Acoustic Emission Monitoring of CFRP Laminated Composites ...

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16 Chapter 2. Carbon Fibre-Reinforced Polymer Composites material aws will grow with repeated loading and cause premature failure. Furthermore, cyclic fatigue is also inuenced by factors such as the loading frequency and the load ratio. 7982 The evolution of composites during cyclic fatigue is discussed separately in the next section. 2.3 Fatigue In order to gain an understanding of the physical processes that occur during the fatigue degradation of FRP composites, the research has been limited to certain laminate sequences, geometries, and specic loading con- 1, 83, 84 ditions. The fatigue of composites has mainly been studied using standardized test coupons. These coupons are sometimes made with notched edges or with a hole. The following loading conditions have been predominant in the published literature: tension-tension, 8, 9, 83, 8589 90, 91 tension-compression, compression-compression 92 and bending. 2, 93 Gamstedt and Sjögren stated that tension-compression loading is common in service applications and is more destructive than tension-tension loading for composites containing transverse plies. 91 They concluded that this was due to more rapid debonding growth around the transverse bres in tension-compression loading, which caused earlier initiation of transverse cracks. Many models have been proposed for the task of predicting fatigue life and failure of composites. Some have been based on laminate analysis, 9496 some on nite element analysis, 65, 97100 and others on measurable quantities such as stiness. Composites are widely used in applications where stiness is critical. Changes in stiness can be monitored by using displacement and loading measurements, both of which are readily obtainable while a composite is in-service. The stiness degradation of composites during cyclic loading can generally be divided into three stages: initial (I), gradual (II), and a nal (III) stage. 2, 87, 101, 102 This division is demonstrated schematically in Fig. 2.4. Dyer and Isaac also observed two stages in glass/polyester composites and concluded that the number of stages and the amount of damage within each stage depends on the laminate structure, matrix material, and bre type. 83 Early during stage I damage starts to accumulate at an increasing rate.
2.3 Fatigue 17 Figure 2.4 Schematic progression of fatigue damage in composites. According to Daniel 103 the duration of this stage, in an idealized case, is from 1 cycle to a few hundred cycles. The damage is mainly microcracks 104 (transverse matrix cracks 2 ) and debonding. Debonding is incipient to micro-cracks transverse cracking. 91 Other damage mechanisms such as the breaking of low strength bres, small edge delamination and 8, 9, 104 bre pullout can take place simultaneously. The reduction in stiness, at this stage, is mainly due to the development of transverse matrix cracks. 82 A drop of the order of 10% in Young's modulus, or stiness, 105, 106 is often observed. However, an increase in stiness has also been reported. 107109 Sung and Jianping observed that stiness increases in E- glass/vinyl ester composites. 107 They attributed the stiness increase to the viscoelasticity of the matrix and the alignment of bres to the loading direction. Kahn et al. 108 tested woven Carbon Fabric with polyester resin. They explained that the stiness was usually observed in the beginning of cyclic testing. Ruggles-Wrenn et al., 109 using a urea/urethane matrix, attributed the stiness increase to the straightening of bres. The fact that some investigations only report two stages (for an example, see the review by Dyer and Isaac 83 ), can be attributed to a short initial stage, i.e. if it is only few cycles, it may not be considered a stage. For a given load level, there is a limit on how many micro cracks can exist in the composite. Hence, the number of micro cracks will eventually
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 29: 2.2 Defects and Damage Mechanisms 1
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 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
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4.1 The Prosthetic Foot 67 (a)The m
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4.2 Test Setup & Procedure 69 the t
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4.3 Data Acquisition 71 Table 4.1 T
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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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