Acoustic Emission Monitoring of CFRP Laminated Composites ...

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34 Chapter 3. Acoustic Emission is related to the Heisenberg's Uncertainty Principle, which can be applied to time-frequency localization of signals. Basically what it says is that we cannot know both the exact localization of time and frequency. In the recent years there has been increasing interest in using methods based on the Discrete Wavelet Transform for analyzing AE signals. The Discrete Wavelet Transform (DWT) was designed to improve the STFT, within the boundaries set by the Heisenberg's Uncertainty Principle. Both these transforms are linear. Instead of using a xed window the DWT uses scaled windows. 138 The DWT has a good time and poor frequency localization at high frequencies, and good frequency and poor time localization at low frequencies. The transform has at least two serious drawbacks. First, it is not time-invariant, which means that small shifts in the signal can generate completely dierent coecients. Second, due to its ability to detect sharp changes, the wavelet transform is sensitive to noise. Waveform analysis of AE signals is the study of fully digitized AE waveforms their shapes and modes of propagation. Full waveform acquisition and analysis of high frequency AE signals places a high demand on the computer memory, the data storage, and on the computing power. At the same time it oers nearly endless possibilities for signal manipulation and feature extraction. Furthermore, the digitized waveform can be reused to study the eects of dierent threshold settings and signal processing on hit-based features. By analyzing the full AE waveform more information about the damage mechanisms can be obtained than when using only hit-based features. Additional information about the damage mechanisms, and the material, can in some cases be obtained by studying Lamb waves, i.e. Lamb wave velocity can be related to the modulus of the material. 139 The two propagation modes of Lamb waves are the lowest order symmetric (S 0 ) and the antisymmetric (A 0 ) Lamb modes. The two modes are also known as exural and extensional modes. These two modes have dierent properties, e.g. the exural mode components are sensitive to dispersion but extensional mode components are not. The analysis technique in which 124, 140 the properties of these modes are studied is known as modal AE.
3.1 Background 35 3.1.4 AE Analysis A number of investigations into the behaviour of AE signals have revealed that the evolution of the cumulative AE count, during cyclic testing of composites, can be divided into three stages: initial (I), gradual (II), and 8, 9, 86, 141 a nal (III) stage. These three stages are analogous to the three stages in the stiness degradation, depicted schematically in Fig. 2.4. Stage I is characterized by high AE activity which is associated with the formation of new damage mechanisms 86 arbitrarily located throughout the material. At the end of the stage the activity decreases and becomes nearly 87, 99 constant. The AE activity during stage II is mainly due to friction 86 and a constant rate of delamination. 8 Occasional new damage will result in high AE activity bursts which will change the AE activity rate temporarily. 8 During stage III the AE activity increases rapidly until failure. During the cyclic testing of composites AE is generated by both actual damage progression and cumulative damage, i.e. friction. AE from cumulative damage can be generated many times but AE from the formation of new damage occurs only once; hence, the majority of the emissions are related to cumulative damage. The values of AE features from cumulated 8, 87 damage usually fall in the same range as the ones from damage growth and it can be very dicult to distinguish between the two types. Because important AE events can get buried in the AE signals generated by friction 6, 86 and the rubbing of crack surfaces attempts have been made to lter out 8, 87, 89 AE events from these damage mechanisms. Several approaches have been used; for example, thresholding the AE features, coupling the AE to the load, 8 limiting the analysis to a part of the loading cycle, 87, 89 and conducting frequency analysis. 86 Thresholding AE features can be dicult both due to attenuation and the fact that the same type of damage can produce AE with dierent amplitudes. 133 As suggested and investigated by Dzenis, 87 it is intuitive to assume that more AE from friction ought to be generated during unloading and more AE from new damage during the loading of a composite. Hence, by splitting the fatigue cycle into loading and unloading Dzenis determined that fatigue damage in unnotched laminate develops mainly during loading. Awerbuch and Ghaari concluded that frictional AE should not be eliminated as it may provide important information about the condition of a composite. 89 They argued that damage detection could be made easier by using frictional AE, since damage
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 and 38: "Enjoy the Silence." Depeche Mode 3
Page 39 and 40: 3.1 Background 25 If a composite is
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: 3.1 Background 33 magnied dierently
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
Page 89 and 90: L-GAGE 4.3 Data Acquisition 75 Usin
Page 91 and 92: "Information is a source of learnin
Page 93 and 94: 5.1 Fatigue Behaviour of the Vari-e
Page 95 and 96: 5.1 Fatigue Behaviour of the Vari-e
Page 97 and 98: 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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