due to the hydrodynamic paradox, such a compressed air shoe will float above the specimen (hovercraft principle). Special attention has to be paid to the noise and turbulences generated by the pressurised air flow. 10.3.11 Electromagnetic-acoustic Transducers (EMAT) EMATs can be used for the acoustic exciting of electric conductors only, as the sound wave is generated in the material itself by electrodynamic coupling. An AC coil induces in the object an eddy current field with a current density J. With an overlaid magnetic field B the Lorentz effect (Hendrik Lorentz, 1853-1928, NP 1902) with the force F results in 202

(10.34)

F∝ J xB

By selecting the magnetic field, longitudinal or transverse waves can be excited and received, as well as Lamb and Rayleigh waves, all without the use of a couplant. The special advantage of EMATs is that as well vertical polarised (shear vertical – sv) as well as horizontal polarised transverse (shear horizontal – sh) waves can be selected. The latter is especially of interest for the inspection of austenitic welds, as due to the dendritic structure the behaviour of sv waves is not predictable without knowing the exact crystal orientation. In the inference of material prop erties from precise velocity or attenuation measurements, use of EMATs can eliminate errors associated with couplant variation, particularly in contact measurements. In opposite to piezoelectric transducers the excitation frequency can be tuned. A design as phased array probes is possible.

(A)

(B)

(C)

N S (D)

(E)

Figure 10-39: Cross-sectional views of possible arrangements for EMATs: (A) spiral coil EMAT exciting radial polarised shear waves propagating normal to the surface, (B) tangential field EMAT for exciting longitudinal waves propagating normal to the surface, (C) normal field EMAT for exciting plane polarised shear waves propagating normal to the surface, (D) meander coil EMAT for exciting obliquely propagating vertically polarised (SV) waves, Rayleigh waves or guided plate waves (such as Lamb waves), (E) periodic permanent magnet EMAT for exciting grazing or obliquely propagating horizontally polarised (SH) or guided SH modes of plate waves

Figure 10-40: Examples of typical EMAT RF coils: spiral, butterfly, meander and meander focussed, coil dimensions typically 10 to 30 mm (Innerspec) 10.3.12 Magnetostrictive Transducers Magnetostriction was discovered 1842 by James Joule (1818-1889). He found that an iron beam produces strains in a magnetic field. These strains are called magnetostriction λ and the effect is known as Joule effect. The physical response of a ferromagnetic material is due to the presence of magnetic moments, and can be understood by considering the material as a collection of domains. When a material is not magnetised, the domains are randomly arranged. When the material is magnetised, the domains are oriented with their axes approximately parallel to one another. This effect can be optimised by controlling the ordering of the domains through alloy selection, thermal annealing, cold working, and magnetic field strength.

203

Table 10-4: Properties of magnetostrictive materials (values are dependant on source) Material

Curie Temperature [°C]

Magnetostriction λ [%]

Fe

770

-0.0014

Ni

360

-0.005

Fe65Ni45 (Permalloy)

440

0.0027

SmFe2

415

-0.234

Fe3O4 (Magnetite)

578

0.006

DyFe2

362

0.065

TbFe2 (Terfenol)

430

0.263

Tb0.3Dy0.7Fe2 (Terfenol-D)

380

0.16 – 0.24 (– 1)

TbZn

-93

0.45 – 0.55

TbDyZn

-23

0.5

Giant magnetostriction was discovered 1965. The most popular giant magnetostrictive material (GMM) at room temperature is Terfenol-D, or simply called GMM, that can show magnetostriction values of up to 1%, dependent on the orientation. The magnetic field of ferromagnetic materials is dependent on the applied strain due to the inverse magnetostrictive effect or Villari effect (Emilio Villari, 1836-1904). Therefore, monitoring the magnetic field during a fatigue experiment can be used to determine stress and strain fields. No external magnetic field needs to be applied, the entire magnetic effect is due solely to cyclic mechanical loading. The characteristics change qualitatively at the moment of appear ance of deformations. For ferromagnetic materials magnetostriction can be used to excite and receive sound waves. This technique is mainly used for guided waves. For wave generation it relies on the Joule Effect, for wave detection on the Villari Effect. Possible modes for cylindrical objects are longit udinal or torsional, for plates transverse (SH) or Lamb waves, dependent on the orientation of the transducer relative to the DC bias magnetic field. The waves are generated by an additional magnetic AC field over several cycles up to a few 100 kHz. As the excited wave propagates in both directions, a pair of exciters and receivers can be used for better location of the echoes or use a kind of phased array technique to produce a positive and negative interference while exciting. For non-insulated parts the possible range can go up to 100 m. The method can even be used for non-ferromagnetic materials applying a magnetostrictive foil onto the object.

Figure 10-41: Basic principle of magnetostrictive ultrasound (MKC Korea) 10.3.13 Phased Array Transducers Phased array transducers consist of several single transducers that can be combined electronic ally. This allows designing the sound field with respect to the angle, focus point and point origin of the field without mechanical motions. A phased array probe consists of a multitude of small transducer elements being activated at different times during transmission and reception.

204

Figure 10-42: Different ways to use phase array technique: scanning (left), focusing and deflection; the more left the signal is marked on the line, the latter is the excitement of the respective element (Imasonic, adapted) The arrangement of the transducers can be used for: ▪

Electronic scanning or sound beam shifting consists of moving a beam in space by activ ating different active apertures in turn, each one made up of several elements of a phased array probe, allows the mechanical axis to be replaced electronically;



Electronic or sound beam focusing is based on the use of electronic delays applied during transmission and reception along each of the channels of the probe; the delays have an effect similar to that of focusing lens and enable focusing to different depths;



Electronic beam steering or sound beam swivelling uses delay laws for electronic focusing, calculated to give the transmitted beam an angle on incidence which can be varied simply by modifying the delay law;



Paintbrush scanning excites all elements at the same time and the location and size of a possible reflector is calculated from the echo structure (often used for tube inspection);



Different techniques can be combined to resolve the desired application.

The probe design therefore can be different, according to the task: ▪

Linear arrays are made of a set of elements aligned along an axis to enable the beam to be moved, focussed and deflected along a plane;



Annular arrays are made of a set of elements of concentric rings, allow the beam to be focussed to different depth along an axis, the elements of each ring can be segmented in form of a daisy type;



Matrix array probes have an active area divided into two dimensions in different elements in form of a chequerboard or segmented rings (rho-theta probes), to allow the ultrasonic beam to be driven in 3D by combining electronic focusing and deflection;



Circular array probes made up of a set of elements in a circle to be directed either towards the interior, or towards the exterior, or along the axis of symmetry of a circle, combined with a mirror to give the beam the required angle of incidence; the arrangement may be in a full or partial circle;



Dual array probes have separate arrays for transmitting and receiving.

Using an appropriate wedge, phased array probes can also be used to produce transverse waves within the object.

205

Figure 10-43: Flat and circular shaped linear phase array probes (GE IT)

Figure 10-44: Fermat phased array transducer with segmented annular array (left) and convex annular array for rotor forged disk inspection from internal bore (Imasonic) When using a phased array transducer, delay laws are applied to each channel to generate a beam with a given refraction angle and focal distance. The ultrasonic beam is generated by the constructive interference of each transducer element’s contribution in the desired direction. In some cases, this interference can also be constructive in other directions. These lobes of energy emitted outside the electronically driven direction are called grating lobes. These energy lobes can interact with the part to be inspected in the same way as the main beam, and thus generate echoes causing interference to the inspection. For electronic deflection parasitic replications of the main beam caused by spatial undersampling may appear. The angle of the position of such grating lobes in relation to the main beam is given by sinθ k=

kλ −sinθ p

(10.35)

where θ is the refracted angle of the main beam, θ k the refracted angle of the grating lobe k (k = integer), p the inter-element pitch of a linear transducer and λ the wavelength of the medium under consideration. From the above formula, the following general rule can be obtained: ▪

If p ≤ λ/2, then no grating lobe is generated whatever the angle of the main beam;



If p > λ, then there is always at least one grating lobe generated whatever the angle of the main beam;



Between these two values, the grating lobes appear progressively according to the angle of the main beam.

Figure 10-45: No grating lobe for p ≤ λ/2 (left), appearance of a grating lobe (shadow on the left) for p > λ even with lower refraction angle of the main beam (Imasonic) Conventional piezoelectric arrays are usually fabricated using a “dice and fill” approach; the piezoelectric material is separated by mechanical dicing and a polymer is infiltrated and cured 206

within the slots. The limits are approximately 100 μm for the active element and 50 μm for the slot, resulting in a pitch of 150 μm. In order to obtain a well collimated acoustic beam, the ele ment pitch must be less than half of the wavelength. For slow materials (e.g. tissue) the max imum frequency, therefore, is limited. 10.3.14 Capacitive Micro-Machined Ultrasonic Transducers (CMUT) The idea of producing capacitive ultrasonic transducers is as old as the early piezoelectric transducers. The necessary electric field, however, is very large (10 6 V cm-1). The possibilities of micro-machining allow two things: The production can be done using standard silicon integrated circuit technology, making it possible to fabricate large arrays using simple photo-lithography and the dimensions can be made smaller compared to standard piezoelectric arrays, allowing higher frequencies. The basic building block of a CMUT is a capacitor cell with a top and bottom electrode and a membrane, the thickness is typically 1 μm or less. A single element in the array consists of many small capacitor cells connected in parallel. For transmission the membrane is driven in flexural vibration by the electrostatic force exerted between the membrane electrode and the back plate; for reception, the membrane vibration excited by the impinging acoustic wave is converted to electrical signals.

Figure 10-46: CMUT array with six rows with a width of 245 μm and a gap of 27 μm (CNR) The resonance frequency of the capacitor is essentially determined by the lateral dimensions, by the thickness and by the tensile stress in the membrane. The bandwidth is typically above 100% (compared to maximum 80% for piezoelectric transducers), allowing short pulses and a good depth resolution. Such technology sounds promising also with respect that arrays of 1'000 x 1'000 elements are in discussion. 10.3.15 Laser Ultrasonics An excitation as well as the sensing of acoustic waves in solids can be made by lasers. In both cases no physical contact is necessary and it can be performed over larger distances and higher objects surface temperatures (up to 1'200 °C are reported). When exciting, the object is heated for a short time locally, thus implying dilatations and resulting broadband sound waves. This is especially useful for the excitation of plate waves, as using adequate filter masks the heating pattern can be chosen and tuned so only the desired modes are resulting. A free heat pulse will result in a lot of different modes with their common disper sion. If the laser energy is enhanced, the surface temperature will reach the boiling point and some ablation will result. In this case mainly longitudinal waves in the direction of the laser are induced. This method is often used for in-line wall thickness measurements. A production speed up to at least 5 m/s is possible. For the detection various methods exist; some measure directly the deformation of the surface due to the ultrasonic wave by the use of interferometres, others use the effect of a change in the air's refraction by the ultrasonic waves. This method is usually called gas-coupled laser acoustic detection (GCLAD).

207

10.3.16 Tone Burst Generators Tone burst generators are used in high power (several kW) ultrasonic applications, like for the inspection of highly attenuative materials or in combination with inefficient transducers, such as EMATs. The receiver is often operated by the super heterodyne principle; a signal at variable frequency is converted to a fixed lower frequency, the intermediate frequency, before detec tion. Heterodyne receivers “mix” all of the incoming signals with an internally generated waveform called the local oscillator and suffer from poor selectivity. The super heterodyne receiver prin ciple overcomes certain limitations of previous receiver designs. 10.3.17 Impact-Echo Equipment For the inspection of concrete and masonry the input energy of a standard piezoelectric transducer is much too low. In this case the impact is made with a hardened steel ball as impactor, generating low-frequency stress waves of up to about 100 kHz. For wave speed measurements a dual transducer set-up is used. The sensing part is usually based on common piezoelectric technology; the data evaluation corresponds to frequency analysis, especially the number and distribution of peaks in the spectra. If flaws are present, these patterns are disrupted and changed, in ways that provide qualitative or quantitative information about the existence and location of the flaws. If necessary, testing device and computer can communicate wireless.

Figure 10-47: Impact-echo equipment: transducer unit (cylindrical device), steel ball impactors (in front of the laptop), A/D converter and laptop and dual pistol grip transducer (right) for wave speed measurements (Impact-Echo) 10.3.18 Dry-Point-Contact Probes Dry-point-contact probes operate similar to impact-echo systems. Every single element has a independent spring load allowing to work on uneven surfaces. The activation of the elements can be done phased array in longitudinal and shear mode. Usual frequencies are between 50 and 100 kHz for concrete and wood testing.

208

Figure 10-48: 6 x 4 dry-point-contact matrix sensor (Acsys)

10.4

Ultrasonic Techniques

10.4.1

Pulse Echo Technique The pulse echo technique utilises the reflected or diffracted signal from any interface of interest within the object under examination. This signal is characterised by its amplitude and position along the time base; the latter related to the distance between the reflector and the probe. The location of the reflector is determined from the knowledge of its distance, the direction of sound propagation and the position of the probe. The amplitude is highly dependent on the reflector (shape, geometry, surface conditions, acoustic impedance, etc.). In some cases the simple knowledge of the position of the probe and the distance, sometimes only the presence of an echo satisfy the needs. This is the case when sheets are examined for delaminations. In most cases, however, it is recommended that the signal amplitude be measured by comparison with either: ▪

Distance amplitude correction curve (DAC) or a series thereof, obtained by using artificial reflectors within one or more reference blocks;



Equivalent reflector diagram (distance – gain – size DGS, AVG);



Echoes from suitable notches;



Echoes from large planar reflectors perpendicular to the acoustic axis (usually back wall echoes).

Figure 10-49: Ultrasonic reflection and reflector characteristics In order to obtain further information about the shape and size of reflectors, other techniques may be used. Such techniques are based e.g. on variations in signal amplitude with movement of the probe, measurement of sound path, or frequency analysis. If possible, the refracted angle should be chosen in such a way that the reflected amplitude is maximised. This often implies that the sound beam has to be reflected at the back wall or other walls of the object to get the best direction. This is called indirect scanning. A typical applica 209

tion is the examination of welds (lack of fusion at X- or V-type weld seams). The distance an angular probe has to be moved to cover the full distance is called the skip distance.

indirect

direct

T

T T

T

L L

skip distance Figure 10-50: Direct and indirect angle beam testing (left), tandem and LLT examination, the skip distance or full skip is the distance measured on the surface of the beam index of an angle probe and the point at which the beam axis impinges on the surface

Figure 10-51: Examination of the continuous electron beam welds of a superconductive component for CERN's Compact Muon Solenoid (CMS) detector with immersion phased array ultrasound in pulsed echo technique (Empa) 10.4.2

Tandem and LLT-Examination Tandem and LLT examination are special pulse echo techniques used for the investigation for discontinuities perpendicular to the surface, especially for vertical welds. Both examination techniques use separate transmitter and receiver and in both cases the range that is examined relies on the position of the probes relative to the weld and relative to each other. While tandem examination uses shear waves only, LLT-examination uses a mode change from longitudinal to shear waves at the second reflection. The latter method can be superior for those angles where the shear-shear reflection coefficient becomes minor. Further possibilities are using mode conversions and neighbour echoes.

10.4.3

Transmission Technique Transmission technique is based on measuring the signal attenuation after the passage of an ultrasonic wave through the examination object. This technique is used: ▪

For flat products (plates or sheets);



Where the shape, dimensions or orientation of possible imperfections are unfavourable for direct reflection;



In materials with high attenuation;



In thin products.

Transmission can be done with normal or angle beam probe, with one single probe or with a transmitter and a receiver, with longitudinal or shear waves, with amplitude or time of flight measurement and in contact (couplant or squirter), in immersion technique or by applying a wheel probe. For some combinations the use of continuous waves is possible. The decrease in amplitude of the transmitted signal can be used to indicate the presence of a discontinuity located in the sound path, or to indicate material attenuation. In addition the pos210

ition of the transmitted signal along the time base of the instrument can be used to indicate material thickness. The technique requires that the geometry of the object under examination and access to its surfaces allow the transmitting and receiving probes to be positioned such that their beam axes are coincident, either with or without intermediate reflection from a surface of the object. The technique is particularly sensitive to variations in probe coupling and misangulation due to surface irregularities, since these coefficients also cause a marked reduction in transmitted sig nal amplitude. To improve the uniformity of coupling immersion or squirter scanning is most frequently used. Dressing of the surface to improve coupling uniformity can be necessary, especially for contact scanning. When using separate transmitting and receiving probes and/or a reflecting object at the opposite side of the object to be examined (mirror), their positions in relation to each other are also critical. The use of a mirror can be necessary if the reflection coefficient of the object material against the immersion liquid is minor. Furthermore, this technique is somewhat compensating for misalignments between the equipment build-up and the object. (A)

(B)

Acoustic mirror Figure 10-52: Examples for transmission technique: angle beam contact (left) with one probe utilising corner reflection, or two probes, double transmission (B) and mirror transmission (A) in immersion 10.4.4

Pitch and Catch Technique (Double Probe Technique) Pitch and catch is an ultrasonic examination technique involving the use of two probes. Both of the probes can be used of transmitter or receiver. Some variations of transmission technique are done this way. Here the distance between the transmitting and the receiving probe are defined by the geometry. Pitch and catch is also a very often applied technique for the examination of plate or surface waves. In this case the relative position of the receiver is no longer determined by the geometry; the wave can be caught at an arbitrary position. Most possibilities of coupling are pos sible, however, if the boundary conditions along the travel path of the sound wave is identical to the catching position (air coupling, immersion), the amount of refracted energy of the leaky waves to the second medium is the same all over and therefore relatively small at the catching position. (A)

L (B)

L

LL L

LRL

R

Figure 10-53: Pitch and catch technique: high amplitude with squirter coupling (A), low amplitude with air coupling (B), LL and LRL signal in focused immersion scanning The pitch and catch effect also happens to a remarkable amount at immersion technique with fairly focused probes. Beneath a direct surface echo (LL) a second echo with changed phase can be observed as the result of a double mode change longitudinal – Rayleigh and back. For such an arrangement this also means, that reflectors directly under the surface cannot become

211

detected properly. Such leaky Rayleigh waves are only possible for certain angles. On the other hand this mode change is often used for very high frequency ultrasonic microscopes (up to 2 GHz), where only surface effects shall be detected. The measurement or comparison of the Rayleigh wave velocity results in much more detailed information about the local elastic properties than changes in the acoustic impedance. The region of interest can be as small as < 10 µm and dependent on the experimental conditions the accuracy is 1 to 10%.

Figure 10-54: Arrangement for pitch and catch technique in immersion, the additional (blue) sensors mounted on the probes are used to measure the angular position (Empa) 10.4.5

Guided Waves

Figure 10-55: Units for the inspection of tubes for corrosion (SwRI, TWI) Testing with guided waves or also called long range ultrasonic testing (LRUT) is often done in combination with magnetostrictive or EMAT transducers. It can be applied to plates or construction elements with a circular shape like ropes or pipes. It ranges up to 100 metres and with a time-of-flight analysis, the position of reflectors can be determined. All elements like distance holders, tube dividers, etc. will also reflect a part of the acoustic energy. The same tech nique is also used for the inspection of rails to detect shelling (head checks).

212

Figure 10-56: LRUT applications for suspension cables for bridges and rails (MKC Korea, Tisec) 10.4.6

Image Presentations As in microwave testing, there are basically three types of scanning and presentation, derived thereof are some more. An A-scan presentation is the display of the ultrasonic signal in which the X-axis represents the time and the Y-axis the amplitude. This is the usual presentation type on the screen of ultrasonic instruments. For an A-scan presentation the probe is not moving. A B-scan presentation is an image of the results of an ultrasonic examination showing a cross section of the test object perpendicular to the scanning surface and parallel to a reference dir ection. The cross section will normally be the plane through which the individual A-scans have been collected. As one axis represents the time, an object with varying ultrasonic velocities will be displayed distorted. A C-scan presentation is an image as result of an ultrasonic examination showing a cross section of the test object parallel to the scanning surface.

1 mm = 0.7 µs

10 mm

Figure 10-57: Example of a B-scan presentation showing a delamination in CFRP plate of 1 mm thickness, top: entrance echo, mid echo of delamination, bottom: back wall echo, note that horizontal and vertical scale are not identical (Empa) Ultrasonic images known from medical examination usually are B-scan type presentations. Generally they are acquired using an array probe. The usual frequency range is like for materials testing a few MHz.

213

Figure 10-58: Medical application of ultrasound: kidney (left) and foetal spine (Siemens) Derived from the types above some more kinds of presentation exist. A D-scan presentation is an image resulting of an ultrasonic examination showing a cross section of the test object perpendicular to the scanning surface and perpendicular to the projection of the beam axis on the scanning surface. Instead of the amplitude the values represent the times of flight. It will normally be perpendicular to the B-scan. An S-scan or sectorial scan image represents a two-dimensional cross-sectional view derived from a series of A-scans that have been plotted with respect to time delay and refracted angle. The horizontal axis corresponds to test piece width, and the vertical axis to depth. The sound beam sweeps through a series of angles to generate an approximately cone-shaped cross-sectional image. A P-scan presentation is a projection view of several B- or C-scans. An F-scan presentation is a modified C-scan; values of a certain feature, e.g. centre frequency are recorded and displayed instead of the amplitude. A volume-scan presentation is a three-dimensional (spatial) representation of the inspected volume. At each inspection point of the scanning surface a complete A-scan has to be recorded.

5

3

2

1 mm

Figure 10-59: Example of a C-scan presentation of a NbTi superconductive flat cable co-extruded into aluminium, top: good bonding, mid: poor bonding, bottom: reference flat bottom holes (Empa)

214

360° = 365 mm

290 mm Figure 10-60: Example of a D-scan presentation of the wall thickness of a compressed air cylinder in winding off view (Empa)

Figure 10-61: Example of an S-Scan of a block with three SDH (Olympus)

Figure 10-62: Ultrasonic images of an electronic device; C-scan (left) and peak-frequency as Fscan showing some different features (Sonix) Finally the information of a whole volume can be used to generate three dimensional views. In medicine surface rendering is an often used tool.

215

Figure 10-63: Ultrasonic surface rendering of a foetal face, 28 weeks (Siemens) For dispersive waves a possible data presentation is in the form of a Wigner-Ville distribution, where the frequency is plotted versus time. Several possibilities of calculation for this spectral analysis exist. The origin is a fully digitised A-scan from which at all times the frequency spec trum is calculated.

Figure 10-64: Frequency vs. time plot of surface waves of a tube (SwRI)

10.5

Characterisation and Sizing of Discontinuities

10.5.1

Techniques for the Classification of Discontinuity Shape The basic discontinuity types and shapes which possibly may be distinguished are as follows: ▪

Point: spherical or planar;



Elongated: cylindrical or planar;



Large: volumetric, smooth planar or rough planar;



Multiple: spherical or planar.

The echo dynamic pattern of a discontinuity is the change in shape and amplitude of its echo when an ultrasonic beam is traversed across it. The observed pattern is a function of the shape and size of the discontinuity, the probe in use and the scanning direction and angle. The term directional reflectivity is used to describe the variation in echo amplitude from a dis continuity in relation to the angle at which the ultrasonic beam is incident upon it. Discontinuities which show relatively constant echo heights over a wide range of incident angels are said to have a low directional reflectivity and vice versa. The echo height from a discontinuity depends upon its size, orientation and surface contour. By measuring the echo height from different dir ections and angles it is possible to obtain information about these characteristics. For this kind of characterisation some knowledge about possible flaws and their respective position, size and characteristic is necessary. 216

10.5.2

Echo Height Evaluation Techniques using the echo height as a reference compare the height of the reflected echo with echoes from known artificial reflectors. Therefore, a real sizing is not possible, but the results give a good comparison and show a good reproducibility. Usual reference reflectors are the back wall, flat bottom holes, side-drilled holes, or notches. Three basic techniques exist: ▪

Single reflector technique: a single reference reflector may be used when evaluating echoes occurring within the same range of sound path distance;



Distance amplitude correction (DAC) technique: the echo heights from a series of identical reflectors at different sound path lengths in suitable reference blocks are used;



Distance – gain – size (DGS)-technique, also called AVG-technique: a series of theoretically derived curves relating the sound path length, the equipment gain and the size of a disc-shaped reflector perpendicular to the beam axis are used.

A DAC reference block shall be either a general purpose block of uniform low attenuation and specified surface finish, or a block of the same acoustic properties, surface finish, shape and curvature as the test object. Unless the test sensitivity is set on a test block which is acoustically representative of the test object, a transfer correction shall be determined and applied if necessary, when setting the test sensitivity or measuring the echo height of any discontinuity. The transfer correction is made up of two parameters: one due to coupling losses at the contact surface and independent of sound path length and the other due to material attenuation and dependent on sound path length. Two methods to determine the transfer correction are common: a simple fixed path length method where compensation is made for coupling loss and for attenuation at the maximum sound path length only; and a comparative method where full compensation is made for both parameters. The result is a plot with the time base as one axis and the screen height of the ultrasonic instrument as the other one. The line corresponds to one type of reference reflector with a cer tain size. For practical reasons the screen height should be used from 20% FSH (full screen height) to 80% FSH, if necessary the line has to be split with a declaration of the additional gain. Each echo can be referenced in position and direction of the probe, time base reading and the necessary change of the gain in dB to reach the same echo height as the reference graph. In the general DGS diagram, distance and reflector size are normalised. Therefore, it is inde pendent of probe size and frequency. It shows distance A as multiples of the near filed length of the probe, reflector size G as multiples of the probe element diameter. s N D G= f D A=

(10.36) (10.37)

Figure 10-65: Screen of ultrasonic instrument showing a split DAC From this general DGS diagram specific DGS diagrams, for common types of probes, are derived for steel which allow the direct reading of equivalent reflector size without calculation. Specific overlays that can be mounted on the screen or program modules that can be blended

217

into the screen electronically are available. Note that the relationship between the differences in gain and sound path in the DGS diagram is logarithmic, whereas the corresponding scaling on the screen is linear.

Figure 10-66: General DGS diagram The recording gain Vr, at which scanning shall be carried out, is calculated from V r= V j+ ΔV+ΔVk +ΔVt

(10.38)

where: ▪

Vj is the gain setting required to set the echo from a reference reflector to a given refer ence height in the screen;



ΔV is the difference in gain between the DGS curve corresponding to the minimum equivalent disc-shaped reflector (i.e. the recording level), measured at the maximum sound path length and the reference reflector, measured at its sound path length;



ΔVk is a correction coefficient when using a concave reference reflector (e.g. using the calibration block No. 1 or No. 2), this value should be given by the probe producer;



ΔVt is the transfer correction.

For materials other than steel the specific DGS diagram can be approximately calculated from the general DGS diagram. The main differences of the DGS method compared with the DAC method are:

218



DGS can use standard calibration blocks and therefore there is no necessity of producing separate reference blocks for different flaw sizes;



DGS allows calculating back every echo to a corresponding size of a disc shaped reflector (bearing in mind to correct for the sound path), the DAC method is applicable for other reference reflectors also;



Echo height evaluation using the DGS method is only applicable if contouring of the probe shoe is not required, that means, for convex surfaces with a diameter of the test object being above ten times the relevant dimensions of the probe, and not for concave surfaces, unless adequate coupling can be achieved due to very large radii of curvature.

Figure 10-67: Example for a specific DGS diagram for an angle beam probe on steel 10.5.3

Probe Movement Sizing Techniques The fixed level amplitude technique measures the dimension of a discontinuity over which the echo is equal to or greater than an agreed amplitude assessment level. The amplitude level may be related to a DGS curve or may be at some dB level in relation to a DAC curve; usual numbers are 6, 12 or 20 dB. To make a measurement the beam is scanned over the discontinuity and the probe position and beam range, at which the echo has fallen to the assessment level, is noted. Application and limitations are as follows: ▪

Measured size depends on the amplitude assessment level;



Technique is simply to apply and gives highly reproducible values;



Technique may be applied to large or small discontinuities but, in the latter case, the measured length is more closely related to the beam width than to the discontinuity size;



Assessment level must be set equal to or below the amplitude level at which a discontinuity of infinite length is acceptable.

The 6 dB drop from maximum technique the amplitude assessment level is 6 dB below the maximum echo height observed at any position along the flaw, rather than a constant, predetermined level as used in the previous technique. That means that a scan has to be done twice, first to find the maximum amplitude and then to find the positions of the drop. This technique is only applicable if the discontinuity to be measured is at least equal to the 6 dB beam width at the relevant sound path range and it not reliable for irregular discontinuities. Instead of 6 dB also values of 12 dB and 20 dB are common. Instead of the maximum echo height of the over all indication also the echo height local to each end of the discontinuity can be taken. This 6 dB drop tip location technique is superior if echo variations occur. The drop to noise level technique measures the dimensions of a discontinuity over which the echo can be observed above the background level. This technique is not very reproducible; however, it is relatively simple to apply and does not require a particular amplitude level to be set. It gives a conservative size measurement, especially where other techniques may carry the risk of undersizing a particular discontinuity. 10.5.4

Time-of-Flight Diffraction (TOFD) Technique The probes used in TOFD are generally angled compressive wave probes having a very wide beam spread. Since the distance 2 S between them is kept constant, two reference echoes, one due to the direct transmission between the probes (lateral wave) and the other due to reflection from the back wall, will be obtained in addition to the diffracted echoes. If the source of

219

diffraction lies at an offset of y between the two probes, the depth d of the emitting crack tip can be calculated from c t= √ d 2 +(S−y)2 +√ d2 +(S+y)2

(10.39)

and in the simplest case, where the discontinuity lies mid-way between the probes c t=2 √ d2+S2

(10.40)

where t is the time of flight between the transmitting to the receiving probe via the crack tip. In this case the time of flight inside the ultrasonic probes has to be subtracted before the calculation of the depth is made. Failure to do so will result in grave errors in the calculated depth. To avoid the errors that may arrive from probe delay estimations, commonly reference is made to the lateral wave response, that is the depth of an indication is calculated from the time of flight difference Δt between the lateral wave and the diffracted pulse. d=

1 √(Δt c)2+4 Δtc S 2

(10.41)

With that much greater sizing accuracies are possible, particularly for larger discontinuities, than can be obtained using methods based on discontinuity echo amplitude. However, in order to interpret the TOFD data, it is desirable to display the observed signals as a B-scan. The dis advantage of the technique is that the presence of two single sources may be misleadingly interpreted as a crack. 2S T d1

R

(A) (B)

d2 (C) (D) mode converted signals

(A)

(B)

(C)

(D)

Figure 10-68: TOFD principle with (A) lateral wave, (B) and (C) crack tip echoes, (D) back wall echo; diffracted waves (B) and (C) are phase-shifted; these four echoes are followed by mode converted signals, which usually are not analysable

220

Figure 10-69: TOFD scanner and B-scan data presentation of a weld inspection; lopen: incomplete penetration, lof: lack of fusion, mode converted echoes are caused by shear wave (US Ultratek, AIS) 10.5.5

Synthetic Aperture Focusing Technique (SAFT) For SAFT reconstruction the same algorithms as for microwaves are applied. For ultrasonics, however, this technique is of less importance, as the focusing of the ultrasonic beam is often possible.

10.6

Aspects of Quality Assurance

10.6.1

Calibration Blocks and Calibration For the calibration of the ultrasonic equipment two official calibration blocks exist. Both consist of ferritic steel and must show a defined ultrasonic velocity (longitudinal and transverse) and low attenuation. Certain characteristics may be adapted. Especially if the thickness of the calib ration block (25 mm for No. 1 EN ISO 2400 and 12.5 mm for No. 2 EN ISO 7963, respectively) for the intended probe is not appropriate, it may be enlarged. For all other applications, the user has to define its own calibration block. Calibration is done before testing, after each pause and after testing and in defined time intervals during the examination. The repetition of the calibration process is necessary due to possible drifts.

100 mm

200 mm

Polymer cylinder Ø 50 mm

Ø 3 mm 75 mm

Ø 5 mm Figure 10-70: Calibration block No. 1 (top, EN ISO 2400) and calibration block No. 2 (EN ISO 7963)

221

10.6.2

Characterisation and Verification of Ultrasonic Examination Equipment The equipment in use has to be checked upon a regular time base. Extended tests have to be carried out usually once per year or after repair. Daily tests concern physical state, the probe index and the beam angle, the latter two for angle probes only. Weekly checks concern the lin earity of time base and of equipment gain of the ultrasonic instrument, the sensitivity and sig nal/noise ratio, as well as pulse duration. The test of the physical state contents a visual inspection of the outside of the ultrasonic instrument, probes, cables and calibration block for physical damage or wear which could influence the system's current operation or future reliability. In particular the face of the probe has to be examined upon damage or wear as well as the stability of electrical contacts. The probe checks are done using a calibration block. The main reason is to guarantee that the calculated position of the reflector's position corresponds to the real one. The checks to perform weekly are relatively simple tests as well, also to be carried out by the operator.

10.6.3

Probe and Sound Field Characterisation The determination of the probe index and the beam angle can be done separately or combined. These checks are important, as the characteristic of the probe can change due to usage and to the roughness of the scanning surface during an examination. For a combined determination a reference block with at least three and preferably four or more side-drilled holes is used.

t1 t2

a2

a3

a4

Depth t

a1

t3 x Shortened projection a Figure 10-71: The slope of the line of shortened projection vs. depth corresponds to the beam angle, the probe index x to the zero crossing of that line Similarly a possible skew angle (sidewards) can be determined. The common way to characterise the complete sound field of ultrasonic probes is the use of electrodynamic sensors as receivers. Such probes use the EMAT principle, therefore measurements are only possible on solids that are electric conductors. Usual ultrasonic instruments can be operated with separate transmitting and receiving channels; the signal of such an electrodynamic probe, however, is small and pre amplifying is necessary. Sound field characterisation can be necessary for high precision localisation of reflectors and it is especially useful for the determination of the effective behaviour of phased array probes.

222

Figure 10-72: Pre amplifier and electrodynamic sensors; longitudinal with coil axis perpendicular (left) and transverse with coil axis parallel to block surface (Empa) 10.6.4

Sound Field Simulation As the real form of a sound field, especially for advanced techniques (like phased array) several software concepts are available to simulate the behaviour of the sound field depending on time and amplitude and its interaction with a flaw. An often used one is CIVA (www-civa.cea.fr). On the other hand it is also possible to simulate such effects with photoelastic visualisation sys tems used on glass objects.

Figure 10-73: Example of a photoelastic visualisation system (Ecliptic Scientific)

10.7

Procedure and Record

10.7.1

Procedure The NDT procedure usually has to define the following technical aspects:

10.7.2



Examination zones and scanning plan;



Inspection and characterisation technique and class;



Special surface preparation;



Coupling medium;



Examination equipment (probe sizes and wave types, angles, frequencies, focal lengths, etc.);



Scanning direction, surface speed, overlap, etc.



Characterisation of imperfections, requirements for signal processing and signal evaluation;



Calibration blocks and verification intervals for the instrument and the probes;



Cleaning and conservation.

Record The record should contain the following information: ▪

Description of the equipment used, including serial numbers and firmware version of instrument and probes, etc.; 223



Identification of the calibration blocks used;



Instrument settings (gain, scale, filters, etc.);



Coupling;



Reference and list of incident directions;



Method if interpretation;



A technical sheet (or equivalent) in cases where the examination procedure allows for a variation of the method of examination, equipment or set-up.

If special calibration blocks were used, they have to be stored in conditions that they may not alter.

224

Figure 10-74: Example of an UT test record for forged products (Empa)

10.8

Special and Advanced Techniques

10.8.1

Special Mode Conversion Applications Using longitudinal double transducer probes with several mode conversions and neighbour echo effects, among them creeping wave generation can be used. Such probes are often found under their German designation SEL (Sender Empfänger Longitudinal). Usually the two crystals are oriented parallel, in ADEPT probes they are oriented in a row. Such applications are often used for the investigation of austenitic welds. The development of such techniques needs detailed knowledge on the structure of sound fields.

225

Figure 10-75: Equivalent wave propagation (www.ndt.net/article/pow1297/schmid/schmid.htm) 10.8.2

Doppler Applications If the ultrasonic velocity is known, the length of the sound path can be determined relatively easy. If the transmitter and the receiver, or the probe and the reflector have a relative move ment towards each other, a frequency shift is to be observed. This can be used to determine the relative velocity. For a moving reflector, the relation is given by f=f o

c o+c

(10.42)

c o−c

and ∣f−f o∣=Δf ≈2

c f co o

(10.43)

where c is the velocity of the reflector and c o the sound velocity of the coupling media, fo the original and f the received frequency. Doppler sonography is often used for medical applications where erythrocytes act as reflectors. The effect is also used for technical applications by adding air-filled hollow spheres or air bubbles of the right size to the liquid.

226

Figure 10-76: Medical application of ultrasonic Doppler effect for flow measurement in coronary artery bypass (Siemens) 10.8.3

Acoustic Microscopy Acoustic microscopy is often compared to optical microscopy as the investigated surface or near surface region, as well as the achieved resolution are similar. From its principle it is done with focused probes in immersion technique. Usually one probe in pulse echo arrangement is used. To reach high resolution the frequency has to be increased up to 2 GHz or more. No clear limit is agreed on the lower limit, 100 MHz would be a reliable value. All components must be designed for high frequencies and adequate mechanical stability. A closer view onto the acoustic lens system shows that there two relevant rays exist: the axial ray, that is reflected from the surface, and a ray accomplishing the Rayleigh condition sinΘ R=

co

(10.44)

cR

with co the longitudinal velocity of the coupling medium (usually water) and c R the Rayleigh wave velocity of the specimen. While all other rays are reflected arbitrarily and possibly do not hit the transducer, the two mentioned waves are reflected and doing so they interfere. (A)

(B) (E) (D)

(F) (C)

ΘR

-z

Figure 10-77: Ray model of an acoustic lens: (A) transducer, (B) acoustic lens, (C) specimen, (D) Rayleigh ray, (E) arbitrary ray, (F) axial ray The interference is periodically dependent on the defocusing distance z. With λ o as wavelength in the coupling medium the period becomes Δz=

λo 2 (1−cosΘR )

(10.45)

As the images are often visualised using the video (V) or envelop detected signal, the effect is called V(z). For the understanding of acoustic microscopy this is fundamental.

227

Figure 10-78: Theoretical V(z) curve for a glass specimen (A. Briggs: Acoustic Microscopy)

Figure 10-79: Examples of acoustic microscopes (Krämer)

Figure 10-80: Examples of acoustic microscopy images of electronic parts, plastic leaded chip carrier (left) and acoustical and optical image of a plastic quad flat pack (Sonoscan) 10.8.4

Advanced Thickness Measurement The usual way to determine the thickness is a measurement of the time difference Δt between two (or more) consecutive back wall echoes. This can be done measuring peak-to-peak or flank-to- flank, where the latter one requires that the measuring points are located at the same relative echo height. D=

c Δt n

with n = 2, 4, ... dependent on the number of echoes.

228

(10.46)

For small thickness the single echoes can no longer became separated clearly, so this method fails. A second possibility uses the Fourier transform of the sequence of several back wall echoes. With Δf as difference between two consecutive frequency peaks, the thickness is given by D=

nc 2 Δf

(10.47)

with n = 1, 2, ... dependent on the number of frequency peaks.

Time domain

Frequency domain

Figure 10-81: Time and frequency domain spectra of 20 mm steel (left) and 4 mm steel, both at 1 MHz, time division for both 5 µs (zero not within range), frequency division left 0.2 MHz, right 0.1 MHz; for 20 mm steel both methods are possible, for 4 mm time difference reading is less accurate than the determination of the frequency peak; note that the main bang (entrance echo) is excluded from FFT (Empa) The methods can also be performed for small thickness, down to below of the half of a wavelength. In this case Δf corresponds to the value of the first frequency peak, which becomes, due to interferences higher than the nominal frequency value of the probe. If only a small number of echoes are present, this method becomes unreliable.

10.9

Summary and Conclusions Due to the nature of wave propagation in solids, ultrasonic examination of structures is generally restricted to applications on rather simple geometries. The a priori information of the ori entation of potential flaws is often an absolute must. As soon as the complexity of the problem grows, more than basic knowledge and training is necessary. Most of the characteristics of the sound beam are defined by the probe; for different applica tions, therefore, often different probes are necessary, this is especially valid for testing in immersion technique. It is almost certain the phased array technology, due to its advantages of beam access and the reduction of moving parts, will cover a wide range of future applications. Signal interpretation is usually based on amplitude analysis in comparison to known standard situations. The declaration in reports, therefore, in most cases is that found indication complies with a known reference. The possibilities of time of flight diffraction are widely accepted so that this defect type can also be regarded as covered. Other advanced methods of signal interpreta tion are used mainly under laboratory conditions. Ultrasonic testing, probably more than all other methods, relies on the qualification of the operator. This begins with an accurate calibration, proceeds with a reliable scanning of the object and ends with a distinct sizing of the found indications. For standard situations this process takes place at the object itself and is not supervised by electronic recording. If no indications were found this can mean that there were none, but it can also mean that they were not found due to miscellaneous errors during the examination process. Phased array technology showed large growth in the past years, if CMUT technology will be the next generation standard is not clear yet.

229

11

ACOUSTIC EMISSION INSPECTION

11.1

Introduction We use our sense of hearing to detect and interpret audible stimuli from the environment. Ancient potters used the cracking sound of clay pots as they cooled in the kiln to monitor the quality of their creations. Through the middle ages and the renaissance period, as metallurgy began to evolve through the work of alchemists, the "cry" of tin as it was deformed was a wellknown phenomenon reported in classic metallurgical texts. The cry of tin is associated with a deformation mechanism called twinning. When this cry takes place inside the tin, it releases audible acoustic energy. Early in the 1900s a similar cry was heard in many other metals including zinc, cadmium, magnesium, steels, lithium, aluminium, vanadium, strontium, titanium, cerium, arsenic and osmium. In the 1930s acoustic emission (AE) was applied to mine timber. A Japanese article describes an experimental method to detect elastic shock waves during bending of quarter sawn boards of Japanese cedar, pine and cypress. The vibrations associated with the fracture of a board were transferred into an electric gramophone through a needle on the board. In the same period the method was used to study the formation of martensite needles. Large-scale AEs from fracture in mines have been identified as the rumbling sound that occurs before an actual cave-in. In the 1940s investigations were performed in mines to determine if rock bursts could be predicted. Using a microphone and amplification, sub audible noises of stressed rock as micro seismic signals were detected for predicting the rock movement preceding failure. In the latter half of the 1900s, acoustic emission developed rapidly as a tool for materials research and as an efficient non-destructive method for inspecting load-bearing structures and pressurised fluid containment systems. Procedures and standards have been established for testing. Equipment has evolved through several generations into sophisticated computer-based systems. Acoustic emission has augmented our sense of hearing and become an accepted and important technique for characterizing materials and their behaviour. AE is related to transient-elastic waves generated by sudden movement in stressed materials. The classic sources of AE are defect-related deformation processes, such as crack growth and plastic deformation. Sudden movements at the source generate a stress wave, which radiates out into the structure and excites a sensitive piezoelectric transducer. As the stress in the material is raised, many of these emissions are generated. The signals from one or more sensors are amplified and measured to generate data for display and interpretation. The source of the AE energy is the elastic stress field in the material. Without stress, there is no emission. Therefore AE is usually carried out during a controlled loading of the structure. In the beginning AE was mainly used for the examination of metal structures, it was even applied to pressure components in nuclear power plants. It was promoted very intensively with a lot of promises that turned out not to be true. It is therefore not astonishing that the general acceptance of this method decreased nearly to null. It turned out that materials that show plastic behaviour are not always very well suited for this method. Today it is successively applied for the determination of structural integrity of components from materials like fibre reinforced plastics or concrete. Other applications are related to melting, phase transformation or cool down cracking. The disadvantage that crack growth must occur can be overcome with some derived methods like acousto-ultrasonics or non-linear acoustics. The measured phenomenon, however, are not identical with those from AE.

11.2

Features and Limits

11.2.1

General The features of AE are:

230



Allows the real-time detection of sources, depending on the materials properties, up to several metres distance and therefore allows a 100% monitoring;



As soon as reflections with or without mode change are mixed with the direct echoes of the sources, the evaluation can become very complex or even impossible;

11.2.2



Subsequent application of load to the same stress level will only identify discontinuities that are still active; this is a reversible phenomenon known as the Kaiser effect;



Can be applied to monitor structures under operating conditions, given that process noise is low or otherwise not interfering;



Capable to locate a growing discontinuity under test by the use of remotely installed sensors and can be therefore used to control the effects of the application of load and prevent catastrophic failure of structures, given that the functioning and coupling of the remotely applied sensors can be controlled.

Definitions For the AE signature the following terms are relevant: ▪

Arrival time (A) is the absolute time when a burst signal first crosses the detection threshold (D);



Burst signal rise time (B) is the time interval between the first threshold crossing and the maximum peak amplitude (E) of the burst signal;



Burst signal duration (C) is the interval between the first and the last time the signal detection threshold was exceeded by a burst signal, a typical values is 100 to 150 μs; the ring down count (F) is the number of times a burst signal crosses the detection threshold.

Figure 11-1: AE signature, definitions see text The burst signal energy given by the instrumentation is dependent on the manufacture type and usually lies between E=∫∣V t ∣dt

11.2.3

and

E=∫ V2 t  dt

(11.1)

Kaiser Effect The Kaiser effect which was first investigated 1950 by Josef Kaiser describes the phenomenon that a material under load emits acoustic waves only after a primary load level is exceeded. During reloading these materials behave elastically before the previous maximum load is reached. If the Kaiser effect is permanent for these materials, little or no AE will be recorded before the previous maximum stress level is achieved.

11.2.4

Felicity Effect For certain materials, especially for composites, there is already significant AE at a load level below the previous maximum applied level. This phenomenon is called the Felicity effect. For calculations, it is indispensable to know if a certain material shows this effect and, if so, to what extent. The Felicity ratio between the applied load at which the AE reappears during the next application of loading and the previous maximum applied load gives some information about the possibility of failure during the next load.

231

Figure 11-2: Example of the Kaiser effect in a cyclically loaded concrete specimen, thick lines indicate the AE activity, thin lines the load and the dashed lines the Kaiser effect; the most right graph corresponds to breaking of the sample (NDT.net)

11.3

Instrumentation

11.3.1

Acoustic Emission Sensors The sensors are normally of the resonant type, where one frequency dominates the response, typical values are between 50 to 250 kHz. Sensors with different resonant frequencies are available. The choice of sensor and operating frequency depends upon the purpose of examination, the type, material of structure or component and its surface structure (insulation, painting, coating, surface corrosion, etc.) and the environment (temperature, background noise, etc.). With such types of sensors little or no information about the real emission frequency spectrum is achieved, however such sensors are more sensitive and smaller in shape. The sensor shall be fixed to the test object using an acoustic couplant and a clamping device or an adhesive bond. In special applications, the sensor is installed on a wave guide. The surface at the sensor positions shall be cleaned and sufficiently flat to ensure adequate and reprodu cible transmission.

Figure 11-3: Acoustic emission sensors (sizes like ultrasonic probes) and application for leak testing (Physical Acoustics) To verify the installations a Hsu-Nielsen source can be used. This device is an aid to simulate an AE event using the fracture of a brittle graphite lead in a suitable fitting. This test consists of breaking a 0.5 mm (alternatively 0.3 mm) diameter pencil lead approximately 3 mm (± 0.5 mm) from its tip by pressing it against the surface of the piece. This generates an intense acoustic signal, quite similar to a natural AE source, detected by the sensors as a strong burst. The purpose of this test is twofold. First, it ensures that the transducers are in good acoustic contact with the part being monitored. Generally, the lead breaks should register amplitudes of at least 80 dBAE. Second, it checks the accuracy of the source location set-up. This last purpose

232

involves indirectly determining the value of the acoustic velocity for the object being monitored. The AE decibel scale dBAE is a dB scale in voltage where the reference voltage U ref is 1 µV referred to the sensor output dBAE =20 log

U Uref

(11.2)

Various sensors for special applications exist, among them sensors that can be directly buried into concrete, dry contact rolling sensors, wireless sensors, sensors that can withstand higher temperature (up to 500 °C, otherwise wave guides may be used), high radiation, etc. 11.3.2

Signal Conditioning and Processing The signal conditioning and processing includes the signal transmission, amplification, filtering and extraction of the AE signal features. The frequency filtering shall be appropriate for the sensor response. The pre-amplifier converts the signal from the sensor into a suitable low impedance signal for transmission over long distances, using coaxial cable. Typical pre-amplifier gains are 34 or 40 dB; the cable length between probe and pre-amplifier should not exceed one metre. The system amplifier provides additional amplification (up to 60 dB), of the signals and the necessary frequency band filtering for the rejection of noise. The amplification section normally provides the power supply for the pre-amplifiers.

Figure 11-4: AE amplifiers (Physical Acoustics, Vallen)

11.4

Data Analysis The data are analysed with the purpose of identifying the AE sources and their grading. The time of occurrence of the AE and the corresponding load are other important characteristics which, together with the location, provide relevant and reliable information on AE source significance. The location depends on the type of application. This can be linear, planar or zonal and in some cases volumetric. An AE simulator may be used to help locate the position of the AE sources on the structure. Examples of evaluation criteria adopted for AE source evaluation are: ▪

AE activity that consistently increases with increasing stimulation or time;



AE during periods of constant stimulus;



Correlation between burst emission and the applied stimulus;



Spatial concentration of AE sources;



Coincidence of AE sources with structurally significant features such as: repairs, welds, nozzles, etc.

Typical grading are non-relevant, to be checked by conventional non-destructive testing methods or critically active.

233

11.5

Applications and Derived Techniques

11.5.1

Power Transformer Partial Discharge Localisation Partial discharge is an electrical phenomenon that occurs within a power transformer whenever the voltage load is sufficient to produce ionisation and partially bridges the insulation between conductors. Partial discharges are pulse-like in nature and cause mechanical stress waves to propagate within the transformer. They propagate throughout the surrounding oil and can be detected at the transformer tank wall. Sensing and evaluation can be done with normal AE equipment. The method is treated in the IEEE Guide for the Detection of Acoustic Emissions from Partial Discharges in Oil-Immersed Power Transformers C57.127-2003.

Figure 11-5: Acoustic emission to localise partial discharges in power transformers (QSL, ERDA) 11.5.2

Applications for Pressure Vessels AE becomes a method that is widely used on above ground storage tanks and pressure vessels that consist of or are wrapped with composites (the latter not yet fully confirmed). The applica tion is also possible for metallic pressure vessels, spheres, columns and tanks. Especially in the US the commercial application to tube trailers and railway tank cars, in Austria the inspection of liquefied petroleum gas (LPG) storage tanks are common and accepted by the authorities. For tanks especially emissions resulting from active corrosion of the floor are monitored, this method, however, does not yet seem to yield consistent results in all cases.

Figure 11-6: Halon bottle re-certification system on AE basis for fire extinguishers used in air crafts (left), AE inspection on a tube trailer (Physical Acoustics, FIBA) 11.5.3

Applications for Structural Integrity Assessment and Health Monitoring Typical applications for structural integrity assessment and health monitoring, besides pressure vessels of all kind, are helicopters and air crafts (namely the F-15, the F-111 and the VC-10) on

234

ground. In-flight monitoring could be a solution for micro-meteorite impact detection for rockets and space crafts.

Figure 11-7: AE used for the determination of the structural integrity of an agricultural silo under hydrostatic load (water filling), AE sensors with pre-amplifiers located on a wing (Empa, Physical Acoustics) Monitoring of construction like pre-stressed concrete cylinder pipelines (PCCP, water and waste water pipelines), buildings and parking structures, suspension bridges, cable-stayed bridges, post-tensioned bridges, nuclear structures, water reservoirs, ground anchors and dams is reported. 11.5.4

Applications for Wood The acoustic emission investigations for wood products can be classified into five fields: monit oring and control during drying, prediction of deterioration, estimation of strength properties and fracture analysis. The use of AE techniques to minimise defects during wood drying has been studied since the 1980s. In early studies, methods were investigated for controlling kiln conditions by monitoring the AE count rate. The AE parameters used in the majority of studies were time-domain AE counts, later more sophisticated methods were applied. With that it was possible to distinguish between AE due to checking (formation of delaminations between wood fibres) and AE due to water movement.

235

Figure 11-8: AE applied to wood (USDA) Decayed wood generates AE at a lower stress level in bending than does sound wood. Usually such decay is due to termites; with appropriate receivers it is also possible to detect termite activity directly. 11.5.5

Micro Acoustics in Geophysics Acoustic emission can also be used for the detection and monitoring of possible crack forma tions during construction and operation of geotechnical structures. Such systems can be used in mining, for monitoring of slopes and slippings, underground disposals or repositories for radio active waste. Good results are reported for salt rock formations of few hundred metres, where some dozen sensors are used. The sensors are applied within boreholes with diameters of typically 40 mm for axial transducers and 100 mm for radial transducers.

~ 50 m

Figure 11-9: Repository for radioactive waste in Gorleben (D), yellow points denote sources of acoustic activity over a period of about 10 years, red ones sensor positions with lengths of boreholes up to 30 m (BGR) 11.5.6

Applications for Process Monitoring Acoustic emission can be also used for process monitoring. In this case the acoustic sources are not growing cracks but all kind of noise that is produced by the process. Reported applications are rotating parts of all kind, welding applications (arc and laser) or the use of AE to determine the chip length in cutting processes. Wärtsilä, e.g. monitors the higher frequency (500 kHz) emissions to detect potential bearing damages, similar to piston seizure, even under the low frequency rumble of running ship engines. Usually process monitoring involves so-called con tinuous acoustic emission, in contrast to applications described above that are based on distinct burst-type signals.

236

Figure 11-10: Machine health monitoring (left), cutting tool with attached AE sensor (AE Consulting, ETHZ-IWF)

11.6

Procedure and Record

11.6.1

Procedure The NDT procedure usually has to define the following special technical aspects:

11.6.2



Examination zones;



On-site situation concerning accessibility, acoustic and thermal background, etc.;



Type of stress (stimulus), method of regulation / control, loading program / sequence;



Temporal coverage of the observation;



Structurally significant features such as repairs, welds, nozzles, etc.;



Source grading and verification criteria.

Record The record should contain the following information:

11.7



Description of the equipment used, including serial numbers and firmware version of instrument and probes, etc.;



Sensor positions, fixing and coupling;



Site operational conditions, background noise level and method of rejection of the interference noise sources;



Results of on-site verification of transducer sensitivity;



Type of analysis carried out;



Spatial concentration of AE sources.

Acousto-Ultrasonics Acousto-ultrasonics is combination of ultrasonics and AE. The technology consists of sending low frequency acoustic pulses at a predetermined angle of incidence into a material under inspection. These acoustic pulses travel through the material and are reflected by the different interfaces inside the sample. If a discontinuity (delamination, debond, etc.) is present inside the material, the reflected acoustic energy changes, revealing the presence of the discontinuity. The technique is less used for the exact localisation of emitting sources rather to get information about the overall characteristics of a certain structure or component. Localisation to a certain amount is possible using e.g. ultrasonic C-scan equipment as receiving part. In its high-end application acousto-ultrasonics is a highly sophisticated and advanced technique using digital signal processing and pattern recognition algorithms. As such it is orders of magnitude superior to any other conventional ultrasonic technique, where the human recognizance capability is much inferior to modern data processing, or where automatic scanning systems are limited by considering only the amplitude in a small portion (gate) of the ultrasonic signal. Acousto-ultrasonics considers the entire ultrasonic response, in time as well as in frequency, of the entirely insonified material. 237

One major task of setting up a fully automated acousto-ultrasonic system consists of the train ing or learning process. Several identical standard specimens are needed for each feature that the system will have to be able to extract. A number of measurements have to be repeated several times on each specimen. After many digital feature extraction processes on all the waveform data, templates are built for each feature. The feature templates are tested by evaluating the same standard specimen and the system is tested for its feature recognition reliabil ity. If the reliability is insufficient, the ultrasonic interrogation system (transducer frequency, number and location) may have to be altered and the training process will have to be repeated. After the part is in place, all transducers are brought into contact; the automated test proced ure is executed. Waveform data are being acquired, controlled by the switch matrix and the acquired waveform data are being processed in the following phases: ▪

Signal enhancement, e.g. increase signal/noise ratio via ensemble averaging, correlation and linear filtering; transducer compensation via Wiener filtering and domain transform;



Feature extraction, e.g. estimate signal properties that relate to the physics of the propagation problem such as time of arrival of certain wave modes, localised density estimates from frequency content, beat-frequency content or specific joint-time-frequency occurrences;



Signal classification, where each feature group is sequentially evaluated against feature group templates from the learning set and decision about acceptable or not acceptable.

Acousto-ultrasonics is useful for the inspection of components that have repeatable shapes, that is for ▪

High volume production with 100% inspection requirement;



Short inspection time;



Complex material systems.

Figure 11-11: Acousto-ultrasonic C-scan of a wrapped pressure bottle, additional the RF signals of a zone with little and with high activity are given (Physical Acoustics) The method has been used for composites strength prediction, wood internal bond strength prediction, rocket motor propellant bond-line adhesion, titanium and other adhesive bond strength prediction, corrosion detection in riveted aluminium plates, tire damage evaluation, thermal-oxidative damage detection.

11.8

Acoustic Resonance Analysis and Non-Linear Acoustics

11.8.1

Acoustic Resonance Analysis When a body is taped it vibrates in certain characteristic forms and frequencies (its natural res onances). The non-destructive acoustic material testing uses this effect to determine the properties of the specimen. Material-specific acoustic parameters can be calculated from the resonant frequencies and correlate to quality characteristics. Resonant analysis is a volume-oriented testing method which includes the whole specimen. The natural eigenfrequencies of the specimen are stimulated by an impact and the sound recorded by a microphone (air-borne sound) or laser vibrometer (structure-borne sound). A detailed and reliable classification is done via time signal analysis and frequency analysis by a comparison with trained patterns. A resolution less than 1.3 Hz can be achieved.

238

For electric conductive materials the excitation can be done using EMAT technology. This allows the choice of the frequency according to the defects to be found. As the sound beam can be focussed, the spatial resolution is highly defined. Similar to acousto-ultrasonics this method is promising for the quality control in mass production. Various applications are reported.

Figure 11-12: Acoustic resonance testing for bricks by tapping and EMAT-resonance testing of the integrity of copper clad aluminium wires (RTE, Resonic) 11.8.2

Tap Testing The term tap testing is often used for the acoustic resonance analysis of sandwich structures for disbonds and core damages. Originally with this technique one takes a hand-held mass, such as a coin or a machined piece of metal, and tap on the surface. A good region without defects or damages will produce a crisp and solid sound whereas a bad or damaged region gives a dull sound. The method is simple and cheap, but is also subjective, inaccurate, and highly affected by the inspector's hearing and background noise. Over the years, a number of instruments have been developed to move the method from a subjective to an objective one. These efforts resulted in several instrumented tap test devices that have proven useful. While some devices measure the acoustic information produced in response to a physical impact, most instruments use force data produced by an impact.

Figure 11-13: Tap tester and semi-automated inspection cart (ASI) By systematically tapping an area and recording the force response an image can be produced representing the surface stiffness of a component. With an image the size, shape and location of the damage can be assessed more accurately and it also provides an electronic record for archiving and later reference. The relation between the local surface stiffness k, the mass of the tap instrument m and the contact time τ is given by k=π2

m τ2

(11.3)

239

Figure 11-14: Tap test image obtained on a B 767 left elevator (ASI) 11.8.3

Non-Linear Acoustics A similar approach is possible with non-linear acoustics. A structure is acoustically exited using a sonotrode. The surface is scanned with an air-coupled ultrasonic receiver or a laser vibrometer tuned to higher harmonics of the excitation frequency. Affected (delaminated, debonded) regions show a higher signal. For some flaw types, not the whole affected region will show an acoustic effect; for cracks e.g. the tips are well detectable, as they can clap together and gen erate AE.

Figure 11-15: Defect selective overtone imaging of a smart structure, the non-affected regions show a signal only at excitation frequency of 50 kHz (Stuttgart University, IKP)

240

Sonotrode

Figure 11-16: Crack tip detection in a steel sample, laser vibrometer image projected over photography of the sample (Stuttgart University, IKP)

11.9

Summary and Conclusions AE is used for the characterisation of structures, mainly from fibre reinforced plastics or con crete and their structural integrity assessment. In certain countries it is also fairly common for the inspection of metallic pressure vessels and storage tanks. The fact that stress application for this method is necessary can be of advantage or disadvantage. Crack growth it not necessary in all cases: for pressure vessels no acoustic emission indicates sufficient structural integrity; in metallic pressure vessels one of two signals of a certain magnitude below or at maximum proof load level may already indicate a critical stage. For the practical examination the comprehension of the Kaiser and the Felicity effect into the interpretation of the results is of basic importance. For certain applications, like in-service inspection of CFRP pressure vessels, AE is the only or at least one of the very few non-destructive examination possibilities. Under certain, well defined circumstances process monitoring or even process control using AE is possible; the method is also used in seismic and mining applications, or to find insects (e.g. termites) in wood or wood-based structures. For a reliable application of AE the user usually should have access to a data base. In general it can be assumed that the larger the data base is, the finer the signal allocation can be performed. This may be the reason why some sources claim to be able to inspect certain struc tures, while others doubt that the results are reliable. Derived techniques, like acousto-ultrasonics or acoustic resonance analysis, are very promising if applied in combination with automated signal interpretation and pattern recognition. The exact quantification of such signals, however, seems to be very complex.

241

12

RADIOGRAPHIC INSPECTION

12.1

Introduction As the propagation of electromagnetic waves is not related to the presence of matter, radiography is suited for nearly all materials. In general, the analysed information is the amount of radiation that penetrated the component under test. The method is suited for examinations of volumes. The main applications of the classical radiography are in welding, soldering, casting, as well as for examinations of content and completeness of components. Non-destructive testing in general was invented in 1895, when Conrad Röntgen (1845-1923) discovered the X-rays. In German related languages, this type of radiation is named after him (“Röntgenstrahlung”). He produced the first medical and technical radiograms and was awarded 1901 the Nobel Prize in physics.

Figure 12-1: Some of the first X-ray images Conrad Röntgen produced: the hand of his wife Bertha December 22, 1895 (left), the hand of Albert von Kölliker January 23, 1896 Influenced by this development, Henri Becquerel (1852-1908) found only short time later, that also radioactive substances were able to influence films. The research on radioactivity was directed by two of his co-workers, Pierre Curie (1859-1906) and his wife Marie Curie, born Sklodowska (1867-1934). The three were awarded 1903 the Nobel Prize in physics (in addition Marie Curie was awarded 1911 the Nobel Prize in chemistry). The developments of the welding techniques and the need for their control led the basis for the technical radiography used today. Radioactive isotopes were not common until after World War II, when neutron treatment in reactors presented the possibility to produce affordable gamma ray sources. The basic investigation for the synthesis of isotopes has been done by Frédéric Joliot (1900-1958) and his wife Irène Joliot-Curie (1897-1956), awarded the Nobel Prize in chemistry in 1935. The studies of Arthur Compton (1892-1962) on X-ray scattering lead to a discovery, nowadays known as Compton effect. He was awarded the Nobel Prize in physics 1927. Oskar Klein (18941988) and Yoshio Nishina (1890-1951) derived 1929 the Klein-Nishina cross section for high energy photon scattering by electrons. Various new possibilities were added with electronic data processing. The mathematical prin ciple of computed tomography originates from Johann Radon (1887-1956). In the early 1960s Allan Cormack (1924-1998) expanded the theoretical basics and Godfrey Hounsfield (19192004) built in the early 1970s the first commercial computed tomography system. He called it computer assisted tomography (CAT) scanner. The two were awarded 1979 the Nobel Prize in medicine. Always related to radiography is radiation protection. At the beginning, the dangerous nature of the new instruments was not known, of course. The first man to die as the result of irradiation was Clarence Dally. He worked as an assistant of Thomas Edison with X-ray tubes and got badly burnt by the radiation. The lesions developed to cancer and he died 1904 at the age of 39. After a lot more cases of death, especially among physicians and patients during the Great War, the first radiation safety standards were established 1928 during an international confer ence on radiology in Stockholm.

242

Hans Geiger (1882-1945) invented 1908 the proportional counter and 1928, together with his doctoral student, the Geiger-Müller counter.

12.2

Physics of Radiography

12.2.1

Attenuation of X-Rays Interaction between penetrating radiation and matter is not a simple process in which the primary X-ray photon changes to some other form of energy and effectively disappears. Attenuation of X-rays is determined by four processes: Photoelectric absorption of X-rays occurs when the X-ray photon is absorbed resulting in the ejection of electrons from the inner shells of the atom, resulting in the ionisation of the atom. Subsequently, the ionised atom returns to the neutral state with the emission of an X-ray char acteristic of the atom. This subsequent emission of lower energy photons is generally absorbed and does not contribute to (or hinder) the image making process. Photo electron absorption is the dominant process for X-ray absorption up to energies of about 50 keV. Compton scattering, also known as incoherent scattering, occurs when the incident X-ray photon ejects an electron from an atom and is scattered of X-ray photon of lower energy from the atom. Relativistic energy and momentum are conserved in this process and the scattered X-ray photon has less energy and therefore greater wavelength than the incident photon. Compton scattering is important for low atomic number specimens. At energies of 100 keV to 10 MeV the absorption of radiation is mainly due to the Compton effect. Pair production can occur when the X-ray photon energy is greater than 1.02 MeV, when an electron and positron are created with the annihilation of the X-ray photon. Positrons are very short lived and disappear with the formation of two photons of 0.51 MeV energy. Pair produc tion is of particular importance when high-energy photons pass through materials of a high atomic number. Under special circumstances two additional phenomena can occur and may need to be considered, but are generally negligible: Rayleigh scattering, also known as coherent, or classical scattering, occurs when the X-ray photon interacts with the whole atom so that the photon is scattered with no change in internal energy to the scattering atom, nor to the X-ray photon. Rayleigh scattering is never more than a minor contributor to the absorption coefficient. The scattering occurs without the loss of energy and is mainly in the forward direction. Photo disintegration is the process by which the X-ray photon is captured by the nucleus of the atom with the ejection of a particle from the nucleus when all the energy of the X-ray is given to the nucleus. Because of the enormously high energies involved, this process may be neglected for the energies of X-rays used in standard radiography. According to Beer law (August Beer, 1825-1863) monochromatic radiation with the intensity J is attenuated over a length x with the linear absorption coefficient μ ln

J =−μ x Jo

(12.1)

For X-rays showing an energy distribution the absorption coefficient can be calculated only approximately and has usually to be determined in an experiment. The energetic composition of an X-ray beam depends furthermore on the matter it has already penetrated, the energy distribution is shifted to higher energies what is called beam hardening. For some applications low energies have to be filtered out and this effect is used by purpose. For industrial radiography, the filters added to the X-ray beam are most often constructed of high atomic number materials such as lead, copper, or brass. Also possible is a combination of lead - tin - aluminium – in this order seen from the source side; the following material filters out the characteristic radiation from the previous one. Filters for medical radiography are usu ally made of aluminium. Gamma rays are at relatively high energy levels at essentially monochromatic radiation; therefore filtration is not a useful technique and is seldom used.

243

1000 100 Overall 10

C

1 PE

0.1 0.01 0.01

PP 0.1 1 Energy [MeV]

10

Linear attenuation coefficient [cm-1]

Cross section [10-28 m2]

10000

10000 W 1000 Pb

100 Fe 10 Al

1 0.1

0.01 0.01

0.1 1 Energy [MeV]

10

Figure 12-2: Simplified attenuation curves for aluminium, PE: photoelectric, C: Compton, PP: pair production (left), linear attenuation coefficients for different metals (for Pb and W including absorption edges) Detailed tables can be found under physics.nist.gov/PhysRefData or including detailed information on filtering www-cxro.lbl.gov/optical_constants. Table 12-1: Half value layers (HVL) of various materials for monochromatic radiation [mm] Material

Density [g cm-3]

200 keV

500 keV

1 MeV

2 MeV

Water

1.0

51

63

90

126

Concrete

2.3

25

23

36

62

Aluminium

2.7

22

29

35

54

Steel

7.8

6

9.5

15

21

Lead

11.4

0.8

3.9

9

Uranium

18.3

0.3

2.1

4.5

12.5 7.2

When the transmitted photon energy reaches the binding energy of a particular shell of elec trons, there is an abrupt increase in the absorption. The energy at which this sharp change occurs for K electrons is called the K absorption edge and is used to identify the situation where kinetic energy of the K electron is zero. Further increase of the photon energy causes the absorption to decrease almost inversely with the cube of the energy. 12.2.2

Thomson Scattering Thomson scattering (Joseph John Thomson, 1856-1940, NP 1906) is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnet ism. The angular energy distribution can be expressed as E' =[1α1−cosθ]−1 E

(12.2)

with the relation of the photon energy to the electron energy α=

E me c2

(12.3)

The differential cross section, that is the relation of the quotients between the emitted energy per time and steradian (P/dΩ) and the incident energy per time and area (P/dσ) is (

dσ 1+cos2 θ ) =r 2e dΩ T 2

(12.4)

with re as the electron radius. The Thomson cross section is independent on the photon energy σ=

244

8π 2 −29 2 r =6.65 ·10 m 3 e

(12.5)

1

400

Relation E'/E

0.8

100 keV 0.6

400 keV

200 keV

0.4

0.2 0

30

60

90

120

150

180

Energy E' [keV]

50 keV 300

400 keV 200 keV

200

100 keV

100

50 keV

0 0

30

Scattering angle θ

60

90

120

150

180

Scattering angle θ

Figure 12-3: Angular energy distribution dependence on incident energy according to Thomson For the low energy range the energy distribution is little affected of the scattering angle, while for higher energies the backscattered photons show a substantially lower energy than the forward scattered ones. 12.2.3

Compton Scattering Compton scattering can be of special interest, if the scattered radiation is of importance.

E=hν

θ φ

Figure 12-4: Compton scattering The cross section due to Thomson is not relativistic and is independent on the energy. The cal culations of Klein and Nishina correct for quantum mechanics and incorporate the electron recoil. (

3 α2 (1−cosθ)2 dσ 1 1+cos2 θ ) =r 2e [ ]( )(1+ ) dΩ KN 1+α(1−cosθ) 2 (1+cos2θ)[1+α(1−cosθ)]

(12.6)

Especially for the lower energy range, there is a substantial amount of backscattered Compton radiation, while for the higher energy range the radiation is mainly scattered forward. For the lower energy range, therefore, it is possible and also realised to get use of the effect. For the higher energy range the scattered radiation is influencing the detector in form of noise.

245

400 keV

200 keV 100 keV Thomson

50 keV

Incident direction

Figure 12-5: Normalised cross sections for Compton and Thomson scattering for several energies 12.2.4

Unsharpness and Spatial Resolution Unsharpness U is an image blurring due to a loss of image definition. It is a combination of geometric unsharpness Ug, inherent unsharpness of the detector U d and movement unsharpness Um due to a movement between source, object and detector during the time the image is made.



2

2

2

(12.7)

U= Ug Ud Um

The geometric unsharpness is the result of the limited size of the source (focal spot or isotope size). The relation between the unsharpness, the source size D s and the magnification m is given by Ug=DS  m−1

(12.8)

For a given object being in direct contact to the detector the maximum geometric unsharpness is given by its total thickness b in radiation direction. As a rule of thumb the source-to-object distance f is given by 2

for lower quality images, b in mm

f≥7.5Ds b 3 f≥15 Ds b

2 3

(12.9)

for higher quality images, b in mm

(12.10)

DS

Object plane Radiation absorption

Ug`

Imaging plane

Ug

Figure 12-6: Influence of the source size on the geometric unsharpness (simplified) If films are used as detectors, their inherent unsharpness is of inferior impact; it is mainly influenced by the energy of the radiation and the processing conditions. However, as soon as the detection is subdivided into pixels (all kinds of digital radiography, digitalisation of films), this can become a limiting factor. A distinct function that is equal in amplitude is only possible if the Nyquist theorem is maintained. It says that the sampling rate must be twice the resolution. Often it is easier to calculate the modulation transfer function (MTF) of the involved systems. The modulation of the focal spot S(f) is given by 246

Sf =sinc π Ds f

m−1  m

(12.11)

and the one for the detector D(f) with the size of a detector element D D D f=sincπ DD

f  m

(12.12)

1.0

1.0

S

Modulation

0.8

Overall

0.6

0.8

D

D 0.6

Overall 0.4

0.4

0.2

0.2

0.0 0

1

2

3

4

Spatial frequency [mm-1]

5

S

0.0

-0.2

-0.4 0

1

2

3

4

5

Figure 12-7: Modulation for two focal spot sizes (S) 0.2 mm (left) and 1 mm with a detector size (D) of 0.2 mm and a magnification of 2.5; the negative modulation (right) can be seen as contrast reversal in some cases; the figure on the right shows clearly, that for larger focal spot sizes the overall modulation is dominated by the source modulation and magnification is useless The overall modulation is given by the multiplication of the single modulations. Increased magnification will also increase the detector modulation; however, it will much more decrease the source modulation. Magnification, therefore, is only useful as long as the overall modulation is dominated by the detector modulation. 12.2.5

Contrast Contrast is determined by the attenuation and the energy dependent detector efficiency. As the detector is often not interchangeable, the contrast can be fine-tuned with the beam energy used. If pair production is not dominating, lower energies lead to enhanced contrast, but as the penetration is limited, they also tend to a reduced signal and therefore to a reduced signal/noise ratio. For X-ray devices the maximum energy usually can be tuned. For gamma-radiation the limits are given in a range of possible wall thickness.

247

Figure 12-8: Maximum X-ray voltage as a function of penetrated thickness and material 12.2.6

Attenuation of Neutrons The attenuation of neutrons is highly different to X-rays. Neutron attenuation is not regularly dependent on the atomic number. Certain elements, as hydrogen, lithium and boron or some rare-earth elements, show relatively high attenuation. Therefore, neutrons e.g. are very convenient to determine organic compounds behind metal sealing, like the distribution of oil in an operating engine. Furthermore, neutron scattering depends on the mass number of the isotope.

Figure 12-9: Comparison of the radiograms of some electronic devices with X-rays (left) and neutrons, both methods show different details (Fuji) The disadvantage of neutrons is the possible activation of the samples and all other objects within the beam path. The decay time depends on the neutron energy and the material, usually it is about one day. In practice alloys containing cobalt (including nickel based super alloys) are seldom investigated by neutrons as the decay time is much longer.

248

Figure 12-10: Comparison of the scattering of X-rays and neutrons; the diameters of the circles correspond to the scattering amplitude (sin Θ = 0) for X-rays and b coh • 10 for neutrons; hatching indicates negative scattering amplitude, inc: incoherent (PSI) Detailed information and further links can be found under www.neutron.anl.gov.

12.3

Radiation Sources

12.3.1

Bremsstrahlung and K-Shell Radiation X-ray tubes generate X-ray photons by accelerating a stream of electrons to energies of several hundred kV with velocities of several hundred km/h and colliding them into a metallic target material. The abrupt deceleration of the charged particles generates bremsstrahlung photons. X-radiation with a continuous spectrum of energies is generated, ranging from a few keV to the maximum of the energy of the electron beam. The target material for industrial tubes is typic ally tungsten with a very high melting point. The inherent filtration of an X-ray tube can be computed, knowing the amount that an electron is penetrating into the surface of the target and by the type of vacuum window present. The bremsstrahlung photons generated within the target material are attenuated as they pass out through typically 50 μm of target material. The beam is further attenuated by the aluminium or beryllium vacuum window. The results are an elimination of the low energy photons, 1 keV through 15 keV, and a significant reduction in the portion of the spectrum from 15 keV through 50 keV. The spectrum from an X-ray tube is further modified by the filtration caused by the selection of filters used in the set-up. Beneath bremsstrahlung also characteristic K-shell and L-shell emission radiation is produced in every X-ray tube. For general non-destructive testing this radiation is not of interest, some crystallographic methods, stress measurement and phase contrast techniques with X-rays, however, depend on this effect. The K-shell is the lowest energy state of an atom. An incoming electron can give a K-shell electron enough energy to knock it out of its energy state. About 0.1% of the electrons generate K-shell vacancies; most produce heat. Then, an electron of higher energy (from an outer shell) can fall into the K-shell. The energy lost by the falling elec tron shows up in an emitted X-ray photon. Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell and so on. K-shell emission produces higher-intensity Xrays than bremsstrahlung and the X-ray photon comes out at a single wavelength. The process for other shells is analogue.

249

Source output [a.u.]

400 kV 1 mA 200 kV 2 mA 200 kV 1 mA 0

50 100 150 200 250 300 350 400 Energy [keV]

Figure 12-11: Typical unfiltered source output for doubled voltage or doubled current The energies of the characteristic X-rays generated are only very weakly dependent on the chemical structure in which the atom is bound, indicating that the non-bonding shells of atoms are the X-ray source. The resulting characteristic spectrum is superimposed on the continuum. An atom remains ionised for a very short time (about 10 -14 s) and thus an atom can be repeatedly ionised by the incident electrons. 12.3.2

Conventional X-Ray Tubes In an X-ray tube the electrons are discharged at a filament by thermal and field emission. The filament is heated by a current of several amperes from a low-voltage source, generally a small transformer. The focusing cup serves to concentrate the stream of electrons onto a small area of the target, called the focal spot. This stream of electrons constitutes the tube current and is measured in milliamperes. The higher the temperature of the filament, the greater is its emission of electrons and the larger is the resulting tube current. The tube current is controlled, therefore, by some device that regulates the heating current supplied to the filament. This is usually accomplished by a vari able-voltage transformer, which energises the primary of the filament transformer. Other conditions remaining the same, the X-ray output is proportional to the tube current. Most of the energy applied to the tube is transformed into heat at the focal spot, only a small portion is transformed into X-rays. The high concentration of heat in a small area imposes a severe burden on the materials and design of the anode. The high melting point of tungsten makes it a very suitable material for the target of an X-ray tube. In addition, the efficiency of the target material in the generation of X-rays is proportional to its atomic number. Circulation of oil in the interior of the anode is an effective method of carrying away the heat. Where this method is not employed, the use of copper for the main body of the anode provides high thermal conductivity and radiating fins on the end of the anode outside the tube transfer the heat to the surrounding medium. The focal spot should be as small as conditions permit, in order to secure the sharpest possible definition in the radiographic image. However, the smaller the focal spot, the less energy it will withstand without damage. To overcome this, tubes are often equipped with two filaments producing focal spots with different sizes depending on the power output. Due to the bevelled target, the effective focal spot is not equal to the radiated field and the intensity distribution is dependent on the direction. For special purposes this has to taken into consideration. Typical focal spot sizes are in the millimetre range. Dependent on the purpose, the tube head can be designed to have a directional or a panoramic emission. For the latter the target has to be cone shaped. The high-voltage supply for the acceleration of the electrons consists of a transformer, an autotransformer and, quite frequently, a rectifier. A transformer makes it possible to change the voltage of an alternating current. The coil con nected to the source of alternating current is called the primary winding, the other the secondary winding. The voltages in the two coils are directly proportional to the number of turns, assuming 100% efficiency. At the same time, the current in the coils is decreased in the same proportion as the voltage is increased. A step-up transformer is used to supply the high voltage to the X-ray tube. An autotransformer is a special type of transformer in which the output

250

voltage is easily varied over a limited range. In an X-ray generator, the autotransformer permits adjustment of the primary voltage applied to the step-up transformer and, hence, of the high voltage applied to the X-ray tube.

Target

Focusing cup

Electrons

Cathode (-) Anode (+)

Actual focal spot

Filament

X-rays

Effective focal spot

Figure 12-12: Schematic view of an X-ray tube and relation between real focal spot area and effective focal spot (A)

(B)

(C)

(D)

Figure 12-13: Basic types of X-ray tubes and typical characteristics: (A) unipolar metal ceramic (up to 225 kV, 3000 W (6000 W), 4 to 11 kg), (B) bipolar metal ceramic (up to 450 kV, 4500 W, 40 to 100 kg), (C) portable metal ceramic (up to 300 kV, 900 W, 2 to 3 kg), (D) unipolar ceramic (up to 150 kV, 1000 W, 1 to 6 kg) (Comet)

Figure 12-14: Mobile tube head and control unit (left), stationary equipment with transformers, cooling and control units (GE IT, Yxlon) The type of the voltage waveform supplied by a high-voltage transformer consists of alternating cycles. Some industrial X-ray tubes are designed for the direct application of this high251

voltage waveform, the X-ray tube then acts as its own rectifier. Such tubes are designed as one pole tubes. As the current can flow in one direction only, the equipment can only make use of one half of the cycle (half wave direct current (HWDC)); the efficiency, therefore, is limited. But such tubes have some other advantages. As the anode serves as earthing it can directly and efficiently be cooled with water. Due to the compact design it can be built into one single housing, which makes such a tube rather mobile. Usually, however, the high voltage is supplied to a unit called a rectifier, which converts the cycles into the unidirectional form (full wave direct current (FWDC)). In this case both poles can be used and the output is considerably larger. Sometimes a filter circuit is also provided to smooth the voltage waves, so that essentially constant potential is applied to the X-ray tube. The newest developments of X-ray tubes are going up to 600 kV. 12.3.3

Microfocus X-Ray Tubes Microfocus X-ray tubes have a very small focal spot of some micrometres and less. They are especially designed for low power output (withstand of the target) and can be used for direct geometric magnification of the radiographic image. The current, therefore is much lower than for standard X-ray tube; it is typically in the range of 0.1 mA. The standard target is tungsten; other metals are possible as well, their power intake, however, is lower. For some transmission target applications also diamond based materials are used. The interior design of such a tube is similar to an electron microscope. To maintain the possibil ity to substitute parts (filament, target, etc.), it is not sealed. That also means that the vacuum (typically < 0.1 μbar) has to be maintained during all operation using a two-step vacuum system with a backing and a turbo molecular vacuum pump. The cathode is designed with a wellcentred filament and a grid; the grid power supply defines if a current between cathode and target is flowing or not. The typical lifetime of a filament is 500 to 1000 h. (B)

(D)

(F)

(H)

(J)

(K)

(L)

(O)

(P) (A)

(C)

(E)

(G)

(I)

(N)

(P) (B)

(D)

(F)

(H)

(J)

(M)

Figure 12-15: Schema of microfocus X-ray tubes, direct (above) and transmission type: (A) high voltage connector, (B) insulator, (C) vacuum pump, (D) grid, (E) filament, (F) anode, (G) centring coils, (H) shutter target, (I) focusing coil, (J) centring blocking medium, (K) cooling, (L) target, (M) transmission target, (N) collimator, (O) tube window, (P) X-rays (Viscom) The reachable focal spot size depends on the system itself, however, also on the experience and carefulness of the operator. Typical values are a few micrometres (dependent of power output). As a rule of thumb for tungsten targets the power (in W) corresponds to the diameter of the focal spot (in μm), for diamond cooling this value is three to four times higher. For the tube head various designs are possible. Some tubes show a standard design with a dir ectional target, some have a rod anode and others have a transmission target. Often the tube

252

head can be changed. Due to the low power output, microfocus X-ray sources can be easily integrated in shielded cabinets.

Figure 12-16: Microfocus tube with transmission target (left), direct target (mid) and as closed system built into a cabinet (Viscom, GE IT, Phoenix) 12.3.4

High Energy X-Ray Sources For high energy applications linear accelerators (linac) and betatrons are used. They also generate bremsstrahlung X-radiation but the difference is in the voltage range and the acceleration of the electrons. The acceleration voltage is in the MeV range. Their typical penetration is in the range of 300 mm steel or 1 m concrete. The betatron may be considered as a high-voltage transformer, in which the secondary consists of electrons circulating in a doughnut-shaped vacuum tube placed between the poles of an alternating current electromagnet that forms the primary. The circulating electrons, accelerated to high speed by the changing magnetic field of the primary, are caused to impinge on a target within the accelerating tube. In the linear accelerator, the electrons are accelerated to high velocities by means of a high frequency electrical wave that travels along the tube through which the electrons travel. Especially linear accelerators, but sometimes also betatrons are also used for the acceleration of other charged particles (protons, ions, etc.). Nowadays high energy X-ray sources are predominantly used for medical applications, including the generation of radio pharmaceutical products. This is possible as X-ray photons with energy of above 6 to 8 MeV can penetrate up to the atomic nucleus and generate an activation of the product. Industrial applications include the inspection of very heavy components and in civil engineering. Accelerators for medical treatment (cancer) can go over 20 MeV.

Figure 12-17: Portable industrial 6 MV betatron (left) and older linac system, note the operator at the right side (JME) 12.3.5

Flash X-Ray Machines Large flash X-ray machines are designed to give extremely short (microsecond) and extremely intense bursts of X-radiation. They are intended for the radiography of objects in rapid motion or the study of transient events. The high-voltage generators of these units give a very short pulse of high voltage, commonly obtained by discharging a condenser across the primary of the high-voltage transformer. The X-ray tubes themselves usually do not have a filament. Rather, the cathode is so designed that a high electrical field "pulls" electrons from the metal of the cathode by field emission or cold emission. Momentary electron currents of hundreds or even

253

thousands of amperes - far beyond the capacity of a heated filament - can be obtained by this process.

Figure 12-18: Flash X-ray: two aluminium 'nose-cones' point towards the centre of an explosive assembly (replaced by a polystyrene foam ball in the photo); these and the heavy steel barrel-type cassettes, protect the sensitive films from damage. The two box shaped objects in the field of view are lead X-ray collimators accurately positioned in line with the two X-ray sources; these and the temporary wooden support structure will be destroyed by the blast from the charge (left), high voltage flash X-ray source (AWE) Portable flash X-ray sources with battery power supply are also available. Their main applications are for homeland security (investigation of abandoned luggage, etc.) and veterinary diagnostics. The exposure time is simply defined by the number of excited pulses, each of a duration of typically 50 ns with a frequency of typically ten pulses per second. The usual detector type is an imaging plate.

Figure 12-19: Portable flash X-ray source with 270 kV (Golden) 12.3.6

Gamma Ray Sources In contradiction to X-ray machines, which emit a broadband of wavelengths, gamma ray sources emit one or a few discrete wavelengths. The energies of radiation emitted by a gamma ray source and their relative intensities depend only on the nature of the emitter. Thus the radiation quality of a gamma ray source is not variable at the will of the operator. A term often used in speaking of radioactive sources is specific activity, a measure of the degree of concentration of a radioactive source, expressed in terms of activity per unit mass. Of two gamma ray sources of the same material and activity, the one having the greater specific activity will be the smaller in physical size. Thus, the source of higher specific activity will suffer less from self-absorption of its own gamma-radiation. In addition, it will give less geometrical unsharpness in the radiogram or, alternatively, will allow shorter source to film distances and shorter exposures. For the reason of specific activity, all isotopes used for non-destructive testing are artificial ones, generated in reactors. The specific activity of a radionuclide can be easily calculated from

254

A spec=

λNA M

with

λ=

ln2 T½

(12.13)

where λ is the decay constant, T½ the half live, NA the Avogadro number and M the atomic weight. The unit for the activity is Becquerel [Bq] = number of decays per second, in non-destructive testing usually used in terms of GBq or TBq. Table 12-2: Characteristics of common gamma ray sources; the range is given for steel, copper and nickel base alloys, the second number (if any) is for aluminium and titanium, the weights of the shielding are typical values, Yt-169 and Tm-170 are seldom used, Tm-170 also emits a bremsstrahlung background due to re-absorbed high-energy β-particles, Cs-137 is used for calibration, etc. only, Am-241 is used for neutron generators but it also emits gamma rays Isotope form

Half live T½

Spectrum [keV]

Range [mm]

Weight of equipment [kg]

Cobalt

Co-60 metallic

5.27 y

1'173, 1'333

40 - 200

130 - 300

Caesium

Cs-137 CsCl

30.0 y

661

not defined

50

Iridium

Ir-192 metallic

73.8 d

341

206 - 612

20 - 100

13 - 22

Selenium

Se-75 metallic

120 d

538

66 - 401

10 - 40 35 - 120

7

Ytterbium

Yb-169 Yb2O3

32.0 d

782

63 - 308

1 - 15 10 - 70

Thulium

Tm-170

128 d

222

52 - 84

≤5

1

Americium

Am-241

432 y

not applicable

3

Element

Specific activity [TBq g-1] 41.9 2.57

0.13

Figure 12-20: Two types of source capsules and cable tip (left) and tungsten collimator for the exposure position, shielding all directions not necessary for the examination (Empa) The sources usually consist of several pellets kept in a sealed metal capsule. Their typical size is one to a few millimetres. The sources are kept in massive containers (usually from depleted uranium) and only put into the radiation position for the duration of the exposure. This movement is done with the aid of a cable, usually manually, sometimes motorised. The advantage of the manual movement is that the operator can easily count the necessary number of turns of the crank and recognises immediately a sudden resistance if the movement is not going smoothly. For reasons of specific shielding, the containers are usually made from depleted uranium, covered with stainless steel.

255

Figure 12-21: Source containers for selenium (left), cobalt (right) and all three common isotopes (Nordion, Sentinel)

Figure 12-22: Self-propelled isotope crawlers for pipeline radiography for different pipe diameters, similar models exist with X-ray tubes (JME, Nordion) The advantages of gamma ray sources are the compactness, the mobility and the independ ence of current supply. The disadvantages are the necessary radiation protection, as the source cannot be turned off and the fact that the source is decaying if used or not. 12.3.7

Neutron Sources Several possibilities exist for the generation of neutrons. Californium Cf-252 shows a spontaneous decay with emission of a neutron. The specific activity, however, is only 0.02 TBq g-1 with a half live of 2.6 y; long exposition is necessary. Several (γ,n) or (α,n) sources are known, the most used among them is the americium-beryl lium source, where the α decay of Am-241 excites Be-9 in such a way that this transforms to C12 under emission of a neutron. This mechanism is often used in equipment designed for the measurement of humidity, so called Troxler gauges. The measuring technique here is based on the thermalisation and backscatter of neutrons. Anion accelerators are more or less mobile. The most known among them are the deuteri um-deuterium source (H-2 (d-n) He-3, 2.5 MeV) and the deuterium-tritium source (H-3 (d,n) He-4, 14 MeV), which shows a rather low acceleration voltage of about 100 kV and generates a substantial neutron flux. Another reaction is Be-9 (d,n) B-10, this, however, needs a high acceleration voltage of typically 2.8 MeV. Such devices produce fast neutrons. Without moderation they show low contrasts and are hard to detect, therefore they are not really suited for ima ging. Moderation can be done with heavy water.

256

Figure 12-23: Troxler gauge (left) and deuterium-tritium neutron tube (Empa, Sodern)

Figure 12-24: Linear accelerator based beryllium neutron generator (Accsys) Electron linear accelerators above 8 MeV generate photo-neutrons, especially in combination with beryllium or uranium. The disadvantage is the high X-ray background. The best choice, however, are reactors (also in sub critical stage) and spallation neutron sources. Here the energy range of neutrons is often given in terms of thermal (2 to 100 meV, typically 25 meV) or cold (0.5 to 2 meV). Neutrons of these energies can be obtained by slowing them down to thermal equilibrium in matter at ambient (thermal) or low temperatures (typically 25 K). Table 12-3: Some European neutron facilities dealing with neutron radiography / tomography Organisation

Facility

Energy

Website

AI, Vienna

TRIGA

cold

www.ati.ac.at

ENEA, Rome

TRIGA

cold

www.enea.it

HMI, Berlin

BER-2

cold + mono energetic

www.hmi.de

IJS, Ljubljana

TRIGA

KFKI, Budapest

WWS

cold

www.kfki.hu

LLB (CEA-CNRS), Paris

ORPHEE

cold

www-llb.cea.fr

LPI, Moscow

RRT

mono energetic

www.icfpm.lpi.ru

PSI, Villigen

SINQ-NEUTRA SINQ-ICON

thermal cold

sinq.web.psi.ch

TU Munich

FRM-II

cold + fast

www.frm2.tumuenchen.de

www-rcp.ijs.si

257

Figure 12-25: Target block of the spallation neutron source SINQ (left) and ICON facility (PSI)

Intensity [rel. units]

100 80

Thermal Maxwellian Cold Source Spectrum B-10 cross-section

60 40 20 0 1.0E-06

1.0E-03 1.0E+00 Neutron energy [eV]

1.0E+03

Figure 12-26: Interaction of thermal and cold neutrons compared to the cross section of B-10 (PSI) 12.3.8

Synchrotrons Synchrotrons are very large sources of X-rays. The size of such a facility depends on energy of the storage ring; typical values are 2.4 GeV with a circumference of roughly 290 m (SLS) and 7 GeV with circumferences of roughly 1'100 m (APS). Their advantage is the very high light intensity and brilliance.

Figure 12-27: Aerial view of ESRF (left) and SLS during construction process (ESRF, PSI) A beam line guides the synchrotron light (X-rays), which is generated by the circulating electrons (or positrons), to the experimental stations. Such a beam line consists of several parts: The source, which is an undulator, a wiggler or a bending magnet. Undulators and wigglers are called insertion devices, because they are "inserted" into straight sections of the storage ring. The most popular source in a modern synchrotron light source is the undulator, which is either composed of an array of permanent magnets or an array of electromagnets. Due to the magnetic forces, the electrons are repeatedly deflected, resulting in electromagnetic radiation.

258

The front end, which contains slits, beam windows and safety components like beam shutter and beam stopper. Mirrors, which deflect and focus the X-rays onto the experimental sample. At the same time they can serve as filters, to cut away the unwanted part of the synchrotron light spectrum. High precision mirrors, such as copper plates or silicon crystals, which reflect the beam under very shallow angles (<1°) can be dynamically bent to focus the beam. The monochromator crystal, which selects a single wavelength with high resolution. The experimental hutch with sample holder and detector equipment, which registers the signals caused by the interaction of the X-rays with the sample.

Figure 12-28: Examples for a wiggler (left) and an undulator (DELTA, HMI)

Figure 12-29: Examples for a mirror (left) and a monochromator (Swiss Neutronics, Accel) Table 12-4: Some European synchrotron facilities with radiography / tomography beam lines (the complete list of European facilities can be found under www.elettra.trieste.it/projects/roundtable) Organisation

Facility / Beam Line

Range [keV] 10 - 30

Website

ANKA, Karlsruhe

TopoTomo

BESSY II, Berlin

ID-02

6 - 50

www.bessy.de

DIAMOND, Oxfordshire

I13L

5 - 30

www.diamond.ac.uk

Elettra, Trieste

SYRMEP

8 - 35

www.elettra.trieste.it

ESRF, Grenoble

ID-19 ID-22

HASYLAB, Hamburg

DORIS III / HARWY DORIS III / BW2 PETRA III / IBL PETRA III / HEMS

PSI, Villigen

SLS / TOMCAT-2D

6 - 120 7 - 60 16 – 250 7 – 24 5 - 50 50 - 150 8 – 45

www.ankaweb.fzk.de

www.esrf.fr

hasylab.desy.de sls.web.psi.ch

259

The source and the front end are stationed inside the storage ring tunnel. The other part of the beam line is usually located in separate experimental hutches, which have lead walls to shield against the X-rays.

12.4

Radiation Detection

12.4.1

X-Ray Film, Film Processing, Film Illuminator Modern X-ray films for general radiography consist of an emulsion - gelatine containing a radi ation-sensitive silver compound - and a flexible transparent, blue-tinted base. Usually, the emulsion is coated on both sides of the base in layers about 10 to 15 µm thick. Putting emul sion on both sides of the base doubles the amount of silver compound and increases the speed. At the same time, the emulsion layers are thin enough so that developing, fixing and drying can be accomplished in a reasonable time. However, some films for radiography in which the highest detail visibility is required have emulsion on only one side of the base (single coated). The grain size of the radiation-sensitive emulsion is from 0.2 to 1.3 µm. Larger grains make the film more sensitive, thus the exposure time can be reduced by a factor of up to 60. Fast films show more blurring and their contrast resolution is lower. When X-rays, gamma rays, or light strike the grains of the sensitive silver compound in the emulsion, a dissolution of the silver bromide takes place and forms a latent image. This change is of such a nature that it cannot be detected by ordinary physical methods. However, when the exposed film is treated with a chemical solution (called a developer); a reaction takes place, causing the formation of black, metallic silver. It is this silver, suspended in the gelatine on both sides of the base that constitutes the image. Although an image may be formed by light and other forms of radiation, as well as by gamma rays or X-rays, the properties of the latter two are of a distinct character and, for this reason, the sensitive emulsion must be different from those used in other types of photography.

Figure 12-30: Radiation-sensitive layer before (left) and after development, black silver accumulations are clearly visible (Kodak) Films are characterised by their sensitometric curve. For an exact comparison of such curves the source type and its range, screens (if any) and processing must by the same. The original curve is showing the optical density D against the dose K in Gray [Gy]. For practical reasons instead of the dose often the relative exposure time is compared. Typical values for the dose are in the range of 1 to 100 mGy. The optical density is the relation of incident L o to transmitted light L. D=log

Lo L



(12.14)

Do is the optical density of the unexposed, processed film, due to film base (also called blue base) and fog density. This value may change with ageing and other processes. Three characteristic numbers are important: the gradient G of the curve at the levels 2 and 4 above Do and the granularity σD at a density level 2 above Do. G=

dD d logK

(12.15)

The granularity is measured by scanning of a film with constant diffuse optical density with a microdensitometer. Both emulsion layers shall be recorded; this means that the focal depth has to include both layers.

260

σ D=



N

1 ̄ 2 ∑ (Di− D) N−1 i=1

(12.16)

Figure 12-31: Sensitometric curves of various films (200 kV, Pb, automatic processing), D2 corresponds to a film system class C1, D8 to C6 (GE IT) Table 12-5: Film system classes according to EN ISO 11699-1 Min. film gradient Gmin at

Min. Max. granularity gradient/noise σD max (G/σD)min at D - Do = 2 at D - Do = 2

Film system class

D - Do = 2

D - Do = 4

C1

4.5

7.5

300

0.018

C2

4.3

7.4

230

0.020

C3

4.1

6.8

180

0.023

C4

4.1

6.8

150

0.028

C5

3.8

6.4

120

0.032

C6

3.5

5.0

100

0.039

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Figure 12-32: X-ray films are available in various forms and formats: as bulk rolls, in various sizes as darkroom packages (the largest commercial size is 30 x 40 cm) or for direct daylight use (GE IT) In the processing procedure, the latent invisible image generated in the film by exposure to Xrays, gamma rays, or light is made visible and permanent. Processing is carried out under subdued light of a colour to which the film is relatively insensitive. The film is first immersed in a developer solution, which causes the areas exposed to radiation to become dark, the amount of darkening for a given degree of development depending on the degree of exposure. After development and a treatment designed to halt the developer reaction abruptly (stop bath in acetic acid, if included), the film passes into a fixing bath. The function of the fixer is to dissolve the darkened portions of the sensitive salt. The film is then washed to remove the fixing chemicals and solubilised salts and is finally dried. In most cases processing is done automatically.

Figure 12-33: Schema of the roller transport system of an automated processor with basic functions (left) and state of the art processor (Kodak, GE IT) Chemicals used for processing may age and temperatures below 5 °C may result in irreversible damage. For optimal results film and processing chemicals must be adjusted to each other. Typical processing times are several minutes plus drying, elevated temperatures increase processing speed, however, if they are elevated too much, quality will suffer. To reduce the silver content in the wash water, new generation processors use a cascade fixing with two successive fixing tanks replenished on the counter flow principle.

262

Figure 12-34: Film illuminators: common version (left) and with iris diaphragm (Wilnos) For the evaluation of films with densities common in non-destructive testing, special viewing equipment is necessary. Light boxes as used in medicine are usually not sufficient. For a lot of application films are replaced by other detection systems. But they are still widely used for on-site inspection and if the film has to accomplish the shape of the object by bending and cutting.

Figure 12-35: For certain applications films cannot easily be replaced by other detection systems, radiography of a pipeline weld (left), radiogram of a weld with an abnormal high number of pores due to welding with a wet electrode (GE IT, Empa) 12.4.2

Intensifying Screens Intensifying screens are used in combination with films. Two basic types exist: metal screens that filter the radiation and emit Compton electrons when exposed to X- or gamma-radiation and fluorescent intensifying screens consisting of a coating of phosphors which fluoresces when exposed to X- or gamma-radiation. For both types the contact between film and screen must be as tight as possible to avoid or at least reduce lateral effects. Metal screens are common for the radiographic inspection of metals. They are used as front and back screen and are made of lead, cooper, steel and tantalum. The useful thickness depends on the radiation, the thickness and material of the object, the film system class and the sensitivity level of the investigation. Their intensifying effect is not more than a factor of five. Metal screen must be very homogeneous in thickness; any small damage on the surface will have an influence on the result. To ease the handling thin screens are glued onto a paper strip. Films that are packed together with disposable screens in vacuum sealed, light-tight bags are available.

263

Figure 12-36: Demonstration of the importance of the contact between film and screen, good contact (left), poor contact results in fuzzy image (mid) and efficiency of a fluorometallic screen (RCF) in comparison to a lead screen (Kodak, GE IT) Fluorescent or fluorometallic intensifying screens, e.g. cadmium tungstate, are only used if very long exposure times have to be avoided. Their intensifying efficiency is between 10 and 100; however, as the light is emitted in any directions, the sharpness quality of the image suffers. Films used together with such screens must show a high enough spectral sensitivity within the emitted range of about 425 nm (CdWO4). Due to dark room processing conditions, usual X-ray films are designed not to be too sensitive for the visual light range. 12.4.3

Film Digitisation The advantages of a film can become combined with the advantages of a digitally stored image in terms of the possibilities of digital image processing, contrast and precise spatial measure ments, zoom, integration into reports, copying and forwarding by e-mail, digital storage, etc. Film digitisers are available that handle optical densities between 0.05 and 4.0 and scan with a pixel size of 50 µm or better with at least 12 bit grey scale resolution. This technique, of course, is not only applicable on new generated films, but especially also on older ones, that were exposed at times when no digital radiography was available. One possible application is the re-examination of earlier films to compare the present damage situation with an earlier stage (e.g. crack growth) using the same image processing steps. However, one has to keep in mind that every such operation, nevertheless the high quality of the digitiser, results in a loss of information. EN 14096-1 and -2 describe in detail the minimum requirements of such systems, what parameters are needed and how they are to be determined. The used standard reference film is equal to the one described in ASTM E 1936. The film contains five types of test targets:

264



Converging spatial resolution targets;



Density contrast sensitivity targets;



Stepped density targets;



Spatial linearity targets;



Parallel line pair target.

Figure 12-37: Film digitiser (left), standard reference film EN 14096-1 (GE IT) 12.4.4

Computed Radiography (CR) with Imaging Plates (IP) A possibility, still similar to film radiography, is the use of imaging plates showing photo stimulated luminescence (PSL). The phenomenon was discovered by Becquerel, but it did not attract much interest until recently. Certain substances are known to emit light when irradiated with radiation, UV rays or an electron beam, when heated or mechanically hit or stimulated by chemical reaction in some cases. Materials of this kind are generally called fluorescent substances. In particular, the substances which are powders with practical applications are often called phosphors. If the light disappears instantaneously when the stimulation ceases, then the phenomenon is called fluorescence. Some of the phosphors, however, continue emitting lights for a while after the stimulation stops, which is called phosphorescence. Luminescence incorporates both of these light emission phenomena. An imaging plate consists of a support coated with an about 5 µm thick layer containing BaFX:Eu2+ (X stands for Br, Cl or I). During the exposure colour centres are formed by the reaction Eu2+ → Eu3+. Stimulating the IP with a pulsed red He-Ne laser, the exposed plate emits blu ish purple light of about 400 nm wavelength. The two wavelengths are distant enough to be separated by a photo multiplier tube scanning the surface of the plate during read out. The readout velocity is limited by the afterglow to about 4 µs/pixel. The IP can be erased by stimu lation with white light. Imaging plates can be re-used up to 1'000 times.

Figure 12-38: Read out of an imaging plate using a red laser and a PMT sensitive to blue light (left), densitometric curve in comparison to a film (Fuji) The input energy of the laser could be reduced if the crystals would be columnar. To grow the standard substances in this way is nearly impossible. A candidate substance for the next generation therefore is CsBr:Eu2+ that shows PSL and can be grown columnar. The advantages of such a system are that the densitometric behaviour is linear over a wide range and the dynamics of 104 to 105 in comparison to a film of 102. Furthermore the exposure time is reduced by a factor of up to 20, due to the linear behaviour it is less critical. The disadvantages are or can be:

265



IP fading, the intensity of the stored image in the imaging plate will decrease over time;



Laser beam jitter, a lack of smooth movement of the plate laser scanning device, which results in lines of the image, which consist of a series of steps;



Scanner slippage, the slipping of an IP in a scanner transport system resulting in fluctu ation of intensity of horizontal image lines;



Aliasing, pre-sampled high spatial frequency signals beyond the Nyquist frequency (given by the pixel distance) reflected back into the image at lower spatial frequencies.

The pixel size of nowadays systems is as low as 12.5 µm at IP sizes of maximum 35 to 45 cm, resulting in maximum image sizes of up to > 30'000 pixels. The spatial resolution corresponds to normal film. IPs are superb when used on microfocus systems.

Figure 12-39: Mobile image plate read out system with integrated erasing unit (Dürr) The classification of CR systems is similar to the film classification. Instead of the ratio G/σ D the normalised signal/noise ratio (SNR) is taken G σD

SNR=log(e )

(12.17)

The classification levels are corresponding IP 1 (C1) to IP 6 (C6). The SNR can be determined by the user.

Figure 12-40: Microfocus imaging plate radiogram of an electronic device (left), detail of the ball grid array (BGA) with soldering flaws (mid) and detailed wiring structure; all three images were extracted from the same original image (Empa) Due to the wide dynamic range, the IP itself cannot be classified. Short exposure can be compensated by a sensitive read-out scan. The classification is usually low (e.g. IP 6). The same plate could be given more exposure and be scanned with low gain of the electronic system. The thickness contrast sensitivity will be improved just by selection of these two parameters. The system can now be classified higher (e.g. IP 1). 12.4.5

Scintillators A scintillator is a compound that absorbs X-rays and converts the energy to visible light that can be collected in a photo diode or directly by a highly light sensitive camera. A good scintillator yields many light photons for each incoming X-ray photon; 20 to 50 visible photons out

266

per 1 kV of incoming X-ray energy are typical. Scintillators usually consist of a high-atomic number material, which has high X-ray absorption and a low-concentration activator that provides direct band transitions to facilitate visible photon emission. Scintillators may be granular like phosphors or crystalline like caesium iodide. Certain sources make a difference between scintillators and phosphors. Lead shielding

Scintillator

Camera

Figure 12-41: Scintillations screen, for light protection covered with a light tight paper in combination with a camera system, an image intensifier is used to mount the equipment but not in use in this combination (Empa) Phosphors are materials which glow when exposed to X-rays. For maximum brightness, the phosphors used in X-ray imaging are made of rare-earth oxysulphides doped with other rareearth elements. The most common are gadolinium, yttrium or lanthanum oxysulphides doped with terbium, commonly called GOS, Gd2O2S(Tb). These typically emit blue to green light which is well-matched to film sensitivity. Various grain sizes and chemical mixtures are used to produce a variety of resolution and brightness varieties. In use, these are mixed with a glue binder and coated on to plastic sheets. These were designed to be pressed against X-ray film to improve sensitivity but they may also be pressed against arrays of amorphous silicon photo diodes to make electronic X-ray detectors with sensitivity at least as good as that of film. Tens of electron volts are needed to produce each visible photon in a phosphor screen and X-ray absorption is good. Light scatter can be a problem if the layers must be thick to stop higher-energy X-rays. If red light matches better the need of the next element in the detector chain, the doping can be done with europium instead of terbium. The above mentioned phosphors have a reaction time of 1 to 3 ms. If a faster reaction is needed – 5 to 10 µs, the doping may be done with praseodymium. For a better combination of resolution and brightness, thallium doted caesium iodide is used. CsI(Tl) has the useful property that it grows as a dense array of fine needles (10 to 20 µm in diameter) under the proper evaporation conditions. This produces crystals which act as light pipes for the visible photons generated near the input side of the layer allowing very thick (up to 1 mm) layers to be used with excellent retention of resolution. Because caesium has a high atomic number, it is an excellent X-ray absorber so this material makes very efficient use of the incoming X-rays. About 20 to 25 eV are needed to generate each light photon. When doped with thallium, CsI emits at about 550 nm, just at the peak of the spectral sensitivity of amorph ous silicon. The combination of CsI and amorphous silicon has the highest efficiency of all materials in production today.

267

Figure 12-42: Scintillators with rare-earth oxysulphides (left) and columnar caesium iodide having a more direct and distinct way for the visible photons, the schema shows the use together with detector panels (Varian) For certain applications instead of CsI ZnSe(Te) can be used. A combination of both is espe cially promising if CsI is used for the higher energy range and ZsSe for the lower one. Further scintillator materials are bismuth germanate BiGe3O12 (BGO), that is still too difficult to make with large areas, the already mentioned cadmium tungstate and plastics, that have low efficiency in detection low energy X-rays. For medical application, especially for positron emission tomography (PET), the cerium dotted lutetium yttrium oxyorthosilicate LuYSiO5:Ce3+ (LYSO) and lutetium oxyorthosilicate Lu2SiO5:Ce3+ (LSO) are used. In comparison to BGO both show a shorter decay time, however, there prize is remarkably higher. It is also possible to use terbium activated luminescent glass fibres. Such scintillating fibre optic faceplates give rather good results as they can be made thick (up to 20 mm) showing a good conversion of the X-rays into green light. Only photons that meet the total internal reflec tion angle requirements are then transmitted along the length of the fibre. Adding a mirror coating to the input side roughly doubles the light output. 12.4.6

Image Intensifying Systems X-rays cannot be intensified directly, but electrons can become accelerated in an electric field, gaining energy and therefore lead to an intensified image. An image intensifier does so in transforming incoming X-rays with a scintillating layer of columnar caesium iodide to light that hits a photo diode, accelerating the generated electrons in a field of up to 30 kV and forming a visible image using a phosphor. Over a mirror the phosphor is read out with a shielded camera system. The mirror and the shielding are necessary to prevent the electronics in the camera from being irradiated and performing noise. The camera gives a real time image, slight afterglow is possible. The field of view can be chosen within the intensifier by focusing the electrons or with the optical system of the camera, the latter, however, is not very efficient, as the resolution on the phosphor is limited.

Figure 12-43: Image intensifier (left), false-colour radiogram of a chip with missing bond (Au 20 µm) at arrows and detail of welding zone (Thales, Empa) The disadvantage of image intensifiers is the multiple signal transform (X-rays → light → elec trons → visible light → electronic signal) with a loss of information in each step, so the resolution is limited. Image intensifiers are a good tool for observing moving parts and for optimum posi tioning of objects in the beam (e.g. for latter examination with imaging plates).

268

Especially for a distortion less images with very high spatial resolution micro-channel plates (MCP) are used. A typical MCP consists of 10 million closely packed channels of a diameter of 10 µm in a lead glass plate, combined with an electric field generated along the axis by elec trodes. Each channel acts as an independent, continuous dynode photo multiplier, concentrating the electrons generated by the Compton effect. The reachable gain factor depends on the length to diameter ratio of the individual channel, usual values are 75:1 or 175:1 that result in a gain of 106 to 108. A further reason for their usage is to prevent or at least reduce a lateral influence of a once entered photon to other channels. At higher energies the channel crossing phenomena becomes important.

Figure 12-44: Schematic construction and operating principle of MCP (left), micrograph of columnar CsI crystals (length of indication bar 100 µm) (Hamamatsu) 12.4.7

Charge-Coupled Device (CCD) Concepts The scintillators can be directly coupled to a commercial CCD chip. To shield the chip from the X-rays that penetrated through the scintillator, a fibre optic plate can be put in between. Such devices are called FOP (fibre optic plate with X-ray scintillator). The fibres have a diameter of several µm and are bundled parallel to one another. The reachable resolution can be more than 20 Lp/mm, the detector size, however, is limited to the chip size of about maximum 50 mm x 50 mm. It is, however, also possible to use fibre optics to use a commercial CCD chip in com bination with a linear array detector, using a fibre optic adapter, also called fibre optic taper.

Figure 12-45: Fibre optic taper (Bologna University) As the yield of visible light is limited, one tends to use cameras working on low light levels; several possibilities exist. The electron-bombarded charge-coupled device (EBCCD) is a hybrid of an image intensifier and a CCD camera. In this device, photons are detected by a photo cathode similar to that in an image intensifier. The released electrons are accelerated across a gap and impact on the rear side of a back-thinned CCD. These energetic electrons generate multiple charges in the CCD resulting in a modest gain of a few hundred.

269

Figure 12-46: Principles of EBCCD and EMCCD (Olympus) An innovative method of amplifying low-light-level signals above the CCD read noise is employed in electron multiplying charge-coupled device (EMCCD) technology. By incorporating on-chip multiplication gain, the EMCCD achieves, in an all solid-state sensor, the single-photon detection sensitivity typical of intensified or electron-bombarded CCDs at much lower cost and without compromising the quantum efficiency and resolution characteristics of the conventional CCD structure. Further information on CCDs is e.g. given in http://microscopy.fsu.edu (category primer). 12.4.8

Detector Panels Three common methods to convert incoming X-rays into charge for electronic readout can be implemented in amorphous silicon. They are the intrinsic, the photoconductor and the scintillator methods (see above). Each method has its performance advantages and disadvantages and each has certain limitations on its use in practical X-ray imagers. In all three methods, the charge is accumulated for a frame period before being read out. Gamma cameras, in contrast, count each X-ray photon as it arrives. That technique is generally not used for X-ray imaging because the X-ray photon arrival rates are too high to permit counting. At the intrinsic method arriving X-rays are captured by the amorphous silicon diode (a-Si) where hole-electron pairs are generated. An applied bias separates the charge to prevent recombination. Because a charge pair is generated for about each 5 eV of X-ray energy, the signals are high. Unfortunately, the X-ray absorption of silicon is very low so the photo diode needs to be 10 to 20 mm thick. Fabricating such devices of amorphous silicon is not feasible. Intrinsic devices have been made from crystalline silicon but only arrays of one or two lines are practical and even these are expensive. Photoconductive materials with higher X-ray absorption than silicon can be coated on an array of conductive charge collection plates each supplied with a storage capacitor. These also generate hole-electron pairs when X-rays are absorbed but the charge generated must be stored out of the layer to avoid lateral cross-talk. The applied field (1.5 to 3 kV) not only separates the charge but directs it towards the collector plate directly below to maintain image sharpness. Currently, the most used photoconductor in production, amorphous selenium (a-Se), has relatively low X-ray absorption and requires about 50 eV to generate a hole-electron pair. These restrict both the minimum dose needed and the size of the signal generated. Other materials with ten times lower energy requirements and higher X-ray absorption are PbI 2, HgI2 and CdTe. All of them, however, are crystalline and therefore more complicated in deposition. The purpose of the sensor panel is to accumulate charge generated by the absorption of X-rays and to provide it row by row during scanning to the charge amplifiers. The charge storage device is a capacitor in photoconductor imagers or a photo diode in panels used with scintillators. The switch used to permit the charge to flow out can be a single diode, a diode pair or a thin-film transistor (TFT). All possible combinations of these storage devices can be made to work but each has a specific set of advantages and disadvantages.

270

Figure 12-47: Intrinsic (left) and photoconductive converter (Varian) In the array the switch can be a TFT much like the switches used in active-matrix liquid crystal displays. An important goal in panel design is maximizing the area of the imager that is taken up by the photo diode (high fill factor) so that a minimum amount of arriving light is wasted. The signals are carried by thin metal lines. A typical pixel centre-to-centre distance (pitch) is between 50 and 200 µm with a fill factor of 35%. In operation, the photo diodes are reversebiased by an external voltage applied to them all. While the TFT switches are off, charge gener ated by light from the scintillator accumulates on the diodes. When readout is wanted, a row line is energised to turn on the switches in that row. The charge from all of the photo diodes in the selected row flows out through all of the data lines simultaneously. In large arrays, this generates several thousand signals that must all be read at the same time. Panels with a small pitch are suited for static applications similar to film radiography or computed radiography with a typical exposition time of 0.5 to 2 seconds. Panels with a larger pitch are used for real-time applications with 25 or 30 images per second. Panels with up to 15 Mpixels are available.

Figure 12-48: Micrograph and schema of an imager array (Varian) Detector panels are available in lines and in arrays. The maximum flat panel size for the moment is about 30 cm x 40 cm, resulting in more than 7 million pixels. Even for the best and most expensive panels not all of the single elements are working (typical 10 to 20 lines at the beginning). Furthermore, some products show ageing effects when exposed to hard X-radiation. A typical lifetime to 75% luminescence is reached after a dose of 10'000 Sv at 100 kV and 1.3 Sv/h (corresponding to 1.6 mA @ 0.5 m). The life time depends very much on the used filter. Panels, therefore, are not really suited for systems that run non-stop and permit no beam shuttering.

271

Figure 12-49: Flat panel detector (Hamamatsu) For mechanical protection of the layer a substance with high X-ray transmittance is needed. Typical materials are amorphous carbon, aluminium or glass. From those three a-C shows by far the best X-ray transmittance, furthermore it is glass like with no particle causing blemish defects and it can be polished to a good flatness. A further possibility is the use of CMOS (complementary metal oxide silicon) detectors. They usually also use a scintillator, but due their design allows a higher fill factor, resulting in higher efficiency. Using fibre optics, the electronics can be oriented in a way that it can be shielded to a certain amount from the X-radiation, resulting in less noise and longer lifetime of the elec tronics, especially at higher energies. A further advantage is that CMOS detectors have less blooming. In conventional detectors image blooming occurs when individual pixels in the detector are over-driven by direct radiation. This happens because the pixels deliver their sig nals to the electronic amplifiers along each row and column. When one or more pixels are overdriven the signals from all other pixels in that row and column can be affected causing blooming or streaks in the observed X-ray image. CMOS detectors eliminate this problem because each pixel is amplified separately and their signals do not affect signals from adjoining pixels. CMOS can also be used in combination with materials that instead of converting X-rays to light directly use the electrons like CdTe or CdZnTe. In this case the detector usually has to be cooled with a Peltier element.

Figure 12-50: CMOS detectors: open array (left) and application of an array digital pipe weld radiography (iXimaging, cmosXray) The various technologies are rather young and new developments coming up very frequently. Some of the concepts will probably disappear after a while being technologically not competitive, too expensive or not rugged enough. 12.4.9

Neutron Detectors Neutrons itself do not react with most of the standard detector systems for X-rays; various converters can be applied. For films and IPs two methods exist: a direct method using con verter foils that are exposed together with the film, or a transfer method where such foils are activated and expose the film or IP later. Converter foils mainly consists from gadolinium, dysprosium or indium and expose the film with the reaction products from the neutron bombardment like gamma or beta radiation or conversion electrons. Neutron sensitive IPs contain gadolinium as neutron absorber. The direct method is faster but has the disadvantage that the usually also present X-radiation exposes the film or IP as well.

272

Figure 12-51: Film and converter foil (left), scintillator with CCD camera (PSI) Another possibility is the use of a highly light sensitive camera system, cooled in most cases, sensing the weak light emission from a neutron sensitive scintillator, containing lithium-6 or gadolinium as neutron absorber. In this case the noise impact from X-radiation is smaller. Further possibilities with flat panels and track-edge-foils exist.

12.5

Radiographic Techniques

12.5.1

Standard Radiographic Techniques Standard radiographic techniques project the shadow image of the object onto a detector like film, image plate, image intensifier, or flat panel. To reduce geometric unsharpness, with the exception of microfocus radiography, one will always try to put the object as close to the detector as possible. This also means that both sides of the object have to be accessible. For flat features, e.g. cracks, it is important that the beam direction coincides with its orienta tion; a material separation like a crack will be not visible if projected perpendicular to its flanks, as in this direction no material is missing. This can be especially relevant if lack of fusion at weld flanks have to be detected with high reliability. In some cases, typically for tubes, one has the choice to perform single wall or double wall radiography. Single wall, here, would mean one part of the source detector system is inside the tube, the other one outside, for double wall both parts are outside of the tube. For the double wall technique by choice of the radiation direction the object can be projected in such a way, that the operator can distinguish between parts that are near to the detector or not. A typical application is the projection of a circular weld in form of an ellipse. Another influence for the radiography of tubes is the difference in wall thickness that can be examined with the same exposure. Standards give distinct information onto this point. An advantage of radiography is that certain information about the part or the location can be imaged together with the object by the use of lead letters, lead scales, etc.

12.5.2

Special Radiographic Techniques without Signal Processing Especially for castings the difference in the penetrated wall thickness can be very large, so it will be not possible to examine the part using on film only. In this case a double film technique can be applied. Two (or more) films of the same or different speed are put together into the same cassette and exposed during the same time. The films can be examined single or together adding their optical density, thus increasing the accessible penetrated thickness. Mainly used in connection with isotopes as radiographic sources is the panoramic exposure technique. Several parts are put in a circle around the source and exposed at the same time. It is even possible to interrupt the exposure to remove certain films and to proceed with the rest. A further possibility is to adjust the distance of the parts to correct for different wall thickness. Sometimes it is necessary to have more detailed information about the depths of certain fea tures or to have a three dimensional overview. If simple depth determination is needed a double film parallax method can be used. Two images of the object are made with a lateral dis placement of the source a. Simply using the theorem of intersecting lines the distance of the feature (measured from the detector side) d is given by d=

bt a+b

(12.18)

273

where b projected distance and t the distance between source and detector. a

t d b Figure 12-52: Schemas of standard radiography (left), single wall radiography of tubes showing large differences in penetrated thickness dependent on position, and parallax method Objects viewed with a normal pair of eyes appear in their true perspective and in their correct spatial relation to each other, largely because of man's natural stereoscopic vision; each eye receives a slightly different view and the two images are combined by the mental processes involved in seeing to give the impression of three dimensions. Because a single radiographic image does not possess perspective, it cannot give the impression of depth or indicate clearly the relative positions of various parts of the object along the direction of vision. Stereo radiography requires two radiograms made from two positions of the X-ray tube, separated by the normal inter-pupillary distance and viewed in a stereoscope. As in ordinary vision, the brain fuses the two images into one in which the various parts stand out in striking relief in true perspective and in their correct spatial relation. The stereoscopic impression is much more distinct if the specimen has a well-defined structure extending throughout its volume. If such a structure does not exist as, for example, in a flat plate of homogeneous material, it is necessary to provide such a structure upon one or more surfaces of the specimen. A widely spaced array of wires mounted on both front and rear surfaces of the specimen will generally suffice, or a similar pattern can be applied in the form of cross lines of lead paint. In stereoscopic radiography, these added structures not only help to secure satisfactory register of the two films but also serve as a reference marking for the loca tion of any details shown stereoscopically within the specimen. The stereoscopic method is not often utilised in industrial radiography, but occasionally it can be of some value in localizing defects or in visualizing the spatial arrangement of hidden structures. 12.5.3

Film Laminography and Film Tomography Film laminography is no longer used nowadays and the film is replaced by a detector that opens together with electronic signal processing much more possibilities. It is, however, a good tool to understand the basics of the technique. The relative positions between source, object and film are changed during the exposure in such a way, that only one plane of the object is projected to the same position on the film during the whole time. All the other information is daubed – and of course influences the image quality also.

Figure 12-53: Schemas of film laminography (left) and film tomography In the early beginning of tomography a flat part of the patient was projected onto a film band. The patient then was turned a bit and the film was moved in order to image the next strip. In the end the physicians were evaluating the film band. As the features are shown in their sinus oidal projection, that kind of image is called a sinogram. Direct film tomography was never really used, as it performs an unfiltered back projection and the images are of poor intensity resolution. 274

Figure 12-54: Unfiltered back projection: original image with two dedicated positions (left), sinogram (showing the two positions) and unfiltered back projection having severe disadvantages in density reproduction

12.6

Tomographic Techniques

12.6.1

Computed Laminography Computed laminography uses the same principle as film laminography. The advantage is that if the data is once taken, it can be calculated back to any plane – flat or in another shape. The data can be taken in several ways. A simple method is to rotate the sample on an axis perpen dicular to the X-ray beam. In this case only one axis has to be manipulated, the resulting image, however, can show some streaky artefacts perpendicular to the rotation axis. Another method rotates the source and the detector synchronously on both sides of the object. This leads to fewer artefacts as the signal is daubed into an area and not along a line. x”

Detector plane y

Image r" plane

x (x,y) = (x',y') Rotation centre (xo,yo) = (x'o,y'o) 

r' y' x' Figure 12-55: Schema of laminographic methods (Phoenix) For the case with the single rotation axis the projection can be calculated by x '=x cosα+ysin α

(12.19)

y '=x sin α+y cosα

(12.20)

x' ' r '' = x ' r '+y' For the z direction similar calculations apply.

(12.21)

275

Figure 12-56: Computed laminogram of missing solder joints at a multilayer electronic device (Empa) 12.6.2

Computed Tomography (CT) Computed tomography allows calculating the structure of planes within an object. The received information is the same as if the object would have been cut at the relevant position and radiographed as a slice. For all imaging methods using photons or particles from sources outside the object the process is basically the same (X-ray, neutrons, microwaves), for magnetic resonance imaging (MRI) and positron emission tomography (PET) it is different. X-ray tube Collimator Detector

1st generation

3rd generation

2nd generation

4th generation

Figure 12-57: Schema of various scanner generations The object is imaged under a rotating angle onto a one or two dimensional detector. If the object is too wide or the pixel density is too low, additional movements are necessary to fill the gaps. From the mathematical principle it is not important if the source and detector or the object are rotating. Medical scanners do the first, industrial scanner generally the latter, as object manipulation is often easer to perform. Several different types of scanners exist; two dimensional scanners are named after their generation:

276



First generation scanners are very slow but have a high spatial resolution due to simple alignment;



Second generation scanners are used for industrial applications with higher energies and higher resolution, the gaps between the detectors are necessary for shielding in order to reduce cross-talk and noise due to Compton scattering;



Third generation scanners are used for medical and industrial applications;



Fourth generation scanners are expensive and have a full circle of detectors, due to their speed some of them are able to acquire the data for tomographic films (electron beam instead of tube is rotated) so they are practically only used for medical applications.

The construction of three dimensional scanners in general is third generation.

Figure 12-58: Industrial CT scanners (Empa, BIR) Beam shutter and slits

Sample handler Standard detector Figure 12-59: Micro CT station at SLS beam line 4S (left) and mountable in-situ mechanical compression and staining device (IMCSD) (Empa) Before being back projected, the acquired data have to be deconvolved. This is usually done in spatial domain (in literature it is often described for frequency domain also, this, however, does not correspond to reality). For discrete data it can be written as Y =F∗X

or

nF

Y i = ∑ F j−k+1 X k

with j = 1 .. nY

(12.22)

k=1

where X is the projection data, Y the result data and F the filter. Various mathematical filters exist. Two often used are according to Shepp-Logan to emphasis contrast and RamachandranLakshiminarayanan (for obvious reasons called RamLak) to emphasis sharpness. f SL =

fRL

−2 2 2 π (4n −1)

=

1 4 0 −1 2 2 π n

(12.23) for n = 0 for n even, ≠ 0

(12.24)

for n odd

277

0.3 0.2 RamLak 0.1

Shepp-Logan

0 -0.1 -0.2 -6

-4

-2

0

2

4

6

Figure 12-60: Shepp-Logan and RamLak filters, for digital processing only the point values are used, the lines are for better comparison only Two main families of reconstruction algorithms have been developed for cone beam X-ray tomography with circular source trajectories: the generalised filtered back projection method (Feldkamp, L. et al., J. Opt. Soc. Am. A 1 No. 6 (1984) pp. 612) and the 3D Radon transform inversion (Grangeat P., CAR'85, Springer (1985) pp. 59). Grangeat algorithm provides a density accuracy within 1% for a cone aperture up to 24° while Feldkamp algorithm is reasonable accurate within 2-3% for cone apertures below 20°. On the other hand, along vertical planes; the 3D back projection has a higher geometrical resolution because it does not filter along this direction.

Figure 12-61: Tomography examples using good contrast or sharpness: new and used 9 V alkaline battery (Zn/MnO2, left), neutron tomogram of a nautilus (Empa, PSI) 12.6.3

Beam Hardening Correction If X-ray sources are used, beam hardening can appear. The reason for this effect is that the weak part of the radiation is filtered out even at a short thickness by the photoelectric effect, simulating a higher density at the edges compared to the inner parts of an object. For a given system, the effect depends on the material, the denser it is, the more accented is the effect at the edges. Four concepts exist to get rid of this effect:

278



Physical beam hardening filters, e.g. a brass plate of several millimetres within the source collimator, reduce substantially the fraction of low energy radiation, they, however, also reduce the relevant photon flux;



The water-bag method – the designation comes from medical applications – uses a wrapping of a similar material around the object, thus pulling the most pregnant part of the effect into a region which is not of interest, the diameter of the part increases and the total useful photon flux is reduced as well;



Mathematical beam hardening correction during reconstruction simulating a monochromatic source, usually a 4 th degree polynomial, can be applied to the data; a wedge of the

same material is used to determine the parameters at a given tube voltage, usually the method can be applied only if the sample consists of one single material; ▪

Dual energy tomography is able to separate the attenuation due to the photoelectric effect and the Compton effect, and therefore it can also be used for density measuring techniques.

Beam hardening correction is of special importance if small density differences have to be detected or for accurate measurements of the geometry.

Figure 12-62: Beam hardening effect on a silicon carbide sample with a diameter of 95 mm, grey scale plot along the centre line (left), mathematically filtered image (Empa) 12.6.4

First Article Inspection, Rapid Prototyping and Reverse Engineering First article inspection is used for the geometrical inspection of new products or products with a changed processing root in casting and extrusion manufacturing. The data acquisition can be done tactile, optical or with computed tomography, only the latter is able to resolve internal structures without destroying the object. For computed tomography this step means that a full 3D data set with the required accuracy of the region of interest has to be acquired. Tomograms

First article part Point cloud

CAD model

Comparison

Figure 12-63: First article inspection (Empa) In a next step a point cloud of the surface information is generated and the amount of data is reduced substantially. The information of this point cloud is then compared to the CAD data set of the intended geometry and a deviation report is created. The critical point is the registration of the two data sets that is to define a common coordinate system (five degrees of freedom) useful for potential corrections. Rapid prototyping and reverse engineering use the same steps up to the formation of the point cloud. The final steps use a lot of brain ware and especially the generation of a CAD model a lot of time. 279

12.6.5

Local Tomography To achieve higher local resolution local tomography may be performed. The object is rotated around the region of interest (ROI) and parts outside of it are suppressed for the reconstruction (similarly to laminography). The spacing between source and detector needs to be large enough and severe artefacts may result.

Figure 12-64: Comparison of global (left) and local tomography for a coated ceramic tube with O.D. of 160 mm and a wall thickness of 5 mm; the global tomography results in a pixel size of 200 µm (0.13% O.D.) , where the local tomography allows one of 60 µm (0.04% O.D.) (Empa)

12.7

Aspects of Quality Assurance

12.7.1

Determination of the Focal Spot Size For the determination of the focal spot size various possibilities exist. EN 12543-1 to -5 series describe them as: ▪

Scanning method, using a shielded scintillation counter, is dedicated to those applications where quantitative values for the intensity distributions and spot sizes are needed, that is calibration and image processing purposes;



Pinhole camera radiographic method, using a kind of a camera obscura, can be used for systems up to 500 kV, the applied voltage should not exceed 200 kV;



Slit camera radiographic system, same as above;



Edge method, determining the geometric unsharpness of an edge, as a very simple method;



A method determining the geometric unsharpness of a tungsten ball using a microdensit ometer, designated for microfocus tubes.

With the exception of microfocus tubes, the focal spot should normally not change in shape. The determination of the size of radiographic sources can be done by X-ray them according to EN 12679. This is possible because the hard radiation of the isotope will less expose the film than the softer radiation from an X-ray tube. The tricky thing is the timing between putting the isotope in its position and turning the tube on and vice versa at the end of the exposure. 12.7.2

Controls Related to Film Radiography If films are used the processing conditions have to be controlled on regular basis. This can be done with films that were pre-exposed with a step wedge. The measured densities then are compared to the nominal values. The archievability of a processed film can depend on its con tent of residual thiosulphate. This can be monitored comparing a colour reaction of a mixture of silver nitrate and acetic acid with the exposed film with a colour table delivered by the producer.

280

Figure 12-65: Tools for controlling film development and residual thiosulphate content (GE IT) Densitometers are used to determine the optical density of radiograms. Densitometers can be calibrated by the user by setting the measured density values to the values of a certified refer ence film traceable to the national standard; thus reducing the allowed archiving time of such reference films to five years. Finally the film viewing equipment has to be checked upon a regular basis to ensure a uniform illumination of the screen. This is done using an illuminance meter capable to measure such high values. 12.7.3

Controls Related to Film-Less Radiography The controls that have to be performed for film-less radiography are similar to the qualifying issues on film scanners. Special attention is needed as image intensifiers and flat panels show ageing that will influence spatial and mainly contrast resolution and also generating dead pixels in flat panels. The degrading can be different over the whole field of view. Special attention has to be paid if the configuration of a system can be changed in its position as local magnetic fields (e.g. caused by massive steel parts) can distort the electron path in image intensifiers or increased scattered radiation can influence the electronic parts of cameras or flat panels.

12.7.4

Image Quality of Radiographs The image quality can be directly compared and documented on the radiogram itself. Image quality indicators (wire type or step/hole type) are put on the object, normally opposite to the film position to simulate the worst case concerning unsharpness. The designation of the wire or the hole that is just visible gives an indication on the quality. Image quality indicators (IQI) are usually made from cooper (for cooper, zinc, tin and its alloys), steel, titanium and aluminium. The wires have diameters from 3.2 mm (W1) down to 0.05 mm (W19), the holes have the same diameter as the thickness of the plates, namely from 6.3 mm (H18) down to 0.125 mm (H1). Wires and holes show an opposite direction in their denomination. A third type of quality indicators uses duplex wires and is used to determine the image unsharpness. The wire pairs are arranged with the same respective distance in between. The wires have diameters from 0.8 mm (1D) down to 0.05 mm (13D) and consist from tungsten (smallest three) and platinum.

BPK 462-1 W10 FE 25

BPK 462-2 H5 AL

Figure 12-66: Image quality indicator wire type EN 462-1 (left) and step/hole type EN 462-2 12.7.5

Secondary Radiation and Undercut Secondary or scattered radiation must often be taken into consideration when generating radiograms. The scattered photons create a loss of contrast and definition. Often secondary radiation is thought of as radiation striking the film reflected from an object in the immediate area, such as a wall, or from the table or floor where the part is laying on. Side scatter origin ates from walls or objects from the source side of the film. Control of side scatter can be

281

achieved by placing a collimator at the exit port, thus reducing the diverging radiation surrounding the central beam and collimating the beam as much as possible to the section under examination. Other possibilities are to move objects away from the film or move the X-ray tube to the centre of the vault. Collimating is especially useful to reduce the noise if detectors or subsequent electronic parts as photo multiplier tubes could be affected. Radiation coming from objects behind the film is called backscatter. Industry codes and standards often require that a lead letter "B" be placed on the back of the film cassette to verify the control of backscatter. If the letter "B" shows as a fair "ghost" image on the film, the letter has absorbed the backscatter radiation indicating a significant amount of radiation reaching the film. Little backscatter is present if the letter “B” is not visible or as dark “ghost” (backscattered radiation from the letter itself). Control of backscatter radiation is achieved by backing the cassette with sheets of lead. Another condition that must often be controlled when producing a radiogram is called undercut. Parts with holes, hollow areas, or abrupt thickness changes are likely to suffer from undercut if controls are not put in place. Undercut appears as lightening of the radiogram in the area of the thickness transition. This results in a loss of resolution or blurring at the transition area. Undercut occurs mainly due to scattering within the film. At the edges of a part or areas where the part transitions from thick to thin, the intensity of the radiation reaching the film is much greater than in the thicker areas of the part. The high level of radiation intensity reaching the film results in a high level of scattering within the film. It should also be noted that the faster the film speed, the more undercut that is likely to occur. Masks are used to control undercut. Sheets of lead cut to fill holes or surround the part and metallic shot and liquid absorbers are often used as masks.

12.8

Procedure and Record

12.8.1

Procedure The NDT procedure usually has to define the following special technical aspects:

12.8.2



Examination zones, film or IP position plan;



Radiographic or radioscopic technique and class;



Examination equipment (source and detector type, filters and screens, etc.);



Minimum image quality indicator reading;



Special restrictions due to radiation protection.

Record For reproducible testing (technical aspects only) the following is necessary, if applicable: ▪

Radiation source, type and size of focal spot and equipment used;



Tube voltage and current or source activity;



Selected film or IP systems, screens, filters and type of processing or type of detector;



Development details or manufacture, serial number and firmware version of IP reader;



Time of exposure or integration time;



Source-to-film or source-to-detector distance, magnification;



Nominal penetrated thickness;



Type and position of image quality indicators;



Reading of image quality indicators and minimum film density or S/N ratio.

Films (if used) are an important part of such an investigation, the responsibility for their stor age should be agreed in advance.

282

Figure 12-67: Example of an RT test record for welds and X-ray source (Empa)

283

Figure 12-68: Example of an RT test record for castings and isotope source (Empa)

12.9

Special and Advanced Techniques

12.9.1

Compton Backscatter Technique Compton backscatter technique detects the backscattered photons of lower energy X-rays (Klein-Nishina). Using an adequate collimation system it is possible to gain information about the depths of the origin of the scattering, the information has to be calculated to an image.

284

Source Line detector

Collimators

Compton backscatter photons

Primary beam Figure 12-69: Schema of Compton backscatter technique, to generate an image, the equipment has to be moved relative to the object

Figure 12-70: Compton backscatter application at an aircraft wing at Jet Aviation Basel (left) demonstration panel with various voids (Yxlon) It is only applicable for materials with low density, as the scattered radiation needs to penet rate back through the surface again. The advantages of this technique are that it is applied from one side only and that it gives automatically information about the depths of a flaw. It is primarily used in air plane maintenance. 12.9.2

Absorption Edge Scanning For certain applications it can be very useful to tune the energy in such a way that certain elements show clear contrasts, hitting their K absorption edge (or other inner shells). Energy tun ing, however, is possible with monochromators only, and due to flux reasons this can usually only be used on synchrotrons.

285

10.8 keV

11.8 keV

14.5 keV

Os Au Os

Au

Au

Figure 12-71: K-edge tomography on bundles of PET filaments (Ø 20 μm) settled with human foreskin fibroblasts after staining with gold and fixation in OsO4, visible is especially the difference between 10.8 keV (Os L3 edge) and 11.8 keV (Au L3 edge), for 14.5 keV the absorption for both elements is equal, pixel size 3.5 µm (Empa) 12.9.3

Phase Contrast Radiography For certain applications the amplitude contrast is that small that an examination is not possible. In such cases sometimes phase contrast radiography can be of help. Like in optical applications, the part of the beam penetrating the object has a phase difference due to slightly reduced particle velocity in material. Phase contrast radiography can be performed with poly chromatic radiation, the usual applications today, however, are done with monochromatic radiation. Future generations of detectors will allow easier applications with polychromatic radiation. In an LLL-interferometer the original beam is split. A phase shifter can adjust the phase difference of the two beams to optimum conditions. To reduce the phase shift modulated by the sample, it is often put into a liquid with similar properties; the reference beam, in this case, is penetrating the liquid as well. Tomographic applications are possible. The method is limited by the interferometer crystals. (E)

(H)

(D)

(G)

(C) (B) (F)

(A)

Figure 12-72: Schema of phase contrast radiography: (A) original beam, (B) LLL-interferometer with four mirrors, (C) phase shifter, (D) beam stopper, (E) sample, (F) optional reference sample, (G) beam containing interference information, (H) detector

Figure 12-73: Amplitude and phase contrast radiograms of a mimosa (Elettra) Differential phase-contrast imaging can be performed on a Talbot-Lau interferometer with a phase grating as beam splitter and an amplitude grating as analyser. This method can be per286

formed on a non-coherent X-ray tube. The limit is the production of the gratings. With tomo graphic methods it is possible to calculate the absorption coefficient, the refraction index and the scattering coefficient (dark field) for every location in a 3D space.

Figure 12-74: Perpendicular tomographic views of a carbon fibre reinforced polymer spring. From left: differential absorption, differential phase and dark field image (Empa) 12.9.4

Dual-Energy X-Ray Absorptiometry Dual-energy X-ray absorptiometry (DEXA or DXA, Dual-emission X-ray absorptiometry) is a technique widely used for medical applications, mostly in the context of calcification of lesions and osteoporosis. Photons of different energies are attenuated differently by bones and soft tis sue. Originally dual energy absorptiometry was developed using gadolinium 153 (44 and 100 keV peaks) in combination with photon counting. With an X-ray tube there are two possibilities: with a filter that generates two energy dependent peaks, or by a rotating disc that measure the absorption for each pixel after the beam has passed first through a known soft tissue equival ent filter and then through a known bone equivalent filter. The precision is given by 1% or less. Analogous applications for industrial products are conceivable, for radiography as well as for tomography. A similar approach is used in some systems for security purposes. The image is scanned with one X-ray tube (usually 70 kV) and two detectors with different efficiency ranges (e.g. CsI and ZnSe) and additional filters to reduce to low energy ratio on one of them. This allows a certain distinction between low, medium and high Z material, or within this context organic, inorganic and overlapping material. Other systems use two X-ray tubes (usually 70 and 140 kV) in serial mode. Dual or multiple energy radiography can also be performed tomographic.

Figure 12-75: DEXA with normal image (left), soft tissue and bone image (GE Medical)

287

Figure 12-76: Grey scale and colour coded image used for security controls, orange: organic, blue: inorganic, green: overlapping material (Heimann) 12.9.5

Electron Radiography and Autoradiography In electron radiography the object itself behaves as a metal screen. The film is exposed to the generated electrons originating from features within or on the object. A close contact between sample and film is necessary, so this technique is mainly used for flat objects. To reduce the exposure of the film of low energy X-rays, the radiation is generally very strongly filtered, using a copper - aluminium combination (sequence seen from the source side) of typically 5 mm thickness, each. Electron radiography is a standard procedure to find and compare watermarks.

Cu – Al filter Cassette with object and film

Figure 12-77: Schema of electron radiography (left) and application on a watermark with bull's head, cross and curved muzzle (www.kb.nl/kb/resources/frameset_inlichtingen-en.html) In autoradiography the object is in a first step activated by neutrons and then put into close contact with a film or an image plate. For biological applications the object can also be activated by radioactive contrast agents. Beneath biology, this technique is often used for art investigations.

288

Figure 12-78: Rembrandt, Tobit and Anna with the Kid (detail), 1626, Amsterdam, Rijksmuseum: visual image (left) and auto-radiogram, the difference that is recognised best is the lower left spinning wheel, missing in the visual image (parallel.park.org/Netherlands/pavilions/culture/rembrandt/invisible) 12.9.6

Emission Techniques Mainly for medical applications radioisotopes can actively be applied to certain regions of the body. Known are two techniques: scintigraphy and in its three-dimensional layout single photon emission computed tomography (SPECT) and positron emission tomography (PET). In both cases radioisotopes are used as markers in substances like sugars. Scintigraphy and SPECT need a strong collimation of the detector, 99.99% of the original signal gets lost. As tracer Tc-99m is often used, where the production and decay chain is: ▪

Slit camera radiographic system, same as above;



Mo-98 + n → Mo-99

(reactor)



Mo-99 → Tc-99m + β-

(T½ = 66 h)



Tc-99m → Tc-99 + γ

(analysed decay, T½ = 6 h, 140 keV)



Tc-99 → Ru-99 + β-

(T½ = 211'000 y)

The critical step is the continuous production of Mo-99, as only a few reactors world-wide are enabled for such operation.

Figure 12-79: SPECT equipment for head investigations (South Western University) For PET a β+ decay is necessary. The resulting two photons of the annihilation process are detected more or less simultaneously and lead to a line of response (LOR) when the two measured signals are within about 10 ns. As they are not exactly emitted 180° apart, the line has a

289

certain width. The detectors are arranged in circles. As the number of detected events in much lower than in conventional CT, the resolution is poorer and reconstruction techniques are more difficult.

Figure 12-80: Schema of a PET acquisition process (Langner) If it possible to reduce the resolving time of the detectors to about 500 ps, the line can be reduced to a chord. Some sources in this case use the depiction tracer concentration imaging in 4D. Two of the most used radioisotopes are F-18 and Ga-68 with the following production and decay chains: ▪

O-18 + p → F-18 + n

(local cyclotron 15 MeV protons)



F-18 → O-18 + β+

(analysed decay, T½ = 110 min, 2 γ ≥ 511 keV)



Ge-68 → Ga-68 + β+

(local Ga-68 generator, T½ = 271 d)



Ga-68 → Zn-68 + β+

(analysed decay, T½ = 68 min, 2 γ ≥ 511 keV)

and

The decay products O-18 and Zn-68, respectively, are stable. As the used substances also show a biological decay, the effective half live is slightly lower. Theoretically such processes can also be used for the investigation of technical processes.

Figure 12-81: Example of a PET scan 12.9.7

X-Ray Flash Technique X-ray flash technique makes sense if processes that happen very fast have to be observed. In most applications this is the case for explosions or related processes like ballistic and impact

290

behaviour of projectiles or air bag operations. For certain basic research like high speed deformation of materials or shock wave behaviour it can also be of use. The usual equipment for such experiments is quite unique. A further, not less unique possibility for the X-ray genera tion is the use of plasma implosions.

Figure 12-82: X-ray flash images of a projectile impact (left) and insects as a demonstration of the capability of the plasma based X-ray flash generation (Fraunhofer EMI, Cornell University) 12.9.8

X-Ray Refractometry For some applications it is useful not to measure the shadow projection of an object, but to determine the amount of refracted photons into certain directions. This will rather result in an overview over the damage situation in certain parts of the object than in detailed information about single flaws.

Figure 12-83: X-ray refraction equipment with two detector cameras, the vertical one is fed over a scattering foil, not the object itself (left), example image of the refraction intensity of a polymer sample before and after fatigue testing (BAM)

12.10

Radiation Protection

12.10.1 Fundamentals In Switzerland, as in other nations, the fundamentals for radiation protection are determined by the law. The respective laws and ordinances can be found under chapter 814.5 of the system atic collection (SR) on http://www.admin.ch. The regulations concerning transportation are under 741.6 and the international regulations under 0.814.5 and 0.741.6, respectively. The systematic collection is administrated in the Swiss governmental languages. Basically the Swiss laws and ordinances are in accordance with the recommendations of the International Commission on Radiological Protection (ICRP), http://www.icrp.org. In addition, Switzerland as member of the European Atomic Energy Community (EAEC – EURATOM) has to overtake also their directives. Some further information is available from the International Atomic Energy Agency (IAEA), http://www.iaea.org. 291

For the operation of equipment discharging ionising radiation a permission of the Swiss Federal Office for Public Health (BAG), http://www.bag.admin.ch is necessary. Technical support is given by the Swiss National Accidence Insurance Organisation, SUVA Pro, http://www.suva.ch. 12.10.2 Dose Rates and Limitations Table 12-6: Radiological units, the radiation weighting factor wR for photons and electrons of any energy is 1, for alpha particles 20, for protons and pions 2, and for neutrons dependent on the energy between 5 and 20 Designation

Symbol

Unit

Conversion

Activity

A

Becquerel [Bq]

1 Bq = 1 s-1

Absorbed dose

D

Gray [Gy]

1 Gy = 1 J kg-1

Equivalent dose

H

Sievert [Sv]

1 Sv = 1 J kg-1 H = D wR

Effective dose

E

Sievert

E = H wT

Dose rate

J

Sievert / hour

The tissue weighting factor wT describes the effect of radiation onto the different organs, for all all of them they add to 1 (whole body dose). The allowed additional dose rate for persons that are not exposed to ionising radiation due to their profession is limited to 1 mSv per year (whole body). This value has to be compared with the averaged exposition due to natural and technical sources. Medical diagnosis lead to an accumulation of: ▪

Dental radiogram: 0.01 mSv;



Cranial radiogram: 0.1 mSv;



Abdominal radiogram: 0.5 mSv;



Mammography: 0.7 mSv;



Abdominal tomogram (series): 10 mSv. Cosmic radiation Other (0.35 mSv) (0.2 mSv) Terrestric radiation (0.45 mSv)

Medical applications (1.2 mSv)

Internal (0.4 mSv)

Radon in buildings (3.2 mSv) Figure 12-84: Averaged dose per year and person in Switzerland, total 5.8 mSv Persons that are exposed due to their profession may accumulate a maximum additional dose rate of 20 mSv per year. Such persons are under regular medical supervision and need to wear a dosimeter when working in controlled areas. Thermoluminescence dosimeters (TLD) usually are read out at regular intervals by a third party (Suva, PSI or others). They consist or LiF, a substance that is similar to biological tissue and emit light dependent on the achieved dose when heated above 250 °C.

292

Figure 12-85: Types of individual dosimeters: thermoluminescence (TLD) type (left), wand type and electronic type with acoustic alarm function (Pedos, Melit) For the measurement of the dose rate several possibilities exist, the lower energy limit and the range must be selected according to the measuring purpose. Table 12-7: Types of dose rate measuring instruments (X- and gamma rays) Lower energy limit [keV]

Range

6 – 10

up to 10 µSv/h

Proportional

25

0.1 µSv/h – 0.1 Sv/h

Geiger-Müller (GM)

60

0.1 µSv/h – 1 Sv/h

50

from 0.01 µSv/h

Type Ionisation chamber Tubes

Scintillation counter

Geiger-Müller tube

Ionisation chamber V

Gas (p < po) Anode Counter

I

R

Mica window

Volume

Scintillation counter PMT

Counter

Visible light Photocathode Figure 12-86: Sketch of a Geiger-Müller tube, ionisation chamber and scintillation counter

293

Figure 12-87: Equipment for dose rate measurement and warning, neutron detector (right) (AEA, Canberra) There are currently three elements in the system of dose limitation recommended by ICRP, namely justification, optimisation and dose limitation. Justification means that any proposed activity that may cause exposure to persons should yield a sufficient benefit to society to justify the risks incurred by the radiation exposure. This feature is based on the assumption that any radiation exposure, no matter how small, carries with it a certain level of risk that is proportional to the level of exposure. This hypothesis is known as the linear non-threshold hypothesis (LNT). An example of an activity that was considered unjustified was the now-discontinued practice of fitting shoes to people's feet in pedoscopes using X-rays. The exposure resulting from this activity was considered to be unjustified and the practice was discontinued. The second element is optimisation, which is also known as the practice of ALARA (As Low As Reasonably Achievable). This means that the radiation exposures resulting from the practice must be reduced to the lowest level possible considering the cost of such a reduction in dose. ALARA is required by nearly all licensing agencies. This feature is also based on the LNT. The third element is dose limitation. This involves setting upper limits on the dose that may be received by any member of the public from all man-made exposures other than medical exposures. These limits are imposed by regulatory agencies as discussed above. 12.10.3 Possibilities of Restrictions against Radiation Basically three concepts are useful to restrict the amount of radiation: distance, time and shielding. The local dose rate depends on the square of the distance J=Jo

2

r r 2o

(12.25)

Keeping distance is especially effective for small absolute distances; e.g. using a gripper instead of the hand directly. But also the step from 5 m to 7 m reduces the radiation by a factor of two. Time has a linear effect on the dose. It would help to hurry when being within a dangerous zone, however, the danger to do mistakes then is often much more dangerous. The method that helps more is think first – act later. It makes a clear difference if the dose rate is calculated for the direct beam or for scattered radiation. Simple disc calculators are available to determine the relation between source parameters, distance and time. For scattered radiation the relevant distance is always also that one to the source. Two calculation examples: in both cases the work is done outside of a radiation room and in both cases one can assume, that the additional dose due to this work will be the only one for this week. If it is clear that persons are not permanently in the area of danger (outside of build ings) the maximum dose is 0.1 mSv per week; while if this is not the case (usually inside buildings) the dose is reduced to 0.02 mSv per week. The minimum time has always be set to one hour. In the first case the source is iridium 192 with an activity of 3.0 TBq. Absolute collimation is difficult, so the unshielded primary beam can leak. The exposure of all films takes 90 minutes. The controlled zone will have a distance from the source of 76 m (!) in direction of the primary beam and 16 m in all others.

294

In the second case a 150 kV X-ray tube is used within a building. It can be placed in such a way, that it points to the ground. The total current time is 5 mA for 25 minutes. Note: min imum time is one hour, the radius of the zone within buildings has to be multiplied by √5. The controlled zone in this case has a radius of 18 m. Therefore, such work is usually done outside working hours. The controlled zone has to be supervised by two trained operators for the whole radiation time. It has to be pointed out, that this zone is not only relevant in horizontal direction, but also in the vertical one. This has to be accounted for especially on construction sites (workers on other levels, cranes, etc.). Shielding, finally, can be done very effectively, however, it uses some technical effort and is usually not easy applicable for mobile missions.

Figure 12-88: Examples for warning signs for radiation protection, general sign (left), designation of a radiation location and information tag for the transport on radioactive substances (SUVA) 12.10.4 Shielding of X-Rays The dimensions of a shielding are usually expressed in the number half value layers n2 or tenth value layers n10 that reduces the radiation intensity to the half or to a tenth, respectively. J=

Jo 2

n2

or

J=

Jo n10

10

with

n2 = 3.3 n10

(12.26)

The respective layer thickness is dependent on the photon energy. Usually the values are given for the tube voltage and for lead. Conversion tables are used to calculate the half value layers for other materials, also dependent on the tube voltage. 300

3.5

Lead equivalent [mm]

Lead half value layer [mm]

4

3 2.5 2 1.5 1 0.5

Gypsum

250 200

Solid brick

150

Concrete

100

Steel Barite concrete

50 0

0 0

100

200 300 400 Tube voltage [kV]

500

0

100

200

300

400

500

Tube voltage [kV]

Figure 12-89: Half value layer for lead for varying tube voltage (left) and equivalent thickness for construction materials, reading example: for a tube voltage of 300 kV, the half value layer for concrete would roughly be 120 mm It makes a clear difference if the shielding is against the primary beam or against scattered radiation. If the source is not clearly fixed and cannot only radiate into one predefined direction (e.g. computed tomography scanners), the shielding layout of the whole installation has to be done considering the primary beam. The scattering factor f also depends on the tube voltage between 0.1% at 50 kV and 0.4% at 300 kV. For the primary beam the factor is 1.

295

Finally the efficiency of the tube, also dependent on the tube voltage, has to be included in form of the X-ray constant hx. Such the dose rate and the dose can be calculated from J=

I hx f r2 F

and

H=

I t hx f r2 F

(12.27)

where r is the distance, I the current of the tube and F is the relation between the unshielded and the shielded dose rate. Well approximated values for the X-ray constant for copper filtered tubes are given by hx =

2

U X

(12.28)

X-ray constant [mSv m2 h-1 mA-1]

where U is the maximum tube voltage in kV and X is a constant of 50 for full wave rectified and 75 for direct current voltage supply. This rule of thumbs gives a value in mSv m 2 h-1 mA-1. Tubes with beryllium filtering (especially tubes for structural analysis like residual stress measurement) have rather higher X-ray constants. Further details can be taken from DIN 54113-3 (in German).

4000

3000

2000

Full wave

1000 Direct current 0 0

100

200

300

400

500

Tube voltage [kV] Figure 12-90: Calculated X-ray constant dependence on tube voltage, measured points with 0.5 mm copper filtering Due to scattering and build-up effects simple calculations often give only approximations and have to be confirmed by measurements. 12.10.5 Shielding of Gamma Rays For gamma rays scattering is assumed to be constant at 4.5%. The equivalent dose rate h 10 [mSv m2 h-1 GBq-1] corresponds to a dose rate at a depth of 10 mm in biological tissue. The values for any isotope can be found in the federal ordinance “Strahlenschutzverordnung”, SR 814.501, appendix 3, column 6. With A as the activity of the source the dose rate and dose are given by J=

296

A h10 f r2 F

and

H=

A t h10 f r2 F

(12.29)

Table 12-8: Equivalent dose rate in mSv m2 h-1 GBq-1 and half value layers in mm for common isotope sources, *) thulium 170 also emits two high energy β-particles that can be partly reabsorbed in the pellet, so a continuous background bremsstrahlung with a maximum of 0.9 MeV and a mean of 0.16 MeV is emitted; the relative intensity of this background and therefore all HVL depend on the physical size of the source and the given values are indicative only, **) americium 241 is usually used in americium-beryllium neutron sources, the given values represent the gamma fraction only Isotope

h10

Uranium

Lead

Steel

Concrete

Water

Co-60

0.366

7

12

20

61

135

Cs-137

0.092

3

7

15

49

94

Ir-192

0.131

4

6

13

41

58

Se-75

0.064

0.45

1.4

7.4

29

51

Yb-169

0.061

0.3

0.6

3

10

20.5

Tm-170 *)

0.001

0.15

0.3

1.8

5.7

12.1

Am-241 **)

0.019

0.1

0.2

1.2

3.9

8.2

12.10.6 Shielding of Neutrons The shielding of neutrons is best done with materials containing hydrogen; the following table, therefore, gives a magnitude only. Table 12-9: Tenth value layer (TVL) of neutrons in cm Water Paraffin Direct neutrons

1st TVL

15

25

following TVLs

10

16

8

13

Scattered neutrons

12.11

Concrete Barite concrete

Steel Lead 47 37

Summary and Conclusions Radiography is probably the most complete non-destructive evaluation method. It is non-contacting, it can be applied to basically all materials and it does not rely on the outer shape or on the surface quality. The attraction of radiography may be based on the fact that in most cases the results are given in form of an image with a content that is more or less understandable to the public. As most people have seen (medical) radiographs before, they feel immediately familiar with the method. A second reason for the popularity may be that radiography has been an imaging method since the discovery of X-rays (in opposite e.g. to ultrasonics or eddy current). The unbeatable advantage of radiography is not, as a lot of people would say, that with a film they have “something in hand”, as the film can get lost, destroyed and will lose quality due to ageing anyway; the real advantage is that an IQI can simultaneously be imaged onto the film or the electronic image allowing an immediate control of the quality of the exposure. For the time being for welds film radiography is still the only technique fulfilling the requested resolution. In the next future imaging plates and flat panels with a similar quality will come to market and then the question if film or film-less will have to be answered differently. The answer will mainly rely on costs, time and reliability. For the moments it seems that the latter could be very dominant, as long term experiences with imaging plates and flat panels under industrial conditions are still missing. The first generation of flat panels had the severe drawback that their lifetime turned out to be very limited. The use of computed tomography outside of the laboratory and for special purposes only is in progress. Increased computer power makes it possible to have such devices directly in the pro duction line. The missing part for the moment are areal detectors that can handle higher X-ray energies over long time and with an acceptable signal/noise ratio, where the noise is mainly generated by scattered radiation. Another approach could be to live with the scattered radiation 297

and to find ways to eliminate their influence mathematically. The use of hybrid arrays with detectors that are sensitive to different energy ranges could be one step in this direction. In the microfocus domain the main steps for the moment are probably reached. The technique is well established, mainly in electronics manufacturing. In the research field the possibilities of synchrotron radiation and powerful neutron sources just opened a wide range of applications. Probably the reachable resolution is less attractive than the possibilities of in-situ altering the object and studying resulting effects real-time in 3D and tuning the energy of the radiation to a range with maximum contrast. The overall disadvantage of radiography, the inherit connection to radiation protection, will be unchanged and an issue to live with.

298

299

13

APPENDIX

13.1

Literature

13.1.1

Selected Books Bauer, N.: Guideline for Industrial Image Processing, Fraunhofer-Allianz Vision, ISBN 3816762964, ISSN 16181565 , electronically: http://www.vision.fraunhofer.de (further booklets in German only) Birolini, A.: Zuverlässigkeit von Geräten und Systemen, Springer 1997, ISBN 4540609970, Reliability Engineering – Theory and Practice, Springer, 2004, ISBN 354040287X (English version is more complete) Blitz, J. and Simpson, G.: Ultrasonic Methods of Non-Destructive Testing, Chapman & Hall, 1996, ISBN 0412604701 Blumich, B.: NMR Imaging of Materials, Oxford, 2000, ISBN 9780198506836 or 019850683X Bray, D.E. and Stanley, R.K.: Nondestructive Evaluation – A Tool in Design, Manufacturing and Service, CRC Press, 1996, ISBN 0849326559 Cartz, L.: Nondestructive Testing, ASM, 1995, ISBN 0871705176 Charlesworth, J.P. and Temple, J.A.G.: Engineering Applications of Ultrasonic Time-of-Flight Diffraction, Research Studies Press, 2001, ISBN 0863802397 Cheecke, J.D.N.: Fundamentals and Applications of Ultrasonic Waves, CRC Press, 2002, ISBN 0849310300 Deutsch, V., Morgner, W., Vogt, M.: Magnetpulver-Rissprüfung – Grundlagen und Praxis, Castell, 2012, ISBN 9783934255517 Deutsch, V. and co-authors: NDT – compact and understandable, 13 information booklets, Castell (also available in German) Fowler, K.A., Hutchkiss, F.H.C., Yamartino, T.V., Nelligan, T.: Important Characteristics of Sound Fields of Ultrasonic Transducers, Web Version 2012, electronically: http://www.olympusims.com/en/knowledge Hellier, C.: Handbook of Nondestructive Evaluation, McGraw-Hill, 2001, ISBN 0070281211, also available as CD-ROM Jol, H.M.: Ground Penetrating Radar: Theory and Applications, Elsevier, 2008, ISBN 0444533486 Kak, A.C. and Slaney, M.: Principles of Computerized Tomographic Imaging, Society of Industrial and Applied Mathematics, 2001, ISBN 089871494X, electronically: http://www.slaney.org/pct Klyuev, V.V. and Zusman, G.V.: Nondestructive Testing and Diagnostics, RSNTTD and Metrix Instrument Co., 2004 Kolobrodov, V.G. 352740130X

and

Schuster,

N.:

Infrarotthermographie,

Wiley-VCH,

1999,

ISBN

Langenberg, K.-H., Markelin, R., Mayer, K.: Theoretische Grundlagen der zerstörungsfreien Materialprüfung mit Ultraschall, Oldenbourg, 2009, ISBN 9783486588811 Lüthi, Th.: Essais Non Destructives, in: Degallaix, S. and Ilschner B. (Edts.): Traité des Matériaux Vol. 2, Caractérisation Expérimentale des Matériaux I: Propriétés physiques, thermiques et mécanicques, PPUR, 2007, ISBN 9782880745677, pp. 353 - 391 Maldague, X.P.V.: Theory and Practice of Infrared Technology for Nondestructive Testing, Wiley, 2001, ISBN 0471181900 Mittleman, D.: Sensing with Terahertz Radiation, Springer, 2003, ISBN 3540431101 Natterer, F.; Wübbeling, F.: Mathematical Methods in Image Reconstruction, SIAM, 2001, ISBN 0898714729 Neumann, E. et al.: Ultraschallprüfung von austenitischen Plattierungen, Mischnähten und austenitischen Schweissnähten, Expertverlag, 1995, ISBN 316910785, partly electronically in English: http://www.ndt.net/publicat/bibliog/expert.htm Olympus: Introduction to Phasd Array Ultrasonic Technology Applications, Olympus, 2007 ISBN 0973593342, electronically: http://www.olympus-ims.com/en/books/

300

Olympus: Advances in Phased Array Ultrasonic Technology Applications, Olympus, 2007, ISBN 0973593342, electronically: http://www.olympus-ims.com/en/books/ Olympus: Phased Array Testing: Basic Theory for Industrial Applications, Olympus, 2010, electronically: http://www.olympus-ims.com/en/books/ Pastorino, M.: Microwave Imaging, Wiley, 2010, ISBN 9780470278000 Rastogi, P.K. (Ed): Optical Measurement Techniques and Applications, Artech House, 1997, ISBN 0890065160 Rastogi, P.K. and Inaudi, D. (Eds): Trends in Optical Nondestructive Testing, Elsevier, 2000, ISBN 0080430201 Richter, H.U.: Chronik der Zerstörungsfreien Prüfung, DGZfP, 1999, ISBN 3871559423 Rose, J.L.: Ultrasonic Waves in Solid Media, Cambridge, 1999, ISBN 0521640431 Sansalone, M.J. and Streett, W.B.: Impact-Echo: Nondestructive Evaluation of Concrete and Masonry, Bullbrier, 1997, ISBN 0961261064 Setter, N. (Ed.): Piezoelectric Materials in Devices, EPFL, 2002, ISBN 2970034603 Shull, P.J.: Nondestructive Evaluation – Theory, Techniques and Applications, Dekker, 2002, ISBN 0824788729, CD-ROM 0824743164 Stadthaus, M. et al.: Theoretical Principles of Liquid Penetrant Testing, DVS, 1999, ISBN 3871559431 Stroppe, H. and 9783934255494

Schiebold,

K.:

Wirbelstrom-Materialprüfung,

Castell,

2011,

ISBN

Vlaardingerbroek, M.T. and den Boer, J.A.: Magnetic Resonance Imaging – Theory and Practice, Springer, 2003, ISBN 3540436812 Wenzel, H.: Health Monitoring of Bridges, Wiley, 2009, ISBN 9780470031755 Nondestructive Testing Handbook; 3rd Edition, Vol. 1 Leak Testing, ASNT, 1998, ISBN 1571170715, CD-ROM 1571170383 (errata available from http://www.asnt.org) ibid.: Vol. 2 Liquid Penetrant Testing, ASNT, 1999, ISBN 1571170286, CD-ROM 1571170391 (errata available from http://www.asnt.org) ibid.: Vol. 3 Infrared and Thermal Testing, ASNT, 2001, ISBN 1571170448, CD-ROM 1571170812 ibid.: Vol. 4 Radiographic Testing, ASNT, 2002, ISBN 1571170456, CD-ROM 1571170987 (errata available from http://www.asnt.org) ibid.: Vol. 5 Electromagnetic Testing, ASNT, 2004, ISBN 1571170464 (incl. CD-ROM) (errata available from http://www.asnt.org) ibid.: Vol. 6 Acoustic Emission Testing, ASNT, 2005, ISBN 1571171061, CD-ROM 1561171371 (errata available from http://www.asnt.org) ibid.: Vol. 7 Ultrasonic Testing, ASNT, 2007, ISBN 1571171054, CD-ROM 151171634 (errata (2) available from http://www.asnt.org) ibid.: Vol. 8 Magnetic Testing, ASNT, 2008, ISBN 0931403030, CD-ROM 9781571171634 (errata available from http://www.asnt.org) ibid.: Vol. 9 Visual Testing, ASNT, 2010, ISBN 9781571171863, CD-ROM 9781571171863 (errata available from http://www.asnt.org) ibid.: Vol. 10 NDT Overview, ASNT, 2012, ISBN 9781571171870, CD-ROM 9781571172341 Review of Progress in Quantitative Nondestructive Evaluation (QNDE), proceedings of the annual conferences, abstracts partially electronically under http://www.cnde.iastate.edu/qnde/qnde.html Further sources see also the web sites of the NDT societies and http://www.ndt.net. 13.1.2

Selected Journals Contrôles Essais Mesures, quarterly, http://www.sogicommunication.com, ISSN 1637-4657 Insight, irregular, The British Institute of Non-Destructive Testing, BINDT, http://www.bindt.org, ISSN 1354-2575 (ISI-Rating: SCIE) Journal of Nondestructive Evaluation, quarterly, Kluwer, http://www.wkap.nl, ISSN 0195-9298 (ISI-Rating: SCI)

301

Journal of Testing and Evaluation, bimonthly, ASTM International, http://www.astm.org, ISSN 0090-3973 (ISI-Rating: SCI) Materialprüfung, monthly, Hanser, http://www.hanser.de, ISSN 0025-5300 (ISI-Rating: SCIE) Materials Evaluation, monthly, American Society for http://www.asnt.org, ISSN 0025-5327 (ISI-Rating: SCIE)

Nondestructive

Testing,

ASNT,

NDT & E International, bimonthly, Elsevier, http://www.elsevier.nl, ISSN 0963-8695 (ISI-Rating: SCI) Research in Nondestructive Evaluation, quarterly, Springer, http://www.springer-ny.com or http://www.asnt.org, ISSN 0934-9847 (ISI-Rating: SCI) Russian Journal of Nondestructive Testing (English translation of Дефектоскопия), monthly, Maik Nauka, http://www.maik.rssi.ru, ISSN 1061-8309 (ISI-Rating: SCIE) Technical Acoustics, completely electronically under http://webcenter.ru/~eeaa/ejta/, ISSN 0869-4583 The e-Journal of Nondestructive Testing & Ultrasonics, monthly, completely electronically under http://www.ndt.net, ISSN 1435-4934 Ultrasonics, monthly, Elsevier, http://www.elsevier.nl, ISSN 0041-624X, (ISI-Rating: SCI) ZfP-Zeitung, quarterly, German, Austrian and Swiss Society for Nondestructive Testing, DGZfP, ÖGfZP, SGZP, http://www.dgzfp.de, ISSN 1616-069X

302

13.2

Web Sites

13.2.1

NDT Societies www.efndt.org

EFNDT - European Federation for Nondestructive Testing

www.icndt.org

ICNDT - International Committee for Non Destructive Testing

www.abende.org.br

ABENDE - Associação Brasileira de Ensiaos Não Destrutivos

www.aend.org

AEND - Asociación Española de Ensayos No Destructivos

www.aindt.com.au

AINDT - Australian Institute for Non Destructive Testing

www.aipnd.it

AIPnD - Associazione Italiana Prove non Distruttive

www.aroend.ro

ARoENd - Asociatia Romana de Examinari Nedistructive

www.asnt.org

ASNT - American Society for Nondestructive Testing

www.bant.be

BANT - Belgian Association for Non Destructive Testing

www.bindt.org

BINDT - The British Institute of Non-Destructive Testing

nntdd.hit.bg

BGSNDT - Bulgarian Society for Nondestructive Testing

www.chsndt.com

CHSNDT - Chinese Society for Nondestructive Testing

www.cinde.ca

CINDE - Canadian Institute for Nondestructive Evaluation

www.cndt.cz

CNDT - Česká společnost pro nedestruktivní testování

www.cofrend.com

COFREND - Confédération Française pour les essais non-destructifs

www.hdkbr.hr

CrSNDT - Hrvatsko društvo za kontrolu bez razaranja

www.dgzfp.de

DGZfP - Deutsche Gesellschaft für zerstörungsfreie Prüfung

www.ndtsweden.com

FOP - Föreningen för Oförstörande Provning

www.hsnt.gr

HSNT - Ελλενικη Εταιρεια μη Καταστροφικων Ελεγχων

www.isnt.org.in

ISNT - Indian Society for Non-Destructive Testing

wwwsoc.nii.ac.jp/jsndi

JSNDI - Japanese Society for Non-Destructive Inspection

www.kint.nl

KINT - Nederlandse Vereniging voor Kwaliteitstoezicht, Inspectie en Niet-destructieve Techniek

www.lnbd.lt

LNBD – Lietuvos neardomųjų bandymų ir techninės diagnostikos draugija

www.marovisz.hu

MAROVISZ - Magyar Roncsolásmentes Vizsgálati Szövetség

www.ndt.no

Norsk Forening for Ikke-destruktiv Prøving

www.ndta.org.nz

NZNDTA – New Zealand Non Destructive Testing Association

www.oegfzp.at

ÖGfZP - Österreichische Gesellschaft für zerstörungsfreie Prüfung

www.ptbnidt.pl

PTBNIDT - Polskie Towarzystwo Badań Nieniszczących i Diagnostyki Technicznej

www.rsnttd.ru

RSNTTD - Российского общества по неразрушающему контролю и технической диагностике

www.saint.org.za

SAINT - South African Institute for Non-Destructive Testing

www.sgzp.ch

SGZP - Schweizerische Gesellschaft für zerstörungsfreie Prüfung

www.ssndt.sk

SSNDT - Slovenskej spoločnosti pre nedeštruktívne testovanie

www.turkndt.org

Turk NDT - Türk Tahribatsız Muayene Cemiyeti

www.usndt.com.ua

USNDT - Українське товариство неруйнівного контролю та технічної діагностики

303

13.2.2

Producers, Service and Trade (extract) www.

AT

ET

LT

MT

NMR

PT

RT

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VT

3d-radar.com

X

3d-shape.com

X

accsys.com

X

acousticideas.com

X

acsys.ru

X

aeconsulting.com

X

aerospace.chemetall.com

X

X

agiltron.com

X

X

aibotix.com

X

aig-imaging.com

X

airmar.com airstar1.com

X X

X

ais4ndt.com

X

aixnmr.com

X

ajat.fi

X

aos-fiber.com

X

applusrtd.com

X

asams.co.uk

X

X

X

X

as-e.com

X

atmmicrowave.com asi-nde.com automation.de

X X X

awe.co.uk

X

bartington.com b-r.ch

X X

X

balteau.com

X

bossanovatech.com

X

breuckmann.com

X

bruker.com

X

buck-mhe.ch

X

X

canberra.com

X

cbs-cbt.com

X X

X

cernex.com

X

circlesafe.com

304

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cmosxray.com

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collimatedholes.com

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comet-xray.com

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corestar-corp.com

X

X

betontest.de

centurionndt.com

WT

X

www.

AT

ET

LT

MT

NMR

PT

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UT

creaform3d.com X

dakotaultrasonics.com

X

danfysik.com

X

dantecdynamics.com

X

dectris.com

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delta-engineering.de

X X

X

dercopter.de

X

diffu-therm.de

X

X

digiray.com digitalwavecorp.com

X X

ditom.com

X

dpix.com

X

dr-hillger.de

X

duerr-ndt.de

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eddy-current.com

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edevis.com

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edmundoptics.com

X

electrophysics.com

X

elektrophysik.com

X

X

X

elp-gmbh.de

X

X

elva-1.com envirocoustics.gr

X X

X

X

X

era.co.uk erda.org

X X

etrema-usa.com

X

feinfocus.com fibatech.com

X X

fischundpartner.ch

X

X

X

flir.com foerstergroup.de

X X

fpw-prueftechnik.de

X

fujifilm.eu ge-mcs.com

WT

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cyberlogic.org

deltatest.de

VT

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X

X

X X

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geophysical.biz

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geophysical.com

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georadar.com

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geoscan-research.co.uk

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geosearches.com geotest.ch

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geozondas.com

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giscogeo.com

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goldenengineering.com

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gom.com

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g-p-r.com

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grecon.de

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X

grindosonic.com

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guided-ultrasonics.com

X

hallcrest.com

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X

hamamatsu.com

X

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heinetech.com

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heliotis.ch

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helling-ndt.de

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hirsch-prueftechnik.de

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hkg-ndt.de ibgndt.de

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X

X

X

X

X

X

X

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icm.be

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imasonic.com

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imce.cit.be

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impact-echo.com

X

inficon.com

X

infratec.de

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innerspec.com innotest.ch

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innovative-test.com

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interflux.de

X

X

intopsys.com intron.ru

X X

X

inuktun.com

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ircon.com

X

isovac.com

X

isravision.com

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jme.co.uk

X

jsrultrasonics.som X

kappa.de

X

kd-flux-technic.de

X

X

X

X

X

keyence.de kodak.com

X X

kaisersystems.com karldeutsch.de

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X

hitachimed.com

306

WT

X X

www.

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konicaminolta.com

VT X

kontrolltechnik.com

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ksi-germany.com

X

labek.at

X

labino.com

X

X

X

landrex.com.tw

X

laser-ndt.com

X

law-ndt.de

X

lazzero.com

X

lecktesten.de

X

lecoeur-electronique.com

X

lixi.com

X

logosimaging.com

X

lytro.com

X

mac-ndt.com

X

X

magnaflux.com

X

magwerks.com

X

X X

matec.com

X

maurermagnetic.ch

X

mcmeister.com

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mdc-vacuum

X

X

X

X

X

X

mds.nordion.com

X

melit.ch

X

metritec.de

X

mfescan.com

X

micro-epsilon.de

X

X

microphotonics.com

X

microtec.eu

X

microwaveeng.com

X

midas-ndt.co.uk

X

millitech.com mistrasgroup.com

X X

X

X

mkckorea.com mr-chemie.de

X

X X

X

X

mtinstruments.com

X

ndtautomation.com

X

ndtsolutions.com

X

ndtsystems.com

X

ndttech.com

X

neo.no nordinkraft.com

WT

X X

307

www.

AT

ET

LT

MT

NMR

PT

nuctech.com

RT

TT

UT

X

oerlikon.com

X

olympus-ims.com

X

X

X

optonor.no

X X

X

X

peakndt.com

X

pecscan.ca

X

pedos.ch

X

penetradar.com

X

perkinelmer.com

X

X

phoenixisl.co.uk

X

phoenix-x-ray.com

X

phynix.com

X

X

picometrix.com

X

piezo.com

X

X

piezotechnologies.com

X

X

pigsunlimited.com

X

plantintegrity.com

X

polytec.com

X

pontax.de

X

positronsystems.com

X

ppic.com

X

proceq.ch

X X

X

X

procon-x-ray.de

X

proscan.de

X

pruftechnik.com

X

X

puchold.de

X

X

pva-analyticalsystems.com

X

qmi-inc.com

X

X

qnetworld.com

X

X

qwip.com

X

radiabeam.com

X

ramayes.com

X

rauschtv.com

X

X

rayscan.eu

X

res-eng.com resonic.com

X X

richard-wolf.com ril-chemie.de ritecinc.com

308

WT

X

nuson.nl

pacndt.com

VT

X X

X

X

X X

www.

AT

rohmann.de rte.de

ET

LT

MT

NMR

PT

RT

TT

UT

VT

X X

sbir.com

X

scanmaster-irt.com

X

scana-msc.com

X

schicktech.com

X

scopes.com

X

sds-systemtechnik.com

X

sensimainsp.com

X

X

sensoft.ca

X

sentinelndt.com

X

silverwinguk.com

X

X

slx-inc.com

X

smithsdetection.com

X

snakeeye.com

X

socomate.com

X

sodern.com

X

sofranel.com sofratest.com

X X

sonaspection.com

X

X

X

X

X

X

X

sonatest-plc.com

X

X

X

X

sonatest.com

X

X

X

X X

sonoscan.com

X

sonotec.de

X

X

sonotronndt.com

X

sonovation.com

X

X

sontec.ch

X X

spectroline.com srem.fr stefan-mayer.com

X

X

X

X

X

X

steinbichler.de

X

step-sensor.de

X

stressphotonics.com stresstech.fi

X X

X

X

studenroth.com

X

swissneutronics.ch

X

synview.de tckglobal.com technologydesign.com

X

X

sonix.com

soundprint.com

WT

X X X

309

www.

AT

ET

LT

MT

NMR

PT

RT

TT

tecnar.com

UT

VT

X

tecscan.ca

X

X

teraview.com

X

terraplus.com

X

testex-ndt.com

X

thalesgroup.com

X

X

thermalwave.com

X

thermosensorik.de

X

tiede.de tisec.com

X

X

X

X

X

tmelectronics.com

X

tqc.co.uk

X

trak.com

X

transmetra.ch

X

troxlerlabs.com

X

thruvision.com

X

tscinspectionsystems.com

X

twi.co.uk

X

X

ulis-ir.com

X

ultrangroup.com

X

ultrasonic-sciences.co.uk

X

unicorn-automation.co.uk

X

usultratek.com

X

utex.com

X X

vacom.de

X

vacuuminst.com

X

vallen.de

X

varian.com varianinc.com

X X

X

vermon.fr

X

vidisco.com

X

viscom.com

X

vizaar.com

X X

vjt.com

X

X

vogt-ndt.de

X

volpi.ch von-der-heyde-gmbh.de vsonomatic.com

X X X

X

wavelineinc.com wazau.de

X

X

uxr.com

310

WT

X X

www.

AT

ET

LT

MT

werthmesstechnik.de

PT

RT

TT

WT

X

X X

X

woodeyeinc.com

X

xenics.com

X

xradia.com

X

xrt.com.au

X

xtekxray.com

X

yxlon.com

X

z-ndt.com

VT

X

wilnos.de

zetec.com

UT

X

westerninstruments.com wins-ndt.com

NMR

X X

311

13.2.3

Selected Research and Information Sites www.wfndec.org

World Federation of NDE Centers

athena.uwindsor.ca

Windsor University

imacwww.epfl.ch

Swiss Federal Institute of Technology (EPFL), Applied Computing and Mechanics Laboratory

nde.mit.edu

Massachusetts Institue of Technology, NDE Lab

physics.nist.gov

National Institute of Standards and Technology

smartsite.immt.pwr.wroc.pl

Wroclaw University of Technology, Institute of Materials and Applied Mechanics

www.anl.gov

Argonne National Laboratory

www.bam.de

Federal Institute for Materials Research and Testing

www.bgr.de

Federal Institute for Geosciences and Natural Resources

www.biomed.ee.ethz.ch

Swiss Federal Institute of Technology (ETHZ), Institute of Biomedical Engineering

www.bnl.gov

Brookhaven National Laboratory

www.csem.ch

Swiss Center for Electronics and Microtechnology

www.cnde.com

Center for Nondestructive Evaluation & Applied Technology, The Johns Hopkins University

www.cnde.iastate.edu

Center for Nondestructive Evaluation, Iowa State University

www.empa.ch

Swiss Federal Laboratories for Materials Testing and Research

www.gel.ulaval.ca

Laval University

www.hse.gov.uk

Health & Safety Executive

www.ifb.ethz.ch

Swiss Federal Institute of Technology (ETHZ), Institute for Building Materials

www.ionactive.co.uk

Ionactive Consulting Ltd. (radiation and radiation protection)

www.izfp.fraunhofer.de

Fraunhofer Institute for Nondestructive Testing

www.lbl.gov

Lawrence Berkeley National Laboratory

www.llnl.gov

Lawrence Livermore National Laboratory

www.me.sc.edu/Research/cmmnde/ University of South Carolina, Center for Mechanics, Materials and Non-Destructive Evaluation www.ndt-ed.org

NDT Resource Center

www.nordtest.org

Nordtest

www.nrc-cnrc.gc.ca

National Research Council Canada

www.ntiac.com

Nondestructive Testing Information Analysis Center

www.ob-ultrasound.net

Obstetric Ultrasound (medical / historical information site)

www.olympus-ims.com/en/ndt-tutorials/

312

Olympus (NDT tutorials site)

www.rcnde.ac.uk

UK Research Centre in Nondestructive Evaluation

www.rsphysse.anu.edu.au

Australian National University, Dept. of Applied Mathematics

www.sandia.gov

Sandia National Laboratories

www.swri.edu

Southwest Research Institute

www.uni-magdeburg.de

Otto-von-Guericke-University of Magdeburg

www.vision.fraunhofer.de

Fraunhofer-Allianz Vision

www.xray.hmc.psu.edu/rci

Penn State University, Milton S. Hershey Medical Center - A Century of Radiology (medical / historical information site)

www.xraytomography.com

Bologna University, Physics Department

www.zfm.ethz.ch

Swiss Federal Institute of Technology (ETHZ), Centre of Mechanics

www.zfp.uni-stuttgart.de

Stuttgart University, Institute for Polymers

www-ece.rice.edu

Rice University, Department of Electrical and Computer Engineering

www-leti.cea.fr

Commissariat d'Energie Atomique, Laboratoire d'Electronique de Technologie de l'Information

13.3

EN Standards (related to NDT) EN 3-8 Portable fire extinguishers – Part 8: Additional requirements to EN 3-7 for the construction, resistance to pressure and mechanical tests for extinguishers with a maximum allowable pressure equal or lower than 30 bar (with amendment AC) EN 3-9 Portable fire extinguishers – Part 9: Additional requirements to EN 3-7 for pressure resistance of CO2 extinguishers (with amendment AC) EN 444 Non-destructive testing – General principles for the radiographic examination of metal lic materials using X- and gamma-rays EN 583-1 Non-destructive testing – Ultrasonic examination – Part 1: General principles (with amendment A1) EN 583-2 Non-destructive testing – Ultrasonic examination – Part 2: Sensitivity and range setting EN 583-3 Non-destructive testing – Ultrasonic examination – Part 3: Transmission technique EN 583-4 Non-destructive testing – Ultrasonic examination – Part 4: Examination for discon tinuities perpendicular to the surface (with amendment A1) EN 583-5 Non-destructive testing – Ultrasonic examination – Part 5: Characterisation and siz ing of discontinuities (with amendment A1) EN 583-6 Non-destructive testing – Ultrasonic examination – Part 6: Time-of-flight diffraction technique as a method for detection and sizing of discontinuities EN 623-1 Advanced technical ceramics – Monolithic ceramics – General and textural properties – Part 1: Determination of the presence of defects by die penetration EN 635-1 Plywood – Classification by surface appearance – Part 1: General EN 635-2 Plywood – Classification by surface appearance – Part 2: Hardwood EN 635-3 Plywood – Classification by surface appearance – Part 3: Softwood EN 635-5 Plywood – Classification by surface appearance – Part 5: Methods for measuring and expressing characteristics and defects EN 975-1 Sawn timber – Appearance grading of hardwoods – Part 1: Oak and beech (with amendment AC) EN 975-2 Sawn timber – Appearance grading of hardwoods – Part 2: Poplars EN 1090-2 Execution of steel structures and aluminium structures: Part 2: Technical require ments for steel structures EN 1090-3 Execution of steel structures and aluminium structures: Part 3: Technical require ments for aluminium structures EN 1147 Portable-ladders for fire service use (with amendment A1) EN 1330-1 Non-destructive testing – Terminology – Part 1: List of general terms EN 1330-2 Non-destructive testing – Terminology – Part 2: Terms common to the non-destructive testing methods EN 1330-3 Non-destructive testing – Terminology – Part 3: Terms used in industrial radio graphic testing EN 1330-4 Non-destructive testing – Terminology – Part 4: Terms used in ultrasonic testing EN 1330-7 Non-destructive testing – Terminology – Part 7: Terms used in magnetic particle testing EN 1330-8 Non-destructive testing – Terminology – Part 8: Terms used in leak tightness test ing EN 1330-9 Non-destructive testing – Terminology – Part 9: Terms used in acoustic emission testing EN 1330-10 Non-destructive testing – Terminology – Part 10: Terms used in visual testing EN 1330-11 Non-destructive testing – Terminology – Part 11: Terms used in X-ray diffraction from polycrystalline and amorphous materials EN 1369 Founding – Magnetic particle testing EN 1370 Founding – Examination of surface condition EN 1371-1 Founding – Liquid penetrant inspection – Part 1: Sand, gravity die and low pressure die castings EN 1371-2 Founding – Liquid penetrant inspection – Part 2: Investment castings

313

EN 1440 LPG equipment and accessories – Periodic inspection of transportable refillable LPG cylinders (with amendment A1) EN 1518 Non-destructive testing – Leak testing – Characterisation of mass spectrometer leak detectors EN 1559-3 Founding – Technical conditions of delivery – Part 3: Additional requirements for iron castings EN 1593 Non-destructive testing – Leak testing – Bubble emission techniques (with amendment A1) EN 1711 Non-destructive examination of welds – Eddy current examination of welds by complex plane analysis (with amendment A1) EN 1779 Non-destructive testing – Leak testing – Criteria for method and technique selection (with amendment A1) EN 1802 Transportable gas cylinders – Periodic inspection and testing of seamless aluminium alloy gas cylinders EN 1803 Transportable gas cylinders – Periodic inspection and testing of welded carbon steel gas cylinders EN 1968 Transportable gas cylinders – Periodic inspection and testing of seamless steel gas cylinders (with amendment A1) EN 1971-1 Copper and copper alloys – Eddy current test for measuring defects on seamless round copper and copper alloy tubes – Part 1: Test with an encircling test coil on the outer surface EN 1971-2 Copper and copper alloys – Eddy current test for measuring defects on seamless round copper and copper alloy tubes – Part 2: Test with an internal test coil on the inner sur face EN 2082-3 Aerospace series – Aluminium alloy forging stock and forgings – Technical specifica tion – Part 3: Pre-production and production forgings EN ISO 2128 Anodizing of aluminium and its alloys – Determination of thickness of anodic oxidation coatings – Non-destructive measurement by split-beam microscope EN 2130 Aerospace series – Bearings, precision ball in corrosion resisting steel for instruments and equipment – Technical specification EN ISO 2143 Anodizing of aluminium and its alloys – Estimation of loss of absorptive power of anodic oxidation coatings after sealing – Dye-spot test with prior acid treatment EN 2157-3 Aerospace series – Steel – Forging stock and forgings – Technical specification – Part 3: Pre-production and production forgings EN ISO 2178 Non-magnetic coatings on magnetic substrates – Measurement of coating thickness – Magnetic method EN 2349-201 Aerospace series – Requirements and test procedures for relays and contactors – Part 201: Visual inspection EN ISO 2360 Non-conductive coatings on non-magnetic electrically conductive basis materials – Measurement of coating thickness – Amplitude-sensitive eddy current method EN ISO 2361 Electrodeposited nickel coatings on magnetic and non-magnetic substrates – Measurement of coating thickness – Magnetic Method EN ISO 2400 Non-destructive testing – Ultrasonic examination – Specification for calibration block No. 1 EN 2591-101 Aerospace series – Elements of electrical and optical connection – Test methods – Part 101: Visual examination EN 2644 Aerospace series – Rod assemblies for flight controls – Technical specification EN ISO 2808 Paints and varnishes – Determination of film thickness EN ISO 3059 Non-destructive testing – Penetrant testing and magnetic particle testing – Viewing conditions EN ISO 3183 Petroleum and natural gas industries – Steel pipe for pipeline transportation systems EN ISO 3452-1 Non-destructive testing – Penetrant testing – Part 1: General principles EN ISO 3452-2 Non-destructive testing – Penetrant testing – Part 2: Testing of penetrant materials EN ISO 3452-3 Non-destructive testing – Penetrant testing – Part 3: Reference test blocks

314

EN ISO 3452-4 Non-destructive testing – Penetrant testing – Part 4: Equipment EN ISO 3452-5 Non-destructive testing – Penetrant testing – Part 5: Penetrant testing at temperature higher than 50 °C EN ISO 3452-6 Non-destructive testing – Penetrant testing – Part 6: Penetrant testing at temperatures lower than 10 °C EN 3475-201 Aerospace series – Cables, electrical, aircraft use; Test methods – Part 201: Visual examination EN ISO 3497 Metallic coatings – Measurement of coating thickness – X-ray spectrometric methods EN ISO 3543 Metallic and non-metallic coatings – Measurement of thickness – Beta backscatter method (with amendment AC) EN ISO 3668 Paints and varnishes – Visual comparison of the colour of paints EN 3745-201 Aerospace series – Fibres and cables, optical, aircraft use; Test methods – Part 201: Visual examination EN 3841-201 Aerospace series – Circuit breakers – Test methods – Part 201: Visual inspection EN ISO 3882 Metallic and other inorganic coatings – Review of methods of measurement of thickness EN 4050-1 Aerospace series – Test methods for metallic materials – Ultrasonic inspection of bars, plates, forging stock and forgings – Part 1: General requirements EN 4050-2 Aerospace series – Test methods for metallic materials – Ultrasonic inspection of bars, plates, forging stock and forgings – Part 2: Performance of tests EN 4050-3 Aerospace series – Test methods for metallic materials – Ultrasonic inspection of bars, plates, forging stock and forgings – Part 3: Reference blocks EN 4050-4 Aerospace series – Test methods for metallic materials – Ultrasonic inspection of bars, plates, forging stock and forgings – Part 4: Acceptance criteria EN 4057-201 Aerospace series – Cable ties for harnesses – Test methods – Part 201: Visual examination EN 4179 Aerospace series – Qualification and approval of personnel for non-destructive testing EN ISO 5817 Welding – Fusion welded joints in steel, nickel, titanium and their alloys (beam welded excluded) – Quality levels for imperfections EN ISO 6157-2 Fasteners – Surface discontinuities – Part 2: Nuts EN ISO 7963 Non-destructive testing – Ultrasonic testing – Specification for calibration block No. 2 EN ISO 9712 Non-destructive testing – Qualification and certification of NDT personnel EN ISO 9809-1 Gas cylinders – Refillable seamless steel gas cylinders – Design, construction and testing – Part 1: Quenched and tempered steel cylinders with tensile strength less than 1'100 MPa EN ISO 9809-2 Gas cylinders – Refillable seamless steel gas cylinders – Design, construction and testing – Part 2: Quenched and tempered steel cylinders with tensile strength greater than or equal to 1'100 MPa EN ISO 9809-3 Gas cylinders – Refillable seamless steel gas cylinders – Design, construction and testing – Part 3: Normalized steel cylinders EN ISO 9934-1 Non-destructive testing – Magnetic particle testing – Part 1: General principles (with amendment A1) EN ISO 9934-2 Non-destructive testing – Magnetic particle testing – Part 2: Detection media EN ISO 9934-3 Non-destructive testing – Magnetic particle testing – Part 3: Equipment EN ISO 10042 Welding – Arc welded joints in aluminium and its alloys – Quality levels for imperfections EN 10160 Ultrasonic testing of steel flat product of thickness equal to or greater than 6 mm (reflection method) EN 10163-1 Delivery requirements for surface conditions of hot-rolled plates, wide flats and sections, Part 1: General requirements (with amendment A1) EN 10163-2 Delivery requirements for surface conditions of hot-rolled plates, wide flats and sections, Part 2: Plate and wide flats EN 10163-3 Delivery requirements for surface conditions of hot-rolled plates, wide flats and sections, Part 3: Sections 315

EN 10208-1 Steel pipes for pipelines for combustible fluids – Technical delivery conditions – Part 1: Pipes of requirement class A EN ISO 10215 Anodizing of aluminium and its alloys – Visual determination of image clarity of anodic oxidation coatings – Chart scale method EN 10216-4 Seamless steel tubes for pressure purposes – Technical delivery conditions – Part 4: Non-alloy and alloy steel tubes with specific low temperature properties EN 10218-1 Steel wire and wire products – general – Part 1: Test methods EN 10219-1 Cold formed welded structural hollow sections of non-alloy and fine grain steels – Part 1: Technical delivery conditions EN 10228-1 Non-destructive testing of steel forgings – Part 1: Magnetic particle inspection EN 10228-2 Non-destructive testing of steel forgings – Part 2: Penetrant testing EN 10228-3 Non-destructive testing of steel forgings – Part 3: Ultrasonic testing of ferritic or martensitic steel forgings EN 10228-4 Non-destructive testing of steel forgings – Part 4: Ultrasonic testing of austenitic and austenitic-ferritic stainless steel forgings EN 10306 Iron and steel – Ultrasonic testing of H beams with parallel flanges and IPE beams En 10307 Non-destructive testing – Ultrasonic testing of austenitic and austenitic-ferritic stain less steels flat products of thickness equal to or greater than 6 mm (reflection method) EN 10308 Non-destructive testing – Ultrasonic testing of steel bars EN ISO 10424-1 Petroleum and natural gas industries – Rotary drilling equipment – Part 1: Rotary drill stem elements EN ISO 10484 Widening test on nuts EN ISO 10485 Cone proof load test on nuts EN ISO 10863 Non-destructive testing of welds – Ultrasonic testing – Use of time-of-flight diffraction technique (TOFD) EN ISO 10893-1 Non-destructive testing of steel tubes – Part 1: Automated electromagnetic testing of seamless and welded (except submerged arc-welded) steel tubes for the verification of hydraulic leak-tightness in substitution of hydrostatic test EN ISO 10893-2 Non-destructive testing of steel tubes – Part 2: Automated eddy current testing of seamless and welded (except submerged arc-welded) steel tubes for the detection of imperfections EN ISO 10893-3 Non-destructive testing of steel tubes – Part 3: Automated full peripheral flux leakage testing of seamless and welded (except submerged arc-welded) ferromagnetic steel tubes for the detection of longitudinal and/or transverse imperfections EN ISO 10893-4 Non-destructive testing of steel tubes – Part 4: Liquid penetrant inspection of seamless and welded steel tubes for the detection of surface imperfections EN ISO 10893-5 Non-destructive testing of steel tubes – Part 5: Magnetic particle inspection of seamless and welded ferromagnetic steel tubes for the detection of surface imperfections EN ISO 10893-6 Non-destructive testing of steel tubes – Part 6: Radiographic testing of the weld seam of welded steel tubes for the detection of imperfections EN ISO 10893-7 Non-destructive testing of steel tubes – Part 7: Digital radiographic testing of the weld seam of welded steel tubes for the detection of imperfections EN ISO 10893-8 Non-destructive testing of steel tubes – Part 8: Automated ultrasonic testing of seamless and welded steel tubes for the detection of laminar imperfections EN ISO 10893-9 Non-destructive testing of steel tubes – Part 9: Automated ultrasonic testing for the detection of laminar imperfections in strip/plate used for the manufacture of welded steel tubes EN ISO 10893-10 Non-destructive testing of steel tubes – Part 10: Automated full peripheral ultrasonic testing of seamless and welded (except submerged arc-welded) steel tubes for the detection of longitudinal and/or transverse imperfections EN ISO 10893-11 Non-destructive testing of steel tubes – Part 11: Automated ultrasonic testing of weld seam of welded steel tubes for the detection of longitudinal and/or transverse imperfections EN ISO 10893-12 Non-destructive testing of steel tubes – Part 12: Automated full peripheral ultrasonic thickness testing of seamless and welded (except submerged arc-welded) steel tubes EN ISO 11463 Corrosion of metals and alloys – Evaluation of pitting corrosion

316

EN ISO 11623 Transportable gas cylinders – Periodic inspection and testing of composite gas cylinders EN ISO 11666 Non-destructive testing of welds – Ultrasonic testing of welded joints – Acceptance levels EN ISO 11699-1 Non-destructive testing – Industrial radiographic film – Part 1: Classification of film systems for industrial radiography EN ISO 11699-2 Non-destructive testing – Industrial radiographic film – Part 2: Control of film processing by means of reference values EN 12079-3 Offshore containers and associated lifting sets – Part 3: Periodic inspection, examination and testing EN 12504-2 Testing concrete in structures – Part 2: Non-destructive testing – Determination of rebound number EN 12504-4 Testing concrete in structures – Part 4: Determination of ultrasonic pulse velocity EN 12517-1 Non-destructive testing of welds – Part 1: Evaluation of welded joints in steel, nickel, titanium and their alloys by radiography – Acceptance levels EN 12517-2 Non-destructive testing of welds – Part 2: Evaluation of welded joints in aluminium and its alloys by radiography – Acceptance levels EN 12542 LPG equipment and accessories – Static welded steel cylindrical tanks, serially pro duced for the storage of liquefied petroleum gas (LPG) having a volume not greater than 13 m 3 – Design and manufacture EN 12543-1 Non-destructive testing – Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing – Part 1: Scanning method EN 12543-2 Non-destructive testing – Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing – Part 2: Pinhole camera radiographic method EN 12543-3 Non-destructive testing – Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing – Part 3: Slit camera radiographic method EN 12543-4 Non-destructive testing – Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing – Part 4: Edge method EN 12543-5 Non-destructive testing – Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing – Part 5: Measurement of the effective focal spot size of mini and micro focus X-ray tubes EN 12544-1 Non-destructive testing – Measurement and evaluation of the X-ray tube voltage – Part 1: Voltage divider method EN 12544-2 Non-destructive testing – Measurement and evaluation of the X-ray tube voltage – Part 2: Constancy check by the thick filter method EN 12544-3 Non-destructive testing – Measurement and evaluation of the X-ray tube voltage – Part 3: Spectrometric method EN 12668-1 Non-destructive testing – Characterisation and verification of ultrasonic examination equipment – Part 1: Instruments EN 12668-2 Non-destructive testing – Characterisation and verification of ultrasonic examination equipment – Part 2: Probes EN 12668-3 Non-destructive testing – Characterisation and verification of ultrasonic examination equipment – Part 3: Combined equipment EN 12679 Non-destructive testing – Determination of the size of industrial radiographic sources – Radiographic method EN 12680-1 Founding – Ultrasonic examination – Part 1: Steel castings for general purposes EN 12680-2 Founding – Ultrasonic examination – Part 2: Steel castings for highly stressed components EN 12680-3 Founding – Ultrasonic examination – Part 3: Spheroidal graphite cast iron castings EN 12681 Founding – Radiographic examination EN ISO 12706 Non-destructive testing – Penetrant testing – Vocabulary EN ISO 12718 Non-destructive testing – Eddy current testing – Vocabulary EN 12735-1 Copper and copper alloys – Seamless, round copper tubes for air conditioning and refrigeration – Part 1: Tubes for piping systems EN 12735-2 Copper and copper alloys – Seamless, round copper tubes for air conditioning and refrigeration – Part 2: Tubes for equipment

317

EN 12799 Brazing – Non-destructive testing of brazed joints (with amendment A1) EN 12805 Automotive LPG components – Containers EN 12807 LPG equipment and accessories – Transportable refillable brazed steel cylinders for liquefied petroleum gas (LPG) – Design and construction EN 12817 LPG equipment and accessories – Inspection and requalification of LPG tanks up to and including 13 m3 EN 12819 LPG equipment and accessories – Inspection and requalification of LPG tanks greater than 13 m3 EN 12863 Transportable gas cylinders – Periodic inspection and maintenance of dissolved acetylene cylinders (with amendment A1) EN 12889 Trenchless construction and testing of drains and sewers EN 12927-8 Safety requirements for cableway installations designed to carry persons – Ropes – Part 8: Magnetic rope testing (MRT) EN 12952-6 Water-tube boilers and auxiliary installations – Part 6: Inspection during construction; documentation and marking of pressure parts of the boiler EN 12953-5 Shell boilers – Part 5: Inspection during construction, documentation and marking of pressure parts of the boiler EN 13018 Non-destructive testing – Visual testing – General principles (with amendment A1) EN 13068-1 Non-destructive testing – Radioscopic testing – Part 1: Quantitative measurement of imaging properties EN 13068-2 Non-destructive testing – Radioscopic testing – Part 2: Check of long term stability of imaging devices EN 13068-3 Non-destructive testing – Radioscopic testing – Part 3: General principles of radioscopic testing of metallic materials by X- and gamma rays EN 13100-1 Non-destructive testing of welded joints of thermoplastics semi-finished products – Part 1: Visual examination EN 13100-2 Non-destructive testing of welded joints of thermoplastics semi-finished products – Part 2: X-ray radiographic testing EN 13100-3 Non-destructive testing of welded joints in thermoplastics semi-finished products – Part 3: Ultrasonic testing EN 13100-4 Non-destructive testing of welded joints in thermoplastics semi-finished products – Part 4: High voltage testing CEN ISO/TR 13115 Non-destructive testing – Methods for absolute calibration of acoustic emis sion transducers by the reciprocity technique EN 13121-3 GRP tanks and vessels for use above ground – Part 3: Design and workmanship (with amendment A1) EN 13121-4 GRP tanks and vessels for use above ground – Part 4: Delivery, installation and maintenance (with amendment AC) EN 13183-2 Moisture content of a piece of sawn timber – Part 2: Estimation by electrical resistance method (with amendment AC) EN 13183-3 Moisture content of a piece of sawn timber – Part 3: Estimation by capacitance method EN 13184 Non-destructive testing – Leak testing – Pressure change method (with amendment A1) EN 13185 Non-destructive testing – Leak testing – Tracer gas method (with amendment A1) EN 13187 Thermal performance of buildings – Qualitative detection of thermal irregularities in building envelopes – Infrared method EN 13192 Non-destructive testing – Leak testing – Calibration of reference leaks for gases (with amendment AC) EN 13445-4 Unfired pressure vessels – Part 4: Fabrication (with amendment A1) EN 13445-5 Unfired pressure vessels – Part 5: Inspection and testing EN 13445-6 Unfired pressure vessels – Part 6: Requirements for the design and fabrication of pressure vessels and pressure parts constructed from spheroidal graphite cast iron EN 13445-8 Unfired pressure vessels – Part 8: Additional requirements for pressure vessels of aluminium and aluminium alloys

318

EN 13477-1 Non-destructive testing – Acoustic emission – Equipment characterisation – Part 1: Equipment description EN 13477-2 Non-destructive testing – Acoustic emission – Equipment characterisation – Part 2: Verification of operating characteristic EN 13480-5 Metallic industrial piping – Part 5: Inspection and testing CEN/TR 13480-7 Metallic industrial piping – Part 7: Guidance on the use of conformity assessment procedures EN 13480-8 Metallic industrial piping – Part 8: Additional requirements for aluminium and alu minium alloy piping EN 13508-2 Conditions of drain and sewer systems outside buildings – Part 2: Visual inspection coding system (with amendment A1) EN 13523-0 Coil coated metals – Test methods – Part 0: General introduction and list of test methods EN 13523-1 Coil coated metals – Test methods – Part 1: Film thickness EN 13530-2 Cryogenic vessels – Large transportable vacuum insulated vessels – Part 2: Design, fabrication, inspection and testing (with amendments A1 and AC) EN 13554 Non-destructive testing – Acoustic emission testing – General principles EN ISO 13588 Non-destructive testing of welds – Ultrasonic testing – Use of automated phased array technology EN 13625 Non-destructive testing – Leak test – Guide to the selection of instrumentation for the measurement of gas leakage EN 13863-1 Concrete pavements – Part 1: Test methods for the determination of the thickness of a concrete pavement by survey method EN 13863-3 Concrete pavements – Part 3: Test methods for the determination of the thickness of a concrete pavement from cores EN 13923 Filament-wound FRP pressure vessels – Materials, design, manufacturing and testing EN 13925-1 Non-destructive testing – X-ray diffraction from polycrystalline and amorphous material – Part 1: General principles EN 13925-2 Non-destructive testing – X-ray diffraction from polycrystalline and amorphous material – Part 2: Procedures EN 13925-3 Non-destructive testing – X-ray diffraction from polycrystalline and amorphous materials – Part 3: Instruments EN 13927 Non-destructive testing – Visual testing – Equipment CR 13935 Non-destructive testing – Generic NDE data format model EN 13938-5 Explosives for civil uses – Propellants and rocket propellants – Part 5: Determina tion of voids and fissures EN 14096-1 Non-destructive testing – Qualification of radiographic film digitisation systems – Part 1: Definitions, quantitative measurements of image quality parameters, standard reference film and qualitative control EN 14096-2 Non-destructive testing – Qualification of radiographic film digitisation systems – Part 2: Minimum requirements EN 14127 Non-destructive testing – Ultrasonic thickness measurement EN 14141 Valves for natural gas transportation in pipelines – Performance requirements and tests EN 14186 Advanced technical ceramics – Ceramic composites – Mechanical properties at room temperature, determination of elastic properties by an ultrasonic technique EN 14197-3 Cryogenic vessels – Static non-vacuum insulated vessels – Part 3: Operational requirements (with amendments AC and A1) EN 14255-4 Measurement and assessment of personal exposures to incoherent optical radi ation – Part 4: Terminology and quantities used in UV-, visible and IR-exposure measurements EN 14398-2 Cryogenic vessels – Large transportable non-vacuum insulated vessels – Part 2: Design, fabrication, inspection and testing (with amendment A2) EN 14398-3 Cryogenic vessels – Large transportable non-vacuum insulated vessels – Part 3: Operational requirements (with amendment A1) EN 14571 Metallic coatings on nonmetallic basis materials – Measurement of coating thickness – Microresitivity method 319

EN 14579 Natural stone test methods – Determination of sound speed propagation EN 14584 Non-destructive testing – Acoustic emission – Examination of metallic pressure equipment during proof testing EN 14624 Performance of portable leak detectors and of room monitors for halogenated refrigerants CEN/TR 14748 Non-destructive testing – Methodology for qualification of non-destructive tests EN 14784-1 Non-destructive testing – Industrial computed radiography with storage phosphor imaging plates – Part 1: Classification of systems EN 14784-2 Non-destructive testing – Industrial computed radiography with storage phosphor imaging plates – Part 2: General principles for testing of metallic materials using X-rays and gamma rays EN 14876 Transportable gas cylinders – Periodic inspection and testing of welded steel pressure drums EN 14893 LPG equipment and accessories – Transportable Liquefied Petroleum Gas (LPG) welded steel pressure drums with a capacity between 150 litres and 1'000 litres (with amendment AC) EN 14894 LPG equipment and accessories – Cylinder and drum marking EN 14912 LPG equipment and accessories – Inspection and maintenance of LPG cylinder valves at time of periodic inspection of cylinders EN 14917 Metal bellows expansion joints for pressure applications (with amendment A1) EN ISO 14922-2 Thermal spraying – Quality requirements of thermally sprayed structures – Part 2: Comprehensive quality requirements EN 15042-1 Thickness measurement of coatings and characterisation of surfaces with surface waves – Part 1: Guide to the determination of elastic constants, density and thickness of films by laser induced surface acoustic waves EN 15042-2 Thickness measurement of coatings and characterisation of surfaces with surface waves – Part 2: Guide to the thickness measurement of coatings by photothermic method CEN/TS 15053 Non-destructive testing – Recommendations for discontinuities-types in test specimen for examination EN 15085-5 Railway applications – Welding of railway vehicles and components – Part 5: Inspection, testing and documentation CEN/TR 15134 Non-destructive testing – Automated ultrasonic examination – Selection and application of systems CEN/TR 15135 Welding – Design and non-destructive testing of welds EN 15305 Non-destructive testing – Test method for residual stress analysis by X-ray diffrac tion (with amendment AC) EN 15317 Non-destructive testing – Ultrasonic testing – Characterisation and verification of ultrasonic thickness measuring equipment EN ISO 15463 Petroleum and natural gas industries – Field inspection of new casing, tubing and plain and end drill pipe (with amendment AC) EN 15495 Non-destructive testing – Acoustic emission – Examination of metallic pressure equipment during proof testing – Zone location of AE sources EN ISO 15548-1 Non-destructive testing – Equipment for eddy current examination – Part 1: Instrument characteristics and verification (with amendment AC) EN ISO 15548-2 Non-destructive testing – Equipment for eddy current examination – Part 2: Probe characteristics and verification EN ISO 15548-3 Non-destructive testing – Equipment for eddy current examination – Part 3: System characteristics and verification EN ISO 15549 Non-destructive testing – Eddy current testing – General principles CEN/TR 15589 Non-destructive testing – Code of practice for the approval of NDT personnel by recognized third party organisation under under the provisions of Directive 97/23/EC EN ISO 15626 Non-destructive testing of welds – time-of-flight diffraction technique (TOFD) – Acceptance levels EN 15856 Non-destructive testing – Acoustic emission – General principles for AE testing for the detection of corrosion within metallic surrounding filled with liquid

320

EN 15857 Non-destructive testing – Acoustic emission – Testing of fibre-reinforced polymers – Specific methodology and general evaluation criteria EN 16016-1 Non-destructive testing – Radiation methods – Computed tomography – Part 1: Terminology EN 16016-2 Non-destructive testing – Radiation methods – Computed tomography – Part 2: Principle, equipment and samples EN 16016-3 Non-destructive testing – Radiation methods – Computed tomography – Part 3: Operation and interpretation EN 16016-4 Non-destructive testing – Radiation methods – Computed tomography – Part 4: Qualification EN 16018 Non-destructive testing – Terminology – Terms used in ultrasonic testing with phased arrays EN 16090 Copper and copper alloys – Estimation of average grain size by ultrasound EN ISO 16148 Gas cylinders – Refillable seamless steel gas cylinders – Acoustic emission (AT) examination and follow-up ultrasonic examination (UT) for periodic inspection and testing CEN/TR 16332 Interpretation of EN ISO/IEC 17024 for NDT personnel certification application EN 16392-2 Non-destructive testing – Characterisation and verification of ultrasonic phased array systems – Part 2: Probes EN 16407-1 Non-destructive testing – Radiographic inspection of corrosion and deposits in pipes by X- and gamma rays – Part 1: Tangential radiographic inspection EN 16407-2 Non-destructive testing – Radiographic inspection of corrosion and deposits in pipes by X- and gamma rays – Part 2: Double wall radiographic inspection EN ISO 16946 Non-destructive testing – Ultrasonic testing – Specification for step wedge calib ration block EN ISO 17405 Non-destructive testing – Ultrasonic testing – Technique of testing claddings produced by welding, rolling and explosion EN ISO 17635 Non-destructive examination of welds – General rules for metallic materials EN ISO 17636-1 Non-destructive testing of welds – Radiographic testing – Part 1: X- and gamma ray techniques with film EN ISO 17636-2 Non-destructive testing of welds – Radiographic testing – Part 2: X- and gamma ray techniques with digital detectors EN ISO 17637 Non-destructive examination of welds – Visual testing of fusion-welded joints EN ISO 17638 Non-destructive testing of welds – Magnetic particle testing EN ISO 17640 Non-destructive testing of welds – Ultrasonic testing – Techniques, testing levels, and assessment EN ISO 19232-1 Non-destructive testing – Image quality of radiographs – Part 1: Image quality indicators (wire type) – Determination of image quality value EN ISO 19232-2 Non-destructive testing – Image quality of radiographs – Part 2: Image quality indicators (step/hole type) – Determination of image quality value EN ISO 19232-3 Non-destructive testing – Image quality of radiographs – Part 3: Image quality classes for ferrous metals EN ISO 19232-4 Non-destructive testing – Image quality of radiographs – Part 4: Experimental evaluation of image quality values and image quality tables EN ISO 19232-5 Non-destructive testing – Image quality of radiographs – Part 5: Image quality indicators (duplex wire type) – Determination of image unsharpness value CEN ISO/TS 21432 Non-destructive testing – Standard test method for determining residual stresses by neutron diffraction (with amendment AC) EN ISO 21968 Non-magnetic metallic coatings on metallic and non-metallic basis materials – Measurement of coating thickness – Phase-sensitive eddy-current method EN ISO 22825 Non-destructive testing of welds – Ultrasonic testing – Testing of welds in austenitic steels and nickel-based alloys EN ISO 23277 Non-destructive testing of welds – Penetrant testing of welds – Acceptance levels EN ISO 23278 Non-destructive testing of welds – Magnetic particle testing of welds – Accept ance levels

321

EN ISO 23279 Non-destructive testing of welds – Ultrasonic testing – Characterisation of indications in welds CEN ISO/TR 25107 Non-destructive testing – Guidelines for NDT training syllabuses CEN ISO/TR 25108 Non-destructive testing – Guidelines for NDT personnel training organiza tions EN 25580 Non-destructive testing – Industrial radiographic illuminators – Minimum requirements

Further standards exist concerning electronic, medical, dental and security equipment.

322

323

14

DICTIONARY English 2D matrix array A-scan presentation absolute arrangement absolute measurement absolute permeability absolute pressure absolute pressure gauge absolute probe absolute signal absolute system absolute value absorption absorption coefficient acceptance criteria acceptance level accumulation test acoustic emission acoustic emission activity acoustic emission burst count acoustic emission burst rate acoustic emission decibel scale; dBAE scale acoustic emission detection threshold acoustic emission event

Nachweisschwelle der Schallemission Schallemissionsereignis

acoustic emission wave energy acoustic emission waveguide

Schallemissionswellenleiter

acoustic shadow

Schallschatten akustische Impedanz; Schallwellenwiderstand Erfassungszyklus; Salve Erfassungssequenz Aktivität adaptive Fokussierung Additionsfluss-Sensor anliegender Leiter Technik mit anliegenden Leiter Alterungsschleier Luftspulensensor Alkali-Ionendetektor Wechselstrom Amperewindungen Amplitude Amplitudenauswertung Amplitudenausgleich Einfallswinkel;

acoustical impedance acquisition cycle; salvo acquisition sequence activity adaptive focusing additive magnetic flux probe adjacent conductor adjacent conductor technique ageing fog air-cored probe alkali-ion detector alternating current ampere turns amplitude amplitude analysis amplitude balance angle of incidence

Français réseau matriciel 2D représentation de type A montage absolu mesurage absolu perméabilité absolue pression absolue manomètre absolu capteur absolu signal absolu système absolu mesure absolu absorption coefficient d’absorption critères d’acceptation niveau d’acceptation contrôle par accumulation émission acoustique activité d'émission acoustique nombre de salves d'émission acoustique taux de salves d'émission acoustique

Dezibelskala für die Schallem- échelle d'émission acoustique ission dBAE

Cluster; Häufung von Schallemissionsortungen Energieinhalt eines Schallemissionsereignisses

acoustic emission location cluster

324

Deutsch zweidimensionales Matrix-Array A-Bild Absolutschaltung Absolutmessung absolute Permeabilität Absolutdruck Absolutdruckmessgerät Absolutsensor Absolutsignal Absolutsystem Absolutmesswert Schallabsorption Schallabsorptionskoeffizient Zulässigkeitskriterien Zulässigkeitsgrenze Akkumulationsprüfung Schallemission Schallemissionsaktivität Anzahl der Schallemissionssignale Rate der Schallemissionssignale

seuil de détection d'émission acoustique événement d'émission acoustique cluster de localisation d'émission acoustique énergie de l'onde d'émission acoustique guide d'ondes d'émission acoustique ombre acoustique impédance acoustique cycle d'acquisition; rafale séquence d'acquisition activité focalisation adaptative capteur à flux additifs conducteur adjacent technique de contrôle à l’aide de conducteur adjacent voile de vieillissement capteur à noyau neutre détecteur d’ions alcalins courant alternatif ampères-tours amplitude analyse en amplitude équilibrage en amplitude angle d’incidence

English angle of reflection angle of refraction angle of vision angle probe angular corrected gain (ACG) angular sensitivity annular array annular sectorial array anode anode current aperture leak approach technique arc strikes archival image recording system area of coverage arrangement; construction array probe arrival time artefact; false indication artificial discontinuity atmospheric pressure attenuation attenuation coefficient automatic scanning auxiliary lighting azimuthal electronic scanning; lateral electronic scanning B-scan presentation back scatter; back scattered radiation back-wall echo background noise backing line; fore-line backing pressure; fore pressure backing pump; fore pump backing volume

Deutsch Auftreffwinkel Reflexionswinkel Brechungswinkel; Einschallwinkel Blickwinkel Winkelprüfkopf winkelkorrigierte Verstärkung (ACG) Richtungsempfindlichkeit Ringarray segmentiertes Ringarray Anode Röhrenstrom Blendenleck Annäherungsverfahren Brandstellen Bildaufzeichnungssystem Wechselwirkungsfläche Schaltung Sensorarray Ankunftszeit Artefakt; Scheinanzeige künstliche Inhomogenität Atmosphärendruck Schwächung Schwächungskoeffizient automatische Prüfung Hilfsbeleuchtung azimutale elektronische Abtastung; laterale elektronische Abtastung B-Bild

angle de réfraction angle d’observation transducteur d’angle gain corrigé angulairement (GCA) sensibilité angulaire réseau annulaire réseau annulaire sectorisé anode courant anodique orifice en paroi mince technique d'approche amorçage d’arc système d’enregistrement des images pour archivage surface d'action montage; construction capteur en réseau temps d'arrivée artefact; pseudo image discontinuité artificielle pression atmosphérique atténuation coefficient d’atténuation contrôle automatique éclairage auxiliaire balayage électronique azimutal; balayage électronique latéral représentation de type B rayonnement rétrodiffusé

Rückwandecho Untergrundrauschen

écho de fond bruit de fond ligne de prévidage; canalisation de vide primaire vide primaire; vide préliminaire pompe primaire; pompe préliminaire volume de prévidage technique de la canalisation de prévidage robinet de canalisation de vide primaire équilibrage technique de mesure par pont filtre passe-bande filtre coupe-bande bande passante; largeur de bande lot bain angle du faisceau

Vorvakuumleitung Vorvakuumdruck; Vordruck Vorvakuumpumpe; Vorpumpe Vorvakuumvolumen Vorvakuumanschlusstechnik

backing-line valve

Vorvakuumventil

balance balanced bridge technique band pass filter band stop filter

Abgleich Brückenmesstechnik Bandpassfilter Bandsperrfilter

bandwidth

Bandbreite

batch bath beam angle

Charge Bad Einschallwinkel Bündelachse; akustische Achse Bündelgrenze

beam edge

angle de réflexion

Rückstreuung

backing-line port technique

beam axis

Français

axe du faisceau bord du faisceau 325

English beam hardening beam index beam profile beam steering betatron black light lamp; Wood’s glass lamp bleedout blocking medium blooming bobbin coil; internal coaxial coil bombing bombing test; pressurising-evacuation test; back pressurising test borescope bubble test bucking signal; compensating signal

Deutsch Strahlenaufhärtung Schalleintrittspunkt Schallbündelf-Profil Steuern des Schallbündels Betatron

build-up factor

Aufbaufaktor

burst emission burst signal duration

transiente Emission Signaldauer

burst signal peak amplitude

Maximalamplitude

burst signal rise-time burst signal; burst C-scan representation calibration calibration block calibration leak capacitive reactance capillary leak

Anstiegszeit Burst; transientes Signal C-Bild Kalibrierung Kalibrierkörper Prüfleck kapazitiver Blindwiderstand Kapillarleck

carrier liquid

Trägerflüssigkeit

cassette cathode CEN speed central conductor centre frequency characteristic curve characteristic frequency characteristic frequency ratio characteristic transfer curve (CTC) circular magnetisation clean up (time) coaxial probe; feed-trough probe coercivity

Kassette Kathode CEN-Empfindlichkeit Zentralleiter Mittenfrequenz charakteristische Kurve Grenzfrequenz Arbeitskonstante charakteristische Übertragungsfunktion Kreismagnetisierung Erholzeit

représentation de type C étalonnage bloc d’étalonnage fuite calibrée réactance capacitive capillaire liquide porteur; liquide support cassette cathode sensibilité CEN conducteur central fréquence centrale courbe caratéristique fréquence caractéristique fréquence réduite courbe de transfert caractéristique du numériseur aimantation circulaire récupération (temps de)

Durchlaufsensor

capteur axial

Koerzitivfeldstärke Spule; Spulenanordnung

coercivité

coil assembly

326

Ausbluten Blende Blooming

Français durcissement de faisceau point d’incidence forme du faisceau conduite du faisceau bétatron lampe à lumière noire; lampe de Wood ressuage matériaux de blocage flash lumineux

Innendurchlaufsensor

sonde axiale

Drucklagerung

pressurisation

Drucklagerungsprüfung

contrôle par pressurisation; contrôle par ressuage

starres Endoskop Blasenprüfung

borescope contrôle à la bulle

Kompensationssignal

signal de compensation

UV-Lampe

coil fill factor

Wicklungsfüllungsgrad

coil length coil separation coil spacing coil technique coil turns coil winding collimation

Spulenlänge Spulenentfernung Spulenbasis Spulentechnik Windungszahl Wicklung Kollimierung

facteur de diffusion; facteur d’accumulation émission discontinue durée de la salve amplitude maximale de la salve temps de montée de la salve salve

disposition d’enroulement(s) taux de remplissage d’un enroulement longueur d’enroulement distance interenroulement écartement moyen aimantation par bobine nombre de tours enroulement collimation

English collimator colour contrast penetrant colour discrimination colour temperature colour vision coloured media combined transmit-receive probe; impedance probe

Deutsch Kollimator Farbeindringmittel Farbunterscheidung Farbtemperatur Farbsehen nichtfluoreszierende Prüfmittel

Français collimateur pénétrant coloré discrimination de couleurs température de couleur vision de la couleur

Doppelfunktionssensor

capteur à double fonction

comparative arrangement

Fremdvergleichsschaltung

comparative measurement comparative measurement with external reference comparative measurement with local reference

Vergleichsmessung

comparator probe

Fremdvergleichssensor

compensation coil

Kompensationsspule

complete encircling array; surrounding array complex permeability

vollständig umschliessendes Array komplexe Permeabilität

complex plane analysis

Vektorauswertung

complex plane display

X/Y-Darstellung

component analysis compression ratio Compton scatter

Komponentenauswertung zeitproportionale Komponentendarstellung Kompressionsverhältnis Compton-Streuung

computed tomography (CT)

Computertomographie (CT)

concentrates conditioning agent conductance

Konzentrate Additive Strömungsleitwert

conductance leak

Leitwertleck

cone beam CT

Kegelstrahl_CT

constant current control

Konstantstrom-Regelung

constant potential circuit contact pad

Gleichspannungsanlage Kontaktplatte

contact testing technique

Kontakttechnik

continuous continuous technique continuous continuous

kontinuierliche Emission simultane Magnetisierungstechnik kontinuierliches Signal kontinuierliches Spektrum Dauerschall; kontinuierliche Welle formangepasster Prüfkopf Kontrast Kontrastfarbe Kontrastmittel Kontrastverhältnis Kontrastempfindlichkeit; Dickenempfindlichkeit Kern Winkelspiegel

component/time display

emission magnetisation signal spectrum

continuous wave contoured probe contrast contrast aid paint contrast medium contrast ratio contrast sensitivity; thickness sensitivity core corner reflector

Fremdvergleich Selbstvergleich

produit indicateur coloré

montage absolu à référence externe mesurage comparatif mesure comparative à référence externe mesure comparative capteur absolu à référence externe enroulement de compensation réseau encerclant complet; réseau environnant perméabilité complexe analyse dans le plan complexe représentation du plan complexe analyse de projection représentation en base de temps taux de compression diffusion Compton tomographie informatisée (TI) concentrés agent de conditionnement conductance défaut d’étanchéité (par conductance) faisceau conique TI contrôle de la constance du courant circuit à potentiel constant touches de contact technique de contrôle par contact émission continue méthode simultanée signal continu spectre continu onde entretenue transducteur de forme contraste peinture de contraste produit de contraste rapport de contraste sensibilité au contraste; sensibilité à l’épaisseur noyau réflecteur coin 327

English counterflow leak detector

Gegenstromleckdetektor

couplant path couplant techniques couplant; coupling medium; coupling film coupling coupling factor coupling losses creeping wave

Koppelstrecke Ankoppeltechnik

Français détecteur de fuites à contrecourant trajet de couplage techniques de couplage

Koppelmittel

couplant couplage coefficient de couplage perte de couplage onde rampante

cupping effect Curie point current driven excitation

Ankopplung Kopplungsfaktor Ankopplungsverluste Kriechwelle kritischer Winkel; Grenzwinkel; Überkoppelecho; Übersprechen Strahlenaufhärtungsartefakt Curie-Punkt stromgesteuerte Erregung

current flow technique

Stromdurchflutung

current generator cut-off frequency; frequency limit cylindrical wave D-scan presentation

Stromgenerator Eckfrequenz; Grenzfrequenz Zylinderwelle D-Bild

DAC-method

Bezugslinienmethode (DAC)

damping capacity dead volume dead zone decay curve

Dämpfungsvermögen Totvolumen tote Zone Zerfallskurve

decrease of sound pressure

Schalldruckabnahme

defect defect sizing; defect size assessment delay law delay line; delay block delayed time-base sweep; correction of zero point

Fehler

effet de tuilage point de Curie injection en courant aimantation par passage de courant générateur de courant fréquence de coupure; fréquence limite onde cylindrique représentation de type D méthode de la courbe amplitude/distance (DAC) capacité d’amortissement volume mort zone morte courbe de décroissance diminution de la pression acoustique défaut

Fehlergrössenabschätzung

dimensionnement

Verzögerungsgesetz Verzögerungsstrecke; Vorlaufstrecke

loi de retards ligne de retard; bloc de retard

Impulsverschiebung

base de temps retardé

critical angle cross-talk

demagnetise demodulated signal demodulator demonstration test piece densitometer

Entmagnetisierungseinrichtung entmagnetisieren demoduliertes Signal Demodulator repräsentativer Testkörper Densitometer

density contrast sensitivity

Dichtekontrastempfindlichkeit

density range

Dichtebereich

density sampling pitch

Dichte-Abtastauflösung

detection detection criteria detection media detection sensitivity detection threshold developer development development time DGS-diagram;

Nachweis Nachweiskriterien Prüfmittel Nachweisempfindlichkeit Nachweisgrenze Entwickler Entwicklung Entwicklungsdauer AVG-Diagram

demagnetisation unit

328

Deutsch

angle critique diaphonie

unité de désaimantation désaimanter signal courant de Foucault démodulateur pièce type témoin densitomètre sensibilité au contraste de densité plage de densité du numériseur pas d'échantillonnage de densité détection critère de détection produit indicateur sensibilité de détection seuil de détection révélateur développement durée de révélation diagramme de réflectivité;

English AVG-diagram

Deutsch

DGS-method; AVG-method

AVG-Methode

diamagnetic substance diaphragm; wear plate differential arrangement differential filter differential measurement

diamagnetischer Werkstoff Verschleissschicht; Schutzschicht Differenzschaltung Differenzfilter Differenzmessung

differential Pirani gauge

Differenzpiranivakuummeter

differential probe differential system differential value diffraction mottle diffuse density digital image processing equipment digital resolution (bit) digitiser unsharpness dip rinse direct current

Differenzsensor Differenzsystem Differenzmesswert Beugungsmuster diffuse optische Dichte Ausrüstung zur digitalen Bildverarbeitung digitale Auflösung (Bit) Digitalisier-Unschärfe Tauchspülen Gleichstrom

direct flow leak detector

Hauptstromleckdetektor

direct scan; single traverse scan direct visual testing directional reflectivity discharge tube leak detector discontinuity echo discontinuity; inhomogeneity display level distance amplitude correction curve (DAC) distance-amplitude compensation divergence angle dose rate meter dosemeter; dosimeter double (twin) transducer probe double aperture technique double differential measurement double differential probe drag effect; speed effect dry developer dry powder technique dual focus tube duplex wire image quality indicator

Direktanschallung; Prüfung im halben Sprungstand direkte Sichtprüfung Richtungsabhängigkeit eines Reflektors Gasentladungsvakuum-prüfgerät Fehlerecho

Français diagramme AVG méthode des diamètres de réflectivité; méthode AVG substance diamagnétique protection de phase avant montage différentiel différentiateur mesurage différentiel manomètre différentiel de Pirani capteur différentiel mesure différentiel signal différentiel moutonnement de diffraction densité diffuse dispositif de traitement numérique de l’image résolution numérique (bit) flou du numériseur rinçage par immersion courant continu détecteur de fuites à flux direct balayage direct; balayage transversal simple contrôle visuel direct réflectivité directionnelle

Abbildungsgrenze

détecteur de fuites à décharge écho de discontinuité discontinué; inhomogénéité niveau de visualisation

Bezugslinie (DAC)

courbe de DAC

Tiefenausgleich

correction amplitude-distance

Divergenzwinkel Dosisleistungsmessgerät

angle de divergence débitmètre de dose

Dosimeter

dosimètre

Inhomogenität

Doppel-Aperture-Technik

transducteur à émetteur et récepteur séparés technique d'ouverture double

Doppeldifferenzmessung

mesure double différentiel

Doppeldifferenzsensor Mitführungseffekt; Geschwindigkeitseffekt Trockenentwickler Trockentechnik Doppelfokusröhre Doppel-Drahtsteg-Bildgüteprüfkörper

capteur double différentiel effet dynamique; effet de vitesse révélateur sec technique de la poudre sèche tube à double foyer indicateur de qualité d’image duplex à fils courants de Foucault dynamiques limite de détection dynamique focalisation électronique dy-

SE-Prüfkopf

dynamic currents

Schleppwirbelströme

dynamic detection limit

dynamische Nachweisgrenze

dynamic electronic focusing:

dynamische elektronische

329

English

dynamic leakage rate measurement dynamic range

Deutsch Fokussierung; dynamische Tiefenfokussierung dynamische Leckageratenmessung Dynamikbereich

dynamic receiving aperture

dynamische Empfangsapertur

E-scan presentation echo echo height; signal amplitude echo receiving point echo width

E-Bild Echo Echohöhe; Signalamplitude Schalleintrittspunkt Echobreite

eddy current distribution

Wirbelstromverteilung

eddy current instrument

Wirbelstrom-Prüfgerät

eddy current testing

Wirbelstromprüfung

eddy current testing system

Wirbelstrom-Prüfsystem

eddy currents

Wirbelstrom Kanteneffekt; Randeffekt

dynamic depth focusing (DDF)

edge effect edge spread function (ESF)

Kantenspreizfunktion

edge-blocking material

Ausgleichskörper

effective coil diameter

effektiver Spulendurchmesser

effective depth of penetration

effektive Eindringtiefe

effective permeability

effektive Permeabilität

effective transducer size

effektive Schwingergrösse

electrical centre electromagnetic induction electromagnetic-acoustic transducer

elektrisches Zentrum elektromagnetische Wechselwirkung elektromagnetische Induktion elektromagnetisch-akustischer Wandler

electronic beam shaping

Elektronische Bündelformung

electromagnetic coupling

electronic beam steering electronic focusing electronic scanning electrostatic spraying elementary A-scan presentation

330

Elektronische Bündelsteuerung Elektronische Fokussierung elektronische Abtastung elektrostatisches Sprühen

Français namique; focalisation dynamique en profondeur mesurage dynamique du flux de fuite étendue dynamique ouverture dynamique en réception représentation de type E écho hauteur d’écho; amplitude du signal point de réception d’écho largeur d’écho distribution des courants de Foucault appareil à courants de Foucault contrôle par courants de Foucault appareillage à courants de Foucault courants de Foucault effet de bord fonction d'étalement d'une marche matériau de blocage des bords diamètre équivalent profondeur de pénétration effective perméabilité effective dimensions efficaces du transducteur centre électrique couplage électromagnétique induction électromagnétique transducteur électromagnétique-acoustique formation de faisceau électronique déflexion électronique

elementary A-scan presentation

elementbezogenes A-Bild: Elementarsignal

elliptical display method empty coil impedance; unloaded impedance emulsification time emulsifier encircling coil end effect endoscope equalizing filter; beam flattener

Ellipsendarstellungsverfahren

focalistaion électronique balayage électronique pulvérisation électrostatique représentation de type A sommée représentation de type A élémentaire; signal élémentaire méthode de l'ellipse

Leerimpedanz

impédance à vide

Emulgierdauer Emulgator Aussendurchlaufsensor Endeneffekt Endoskop

durée d’émulsification émulsifiant bobine encerclante effet d’extrémité endoscope

Ausgleichsfilter

filtre égalisateur

Summen-A-Bild

English equivalent nitrogen pressure

Deutsch Stickstoffäquivalentdruck

equivalent X-ray voltage

äquivalente Röhrenspannung

evaluation

Bewertung

evaluation level

Beobachtungsschwelle

evaluation of indications examination levels examination object examination volume

Anzeigebewertung Prüfempfindlichkeit Prüfgegenstand Prüfvolumen

excess penetrant removal

Zwischenreinigung

excitation current excitation element; primary coil excitation field; primary field excitation frequency excitation power amplifier excitation; induction excitation; induction exhaust pressure exposure exposure chart exposure latitude exposure time extenders

Erregerstrom

Français pression équivalente d’azote tension de rayonnement équivalente évaluation niveau d’évaluation; niveau de caractérisation évaluation des indications sensibilité d’examen pièce d’examiner zone contrôlée élimination de l’excès de pénétrant courant d’excitation

Erregerwicklung

enrouement d’excitation

Erregerfeld

champ d’excitation

Erregerfrequenz Senderverstärker

external tracer gas drift

externe Prüfgasdrift

F-scan representation false indication fan beam CT far field; Fraunhofer-zone far vision Felicity effect Felicity ratio ferrite ferromagnetic cored probe ferromagnetic substance fibre optics fibrescope field of view film base film density film gradient film illuminator; viewing screen film processing film system film system class filter fixed installation fixing

F-Bild Scheinanzeige Fächerstrahl-CT Fernfeld; Fraunhofer-Zone Fernsehfähigkeit Felicity-Effekt Felicity-Verhältnis Ferrit Ferromagnetkernsensor ferromagnetischer Werkstoff Faseroptik flexibles Endoskop Blickfeld Filmbasis Filmschwärzung Filmgradient

fréquence d’excitation ampliateur d'injection excitation; induction excitation; induction pression de refoulement exposition abaque d’exposition latitude de pose temps d’exposition rallonges magnétiques dérive due au gaz traceur externe représentation de type F indication fallacieuse TI à faisceau en éventail champ éloigné; zone de Fraunhofer vision lointaine effet Felicity rapport Felicity ferrite capteur à circuit magnétique substance ferromagnétique fibres optiques fibroscope champ d‘observation support de film densité de film gradient du film

Filmbetrachtungsgerät

négatoscope traitement du film système film classe de système film filtre installation en poste fixe fixage

flaw detection equipment sensitivity

Filmverarbeitung Filmsystem Filmsystemklasse Filter stationäre Anlage Fixierung Fehlertiefe; Reflektortiefe; Tiefenlage Fehlernachweisempfindlichkeit

flaw sensitivity

Anzeigeempfindlichkeit

flaw depth; reflector depth

Erregung Erregung Auspuffdruck Belichtung Belichtungsdiagramm Belichtungsumfang Belichtungszeit Verlängerungen

profondeur sensibilité de détection sensibilité de détection des défauts 331

English flexible coil technique flow rate fluorescence fluorescent detection media fluorescent intensifying screen

Deutsch Kabelspulentechnik; Kabeltechnik

fluorescent stability fluoroscopy

Gasströmungsrate Fluoreszenz fluoreszierende Prüfmittel fluoreszierende Verstärkerfolie fluoreszierendes Eindringmittel Fluoreszenzbeständigkeit Fluoroskopie

flux gate sensor

Fluxgate-Sensor

flux indicator

Magnetisierungsindikator

focal spot

Brennfleck

focal spot size focal zone focus-to-film distance focus; focal point; focusing probe focusing transducer fog density frequency frequency spectrum

gamma radiography gamma rays

Brennfleckgrösse Fokusbereich Abstand Fokus-Film Fokus; Fokuspunkt fokussierender Prüfkopf fokussierender Wandler Schleierschwärzung Frequenz Frequenzspektrum Erfassung der gesamten Matrix Verstärkungssteller Apodisierung der Verstärkung; Gewichtung der Verstärkung Gammaradiographie Gammastrahlen

gamma-ray source

Gammastrahlenquelle

fluorescent penetrate

full matrix capture gain adjustment gain apodisation

Pfützentechnik; Fliessspalttechnik gas ballast device Gasballastvorrichtung gate level; Blendenschwelle; monitor level Monitorschwelle gate; Monitorblende; time gate Zeitblende gating technique Blendentechnik gauge pressure Überdruck allgemeine Sichtprüfung; general visual testing Übersichtsprüfung generator unit Generatoreinheit geometric effect Geometrieeffekt geometric unsharpness geometrische Unschärfe gettering Aufzehrung ghost echo Phantomecho Giant magnetoresistiver giant magnetoresistive sensor Sensor glare Blendung gradient/noise ratio Gradient/Rausch-Verhältnis graininess Körnigkeit granularity Körnung graticule Skala grey scale Grauwertskala group velocity Gruppengeschwindigkeit gap testing technique

332

Français technique de la bobine souple; technique des spires enroulées débit fluorescence produit indicateur fluorescent écran renforçateur fluorescent pénétrant fluorescent stabilité de la fluorescence fluoroscopie capteur à effet de vanne de flux témoin d’aimantation foyer émissif; foyer optique dimension du foyer émissif tache focale distance foyer-film foyer: point focal transducteur focalisant transducteur focalisant densité de voile fréquence spectre de fréquence acquisition de la matrice inter-éléments réglage du gain apodisation en gain gammagraphie rayonnement gamma source de rayonnement gamma technique d'essai par le vide lest d’air hauteur de porte; seuil du moniteur porte de sélection; largeur de porte sélection par porte(s) pression effective contrôle visuel global; contrôle visuel complet générateur effet de géométrie flou géométrique effet getter écho fantôme capteur à magnétorésistance géante éblouissement rapport gradient/bruit granulation granularité réticule échelle des gris vitesse de groupe

English guard sensor

Deutsch Abschirmsensor

half value thickness (HVT)

Halbwertsschicht (HWS)

half-amplitude technique; 6 dB drop technique Hall effect sensor

Halbwertsverfahren; – 6 dB Technik Halleffektsensor

halogen leak detector

Halogenleckdetektor

helium leak detector high-pass filter hit holding pump hood test Hsu-Nielsen source; pencil lead break hydrophilic emulsifier illuminance

image contrast image definition image enhancement

Heliumleckdetektor Hochpassfilter Hit Haltepumpe Hüllenprüfung Hsu-Nielsen Quelle; Bleistiftminenbruch hydrophiler Emulgator Beleuchtungsstärke Beleuchtungsstärkemesser; Beleuchtungsmesser; Luxmeter Bildkontrast Bildauflösung Bildverbesserung

image intensifier

Bildverstärker

image quality image quality indicator (IQI) image quality value; IQI sensitivity image sensor immersion technique impedance impedance plane diagram in phase demodulation in-phase demodulation incident beam axis incremental permeability technique indication

Bildgüte Bildgüteprüfkörper (BPK) Bildgütezahl; BPK-Empfindlichkeit Bildsensor Tauchtechnik Impedanz Impedanzortskurve Demodulation in Phase Demodulation in Phase Zentralstrahl Überlagerungspermeabilitätstechnik Anzeige indirekte Anschallung; Prüfung im ganzen Sprungstand Induktionsdurchflutungstechnik Induktivität induktiver Sensor industrielle Radiologie Eigenfilterung innere Unschärfe Einlassleitung Einlasssystem Einlassventil Einlaufeffekt Gerätedrift Verstärkungsfaktor Aufnahmefolie Grenzfläche Grenzflächenecho

illuminance meter; luxmeter

indirect scan induced current flow technique inductance inductive sensor industrial radiology inherent filtration inherent unsharpness inlet line inlet system inlet valve input effect instrument drift intensifying factor intensifying screen interface interface echo internal coaxial probe; bobbin coil internal probe ion source ionisation potential

Français capteur de garde épaisseur (couche) de demi-absorption (CDA) technique de demi-amplitude technique à – 6 dB capteur à effet Hall détecteur de fuites d’halogènes détecteur de fuites à l’hélium filtre passe-haut hit pompe de maintien contrôle sous enveloppe source Hsu-Nielsen; rupture d'une mine émulsifiant hydrophile intensité lumineuse luxmètre contraste image définition d’image amélioration d’image intensificateur d’image; amplificateur de luminance qualité d’image indice de qualité d’image indice de qualité d’image capteur d’image technique en immersion impédance diagramme d’impédance démodulation en phase démodulation en phase axe du faisceau incident technique de perméabilité incrémentale indication balayage indirect technique d'aimantation par passage de courant induit inductance capteur inductif radiologie industrielle filtration inhérente flou interne canalisation d’aspiration système d’aspiration vanne d’aspiration effet d’entrée dérive d’un instrument facteur de renforcement écran renforçateur interface écho d’interface

Innendurchlaufsensor

sonde axiale

Innensensor Ionenquelle Ionisationspotential

sonde source d’ions potentiel d’ionisation 333

English isolated pressure test

Druckhalteprüfung

Kaiser effect Lamb wave; plate wave latent image law of similarity leak leakage rate length of coverage lens lift test lift-off light source line chart

Kaiser-Effekt Lamb-Welle; Plattenwelle latentes Bild Ähnlichkeitsgesetz Leck Leckagerate Wirkbreite Linse Abhebetest Abhebeeffekt Lichtquelle Linientafel

line spread function (LSF)

Linienspreizfunktion

linear array; 1D linear array

Lineares Array; eindimensionales lineares Array

linear electron accelerator (LINAC)

Linearbeschleuniger

linear electronic scanning; E-scan linear location linearity indicators lipophilic emulsifier loaded coil impedance; apparent impedance local tomography; region of interest CT location longitudinal wave; compressional wave low-pass filter

barrette linéaire; réseau linéaire 1D

lineare elektronische Abtastung; E-Abtastung lineare Ortung Linearitätsanzeiger lipophiler Emulgator Arbeitsimpedanz

impédance apparente

lokale Tomographie; Ausschnitt-CT Ortung Longitudinalwelle; Druckwelle Tiefpassfilter

tomographie locale; TI de région d'intérêt localisation onde longitudinale; onde de compression filtre passe-bas banc de contrôle par magnétoscopie champ magnétique intensité du champ magnétique aimantation par passage de flux magnétique flux magnétique

Magnetpulverprüfbank

magnetic field

Magnetfeld

magnetic field strength

Magnetfeldstärke

magnetic flow technique

Felddurchflutung

magnetic magnetic magnetic magnetic

magnetischer Fluss magnetische Flussdichte; Induktion magnetische Hysterese

flux flux density; induction hysteresis

Français contrôle par maintien de pression effet Kaiser onde de Lamb; onde de plaque image latente loi de similtude défaut d’étanchéité flux de fuite longueur d’action lentille force de soulèvement effet d’éloignement source lumineuse mire linéaire fonction d'étalement d'une ligne

accélérateur électronique linéaire balayage électronique linéaire; E-balayage localisation linéaire indicateur de linéarité émulsifiant lipophile

magnetic bench

magnetic ink

Nassprüfmittel

magnetic particle content

Magnetpulveranteil

magnetic particle inspection magnetic particles magnetic remanence magnetic saturation magnetic writing magnetising coil magnetostrictive transducer masking

Magnetpulverprüfung Magnetpulver magnetische Remanenz magnetische Sättigung Magnetschrift Magnetisierungsspule magnetostriktiver Wandler Handprüfung; manuelle Prüfung Ausblendung

mass spectrometer

Massenspektrometer

manual scanning

334

Deutsch

induction magnétique hystérésis magnétique liqueur magnétique; encre magnétique concentration en particules magnétiques examen par magnétoscopie particules magnétiques rémanence magnétique saturation magnétique écriture magnétique bobine transducteur magnétostrictif contrôle manuel masquage cellule de spectromètre de masse

English material effect metal screen microfocus radiography mirror mode conversion; wave conversion modulation transfer function (MTF) monitor movement unsharpness multichannel instrument multidirectional magnetisation multifrequency examination

Deutsch Werkstoffeffekt Metallfolie Mikrofokusradiographie Spiegel

Français effet de matériau écran renforçateur métallique radiographie microfocale miroir

Wellenumwandlung

conversion de mode

Modulationsübertragungsfunktion Blendenmodul Bewegungsunschärfe Mehrkanalgerät

fonction de transfert par modulation moniteur flou cinétique appareil multivoie aimantation multidirectionnelle examen multifréquence

kombinierte Magnetisierung

multiple-echo technique

Mehrfrequenzprüfung Mehrfachecho; Echofolge Mehrfachrekonstruktion der empfangenen Signale Anschallung mit mehrfacher Umlenkung Mehrfachecho-Methode

NDT instruction

ZfP-Prüfanweisung

NDT method

ZfP-Verfahren

NDT procedure

ZfP-Verfahrensbeschreibung

NDT technique

ZfP-Technik

near field; Fresnel-zone near vision noise noise signal nominal angle of probe nominal frequency normal probe; straight beam probe normalised impedance plane diagram normalised leakage rate normalised reactance normalised resistance object contrast object-to-film distance occlusion (of gas) operating point optical attenuator optical density optical device optical test chart opto-electronic device orbital scanning output effect P-scan presentation paintbrush method panoramic exposure parallel beam CT paramagnetic substance

Nahfeld; Fresnel-Zone Nahsehfähigkeit Störuntergrund Rauschanzeige Nennwinkel Nennfrequenz Normalprüfkopf; Senkrechtprüfkopf

multiple echo multiple reconstruction of the received signals multiple transverse technique

partial annular sectorial array (type “daisy”) partial pressure

normierte Impedanzortskurve Normleckagerate normierter Blindwiderstand normierter Wirkwiderstand Objektkontrast Abstand Prüfgegenstand-Film Okklusion (von Gas) Arbeitspunkt optischer Abschwächer optische Dichte optische Vorrichtung optische Prüftafel optotronisches Bauelement Kreisabtastung Auslaufeffekt P-Bild Paint-Brush-Verfahren Karussell-Aufnahme Parallelstrahl-CT paramagnetischer Werkstoff Segmentiertes partielles Ringarray (Typ „Gänseblümchen“) Partialdruck

échos multiples reconstruction multiple des signaux reçus contrôle en bonds multiples technique à échos multiples instruction d’essai non destructif méthode d’essai non destructif procédure d’essai non destructif technique d’essai non destructif champ proche; zone de Fresnel vision proche bruit bruit de fond angle de réfraction nominal fréquence nominale transducteur normal; transducteur droit diagramme d’impédance normé flux de fuite normalisé réactance réduite résistance réduite contraste objet distance film-objet occlusion (de gaz) point de fonctionnement atténuateur optique densité optique instrument d’optique mire optique instrument optoélectronique contrôle orbital effet de sortie représentation de type P méthode du pinceau exposition panoramique TI à faisceau parallèle substance paramagnétique réseau annulaire sectorisé partiel (type "marguerite") pression partielle 335

English partial volume effect partially encircling array path-synchronous display peak frequency peelable developer penetrant penetrant system; test system; product family penetrant testing penetration time

Français effet de volume partiel

Eindringsystem; Produktefamilie

système de ressuage; famille de produits

Eindringprüfung Eindringdauer

réseau encerclant partiel représentation en fonction du trajet d'examen fréquence crête révélateur pelliculaire pénétrant

permanent magnet probe

Permanentmagnetsensor

permeability permeability coefficient permeation leak phase phase adjustment; phase setting phase analysis phase reference phase shifter phase velocity phased array probe; transducer array probe

Gaspermeabilität Permeationskoeffizient Permeationsleck Phase

contrôle par ressuage durée de pénétration capteur à aimant(s) permanent(s) perméabilité coefficient de perméabilité fuite de perméation phase

Phasenjustierung

calage de phase

Phasenauswertung Referenzphase Phasensteller Phasengeschwindigkeit

analyse en phase référence de phase déphaseur vitesse de phase transducteur matriciel multiéléments

phased array technique piezoelectric crystal pitch pitch and catch technique; double probe technique pixel size planar location plane wave point of return technique portable electromagnet (yoke) post-cleaning

Gruppenstrahler Technik mit phasengesteuerten Arrays piezoelektrischer Schwinger Elementabstand; Pitch Zweikopftechnik Pixelgrösse planare Ortung ebene Welle Umkehrpunkttechnik

technique multi éléments élément piézo-électrique pas inter-éléments technique à deux transducteurs taille de pixel localisation planaire onde plane technique du point de rebroussement

Handmagnet (Joch)

électroaimant portatif nettoyage après examen

post-magnetisation time precleaning

Nachreinigung nachemulgierbares Eindringmittel Nachmagnetisierungszeit Vorreinigung

pressure change test

Druckänderungsprüfung

post-emulsifiable penetrant

probe array probe axis probe clearance

Flüssigkeits-Durchdringprüfung Primärstrahlung Prüfkopf; Sensor Gruppensensor Prüfkopfachse Sensorabstand

probe damping factor

Dämpfungsfaktor

probe fill factor

Sensorfüllungsgrad

probe index probe orientation probe pusher puller unit

Schallaustrittspunkt Prüfwinkel Sensorvorschubeinheit

pressure dye test primary radiation probe

336

Deutsch Teilvolumen-Effekt teilweise umschliessendes Array wegproportionale Signaldarstellung Spitzenfrequenz abziehbarer Entwickler Eindringmittel

pénétrant à post-émulsion durée de post-aimantation nettoyage avant ressuage contrôle par mesure de variation de pression contrôle par liquides traceurs sous pression rayonnement primaire transducteur; capteur capteurs en réseau axe du transducteur entrefer facteur d’amortissement du transducteur taux du remplissage du capteur point d’émergence orientation du transducteur tireur-pousseur

English probe shoe prods product family proportioning spray gun pulse pulse amplitude pulse echo technique; reflection technique pulse energy pulse envelope pulse length pulse repetition frequency; PRF; rate pulse reverberation pulse shape pulse technique pulsed eddy currents pulser pV-throughput quadrature demodulation radiation contrast radiation source radiograph radiographic film radiographic film digitisation system radiography radioisotope

Deutsch Prüfkopfschuh Aufsetzelektroden Eindringsystem Dosiersprühpistole Impuls; Schallimpuls Impulshöhe Impuls-Echo-Technik Impulsenergie Einhüllende des Impulses Impulsdauer Impulsfolgefrequenz PRF Nachschwingen des Impulses Impulsform Impulstechnik Impulswirbelstrom Pulser pV-Durchfluss Quadratur-Demodulation Strahlenkontrast Strahlenquelle Durchstrahlungsbild radiographischer Film Röntgenfilm-Digitalisierungssystem Radiografie Radioisotop

radionuclide leakage test

Radionukliddichtheitsprüfung

radioscopy Rayleigh wave; surface wave

Radioskopie Rayleigh-Welle; Oberflächenwelle Blindwiderstand; Reaktanz Messspule; Messelement Rekonstruktion Bildaufzeichnung Registrierung

reactance receiving element; secondary coil reconstruction record of image recording recording level; reporting level rectified signal reference block method reference block; test panel reference echo reference level reference probe reference reflector reference test piece reflection reflection assembly reflection coefficient reflectivity reflector refracting prism; wedge refraction

Registrierschwelle gleichgerichtetes Signal Vergleichskörpermethode Kontrollkörper; Referenzkörper Vergleichskörper Bezugsanzeige Bezugshöhe Vergleichssensor Bezugsreflektor Bezugskörper Reflexion Reflexionsanordnung Reflexionsfaktor Reflexionsvermögen Reflektor Brechungsprisma; Vorsatzkeil Brechung

Français sabot touches famille de produits pistolet-doseur d’aspersion impulsion amplitude d’impulsion technique par échos; technique par réflexion énergie d’impulsion enveloppe de l’impulsion durée de l’impulsion fréquence de récurrence des impulsions; FRI réverbération d'une impulsion forme de l’impulsion technique pulsée courants de Foucault pulsés pulser flux gazeux démodulation en quadrature contraste rayonnement source de rayonnement radiogramme film radiographique système de numérisation des films radiographiques radiographie radio-isotope contrôle d’étanchéité aux radionucléides radioscopie onde de Rayleigh; onde de surface réactance enroulement récepteur; élément récepteur reconstruction enregistrement d’image enregistrement seuil de notation; seuil d’enregistrement signal redressé méthode d’évaluation par comparaison directe échantillon de référence; éprouvette de référence; bloc de référence écho de référence amplitude de référence capteur de référence réflecteur de référence pièce de référence réflexion dispositif en réflexion coefficient de réflexion réflectivité réflecteur prisme de réfraction; sabot réfraction 337

English refractive index

relative reflectivity remote field technique remote visual testing replication residual field resistance to flow resolution resolution capability

Deutsch Brechungsindex interessierender Objektbereich (IOB) relative Permeabilität; Permeabilitätszahl relativer Reflexionsgrad Fernfeldtechnik indirekte Sichtprüfung Abdrucktechnik Restfeld Strömungswiderstand Auflösung Auflösungsvermögen

resolution target

Auflösungstestplatte

response factor response time reticule

Empfindlichkeitsfaktor Ansprechzeit resultierendes magnetisches Wechselfeld Skale

reverse phasing technique

Umkehrphasentechnik

region of interest (ROI) relative permeability

resultant magnetic field

RF signal rigid coil technique ring down count rinse rod anode tube roof angle; toe-in-semi-angle rotating field technique rotating head rotating probe roughing pump S-scan presentation sagittal electronic scanning sampling phased array technique sampling probe saturation coil saturation unit scale expansion; expanded time-base sweep scanning scanning aperture scanning direction scanning surface scattered energy scattered radiation screen; shield secondary field sectorial analysis sectorial electronic scanning; S-scan 338

Hochfrequenzsignal; HF-Signal Spulentechnik mit fester Spule Anzahl der Überschwingungen Spülen Stabanodenröhre

Français indice de réfraction région d'intérêt (ROI) perméabilité relative réflectivité relative technique du champ lointain contrôle visuel indirect prise d’empreinte par réplique champ résiduel résistance à l’écoulement résolution capacité de résolution cible pour le contrôle de la résolution facteur de réponse temps de réponse champ magnétique résultant réticule technique du retournement temporel signal H.F. aimantation par bobine rigide nombre de coups rinçage tube à anode longue

Dachwinkel

angle de toit

Rotierfeldtechnik Rotierkopf Rotiersensor Hilfspumpe S-Bild; Sektorbild sagittale elektronische Abtastung Technik mit phasengesteuerten Arrays Schnüffelsonde Vormagnetisierungswicklung Einrichtung zur magnetischen Sättigung Tiefenlupe; gedehnte Zeitachse Abtastung; Prüfung Abtast-Apertur Prüfrichtung Kontaktfläche; Prüffläche gestreute Energie; Schallstreuung Streustrahlung

technique du champ tournant tête tournante sonde tournante pompe de prévidage

Abschirmung Sekundärfeld Sektorauswertung elektronische Sektorabtastung; S-Abtastung

représentation de type S balayage électronique sagittal technique d’échantillonnage de multi éléments sonde de reniflage enroulement de saturation unité de saturation loupe de profondeur contrôle; balayage ouverture de balayage direction de balayage surface balayée énergie diffusée rayonnement diffusé blindage; masque champ en retour analyse sectorielle balayage électronique sectoriel; S-balayage

English segmental probe sensitivity sensitivity curve sensor array separate transmit receive probe shielded probe shot signal amplifier signal locus signal phase; phase angle of a signal signal/noise ratio signature single single single single single single single single

aperture technique channel instrument frequency examination frequency instrument frequency technique parameter examination parameter instrument parameter technique

Deutsch Segmentsensor Empfindlichkeit Empfindlichkeitskurve Sensorgruppe

Français capteur sectoriel sensibilité courbe de sensibilité maillage de capteurs

transformatorischer Sensor

capteur à fonctions séparées

abgeschirmter Sensor Schuss Signalverstärker Signalschleife

capteur à masque tir amplificateur du signal enveloppe du signal

Signalphase

phase d’un signal

Signal/Rausch-Verhältnis charakteristisches Signalmuster Einzel-Apertur-Technik Einkanalgerät Einfrequenzprüfung Einfrequenzgerät Einfrequenztechnik Einparameterprüfung Einparametergerät Einparametertechnik

rapport signal/bruit

single probe technique

Einkopftechnik

skew angle

Schielwinkel Skineffekt; Stromverdrängung

skin effect skip distance; full skip slot sniffing test solid state camera

Sprungabstand

sorting class sound attenuation sound field sound generation sound path length sound path travel distance sound path travel time sound propagation sound velocity; velocity of propagation source holder source location source-to-film distance

Schlitz Schnüffelprüfverfahren Halbleiterkamera Zwischenreiniger auf Lösemittelbasis Nassentwickler auf Lösemittelbasis; nichtwässriger Nassentwickler lösemittelentfernbares Eindringmittel Prüfklasse Schallschwächung Schallfeld Schallerzeugung Schallweg Schalllaufweg Schalllaufzeit Schallausbreitung Schallgeschwindigkeit; Ausbreitungsgeschwindigkeit Strahlenhalter Quellenordnung Abstand Strahlenquelle-Film

spark coil leak tester

Hochfrequenzleckprüfgeräte

spatial frequency spatial resolution specific activity specification specular density spherical wave spiral scanning

Ortsfrequenz Ortsauflösung spezifische Aktivität Spezifikation gerichtete optische Dichte Kugelwelle spiralförmige Abtastung

solvent remover solvent-based developer; non-aqueous wet developer solvent-removable penetrant

signature technique d'ouverture unique appareil monovoie examen monofréquence appareil monofréquence technique monofréquence examen monoparamètre appareil monoparamètre technique monoparamètre technique du transducteur unique angle de skew effet de peau longueur du bond; bond complet fente contrôle par reniflage caméra intégrée solvant révélateur à base de solvant; révélateur humide non aqueux pénétrant éliminable par solvant classe de tri atténuation acoustique champ acoustique génération du son trajet ultrasonore parcours ultrasonore temps de parcours propagation des ondes vitesse de propagation de l’onde porte-source localisation de la source distance source-film détecteur de fuites à bobines d’induction fréquence spatiale résolution spatiale activité spécifique spécification densité spèculaire onde sphérique contrôle en spirale

339

English split coil probe spray gun spurious echo; parasitic echo spurious noise SQUID sensor squint angle standard (calibrated) leak

Deutsch teilbarer Sensor Sprühpistole Störecho; Störanzeige Störgeräusch SQUID-Sensor Schielwinkel Standardleck

standard depth of penetration Standardeindringtiefe standard reference film

Standard-Referenzfilm

static electronic focusing

statische elektronische Fokussierung

stationary wave; standing wave steering angle step wedge stereo radiography subtractive magnetic flux probe suppression; grass cutting surface probe

onde stationnaire

Steuerwinkel Stufenkeil Stereoradiografie

angle de déflexion cale à gradins stéréoradiographie

Subtraktionsfluss-Sensor

capteur à flux soustractifs

Unterdrückung; Verstärkerschwelle Tastsensor Oberflächenwellenprüfkopf

swivel scanning

Wedeln phasenselektive Demodulation T-Sensor T-sensor

T probe T-probe tandem technique; tandem scanning tangential field

seuil de rejet palpeur transducteur d’ondes de surface contrôle en rotation démodulation synchrone capteur en T capteur en T

Tandemtechnik

méthode tandem

Tangentialfeld

tangential field strength

Tangentialfeldstärke

target test test equipment testing configuration thermal imaging camera

Target Prüfung Prüfausrüstung Prüfanordnung Infrarotkamera Magnetisierungstechnik mit durchgestecktem Hilfsleiter durchgesteckter Leiter Drosselung

champ magnétique tangentiel intensité du champ magnétique tangentiel cible essais appareillage de contrôle configuration d’examen caméra infrarouge technique d’aimantation par conducteur traversant conducteur traversant étranglement

Durchschallungstechnik

technique par transmission

Kippwinkel Kippeffekt Grundlinie; Zeitachse Einstellen des Justierbereichs Justierbereich

angle de tilt effet de basculement

threaded conductor technique threading conductor throttling through transmission technique tilt angle tilt effect time base time base adjustment time base range time-of-flight diffraction technique (TOFD) time of flight technique time synchronous display total (integral) leakage rate total electronic focusing total reflection 340

stehende Welle

surface wave probe synchronous demodulation

Français bobine ouvrante pistolet d’aspersion écho brouilleur écho parasite bruit parasite capteur SQUID angle de bigle fuite de référence profondeur de pénétration conventionnelle film de référence normalisé focalisation électronique statique; focalisation électronique en un seul point focal

Beugungslaufzeit Technik Laufzeittechnik zeitproportionale Signaldarstellung integrale Leckagerate elektronische Gesamtfokussierung Totalreflexion

base de temps réglage de la base de temps échelle de la base de temps technique de diffraction du temps de vol technique de temps du vol représentation en fonction de la durée de l'examen flux de fuite global focalisation électronique totale réflexion totale

English tracer fluid tracer gas background transceiver transducer backing transducer mosaic transducer size; nominal size transducer; crystal transfer correction transmission assembly transmission coefficient transmission point; zero point transmission pulse indication transmission technique transverse wave probe transverse wave; shear wave tube camera tube diaphragm tube head tube shield tube shutter tube voltage tube window ultrasonic beam; sound beam

Deutsch Prüffluid Prüfgasuntergrund Einschwinger-Prüfkopf Dämpfungskörper Mosaikschwinger Schwingergrösse; Nenngrösse Schwinger; Wandler Transferkorrektur Transmissionsanordnung Durchlässigkeitsfaktor

Français fluide traceur bruit de fond du gaz traceur transducteur mono-élément amortisseur transducteur mosaïque

Nullpunkt

point zéro

Sendeimpuls Durchschallungstechnik, Transmissionstechnik Transversalprüfkopf; Transversalwellenprüfkopf Transversalwelle: Scherwelle Röhrenkamera Röhrenblende Röntgenstrahler Röhrenschutzgehäuse Röhrenverschluss Röhrenspannung Röhrenfenster

signal d’émission

Schallbündel

faisceau acoustique

ultrasonic leak tester

Ultraschallleckprüfgerät

ultrasonic test equipment

Ultraschallprüfgerät

ultrasonic wave ultraviolet radiation unsealed source unsharpness V-transmission vacuum box vacuum cassette

Ultraschallwelle UV Strahlung offene Strahlenquelle Unschärfe V-Durchschallung Vakuumglocke Vakuumkassette

vacuum test

Vakuumprüfverfahren

dimensions du transducteur transducteur correction de transfert dispositif en transmission coefficient de transmission

technique par transmission transducteur d’ondes transversales onde transversale; onde de cisaillement tube analyseur diaphragme du tube tête du tube gaine du tube cache tension du tube fenêtre du tube détecteur de fuites par ultrasons appareil de contrôle par ultrasons onde ultrasonore rayonnement ultraviolet source non scellée flou transmission en V boîte à dépression cassette à vide contrôle d’étanchéité sous vide

vent valve verification videoscope viewing mask

Prüfkopf mit variablem Winkel; Universalprüfkopf Belüftungsventil Funktionskontrolle Videoskop Blendschutz

viscosity coefficient

Viskosität

viscous leak visible radiation visual contrast visual testing visual testing

viskoses Leck sichtbare Strahlung sichtbarer Kontrast Sichtprüfung Sichtprüfung Apodisierung der Anregespannung; apodisation en tension Gewichtung der Anregespannung spannungsgesteuerte Erreinjection en tension gung

variable angle probe

voltage apodisation voltage driven excitation

transducteur à l’angle variable entrée d’air vérification vidéoscope cache de lecture coefficient de viscosité (dynamique) fuite visqueuse rayonnement visible contraste visuelle contrôle visuel examen visuel

341

English volume scan presentation W-transmission

waveguide wavelength wedge

Deutsch Volumenbild W-Durchschallung Nassentwickler auf Wasserbasis Nassentwickler auf Wasserbasis, suspendiert wasserabwaschbares Eindringmittel Welle Wellenfront Interferenz Wellenzug Bildschirm mit Impulsdarstellung Wellenleiter Wellenlänge Vorsatzkeil

wet technique

Nassprüfung

water-soluble developer water-suspendable developer water-washable penetrant wave wave front wave interference wave train waveform display

wide beam transmitting width of coverage window wobble

Radprüfkopf; Rollenprüfkopf Senden eines breiten Bündels Spurbreite Fenster Wackeleffekt

working range

Arbeitsbereich

wheel probe

X-ray film X-ray tube

Dickenschwinger; X-Schwinger Röntgenfilm Röntgenröhre

X-rays

Röntgenstrahlen

Y-cut crystal

Scherschwinger; Y-Schwinger

yoke

Joch

yoked coil zone location zone of influence of the probe zone of interaction

Jochspule Zonenortung Sensoreinflusszone Wechselwirkungsvolumen

X-cut crystal

342

Français représentation volumique transmission en W révélateur hydrosoluble révélateur en suspension dans l’eau pénétrant éliminable à l’eau onde front d’ondes interférence train d’ondes affichage de la forme d'onde guide d'ondes longueur d’onde sabot méthode de contrôle par voie humide transducteur-roue émission large champ largeur d’action fenêtre ballottement plage de fonctionnement du numériseur cristal-taille X film à rayons X tube radiogène rayonnement X; rayons X cristal-taille Y aimant portatif; “fer à cheval” capteur en circuit en fer localisation par zone zone d’influence du capteur zone d’action du capteur

343

INDEX A A-scan .............................................. 158, 213 Abbot, C. .................................................... 41 Absolute probe .......................................... 135 Absorbed dose ........................................... 292 Absorption edge scanning ........................... 285 Acceptance level ........................................... 3 Acoustic emission (AE) ................................ 230 Acoustic emission decibel scale .................... 233 Acoustic emission signature ......................... 231 Acoustic impedance .................................... 182 Acoustic microscopy ................................... 227 Acoustic mirror .......................................... 201 Acoustic shadow ........................................ 190 Acousto-ultrasonics .................................... 237 Active thermography .................................... 51 Activity ..................................................... 292 ADEPT probe ............................................. 225 Adjacent cable ............................................ 99 Adjacent conductor ...................................... 99 Air coupling transducer ............................... 202 ALARA ...................................................... 294 Alkali-ion diode ........................................... 79 Alternating current field measurement (ACFM) . . . . 149 American Society of Mechanical Engineers (ASME) .................................................................. 20 Ammonia leak detection ............................... 81 Amorphous carbon ..................................... 272 Amorphous selenium .................................. 270 Amorphous silicon ...................................... 270 Angle double transducer probe ..................... 197 Angle probe .............................................. 196 Anisotropic magnetoresistive (AMR) ............... 89 Annihilation ............................................... 289 Aperture leak .............................................. 77 Appleton, E. .............................................. 151 Arago, D. .................................................. 119 Arc blow ................................................... 104 ASTM International ...................................... 20 Attenuation coefficient ................................ 183 Attenuator ................................................ 160 Automated optical inspection (AOI) ................ 31 Autoradiography ........................................ 288 Axial current flow ......................................... 97

B B-scan .............................................. 158, 213 Back pressurising test .................................. 81 Background limitation performance (BLIP) ....... 46 Backing .................................................... 193 Backscatter radiation .................................. 282 Bandwidth ................................................ 192 Barium strontium titanate (BST) .................... 45 Barkhausen noise ....................................... 113 Barkhausen, H. .......................................... 114 Beam angle ............................................... 222 Beam hardening ........................................ 278 Becquerel [Bq] ................................... 255, 292 Becquerel, H. ............................................ 242

344

Beer law ................................................... 243 Beer, A. .................................................... 243 Bell pressure change .................................... 83 Belm, A. ................................................... 179 Berthold penetrameter ................................ 106 Beryllium neutron generator ........................ 257 Betatron ................................................... 253 Biot-Savart law ........................................... 93 Biot, J.-B. ................................................... 93 Bismuth germanate (BGO) .......................... 268 Black light lamp ........................................... 66 Blackbody ............................................. 41, 60 Bleed back .................................................. 71 Bleed out .............................................. 63, 71 Bloch wall ................................................. 113 Bloch, F. ............................................ 113, 171 Blower door ................................................ 86 Blue base .................................................. 260 Bolometer .................................................. 41 Boltzmann constant ..................................... 41 Boltzmann, L. .............................................. 41 Bombing test .............................................. 81 Borescope .................................................. 25 Box car technique ........................................ 56 Bremsstrahlung ......................................... 249 Brewster angle .......................................... 154 Brewster, D. .............................................. 154 Bubble emission ................................... 84, 314 Bubbler .................................................... 199 Bureau de Normalisation des Industries de la Fonderie (BNIF) ........................................... 25 Buys-Ballot, C. .......................................... 179

C C-scan .............................................. 158, 213 Caesium iodide .......................................... 267 Calibration block ....................... 218, 221, 314p. Capacitive micro-machined ultrasonic transducers (CMUT) .................................................... 207 Capillary leak .............................................. 77 Centre frequency ....................................... 192 Certification ......................................... 14, 315 Characteristic frequency .............................. 124 Charcoal gettering ....................................... 87 Charge-coupled device (CCD) ...................... 269 Chilowsky, C. ............................................ 179 CIE 1931 chromaticity diagram ...................... 28 Circulator .................................................. 160 Coaxial cable ............................................. 191 Coercitivity ................................................. 90 Coil fill factor .............................. 122, 126, 145 Colladon, J.-D. ........................................... 179 Colour temperature ...................................... 31 Colour vision deficiency (CVD) ....................... 17 Complementary metal oxide silicon (CMOS) ... 272 Complex impedance plane .................... 124, 126 Complex plane ................................... 133, 314 Compton backscatter technique ................... 284 Compton effect ....................................... 242p. Compton scattering .................................... 243

Compton, A. .............................................. 242 Computed laminography ............................. 275 Computed radiography (CR) ........................ 265 Computed tomography (CT) ........................ 276 Conductance ............................................. 119 Contact angle .............................................. 65 Contact probe ........................................... 197 Contact testing technique ............................ 195 Contrast aid paint ...................................... 100 Converter foil ............................................ 272 Cormack, A. .............................................. 242 Corner reflection ........................................ 185 Couplant ................................................... 197 Coupler .................................................... 161 Creeping wave ................................... 185, 225 Critical angle ............................................. 154 Cross-talk .......................................... 270, 276 Curie temperature ....................... 104, 120, 192 Curie, J. .................................................... 179 Curie, M. .................................................. 242 Curie, P. ..................................... 104, 179, 242 Cut-off frequency ....................................... 192

D D-scan .............................................. 158, 214 Damadian, R. ............................................ 171 Damping ................................................... 193 Dark adaptation .......................................... 18 Decay constant .......................................... 255 Decibel [dB] ....................................... 183, 188 Deconvolution ........................................... 277 Delay line transducer .................................. 196 Demagnetisation ........................................ 104 Densitometer ............................................ 281 Detector panel ........................................... 270 Deuteranomaly ............................................ 17 Deuterium-tritium neutron tube ................... 257 Developer ...................................... 64, 70, 260 Dice-and-fill technique ................................ 193 Differential blackbody ................................... 60 Differential phase-contrast imaging .............. 286 Differential probe ....................................... 135 Diffuse scattering ....................................... 183 Digital holography ....................................... 34 Dirac pulse ................................................. 53 Dirac, P. ..................................................... 53 Direct coupling zone ................................... 146 Direct visual testing ..................................... 25 Disc shaped reflector (DSR) ................. 189, 218 Distance – gain – size (DGS) ......... 189, 209, 217 Distance amplitude correction (DAC) ..... 209, 217 Doppler sonography ................................... 226 Doppler, C. ............................................... 179 Dose ................................................. 260, 271 Dose rate .................................................. 292 Double film technique ................................. 273 Double transducer probe ............................. 195 Double transmission ................................... 211 Dry magnetic particles ................................ 100 Dual energy tomography ............................. 279 Dual energy X-ray absorptiometry (DEXA or DXA) . 287

E

Echo planar imaging (EPI) .................... 171, 177 Eddy current probe testing .......................... 128 Eddy current through-transmission testing ..... 128 EddyTherm ................................................. 59 Edge spread function (ESF) ............................. 9 Effective dose ............................................ 292 Effective permeability ................................. 124 Eigenfrequency ................................... 192, 238 Electon-bombarded charge-coupled device (EBCCD) ................................................... 269 Electric conductivity ............................ 119, 152 Electrocardiography (ECG) .......................... 118 Electromagnetic-acoustic transducer (EMAT) . 112, 202, 239 Electron multiplying charge-coupled device (EMCCD) .................................................. 270 Electron paramagnetic resonance (EPR) ... 75, 177 Electron radiography .................................. 288 Electron spin resonance (ESR) ..................... 177 Electronic speckle pattern interferometry (ESPI) . . 30, 35 Electrostatic spray gun ................................. 66 Emulsifier ................................................... 64 Encircling probe ......................................... 125 Endoscope .................................................. 25 Equivalent dose ......................................... 292 Equivalent dose rate ................................... 297 Ernst, R. ................................................... 171 European Atomic Energy Community (EAEC – EURATOM) ................................................ 291 European Aviation Safety Agency (EASA) ........ 21 European Co-operation for Accreditation (EA) . . 21 European Committee for Standardization (CEN) ... 20 European Federation for Nondestructive Testing (EFNDT) ..................................................... 12 European Synchrotron Radiation Facility (ESRF) ... 258 Excess penetrant remover ....................... 64, 70 Experimental hutch .................................... 260 Extensional mode ...................................... 182

F F-scan ...................................................... 214 False call ...................................................... 3 Far field .................................................... 187 Far vision acuity .......................................... 39 Faraday rotation ........................................ 103 Faraday, M. ............................................... 119 Farnsworth-Munsell ...................................... 14 Fast Fourier transform (FFT) .......................... 11 Feed-through attenuator ............................. 191 Felicity effect ............................................. 231 Felicity ratio .............................................. 231 Femtosecond laser ..................................... 163 Fermat transducer ..................................... 198 Fermat, P. ................................................. 198 Fert, A. ...................................................... 89 Fessenden, R. ............................................ 179 Fibre optic plate with X-ray scintillator (FOP) . 269 Fibre optic taper ........................................ 269 Fibrescope .................................................. 25 Fick law ...................................................... 41 Fick, A. ...................................................... 41 345

Field effect transistor (FET) ......................... 160 Field of view (FOV) ...................................... 44 Film base .................................................. 260 Film digitisation .................................. 264, 319 Film illuminator .................................. 263, 322 Film laminography ..................................... 274 Film processing .......................................... 262 Filtered backprojection ............................... 278 Filtered particle testing ................................. 74 Firestone, F. .............................................. 179 First article inspection ................................. 279 Fixed-frequency continuous-wave transmission .... 156 Flash X-ray machine ................................... 253 Flat bottom hole (FBH) ........................ 189, 217 Flexible coil ................................................. 99 Flexural mode ........................................... 182 Flow gap coupling ...................................... 199 Fluorometallic intensifying screen ................. 264 Fluxgate magnetometer .............................. 117 Flying line thermography .............................. 56 Flying spot thermography ............................. 56 Focal plane array (FPA) ........................... 45, 47 Focal spot size .................................... 280, 317 Fog density ............................................... 260 Förster, F. ........................................... 90, 119 Foucault, L. ............................................... 119 Fourier transform ........................................ 11 Fourier, J. ................................................... 11 Fraunhofer-zone ........................................ 187 Fraunhofer, J. ............................................ 187 Frequency encode gradient .......................... 175 Frequency limit .......................................... 192 Frequency scanning eddy current technique (FSECT) .................................................... 145 Fresnel equations ....................................... 153 Fresnel-zone ............................................. 187 Fresnel, A. ................................................ 153 Fringe projection .................................... 29, 34 Fuchsine dye test ......................................... 65 Full screen height (FSH) .............................. 217 Full wave direct current (FWDC) ............. 93, 252 functional MRI (fMRI) .......................... 171, 177

G Gadolinium oxisulphide (GOS) ..................... 267 Gamma-ray source ............................. 254, 297 Gas-coupled laser acoustic detection (GCLAD) ..... 207 Geiger-Müller (GM) .................................... 293 Geiger, H. ................................................. 243 General visual testing ................................... 25 Geometric domain ...................................... 183 Giant magnetoresistance (GMR) ..................... 89 Giant magnetostrictive material (GMM) ......... 204 Granularity ............................................... 260 Grating lobe .............................................. 206 Gray [Gy] .......................................... 260, 292 Ground penetrating radar (GPR) ................... 161 Grünberg, P. ............................................... 89 Gyromagnetic ratio .................................... 171

H Halbach .................................................... 173

346

Half live .................................................... 255 Half value layer (HVL) ......................... 244, 295 Half wave direct current (HWDC) ............ 93, 252 Hall effect ................................................. 102 Hall, E. ..................................................... 102 Head wave ................................................ 185 Henry [H] ................................................. 121 Hermetically sealed device ............................ 87 Herschel, W. ............................................... 41 Hertz, H. ................................................... 151 Heterodyne ............................................... 156 Homodyne ................................................ 156 Horn ........................................................ 161 Hounsfield, G. ........................................... 242 Hsu-Nielsen source .................................... 232 Hydrodynamic paradox ............................... 202 Hydrophilic emulsifier ................................... 65 Hydrophone .............................................. 179

I Illuminance ................................................. 39 Illuminance meter .................................. 39, 72 Image intensifying system ........................... 268 Image quality indicator (IQI) ................ 281, 321 Imaging plate (IP) .............................. 265, 320 Immersion transducer ................................ 200 Impact-echo .............................................. 208 Impedance ................................................ 122 Impedance bridge ...................................... 136 In phase component ................................... 156 In phase demodulation ............................... 132 Index of refraction .............................. 152, 169 Induced current flow .................................... 98 Inductance ................................................ 121 Induction .................................................. 120 Induction coil ............................................ 102 Inductive reactance .................................... 121 Instantaneous field of view (IFOV) ............ 45, 61 Instantaneous measurement field of view (IFOVmeas) ........................................... 45, 62 Insulation component test (INCOTEST) ......... 148 Intensifying screen ..................................... 263 Internal coaxial probe ................................. 127 International Annealed Copper Standard (IACS) . . 120 International Atomic Energy Agency (IAEA) . . . 291 International Commission on Radiological Protection (ICRP) ....................................... 291 International Committee for Nondestructive Testing (ICNDT) .......................................... 12 International Organization for Standardization (ISO) ......................................................... 20 Intrinsic impedance .................................... 152 Inverse Doppler effect ................................ 165 Inverse fast Fourier transform (IFFT) .............. 11 Ionisation gauge .......................................... 79 Ishihara test plate ....................................... 14 Isotope crawler .......................................... 256

J Japanese Industrial Standards Committee (JISC)... .................................................................. 20 Jet technique ............................................. 199 Joliot, F. ................................................... 242

Joliot, I. .................................................... 242 Josephson junction ..................................... 117 Josephson, B. ............................................ 117 Joule effect ............................................... 203 Joule-Thomson effect ................................... 46 Joule, J. .............................................. 46, 203

K K-shell radiation ........................................ 249 k-space .................................................... 177 Kaiser effect .............................................. 231 Kelvin, Lord ................................................ 46 Kirchhoff law ............................................... 42 Kirchhoff, G. ............................................... 42 Klein-Nishina cross section .......................... 284 Klein, O. ................................................... 242

L Laboratory blackbody ................................... 60 Lamb wave ............................................... 180 Lamb, H. .................................................. 180 Langevin, P. .............................................. 179 Langley, S. ................................................. 41 Larmor equation ........................................ 171 Larmor frequency ....................................... 171 Larmor, J. ................................................. 171 Laser shearography ................................ 30, 36 Latent image ............................................. 260 Lauterbur, P. ............................................. 171 Lead zirconate titanate (PZT) ....................... 192 Leakage rate ............................................... 77 Leaky Rayleigh wave .................................. 180 Lenz law ................................................... 121 Lenz, H. .................................................... 121 Lidar (light detection and ranging) ................. 25 Lift-off .................................. 90, 129, 134, 145 Light field camera ........................................ 40 Line chart ................................................... 38 Line of response (LOR) ............................... 289 Line spread function (LSF) .............................. 9 Linear accelerator (linac) ............................ 253 Linear non-threshold hypothesis (LNT) .......... 294 Lipophilic emulsifier ..................................... 65 Lippmann, G. ............................................ 179 Liquefied petroleum gas (LPG) ........... 234, 317p. Lissajous figure ......................................... 119 Lissajous, J. .............................................. 119 LLL-interferometer ..................................... 286 LLT examination ................................. 185, 210 Local default (LD) ...................................... 102 Lock-in thermography .................................. 56 Longitudinal wave ...................................... 180 Lorentz effect ............................................ 202 Lorentz, H. ................................................ 202 Loss of metallic area (LMA) ......................... 102 Love wave ................................................ 181 Low frequency eddy current (LFEC) .............. 115 Lumen [lm] ................................................ 39 Lutetium oxyorthosilicate (LSO) ................... 268 Lutetium yttrium oxyorthosilicate (LYSO) ...... 268 Lux [lx] ...................................................... 39 Luxmeter .................................................... 39

M

Magnetic Bench ........................................... 96 Magnetic field constant ................................. 90 Magnetic field meter ................................... 106 Magnetic flux leakage (MFL) ............ 89, 109, 133 Magnetic hysteresis loop ............................... 90 Magnetic remanence method (MRM) ............. 115 Magnetic resonance imaging (MRI) ........ 171, 276 Magnetic rope testing (MRT) ................. 110, 318 Magnetic saturation .............................. 90, 144 Magneto-optical (MO) ................................. 103 Magneto-optical imaging (MOI) .................... 149 Magnetocardiography (MCG) ....................... 118 Magnetostriction ........................................ 203 Magnetovision ........................................... 116 Mandrel ...................................................... 95 Manhattan Project ........................................ 77 Mansfield, P. ............................................. 171 Mass spectrometer ............................... 79, 314 Maxwell, J. ................................................ 151 Mercury cadmium telluride (MCT) ................... 48 Metal screen ............................................. 263 Metal-semiconductor field effect transistor (MESFET) .................................................. 151 Metamaterial ............................................. 169 Michelson interferometer .............................. 36 Micro power xenon light (MPXL) ..................... 66 Micro-channel plate (MCP) ........................... 269 Microbolometer ........................................... 45 Microfocus X-ray tube ................................. 252 Microwave reflectometer ............................. 158 Microwave transmission .............................. 156 MIL-standards ............................................. 20 Minimum detectable leakage rate ................... 78 Minimum resolvable temperature difference (MRTD) ................................................. 44, 61 Mirror transmission .................................... 211 Modulation ................................................... 9 Modulation transfer function (MTF) ...... 8, 45, 246 Moiré interferometry ............................... 29, 37 Molecular mobility ...................................... 172 Monochromator ......................................... 285 Multi-quantum well (MQW) ............................ 48 Mumetal ................................................... 120 Mutual inductance ...................................... 121

N Near field ..................................... 34, 168, 187 Near vision acuity ........................................ 14 Néel temperature ....................................... 120 Néel, L. .................................................... 120 Neutron detector ....................................... 272 Nishina, Y. ................................................ 242 Noise equivalent temperature difference (NETD) . . 45 Noise vibration harshness (NVH) .................... 36 Non-linear acoustics ................................... 240 Nuclear magnetic resonance (NMR) .............. 171 Nyquist frequency ........................................ 10 Nyquist-Shannon sampling theorem ............... 10 Nyquist, H. ................................................. 10

O Ohm, G. ................................................... 119 Oil and whiting ............................................ 63

347

One-sided access NMR (OSA-NMR) ............... 172 Operating point ......................................... 124 Optical approximation ................................. 155 Optical coherence tomography (OCT) ............. 33 Optical density .......................................... 260 Optical region ............................................ 155

P P-scan ...................................................... 214 Paintbrush scanning ................................... 205 Pair production .......................................... 243 Panoramic exposure technique ..................... 273 Parallel optical low-coherence tomography (pOCT) .................................................................. 33 Partial discharge localisation ........................ 234 Passive thermography .................................. 51 Peak frequency .......................................... 192 Peelable detection media ............................ 101 Peelable developer ....................................... 65 Peltier element ..................................... 46, 272 Peltier, J. .................................................... 46 Penetrant .............................................. 69, 77 Permalloy ................................................. 204 Permanent magnet ...................................... 93 Permeability .......................... 90, 120, 152, 169 Permittivity ........................................ 152, 169 Phase contrast radiography ......................... 286 Phase encode gradient ................................ 175 Phase lag .................................................. 123 Phase scattering ........................................ 183 Phase shifter ............................................. 160 Phased array ...................................... 151, 204 Photo disintegration ................................... 243 Photo stimulated luminescence (PSL) ............ 265 Photochromatic spectacles ............................ 72 Photoconductor ............................. 46, 163, 270 Photoelectric absorption .............................. 243 Photoelectric detector ................................... 46 Photoemissive detector ................................. 46 Photometer ................................................. 79 Photonic detector ......................................... 46 Photopic ..................................................... 16 Photovoltaic detector ................................... 46 Pierce, G. .................................................. 179 Piezo-composite ........................................ 193 Piezoelectric element ........................... 192, 208 Pipeline inspection gauge (PIG) .................... 112 Pipeline pig ............................................... 113 Pirani gauge ................................................ 79 Pirani, M. .................................................... 79 Pitch and catch .......................................... 211 Planck constant ........................................... 41 Planck law .................................................. 41 Planck, M. ................................................... 41 Plenoptic camera ......................................... 40 Point spread function (PSF) ............................. 9 Poisson ratio ........................................ 35, 181 Polyvinylidene fluoride (PVDF) ..................... 200 Positron emission tomography (PET) .... 268, 276, 289 Post-emulsifiable penetrant ........................... 65 Post-magnetisation time ............................. 100 Power divider ............................................ 161 Pressure change method .................. 78, 82, 318 348

Pressure decay ............................................ 83 Pressure dye test ......................................... 77 Pressure rise ............................................... 83 Pressurisation-evacuation test ....................... 81 Probability of detection (POD) ......................... 3 Probability of false alarms (POFA) .................... 4 Probe array ............................................... 137 Probe index ............................................... 222 Prod ..................................................... 95, 98 Product family ............................................. 64 Protanomaly ............................................... 17 Pulse echo technique .................................. 209 Pulse envelope .......................................... 192 Pulse phase thermography ............................ 58 Pulse shape ............................................... 192 Pulse thermography ..................................... 53 Pulsed eddy current (PEC) ........................... 146 Purcell, E. ................................................. 171 Pyroelectric detector .................................... 45

Q Quadrature component ............................... 156 Quadrature demodulation ............................ 132 Qualitative thermography ............................. 51 Quantitative thermography ........................... 51 Quantum detector ........................................ 46 Quantum well infrared photodetectors (QWIP) . 48

R Radar (radio detection and ranging) ............. 151 Radargram ................................................ 158 Radiation weighting factor ........................... 292 Radionuclide leakage .................................... 87 Radon transform inversion .......................... 278 Radon, J. .................................................. 242 RamLak filter ............................................. 277 Rapid prototyping ................................. 34, 279 Rayleigh approximation .............................. 155 Rayleigh domain ........................................ 183 Rayleigh region .......................................... 155 Rayleigh scattering ..................................... 243 Rayleigh wave .............................. 36, 180, 185 Rayleigh, Lord ........................................... 179 Reactance ................................................. 122 Receiver operation characteristics (ROC) ........... 6 Reference vessel technique ........................... 83 Reflection coefficient ...................... 43, 153, 183 Refractometer ............................................. 72 Region of interest (ROI) .............................. 280 Relative bandwidth ..................................... 192 Remanence ................................................. 90 Remote field eddy current (RFEC) ......... 112, 146 Remote field zone ...................................... 146 Remote visual testing ................................... 27 Replica ....................................................... 25 Residual thiosulphate ................................. 280 Resistance ................................................ 119 Resistivity ................................................. 119 Resolution target .................................. 38, 169 Retentivity .................................................. 90 Reverse engineering ............................. 34, 279 Rho-theta probe ........................................ 205 Richardson, L. ........................................... 179 Rigid coil .................................................... 99

Roller probe .............................................. 198 Röntgen, C. ............................................... 242 Rotating probe ........................................ 130p. Roughness .................................................. 25

S S-scan ...................................................... 214 Sampling probe ........................................... 81 Saturation low frequency eddy current (SLOFEC) . 144 Savart, F. ................................................... 93 Scanning near field optical microscopy (SNOM) .... 34 Schottky-barrier detector .............................. 47 Schottky, W. ............................................. 151 Scintigraphy .............................................. 289 Scintillator ................................................ 266 Scotopic ..................................................... 16 Sedimentation tube .................................... 107 Self-inductance .......................................... 121 Sensitometric curve ................................... 260 Shear horizontal – sh .................................. 203 Shear vertical – sv ..................................... 203 Shear wave ............................................... 180 Shepp-Logan filter ..................................... 277 Short ....................................................... 161 Shortened projection .................................. 222 Side lobe .................................................. 188 Side-drilled hole (SDH) ........................ 189, 217 Siemens [S] .............................................. 119 Sievert [Sv] .............................................. 292 Signal-to-noise ratio ...................................... 7 Single photon emission computed tomography (SPECT) .................................................... 289 Sinogram .................................................. 274 Six sigma ..................................................... 6 Skin effect ........................................... 59, 122 Skip distance ............................................. 210 Slice select gradient ................................... 175 Slit response function (SRF) ..................... 45, 62 Smith chart ............................................... 151 Smith, P. .................................................. 151 Snell law ........................................... 153, 183 Snell, W. .................................................. 153 Sniffing ...................................................... 81 Sokolov, S. ............................................... 179 Sonar (sound navigation and ranging) ........... 179 Sorting ..................................................... 143 Source location .......................................... 232 Spallanzani, L. ........................................... 179 Spallation neutron source ............................ 257 Spark coil leak tester ................................... 77 Specific activity ......................................... 254 Specific heat capacity ................................... 43 Spherical wave .......................................... 188 Spin-lattice relaxation ................................. 172 Spin-spin relaxation ................................... 172 Split ring resonator (SRR) ........................... 169 SPRITE detector .......................................... 48 Sproule, D. ............................................... 179 Squirter .................................................... 199 Standard depth of penetration ..................... 123 Standard reference film ....................... 265, 319 Standing wave ........................................... 159

Steel Castings Research and Trade Association (SCRATA) ................................................... 25 Stefan-Boltzmann constant ........................... 42 Stefan-Boltzmann law .................................. 42 Stefan, J. .................................................... 42 Step heating ............................................... 58 Stereo radiography .................................... 274 Stirling cooler ............................................. 46 Stirling, R. .................................................. 46 Stochastic domain ...................................... 183 Stoneley wave ........................................... 180 Stress pattern analysis by thermal emission (SPATE) ..................................................... 59 Sturm, J.-F. .............................................. 179 Super lattice detector ................................... 47 Superconductive quantum interference device (SQUID) ................................................... 117 Superheterodyne ....................................... 208 Swept frequency continuous-wave transmission technique ................................................. 156 Swept frequency technique .......................... 145 Swiss Accreditation Service (SAS) ............. 12, 21 Swiss Association for Standardisation (SNV) .... 20 Swiss Association for Technical Inspections (SVTI)... ..................................................... 20 Swiss Federal Office for Public Health (BAG) ... 292 Swiss Light Source (SLS) ............................ 258 Swiss National Accidence Insurance Organisation (SUVA) ..................................................... 292 Swiss Society for Nondestructive Testing (SGZP)... .................................................................. 12 Swiss Society of Engineers and Architects (SIA) ... 20 Swiss Spallation Neutron Source (SINQ) ........ 258 Swiss Water Pollution Control Association (SVA) . . 86 Synchronous demodulation .......................... 132 Synchrotron ....................................... 258, 285 Synthetic aperture focusing technique (SAFT) ...... 167, 221

T T-probe .................................................... 136 T1 relaxation ............................................. 172 T2 relaxation ............................................. 172 Talbot-Lau interferometer ........................... 286 Tandem examination .................................. 210 Tap testing ............................................... 239 Tenth value layer (TVL) .............................. 295 Terahertz pulse imaging (TPI) ...................... 158 Terfenol .................................................... 204 Tesla, N. ................................................... 151 Thermal conductivity .................................... 43 Thermal diffusion length ............................... 57 Thermal diffusivity .................................. 41, 43 Thermal effusivity ........................................ 43 Thermal mismatch factor .............................. 43 Thermochromic liquid crystal (TLC) ................ 49 Thermoelastic constant ................................. 60 Thermoelastic stress analysis (TSA) ................ 59 Thermoelectric method with magnetic readout (TEM) ....................................................... 115 Thermoluminescence dosimeter (TLD) .......... 292 Thin-film inductive (TFI) ........................ 89, 115 349

Thin-film transistor (TFT) ............................ 270 Thomson scattering .................................... 244 Thomson, J. .............................................. 244 Threading conductor .................................... 98 Time of flight ............................................. 158 Time-of-flight diffraction (TOFD) ........... 219, 313 Time-resolved infrared radiometry (TRIR) ....... 58 Tissue weighting factor ............................... 292 Tone burst generator .................................. 208 Torrent permeability measurement technique . . 84 Total reflectance ........................................ 185 Tracer gas method ............................. 78p., 318 Track-edge-foil .......................................... 273 Transceiver ............................................... 193 Transfer correction ..................................... 217 Transient thermography ............................... 53 Transition zone .......................................... 146 Transmission coefficient ....................... 153, 183 Transmission technique ....................... 210, 313 Transmission thermography .......................... 52 Transreflective ........................................... 190 Transverse wave ........................................ 180 Tritanomaly ................................................ 17 Trombone line ........................................... 160 Trost, A. ................................................... 179 Troxler gauge ............................................ 256 TV holography ............................................. 34

U Undercut .................................................. 282 Unmanned aerial vehicle (UAV) ...................... 27 Unsharpness ........................ 246, 273, 280, 321 UV-A radiometer .......................................... 72

V V(z) curve ................................................. 227 Vacuum box ................................................ 81 Vacuum chamber technique .......................... 82 Vacuum technique ....................................... 80

350

Verdet constant ......................................... 103 Verdet, E. ................................................. 103 Veselago, V. .............................................. 169 Vibro-thermography ..................................... 59 Viewing conditions ......................... 72, 107, 314 Villari effect ....................................... 116, 204 Villari, E. .................................................. 204 Virgin curve ................................................ 90 Visual tactile comparator .............................. 25 Volume-scan ............................................. 214

W Water-air-gun ............................................. 65 Water-bag method ..................................... 278 Waterfall display ........................................ 130 Watson-Watt, R. ........................................ 151 Waveguide ................................................ 233 Wet magnetic particles ............................... 100 Wheel probe .............................................. 198 Wien law .................................................... 41 Wien, W. .................................................... 41 Wigner-Ville distribution ...................... 147, 216 Wüthrich ,K. .............................................. 171

X X-ray X-ray X-ray X-ray X-ray X-ray

constant ........................................... 296 film ................................................. 260 flash technique .................................. 290 limit .................................................. 80 refractometry .................................... 291 tube ................................................ 250

Y Y-cut crystal .............................................. 196 Yoke ..................................................... 95, 99 Young modulus .................................... 35, 181 Yttrium iron garnet (YIG) ............................ 103

Z Z plane technology ...................................... 47

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Non-Destructive Evaluation Methods

Non-Destructive Evaluation Methods Thomas Lüthi Cover image: Anax imperator, micro X-ray tomographic view, Iwan Jerjen, Empa Version 2013 Copyright...

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