Acoustic Emission Monitoring of CFRP Laminated Composites ...

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24 Chapter 3. Acoustic Emission the entropy. The entropy is studied here for its use for condition monitoring. The features based on the time intervals between hits were presented and studied in On Using AE Hit Patterns for Monitoring Cyclically Loaded CFRP 15, 18 and in An AE Feature for Issuing Early Failure Warning of CFRP Subjected to Cyclic Fatigue. 17 The results of the entropy study were presented in AE Entropy for Condition Monitoring CFRP Subjected to Cyclic Fatigue. 19 The last section, or Sect. 3.4, presents the main contribution of this thesis. It is a methodology for processing, presenting, and quantifying AE data for the purpose of identifying and tracking the locations of multiple AE sources relative to a reference signal. This methodology was presented in Monitoring The Evolution of Individual AE Sources in Cyclically loaded 16, 20 FRP Composites. 3.1 Background Acoustic Emission (AE) is a term used for transient stress waves which are generated by the energy released when microstructural changes occur in a material. 5, 6 AE is measured using a passive, non-destructive measurement technique. The energy is provided by an elastic stress eld in the material. The stress eld can be generated by stressing the material, for instance using mechanical-, thermal-, pressure- and chemical stressing. These types of stress all contribute to fatigue failure and are commonly encountered in-service. The stress waves travel through the material and, when they reach the surface, cause it to vibrate. AE signals are acquired by measuring the minute surface displacements with sensitive transducers. From the acquired AE signal it is possible to detect delamination, matrix cracking, 5, 710 debonding, bre cracking and bre pull-outs. Acoustic Emission signals can be roughly divided into three types: bursts, continuous and mixed. 119 Bursts are transient signals generated by the formation of damage, e.g. ber breaking and delamination. Continuous AE signals are generated when multiple transients overlap so that they cannot be distinguished and the envelope of the signal amplitudes becomes constant. Continuous AE can be generated by electrical noise and rubbing. The mixed type signal contains both bursts and continuous signals and it is the type which is normally encountered in-service.
3.1 Background 25 If a composite is stressed and microstructural changes occur, then an AE is generated. If the stress is removed and then reapplied, then no AE should be generated unless higher stress is applied. This phenomenon is called Kaiser Eect and is valid for most materials. AE, however, can be generated at stress levels below those previously applied. This is known as the Felicity Eect. One reason for this eect is material degradation, i.e. damages can occur and grow at lower stress levels. Other sources which can contribute to the eect are the opening and closing of cracks, the rubbing of delamination surfaces and the rubbing of parts. AE is also generated under loading because of the dierent material properties of the bers and the matrix. 120 AE that comes from other sources, such as machinery and electricity, are considered to be disturbances or noise. AE noise from the environment is generally strong; hence, it has the potential to be a huge problem. However, the noise is commonly located at frequencies that are much lower than the AE generated by a material damage, and therefore it is not as big problem as it could be. 7 3.1.1 Propagation As the stress waves propagate from the AE source they are inuenced by a variety of factors. These factors include propagation velocities, attenuation, reection, refraction, discontinuities and the geometry of the material. The propagation velocity of an elastic stress wave depends on the wave type, material properties and frequency. Stress waves can be split into two classes depending on where in the material they travel: body waves and surface waves. The body waves travel inside the material as either P-waves or S-waves. P-waves are compressional waves and S-waves are shear waves. P-waves generally propagate at higher velocities than S-waves. 121 Two special types of shear waves can occur: SH-waves and SV-waves. These two types of waves are polarized so that the wave propagations are respectively on a horizontal and a vertical plane. The surface waves, as their name indicates, travel along the surface of the material, hence they are in essence plane waves. Their propagation velocities are typically smaller than the velocity of S-waves 122 and since they only spread out in two directions their amplitude attenuation is less than body waves. Rayleigh and Love waves are two special surface waves. Rayleigh waves are vertically polar-
Page 1 and 2: Acoustic Emission Monitoring of CFR
Page 3 and 4: To my family.
Page 5 and 6: Abstract The condition monitoring o
Page 7 and 8: Ágrip (in Icelandic) Líftími vé
Page 9 and 10: Preface This dissertation has been
Page 11 and 12: Acknowledgements There are many peo
Page 13 and 14: Contents Abstract Ágrip (in Icelan
Page 15 and 16: "If all diculties were known at the
Page 17 and 18: 1.2 Contributions 3 thermore, the A
Page 19 and 20: 1.3 Outline 5 description of how AE
Page 21 and 22: "Prediction is very dicult, especia
Page 23 and 24: 2.2 Defects and Damage Mechanisms 9
Page 31 and 32: 2.3 Fatigue 17 Figure 2.4 Schematic
Page 33 and 34: 2.3 Fatigue 19 Van Paepegem and Deg
Page 35 and 36: 2.4 Discussion and Summary 21 how t
Page 37: "Enjoy the Silence." Depeche Mode 3
Page 41 and 42: 3.1 Background 27 3.1.2 Measurement
Page 43 and 44: 3.1 Background 29 that of velocity
Page 45 and 46: 3.1 Background 31 Hit-based Analysi
Page 47 and 48: 3.1 Background 33 magnied dierently
Page 49 and 50: 3.1 Background 35 3.1.4 AE Analysis
Page 51 and 52: 3.1 Background 37 300 kHz. Iwamoto
Page 53 and 54: 3.2 AE Hit Determination 39 whereby
Page 55 and 56: 3.2 AE Hit Determination 41 signal,
Page 57 and 58: 3.2 AE Hit Determination 43 1 2 3 4
Page 59 and 60: 3.2 AE Hit Determination 45 1 2 3 4
Page 61 and 62: 3.2 AE Hit Determination 47 Figure
Page 63 and 64: 3.2 AE Hit Determination 49 (a)The
Page 65 and 66: 3.3 Features 51 is a function of th
Page 67 and 68: 3.3 Features 53 the data compressio
Page 69 and 70: 3.3 Features 55 An intuitive rhythm
Page 71 and 72: 3.4 AE Source Tracking 57 H P , is
Page 73 and 74: 3.4 AE Source Tracking 59 The AE si
Page 75 and 76: 3.4 AE Source Tracking 61 dierent s
Page 77 and 78: 3.5 Summary 63 quantifying and visu
Page 79 and 80: "Door meten tot weten" [Through mea
Page 81 and 82: 4.1 The Prosthetic Foot 67 (a)The m
Page 83 and 84: 4.2 Test Setup & Procedure 69 the t
Page 85 and 86: 4.3 Data Acquisition 71 Table 4.1 T
Page 87 and 88: 4.3 Data Acquisition 73 (a)The shim
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L-GAGE 4.3 Data Acquisition 75 Usin
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"Information is a source of learnin
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5.1 Fatigue Behaviour of the Vari-e
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5.1 Fatigue Behaviour of the Vari-e
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5.2 Load-Displacement 83 (a)An area
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5.2 Load-Displacement 85 (a)The she
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5.2 Load-Displacement 87 Figure 5.7
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5.2 Load-Displacement 89 QTest/10LP
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5.2 Load-Displacement 91 Figure 5.1
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5.3 Impending Failure Warning 93 Ba
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5.3 Impending Failure Warning 95 (a
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5.3 Impending Failure Warning 97 (a
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5.3 Impending Failure Warning 99 bi
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5.3 Impending Failure Warning 101 T
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5.3 Impending Failure Warning 103 F
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5.3 Impending Failure Warning 105 5
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5.4 AE-Based Failure Criterion 107
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5.5 Case Study 113 5.5 Case Study I
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5.5 Case Study 115 5.5.2 AE Feature
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5.5 Case Study 117 (a)The evolution
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5.5 Case Study 119 (a)The evolution
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5.5 Case Study 121 5.5.3 AE Feature
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5.5 Case Study 123 the woven layers
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5.5 Case Study 125 vertical asympto
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5.5 Case Study 127 lengths are used
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5.5 Case Study 129 ISI/Peak Amplitu
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5.5 Case Study 131 Figure 5.32 A co
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"All great truths are simple in nal
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6.1 Contributions 135 combining the
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6.2 Directions for Future Research
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Bibliography [1] J. Degrieck and W.
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BIBLIOGRAPHY 141 [18] R. Unnthorsso
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BIBLIOGRAPHY 143 [41] T. D. P. S. C
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BIBLIOGRAPHY 145 [61] C. N. Della a
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BIBLIOGRAPHY 147 of Reinforced Plas
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BIBLIOGRAPHY 149 [102] J. P. Dunker
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BIBLIOGRAPHY 151 [125] H. C. Kim an
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BIBLIOGRAPHY 153 [147] M. Iwamoto,
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BIBLIOGRAPHY 155 [170] Newport 301
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List of Tables 4.1 The measurement
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List of Figures 2.1 Shows matrix cr
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LIST OF FIGURES 161 4.10 The resolu
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LIST OF FIGURES 163 5.25 The evolut
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List of Algorithms 1 STFT-based det
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