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