Application of smart classification techniques to non destructive testing of composites

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TECHNIQUES TO NON-DESTRUCTIVE TESTING

OF COMPOSITES

BY: M.Z. ISKANDARANI

(B.Eng (Hon.), M.Sc, AMIEE (GB), MIEEE (USA))

Submitted for the degree of

DOCTOR OF PHILOSOPHY in Engineering

at

University of Warwick

Advanced Technology Centre

Department of Engineering

Coventry, UK

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DECLARATION

I declare that all the work appeared in this thesis was carried out by myself

unless otherwise acknowledged in the text.

r

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ACKNOWLEDGEMENTS

I would like to express my gratitude and thanks to my excellent supervisor, Dr. Gordon F.

Smith for the Total confidence that he placed in me, the full academical and financial support

and for his constant encouragement and help.

I would also like to thank Dr. Peter T. Davies (Director of the Advanced Technology Centre)

for supporting this project and for valuable comments and disscussions.

My appreciation extends to all the staff of the Advanced technology Centre and Warwick

Manufacturing Group for their help and contribution, and to the following companies for their

co-operation, collaboration and technical support:

(1) SOFRATEST U.K (SPEEDTRONIC DIVISION).

(2) AGEMA INFRARED SYSTEMS.

(3) NCS (NEURAL COMPUTER SCIENCES).

(4) ROVER GROUP.

(5) OMETRON.

(6) COMFORD DESIGN LIMITED.

My personal appreciation goes to Dr.Mike Arrington (SOFRATEST U.K) and Alan Grant

(AGEMA INFRARED SYSTEMS) for their personal contribution to this work.

However, my everlasting gratitude and thanks to Ian Pearson of the Advanced Technology

Centre for many hours that he spent with me correcting and editing this document and for his

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SUMMARY

Composites manufactured for applications in the automotive industry were non-destructively

tested to determine damage using the following techniques:

(1) Low frequency tapping.

(2) High frequency (C-Scan).

(3) Visual imaging.

(4) Low and high temperature pulse video thermography.

Various levels of impact energy were applied to the following types of composites

(I) RIM: Reaction injection moulded.

(II) Woven Glass.

(III) GMT: Glass mat thermoplastic.

Some interesting results were obtained which could be explained through analytical and

numerical modelling. These results were analyzed through developments of the following

algorithms:

(a) A novel approach to damage detection using wavelength variation and sequence grouping

software.

(b) Correlation of the various NDT techniques through one mathematical equation and

software.

(c) The introduction of the uniformity factor concept and software to account for variations

among samples quality in relation to experimental results.

(d) The development of smart classification system together with standard neural network

algorithms for prediction and classification.

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LIST OF FIGURES

Figure 2.1 An intelligent image capturing, processing and recognition system [214], 3

Figure 3.1 Hybridization of composites [5], 5

Figure 3.2 Concepts of material design [5]. 6

Figure 3.3 Process employed in the design of composites [6]. 7

Figure 4.1 Energy Band Model. 21

Figure 4.2 Geometrical and Electrical Transformations. 25

Figure 4.3 Field function of interactive energy states. 30

Figure 6.1 Low frequency tapping system [46]. 37

Figure 6.2 Frequency response of a tap tested CFRP tubes [46]. 38

Figure 6.3 Measurement and modelling of composite mechanical impedance. 43

Figure 6.4 Response of defective and non-defective structures to tappings [46]. 44

Figure 6.5 Response system block diagram. 46

Figure 6.6 Shape function for 3mm GMT (Perpendicular). 54

Figure 6.7 Shape function for 3mm woven glass. 54

Figure 6.8 An ultrasonic A-scan system [50], 62

Figure 6.9 B-scan ultrasonic system [50]. 64

Figure 6.10 C-scan ultrasonic system [50]. 66

Figure 6.11 Acoustic Emission system and Defect frequency response spectrum [60-61]

72

Figure 6.12 Stress Waves interpretation through ring and event counting [64]. 73

Figure 6.13 Effect of load on stress wave amplitudes in composites [64], 74

Figure 6.14 AE response to different types of composite damage [64]. 77

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Figure 7.1 Luminosity function used in visual NDT [41], 81

Figure 7.2 IR emissions and possible wavelengths recordings, reproduced from KODAK

film company, U.K. 83

F%ure 7.3 Types and structures of Liquid Crystals [106]. 85

Figure 7.4 Concept of Thermal Colour OR and its response.Reproduced from the work at

SIBER HEGNER Ltd, U.K. 87 '

Figure 7.5 Visual processing of Liquid Crystals profile [103], 89

Figure 7.6 Passive and Active Thermography [79-82]. 96

Figure 7.7 Generation of holograms using Holography [124], 102

Figure 7.8 Generation of holograms using Shearography [124], 103

Figure 7.9 X-rays backscattering concept [137], 106

Figure 7.10 The CAT system [137]. 107

Figure 7.11 Measurement o f specimen reflection coefficient using Microwaves [132]. 109

Figure 7.12 Detection of defects using Microwaves in Transmission mode [132], 110

Figure 7.13 A look-into microwave detecting system [132]. 111

Figure 8.1 Classification and Discrimination concept. 120

Figure 8.2 Discrimination using LC classification. 120

Figure 8.3 Classification of different categories using linear surfaces. 123

Figure 8.4 Use o f NNC as a discriminating method. 124

Figure 8.5 Misclassification as a drawback in NNC. 125

Figure 8.6 Illustration of the structural technique. 128

Figure 8.7 Concept of information processing using Neural interconnections. 134

Figure 8.8 Representation of neuron activity. 135

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Figure 8.10 Unsupervised learning mechanism. 137

Figure 8.11 Supervised learning mechanism. 138

Figure 8.12 Smart classification system block diagram. 139

Figure 8.13 Data file source matrix. 140

Figure 8.14 Matrix-Column classification concept. 143

Figure 8.15 Uniformity Factor concept. 144

Figure 8.16 The overall Smart Classification block diagram. 146

Figure 8.17 Back Propagation method. 148

Figure 9.1 Drop Weight Impacting System. 155

Figure 9.2 Design and shape of used impact weights. 156

Figure 9.3 An Electronic Tapping System. 158

Figure 9.4 The scanning arrangement adopted in the experiments. 159

Figure 9.5 The Ultrasonic C-scan system used in the experiments. 160

Figure 9.6 Dual Visualand IR Imaging setup. 161

Figure 9.7 High Temperature PVT setup. 161

Figure 9.8 Low Temperature PVT set up. 162

Figure 10.1 Low Frequency tapping response of 3mm and 5mm W.Glass. Top: Experimental.

Bottom: Fitted. 166

Figure 10.2 Low Frequency tapping response of 3mm GMT. Top: Experimental. Bottom:

Fitted. 167

Figure 10.3 Low Frequency taping response of 2mm RIM. Top: Reference factor. Bottom:

Average factor. 168

Figure 10.4 Low Frequency tapping response of 5mm RIM. Top: Experimental. Bottom:

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Figure 10.5 Effect of impact energy on C-Scan response of W.Glass. Top: 3mm. Bottom:

5mm. 170

Figure 10.6 C-Scan images of 5mm W.Glass impacted at 14.3J using Search Through

Technique. 171

Technique. 172

Technique 173

Technique. 174

Figure 10.10 C-Scan images of 5mm W.Glass with a hole drilled through it using Search

Through Technique. 175

Figure 10.11 C-Scan of A W.Bone. Left: Inner parts. Right: Outer parts. 176

Figure 10.12 First stage classification of 5mm W.Glass impacted at 4.76J and tested using

Low Temperature Visual Camera. 176

Figure 10.13 First stage classification of 5mm W.Glass impacted at 7 .14J and tested using

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Figure 10.18

Figure 10.19

Figure 10.20

Figure 10.21

Figure 10.22

Figure 10.23

Figure 10.24

Figure 10.25

Figure 10.26

Figure 10.27

Figure 10.28

Figure 10.29

Figure 10.30

Thw O.I.W.V. search through technique applied to 5mm RIM impacted at 28.6J

and 42J. 180

The O.I.W.V. search through technique applied to 5mm RIM impacted at 14.3J

and 30.8J. 181

and 28.6J with load angle. 182

The O.I.W.V. search through technique applied to 5mm RIM impacted at 47.61

and 47.6J with reflective paint. 183

with reflective paint and part extraction. 184

Visual first stage classification of 5mm RIM. (Reference Sample). 185

Visual first stage classification of 5mm RIM impacted at 14.3J and 30.8J.

185

Visual first stage classification of 5mm RIM impacted at 28.6J and 28.61 with

load angle. 186

Visual first stage classification of 5mm RIM impacted at 28.6J and 42J.

186

Visual First stage classification of 5mm RIM impacted at 47.6J and 47.6J

187

Visual first stage classification of 5mm RIM impacted at 55.6J with reflective

paint and part extraction. 187

Visual second stage classification of 5mm RIM. (Reference sample). 188

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Figure 10.31

Figure 10.32

Figure 10.33

Figure 10.34

Figure 10.35

Figure 10.36

Figure 10.37

Figure 10.38

Figure 10.39

Figure 10.40

Figure 10.41

Figure 10.42

Figure 10.43

Figure 10.44

188

Visual second stage classification of 5mm RIM impacted at 28.6J and 28.6J

with load angle. 189

Visual second stage classification of 5mm RIM impacted at 28.6J and 42J.

189

Visual first stage classification of 5-layers, 5mm RIM. 190

Visual First stage classification of 10-layers, 5mm RIM. 190

Visual first stage classification o f 5mm RIM with plastic inclusions. 191

Visual first stage search through classification of 2mm RIM impacted at

4.76J. 191

Visual first stage classification of 2mm RIM impacted at 7.14J. 192

Visual first stage classification of 2mm RIM impacted at 14.3J. 192

Images of the I.I.W.V. technique applied to 5mm RIM impacted at 28.6J and 421.

(Light source 1). 193

Images of the I.I.W.V. technique applied to 5mm RIM impacted at 14.3J and

30.8J (shown). (Light source 1). 194

28.6J with load angle (shown). (Light source 1). 195

28.6J with load angle (both shown). (Light source 2). 196

47.6J with reflective paint. (Light source 1). 197

Images of the I.I.W.V technique applied to 5mm RIM impacted at 55.6J with

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Figure 10.45

Figure 10.46

Figure 10.47

Figure 10.48

Figure 10.49

Figure 10.50

Figure 10.51

Figure 10.52

Figure 10.53

Figure 11.1

Figure 11.2

Figure 11.3

Figure 11.4

Images of the I.I.W.V. technique applied to 5mm RIM with reflective paint and

part extraction. (Cycles 7-12, light source 1). 199

Thermal Visual Imaging applied to detect damage in 5mm RIM impacted at 28.6J

& 42J, 14.3J & 30.8J, 28.6J & 28.6J with load angle. 200

First stage classification of 5mm RIM tested using High Temperature Visual

Imaging. (Reference sample). 201

First stage classification of 5mm RIM impacted at 28.6J and 42J and tested using

High Temperature Visual Imaging. 201

First stage classification of 5mm RIM impacted at 14.3J and 30.8J and tested

using High Temperature Visual Imaging. 202

First stage classification of 5mm RIM impacted at 28.6J and 28.6J with load

angle and tested using High Temperature Visual Imaging. 202

The I.I.W.V. technique applied to 5mm RIM (with reflective paint) and 5mm

W.Glass impacted at 55.6J and 28.6J respectively and tested using High

Temperature PVT. 203

The I.I.W.V. technique applied to 3mm Perpendicular and Parallel GMT

impacted at 14.3J and tested using High Temperature PVT. 204

The I.I.W.V. technique applied to W.Bone tested using High temperature

PVT. 205

Tapping damage detection system. 219

Comparison between responses of 3mm GMT and W.Glass to tapping. 221

Curve fit for figure 11.2. 221

Comparison between Tap tested 3mm composites. 224

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Figure 11.6 The complete tap testing and classification system. 226

Figure 11.7 Comparison between the response of 3mm & 5mm 4.76J damaged W.Glass to

C-scan. 229

Figure 11.10 Comparison between the effect of a 28.6J impact and hole on the integrity of

5mm W.Glass component. 230

Figure 11.11 Damage detection in a W.Bone using C-scan. 230

Figure 11.12 the human visual spectrum. 232

Figure 11.13 The Tristimulus functions. 234

Figure 11.14 Cone relative sensitivity spectrum. 235

Figure 11.15 Cone spectral sensitivity curve. 235

Figure 11.16 The Optical Map. 236

Figure 11.17 Relationship between Damage level and C-scan image intensities as a function

of impact energy in 5mm W.Glass. 238

Figure 11.18 Relationship between Damage level and Impact energy in 5mm W.Glass using

C-scan. 239

Figure 11.19 Effect of impact energy on the size if defect in 3mm W.Glass. 239

Figure 11.20 5mm W.Glass C-scan output signals, the component suffered a 28.6J impact and

a drilled hole. 241

Figure 11.21 A search Through Technique applied to the C-scanned 5mm W.Glass with 28.6J

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Figure 11.23

Figure 11.24

Figure 11.25

Figure 11.26

Figure 11.27

Figure 11.28

Figure 1 1.29

Figure 11.30

Figure 1 1.31

Figure 1 1.32

Figure I 1.33

Figure 11.34

Figure 11.35

Figure 11.22 Detection of level and direction of damage in a C-scan image of 3mm W.Glass

using the Uniformity Factor algorithm. 242

Using the Uniformity Factor algorithm to establish defect level and orientation

in 5mm W.Glass through C-scans. 243

Visual detection of damage (4.76J) in W.Glass using Matrix- Column search

through algorithm. 246

Effect of impact energy on the visual response of 3mm W.Glass . 248

Effect of impact energy on the visual response of 5mm W.Glass. 248

Correlation between C-scan & Visual detection of damage(4.76J) in 5mm

W.Glass. 249

Correlation of C-scan & Visual detection of damage (4.76J) in 3mm W.Glass.

250

Correlation between C-scan & Visual detection of damage (7.14J) in 5mm

W.Glass. 250

Correlation of C-scan & Visual detection of damage (7.14J) in 3mm

W.Glass. 251

Correlation of C-scan & visual detection of damage (14.3J) in 5mm W.Glass.

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Figure 11.36 Correlation of C-scan & Visual detection of damage (14.3J)in 3mm W.Glass.252

Figure 11.37 The Classifying Optical Map (COM). 255

Figure 11.38 The Damage Classifying Function (DCF). 257

Figure 11.39 Visual damage detection in 5mm RIM. 258

Figure 11.40 Effect o f impact energy on the visual response of 5mm RIM. 259

Figure 11.41 Effect of impact energy on the visual response of 2mm RIM. 260

Figure 11.42 Comparison between the responses of damaged (4.76J), 5mm thick RIM &

W.Glass. 260

W.Glass. 261

Figure 11.45 Effect of impact damage on the uniformity of a 5mm RIM component (ROW

SPREAD), refer to figure 11.26. 262

Figure 11.46 Curve fit to figure 11.45. 262

Figure 11.47 Effect of impact damage on the uniformity of 5mm RIM component (COLUMN

SPREAD) , refer to figure 11.26. 262

Figure 11.48 Curve fit for figure 11.47. 263

Figure 11.49 Effect of using different optical sources and impact energy (28.6J) on detection

of damage level in 5mm RIM component. 264

Figure 11.50 Effect of application of same impact energy (figure 11.49) at an angle on level

of damage induced in 5mm RIM component using different optical sources.

264

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Figure 11.52

Figure 11.53

Figure 11.54

Figure 11.55

Figure 11.56

Figure 11.57

Figure 11.58

Figure 11.59

Figure 11.60

Figure 11.61

Figure 11.62

on the detection of damage in 5mm RIM (refer to figure 11.50). 265

Effect of applying 42J impact energy and use of different optical sources on

damage detection in 5mm RIM. 265

Effect of painting (reflective coating) a 47.6J impact damaged component on

limiting the depth o f detection in 5mm RIM (refer to figure 11.50). 266

Effect of increasing the impact energy (55.6J) with part of the component

surface extracted due to impact on depth of detection in 5mm reflective painted

RIM. 266

Effect of impact energy on damage spread. 267

The T-MAP. The range of values corresponds to pixel intensities. 269

Application of I.I.W.V. to damage detection (28.6J & 42J) in 5mm RIM tested

using high temperature source. (Cycles 1-4). 270

Application of I.I.W.V.to damage detection (28.6J & 42J) in 5mm RIM using

high temperature source. (Cycles 5-8). 271

Application of I.I.W.V. to damage detection (28.6J & 42J) in 5mm RIM using

Application of I.I.W.V. to damage detection (28.6J & 28.6J applied at an angle

to the normal) in 5mm RIM using high temperature source.

(Cycles 1-4). 274

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Figure 11.63

Figure 11.64

Figure 11.65

Figure 11.66

Figure 11.67

Figure 11.68

Figure 11.69

Figure 11.70

Figure 11.71

Figure 11.72

Figure I 1.73

(Cycles 9-12). 276

Comparison between the effect of 28.6J and 42J impact energies on the level of

damage in 5mm RIM. (Refer to figures 11.57-11.60). 278

Comparison between the effect of 28.6J and 28.6J applied at an angle to the

normal on the level of damage in 5mm RIM. (Refer to figures 11.61-11.63).278

Classification of damage in 5mm RIM impacted at 55.6J and tested using High

Temperature PVT. (Refer to figure 10.51). 280

Using the uniformity factor to detect damage in 5mm RIM impacted at 55.6J by

evaluating rate of energy propagation in a High Temperature PVT test.

280

Modelling of energy pulse propagation in 5mm RIM impacted at 55.6J with part

of the component removed. 281

Classification of damage in 5mm W.Glass impacted at 28.6J using High

Temperature PVT. (Refer to figure 10.51). 283

Application of the uniformity factor to detect damage in 5mm W.Glass impacted

at 28.6J by evaluating the rate of energy pulse propagation in High

Temperature PVT test. 284

Classification of damage in perpendicular 3mm GMT impacted at 14.3J and

tested using High Temperature PVT. (Refer to figure 10.52). 285

Application of the uniformity factor to damage detection in perpendicular 3mm

GMT impacted at 14.3J using H.T.P.V.T. test. 285

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Figure 11.74

Figure 11.75

Figure 11.76

Figure 11.77

Figure 11.78

Figure 11.79

Figure 11.80

Figure 11.81

Figure 1 1.82

Figure 11.83

Figure 1 1.84

Figure 1 1.85

using High Temperature PVT. (Refer to figure 10.52). 286

Application of the uniformity factor to damage detection in 3mm parallel GMT

impacted at 14.3J using H.T.P.V.T. test. 286

Classification of damage in W.Bone using High Temperature PVT. (Refer to

figure 10.78). 287

Application of the uniformity factor to damage detection in W.Bone by

evaluating the rate of energy pulse propagation in High Temperature PVT

test. 287

Low Temperature PVT applied to an undamaged 5mm RIM component with two

different thermal ranges. 290

Low Temperature PVT applied to 5mm RIM impacted at 42J and tested at two

thermal ranges after coating with absorbing paint. 291

Low Temperature PVT applied to 5mm RIM impacted at both 28.6J and 42J

with the sample black painted. Notice the larger size of the cold spot in

proportion to higher impact energy (42J). 292

Low Temperature PVT applied to 5mm RIM impacted at 28.6J with the load

applied at angle.Two thermal ranges. 294

Low Temperature PVT applied to 5mm RIM impacted at 28.6J. 295

Low Temperature PVT applied to 5mm RIM impacted at 55.6J and painted with

reflective paint. 296

Low temperature applied to 5mm W.Glass impacted at 28.6J. 297

Neural Networks prediction of the effect of impact energy on the normalized

response of 5mm RIM. (Complete cycle). 298

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response of 5mm RIM. (first cycle). 298

Figure 11.86 Neural Network prediction of effect of impact energy on the normalized response

of 5mm RIM.(Second cycle ).

299

of 5mm RIM. (Third cycle). 299

of 8mm RIM. (Complete cycle). 300

of 8mm RIM.(First cycle). 300

of 11mm RIM. (Complete cycle). 301

Figure 11.91 Neural Network prediction on effect of impact energy on the normalized

response of 11mm RIM. (First cycle). 301

Figure 1 1.92 Relationship between threshold impact damage energy and component

thickness. 303

Figure 13.1 An advanced impact system. 307

Figure 13.2 An accurate electronic tapping device. 310

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LIST OF TABLES

Table 6.1: Ultrasonic Interaction. 61

Table 7.1: Comparison of NDT techniques. 116

Table 10.1: Image Decision Data (RIM). 206

Table 10.2: Image Decision Data (W.Glass). 207

Table 10.3: Neural Network Prediction Data. 208

Table 10.4: Neural Network Training Data (W.Glass). 209

Table 10.5: Neural Network Training Data (RIM). 210

Table 10.6: Neural Network Training Data (GMT). 213

Table 11.1: Comparison of composites. 222

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NOMENCLATURE

E, 5, E,h Er

Z.Ç

r.

e

w

g dL f K„ Kc d U.p.q E h P

Applied impact energy.

Defect energy, i= l....n.

Balancing energy factors accounts for frictional forces.

Impulse functions.

Threshold Energy.

Response energy.

Complex geometrical plans.

Polar coordinates.

Complex potential plan.

Complex potential variables.

Distance between energy states.

Length of an energy state.

Field function.

Effective contact stiffness.

Stiffness over good area.

Stiffness over defective area.

Defect diameter.

Constants depend on effective boundary conditions.

Flexural modulus of a material and modulus of elasticity.

Depth of a defect.

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X

y.. y2 :

a , (3 :

^G eneral •

Kref

Vef* Lvg’ f

I

K, X

c

M

P(x) :

4 a 4s 4 z r T I V T N f Ea

Input signal-low frequency model.

Output signals-low frequency model.

Gain functions.

General normalized response factor-low frequency model.

Normalized time response of low frequency tapping model.

Normalized average time response o f low frequency tapping.

Multiplication function-low frequency model.

Reference, average and defect time responses-low frequency.

Differential parameters of tolerance-low frequency model.

Acoustic velocity.

Poissons ratio.

Pressure amplitude.

Absorption coefficient.

Scattering coefficient.

Attenuation coefficient.

Acoustic impedance.

Reflection coefficient

Transmission coefficient.

Acoustic energy.

Analogue voltage.

Decay time.

Number of ring counts.

Transducer resonant frequency.

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a b TD TE L

Q

L’ T

0

e„ P r v hp K I(v) g(X) X,

Wj

n

h(X) Tc

proportionality constant.

Distribution characteristic constant.

Thermal diffusivity.

Thermal effusivity.

Material thickness.

Heat flux.

Radiant emittance.

Atmosphere transmission coefficient.

Object emissivity.

Atmosphere radiant emittance.

Object radiant emittance.

Absorbed radiation.

Reflected radiation.

Transmitted radiation.

Frequency of radiation.

Plank’s constant.

Boltzmans constant.

Intensity of radiation.

decision discriminating function-LC.

Input vector.

Weight vector.

Bias value.

Classification function-NNC.

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S, Th q,

s

W„, C, fT hT g(x) h, e, o t,

r

0 th, Slh L, AP N, Nh D,

Grouped classification sequences.

Amplitude factor.

Normalizing factor.

Hybrid column matrix.

Quantizing vector.

Data source.

Weight or response factor.

Grouped subsequences.

Uniformity factor-column.

Uniformity factor-row.

Activation classification function.

Hidden layer(s).

Vector of errors.

Output layer vector(s).

Target activation of output layer.

Learning rate.

Momentum factor.

Threshold function.

Threshold level sequence.

Optical source.

Appearance function.

Number of defective layers-low temperature.

Number of defective layers-high temperature.

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CHAPTER 1 INTRODUCTION.

C o m p o site s a re in c re a sin g ly b ein g u sed a s s tr u c tu r a l m a te ria ls. O ne o f th e

m a jo r a d v a n ta g e s o f com posite m a te ria ls is th e ir a b ility to be ta ilo re d to m e e t

c e r ta in ap p lic a tio n s. T h u s, a p a rtic u la r type o f fibre a n d m a trix ca n be u s e d to

p ro d u c e co m p o n en ts a c q u irin g d ifferen t lay -u p s, th ic k n e sse s a n d v olum e

fra c tio n s . This im p lies t h a t a defect w hich is c o n sid e re d to be critic a l in one

c o m p o n e n t m ig h t be to ta lly acceptable in a n o th e r. H ence, s e ttin g q u a lity

c o n tro l a n d in sp e c tio n s ta n d a r d s is a v ery com plex t a s k , and is n o t m ad e a n y

s im p le r o r econom ical due to th e d iffe re n t N D T te c h n iq u e s, each of w h ich is

c a p a b le o f d e te c tin g c e rta in defects. T o g eth er w ith em ploying s ta n d a r d b u t

v a r ia b le in te rp re ta tio n m eth o d s, efforts h av e been d ire c te d to w ard b a la n c in g

th e c o s t a n d co m plexity o f a n in sp ectio n te c h n iq u e a g a in s t th e q u a lity of

in fo rm a tio n o b ta in e d an d th e flexibility of th e te c h n iq u e in d e te c tin g v a rio u s

ty p e s o f defects.

In th e a u to m o tiv e in d u s try th e re is a n eed to m a n u fa c tu re w ith la rg e r

p ro d u c tio n ra te b u t tig h te r q u a lity control w hich re q u ire s inspection te c h n iq u e s

w ith h i g h e r speed a n d b e tte r re so lu tio n w ith m ore in tellig en ce a n d accu racy

c o m p a re d to th e c u r r e n t co n v en tio n al ones. T h is w ork w ill be looking a t p a s t,

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CHAPTER 2 OBJECTIVES.

As a so lu tio n to th e p roblem s a sso c ia te d w ith N D T a n d its ap p lic a tio n in th e

a u to m o tiv e in d u s tr y , th e re is a n eed to e sta b lish a n in te llig e n t a n a ly sis sy stem

coupled to a p o rta b le detectio n e n v iro n m e n t th a t un ifies all d iffe re n t ty p e s of

in p u ts a n d a u to m a tic a lly co n sid ers v a rio u s fib re /m a trix c o m b in atio n s w h ich

le a d s to e lim in a tio n o f any a m b ig u ity re g a rd in g defect se v e rity a n d its effect on

th e co m p o n en t u n d e r co n sid eratio n . T h is su g g e sts th e u se o f a n im ag in g

sy stem .

W e propose t h a t conversion o f v a rio u s d a ta s tru c tu re s a n d im ag es in

co n ju n ctio n w ith smart classification (a specially devised fin g e r p rin tin g

a lg o rith m b a se d o n p a tte r n reco g n itio n tech n iq u es) a n d neural networks

(p a ra lle l, h ig h ly in te rc o n n e c te d sy ste m s m ade o f m a n y in d iv isib le p ro cessin g

u n its in te ra c tin g w ith ea c h o th e r v ia a d a p tiv e w eig h ts) to g e th e r w ith

wavelength variations (in co rp o ratio n o f d iffe re n t e n erg y so u rces to u ncover

th e p resen ce o f d a m a g e ) to be a p ro m isin g sy stem for n o n -d e stru c tiv e te stin g .

F ig u re 1.1 show s s u c h a sy stem . T h is a u to m a te d im ag e c a p tu rin g sy stem

p o sse ss th e follow ing m ain c h a ra c te ris tic s :

(1) T h e a b ility to e x tra c t im p o rta n t in fo rm atio n from a b ac k g ro u n d of

ir r e le v a n t d e tails.

(2) T he c a p a c ity to le a rn from p a s t ex p erien ce a n d ap p ly its

k n o w led g e to new d iffe re n t s itu a tio n s.

[image:32.586.1.542.12.659.2]

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I .it s.'mediale-level processing

Representation

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[image:33.590.17.540.11.653.2]

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CHAPTER 3 COMPOSITES

C om posite m a te r ia ls h a v e developed ra p id ly over th e la s t few y e a r s a n d it is

a ssu m e d now t h a t th e y a r e u se d in a lm o s t every in d u s tria l sector. T h e science

a n d technology o f com posite m a te ria ls a re b a sed on a design concept w h ich is

p rin c ip a lly d iffe re n t from th a t of co n v en tio n al m a te ria ls [1],[2],[3]. In th e

process o f d e s ig n in g su ch m a te ria ls , w e n eed to e sta b lish p erfo rm an ce objectives

for m a te ria ls se lectio n , s tr u c tu r a l co n fig u ratio n a n d fab ricatio n ro u te .

W ith fibre c o m p o site s, th e re la tio n s h ip b etw een m ic ro -s tru c tu re an d

m acroscopic a r r a n g e m e n t a n d th e ir d ep en d en ce on p rocessing te c h n iq u e s are

m u ch stro n g e r t h a n co n v en tio n al m a te ria ls . T h u s, com posites tech n o lo g y offers

th e p o te n tia l to d e sig n m a te ria ls for p a rtic u la r u se s a t d ifferen t scale levels.

F irstly , a t th e m icroscopic level, th e in te rn a l s tru c tu re of a co m p o n en t c a n be

controlled th r o u g h processing. Som e ex am p les

are:-(1) M oulding o f sh o rt-fib re com posites, w h ere fibre o rie n ta tio n , le n g th and

d is trib u tio n m ay be con tro lled to yield th e d esired local p ro p e rtie s .

(2) F ila m e n t w in d in g o f co n tin u o u s fibres.

(3) H y b rid iz a tio n of fib res a n d te x tile s tr u c tu r a l p a tte r n s b ased on w eav in g ,

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F ig u re 3.1 H y b rid izatio n of com posites [5],

In all th e above cases, th e re q u ire d local stiffn ess, s tre n g th , to u g h n e ss and

o th e r pre-specified p ro p e rtie s c a n be o b ta in e d by co n tro llin g th e fib re type,

o rie n ta tio n a n d volum e fra c tio n th ro u g h o u t th e s tru c tu ra l com ponent.

Secondly, th e e x te rn a l g e o m etrical sh ap e o f a s tr u c tu r a l com ponent ca n also be

d esig n ed , for exam

ple:-(1) A dvances in th e tech n o lo g y of fila m e n t w in d in g en ab le th e a u to m a te d

p ro d u ctio n of com ponents w ith com plex contours.

(2) T he ab ility to fa b ricate th re e -d im e n sio n a l fibre u sin g ad v a n c e d tex tile

technology.

[image:35.584.18.523.13.651.2]

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co m p o n en ts o f n e t sh a p e is en h a n c e d even more.

The in te g ra te d a n d system ap p ro ach , ra n g in g from m ic ro stru c tu re to com ponent

n et, offers alm o st u n lim ite d o p p o rtu n ity in com posites p ro c e ssin g a n d

m a n u fa c tu rin g a s discussed by v ario u s rese a rc h ers [4-8],

As a re s u lt, th e concept of ta ilo rin g m a te ria ls a t a fu n d a m e n ta l level to m e e t

specific desig n c rite rio n h a s b een a topic of in te re s t to m any E n g in e e rs a n d only

re c e n tly it h as b een fully in c o rp o ra te d in th e overall design ( F ig u re 3.2).

F ibre rein fo rced com posites provide a lte rn a tiv e s often only lim ite d by th e

d e sig n e rs im a g in a tio n . T hey re p re s e n t excellent exam ples of t h e p rin cip les of

m a te ria l design w h e re th e p erfo rm an ce of th e w hole is g re a te r t h a n th e su m of

its p a r ts [9] & [10]. F ib res, lig h t-w eig h t, very stro n g and s t i f f b u t easily

d am ag ed w ith lim ited e n g in e e rin g application, and the m atrix , co m p a ra tiv e ly

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T o g eth er, how ever, th e y provide a w ide ra n g e o f m a te ria ls a n d ch an ce for

o p tim iz a tio n w hich is n o t m ech an ical p ro p e rtie s exclusive b u t e x te n d s to

th e rm a l, acoustic, elec tro m a g n etic , creep an d fa tig u e ch ara c teristic s.

F ro m th e above, it is e x p e c te d th a t th e n u m b e r o f m a te ria l co n fig u ratio n s will

c o n s ta n tly in c rease th r o u g h th e u se of h y b rid s a n d o th e r in n o v ativ e m a te ria l

co n stru c tiv e form s [11]. H en ce, we m u s t choose th e p a th of u n d e rs ta n d in g th e

p ro p e rtie s of th e basic m a te r ia l w hich w ill re q u ire new te s t a n d in sp ectio n

p ro c e d u re s (F ig u re 3.3).

[image:37.581.24.527.11.658.2]

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CHAPTER 4 DAMAGE IN COMPOSITES

4.1 Introduction.

W h e n e v a lu a tin g th e c o n trib u tio n o f defects to th e failu re pro cess it is

im p o r ta n t to d is tin g u is h b etw e e n tw o classes o f defects:

(a) D efects t h a t act on a m icroscopic level,

(b) C ritical defects in a p ra c tic a l o r m u lti-d im e n sio n a l com ponent.

F o r (a) th e ir role can be re g a rd e d a s s u b s ta n tia l from th e p o in t of view o f

q u a lity control b a se d on e x p e rim e n ts b u t w ith little o r no re lev an ce to th e

s ta n d a r d s s e t fo r p ractical co m p o n en ts. T hese a im to com bine th e fibre a n d

r e s in in to a w ell co n so lid ated p ro d u ct. The fibre a n d re s in m ay s e p a ra te before

m a n u fa c tu re or, m ore u su a lly , th e y m a y a lre a d y be com bined in th e form o f a

p re -p re g m a te ria l.

T h e ty p e s of d e fects in tro d u ced can be classified a s follows:

(1) Manufacturing Defects.

C om posite m a te ria ls can be m a n u fa c tu re d by a n u m b e r o f te c h n iq u e s a s

m en tio n e d in th e p rev io u s c h a p te r. T he m a n u fa c tu rin g te c h n iq u e

selected d e p en d s p a r tly up o n th e size a n d q u a lity o f th e re q u ire d

com posite. D u rin g th e s e m a n u fa c tu rin g p ro cesses, defects c a n be

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o c c u rre n ce s o f e ach type d e p e n d s u p o n th e p a rtic u la r process cycle. A

n u m b e r o f defect ty p es h av e b e e n id en tified

including:-(a) In c o m p le te ly cu red m a trix d u e to in co rrect c u rin g cycle o r fa u lty

m a te ria l.

(b) In c o rre c t fibre volum e fra c tio n d ue to excess or in su ffic ie n t resin .

(c) V o id s d ue to v o latile r e s in co m p o n en ts or im p ro p e r c o n tro l o f a ir

d u r in g cure.

(d) E x iste n c e o f foreign bodies.

(e) F ib r e m isa lig n m e n t. T h is c a u s e s local ch an g es in volum e fra c tio n

b y p re v e n tin g id eal p a c k in g o f fibres.

(f) P ly m isa lig n m e n t. T h is is p ro d u ced a s a consequence of m is ta k e s

m a d e in lay -u p of th e c o m p o n e n t plies.

(g) W a v y fibres. T h ese a re p ro d u c e d by in -p la n e k in k in g of th e fib res

in a ply a n d c a n se rio u sly a ffect la m in a te s tre n g th .

(h) P ly cracking. T h e rm a lly in d u c e d crack s occur w ith c e rta in p ly la y

u p s d ue to d iffe re n tia l c o n tra c tio n o f th e plies a f te r cure.

(i) D e la m in a tio n s. T hese a re p la n a r defects u su a lly a t ply b o u n d a rie s

a n d fairly r a r e d u rin g th e m a n u fa c tu re of th e b asic m a te r ia l b u t

m a y be p roduced by c o n ta m in a tio n d u rin g lay-up or by m a c h in in g .

(j) F ib r e defects. T he p re se n c e o f defects in th e fibres th e m s e lv e s is

o n e o f th e u ltim a te lim itin g fa c to rs in d e te rm in in g s tre n g th , w h e re

so m e tim e s fa u lty fibres c a n b e id en tified as th e s ite s from w h ic h

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fibres supplied, a re alw a y s likely to be co n sid ered a s one o f the

b asic m a te ria l p ro p e rtie s.

(k) B onding defects. D u rin g m a n u fa c tu re co m p o n en ts m ay be bonded

to g e th e r a n d it is p o ssib le for defects to occur in th e b o n d lin e due

to in co rrect cure c o n d itio n s for th e adhesive o r c o n ta m in a tio n of

th e su rfaces to be b o n d ed .

(2) In-Service Defects.

C om posites can be d eg rad ed in service by a n u m b e r o f m e c h a n ism s an d

th o se of p rim e im p o rtan ce w ill depend up o n th e e n v iro n m e n t experienced

a n d th e se n sitiv ity of the p a r tic u la r m a te ria ls u sed . The m e c h an ism s

a n d d e g ra d a tio n include s ta tic overload, im pact, fa tig u e , h y g ro th e rm a l

effects, o v erh eatin g , lig h tn in g strik e a n d creep. H ow ever, a lth o u g h th e

m e c h a n ism s by w hich d efects a re in itia te d and grow a r e v aried , only th e

follow ing ty p es o f defects r e s u lt:

(a) F ra c tu re or b u ckling o f fibres.

(b) F a ilu re of th e in te rfa c e betw een th e fibres a n d m atrix .

(c) C racks.

(d) D elam in atio n s.

(e) B ond failures.

(f) In g re ss of m oisture.

I t is obvious th a t com posites c a n differ from th e id e a l e ith e r d u rin g

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id e a l sh o u ld be co n sid ered as defect is a fu n ctio n o f th e in te n d e d u s e of th e

m a te r ia l an d th e significance o f th e d ev iatio n on th e req u ire d p e rfo rm a n c e. All

d efect ty p es a re k n o w n to a d v e rse ly affect p erfo rm an ce in som e w ay.

F ro m p rev io u s s tu d ie s [15-20], th e following defects a re th o u g h t to b e o f prim e

im p o rta n c e in c u r r e n t m a te ria ls .

(a) Manufacturing Defects.

(i) Voids

(ii) F o re ig n bodies

(iii) In c o rre c t fibre v olum e fractio n

(iv) B o n d in g defects

In p ra c tic e th e m o st im p o rta n t m a n u fa c tu rin g d efect a n d likely to o c c u r is th e

p re se n c e of voids. S om e of th e o th e r defects occur v ery ra re ly o r n o t a t all in

iso latio n . T h u s th e in c o rre c t fib re volum e d u e to insu fficien t re sin w ill u su a lly

be acco m p an ied by voids as w ell as in co rrectly c u re d resin.

(b) In-Service Defects.

(i) D e la m in a tio n

(ii) B on d in g defects

(iii) C rack s

(iv) M o istu re in g re ss

The m a jo r in-service d efect to be found is th e p resen ce of d e la m in a tio n s. T hese

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D eb o n d in g can also be fo u n d b u t as y e t no m e th o d is av ailab le to m e a s u re

a d h e s iv e s tre n g th . I t is n o t g en erally e x p e c te d th a t crack s will n eed to b e

fo rm ed since th e y w ill le a d to d e la m in a tio n g ro w th before a critical sta g e is

re a c h e d .

4.2 Im pact Damage.

S tr u c tu r a l d am ag e of co m p o sites as a r e s u lt o f im p a c t is re g a rd e d as one of th e

m o st c ritic a l asp ects of fu n ctio n in g t h a t r e s tr ic ts w ider ap p licatio n s. I n

co m p o sites, th e po ssib ility o f p lastic d e fo rm a tio n is lim ited w hich can lead to a

s u b s ta n tia l a m o u n t of d a m a g e upon im p act, th e influence of w hich on re s id u a l

p ro p e rtie s is h a rd to p red ict.

D u rin g a n im p act c e rta in fa cto rs affect th e outcom e of su c h a n o p eratio n ,

n am ely ,

(1) M a te ria l P ro p e rtie s (2) B o undary C onditions

(3) F a ilu r e M echanism (4) E n v iro n m e n ta l F acto rs

(5) Im p o se d C o n stra in ts (6) Im pact M echanism

W hen a m a te ria l is im p acted , a stre ss field is re a liz e d on co ntact. A series o f

s tre s s w a v e s is th e n p ro p a g a te d th ro u g h th e m a te r ia l w hich m ay or m ay not

cau se d a m a g e .

D u rin g th e im p act a larg e n u m b e r of d a m a g e m e c h a n ism s m ay occur affecting

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c h a ra c te ris tic s . D am ages in clu d e, s p littin g , debonding, m a trix cra c k in g , fibre

p u ll-o u t, fib re-b reak ag e a n d d e la m in a tio n [21] & [22].

T he e x te n t to w hich a specific f a ilu re m ech an ism occurs d e p e n d s on th e

p ro p e rtie s of th e fibre, m a trix a n d in te r p h a s e a s w ell a s th e g eo m etric form a n d

a r r a n g e m e n t o f th e fibres, hence, efficiency. T h is efficiency in lo a d tra n s fe r

b e tw e e n a fibre a n d its s u rro u n d in g m a trix d ep en d s on th e le n g th o f th e fibre

re la tiv e to its critical len g th . I f t h e le n g th of th e fib re is s h o r te r th a n th e

c ritic a l le n g th , th e n fibre p u ll-o u t a n d m a trix fra c tu re are th e d o m in a tin g

m e c h a n is m s of e n erg y ab so rp tio n a f t e r a n im pact. O n th e o th e r h a n d , w hen th e

fibre le n g th is longer th a n th e c ritic a l one, th e fibres w ill, in som e c a se s, b re a k

a n d in o th e rs be p u lled out. B oth fib re fa ilu re m e c h a n ism s a re in flu e n c e d by

th e fib re location a n d o rie n ta tio n w i t h re sp ect to th e crack.

I t h a s b e e n show n [23] th a t for a fib re perfectly bonded to th e m a trix , th e s h e a r

s tre s s c o n c e n tra tio n p a ra lle l to th e in te rfa c e is a m a x im u m a t th e e n d of th e

fibre. I f th e s h e a r s tre n g th o f th e in te rfa c e or th e s u rro u n d in g m a tr ix m a te ria l

is ex ceed ed , fra c tu re will occur a t o r close to th e in terface.

T he f r a c tu r e w ill s t a r t a t th e fibre e n d an d grow along th e fibre a s th e stre ss

on th e sy s te m in creases. T he m a in processes w hich c a n occur a t th e broken

fib re a

re:-(1) T h e crack in th e fibre p ro p a g a te s into th e s u rro u n d in g m a trix a s a b rittle

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(2) A yield zone is s p re a d alo n g th e fibre as a r e s u lt o f th e m a trix b e in g

yielded.

(3) T h e in te rfa c e or th e m a tr ix im m e d ia te ly a d ja c e n t to th e fibre or th e fib re

im m e d ia te ly a d ja c e n t to th e m a trix failin g in s h e a r allow ing th e

u n lo a d e d fib re to s h r in k b ack in to th e m a trix (s h e a r y ield in g or s h e a r

fa ilu re d o es n o t n e c e ssa rily im ply t h a t th e m a trix is u n a b le to tr a n s f e r

lo a d to th e fibre).

Also th e fric tio n a l forces a t th e in te rfa c e le a d to a n o n -lin e a r b uild-up in s tre s s

in th e fib re from th e b ro k en e n d s. T he re la tiv e im p o rta n c e of th e se p ro cesses

d e p e n d s on th e p ro p e rtie s o f th e fibres a n d th e resin.

T he re la tiv e a m o u n ts of in te rfa c e fa ilu re a n d th e m a g n itu d e of th e frictio n al

forces d e te rm in e th e overall a p p e a ra n c e o f th e fra c tu re su rface. F o r stro n g ly

bonded c a rb o n fibre-epoxy re s in sy stem w ith sm all te n sile a n d s h e a r v a lu e s,

th e re is on ly a s m a ll am o u n t of debonding.

In c o n tra s t, th e fra c tu re su rfa c e s o f g lass fib re-p o ly ester re s in w ith h ig h te n s ile

a n d s h e a r v a lu e s u n d erg o m a ssiv e d eb o n d in g w ith larg e a m o u n ts of fibre p u ll

out.

E x p e rim e n ta lly , i t is found t h a t for fib res em bedded in epoxy m a tric e s, th e

s tre s s re q u ire d to e x tra c t th e fib re does n o t drop to zero a fte r deb o n d in g h a s

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fib re o u t o f th e re s in s h e a th .

In a d d itio n th e re d u c tio n in th e s tr e s s on th e fib re a t debonding re s u lte d in an

in c re a s e in fibre d ia m e te r ow ing to th e poisson ex p an sio n a n d th is le a d s to an

in c re a s e in th e p re s s u re on th e fib re su rfa c e [24]. The re la tiv e co n trib u tio n of

th e in te rfa c e b o n d in g a n d frictio n al forces to th e w o rk re q u ire d to p ull th e fibre

o u t of th e m a tr ix d ep en d s on a la rg e n u m b e r o f m a te ria l p a ra m e te rs w ith

fric tio n a l forces m a k in g a m ajo r c o n trib u tio n for a larg e p u ll-o u t d istance.

B efore m a k in g a n e s tim a te of th e w o rk done in fib re debonding an d fibre pull

o u t, re fe re n ce m u s t be m ad e to th e s tr e s s d is trib u tio n alo n g th e fibre afte r

d eb o n d in g h a s occurred. T h is d e b o n d in g will in itia te a t th e fibre en d s and

s p re a d alo n g th e fibre n o t forbidding th e frictio n al forces from o p e ra tin g and

tr a n s f e r r in g s tr e s s to th e fibre. T h is s tre s s w h ich is re q u ire d to e x tra c t th e

fib re in c re a se s a s th e em bedded le n g th in creases.

I t w a s show n [25] & [26] th a t p u ll o u t is far m o re sig n ifican t th a n debonding

a s a n e n e rg y ab so rb er.H o w ev er, d e b o n d in g m u s t occur before pull out. I f th e

s h e a r s tre s s for d eb o n d in g is larg e t h e n fibre fra c tu re occurs before extensive

d e b o n d in g a n d th e a m o u n t of p u ll o u t, a n d h en ce en erg y absorbed is sm all.

F ib re s a re th e d o m in a n t c o n s titu e n t w h en consid erin g m ost p ro p erty

c h a ra c te ris tic s a n d th e sam e is tru e w h e n a sse ssin g im p act perform ance. For

low velocity im p a c t [27] th e sto re d e n e rg y c a p a b ility o f th e fibre is o f key

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th e ir s tr e s s /s tr a in cu rv es, offer good p e rfo rm a n c e. C om posites m a d e from th ese

m a te r ia ls te n d to fail in a p ro g re ssiv e m a n n e r th ro u g h d e la m in a tio n . C arb o n

fib re sy ste m s, in th e o th e r h a n d , c a n be b rittle a n d fail su d d e n ly a t th e

m a x im u m load.

I n c a lc u la tin g d e la m in a tio n a n d its e x te n t, one w ould co n sid er th e a v a ilab le

s tr a in e n e rg y p e r u n it a re a a t every p o in t of a la m in a te in te rfa c e. T he region

o f h ig h e s t en erg y is id en tified a n d co m p ared to th a t o f d e la m in a tio n

(d e la m in a tio n can be view ed a s a p la n e crack p ro p a g a tin g in th e in terface

b e tw e e n tw o a d ja c e n t plies p a ra lle l to th e ap p lied te n sio n a n d re s u ltin g from

t h e in te r la m in a r stre sse s). I f a n a re a h a s a n excess of en erg y , it is considered

to be d e la m in a te d .

4.3 Impact Damage: A Closer Look.

I m p a c t is a key issu e in th e desig n o f com posite s tr u c tu r e s a n d m a y be th e

lim itin g d e sig n issu e in m a n y cases. I t is th e re fo re e s s e n tia l t h a t a com plete

u n d e r s ta n d in g of th e re sp o n se of co m p o site s tru c tu re s to im p a c t be obtain ed .

T h is co m p rises two s e p a ra te areas:

(1) D a m a g e R esistan ce. W here it is re la te d d irectly to th e im p a c t a s it

m e a s u re s th e d a m a g e in c u rre d b y a m a te ria l or a com posite s tru c tu re

[28],

(2) D a m a g e T olerance. W here it h a s only a se c o n d ary re la tio n s h ip to th e

im p a c t ev e n t since it is th e a m o u n t a n d d is trib u tio n o f th e dam ag e

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th e im p a c t ev en t.

T h e im p act e v e n t is re la te d to dam age to le ra n c e th ro u g h th e d a m a g e re sista n c e .

U n d e rs ta n d in g an d a n a ly z in g th e d a m a g e re sista n c e o f com posite s tr u c tu r e s is

th e re fo re im p o rta n t to d eterm in e th e d a m a g e c a u sed by a p a r tic u la r im pact.

T h is d am ag e s ta te can th e n be used to a s s e s s th e d am ag e to le ra n c e a n d th u s

t h e c a p ab ility o f th e s tru c tu re to co n tin u e to m eet p erfo rm an ce re q u ire m e n ts .

T h e dam ag e t h a t occurs d u rin g a n im p a c t is a p ro g ressiv e p h en o m en o n . As

d a m a g e occurs d u rin g th e im pact e v e n t, th e p a ra m e te rs th a t g overn th e

re sp o n se m a y change. T h is can influence th e overall response. T h is, in tu rn ,

c a n have a n effect on th e to tal am o u n t o f d am ag e in c u rre d d u rin g th e im p act

e v e n t. It is th e re fo re e s s e n tia l to u n d e rs ta n d how im p a c t resp o n se p a ra m e te rs

c h a n g e as d a m a g e occurs.

T h is g e n erally involves considering th e global a n d local re sp o n se s o f th e

m a te r ia l p ro m o ted by th e im pactor. T h is w ill en ab le u s to an aly ze th e dynam ic

s tr u c tu r a l re sp o n se a n d in d e n ta tio n in th e m a te ria l caused th ro u g h im p act

d a m a g e . T h is d am ag e w a s generally found to consist of sm all m a trix crack s on

t h e back su rfa c e an d in te rn a l dam age, co n sistin g of m a trix cra c k s and

d e la m in a tio n , a b o u t th e size of the im p acto r.

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s tr e s s a t th e p o in t o f im p act e x ists d u e to th e d ifferen ce in co n tact a re a w hich

r e s u lts in in d e n ta tio n s of v a rio u s levels in re la tio n to a re a o f coverage. T h is

r e s u lts in th e local dam ag e n o t h a v in g a noticeable e ffe c t on th e global s tru c tu re

re s p o n s e if th e a r e a o f co n tact w hich is re la te d to th e im p act en erg y is below a

c e r ta in level a n d hence th e d am a g e size an d d e p th o f defect p e n e tra tio n .

In a d d itio n low -speed im p act d a m a g e can d eg rad e th e com pression s tre n g th of

co m p o site s tr u c tu r e s w ith tra n s v e rs e s h e a r d e fo rm a tio n possible d e p e n d in g on

th e m a s s ra tio o f im p acto r to sa m p le u n d e r te s t w h ich affects th e re sp o n se tim e

a n d m a g n itu d e a n d p h ase o f s tr a in response. T h is s t r a i n is found to be low er

for c la m p e d sp ecim en s co m p ared w ith th e u n s u p p o rte d ones. H ence it is

fo re c a ste d th a t b o u n d a ry con d itio n s sig n ifican tly a ffect s tr a in resp o n se of

im p a c te d s tru c tu re s .

T h e a p p lic a tio n o f a n im p act ca n re s u lt in a dynam ic s tr e s s sy stem w h ich w hen

e s ta b lis h e d can in d u ce a d a m a g e th a t p ro p a g a te s a t a n u m b e r o f site s w ith in

th e m a te r ia l th ic k n e ss. C om posites w ith th e ir low tr a n s v e r s e te n sile s tre n g th

can be p ro n e to th is ty p e of effect. In th e case of c a rb o n com posites w hich can

be o p a q u e th e d a m a g e m ay n o t be a p p a re n t w ith o u t th e ap p licatio n of

so p h is tic a te d in sp e c tio n m ethod.

It h a s b e e n re p o rte d [29] th a t u p o n im p a c t s h e a r fa ilu r e is found to be th e

d o m in a n t m ode o f failu re, followed by com pression w h ic h can ta k e th e form of

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m a te ria ls.

C o n v en tio n al f a ilu r e c rite ria for com posites do n o t ad d ress all o f th e a sp e c ts of

b e h a v io u r e n c o u n te re d in a n im pact. Both th e d iffe re n t ty p e s o f fa ilu re m ode

a n d th e d y n a m ic s o f th e s itu a tio n need to be acco m m o d ated in th e a n a ly sis.

H ence, a n y a n a ly s is w hich is in te n d e d to e v a lu a te im pact p erfo rm an ce m u st

co n sid er e n erg y d is s ip a tio n in all fa ilu re processes. In a d d itio n , th e re sid u a l

s tre n g th w hich r e p r e s e n t th e load-carrying c a p a b ility a fte r a n im p act event.

S u ch a v a lu e is o f te n used a s a m ean s of q u a n tify in g m a te ria l beh av io u r.

M a trix p ro p e rtie s are , w ith o u t a doubt, a k e y co n sid e ra tio n for im p act

p erfo rm an ce, a s n o t only do th e y provide th e m e c h a n ism o f lo a d tra n s fe r as

m e n tio n e d before, b u t if d a m ag ed d u rin g th e im p a c t th e re s u ltin g cracks could

allow in g re ss of m o is tu re w hich could cause d e g ra d a tio n . T he p ro p e rtie s o f th e

in te rfa c e a re also im p o rta n t, b u t m ore difficult to control g iv en a p reselected

m atrix /fib re c o m b in a tio n . F o r w eak in terfaces f a ilu re is g e n e ra lly th ro u g h larg e

a re a s o f d e la m in a tio n . L a m in a te co n stru ctio n a n d o rie n ta tio n can be a n

im p o rta n t facto r i n th e desig n for im p act p erfo rm an ce. S im ple u n id ire c tio n a l

m a te ria ls do n o t p erfo rm w ell, m ain ly because o f high s tre s s e s g e n e ra te d

tra n s v e rs e to th e fibre direction. Also the te n d e n c y for d e la m in a tio n is

in c re a se d w h ere t h e disp o sitio n o f in d iv id u al plies le a d s to larg e d isc o n tin u itie s

in stiffn ess. A d o p tin g w oven m a te ria ls or th re e -d im e n sio n a l s titc h e d fabrics

w hich te n d to p ro m o te in creased th ro u g h -th ic k n e ss tensile s tr e n g th can be a

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F o r a n u m b e r o f ap p lic a tio n s, especially in th e tra n s p o rt in d u s try , th e control

o f e n e rg y ab so rp tio n u n d e r im p act conditions is a n im p o r ta n t design fe a tu re .

B ec a u se o f th is, th e re h a v e been a n u m b e r of a tte m p ts to im p ro v e th e e n e rg y

a b s o rb in g c h a ra c te ris tic s o f m a trix m a te ria ls.

T h ese in c lu d e th e use of plasticizo rs, th e a d d itio n of ru b b e r or th e rm o p la stic

p a rtic le s , co n tro l o f cro ss-lin k d en sity , th e use o f th e rm o p la stic m a tric e s a n d th e

u se of in te r la y e r s w ith in plies.

4,4

Im pact Damage: An Energy Band Model

(ORIGINAL CONTRIBUTION)

U n d e r n o rm a l conditions (no im p act e n e rg y applied), m a te r ia l c o n s titu e n ts a re

bound to th e ir resp ectiv e p o te n tia l levels w ith re la tiv e s ta b ility u n d e r sm all

v ib ra tio n s . As im p act e n e rg y is a p p lied , shock w aves (im p u lse s) m ay cau se

d a m a g e s u c h as fibre b re a k a g e or cra c k s ( th a t can p ro p a g a te over tim e).

W hen a d e fe c t is induced, th e orig in al en erg y d istrib u tio n could be affected,

hence, n e w en erg y levels a n d pockets o f en erg y sub-levels m a y be form ed. T h is

e n erg y re -m a p p in g can be co rre la ted to th e ap p lied force o f im p act v ia th e

proposed e n e rg y b a n d m odel described below.

F o r a n o n -d efectiv e or p ra c tically accep tab ly defective co m p o n en t w e will

a ssu m e a n e n erg y level E th. T he im p act will induce a larg e im p u lse su ch th a t

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su b s e q u e n t im p a c ts on th e sam e a re a follows a p o w er accu m u lativ e or

ex p o n e n tia l law in a context t h a t will m a k e th e re q u ire d im p a c t en erg y for the

sam e re sp o n se lo w e r locally b u t h ig h e r globally im p ly in g th e fo rm atio n of

in te rm e d ia te e n e rg y sta te s or b an d s of e n erg y in a less s ta b le e n v iro n m en t

co m p ared to th e o rig in a l s tru c tu re , hence, p ro p ag atio n of a defect is a stro n g

p o ssib ility over a m u c h longer period of tim e re la tiv e to th e im p a c t c o n ta c t tim e

(as sh o w n in F ig u re 4.1.)

F ig u re 4.1: E n erg y B a n d Model.

From th e g rap h , th r e e cases c a n be estab lish ed :

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