Acoustic Emission Monitoring of CFRP Laminated Composites ...

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30 Chapter 3. Acoustic Emission 3.1.3 AE Features Over the years many research projects have been conducted with the aim of extracting useful information from AE signals. The extracted information is stored in n-dimensional data structures, known as features. A number of techniques can be used for studying AE features. These techniques include trending-, statistical- and classication techniques. Here, the AE feature extraction approaches, the corresponding features, and the rst interpretation steps are described. For this, the approaches are grouped into four types: activity-, hit-based-, frequency-, and waveform analysis. Classication approaches will be discussed later in this chapter. Activity analysis is the process of detecting trends and abrupt changes in AE features over time. Common features used in this analysis include 135, 136 the number of AE hits and the signal's energy. The features are extracted and compared regularly. Additional parameters are sometimes acquired at the same time and used for reference. Activity analysis is mainly represented graphically by plotting the features against time. The activity can be studied by plotting either the absolute feature values or their cumulative sum against time. 135 Two ratios are often used when load is used as a reference parameter, namely the Felicity ratio and the Shelby ratio. The Felicity ratio is the ratio of the lowest load that generates a certain amount of AE activity, during loading, against the highest load in last cycle. 137 The ratio was described by Dr. Timothy Fowler and has been used as an indicator of damage with mixed results. 137 Instead of looking at the loading of a composite the Shelby ratio looks at the unloading. The Shelby ratio is therefore analogous to the Felicity ratio. The Shelby ratio is dened as the ratio of the lowest load that generates a certain amount of AE activity, during unloading, against the previous maximum load. 120 If the level of AE activity isn't reached then the ratio is dened as 1.0. The Shelby ratio, described by Dr. Marvin A. Hamstad, is useful for detecting damage that generates AE from friction. This ratio hasn't been used much in the literature, but if an AE is generated from friction it is believed that it can provide important information about the condition of a composite. 89
3.1 Background 31 Hit-based Analysis is the analysis of AE hits using a certain set of features, or waveform parameters. AE hits are the portions of a measured AE waveform which satisfy a given detection criterion. The purpose of the detection criterion is to determine the presence of AE and discriminate it from background noise, or continuous AE. Because AE is mainly transient stress waves, the term AE hit is usually understood as an isolated and separated transient from the acquired waveform. There are many detection techniques which can be used for determining AE hits. A common technique used in real-time commercial parameterbased AE systems is to compare the AE signal against a certain threshold. The threshold is typically set on the positive side of the signal and held in a xed position, but it can also be oating. A hit is detected by comparing the AE signal against the threshold, and if the signal surpasses the threshold a hit is detected. Three parameters are commonly used with this detection technique: a hit denition time (HDT), the hit lockout time (HLT), and the peak denition time (PDT). A timing parameter is triggered every time the threshold is crossed. If the time which has elapsed since the timing parameter was last triggered equals the HDT parameter, then the hit has ended. The HLT parameter species the amount of time which must pass after a hit has been detected and before a new hit can be detected. The PDT parameter species the time allowed, after a hit has been detected, to determine the peak value. Conventional AE hit-based features include amplitude, duration, energy, number of peaks above certain threshold (ring-down count) and rise time. 136 Figure 3.3 illustrates how these and other common hit based features are related. New features can be designed by processing existing ones. The processing includes, but is not limited to: adding, subtracting, multiplying, and dividing two or more features. New features can also be made by ltering and extracting statistical information from the features in the set, e.g. variance, skewness and kurtosis. Trend analysis of hit-based features is widely used. Sometimes trending is carried out by plotting the cumulative sum of the feature. In some applications trending can be sucient, e.g. when monitoring of the AE signal's power alone is of interest. In many cases, however, further analysis is required. In some cases more information about a feature can be gleaned by
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: 3.1 Background 29 that of velocity
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
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
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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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