In Situ Monitoring of Fiber-Reinforced Composites (eBook)

Dr. habil. Markus Sause studied Physics at the University of Augsburg and earned his doctoral degree in 2010 in Experimental Physics at the same institution. He received the “Erich-Krautz-Preis” in 2010 for his outstanding contribution to the interpretation of acoustic emission of fiber-reinforced materials. In 2015 he was awarded his Habilitation in Experimental Physics. He is lecturer at University of Augsburg and head of the Materials Testing Research Group at the Chair for Experimental Physics II. Since 2014 he has been a member of the EWGAE executive committee and active in several other committees dedicated to the testing and analysis of fiber-reinforced materials. His research interests span the mechanics of fiber-reinforced composites, their destructive and non-destructive testing as well as numerical methods to interpret the materials behavior. A special focus is given to bridge the gap between destructive testing approaches and non-destructive inspection to perform insitu analysis of materials failure.

Preface 8
Acknowledgments 10
Contents 12
Financial Support 16
About the Author 18
Chapter 1: Introduction 19
References 22
Chapter 2: Failure of Fiber-Reinforced Composites 23
2.1 Classification of Failure Mechanisms 24
2.1.1 Microscale 25
2.1.2 Mesoscale 31
2.1.3 Macroscale 34
2.2 Failure Theories for Fiber-Reinforced Composites 36
2.2.1 Quasi-Static Failure 36
2.2.2 Quasi-Static Failure Including Growth of Damage, Damage Mechanics, and Degradation 39
2.2.3 Long-Term Behavior 46
2.2.3.1 Creep and Stress Rupture 46
2.2.3.2 Fatigue 48
2.2.4 High Velocity 50
2.3 Challenges in Mechanical Testing of Fiber-Reinforced Materials 51
2.3.1 Detection of First Failure Onsets 52
2.3.2 Tracking Failure Evolution 53
2.3.3 Ductile Matrix Materials 54
2.4 What Can In Situ Methods Contribute to Mechanical Testing? 54
2.4.1 In Situ Microscopy 55
2.4.2 Digital Image Correlation 55
2.4.3 X-Ray Methods 56
2.4.4 Thermography 58
2.4.5 Shearography 59
2.4.6 Ultrasonic Measurements 60
2.4.7 Acoustic Emission 61
2.4.8 Electromagnetic Emission 61
References 62
Chapter 3: Digital Image Correlation 74
3.1 Principle of Operation 75
3.2 System Accuracy 80
3.2.1 Error Sources 81
3.2.1.1 Systematic Errors 81
3.2.1.2 Random Errors 83
3.2.1.3 Stereo Vision 84
3.2.2 Resolution 84
3.3 Strain Concentration 86
3.3.1 Measurement of Strain Concentration Due to Internal Defects 88
3.3.2 FEM Modeling of Strain Concentration Due to Internal Defects 97
3.3.2.1 Full-Field Comparison 98
3.3.2.2 Signatures of Artificial Defects 104
3.3.3 Detectability of Defects Using DIC 114
3.3.3.1 Experimental Parameters 116
3.3.3.2 Applicability of the Modeling Approach to Real Defects 120
3.3.3.3 Limitations Due to System Accuracy 122
3.4 Application to Composites 128
3.4.1 DIC as Optical Extensometer 128
3.4.1.1 Tensile Testing of Unidirectional Fiber Reinforced Polymers 129
3.4.1.2 Compressive Testing of Unidirectional Fiber Reinforced Polymers 131
3.4.1.3 V-Notched Rail Shear Testing of Unidirectional Fiber Reinforced Polymers 133
3.4.2 Detection of Failure Onsets 135
3.4.2.1 Short-Beam Shear Tests 136
3.4.2.2 End-Notched Flexure Tests 138
3.4.3 Detectability of Failure Mechanisms 141
References 143
Chapter 4: Acoustic Emission 147
4.1 Principle of Operation 147
4.2 Source Mechanics 149
4.2.1 AE Rise-Times and Plate Waves 151
4.2.2 AE Source Model Implementation for Fiber Reinforced Materials 154
4.2.2.1 Source Models for Microscale 159
4.2.2.2 Source Models for Meso- and Macroscale 162
4.2.3 Case Studies for Acoustic Emission Sources in Fiber Reinforced Materials 163
4.2.3.1 Inter-fiber Failure 166
Tensile Perpendicular 167
Compression Perpendicular 172
Shear Parallel 174
Bending sigman 177
4.2.3.2 Delamination 181
Shear Parallel 183
Bending sigman 185
4.2.3.3 Fiber Failure 189
Tension Parallel 189
Compression Parallel 192
4.2.3.4 Fiber Bridging 195
4.2.3.5 Influence of Crack Length 200
4.2.3.6 Influence of Fracture Surface 201
4.2.3.7 Influence of Depth Position 202
4.2.4 Detectability of Failure Mechanisms 203
4.3 Wave Propagation 208
4.3.1 Attenuation 218
4.3.1.1 Geometric Spreading 219
4.3.1.2 Thermoelastic and Akhiezer Dissipation 219
4.3.1.3 Dispersion 220
4.3.1.4 Scattering 220
4.3.1.5 Dissipation into Adjacent Media 220
4.3.1.6 Modeling Attenuation 220
4.3.2 Influence of Geometry 222
4.3.3 External and Internal Obstacles 230
4.3.3.1 Reference Case 232
4.3.3.2 Rivets and Bolts 234
4.3.3.3 Holes 236
4.3.3.4 Inter-fiber Cracks 236
4.3.3.5 Broken Fibers 239
4.3.3.6 Delamination 240
4.3.3.7 Influence on Signal Arrival Times 241
4.4 Detection of Acoustic Emission Signals 242
4.4.1 Comparison of Sensor Types 246
4.4.2 Sensor Modeling 249
4.4.2.1 Influence of Attached Circuit 251
4.4.2.2 Influence of Sensor Aperture 253
4.4.2.3 Influence of Impedance Mismatch 254
4.4.3 Waveguides 255
4.4.3.1 Shape of Waveguide 258
4.4.3.2 Diameter of Waveguide 259
4.4.3.3 Material of Waveguide 259
4.4.3.4 Influence of Temperature Gradient 261
4.4.4 Other Factors Affecting Sensor Sensitivity 261
4.4.4.1 Coupling Medium 261
4.4.4.2 Sensor Fixation 264
4.5 Signal Classification 266
4.5.1 Recommended Practices Before Starting Signal Classification 268
4.5.2 Pattern Recognition Method to Detect Natural Clusters 272
4.5.3 Uncertainty of Classification 276
4.5.4 Factors of Influence 280
4.5.4.1 Source-Sensor Distance 281
4.5.4.2 Plate Thickness 284
4.5.4.3 Stacking Sequence 285
4.5.4.4 Internal Damage 292
4.5.4.5 Sensor Type 294
4.5.4.6 Signal-to-Noise Ratio 297
4.6 Source Localization 298
4.6.1 Determination of Signal Onset 301
4.6.2 Classical Localization Methods 303
4.6.3 Source Localization Methods Based on Neural Networks 307
4.6.3.1 Training Stage Using Test Sources 309
4.6.3.2 Compensation of Acoustic Anisotropy 314
4.6.3.3 Discontinuous Deltat-Fields 318
4.6.3.4 Scale Invariance and Portability 319
4.7 Application to Composites 321
4.7.1 AE Source Identification Using FEM Results 321
4.7.2 Detection of Failure Onset 325
4.7.2.1 Apparent Interlaminar Shear Strength Tests 325
4.7.2.2 End-Notched Flexure Tests 327
4.7.2.3 Transverse Crack Tension Tests 328
4.7.3 Comparison to Failure Criteria Predictions 333
4.7.3.1 Unidirectional Laminates 333
4.7.3.2 Cross-Ply Laminates 336
4.7.4 Tracking Failure Evolution 343
4.7.4.1 Double-Cantilever Beam Testing 343
4.7.4.2 Tensile Testing 350
4.7.4.3 Structural Components 355
References 365
Chapter 5: Electromagnetic Emission 376
5.1 Principle of Operation 376
5.2 Source Mechanism 378
5.2.1 Modeling of Electromagnetic Emission Sources 386
5.2.1.1 Implementation and Validation 387
5.2.1.2 Source Radiation Pattern 396
5.2.1.3 Influence of Distance Between Source and Sensor 398
5.2.1.4 Influence of Electrical Properties 400
5.2.1.5 Sources in Fiber Reinforced Composites 400
5.2.2 Test Sources for Electromagnetic Emission 404
5.3 Signal Propagation 411
5.4 Detection of Electromagnetic Emission Signals 415
5.4.1 EME Detector Concepts 416
5.4.1.1 Capacitance Plate Sensors 418
5.4.1.2 Wire Sensors 421
5.4.1.3 Coil Sensors 422
5.4.1.4 Comparison and Common Aspects 423
Material 426
Positioning 426
Preamplifier 427
Acquisition 427
5.4.2 Electromagnetic Shielding 428
5.4.2.1 Apertures 435
5.4.2.2 Waveguides Below Cutoff 438
5.4.2.3 Seams 440
5.4.2.4 Cable Penetrations 441
5.5 Application to Composites 442
5.5.1 Measurement of EME Due to Crack Formation 442
5.5.1.1 Polymer Failure 443
5.5.1.2 Fiber Filament Failure 448
5.5.1.3 Composites 451
5.5.2 Measurement of Fracture Surface Orientation 460
5.5.3 Detectability of Failure Mechanisms 465
5.5.3.1 Absolute System Limits 465
5.5.3.2 Signal-to-Noise Ratio 466
5.5.3.3 Acquisition Mode 467
References 468
Chapter 6: Computed Tomography 472
6.1 Principle of Operation 473
6.2 Detail Visibility 476
6.2.1 Object Resolution 477
6.2.2 Artifacts 479
6.2.2.1 Physics-Based Artifacts 481
Beam Hardening 481
Cupping Artifact 481
Dark Bands 482
High-Density Foreign Material Artifact 482
Partial Volume Averaging 483
Quantum Mottle (Noise) 484
Photon Starvation 486
Undersampling 486
6.2.2.2 Hardware-Based Artifacts 487
Ring Artifact 487
Tube Arcing 487
6.2.2.3 Reconstruction Artifacts 488
Cone-Beam Effect 488
Windmill Artifacts 488
6.2.2.4 Motion Artifacts 489
6.2.3 Detectability of Defects in Fiber Reinforced Materials 490
6.3 Volumetric Inspection of Materials 496
6.3.1 Concepts for Ex Situ Loading 496
6.3.2 Concepts for In Situ Loading 497
6.3.2.1 Type of X-ray Source 498
6.3.2.2 Type of X-ray Detector 499
6.3.2.3 Type of Load Rig 499
Tensile or Compressive Load Test 501
Mode I Test 504
Flexural Test and Mode II Test 505
Thermal Loading 506
Imaging Requirements 507
6.4 Digital Volume Correlation 508
6.5 Application to Composites 513
6.5.1 Ex Situ Testing 517
6.5.1.1 Visualization of Damage Progress 517
6.5.1.2 Extraction of Geometries from Volume Data 521
6.5.2 In Situ Testing 530
6.5.2.1 Inter-fiber Failure 533
6.5.2.2 Fiber Failure 535
6.5.2.3 Interlaminar Failure 540
References 542
Chapter 7: Combination of Methods 548
7.1 Comparison of In Situ Methods 549
7.1.1 In Situ Capabilities 551
7.1.2 Detectability of Failure Mechanisms 553
7.1.3 Detection Sensitivity 555
7.1.4 Extension of Methods to Large Scale and Field Testing 558
7.2 Established Method Combinations 563
7.2.1 Imaging Methods and Acoustic Emission 563
7.2.2 Digital Image Correlation and Acoustic Emission 570
7.2.3 Thermography and Acoustic Emission 573
7.2.4 Computed Tomography and Acoustic Emission 578
7.2.4.1 Ex Situ Computed Tomography 578
7.2.4.2 In Situ Computed Tomography 588
Inter-Fiber Failure 593
Fiber Failure 602
7.2.5 Computed Tomography and Digital Image Correlation 605
7.2.6 Electromagnetic Emission and Acoustic Emission 605
7.2.7 Acousto-Ultrasonics 611
7.3 Future Developments 616
7.3.1 Digital Image Correlation 617
7.3.2 Acoustic Emission 617
7.3.3 Electromagnetic Emission 619
7.3.4 In Situ Computed Tomography 620
7.3.5 Method Combinations 621
References 621
Appendix A: Acoustic Emission-Parameters of Influence 625
A.1 Acquisition System Electronics 625
A.2 Sensing Technology 626
A.3 Material and Geometry 627
A.4 Experimental Configuration 629
Appendix B: Material Properties 631
Appendix C: Acoustic Emission Signal Parameters 635
Appendix D: Definitions and Abbreviations 639
Index 643