3D Fibre Reinforced Polymer Composites Liyong Tong School of Aerospace,Mechanical and Mechatronic Engineering, University of Sydney,Sydney,Australia Adrian P.Mouritz Department of Aerospace Engineering, Royal Melbourne Institute of Technology,Melbourne,Australia Michael K.Bannister Cooperative Research Centre for Advanced Composite Structures Ltd & Department of Aerospace Engineering, Royal Melbourne Institute of Technology,Melbourne,Australia 2002 ELSEVIER AMSTERDAM-BOSTON-LONDON-NEW YORK-OXFORD-PARIS SAN DIEGO-SAN FRANCISCO-SINGAPORE-SYDNEY-TOKYO
3D Fibre Reinforced Polymer Composites Liyong Tong School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, Sydney, Australia Adrian P. Mouritz Department of Aerospace Engineering, Royal Melbourne Institute of Technology, Melbourne, Australia Michael K. Bannister Cooperative Research Centre for Advanced Composite Structures Ltd & Department of Aerospace Engineering, Royal Melbourne Institute of Technology, Melbourne, Australia 2002 ELSEVIER AMSTERDAM - BOSTON - LONDON -NEW YORK - OXFORD - PARIS SAN DIEGO - SAN FRANCISCO - SINGAPORE - SYDNEY - TOKYO
Preface Fibre reinforced polymer(FRP)composites are used in almost every type of advanced engineering structure,with their usage ranging from aircraft,helicopters and spacecraft through to boats,ships and offshore platforms and to automobiles,sports goods, chemical processing equipment and civil infrastructure such as bridges and buildings. The usage of FRP composites continues to grow at an impressive rate as these materials are used more in their existing markets and become established in relatively new markets such as biomedical devices and civil structures.A key factor driving the increased applications of composites over recent years is the development of new advanced forms of FRP materials.This includes developments in high performance resin systems and new styles of reinforcement,such as carbon nanotubes and nanoparticles.A major driving force has been the development of advanced FRP composites reinforced with a three-dimensional(3D)fibre structure.3D composites were originally developed in the early 1970s,but it has only been in the last 10-15 years that major strides have been made to develop these materials to a commercial level where they can be used in both traditional and emerging markets. The purpose of this book is to provide an up-to-date account of the fabrication. mechanical properties,delamination resistance,impact damage tolerance and applications of 3D FRP composites.The book will focus on 3D composites made using the textile technologies of weaving,braiding,knitting and stitching as well as by z- pinning.This book is intended for undergraduate and postgraduate students studying composite materials and also for the researchers,manufacturers and end-users of composites. Chapter I provides a general introduction to the field of advanced 3D composites. The chapter begins with a description of the key economic and technology factors that are providing the impetus for the development of 3D composites.These factors include lower manufacturing costs,improved material quality,high through-thickness properties,superior delamination resistance,and better impact damage resistance and post-impact mechanical properties compared to conventional laminated composites. The current and potential applications of 3D composites are then outlined in Chapter 1, including a description of the critical issues facing their future usage. Chapter 2 gives a description of the various weaving,braiding,knitting and stitching processes used to manufacture 3D fabrics that are the preforms to 3D composites.The processes that are described range from traditional textile techniques that have been used for hundreds of years up to the most recent textile processes that are still under development.Included in the chapter is an examination of the affect the processing parameters of the textile techniques have on the quality and fibre architecture of 3D composites. The methods and tooling used to consolidate 3D fabric preforms into FRP composites are described in Chapter 3.The liquid moulding methods used for consolidation include resin transfer moulding,resin film infusion and SCRIMP.The benefits and limitations of the different consolidation processes are compared for producing 3D composites.Chapter 3 also gives an overview of the different types of processing defects (eg.voids,dry spots,distorted binder yams)that can occur in 3D composites using liquid moulding methods
Preface Fibre reinforced polymer (FRP) composites are used in almost every type of advanced engineering structure, with their usage ranging from aircraft, helicopters and spacecraft through to boats, ships and offshore platforms and to automobiles, sports goods, chemical processing equipment and civil infrastructure such as bridges and buildings. The usage of FRP composites continues to grow at an impressive rate as these materials are used more in their existing markets and become established in relatively new markets such as biomedical devices and civil structures. A key factor driving the increased applications of composites over recent years is the development of new advanced forms of FRP materials. This includes developments in high performance resin systems and new styles of reinforcement, such as carbon nanotubes and nanoparticles. A major driving force has been the development of advanced FRP composites reinforced with a three-dimensional (3D) fibre structure. 3D composites were originally developed in the early 1970s, but it has only been in the last 10- 15 years that major strides have been made to develop these materials to a commercial level where they can be used in both traditional and emerging markets. The purpose of this book is to provide an up-to-date account of the fabrication, mechanical properties, delamination resistance, impact damage tolerance and applications of 3D FRP composites. The book will focus on 3D composites made using the textile technologies of weaving, braiding, knitting and stitching as well as by zpinning. This book is intended for undergraduate and postgraduate students studying composite materials and also for the researchers, manufacturers and end-users of composites. Chapter 1 provides a general introduction to the field of advanced 3D composites. The chapter begins with a description of the key economic and technology factors that are providing the impetus for the development of 3D composites. These factors include lower manufacturing costs, improved material quality, high through-thickness properties, superior delamination resistance, and better impact damage resistance and post-impact mechanical properties compared to conventional laminated composites. The current and potential applications of 3D composites are then outlined in Chapter 1, including a description of the critical issues facing their future usage. Chapter 2 gives a description of the various weaving, braiding, knitting and stitching processes used to manufacture 3D fabrics that are the preforms to 3D composites. The processes that are described range from traditional textile techniques that have been used for hundreds of years up to the most recent textile processes that are still under development. Included in the chapter is an examination of the affect the processing parameters of the textile techniques have on the quality and fibre architecture of 3D composites. The methods and tooling used to consolidate 3D fabric preforms into FRP composites are described in Chapter 3. The liquid moulding methods used for consolidation include resin transfer moulding, resin film infusion and SCRIMP. The benefits and limitations of the different consolidation processes are compared for producing 3D composites. Chapter 3 also gives an overview of the different types of processing defects (eg. voids, dry spots, distorted binder yams) that can occur in 3D composites using liquid moulding methods
A review of micro-mechanical models that are used or have a potential to be used to theoretically analyse the mechanical properties of 3D textile composites is presented in Chapter 4.Models for determining the in-plane elastic modulus of 3D composites are described,including the Eshlby,Mori-Tanaka,orientation averaging,binary and unit cell methods.Models for predicting the failure strength are also described,such as the unit cell,binary and curved beam methods.The accuracy and limitations of models for determining the in-plane properties of 3D composites are assessed,and the need for more reliable models is discussed. The performance of 3D composites made by weaving,braiding,knitting,stitching and z-pinning are described in Chapters 5 to 9,respectively.The in-plane mechanical properties and failure mechanisms of 3D composites under tension,compression, bending and fatigue loads are examined.Improvements to the interlaminar fracture toughness,impact resistance and damage tolerance of 3D composites are also described in detail.In these chapters the gaps in our understanding of the mechanical performance and through-thickness properties of 3D composites are identified for future research. We thank our colleagues with whom we have researched and developed 3D composites over the last ten years,in particular to Professor I.Herszberg,Professor G.P. Steven,Dr P.Tan,Dr K.H.Leong,Dr P.J.Callus,Dr P.Falzon,Mr K.Houghton,Dr L.K.Jain and Dr B.N.Cox.We are thankful to many colleagues,in particular to Professors T.-W.Chou,O.O.Ochoa,and P.Smith,for their kind encouragement in the initiation of this project.We are indebted to the University of Sydney,the Royal Melbourne Institute of Technology and the Cooperative Research Centre for Advanced Composite Structures Ltd.for allowing the use of the facilities we required in the preparation of this book.LT and APM are grateful for funding support of the Australian Research Council (Grant No.C00107070,DP0211709),Boeing Company, and Boeing (Hawker de Havilland)as well as the Cooperative Research Centre for Advanced Composite Structures Ltd.We are also thankful to the many organisations that kindly granted permission to use their photographs,figures and diagrams in the b00k. L.Tong School of Aerospace,Mechanical Mechatronic Engineering University of Sydney A.P.Mouritz Department of Aerospace Engineering Royal Melbourne Institute of Technology M.K.Bannister Cooperative Research Centre for Advanced Composite Structures Ltd & Department of Aerospace Engineering Royal Melbourne Institute of Technology
A review of micro-mechanical models that are used or have a potential to be used to theoretically analyse the mechanical properties of 3D textile composites is presented in Chapter 4. Models for determining the in-plane elastic modulus of 3D composites are described, including the Eshlby, Mori-Tanaka, orientation averaging, binary and unit cell methods. Models for predicting the failure strength are also described, such as the unit cell, binary and curved beam methods. The accuracy and limitations of models for determining the in-plane properties of 3D composites are assessed, and the need for more reliable models is discussed. The performance of 3D composites made by weaving, braiding, knitting, stitching and z-pinning are described in Chapters 5 to 9, respectively. The in-plane mechanical properties and failure mechanisms of 3D composites under tension, compression, bending and fatigue loads are examined Improvements to the interlaminar fkacture toughness, impact resistance and damage tolerance of 3D composites are also described in detail. In these chapters the gaps in our understanding of the mechanical performance and through-thickness properties of 3D composites are identified for future research. We thank our colleagues with whom we have researched and developed 3D composites over the last ten years, in particular to Professor I. Herszberg, Professor G.P. Steven, Dr P. Tan, Dr K.H. Leong, Dr P.J. Callus, Dr P. Falzon, Mr K. Houghton, Dr L.K. Jain and Dr B.N. Cox. We are thankful to many colleagues, in particular to Professors T.-W. Chou, 0.0. Ochoa, and P. Smith, for their kind encouragement in the initiation of this project. We are indebted to the University of Sydney, the Royal Melbourne Institute of Technology and the Cooperative Research Centre for Advanced Composite Structures Ltd. for allowing the use of the facilities we required in the preparation of this book. LT and APM are grateful for funding support of the Australian Research Council (Grant No. C00107070, DP0211709), Boeing Company, and Boeing (Hawker de Havilland) as well as the Cooperative Research Centre for Advanced Composite Structures Ltd. We are also thankful to the many organisations that kindly granted permission to use their photographs, figures and diagrams in the book. L. Tong School of Aerospace, Mechanical & Mechatronic Engineering University of Sydney A.P. Mouritz Department of Aerospace Engineering Royal Melbourne Institute of Technology M.K. Bannister Cooperative Research Centre for Advanced Composite Structures Ltd & Department of Aerospace Engineering Royal Melbourne Institute of Technologv
Table of Contents Preface vii Chapter 1 Introduction 1.1 Background 1 1.2 Introduction to 3D FRP Composites 6 1.2.1 Applications of 3D Woven Composites 1.2.2 Applications of 3D Braided Composites 10 1.2.3 3D Knitted Composites 1.2.4 3D Stitched Composites 11 1.2.5 3D Z-Pinned composites 12 Chapter 2 Manufacture of 3D Fibre Preforms 2.1 Introduction 1 2.2 Weaving 2.2.1 Conventional Weaving 2.2.2 Multilayer or 3D Weaving 2.2.3 3D Orthogonal Non-Wovens 2.2.4 Multiaxial Weaving 2.2.5 Distance Fabrics 2.3 Braiding 2.3.1 2D Braiding 2.3.2 Four-Step 3D Braiding 2.3.3 Two-Step 3D Braiding 2.3.4 Multilayer Interlock Braiding 2.4 Knitting 2.4.1 Warp and Weft Knitting 13151922245912267 2.4.2 Three-Dimensional Shaping 2.4.3 Non-Crimp Fabrics 2.5 Stitching 2.5.1 Traditional Stitching 2.5.2 Technical Embroidery 2.5.3 Z-Pinning 005355 2.6 Summary Chapter 3 Preform Consolidation 41 3.1 Introduction 3.2 Liquid Moulding Techniques 48 3.2.1 Resin Transfer Moulding 48 3.2.2 Resin Film Infusion 3.2.3 SCRIMP-based Techniques 51 3.3 Injection Equipment 3.4 Resin Selection 3.5 Preform Considerations 2467 3.6 Tooling
Table of Contents Preface vii Chapter 1 Introduction 1.1 Background 1.2 Introduction to 3D FRP Composites 1.2.1 Applications of 3D Woven Composites 1.2.2 Applications of 3D Braided Composites 1.2.3 3D Knitted Composites 1.2.4 3D Stitched Composites 1.2.5 3D 2-Pinned composites Chapter 2 Manufacture of 3D Fibre Preforms 2.1 Introduction 2.2 Weaving 2.2.1 Conventional Weaving 2.2.2 Multilayer or 3D Weaving 2.2.3 3D Orthogonal Non-Wovens 2.2.4 Multiaxial Weaving 2.2.5 Distance Fabrics 2.3.1 2D Braiding 2.3.2 Four-Step 3D Braiding 2.3.3 Two-step 3D Braiding 2.3.4 Multilayer Interlock Braiding 2.4.1 Warp and Weft Knitting 2.4.2 Three-Dimensional Shaping 2.4.3 Non-Crimp Fabrics 2.5.1 Traditional Stitching 2.5.2 Technical Embroidery 2.5.3 2-Pinning 2.3 Braiding 2.4 Knitting 2.5 Stitching 2.6 Summary Chapter 3 Preform Consolidation 3.1 Introduction 3.2 Liquid Moulding Techniques 3.2.1 Resin Transfer Moulding 3.2.2 Resin Film Infusion 3.2.3 SCRIMP-based Techniques 3.3 Injection Equipment 3.4 Resin Selection 3.5 Preform Considerations 3.6 Tooling 1 1 6 7 10 11 11 12 13 13 13 13 15 19 22 22 22 24 25 29 31 32 32 36 37 40 40 43 45 45 47 47 48 48 49 51 52 54 56 57
3.6.1 Tool Materials 3.6.2 Heating and Cooling 3.6.3 Resin Injection and Venting 3.6.4 Sealing 3.7 Component Quality 7383869606 3.8 Summary Chapter 4 Micromechanics Models for Mechanical Properties 63 4.1 Introduction 4.2 Fundamentals in Micromechanics 4.2.1 Generalized Hooke's Law 4.2.2 Representative Volume Element and Effective Properties 4.2.3 Rules of Mixtures and Mori-Tanaka Theory 4.2.4 Unit Cell Models for Textile Composites 4.3 Unit Cell Models for 2D Woven Composites 4.3.1 One-Dimensional (ID)Models 4.3.2 Two-Dimensional(2D)Models 4.3.3 Three-Dimensional (3D)Models 4.3.4 Applications of Finite Element Methods 4.4 Models for 3D Woven Composites 4.4.1 Orientation Averaging Models 4.4.2 Mixed Iso-Stress and Iso-Strain Models 4.4.3 Applications of Finite Element Methods 4.4.3.1 3D Finite Element Modelling Scheme 4.4.3.2 Binary Models 6446600738809269901004 4.5 Unit Cell Models for Braided and Knitted Composites 4.5.1 Braided Composites 4.5.2 Knitted Composites 4.6 Failure Strength Prediction Chapter 5 3D Woven Composites 5.】Introduction 5.2 Microstructural Properties of 3D Woven Composites 5.3 In-Plane Mechanical Properties of 3D Woven Composites 00W3 5.3.1 Tensile Properties 5.3.2 Compressive Properties 5.3.3 Flexural Properties 126 5.3.4 Interlaminar Shear Properties 5.4 Interlaminar Fracture Properties of 3D Woven Composites 128 5.5 Impact Damage Tolerance of 3D Woven Composites 132 5.6 3D Woven Distance Fabric Composites 133 Chapter 6 Braided Composite Materials 137 6.1 Introduction 137 6.2 In-Plane Mechanical Properties 138 6.2.1 Influence of Braid Pattern and Edge Condition 138 6.2.2 Influence of Braiding Process 6.2.3 Influence of Yarn Size 9 6.2.4 Comparison with 2D Laminates 143
3.6.1 Tool Materials 3.6.2 Heating and Cooling 3.6.3 Resin Injection and Venting 3.6.4 Sealing 3.7 Component Quality 3.8 Summary Chapter 4 Micromechanics Models for Mechanical Properties 4.1 Introduction 4.2 Fundamentals in Micromechanics 4.2.1 Generalized Hooke’s Law 4.2.2 Representative Volume Element and Effective Properties 4.2.3 Rules of Mixtures and Mori-Tanaka Theory 4.2.4 Unit Cell Models for Textile Composites 4.3 Unit Cell Models for 2D Woven Composites 4.3.1 One-Dimensional (1D) Models 4.3.2 Two-Dimensional (2D) Models 4.3.3 Three-Dimensional (3D) Models 4.3.4 Applications of Finite Element Methods 4.4 Models for 3D Woven Composites 4.4.1 Orientation Averaging Models 4.4.2 Mixed Iso-Stress and Iso-Strain Models 4.4.3 Applications of Finite Element Methods 4.4.3.1 3D Finite Element Modelling Scheme 4.4.3.2 Binary Models 4.5.1 Braided Composites 4.5.2 Knitted Composites 4.6 Failure Strength Prediction 4.5 Unit Cell Models for Braided and Knitted Composites Chapter 5 3D Woven Composites 5.1 Introduction 5.2 Microstructural Properties of 3D Woven Composites 5.3 In-Plane Mechanical Properties of 3D Woven Composites 5.3.1 Tensile Properties 5.3.2 Compressive Properties 5.3.3 Flexural Properties 5.3.4 Interlaminar Shear Properties 5.4 Interlaminar Fracture Properties of 3D Woven Composites 5.5 Impact Damage Tolerance of 3D Woven Composites 5.6 3D Woven Distance Fabric Composites Chapter 6 Braided Composite Materials 6.1 Introduction 6.2 In-Plane Mechanical Properties 6.2.1 Influence of Braid Pattern and Edge Condition 6.2.2 Influence of Braiding Process 6.2.3 Influence of Yarn Size 6.2.4 Comparison with 2D Laminates 57 58 58 59 60 61 63 63 64 64 66 68 70 70 71 78 81 88 90 91 92 96 97 99 100 100 103 104 1 07 1 07 108 113 113 123 126 127 128 132 133 137 137 138 138 140 141 143
6.3 Fracture Toughness and Damage Performance 143 6.4 Fatigue Performance 145 6.5 Modelling of Braided Composites 145 6.6 Summary 146 Chapter 7 Knitted Composite Materials 147 7.1 Introduction 147 7.2 In-Plane Mechanical Properties 149 7.2.1 Tensile Properties 149 7.2.2 Compressive Properties 154 7.2.3 In-Plane Properties of Non-Crimp Fabrics 156 7.3 Interlaminar Fracture Toughness 158 7.4 Impact Performance 159 7.4.1 Knitted Composites 159 7.4.2 Non-Crimp Composites 161 7.5 Modelling of Knitted Composites 161 7.6 Summary 162 Chapter 8 Stitched Composites 163 8.1 Introduction to Stitched Composites 163 8.2 The Stitching Process 164 8.3 Mechanical Properties of Stitched Composites 169 8.3.1 Introduction 169 8.3.2 Tension,Compression and Flexure Properties of Stitched Composites 170 8.3.3 Interlaminar Shear Properties of Stitched Composites 176 8.3.4 Creep Properties of Stitched Composites 178 8.3.5 Fatigue Properties of Stitched Composites 179 8.4 Interlaminar Properties of Stitched Composites 182 8.4.1 Mode I Interlaminar Fracture Toughness Properties 182 8.4.2 Mode II Interlaminar Fracture Toughness Properties 189 8.5 Impact Damage Tolerance of Stitched Composites 195 8.5.1 Low Energy Impact Damage Tolerance 195 8.5.2 Ballistic Impact Damage Tolerance 199 8.5.3 Blast Damage Tolerance 200 8.6 Stitched Composite Joints 201 Chapter 9 Z-Pinned Composites 205 9.1 Introduction 205 9.2 Fabrication of Z-Pinned Composites 206 9.3 Mechanical Properties of Z-Pinned Composites 209 9.4 Delamination Resistance and Damage Tolerance of Z-Pinned Composites 211 9.5 Z-Pinned Joints 216 9.6 Z-Pinned Sandwich Composites 217 References 219 Subject Index 237
6.3 Fracture Toughness and Damage Performance 6.4 Fatigue Performance 6.5 Modelling of Braided Composites 6.6 Summary Chapter 7 Knitted Composite Materials 7.1 Introduction 7.2 In-Plane Mechanical Properties 7.2.1 Tensile Properties 7.2.2 Compressive Properties 7.2.3 In-Plane Properties of Non-Crimp Fabrics 7.3 Interlaminar Fracture Toughness 7.4 Impact Performance 7.4.1 Knitted Composites 7.4.2 Non-Crimp Composites 7.5 Modelling of Knitted Composites 7.6 Summary Chapter 8 Stitched Composites 8.1 Introduction to Stitched Composites 8.2 The Stitching Process 8.3 Mechanical Properties of Stitched Composites 8.3.1 Introduction 8.3.2 Tension, Compression and Rexure Properties of Stitched Composites 8.3.3 Interlaminar Shear Properties of Stitched Composites 8.3.4 Creep Properties of Stitched Composites 8.3.5 Fatigue Properties of Stitched Composites 8.4 Interlaminar Properties of Stitched Composites 8.4.1 Mode I Interlaminar Fracture Toughness Properties 8.4.2 Mode 11 Interlaminar Fracture Toughness Properties 8.5.1 Low Energy Impact Damage Tolerance 8.5.2 Ballistic Impact Damage Tolerance 8.5.3 Blast Damage Tolerance 8.5 Impact Damage Tolerance of Stitched Composites 8.6 Stitched Composite Joints Chapter 9 Z-Pinned Composites 9.1 Introduction 9.2 Fabrication of Z-Pinned Composites 9.3 Mechanical Properties of Z-Pinned Composites 9.4 Delamination Resistance and Damage Tolerance of Z-Pinned Composites 9.5 Z-Pinned Joints 9.6 Z-Pinned Sandwich Composites 143 145 145 146 147 147 149 149 154 156 158 159 159 161 161 162 163 163 164 169 169 170 176 178 179 182 182 189 195 195 199 200 20 1 205 205 206 209 21 1 216 217 References 219 Subject Index 237
Chapter 1 Introduction 1.1 BACKGROUND Fibre reinforced polymer(FRP)composites have emerged from being exotic materials used only in niche applications following the Second World War,to common engineering materials used in a diverse range of applications.Composites are now used in aircraft,helicopters,space-craft,satellites,ships,submarines,automobiles,chemical processing equipment,sporting goods and civil infrastructure,and there is the potential for common use in medical prothesis and microelectronic devices.Composites have emerged as important materials because of their light-weight,high specific stiffness, high specific strength,excellent fatigue resistance and outstanding corrosion resistance compared to most common metallic alloys,such as steel and aluminium alloys.Other advantages of composites include the ability to fabricate directional mechanical properties,low thermal expansion properties and high dimensional stability.It is the combination of outstanding physical,thermal and mechanical properties that makes composites attractive to use in place of metals in many applications,particularly when weight-saving is critical. FRP composites can be simply described as multi-constituent materials that consist of reinforcing fibres embedded in a rigid polymer matrix.The fibres used in FRP materials can be in the form of small particles,whiskers or continuous filaments.Most composites used in engineering applications contain fibres made of glass,carbon or aramid.Occasionally composites are reinforced with other fibre types,such as boron, Spectra or thermoplastics.A diverse range of polymers can be used as the matrix to FRP composites,and these are generally classified as thermoset (eg.epoxy,polyester) or thermoplastic (eg.polyether-ether-ketone,polyamide)resins. In almost all engineering applications requiring high stiffness,strength and fatigue resistance,composites are reinforced with continuous fibres rather than small particles or whiskers.Continuous fibre composites are characterised by a two-dimensional(2D) laminated structure in which the fibres are aligned along the plane (x-&y-directions)of the material,as shown in Figure 1.1.A distinguishing feature of 2D laminates is that no fibres are aligned in the through-thickness (or z-)direction.The lack of through- thickness reinforcing fibres can be a disadvantage in terms of cost,ease of processing, mechanical performance and impact damage resistance. A serious disadvantage is that the current manufacturing processes for composite components can be expensive.Conventional processing techniques used to fabricate composites,such as wet hand lay-up,autoclave and resin transfer moulding,require a high amount of skilled labour to cut,stack and consolidate the laminate plies into a preformed component.In the production of some aircraft structures up to 60 plies of carbon fabric or carbon/epoxy prepreg tape must be individually stacked and aligned by hand.Similarly,the hulls of some naval ships are made using up to 100 plies of woven
Chapter 1 Introduction 1.1 BACKGROUND Fibre reinforced polymer (FRP) composites have emerged from being exotic materials used only in niche applications following the Second World War, to common engineering materials used in a diverse range of applications. Composites are now used in aircraft, helicopters, space-craft, satellites, ships, submarines, automobiles, chemical processing equipment, sporting goods and civil infrastructure, and there is the potential for common use in medical prothesis and microelectronic devices. Composites have emerged as important materials because of their light-weight, high specific stiffness, high specific strength, excellent fatigue resistance and outstanding corrosion resistance compared to most common metallic alloys, such as steel and aluminium alloys. Other advantages of composites include the ability to fabricate directional mechanical properties, low thermal expansion properties and high dimensional stability. It is the combination of outstanding physical, thermal and mechanical properties that makes composites attractive to use in place of metals in many applications, particularly when weight-saving is critical. FRP composites can be simply described as multi-constituent materials that consist of reinforcing fibres embedded in a rigid polymer matrix. The fibres used in FRP materials can be in the form of small particles, whiskers or continuous filaments. Most composites used in engineering applications contain fibres made of glass, carbon or aramid. Occasionally composites are reinforced with other fibre types, such as boron, Spectra@ or thermoplastics. A diverse range of polymers can be used as the matrix to FRP composites, and these are generally classified as thermoset (eg. epoxy, polyester) or thermoplastic (eg. polyether-ether-ketone, polyamide) resins. In almost all engineering applications requiring high stiffness, strength and fatigue resistance, composites are reinforced with continuous fibres rather than small particles or whiskers. Continuous fibre composites are characterised by a two-dimensional (2D) laminated structure in which the fibres are aligned along the plane (x- & y-directions) of the material, as shown in Figure 1.1. A distinguishing feature of 2D laminates is that no fibres are aligned in the through-thickness (or z-) direction. The lack of throughthickness reinforcing fibres can be a disadvantage in terms of cost, ease of processing, mechanical performance and impact damage resistance. A serious disadvantage is that the current manufacturing processes for composite components can be expensive. Conventional processing techniques used to fabricate composites, such as wet hand lay-up, autoclave and resin transfer moulding, require a high amount of skilled labour to cut, stack and consolidate the laminate plies into a preformed component. In the production of some aircraft structures up to 60 plies of carbon fabric or carbodepoxy prepreg tape must be individually stacked and aligned by hand. Similarly, the hulls of some naval ships are made using up to 100 plies of woven
2 3D Fibre Reinforced Polymer Composites glass fabric that must be stacked and consolidated by hand.The lack of a z-direction binder means the plies must be individually stacked and that adds considerably to the fabrication time.Furthermore,the lack of through-thickness fibres means that the plies can slip during lay-up,and this can misalign the fibre orientations in the composite component.These problems can be alleviated to some extent by semi-automated processes that reduce the amount of labour,although the equipment is very expensive and is often only suitable for fabricating certain types of structures,such as flat and slightly curved panels.A further problem with fabricating composites is that production rates are often low because of the slow curing of the resin matrix,even at elevated temperature. Figure 1.1 Schematic of the fibre structure to a 2D laminate Fabricating composites into components with a complex shape increases the cost even further because some fabrics and many prepreg tapes have poor drape.These materials are not easily moulded into complex shapes,and as a result some composite components need to be assembled from a large number of separate parts that must be joined by co-curing,adhesive bonding or mechanical fastening.This is a major problem for the aircraft industry,where composite structures such as wing sections must be made from a large number of smaller laminated parts such as skin panels,stiffeners and stringers.These fabrication problems have impeded the wider use of composites in some aircraft structures because it is significantly more expensive than using aircraft- grade aluminium alloys. As well as high cost,another major disadvantage of 2D laminates is their low through-thickness mechanical properties because of the lack of z-direction fibres.The two-dimensional arrangement of fibres provides very little stiffness and strength in the through-thickness direction because these properties are determined by the low mechanical properties of the resin and fibre-to-resin interface.A comparison of the in- plane and through-thickness strengths of 2D laminates is shown in Figure 1.2.It is seen that the through-thickness properties are often less than 10%of the in-plane properties
2 30 Fibre Reinforced Polymer Composites glass fabric that must be stacked and consolidated by hand. The lack of a z-direction binder means the plies must be individually stacked and that adds considerably to the fabrication time. Furthermore, the lack of through-thickness fibres means that the plies can slip during lay-up, and this can misalign the fibre orientations in the composite component. These problems can be alleviated to some extent by semi-automated processes that reduce the amount of labour, although the equipment is very expensive and is often only suitable for fabricating certain types of structures, such as flat and slightly curved panels. A further problem with fabricating composites is that production rates are often low because of the slow curing of the resin matrix, even at elevated temperature. Y Figure 1.1 Schematic of the fibre structure to a 2D laminate Fabricating composites into components with a complex shape increases the cost even further because some fabrics and many prepreg tapes have poor drape. These materials are not easily moulded into complex shapes, and as a result some composite components need to be assembled from a large number of separate parts that must be joined by co-curing, adhesive bonding or mechanical fastening. This is a major problem for the aircraft industry, where composite structures such as wing sections must be made from a large number of smaller laminated parts such as skin panels, stiffeners and stringers. These fabrication problems have impeded the wider use of composites in some aircraft structures because it is significantly more expensive than using aircraftgrade aluminium alloys. As well as high cost, another major disadvantage of 2D laminates is their low through-thickness mechanical properties because of the lack of z-direction fibres. The two-dimensional arrangement of fibres provides very little stiffness and strength in the through-thickness direction because these properties are determined by the low mechanical properties of the resin and fibre-to-resin interface. A comparison of the inplane and through-thickness strengths of 2D laminates is shown in Figure 1.2. It is seen that the through-thickness properties are often less than 10% of the in-plane properties
Introduction 3 and for this reason 2D laminates can not be used in structures supporting high through- thickness or interlaminar shear loads 150 145 GPa In-Plane Property Through-Thickness Property 125 100 76 GPa 75 50 45 GPa 25 10 GPa 12 GPa 6 GPa Carbon/Epoxy E-glass/Epoxy Kevlar/Epoxy (a) In-Plane Proper ty 1400 Through-Thickness Proper ty 1240 MPa 1240nP3 1200 102084Pa (edW) 1000 800 600 400 200 41MPa 40 MPa 30MP9 Carbon/Epoxy E-glass/Epoxy Kevlar/Epoxy (b) Figure 1.2 Comparison of in-plane and through-thickness mechanical properties of some engineering composites
Introduction 3 1200 2 1000- 2 5 800 v cn c ; 000- 200: W S - '5 400 0- and for this reason 2D laminates can not be used in structures supporting high throughthickness or interlaminar shear loads. - - - W cn c W - .- I- 125 - 100 - 75 - 50 - 25 - 0- + L 6 GPa CarbodEpoxy E-glass/Epoxy Kevlar/Epoxy 1400 r In-Plane Property 0 Through-Thickness Property 1240 MPa CarbonIEpoxy + E-glass/Epoxy + 1240 MPa Kevlar/Epoxy Figure 1.2 Comparison of in-plane and through-thickness mechanical properties of some engineering composites
4 3D Fibre Reinforced Polymer Composites 1400 In-Plane Property 1240Pa 1200 Through-Thickness Property 1000 800 620 MPa 600 400 280 MPa 200 170 MPa 140 MPa 140 MPa 0 Carbon/Epoxy E-glass/Epoxy Kevlar/Epoxy (c) Figure 1.2 (continued)Comparison of in-plane and through-thickness mechanical properties of some engineering composites. A further problem with 2D laminates is their poor impact damage resistance and low post-impact mechanical properties.Laminates are prone to delamination damage when impacted by low speed projectiles because of the poor through-thickness strength.This is a major concern with composite aircraft structures where tools dropped during maintenance,bird strikes,hail impacts and stone impacts can cause damage.Similarly, the composite hulls to yachts,boats and ships can be damaged by impact with debris floating in the water or striking a wharf while moored in heavy seas.This damage can be difficult to detect,particularly when below the waterline,and can affect water- tightness and structural integrity of the hull.Impact damage can seriously degrade the in-plane mechanical properties under tension,compression,bending and fatigue loads. For example,the effect of impact loading on the tension and compression strengths of an aerospace grade carbon/epoxy laminate is shown in Figure 1.3.The strength drops rapidly with increasing impact energy,and even a lightweight impact of several joules can cause a large loss in strength.The low post-impact mechanical properties of 2D laminates is a major disadvantage,particularly when used in thin load-bearing structures such as aircraft fuselage and wing panels where the mechanical properties can be severely degraded by a small amount of damage.To combat the problem of delamination damage,composite parts are often over-designed with extra thickness. However,this increases the cost,weight and volume of the composite and in some cases may provide only moderate improvements to impact damage resistance. Various materials have been developed to improve the delamination resistance and post-impact mechanical properties of 2D laminates.The main impact toughening methods are chemical and rubber toughening of resins,chemical and plasma treatment of fibres,and interleaving using tough thermoplastic film.These methods are effective in improving damage resistance against low energy impacts,although each has a number of drawbacks that has limited their use in large composite structures.The
4 30 Fibre Reinforced Polymer Composites 1400 r 1200 m a z 1000 - In-Plane Property L---l Through-Thickness Property 620 MPa + Carbo n/Epoxy E-glass/Epoxy Kevlar/Epoxy (c> Figure 1.2 (continued) Comparison of in-plane and through-thickness mechanical properties of some engineering composites. A further problem with 2D laminates is their poor impact damage resistance and low post-impact mechanical properties. Laminates are prone to delamination damage when impacted by low speed projectiles because of the poor through-thickness strength. This is a major concern with composite aircraft structures where tools dropped during maintenance, bird strikes, hail impacts and stone impacts can cause damage. Similarly, the composite hulls to yachts, boats and ships can be damaged by impact with debris floating in the water or striking a wharf while moored in heavy seas. This damage can be difficult to detect, particularly when below the waterline, and can affect watertightness and structural integrity of the hull. Impact damage can seriously degrade the in-plane mechanical properties under tension, compression, bending and fatigue loads. For example, the effect of impact loading on the tension and compression strengths of an aerospace grade carbodepoxy laminate is shown in Figure 1.3. The strength drops rapidly with increasing impact energy, and even a lightweight impact of several joules can cause a large loss in strength. The low post-impact mechanical properties of 2D laminates is a major disadvantage, particularly when used in thin load-bearing structures such as aircraft fuselage and wing panels where the mechanical properties can be severely degraded by a small amount of damage. To combat the problem of delamination damage, composite parts are often over-designed with extra thickness. However, this increases the cost, weight and volume of the composite and in some cases may provide only moderate improvements to impact damage resistance. Various materials have been developed to improve the delamination resistance and post-impact mechanical properties of 2D laminates. The main impact toughening methods are chemical and rubber toughening of resins, chemical and plasma treatment of fibres, and interleaving using tough thermoplastic film. These methods are effective in improving damage resistance against low energy impacts, although each has a number of drawbacks that has limited their use in large composite structures. The