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x
TABLE OF CONTENTS
CHAPTER NO. TITLE PAGE NO.
ABSTRACT v
LIST OF TABLES xvii
LIST OF FIGURES xix
LIST OF SYMBOLS AND ABBREVIATIONS xxxiii
1 INTRODUCTION 1
1.1 COMPOSITE MATERIALS 1
1.2 MATERIAL SELECTION 2
1.2.1 Fibers 2
1.2.1.1 Glass fiber 3
1.2.1.2 Basalt fiber 4
1.2.1.3 Characteristics of basalt fiber 5
1.3 Matrix Materials 5
1.3.1 Polyester 6
1.3.2 Epoxy 7
1.3.2.1 Types of adhesives 7
1.3.2.2 Adhesive layer 9
1.3.2.3 Adhesive preparation 10
1.4 OVERVIEW OF COMPOSITE JOINTS 10
1.4.1 Bonded Joint 13
1.4.2 Riveted Joint 14
1.4.3 Hybrid Joint 16
1.5 PLANNING FOR THE BONDING PROCESS 16
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CHAPTER NO. TITLE PAGE NO.
1.6 INTRODUCTION TO ACOUSTIC EMISSION (AE)
TECHNIQUE 17
1.6.1 Concept of Acoustic Wave Propagation 18
1.6.2 Advantages of AE Monitoring 20
1.6.3 Challenges and Complexities of AE on
Composite Materials 21
1.6.4 Equipment Used in AE Monitoring 22
1.6.4.1 Sensors 22
1.6.4.2 Couplants and holders 24
1.6.4.3 Pre-amplifiers 24
1.6.4.4 Data acquisition system 24
1.7 AE PARAMETERS AND THEIR
DESCRIPTIONS 25
1.8 FAILURE MECHANISMS IN COMPOSITE TO
COMPOSITE ADHESIVE-BONDED JOINTS
AND MECHANICALLY FASTENED JOINTS 27
1.9 IDENTIFICATION OF FAILURE MODES
USING AE 30
1.9.1 Parametric Analysis 30
1.9.2 Fast Fourier Transform Analysis 32
1.9.3 Wavelet Transform analysis 34
1.10 INTRODUCTION TO FINITE ELEMENT
ANALYSIS 36
1.10.1 Structural Static Analysis 37
1.10.2 Performing of Static Analysis 37
1.11 NEED FOR THE STUDY AND OBJECTIVE
OF THE RESEARCH WORK 38
1. 12 SUMMARY 40
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CHAPTER NO. TITLE PAGE NO.
1.13 FLOW DIAGRAM FOR ORIGANISATION
OFTHE RESEARCH WORK 41
1.14 THESIS ORGANIZATION 44
2 LITERATURE SURVEY 46
2.1 INTRODUCTION 46
2.1.1 Review of Mechanical Characterization of
Lap Joints in Composite Materials 47
2.1.2 Failure Modes on Composite Materials
Using AE Monitoring 51
2.1.3 Failure Mode Identification Using AE Fast
Fourier Transform (FFT) Analysis 59
2.1.4 Failure Mode Identification Using AE
Wavelet Transform (WT) Analysis 61
2.2 SUMMARY OF LITERATURE REVIEW 63
3 FABRICATION AND TESTING METHODS 66
3.1 FABRICATION OF COMPOSITE LAMINATE 66
3.1.1 Hand Lay-Up Method 66
3.1.2 Compression Molding 67
3.2 SPECIMEN PREPARATION FOR MECHANICAL
CHARACTERIZATION STUDIES 67
3.2.1 Preparation of Tensile and Flexural
Specimens from Composite Laminates 67
3.2.2 Tension and Flexural Testing Set-up 69
3.2.3 Impact Test Setup 69
3.2.4 Double Cantilever Beam (DCB) Test Setup 70
3.3 SPECIMEN PREPARATION FOR SINGLE
LAP JOINTS AND DOUBLE LAP JOINTS 71
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3.3.1 Bonded Joints 71
3.3.2 Riveted Joints 74
3.3.3 Hybrid Joints 75
3.4 AE MONITORING 76
3.4.1 A Generic AE Setup 76
3.4.2 Pencil Lead Break Test 77
3.5 INPUT PARAMETERS TO AE WIN SOFTWARE 79
3.5.1 Wave Velocity 79
3.5.2 Hit Definition Time (HDT) 81
3.5.3 Hit Lockout Time (HLT) 81
3.5.4 Peak Definition Time (PDT) 81
3.5.5 Sample Rate 82
3.5.6 Pre-Trigger 82
3.5.7 Hit Length 82
3.6 SUMMARY OF EXPERIMENTAL
PROCEDURE 82
4 RESULTS AND DISCUSSION 84
4.1 MECHANICAL CHARACTERIZATION OF
COMPOSITE LAMINATE 84
4.1.1 Introduction 84
4.1.2 Tensile Test 84
4.1.3 Flexural Test at Different Temperature 86
4.1.4 Impact Test Results at Different
Temperature 88
4.1.5 Brinell Hardness Test 90
4.1.6. Identification of Volume Fraction for BFRP 90
4.1.7 Summary of Mechanical Characterization 92
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CHAPTER NO. TITLE PAGE NO.
4.2 MECHANICAL CHARACTERIZATION OF
SINGLE LAP JOINTS OF GFRP LAMINATE 93
4.2.1 Introduction 93
4.2.2 Pure resin, Single layer and Double layer of
Adhesive-Bonded Lap Joints 94
4.2.3 Bonded, Riveted and Hybrid Single
Lap Joints 96
4.3 IDENTIFICATION OF FAILURE MODES FOR
SINGLE LAP JOINTS USING AE DATA 100
4.3.1 Introduction 100
4.3.2 AE Parametric Analysis and Fast Fourier
Transform (FFT) Data 101
4.3.2.1 Pure resin bonded joints 101
4.3.2.2 Identification of failure modes
under Mode I loading 104
4.3.2.3
layer thickness 106
4.3.2.3.1 Single layer specimen 106
4.3.2.3.2 Double layer specimen 111
4.3.2.4 Single Lap Bonded joints 116
4.3.2.5 Single Lap Riveted joints 120
4.3.2.6 Single Lap Hybrid joints 124
4.4 MECHANICAL CHARACTERIZATION OF
DOUBLE LAP JOINTS OF GFRP LAMINATE 128
4.4.1 Bonded, Riveted and Hybrid Double
Lap Joints 128
4.5 IDENTIFICATION OF FAILURE MODES FOR
DOUBLE LAP JOINTS USING AE DATA 132
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4.5.1 Double Lap Bonded joints 132
4.5.2 Double Lap Riveted joints 136
4.5.3 Double Lap Hybrid joints 138
4.6 MECHANICAL CHARACTERIZATION OF
SINGLE LAP JOINTS OF BFRP LAMINATE 143
4.6.1 Introduction 143
4.6.2 Bonded, Riveted and Hybrid Single
Lap Joints 144
4.7 IDENTIFICATION OF FAILURE MODES
ON SINGLE LAP JOINTS USING AE DATA 146
4.7.1 AE Parametric Analysis and Fast Fourier
Transform Data 146
4.7.1.1 Single Lap Bonded joints 147
4.7.1.2 Single Lap Riveted joints 151
4.7.1.3 Single Lap Hybrid joints 155
4.8 MECHANICAL CHARACTERIZATION OF
DOUBLE LAP JOINTS OF BFRP LAMINATE 159
4.8.1 Bonded, Riveted and Hybrid Joints 159
4.9 IDENTIFICATION OF FAILURE MODES FOR
DOUBLE LAP JOINTS USING AE DATA 162
4.9.1 Double Lap Bonded joints 162
4.9.2 Double Lap Riveted joints 165
4.9.3 Double Lap Hybrid joints 166
4.10 WAVELET TRANSFORM ANALYSIS OF
AE DATA 170
4.10.1 Introduction 170
4.10.2 Continuous and Discrete Wavelet
Transform of AE Signal 170
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CHAPTER NO. TITLE PAGE NO.
4.11 SUMMARY OF FAILURE MODES USING
AE DATA 178
5 FINITE ELEMENT ANALYSIS OF
COMPOSITE JOINTS 182
5.1 INTRODUCTION 182
5.2 FINITE ELEMENT MODELS OF SINGLE
AND DOUBLE LAP JOINTS 183
5.2.1 Bonded, Riveted and Hybrid Joints 183
5.2.2 Boundary Conditions and Loads 185
5.3 FEA RESULTS 186
5.3.1 Bonded Joints 186
5.3.2 Riveted Joints 187
5.3.3 Hybrid joints 189
5.4 COMPARISON BETWEEN BONDED,
RIVETED AND HYBRID JOINTS 190
6 CONCLUSIONS 192
6.1 SCOPE FOR FUTURE WORK 195
APPENDIX: MAT LAB CODE 196
REFERENCES 203
LIST OF PUBLICATIONS 216
xvii
LIST OF TABLES TABLE NO.
TITLE PAGE NO.
1.1 Description of sensors 23
2.1 Characterisation of failure modes based on amplitude parameter 59
2.2 Characterisation of failure modes using frequency analysis 61
3.1 Wave velocity for composite lap joints 80
4.1 Standard deviation values of ultimate tensile strength and flexural strength of GFRP and BFRP test specimens 85
4.2 Flexural test results at different temperatures 88
4.3 Impact test results at different temperatures 89
4.4 Brinell hardness number for basalt composite laminates 90
4.5 Volume fraction of basalt laminates 92
4.6 Standard deviation values of ultimate load and displacement for single lap joints in GFRP and BFRP laminates
98
4.7 Standard deviation values of ultimate load and displacement for double lap joints in GFRP and BFRP laminates 131
4.8 Standard deviation values of ultimate load and displacement for single lap bonded, riveted and hybrid joints in GFRP and BFRP laminates
146
4.9 Standard deviation values of ultimate load and displacement values for double lap joints in GFRP and BFRP laminates 160
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TABLE NO.
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4.10 Frequency ranges for each failure modes using AE monitoring 181
4.11 Frequency ranges for each failure modes using AE monitoring 181
5.1 Properties of epoxy resin 182
5.2 Properties of glass/epoxy composite and basalt/epoxy composite
183
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LIST OF FIGURES
FIGURE NO.
TITLE PAGE NO.
1.1 Comparative characteristics of metals and composites 02
1.2 Conversion of natural volcanic rock to basalt fiber 04
1.3 Schematic diagram of single lap joint specimen with dimensions 12
1.4 Schematic diagram of double lap joint specimen with dimensions 12
1.5 Adhesive-bonded single lap joints and double lap joints used in structural applications 14
1.6 Riveted single lap joints and double lap joints used in structural applications 15
1.7 Bonding process involved in the design 17
1.8 (a) Acoustic wave propagation (b) Attenuation Graph 20
1.9 Calibration certificate for Wide band sensor 23
1.10 Pre-Amplifier 24
1.11 Acoustic emission monitoring process 25
1.12 Typical AE waveform and its parameters 25
1.13 Different types of failure in composite to composite adhesive joints 29
1.14 Possible failure modes in mechanical fastened joints between FRP composite adherends 29
xx
FIGURE NO.
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1.15 Cumulative counts versus time for adhesive-bonded joints 32
1.16 Methodology of research work 43
3.1 Hand lay-up manufacturing process 66
3.2 Composite laminate: (a) Glass fiber reinforced plastics (b) Basalt fiber reinforced plastics 67
3.3 ASTM D 638 standard tensile specimen-GFRP 68
3.4 ASTM D3039 standard tensile specimen-BFRP 68
3.5 ASTM D790 standards flexural test specimen (a) GFRP (b) BFRP 68
3.6 (a) Tensile test set up (b) Flexural test set up 69
3.7 Impact test setup with specimen 70
3.8 (a), (b) DCB with piano hinge (c) DCB under Mode I loading 71
3.9 Photographs of single lap bonded joints: (a) GFRP (b) BFRP 72
3.10 Photographs of double lap bonded joints: (a) GFRP (b) BFRP 72
3.11 Photographs of single lap joints and double lap pure resin bonded joints 72
3.12 Photographs of single lap bonded joints: (a) single layer specimen (b) two layer specimen
73
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FIGURE NO.
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3.13 Photograph of double lap bonded joints: (a) single layer specimen (b) two layer specimen 73
3.14 Photographs of the lap joints: (a) Front view (b) Top view 73
3.15 (a) INSTRON 3367 Universal Testing Machine with AE setup (b) Specimen with AE sensor using magnifying lens 74
3.16 Photographs of single lap riveted joints: (a) GFRP (b) BFRP 74
3.17 Photographs of double lap riveted joints: (a) GFRP (b) BFRP 75
3.18 Photographs of single lap hybrid joints: (a) GFRP (b) BFRP 75
3.19 Photographs of double lap hybrid joints: (a) GFRP (b) BFRP 76
3.20 Schematic of acoustic emission (AE) 76
3.21 Hsu-Nielsen source (NDT.net, 2007) 79
3.22 Wave velocity using pencil lead break test 80
3.23 Standard deviation values of wave velocity for different single lap joints 81
4.1 Stress-Strain behavior of tensile test specimens: (a) GFRP (ASTM D 638 and D 3039) (b)BFRP(ASTM 3039) 85
4.2 Standard deviation values of ultimate
tensile strength for composite laminates 86
4.3 Tensile test specimens: (a) GFRP (ASTM D 638) (b) BFRP (ASTM 3039) 86
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FIGURE NO.
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4.4 Fractured surface of flexural test specimens: (a) GFRP (b) BFRP 87
4.5 Standard deviation values of flexural strength for composite laminates 87
4.6 Fractured specimens of BFRP after impact testing 89
4.7 Temperature (00C) vs impact Energy for GFRP and BFRP laminate 89
4.8 Burnout test for basalt laminate 91
4.9 Photographs of UTM fixture for flat specimens at MIT campus, Anna University 94
4.10 Displacement vs AE count rate: (a) pure resin bonded joints (b) single layer adhesive-bonded joints 95
4.11 Cumulative counts versus time for adhesive-bonded joints 95
4.12 Standard deviation values of displacment for single lap joints of GFRP and BFRP laminates 98
4.13 Load vs displacement diagram of bonded, riveted and hybrid single lap joints 99
4.14 AE count rate and cumulative counts vs displacement for single lap joints 99
4.15 Visual images of failure modes observed in pure resin specimens: (a) single lap joint (b) double lap joint 102
4.16 Peak frequency and cumulative counts versus time for (a) pure resin single bonded lap joint (b) pure resin double bonded lap joint
102
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4.17 Amplitude and duration versus time for (a) Pure resin single bonded lap joint (b) Pure resin double bonded lap joint
103
4.18 FFT frequency domain of adhesive failure signal in pure resin specimens: (a) single lap bonded Joint (b) double lap bonded joint 103
4.19 SEM image representation of the adhesive failure modes in pure resin specimens
104
4.20 Different stages of delamination propagation under Mode I loading
105
4.21 SEM image representation of (a) matrix cracking (b) fiber-matrix debonding (c) delamination fracture
105
4.22 AE parametric analysis of (a) peak frequency and cumulative counts versus time (b) Amplitude and duration vs time for Mode I loading
106
4.23 Typical AE signal for associated failure mode on GFRP under Mode I loading: (a) time domain (b) frequency domain
106
4.24 Visual images of failure modes observed in Single layer specimen: (a) photographic image of the specimen subjected to tensile testing (b) single lap joint (c) double lap joint 108
4.25 Peak frequency and cumulative counts versus time for (a) single layer single bonded lap joint (b) single layer double bonded lap joint
108
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4.26 Amplitude and duration versus time for (a) single layer single bonded lap joint (b) single layer double bonded lap joint
109
4.27 Location versus peak frequency for (a) single layer single bonded lap joint (b) single layer double bonded lap joint
109
4.28 Typical AE signal for individual failure mode in single layer specimen: (a) adhesive (b) light fiber tear failure (c) fiber tear failure
111
4.29 SEM image representation of failure modes: (a) adhesive failure and fiber tear failure (b) light fiber tear failure
111
4.30 Failure modes observed in double layer specimen: (a) photograph of the specimen subjected to tensile testing (b) single lap joint (c) double lap joint
112
4.31 Peak frequency and cumulative counts versus time for (a) double layer single bonded lap joint (b) double layer double bonded lap joint
113
4.32 Amplitude and duration versus time for (a) double layer single bonded lap joint (b) double layer double bonded lap joint
113
4.33 Location versus peak frequency for (a) double layer single bonded lap joint (b) double layer double bonded lap joint 114
4.34 Typical AE signal for individual failure mode on double layer specimen: (a) adhesive (b) light fiber tear failure (c) fiber tear failure
115
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4.35 SEM image representation of fiber tear failure mode 116
4.36 Peak frequency and cumulative counts versus time for single lap bonded joint 117
4.37 Amplitude and duration versus time for single lap joint bonded specimen
118
4.38 Failure modes observed in single lap bonded joints 118
4.39 Location versus peak frequency for single lap bonded joint
119
4.40 Typical time domain and frequency domain AE signal for individual failure mode in single lap bonded joint: (a) adhesive failure (b) light fiber tear failure (c) fiber tear failure 120
4.41 SEM image representation of the typical failure modes after tensile test: (a) failure region (b) adhesive failure (c) light fiber tear failure (d) fiber tear failure
120
4.42 Peak frequency and cumulative counts versus time for single lap riveted joint 122
4.43 Amplitude and duration versus time for single lap riveted joint 122
4.44 Location versus peak frequency for single lap riveted joint 123
4.45 Typical AE signal for shear failure mode in single lap riveted joint: (a) time domain (b) frequency domain
123
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4.46 Visual and SEM image repesentation of shear failure mode on single lap riveted joint 123
4.47 Peak frequency and cumulative counts versus time for single lap hybrid joint 125
4.48 3 D histogram of amplitude and duration versus location for single lap hybrid joint 125
4.49 Location versus peak frequency for single lap hybrid joint 126
4.50 Typical time domain and Frequency domain AE signal for individual failure mode in single lap hybrid joint: (a) adhesive failure (b) shear failure (c) fiber tear failure
127
4.51 Visual and SEM image repesentation of failure mode in single lap hybrid joint 127
4.52 Location versus number of events for single lap: (a) bonded joint (b) riveted joint (c) hybrid joint
127
4.53 Load vs displacement diagram of bonded, riveted and hybrid double lap joints-GFRP 130
4.54 Standard deviation values of displacment for double lap joints in GFRP and BFRP laminates 130
4.55 AE count rate and cumulative counts versus displacement for double lap joints 131
4.56 Peak frequency and cumulative counts versus time for double lap bonded joint 133
4.57 Amplitude and duration versus time for double lap bonded joint 133
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4.58 Location versus peak frequency for double lap bonded joint 134
4.59 Typical AE time domain and FFT signal of individual peak frequency values for failure modes on double lap bonded joint: (a) adhesive failure (b) light fiber tear failure (c) fiber tear failure/ delamination 135
4.60 Visual and SEM image representation of the typical failure modes after tensile test: (a) overlap failure surface (b) adhesive failure and light Fiber tear failure (c) fiber tear failure 135
4.61 Peak frequency and cumulative counts versus time for double lap riveted joint 136
4.62 Amplitude and duration versus time for double lap riveted joint 137
4.63 Location versus peak frequency for double lap riveted joint 137
4.64 Typical AE time domain and FFT signal of individual peak frequency value for shear failure mode in double lap riveted joint 138
4.65 (a), (b) Visual and (c) SEM image representation of shear failure mode on double lap riveted joint 138
4.66 Peak frequency and cumulative counts versus time for double lap hybrid joint 140
4.67 Amplitude and duration versus time for double lap hybrid joint
140
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4.68 Location versus peak frequency for double lap hybrid joint 141
4.69 Typical AE time domain and FFT signal of individual peak frequency values for failure modes on double lap hybrid joint: (a) adhesive failure (b) shear failure (c) light fiber tear failure (d) fiber tear failure mode
143
4.70 Visual and SEM image representation of the typical failure modes: (a) photograph of hybrid joint specimen during tensile test with AE sensor (b) failure surface (c) fiber tear failure
143
4.71 Load vs displacement graph (b) AE count rate and cumulative counts vs displacement for lap joints 145
4.72 Peak frequency and cumulative counts versus time for single lap bonded joint 147
4.73 Amplitude and duration versus time for single lap bonded joint 148
4.74 Location versus peak frequency for single lap bonded joint 150
4.75 Typical AE time domain and FFT signal for individual failure mode on single lap bonded joint: (a) adhesive failure (b) delamination /light fiber tear failure 150
4.76 Visual failure modes observed in single lap bonded joints: (a) photographs of specimen during tensile loading (b) overlap failure region
151
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4.77 SEM image representation of the typical failure modes on bonded joints in basalt laminate: (a) adhesive failure (b) light fiber tear failure
151
4.78 Peak frequency and cumulative counts versus time for single lap riveted joint 152
4.79 Amplitude and duration versus time for single lap riveted joint 153
4.80 Location versus peak frequency for single lap riveted joint 154
4.81 Typical AE time domain and FFT signal for individual failure mode in single lap riveted joint (a) adhesive failure (b) shear failure 154
4.82 (a) Visual and (b), (c) SEM image representation of shear failure mode on single lap riveted joint in Basalt laminate 155
4.83 Peak frequency and cumulative counts versus time for single lap hybrid joint 156
4.84 Amplitude and duration versus time for single lap hybrid joint 156
4.85 Location versus peak frequency for single lap hybrid joint 157
4.86 Typical AE time domain and FFT signal for individual failure mode in single lap hybrid joint: (a) adhesive Failure (b) shear failure (c) light fiber tear failure 158
4.87 Visual failure modes observed on single lap hybrid joints in basalt laminate: (a) photographic images of specimen during tensile loading (b) overlap failure region
158
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4.88 SEM image representation of typical failure modes observed in single lap hybrid joint: (a) BFRP failure surface (b) adhesive failure (c) light fiber tear failure 159
4.89 Standard deviation values of ultimate load of bonded, riveted and hybrid double lap joints in GFRP and BFRP laminates 161
4.90 AE count rate and cumulative counts versus Displacement for lap joints 161
4.91 Peak frequency and cumulative counts versus time for double lap bonded joint 163
4.92 Amplitude and duration versus time for double lap bonded joint 163
4.93 Location versus peak frequency for double lap bonded joint 164
4.94 Typical AE time domain and FFT signal for individual failure mode on double lap bonded joint (a) adhesive failure (b) light fiber tear failure/ delamination 165
4.95 Visual and SEM image representation of the typical failure modes: (a) photograph of specimen with AE setup during test (b) adhesive failure (b) mixed failure mode 165
4.96 Peak frequency and cumulative counts versus time for lap riveted joint 166
4.97 Typical AE time domain and FFT signal for shear failure of the fastener on lap riveted joint
166
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4.98 Peak frequency and cumulative counts versus time for double lap hybrid joint 167
4.99 Amplitude and duration versus time for double lap hybrid joint 168
4.100 Location versus peak frequency for double lap hybrid joint
168
4.101 Typical AE time domain and FFT signal of individual peak frequency values for failure modes on double lap hybrid joint: (a) adhesive failure (b) shear failure (c) light fiber tear failure (d) fiber tear failure 169
4.102 Visual and SEM image repesentation of failure mode observed in double lap hybrid joint 170
4.103 A signal representative of adhesive (Matrix) failure: Time-frequency representation provided by CWT, four levels of decomposition (D1-D4), FFT modulus and its level of decomposition 174
4.104 B signal representative of light fiber tear failure: Time-frequency representation provided by CWT, four levels of decomposition (D1-D4), FFT modulus and its level of decomposition 175
4.105 C signal representative of fiber tear failure: Time-frequency representation provided by CWT, four levels of decomposition (D1-D4), FFT modulus and its level of decomposition
176
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4.106 D signal representative of mixed (Shear and light fiber tear) failure: Time-frequency representation provided by CWT, four levels of decomposition (D1-D4), FFT modulus and its level of decomposition
177
5.1 Meshed models of bonded joint: (a) single lap joints (b) double lap joints 184
5.2 Meshed model: (a) riveted joint (b) hybrid joint 185
5.3 Analysis result of single lap bonded joint: (a) GFRP (b) BFRP 186
5.4 Analysis result of double lap bonded joint: (a) GFRP (b) BFRP 187
5.5 Analysis result of single lap riveted joint: (a) GFRP (b) BFRP 188
5.6 Analysis result of double lap riveted joint: (a) GFRP (b) BFRP 188
5.7 Analysis result of single lap hybrid joint: (a) GFRP (b) BFRP 190
5.8 Analysis result of double lap hybrid joint: (a) GFRP (b) BFRP
190
xxxiii
LIST OF SYMBOLS AND ABBREVIATIONS
AE - Acoustic Emission
ASTM - American Society for Testing and Materials
BVID - Barely Visible Impact Damage
BFRP - Basalt Fiber Reinforced Plastics
GFRP - Glass Fiber Reinforced Plastics
FMI - Failure Mode Identification
FFT - Fast Fourier Transform
FRP - Fiber Reinforced Plastics
FEA - Finite Element Analysis
HDT - Hit Definition Time
HLT - Hit Lockout Time
NDE - Non Destructive Evaluation
NDT - Non Destructive Testing
PDT - Peak Definition Time
PAC - Physical Acoustics Corporation
WT - Wavelet Transform
WD - wide band
CWT - Continous Wavlet Transform
DWT - Discrete Wavlet Transform
- Density of Fiber
- Density of Matrix
c - Density of Composite
- Volume Fraction of Fiber
- Volume Fraction of Matrix