Whole

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IMPROVEMENT OF BEARING STRENGTH OF LAMINATED

COMPOSITES BY NANOCLAY AND Z-PIN REINFORCEMENT

by

TRAN PHUONG NAM HUONG

A thesis submitted in fulfillment

Of the requirement for the degree of

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Surname or Family name: HUONG

First name: Tran Phuong Nam Other name/s: N/A

Abbreviation for degree as given in the University calendar: Ph.D

School: Materials Science and Engineering Faculty: Science

Title: Improvement of bearing strength of laminated composites by nanoclay and z-pin reinforcement

Abstract

The bearing behavior of bolted composite joints is significantly poorer than that of their metallic counterparts. The objective of the present study was to examine ways of improving the bearing performance of bolted joints in carbon fibre reinforced epoxy laminates. Two strategies were examined, namely stiffening of the matrix using nanofillers and through- thickness reinforcement of the laminates using z- pins.

The development of a nanoparticle reinforced matrix resin, and its performance in a composite loaded in bearing, was the focus of the first part of the study. A commercial nanoclay, I30E from Nanocor, was chosen as the reinforcement since the nanoclay particles are modified with long alkyl chain amines which improves dispersion in the epoxy resin. The c onditions for preparing nanocomposites based on the nanoclay were examined for two epoxy resin systems , DGEBA and TGDDM. Significant improvements in the elastic modulus were obtained , with a 20% increase being recorded w ith 8.4 phr nanoclay content in the DGEBA resin and a 50% increase with 20 phr nanoclay content in the TGDDM system.

Carbon fibre reinforced laminates w ere prepared from nanoclay reinforced TGGDM matrix resin, using a vacuum assisted prepregging process. The bearing strength and stiffness of the laminated composites was improved by 7% and 15% respectively but the strain to failure was reduced. The addition of nanoclay to the matrix resin was found to change the failure mode.

Enhancement of the bearing performance of laminates by through-thickness reinforcement (z-pins) was examined as the second strategy trialed in this thesis. The bearing strength, stiffness and energy to failure of carbon fibre laminates was found to increase progressively with increasing volume content of z-pins , with increases of 10%, 10% and 16%, respectively , being achieved at a z-pin volume content of 4%. No significant change in strength, stiffness or failure energy was observed when the z- pin diameter was changed from 0.28 to 0.51 mm at the same volume content.

Declaration relating to disposition of project thesis/dissertation

I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or parts of this thesis or dissertation.

I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only)

……… ……… ………

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The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research.

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‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged.’

Signed...

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The bearing behavior of bolted composite joints is significantly poorer than that of their metallic counterparts. The objective of the present study was to examine ways of improving the bearing performance of bolted joints in carbon fibre reinforced epoxy laminates. Two strategies were examined, namely stiffening of the matrix using nanofillers and through-thickness reinforcement of the laminates using z-pins.

Stiffening of the resin matrix was carried out by reinforcement with nanoclay partic les. It was necessary to establish a technique for uniformly dispers ing the nanoclay particles in the epoxy resin and a systematic study was conducted. The effect of surfactant modification of the nanoclay particles was examined first. The effect of surfactant structure was examined using the surfactants octylamine, diaminoctane and methylbenzylamine, which all have the same chain length (8 carbon atoms in the chain), while the effect of chain length was examined by including hexadecylamine (16 carbon atoms in the chain). The short chain surfactants did not expand the layers in the nanoclay appreciably, but the longer chain surfactant produced a substantial increase in the interlayer spacing (from 11 Å to 18 Å) and was considered a suitable surfactant for modifying the nanoclay for reinforcing epoxy resin. However, similar results were obtained using the commercially available octadecylamine modified nanoclay, N anomer I30E, and this was used in the remainder of the work.

DGEBA epoxy resin nanocomposites with up to 8.4 phr Nanomer I30E nanoclay were prepared and the effect of varying the processing parameters explored. It was found that exfoliation of the nanoclay particles was insensitive to the processing conditions within the range examined. Exfoliated nanoc omposites with a d-spacing of 100 Å were

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highest nanoclay loading.

Nanocomposites were also prepared using the higher performance aircraft grade TGDDM epoxy resin with loadings of up to 20 phr Nanomer I30E nanoclay. Fully exfoliated nanocomposites with an interlayer spacing greater than 120 Å were obtained up to 5 phr nanoclay, while pre-exfoliated or intercalated nanocomposites, with interlayer spacings from 85 Å (7.5 phr nanoclay) to 60 Å (20 phr nanoclay), were obtained at higher loadings. The compression modulus increased linearly with clay content, with an increase of 50% being achieved over that of the pristine resin at 20 phr clay. The results were in good agreement with the predictions of the Halpin-Tsai model for an aspect ratio of 13.

Carbon fibre reinforced laminates with a fibre volume fraction of approximately 55% were prepared using I30E nanoclay modified TGGDM as the matrix resin with clay loadings of 7.5 and 12.5 phr. The addition of nanoclay to the matrix produced a progressive increase in bearing stiffness with improvements of 10% and 23% being obtained at 7.5 and 12.5 phr nanoclay respectively. A more modest improvement in bearing strength was obtained with a 7% increase being achieved at 12.5 phr nanoclay. The strain to failure was reduced by the addition of nanoclay. The relatively poor increase in bearing strength and the reduced strain to failure are attributed to a change in failure mode brought about by the introduction of the nanoclay into the matrix resin. The spacing of the nanoclay layers in the laminates was only half that obtained in the clay-resin nanocomposites with the same clay content. This is attributed to the resin curing prematurely within the clay galleries during the prepreg drying stage.

The effect of through thickness reinforcement on bearing strength was examined by reinforcing carbon fibre epoxy laminates locally in the vicinity of the hole with z-pins. Three different volume contents of z-pins were examined (0.5, 2, and 4%) and two different pin diameters (0.28 and 0.51 mm). The insertion of z-pins increased the bearing strength, stiffness and energy to failure. No change in the strain to failure was however observed. The increases are attributed to the bridging effect of the z-pins. The

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and 10% at the same pin contents while the energy absorbed to failure was correspondingly increased by 9% and 16% . These increases are attributed to the bridging effect of the z-pins. No significant change in strength, stiffness or failure energy was observed when the pin diameter was changed from 0.28 to 0.51 mm at the same volume content.

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Doing a PhD thesis could be compared to navigation of a boat through the ocean till reaching the land. It is a result of interdependence, a cooperation of knowledge and experience of many people. Now I have a pleasant opportunity to express my gratitude for them who have given me uncountable help to complete this project.

The first person I would like to gratefully thank with all my heart is Professor Alan Crosky who is not only my direct excellent supervisor, but also as close as my “father”. I owe him a lot of gratitude for giving me great supervision and valuable advice in both fields - research and life. No boundary is drawn between us, a supervisor and a student , only family sense existed instead. He may not realize how much happy I have come to know him in my life. Furthermore, I would like to express my deep gratitude to his wife, Peta, who always shared with me difficult times happening in my life during research period.

I would like to thank my co-supervisors, Professor Don Kelly (from UNSW) and Dr. Ben Qi (from CRC-ACS) who kept an eye on the progress of my work as well as gave me great support during project time. I really appreciated valuable comments they made for my thesis.

A great thank to Dr. Brian who always sent his great friendship with precious help in improving my writing skill as well as how to make a good structure of thinking. Joining with him, Gavin, Honghua, Singh, Fuhai, all my friends in School of Materials Research and Engineering, UNSW and my flatmates, Hoang, Nadia, Julia, created for

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I am grateful for Katie Levick, Jenny Norman, Veira and all staff in EM Unit who trained and helped me using varieties of electron microscopes (TEM, SEM, AFM and FIB). Their support straight distributed to my successful work in research.

I would like to thank Professor Chris Sorrel who installed a first brick for me to get into UNSW for studying. Special thanks to you, Mrs. Lana, who always took care of me with all her sympathy, to all staffs in the School of MSE for their valuable help and pleasure.

The Australian Government and CRC-ACS Ltd. are acknowledged for the supply of IPRS and Nanocomposites scholarships, supporting in finance for my research till fully completed. I would also like to thank Professor Adrian Mouritz, and Dr Paul Chang, RMIT University, Melbourne, Australia for their assistanc e with the z-pinning.

Deeply, I am very grateful to my beautiful wife for her love and patience, and to my parents, my brothers and my sisters for their uninterrupted encouragement.

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2.3.1.1. Epoxy resin ... 27 2.3.1.2. Nanoclay particles ... 33 2.3.2. Structure of nanocomposites ... 36 2.3.3. Properties of nanocomposites... 38 2.3.3.1. Tensile properties... 38 2.3.3.2. Compressive properties ... 39 2.3.3.3. Impact properties ... 40 2.3.3.4. Flexural properties ... 41 2.3.3.5. Toughness ... 42 2.3.3.6. Barrier performance ... 42 2.3.3.7. Other properties ... 43 2.3.4. Synthesis ... 45 2.3.4.1. Physical methods ... 45 2.3.4.2. Chemical methods ... 46 2.3.4.3. Curing procedure ... 49

2.3. 5. Factors affecting nanocomposite structure... 53

2.3.5.1. Effect of nanoclay... 54

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2.3.5.4. Effect of hardeners... 56 2.3.5.5. Effect of temperature ... 58 2.3.5.6. Effect of time ... 59 2.3.5.7. Effect of pressure ... 60 2.4. Z -pin reinforcement... 61 2.4.1. Concepts... 61 2.4.2. Mechanism... 63 2.4. 3. Important factors ... 67 2.4.3.1. Pin density ... 67 2.4.3.2. Pin diameter ... 68 2.4.3.3. Other factors ... 69 2.4. 4. Pinning process ... 69 2.5. Characterization ... 71 2.5.1. Structure Characterization... 71

2.5.1.1. Wide Angle X-ray Diffraction ... 71

2.5.1.2. Transmission Electron Mic roscopy ... 72

2.5.1.3. Scanning Electron Microscopy... 73

2.5.1.4. Optical Microscopy ... 74

2.5.2. Thermal property analysis... 75

2.5.2.1. Differentional Scanning Calorimetry ... 75

2.5.3. Mechanical Behaviour ... 76

2.5.3.1. Compression Testing ... 76

2.5.3.2. Bearing Test ... 77

2.5.3.3. Pin-contact Bearing Test... 78

2.6. Aims and Objectives of Thesis ... 80

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References ... 82 Chapter 3 Nanocomposites... 92 3.1. Introduction... 92 3.2. Experimental procedures... 93 3.2.1. Nanoclay modification ... 93 3.2.1.1. Materials ... 93 3.2.1.2. Sample preparation ... 94 3.2.1.3. Analysis ... 95 3.2.2. DGEBA nanocomposites ... 96 3.2.2.1. Materials ... 96 3.2.2.2. Sample preparation ... 97 3.2.2.3. Analysis ... 97 3.2.3. TGDDM nanocomposites ... 100 3.2.3.1. Materials ... 100 3.2.3.2. Sample preparation ... 100 3.2.3.3. Analysis ... 101 3.2.4. Experimental variables ... 101 3.3. Results ... 102 3.3.1. Nanoclay modification ... 102

3.3.1.1. Effect of surfactant types ... 104

3.3.1.2. Effect of acid/amine ratio ... 105

3.3.1.3. Effect of surfactant concentration... 106

3.3.1.4. Effect of mixing time... 107

3.3.2. DGEBA Nanocomposites ... 108

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3.3.2. 3. Effect of mixing temperature... 116

3.3.2. 4. Effect of mixing speed... 118

3.3.2. 5. Effect of mixing time... 119

3.3.2.6. Effect of hardener concentration ... 121

3.3.2.7. Effect of curing temperature... 123

3.3.2. 8. Effect of curing time... 124

3.3.2. 9. Effect of nanoclay content ... 125

3.3.2. 10. Optical properties ... 128

3.3.3. TGDDM Nanocomposites... 129

3.3.3.1. Effect of surfactant on curing behaviour ... 128

3.3.3.2. Effect of mixing speed... 131

3.3.3.3. Effect of mixing temperature... 134

3.3.3.4. Effect of mixing time... 135

3.3.3.5. Effect of degassing time ... 138

3.3.3.6. Effect of curing temperature... 140

3.3.3.7. Effect of curing time ... 143

3.3.3.8. Effect of nanoclay content ... 146

3.4. Discussion ... 150

3.4.1. Nanoclay modification ... 150

3.4.1.1. Effect of surfactants ... 150

3.4.1.2. Effect of acid/amine ratio ... 151

3.4.1.3. Effect of surfactant concentration... 152

3.4.1. 4. Effect of mixing time... 153

3.4.1. 5. Conclusion ... 153

3.4.2. DGEBA Nanocomposites ... 153

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3.4.2.2. Effect of mixing conditions ... 156

3.4.2.2.1. Mixing temperature... 156

3.4.2.2.2. Mixing speed... 157

3.4.2. 2.3. Mixing time... 157

3.4.2. 3. Effect of curing conditions ... 157

3.4.2.3.1. Hardener concentration ... 157

3.4.2.3.2. Cure temperature... 158

3.4.2.3.3. Cure time... 159

3.4.2. 4. Effect of nanoclay content ... 159

3.4.2.5. Optical properties... 160

3.4.2. 6. Conclusion ... 161

3.4.3. TGDDM Nanocomposites... 161

3.4.3.1. Effect of surfactant on cure ... 162

3.4.3.2. Effect of mixing conditions ... 162

3.4.3.2.1. Mixing speed... 162

3.4.3.2.2. Mixing temperature... 162

3.4.3.2.3. Mixing time... 163

3.4.3.3. Effect of curing conditions ... 163

3.4.3.3.1. Degassing time ... 163

3.4.3.3.2. Effect of curing temperature ... 164

3.4.3.3.3. Effect of curing time ... 164

3.4.3.4. Effect of nanoclay content ... 165

3.4.3. 5. Conclusions... 170

3.5. Conclusions ... 172

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4.1. Introduction... 178

4.2. Experimental procedure ... 179

4.2.1. Materials ... 179

4.2.2. Sample preparation... 179

4.2.3. Testing and analysis ... 182

4.2.3.1. Pin-contact bearing test... 182

4.2.3.2. Microstructure ... 184

4.2.3.3. Fracture surfaces ... 184

4.2.3.4. Wide angle X-ray diffraction... 185

4.3. Results ... 185

4.3.1. Pin-contact bearing test ... 185

4.3.2. Laminate quality... 192 4.3. 3. Microstructural examination ... 194 4.3. 4. Fracture surfaces ... 202 4.3. 5. Interlayer spacing ... 204 4.4. Discussion ... 206 4.5. Conclusions ... 211 References ... 213

Chapter 5 Z-pin reinforcement... 214

5.1. Introduction... 214 5.2. Experimental procedure ... 215 5.2.1. Materials ... 215 5.2.2. Sample preparation... 215 5.2.3. Bearing testing... 219 5.2.4. Microstructural examination ... 221 5.3. Results ... 221

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5.3.1. Bearing testing... 221 5.3.2. Microstructure ... 231 5.4. Discussion ... 239 5.5. Conclusions ... 244 References ... 245 Chapter 6 Conclusions ... 246 Appendix A Compression Tests

Appendix B Pin -contact test results for laminated nanocomposites Appendix C Students’ T-Test for laminated nanocomposites Appendix D Bearing test results for Z-pin reinforcement

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LIST OF FIGURES

Figure 2.1 Materials used in F/A 18 fighter aircraft ... 10

Figure 2.2 Schematic of joints used in hybrid metal/composite wing ... 11

Figure 2.3 Joint efficiency of different materials ... 12

Figure 2.4 Bolted joint failure modes... 13

Figure 2.5 Comparison of compression and bearing failure mechanisms ... 13

Figure 2.6 Bearing damage process detected by acoustic emission (AE)... 14

Figure 2.7 Schematic illustration of the failure process... 15

Figure 2.8 Initial configuration and buckling modes investigated by Rosen... 17

Figure 2.9 In-plane buckling of fibres and fibre kink band geometry ... 18

Figure 2.10 Schematic of fibre failure sequence in shear triggere d kink-band formation ... 19

Figure 2.11 Geometry of kink-band formation... 20

Figure 2.12 Micrograph of shear cracking in bearing failure ... 21

Figure 2.13 Shear cracking and delamination ... 21

Figure 2.14 Open hole average compressive strength as a function of specimen thickness for multidirectional laminates ... 24

Figure 2.15 Schematic of curing reaction of epoxy/amine system ... 32

Figure 2.16 Schematic of curing reaction of epoxy/anhydride system... 32

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Figure 2.18 Schematic illustrations of conventional composite and

nanocomposites... 36 Figure 2.19 TEM images of intercalated (left) and exfoliated (right)

nanocomposites... 37 Figure 2.20 Representative scattering curves for intercalated and exfoliated

morphology for epoxy/SC18 layered silicate nanocomposites

at several stages of cure ... 37 Figure 2.21 Effect of nanoclay content on the modulus of nanocomposites ... 39 Figure 2.22 Compressive modulus of nanocomposite and filler composites

with clay loading ... 40 Figure 2.23 Effect of nanoclays on impact strength of nanocomposite ... 41 Figure 2.24 Illustration of Neilson’s tortuous path model for barrier

enhancement of nanocomposites ... 43 Figure 2.25 Optical image of 5 mm thick plaques of nanocomposites based

on EPON 828 cured by D400 and contains different loadings

of nanoparticles... 44 Figure 2.26 Schematic figures for the exfoliation process of clay layers

with DGEBA on the mixing process ... 50 Figure 2.27 D-spacing of clay layers with the degree of conversion at some

isothermal curing temperatures of 120, 130, 140°C in the

DGEBA-DDS-C18 clay (5 phr) ... 51 Figure 2.28 Schematic illustration of the intercalated state and exfoliation

process showing the forces acting on a pair of clay layers: (a) organically modified clay, (b) epoxy-intercalated state, (c)

forces acting on a two-particle tactoid ... 52 Figure 2.29 Schematic diagram showing the relationship between the ionic

bonding energy and the location of the layers in the tactoid: (a) tactoid, (b) variation of bonding energy along the thickness of

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Figure 2.30 Transmission electron micrographs of 10 phr C18 clay reinforced epoxy nanocomposites cured by (a) MDA (4,4’-methylene -dianiline) and (b) DDS (4,4’-diaminodiphenyl

sulfone)... 56 Figure 2.31 Small angle X-ray scattering patterns of 1% montmorillonite

containing DGEBA epoxy cured by MPDA with (a) 25 phr; (b)

14.5 phr; (c) 5 phr ... 57 Figure 2.32 Isothermal time-resolved SXRD patterns of nanocomposites at

120°C with scan time (minute) from bot tom to top as follow: 0,

5, 10, 11, 15, 20, 25, 30, 40, 60 and 89 min... 59 Figure 2.33 Time-dependent small angle synchrotron X-ray scattering

patterns of the epoxy/montmorillonite mixture at 1350C ... 60 Figure 2.34 Z-pin performs containing 0.28 mm diameter pins at densities

of 0.5, 2 and 4% ... 62 Figure 2.35 Micrograph of z-pin reinforcement ... 63 Figure 2.36 Typical morphology of local area around a z-pin... 64 Figure 2.37 Sketch and SEM micrograph showing fibres deflecting around

z-pins and weaving through a field of z-pins... 64 Figure 2.38 Crack around z-pin in laminated composite ... 65 Figure 2.39 Scanning electron micrograph showing a crack that initiated

near a pin under flexural load ... 66 Figure 2.40 Delamination of laminate pinned with (a) 0.28 mm and (b) 0.51

mm pins... 68 Figure 2.41 Schematic of the z-fibre reinforcement process ... 69 Figure 2.42 Schematic of ultrasonically assisted z-fibre insertion... 70 Figure 2.43 Schematic depicting the expected X-ray diffraction patterns for

various types of hybrid structures ... 71 Figure 2.44 TEM images of clay nanocomposite at low (left) and high

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Figure 2.45 SEM fractograph of 7 wt.% clay nanocomposite ... 73

Figure 2.46 Optical micrograph showing large particles the DETDA cured DGEBA system containing 5 wt.% clay... 74

Figure 2.47 DSC of Epon 826/W, 3% SC8/Epon 826/W, and 3% SC18/Epon 862/W at 2°C/min ... 75

Figure 2.48 Typical stress - strain curve for a compression test... 76

Figure 2.49 Typical stress - strain curve for a bearing test ... 77

Figure 2.50 Fixture assembly for a bearing test ... 78

Figure 2.51 Schematic diagram of pin-contact test... 79

Figure 2.52 Typical load - displacement curve for cross-ply laminate ... 79

Figure 3.1 Schematic of nanoclay modification... 94

Figure 3.2 XRD patterns of modified nanoclay... 105

Figure 3.3 XRD patterns of modified nanoclay in different acid/amine ratios... 106

Figure 3.4 XRD patterns of nanoclay modified with various concentration of hexadecylamine ... 107

Figure 3.5 XRD patterns of nanoclay modified with hexadecylamine for various mixing time ... 108

Figure 3.6 Exothermal curves of curing reaction for SP Systems amine/epoxy system with different nanoclay types ... 110

Figure 3.7 Compression modulus for nanocomposites reinforced with modified nanoclay particles and pure nanoclay (CNa+) as well as for neat epoxy resin (PR) showing (1) effect of surfactants, (2) effect of acid/amine ratio, (3) effect of surfactant concentration and (4) effect of mixing time ... 113

Figure 3.8 WAXD spectra for nanocomposites reinforced with C30B and I30E nanoclay ... 114

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Figure 3.9 TEM micrographs of nanocomposite reinforced with C30B (a)

and I30E (b) ... 115 Figure 3.10 Compression modulus of pure resin and nanocomposites

reinforced w ith different nanoclay particles ... 115 Figure 3.11 WAXD spectra for nanocomposites made with varying mixing

temperatures ... 116 Figure 3.12 Compression modulus of nanocomposites based on I30E

nanoclay made with different mixing temperatures... 117 Figure 3.13 WAXD spectra of nanocomposites made with different mixing

speeds... 118 Figure 3.14 Compression modulus of nanocomposites for different mixing

speeds... 119 Figure 3.15 WAXD spectra of nanocomposites made with different mixing

times ... 120 Figure 3.16 Compression modulus of nanocomposites for different mixing

times ... 120 Figure 3.17 Diagrams showing (a) compression modulus of

nanocomposites for varying hardener concentrations and (b) %

increase over that of neat resin... 122 Figure 3.18 WAXD spectra of nanocomposite for varying hardener

concentrations ... 122 Figure 3.19 WAXD spectra of nanoc omposites for different curing

temperatures ... 123 Figure 3.20 Compression modulus of nanocomposites for different curing

temperatures ... 124 Figure 3.21 Compression modulus of nanocomposites for different curing

times ... 125 Figure 3.22 WAXD spectra of nanocomposites for different nanoclay

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Figure 3.23 Diagrams showing (a) compression modulus of

nanocomposites for different nanoclay contents and (b) %

increase over that of neat resin... 127 Figure 3.24 Optical appearance of pure resin and nanocomposites based on

I30E nanoclay ... 128 Figure 3.25 EDS analysis of nanocomposite: (A) montmorillonite nanoclay

particles, (B) aluminum oxide and (C) resin... 129 Figure 3.26 Exothermal curves for cure reaction of TGDDM/DETDA with

various nanoclay loadings... 130 Figure 3.27 Compress ion modulus of nanocomposites made w ith different

mixing speeds ... 132 Figure 3.28 WAXD spectra of nanocomposites with (a) 2.5 phr and (b) 7.5

phr nanoclay... 133 Figure 3.29 Compress ion modulus of nanocomposites for different mixing

temperatures ... 134 Figure 3.30 WAXD spectra of nanocomposites with (a) 2.5 phr and (b) 7.5

phr nanoclay... 135 Figure 3.31 Diagrams showing (a) compression modulus of

nanocomposites for different mixing times and (b) % increase

over that of neat resin... 136 Figure 3.32 WAXD spectra of nanocomposites with (a) 2.5 phr and(b) 7.5

phr nanoclay... 137 Figure 3.33 Diagrams showing (a) compression modulus of nanocomposite

made with 7.5 phr nanoclay for various additional degassing

times and (b) % increase over that of neat resin ... 139 Figure 3.34 WAXD spectra of nanocomposites containing 7.5 phr

nanoclay for various additional degassing times ... 139 Figure 3.35 Compression modulus of nanocomposites with varying initial

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Figure 3.36 Percentage increase of compressive modulus over that of neat resin for nanocomposites cured at various initial (I),

intermediate (II) and final (III) curing temperatures... 141 Figure 3.37 WAXD spectra of nanocomposites for various initial cure

temperatures ... 142 Figure 3.38 WAXD spectra of nanocomposites for various intermediate

cure temperatures ... 142 Figure 3.39 WAXD spectra of nanocomposites for various final cure

temperatures ... 143 Figure 3.40 Compression modulus of nanocomposites cured for different

initial (I), intermediate (II) and final (III) cure times ... 144 Figure 3.41 Percentage increase of compression modulus over that of neat

resin for nanocomposites cured at various initial (I),

intermediate (II) and final (III) cure times... 144 Figure 3.42 WAXD spectra of nanocomposites for various initial curing

times ... 145 Figure 3.43 WAXD spectra of nanocomposites for various final curing

times ... 145 Figure 3.44 WAXD spectra for nanocomposites for varying I30E nanoclay

contents ... 146 Figure 3.45 High magnification TEM images of nanocomposites with

various nanoclay contents: (a) 2.5 phr, (b) 7.5 phr, (c) 12.5 phr

and (d) 20 phr ... 147 Figure 3.46 Compression modulus of TGDDM/DETDA/I30E

nanocomposites ... 148 Figure 3.47 Percentage increase in compression modulus over that of neat

resin for nanocomposites made with varying nanoclay contents... 148 Figure 3.48 Tg of nanocomposites with varying nanoclay contents ... 149

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Figure 3.49 SEM images of various nanoclay contents in the resin matrix:

(a) 2.5 phr, (b) 7.5 phr, (c) 12.5 phr and (d) 20 phr ... 150 Figure 3.50 Light transmission spectra of pure epoxy, Ep/CM

nanocomposites with different CM contents ... 160 Figure 3.51 Comparison of experimental results with pre dicted results for

aspect ratios of 13, 40 and 100... 170

Figure 4.1 Wet prepreg plies drying in laboratory... 180 Figure 4.2 Prepreg lay-up (a) and sealed vacuum bag (b) ... 180 Figure 4.3 Vacuum bagging of lay up (a) and layup being inserted into

hot press (b) ... 181 Figure 4.4 Procedure for producing pin-contact bearing test specimens:

(a) specimen with drilled hole, (b) testpiece produced by

slitting specimen through hole ... 182 Figure 4.5 Schematic of the pin-contact bearing test employed by Wu and

Sun ... 183 Figure 4.6 Diagram showing positions where thickness was measured on

specimens ... 183 Figure 4.7 Production of Mode 1 fracture surface ... 184 Figure 4.8 Typical pin-contact load-displacement curves for laminates

made with neat resin (baseline) and nanoclay reinforced resin ... 185 Figure 4.9 Optical micrograph showing un-reinforced laminate ... 192 Figure 4.10 Optical micrograph showing laminate reinforced with 7.5 phr

nanoclay (55 vol.% fibres) ... 193 Figure 4.11 Optical micrograph showing laminate reinforced with 12.5 phr

nanoclay... 193 Figure 4.12 Optical micrograph showing un-reinforced 6K fabric laminate ... 194

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Figure 4.13 Optical micrograph showing 6K fabric laminate reinforced

with 7.5 phr nanoclay... 194 Figure 4.14 Optical micrographs showing local failure modes in the

laminated composites: kinking (K), matrix cracking (M),

shearing cracking (S), delamination (D) ... 195 Figure 4.15 Optical micrograph showing bearing damage in unreinforced

laminate containing 56.9 vol.% 3K carbon fibres (a) low

magnification view and (b) enlarged view of region A in (a) ... 196 Figure 4.16 Optical micrograph showing bearing damage in laminate

reinforced with 7.5 phr nanoclay containing 57.6 vol.% 3K carbon fibres (a) low magnification view and (b) enlarged view

of region A in (a) ... 197 Figure 4.17 Optical micrograph showing bearing damage in laminate

reinforced with 7.5 phr nanoclay containing 55 vol.% 3K carbon fibres (a) low magnification view and (b) enlarged view

of region A in (a) ... 198 Figure 4.18 Optical micrograph showing bearing damage in laminate

reinforced with 12.5 phr nanoclay containing 51.7 vol.% 3K carbon fibres (a) low magnification view and (b) enlarged view

of region A in (a) ... 199 Figure 4.19 Optical micrograph showing bearing damage in unreinforced

laminate containing 61.3 vol.% of 6k carbon fibres ... 200 Figure 4.20 Optical micrograph showing bearing damage in laminate

reinforced with 7.5 phr nanoclay and containing 61.1 vol.% 6K carbon fibres (a) low magnification view and (b) enlarged view

of region A in (a) ... 201 Figure 4.21 Mode 1 fracture surface of baseline (left) and 7.5 phr nanoclay

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Figure 4.22 SEM images of the fracture surfaces of unreinforced (a, b) and nanoclay reinforced (7.5 phr I30E) (c, d) laminates with 3K

tow carbon fibres ... 203 Figure 4.23 SEM images of fracture surfaces of unreinforced (a, b) and

nanoclay reinforced (7.5 phr I30E) (c, d) laminates with 6K

tow carbon fibres ... 203 Figure 4.24 WAXD spectra of unreinforce d and reinforced (7.5 phr and

12.5 phr I30E nanoclay) laminates with 3K tow carbon fibres ... 204 Figure 4.25 WAXD spectra of unreinforced and reinforced (7.5 phr I30E

nanoclay) laminates with 6K tow carbon fibres ... 205 Figure 4.26 WAXD spectra of uncured 3K tow carbon fibre prepreg

containing 7.5 phr I30E nanoclay... 205 Figure 4.27 Normalised bearing strength from pin-contact test for

laminates reinforced with 3K tow fabric (unfilled) and 6K tow fabric (filled) with various nanoclay contents (0, 7.5 and 12.5

phr) ... 207 Figure 4.28 Slope of load-displacement curves of unreinforced and

reinforced laminates with 3k tow fabric (unfilled) and 6k tow

carbon fibres (filled) ... 208 Figure 4.29 Displacement at failure of unreinforced and reinforced

laminates with 3K tow carbon fibres (unfilled) and 6K tow

carbon fibres (filled) ... 209

Figure 5.1 Area reinforced by z-pins around 10 mm bolthole ... 216 Figure 5.2 Photographs showing (a) pins embedded in foam preform and

(b) use of ultrasonic horn to insert pins into prepreg stack... 217 Figure 5.3 Photographs showing (a) completely inserted pins in prepreg

and (b) cutting the preform and remaining portion of the pins

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Figure 5.5 Schematic representation of fixture assembly for pin loaded

bearing test ASTM D5961... 220 Figure 5.6 Positions at which thickness measurements were made ... 221 Figure 5.7 Z-pinned and baseline laminates after bearing test... 221 Figure 5.8 Typical load - displacement curves for baseline and z-pinned

laminates ... 222 Figure 5.9 Variation in bearing strength with pin density for 0.28 mm

pins... 227 Figure 5.10 Variation in bearing stiffness (slope to failure) with pin density

for 0.28 mm pins ... 228 Figure 5.11 Variation in energy absorbed to failure with pin density for

0.28 mm pins... 229 Figure 5.12 Comparison of bearing strength for 0.28 and 0.51 mm z-pins at

2% volume density... 230 Figure 5.13 Comparison of bearing stiffness for 0.28 and 0.51 mm z-pins

at 2% volume density... 230 Figure 5.14 Comparison of energy absorbed to failure for 0.28 and 0.51

mm z-pins at 2% volume density... 231 Figure 5.15 Optical micrograph of unpinned laminate loaded to 95% of

ultimate failure load ... 232 Figure 5.16 Optical micrograph of unpinned laminate at final failure... 232 Figure 5.17 Optical micrograph of laminate containing 0.5 % 0.28 mm

z-pins loaded to 95% of ultimate failure load (a) full section view and (b) detailed view of region marked A in (a ) showing

fibre kinking ... 233 Figure 5.18 Optical micrograph of laminate containing 0.5 % 0.28 mm

z-pins loaded to ultimate failure ... 234 Figure 5.19 Optical micrograph of laminate containing 2 % 0.28 mm

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view and (b) detailed view of region marked A in (a) showing

shear bands ... 235 Figure 5.20 Optical micrograph of laminate containing 2 % 0.28 mm

z-pins loaded to ultimate failure load (a) full section view and

(b) - (e) detailed views of regions marked A – D in (a)... 236 Figure 5.21 Optical micrograph of laminate containing 4 % 0.28 mm

z-pins loaded to ultimate failure ... 237 Figure 5.22 Optical micrograph of laminate containing 2 % 0.51 mm

z-pins loaded to 95% of ultimate failure load (a) full section view and (b) and (c) detailed views of regions marked A and B

in (a) ... 238 Figure 5.23 Optical micrograph of laminate containing 2 % 0.51 mm

z-pins loaded to ultimate failure load (a) full section view and (b)

detailed views of region marked A in (a)... 239 Figure 5.24 Optical micrograph of curvature of 0° fibres in pinned laminate ... 241 Figure 5.25 SEM micrograph of a crack beside a z-pin ... 243

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LIST OF TABLES

Table 2.1 Classification and generalized structural formulae of phyllosilicates... 33 Table 2.2 Notional structure and chemistry of smectites... 34

Table 3.1 Effect of surfactant on nanoclay modification... 101 Table 3.2 E ffect of nanoclay type on properties of DGEBA composites ... 101 Table 3.3 Effect of processing conditions on properties of

I30E/DGEBA composites ... 102 Table 3.4 Effect of processing conditions on properties of

I30E/TGDDM composites ... 102 Table 3. 5 S urfactants and experimental conditions used... 104 Table 3.6 Effect of acid/amine ratio on d-spacing of nanoclay ... 105 Table 3.7 Effect of surfactant concentration on d-spacing of nanoclay... 107 Table 3.8 Effect of the mixing time on d-spacing of nanoclay... 108 Table 3.9 D-spacing in modified CNa+nanoclay nanocomposites ... 111 Table 3.10 Compression modulus for modified CNa+nanoclay

nanocomposites ... 112 Table 3.11 Effect of nanoclay type on d-spacing and compression modulus ... 114 Table 3.12 Effect of mixing temperature on compression modulus ... 117 Table 3.13 Effect of hardener concentration on compression modulus... 121

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Table 3.14 Effect of curing time on d-spacing and compression modulus ... 124 Table 3.15 Effect of nanoclay content on d-spacing and modulus of

nanocomposites ... 125 Table 3.16 Peak and onset temperatures of nanocomposite s having varying

nanoclay contents ... 131 Table 3.17 Effect of cure temperature on compression modulus of

nanocomposites ... 140 Table 3.18 Effect of curing time on compression modulus of

nanocomposites ... 143

Table 4.1 Measured values of thickness, failure load, bearing strength, slope to failure and displacement to failure for neat epoxy

laminates (baseline) containing 56.9 vol.% carbon fibre... 186 Table 4.2 Measured values of thickness, failure load, bearing strength,

slope to failure and displacement to failure for laminates with

7.5 phr I30E nanoclay and 57.6 vol.% carbon fibre... 187 Table 4.3 Measured values of thickness, failure load, bearing strength,

7.5 phr I30E nanoclay and 55 vol.% carbon fibre ... 188 Table 4.4 Measured values of thickness, failure load, bearing strength,

7.5 phr I30E nanoclay and 51.7 vol.% carbon fibre... 189 Table 4.5 Summary of results for baseline and nanocomposite laminates

(3K tow fabric) ... 189 Table 4.6 Measured values of thickness, failure load, bearing strength,

slope to failure and displacement to failure for baseline (neat

resin) laminates containing 61.3 vol.% carbon fibres ... 190 Table 4.7 Measured values of thickness, failure load, bearing strength,

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Table 4.8 Summary of results for baseline and nanocomposite laminates

(6K tow fabric) ... 191 Table 4.9 Bearing strength of CFRPs with or without nanoclay

reinforcement ... 206 Table 4.10 Toughness estimated from area under stress strain curve ... 210

Table 5.1 Measured values of thickness, failure load, bearing strength, energy absorbed to failure, slope to failure and displacement to

failure for baseline samples... 223 Table 5.2 Measured values of thickness, failure load, bearing strength,

energy absorbed to failure, slope to failure and displacement to

failure for laminates pinned with 0.5% 0.28 mm pins ... 224 Table 5.3 Measured values of thickness, failure load, bearing strength,

failure for laminates pinned with 2% 0.28 mm pins ... 224 Table 5.4 Measured values of thickness, failure load, bearing strength,

failure for laminates pinned with 4% 0.28 mm pins ... 225 Table 5.5 Measured values of thickness, failure load, bearing strength,

failure for laminates pinned with 2% 0.51 mm pins ... 225 Table 5.6 Summary of changes in mechanical behaviour produced in

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Appendix A Compression Tests

Appendix B Pin-contact test results for laminated nanocomposites Appendix C Students’ T-Test for laminated nanocomposites Appendix D Bearing test results for Z-pin reinforcement

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Arabic Description d ... hole diameter

d001... d-spacing of nanoclay layers E ... elastic modulus

e ... edge distance G ... shear modulus

FB... bearing strength of laminate FTU... tensile strength

J ... joint efficiency l... length

m... weight P... bearing load

PB... maximum bearing load PTU... ultimate tensile load

t... thickness

Tg... glass Transition Temperature V ... volume fraction

w ... width

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Greek Description ? ... shape parameter a ... aspect ratio s ... stress Subscripts Descriptions c ... clay property cr... microbuckling property ep ... elastic-plastic property f... fibre property L... longitudinal property m ... matrix property n ... nanocomposite property p ... platelet property T... transverse property xx ... compressive direction Abbreviations Descriptions AE... Acoustic Emission

ASTM... American Society for Testing and Materials CEC... Cation-Exchange Capacity of clay

CFRP ... Carbon Fibre Reinforced Polymer CNa+... Pure sodium nanoclay

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DETA ... Diethylene Triamine DETDA ... Diethyltoluene Diamine

DGEBA... Di-glycidyl ether of bisphenol A DSC... Differential Scanning Calorimetry hrs... hours

K ... Kinking

M ... Matrix cracking MDA... Methylene Diamine min ... minute

mPDA ... m-Phenylene Diamine PEEK... Poly(ether ether ketone) PES ... Poly(ether sulfone) phr ... per hundred of resin rpm ... rounds per minute S... Shear cracking

SEM... Scanning Electron Microscopy STDEV... Standard Deviation

TEM ... Transmission Electron Microscopy TETA... Triethylene Tetramine

TGAP ... Triglycidyl p-amino phenol

TGDDM... Tetra-glycidyl diamino diphenyl methane TGMDA ... Tetraglycidyl methylene dianiline

WAXD ... Wide Angle X-Ray Diffraction XRD ... X-ray Diffraction

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CHAPTER 1

INTRODUCTION

The development of high performance structur al materials is particularly attractive to the aerospace industry. Fibre polymer composites are one such class of materials that have become increasingly popular in this industry in view of their exceptional strength and stiffness to density ratios. A typical composite consists of strong stiff fibres, such as glass or carbon fibres, embedded in a tough resin matrix, such as epoxy resin. The fibres react the loads while the matrix maintains the fibre orientation, distributes the load, and protects the structure against environmental factors such as moisture and chemical attack.

One of the first applications of modern composites in the aerospace industry was in the skins of the empennage of the F14 and F15 fighters, but the structural weight of the composites used was only around 2% [1]. With improved understanding of the material behaviour , the percentage of composites used in military aircraft increased rapidly rising to 19% by weight in the F18 and 24% in the F22. C omposites have also seen growing use in civil transport aircraft where reduced airframe weight can reduce fuel consumption, thereby lowering operating costs. A irbus in particular has made significant use of composite materials for commercial aircraft starting with the A300 and A310. The new A380, one of the largest commercial aircraft ever, contains around 30% composite material by weight. T he Boeing 777 is about 20% composite while the new generation Boeing 787 is 50% composite by weight and boasts high efficiency and performance with reduced weight. Many different parts of the aircraft are now made of

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composites including wing skins, forward fuselage, flaperons, trailing-edge flaps, spoilers, airlerons, wheel doors, rear pressure bulkhead, and top and bottom skin panels.

Effort is now focused on more extensive use of composites in primary airc raft structure such as wing and fuselage structure to further reduce weight and operating costs. This introduces new technical challenges. One of the critical areas is bolted composite joints. Size limitations in fabrication, in addition to economic factors, have necessitated that structur es are manufactured as subcomponents which are subsequently assembled to produce a final vehicle. Due to the inability of adhesive bonds to transfer high mechanical loads between structural components, mechanically fastened joints are often used in design.

The joints, however, may be potential weak points in the structure as the bolt hole produces a stress concentration with the potential for crack initiation. The bolts impose a compressive load on the laminate , which can result in bearing failure by a combination of fibre buckling, shear and interlaminar splitting [2]. Since joints have such a critical effect on the safety and efficiency of the aircraft structure, it is vital that the most advanced design methods are used. The need to join composite panels through bolted joints has raised many issues not encountered with metallic joints and has been a major topic of research since the 1970s.

For composite laminates, the joint efficiency is substantially lower than for structural metals, being reduced by almost 50% [3]. To avoid catastrophic failure, joints are designed to fail in bearing and the design strength is then limited by the bearing performance of the composite [4, 5]. Bearing results in compression loading of the hole indicating that improved bearing performance should be achieved by increasing the compression strength of the composite. While the stiffness and tensile strength of a composite are influenced principally by the reinforcement , the shear and compressive strength are depende nt more on the resin matrix.

Rosen [6] , in a composite analysis, proposed that the compressive strength of a composite is proportional to the shear modulus of the matrix. By taking into account the matrix non-linearity and fibre misalignment, Sun and Jun [7] concluded that the

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modulus and to the elastic-plastic shear modulus of the composite. This implies that improved bearing performance could be achieved by using a matrix resin with a higher modulus. However the conventional method for improving the modulus is to increase the level of cross linking which reduces toughness. As a result the increased elastic performance is offset by decreased resistance to matrix cracking.

In recent years there has been intense interest in the development of polymer nanocomposites. These are a combination of a polymer matrix and nanoparticles having at least one dimension of less than 100 nm. A variety of polymers ha s been used as the matrix, including polyurethane [8, 9], polyimide [10, 11] , polyamide [12, 13] , polyester [14, 15] , and epoxy resin [16, 17]. Many nanometer-sized particles have been introduced into the matrix, including metal oxides, such as T iO2[18, 19] , SiO2[20, 21] , Al2O3[22] and ZnO [23] and metallic powders, such as silver [24, 25] and gold [26, 27]. In particular, nanoclay particles have been widely used as the reinforcement in nanocomposites because of their special structure, high aspect ratio, high surface area, high strength and low cost [28-31]. Nanocomposites are promising materials for many industries and are being considered for automotive applications [32, 33], flame retardant materials [34, 35], electronics and electrical engineering [36-38] , packaging and containers for food [39].

When uniformly dispersed at a molecular scale, these nanometer-sized fillers can provide substantial improvements in mechanical properties [40] , thermal properties [35], and barrier performance [17, 41] even at very low volume fraction loadings (1-5%). These contrast with the high volume fraction loadings (50-70%) of micro-particles used in conventional composites. In particular, the introduction of nanoparticles into a polymer matrix is reported to simultaneously increase both stiffness and toughness providing a potential strategy for improving bearing performance. Additionally, because of the low loadings, there is only minimal change in weight. However, to date no studies have been conducted to examine whether bearing performance can be improved.

Bearing fa ilure involves shear displacement beneath the hole resulting in outward lateral movement [7]. This suggests that an alternative strategy for improving bearing performance would be the use of through thickness reinforcement. 3D reinforcement

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can be produced by techniques such as weaving [42, 43], braiding [44, 45] , stitching [46, 47] , and knitting [48, 49] and its use in composite aircraft components has attracted significant attention since the late 1960s , particularly because of the potential for producing complex shaped structures [50]. However, the cost of manufacture, poor understanding of the in-plane properties and a lack of characterisation of the failure mechanisms have greatly limited the use of 3D fibre composites to date.

An effective alternative to through-thickness stitching of composites in the z-direction is through-thickness z-pin reinforcement of traditional two dimension composites [51]. The z-pins are short fibres made of carbon or metal, inserted perpendicular to the stack of prepreg plies during the fabrication pr ocess prior to cur ing the laminate. The presence of z-pins improves the out-of-plane properties of the composite as they effectively link the individual layers together, preventing delamination crack growth. However, the high cost has limited their application. Only limited studies on the use of z-pins have been made and these have been mostly restricted to T-joints [52-54] or areas subject to impact damage [55-57]. However one study has been made very recently on the effect of z-pinning on bearing behaviour [58]. While the z-pins increased the load to failure , the bearing strength was not increased because of local thickening around the hole. None the less the technology looked promising.

The present study examines both the use of nanoparticle matrix resin and the use of through thickness reinforcement using z-pinning to improve bearing strength of fibre reinforced composites. While the z-pins were available commercially, nanoreinforced matrix resin was not, and it was therefore necessary to develop the nanoreinforced resin as part of the work.

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CHAPTER 2

LITERATURE REVIEW

2.1. INTRODUCTION

This chapter reviews the literature pertinent to this thesis. It begins with a discussion of bearing behaviour of composites with a review of the mechanisms that have been proposed for bearing failure and an examination of the factors that affect bearing performance. The use of nanoparticle reinforced matrix resin was considered as a potential strategy for improving bearing performance and a detailed review of nanoparticle reinforced composites (nanocomposites) is then presented. A second potential strategy, namely through thickness reinforcement using z-pins, was also trialed in this study and a detailed review of this technology is also included. Finally, the goals of the project are presented.

2.2. BEARING PERFORMANCE

2.2.1. Introduction

For over 30 years, there has been a steady increase in the use of composite materials , typically carbon and glass fibre reinforced plastics (CFRP and GFRP respectively), in the aerospace industry. Initially, composites were used only in non-structural parts and

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secondary structures. With further development and improved understanding of the materials, polymer composites have increasingly found use in primary aircraft structures, such as wings, and fuselages, Figure 2.1 [1]. Some of the potential benefits of using composite materials are substantial weight savings, a reduced number of joining operations during assembly, reduced inspection and reduced parts storage and movement, which collectively result in increased reliability and lower operating costs.

Fig. 2.1 Materials used in F/A 18 fighter aircraft (composites are shown in pink) [1].

However, realizing the full value of this potential still presents many technical challenges. One of them is mechanically fastened composite joints. Bolted connections are commonly used in preference to other joining techniques because they allow greater freedom in assembly and repair. An example is shown in Figure 2.2 [2]. The holes in the joints are however potential weak points in the structure. Additionally, introducing a bolted joint into a laminate introduces a new potential failure mode against which the laminate is relatively weak. The bolt imposes a compression load on the laminate and this can result in failure in bearing by a combination of fibre buckling, shear and interlaminar splitting.

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Fig. 2.2 Schematic of joints used in hybrid metal/composite wing [2].

The efficiency J of a single row fastened joint in a panel is a the ratio of the maximum bearing load PB that can be transferred by the bolt to the laminate and the ultimate tensile

load PTU for the laminate remote from the hole. It is a function of the tensile strength FTU

and bearing strength FB of the laminate, the width w and thickness t of the specimen, and

the diameter d and local thickness tB of the fastener hole. The joint efficiency J is given by:

wt F dt F wt F P P P J TU B TU B TU B = = = (1)

For composite laminates, the joint efficiency is substantially lower than for structural metals [3] , as shown in Figure 2.3. The lower efficiency is caused by many factors, such as brittleness, which leads to only minimal stress relief around the highest loaded holes, anisotropy, which leads to higher stress concentration factors, low transverse strength, susceptibility to delamination, and sensitivity to environmental conditions. Combined with these factors, the composite failure modes make the analysis and design of composites

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Fig. 2.3 Joint efficiency of different materials (after Hart-Smith [3]).

Indeed, there are three different failure modes that can occur in bolted composite joints, namely net tension, shear-out and bearing failure, as shown in Figure 2. 4 [4]. Net tension and shear-out failures are total structural failures in which the joint components are separated. The net tension failure mode is characterized by fracture of the laminate across its width from the hole to the edges. The load required to fracture the laminate through a cross-section containing holes is less than for a section where there are no geometric irregularities [5, 6]. The shear-out failure mode is characterized by a “pull-out” fracture between the hole and laminate end, as a result of a shear stress concentration that the laminate is unable to adequately support [7]. The bearing failure mode is observed as a form of accumulated compressive damage in the laminate adjacent to the hole [8] and is much less catastrophic.

P

B

w

d

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.2 0.4 0.6 0.8 1 d/w PB /FTU wt Ductile metal Brittle material Composite

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Fig. 2.4 Bolted joint failure modes [4].

Geometric factors usually render one of the failure modes dominant. These factors include specimen width (w), edge distance (e), hole diameter (d), and thickness (t). For a given laminate, the net tension failure mode is associated with insufficient w/d ratio, where the specimen is not wide enough to prevent a tension failure. The shear-out failure mode is dependant on the e/d ratio; the edge distance can be made larger to prevent this type of failure. For sufficiently large e/d and w/d, the bearing failure mode will result. Various types of compression and bearing failures are illustrated in Figure 2. 5.

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An understanding of the mechanisms, as well as the factors affecting bearing failure of composites, is important in order to develop suitable strategies for improving bearing performance.

2.2.2. Failure mechanism

The mechanism of bearing failure is very complicated, being a combination of several different fracture mechanisms occurring in the individual component materials and their interfaces, such as microbuckling, kinking, shear-cracking and delamination. Bearing failure in bolted composites is a process of damage accumulation, which is divided into four stages: damage onset, damage growth, local fracture and final structural fracture, as shown in Figure 2. 6 [9].

Fig. 2.6 Bearing damage process as detected by acoustic emission (AE) [9].

In a study by Xiao and Ishikawa [9] , it was found that micro-damage, such as fibre microbuckling and matrix cracking started to occur when the applied load reached 50% of the maximum load. Only slight local delamina tion was detected in the epoxy composite laminate. When the load reached 60% of the failure load, the regions of micro-damage

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became more extensive around the hole because of the propagation of kink-bands and the formation of local delaminations. At 75% of the failure load, the extent of the damage became more pronounced as distinct through-thickness shear cracks expanded. In the final fracture, large-scale delaminations and shear cracks dominated.

These results were similar to the previous findings of Ireman [10] in his study of the damage process, as illustrated in Figure 2. 7. Matrix cracks occurred in the resin rich surface layer at 25% of the failure load. The resin-rich layer chipped off at the hole edge. More severe damage was observed at approximately 35% of the failure load, where fibre fracture