10 3D Fibre Reinforced Polymer Composites to Mach 8(~2600 m/s)(Kandero,2001).The 3D material is a ceramic-based composite consisting of 3D woven carbon fibres in a silicon carbide matrix.The 3D composite is used in the combustion chamber to the Scramjet engine.A key benefit of using 3D woven composite is the ability to manufacture the chamber as a single piece by 3D weaving,and this reduces connection issues and leakage problems associated with conventional fabrication methods. Apart from these aerospace applications,the only other uses of 3D woven composite is the very occasional use in the repair of damaged boat hulls,I-beams in the roof of a ski chair-lift building in Germany (Muller et al.,1994),manhole covers,sporting goods such as shin guards and helmets,and ballistic protection for police and military personnel (Mouritz et al.,1999).3D woven composite is not currently used as a biomedical material,although its potential use in leg prosthesis has been explored (Limmer et al.,1996). 1.2.2 Applications of 3D Braided Composites The braiding process is familiar to many fields of engineering as standard 2D braided carbon and glass fabric have been used for many years in a variety of high technology items,such as golf clubs,aircraft propellers and yacht masts (Popper,1991).3D braided preform has a number of important advantages over many types of 2D fabric preforms and prepreg tapes,including high levels of conformability,drapability, torsional stability and structural integrity.Furthermore,3D braiding processes are capable of forming intricately-shaped preforms and the process can be varied during operation to produce changes in the cross-sectional shape as well as to produce tapers, holes,bends and bifurcations in the final preform. Potential aerospace applications for 3D braided composites are listed in Table 1.2, and include airframe spars,F-section fuselage frames,fuselage barrels,tail shafts,rib stiffened panels,rocket nose cones,and rocket engine nozzles(Dexter,1996;Brown, 1991;Mouritz et al.,1999).A variety of other components have been made of 3D braided composite as demonstration items,including I-beams (Yau et al.,1986;Brown, 1991;Chiu et al.,1994;Fukuta,1995;Wulfhorst et al.,1995),bifurcated beams (Popper and McConnell,1987),connecting rods (Yau et al.,1986),and C-,J-and T-section panels (Ko,1984;Crane and Camponesch,1986;Macander et al.,1986;Gause and Alper,1987;Popper and McConnell,1987;Malkan and Ko,1989;Brookstein,1990; Brookstein,1991;Fedro and Willden,1991;Gong and Sankar,1991;Brookstein,1993; Dexter,1996). Table 1.2 Demonstrator components made with 3D braided composite. Airframe spars,fuselage frames and barrels Tail shafts Rib-stiffened,C-,T-and J-section panels Rocket nose cones and engine nozzles Beams and trusses Connecting rods Ship propeller blades Biomedical devices
10 3D Fibre Reinforced Polymer Composites to Mach 8 (-2600 ds) (Kandero, 2001). The 3D material is a ceramic-based composite consisting of 3D woven carbon fibres in a silicon carbide matrix. The 3D composite is used in the combustion chamber to the Scramjet engine. A key benefit of using 3D woven composite is the ability to manufacture the chamber as a single piece by 3D weaving, and this reduces connection issues and leakage problems associated with conventional fabrication methods. Apart from these aerospace applications, the only other uses of 3D woven composite is the very occasional use in the repair of damaged boat hulls, I-beams in the roof of a ski chair-lift building in Germany (Mtiller et al., 1994), manhole covers, sporting goods such as shin guards and helmets, and ballistic protection for police and military personnel (Mouritz et al., 1999). 3D woven composite is not currently used as a biomedical material, although its potential use in leg prosthesis has been explored (Limmer et al., 1996). 1.2.2 Applications of 3D Braided Composites The braiding process is familiar to many fields of engineering as standard 2D braided carbon and glass fabric have been used for many years in a variety of high technology items, such as golf clubs, aircraft propellers and yacht masts (Popper, 1991). 3D braided preform has a number of important advantages over many types of 2D fabric preforms and prepreg tapes, including high levels of conformability, drapability, torsional stability and structural integrity. Furthermore, 3D braiding processes are capable of forming intricately-shaped preforms and the process can be varied during operation to produce changes in the cross-sectional shape as well as to produce tapers, holes, bends and bifurcations in the final preform. Potential aerospace applications for 3D braided composites are listed in Table 1.2, and include airframe spars, F-section fuselage frames, fuselage barrels, tail shafts, rib stiffened panels, rocket nose cones, and rocket engine nozzles (Dexter, 1996; Brown, 1991; Mouritz et al., 1999). A variety of other components have been made of 3D braided composite as demonstration items, including I-beams (Yau et al., 1986; Brown, 1991; Chiu et al., 1994; Fukuta, 1995; Wulfhorst et al., 1995), bifurcated beams (Popper and McConnell, 1987), connecting rods (Yau et al., 1986), and C-, J- and T-section panels (KO, 1984; Crane and Camponesch, 1986; Macander et al., 1986; Gause and AIper, 1987; Popper and McConnell, 1987; Malkan and KO, 1989; Brookstein, 1990; Brookstein, 1991; Fedro and Willden, 1991; Gong and Sankar, 1991; Brookstein, 1993; Dexter, 1996). Table 1.2 Demonstrator components made with 3D braided composite. Airframe spars, fuselage frames and barrels Tail shafts Rib-stiffened, C-, T- and J-section panels Rocket nose cones and engine nozzles Beams and trusses Connecting rods Ship propeller blades Biomedical devices
Introduction 11 In the non-aerospace field,3D braided composite has been used in propeller blades for a naval landing craft (Maclander et al.,1986;Maclander,1992).There is also potential application for 3D braided composite on ships,such as in propulsion shafts and propellers(Mouritz et al.,2001).3D braided composite has been used in truss section decking for light-weight military bridges capable of carrying tanks and tank carriers (Loud,1999).Other potential applications include military landing pads,causeways, mass transport and highway bridge structures when bonded to pre-stressed concrete.3D braided composite also has potential uses in the bodies,chassis and drive shafts of automobiles because they are about 50%lighter than the same components made of steel but with similar damage tolerance and crashworthiness properties (Brandt and Drechsler,1995).3D braided composite has also been manufactured into a number of biomedical devices (Ko et al.,1988). 1.2.3 3D Knitted Composites 3D knitted composite has a number of important advantages over conventional 2D laminate,particularly very high drape properties and superior impact damage resistance. Despite these advantages,there are some drawbacks with 3D knitted material that has limited its application.A number of aircraft structures have been made of 3D knitted composite to demonstrate the potential of these materials,such as in wing stringers (Clayton et al.,1997),wing panels (Dexter,1996),jet engine vanes (Gibbon,1994; Sheffer Dias,1998).T-shape connectors (King et al.,1996)and I-beams (Sheffer Dias,1998).This composite is under investigation for the manufacture of the rear pressure bulkhead to the new Airbus A380 aircraft(Hinrichsen,2000).The potential use of 3D knitted composite in non-aerospace components includes bumper bars,floor panels and door members for automobiles (Hamilton and Schinske,1990),rudder tip fairings,medical prothesis (Mouritz et al.,1999),and bicycle helmets (Verpoest et al., 1997). 1.2.4 3D Stitched Composites The stitching of laminates in the through-thickness direction with a high strength thread has proven a simple,low-cost method for producing 3D composites.Stitching basically involves inserting a fibre thread (usually made of carbon,glass or Kevlar)through a stack of prepreg or fabric plies using an industrial grade sewing machine.The amount of through-thickness reinforcement in stitched composites is normally between I to 5%, which is a similar amount of reinforcement in 3D woven,braided and knitted composites. Stitching is used to reinforce composites in the z-direction to provide better delamination resistance and impact damage tolerance than conventional 2D laminates. Stitching can also be used to construct complex three-dimensional shapes by stitching a number of separate composite components together.This eliminates the need for mechanical fasteners,such as rivets,screws and bolts,and thereby reduces the weight and possibly the production cost of the component.If required,stitches can be placed only in areas that would benefit from through-thickness reinforcement,such as along the edge of a composite component,around holes,cut-outs or in a joint. A variety of 3D composite structures have been manufactured using stitching,and the more important stitched structures are lap joints,stiffened panels,and aircraft wing-
Introduction 11 In the non-aerospace field, 3D braided composite has been used in propeller blades for a naval landing craft (Maclander et al., 1986; Maclander, 1992). There is also potential application for 3D braided composite on ships, such as in propulsion shafts and propellers (Mouritz et al., 2001). 3D braided composite has been used in truss section decking for light-weight military bridges capable of carrying tanks and tank carriers (Loud, 1999). Other potential applications include military landing pads, causeways, mass transport and highway bridge structures when bonded to pre-stressed concrete. 3D braided composite also has potential uses in the bodies, chassis and drive shafts of automobiles because they are about 50% lighter than the same components made of steel but with similar damage tolerance and crashworthiness properties (Brandt and Drechsler, 1995). 3D braided composite has also been manufactured into a number of biomedical devices (KO et al., 1988). 1.2.3 3D Knitted Composites 3D knitted composite has a number of important advantages over conventional 2D laminate, particularly very high drape properties and superior impact damage resistance. Despite these advantages, there are some drawbacks with 3D knitted material that has limited its application. A number of aircraft structures have been made of 3D knitted composite to demonstrate the potential of these materials, such as in wing stringers (Clayton et al., 1997), wing panels (Dexter, 1996), jet engine vanes (Gibbon, 1994; Sheffer & Dias, 1998), T-shape connectors (King et al., 1996) and I-beams (Sheffer & Dias, 1998). This composite is under investigation for the manufacture of the rear pressure bulkhead to the new Airbus A380 aircraft (Hinrichsen, 2000). The potential use of 3D knitted composite in non-aerospace components includes bumper bars, floor panels and door members for automobiles (Hamilton and Schinske, 1990), rudder tip fairings, medical prothesis (Mouritz et al., 1999), and bicycle helmets (Verpoest et al., 1997). 1.2.4 3D Stitched Composites The stitching of laminates in the through-thickness direction with a high strength thread has proven a simple, low-cost method for producing 3D composites. Stitching basically involves inserting a fibre thread (usually made of carbon, glass or Kevlar) through a stack of prepreg or fabric plies using an industrial grade sewing machine. The amount of through-thickness reinforcement in stitched composites is normally between 1 to 5%, which is a similar amount of reinforcement in 3D woven, braided and knitted composites. Stitching is used to reinforce composites in the z-direction to provide better delamination resistance and impact damage tolerance than conventional 2D laminates. Stitching can also be used to construct complex three-dimensional shapes by stitching a number of separate composite components together. This eliminates the need for mechanical fasteners, such as rivets, screws and bolts, and thereby reduces the weight and possibly the production cost of the component. If required, stitches can be placed only in areas that would benefit from through-thickness reinforcement, such as along the edge of a composite component, around holes, cut-outs or in a joint. A variety of 3D composite structures have been manufactured using stitching, and the more important stitched structures are lap joints, stiffened panels, and aircraft wing-
12 3D Fibre Reinforced Polymer Composites to-spar joints (Cacho-Negrete,1982;Holt,1992;Lee and Liu,1990;Liu,1990;Sawyer, 1985;Tada and Ishikawa,1989;Tong et al.,1998;Whiteside et al.,1985).The feasibility of joining and reinforcing the wing and fuselage panels for large commercial aircraft using stitching has been evaluated as part of the ACT program (Palmer et al., 1991;Dexter,1992;Deaton et al.,1992;Jackson et al.,1992;Kullerd and Dow,1992; Markus,1992;Suarez and Dastin,1992;Jegley and Waters,1994;Smith et al.,1994). Stitching is being evaluated as a method for manufacturing the centre fuselage skin of Eurofighter (Bauer,2000).Stitching may be used for joining the stiffeners to fuselage panels on Eurofighter,and it is expected to reduce the component cost by 50% compared with similar stiffened panels made of prepreg laminate.Stitching is also being evaluated for the fabrication of the rear pressure bulkhead to the Airbus A380 aircraft,a component measuring 5.5 m by 6.2 m(Hinrichsen,2000). 1.2.5 3D Z-Pinned Composites In the early 1990s the Aztex Corporation developed and patented Z-fiberTM technology for reinforcing 2D laminates in the through-thickness direction(Freitas et al.,1994).Z- fibersTM are short pins made of metal wire or pultruded composite that can be inserted through uncured prepreg tapes or dry fabrics to create 3D composites. Z-pinning is a relatively new technology,and its full potential and applications is still being evaluated.Composite structures such as hat-stiffened and T-stiffened panels have been reinforced in the flange region with Z-fibresTM to demonstrate the effectiveness of z-pinning to increase joint strength.The localised reinforcement of flanges and joints with Z-fibersTM removes the need for fasteners or rivets and produces a more even load distribution over the joined area.Z-pinning is also being used to reinforce inlet duct skin panels and to fasten hat-shaped stiffeners to selected composite panels on the F/A-18 SuperHornet fighter aircraft
12 30 Fibre Reinforced Polymer Composites to-spar joints (Cacho-Negrete, 1982; Holt, 1992; Lee and Liu, 1990; Liu, 1990; Sawyer, 1985; Tada and Ishikawa, 1989; Tong et al., 1998; Whiteside et al., 1985). The feasibility of joining and reinforcing the wing and fuselage panels for large commercial aircraft using stitching has been evaluated as part of the ACT program (Palmer et al., 1991; Dexter, 1992; Deaton et al., 1992; Jackson et al., 1992; Kullerd and Dow, 1992; Markus, 1992; Suarez and Dastin, 1992; Jegley and Waters, 1994; Smith et al., 1994). Stitching is being evaluated as a method for manufacturing the centre fuselage skin of Eurofighter (Bauer, 2000). Stitching may be used for joining the stiffeners to fuselage panels on Eurofighter, and it is expected to reduce the component cost by 50% compared with similar stiffened panels made of prepreg laminate. Stitching is also being evaluated for the fabrication of the rear pressure bulkhead to the Airbus A380 aircraft, a component measuring 5.5 m by 6.2 m (Hinrichsen, 2000). 1.2.5 3D %Pinned Composites In the early 1990s the Aztex Corporation developed and patented Z-fiberm technology for reinforcing 2D laminates in the through-thickness direction (Freitas et al., 1994). 2- fibersTM are short pins made of metal wire or pultruded composite that can be inserted through uncured prepreg tapes or dry fabrics to create 3D composites. Z-pinning is a relatively new technology, and its full potential and applications is still being evaluated. Composite structures such as hat-stiffened and T-stiffened panels have been reinforced in the flange region with Z-fibresTM to demonstrate the effectiveness of z-pinning to increase joint strength. The localised reinforcement of flanges and joints with Z-fibersTM removes the need for fasteners or rivets and produces a more even load distribution over the joined area. 2-pinning is also being used to reinforce inlet duct skin panels and to fasten hat-shaped stiffeners to selected composite panels on the F/A-18 SuperHornet fighter aircraft
Chapter 2 Manufacture of 3D Fibre Preforms 2.I INTRODUCTION In spite of the demonstrated advantages of 3D composites in their through-thickness and impact performance,the use of these materials is not yet widespread.A major reason for this limited use is related to the maturity of the manufacturing processes being used to produce the preforms and the understanding and process control required to design and cost-effectively manufacture a preform for a specific application.The manufacture of 3D fibre preforms for composite structures can be accomplished in a variety of ways, however,all the processes that have been developed for composite applications are essentially derived from one of the following four groups of traditional textile procedures;Weaving,Braiding,Knitting and Stitching. The aim of this chapter is not to give an exhaustive description of each manufacturing process but rather to be a lay-persons introduction to the various techniques being developed and used within the composites industry and to illustrate their advantages and limitations. 2.2 WEAVING Weaving is a process that is already used extensively within the composite industry as it is the manufacturing method that produces the vast majority of the single-layer,broad- cloth carbon and glass fabric that is currently used as a reinforcement material for composite components.However,the same weaving equipment can also be used to manufacture more intricate,net-shaped preforms that have a three-dimensional fibre architecture.To understand how 3D preforms can be produced through weaving,it is necessary to first understand the conventional 2D weaving process. 2.2.1 Conventional Weaving Weaving is essentially the action of producing a fabric by the interlacing of two sets of yarns:warp and weft.The basic weaving process is illustrated in Figure 2.1.The warp yarns run in the machine direction,the 0 direction,and are fed into the weaving loom from a source of yarn.This source can consist of a multitude of individual yarn packages located on a frame (a creel),or as one or more tubular beams onto which the necessary amount of yarn has been pre-wound (warp beams).The warp yarns may then go through a series of bars or rollers to maintain their relative positioning and apply a small amount of tension to the yarns,but are then fed through a lifting mechanism which is the crucial stage in the weaving process.The lifting mechanism may be
Chapter 2 Manufacture of 3D Fibre Preforms 2.1 INTRODUCTION In spite of the demonstrated advantages of 3D composites in their through-thickness and impact performance, the use of these materials is not yet widespread. A major reason for this limited use is related to the maturity of the manufacturing processes being used to produce the preforms and the understanding and process control required to design and cost-effectively manufacture a preform for a specific application. The manufacture of 3D fibre preforms for composite structures can be accomplished in a variety of ways, however, all the processes that have been developed for composite applications are essentially derived from one of the following four groups of traditional textile procedures; Weaving, Braiding, Knitting and Stitching. The aim of this chapter is not to give an exhaustive description of each manufacturing process but rather to be a lay-persons introduction to the various techniques being developed and used within the composites industry and to illustrate their advantages and limitations. 2.2 WEAVING Weaving is a process that is already used extensively within the composite industry as it is the manufacturing method that produces the vast majority of the single-layer, broadcloth carbon and glass fabric that is currently used as a reinforcement material for composite components. However, the same weaving equipment can also be used to manufacture more intricate, net-shaped preforms that have a three-dimensional fibre architecture. To understand how 3D preforms can be produced through weaving, it is necessary to first understand the conventional 2D weaving process. 2.2.1 Conventional Weaving Weaving is essentially the action of producing a fabric by the interlacing of two sets of yarns: warp and weft. The basic weaving process is illustrated in Figure 2.1. The warp yarns run in the machine direction, the 0" direction, and are fed into the weaving loom from a source of yarn. This source can consist of a multitude of individual yarn packages located on a frame (a creel), or as one or more tubular beams onto which the necessary amount of yarn has been pre-wound (warp beams). The warp yarns may then go through a series of bars or rollers to maintain their relative positioning and apply a small amount of tension to the yarns, but are then fed through a lifting mechanism which is the crucial stage in the weaving process. The lifting mechanism may be
14 3D Fibre Reinforced Polymer Composites mechanically or electronically operated and may allow individual yarns to be selectively controlled (jacquard loom)or control a set of yarns simultaneously (loom with shafts,as shown in Figure 2.1).The crucial point is that the lifting mechanism selects and lifts the required yarns and creates a space (the shed)into which the weft yarns are inserted at right angles to the warp (the 90 direction).The sequence in which the warp yarns are lifted controls the interlinking of the warp and weft yarns and thus the pattern that is created in the fabric (see Figure 2.2).It is this pattern that influences many of the fabric properties,such as mechanical performance,drapability,and fibre volume fraction. Therefore to manufacture a suitable 2D or 3D preform an understanding of how the required fibre architecture can be produced through the design of the correct lifting pattern is crucial in the use of this manufacturing process. Lifting Shafts Shed Fabric and Warp yarns take-up rollers Warp Beam Heddles Reed Shuttle with weft yarn Figure 2.1 Illustration of conventional weaving process PLAIN TWILL SATIN WEAVE 2x2 5HS Figure 2.2 Typical 2D weave patterns
14 30 Fibre Reinforced Polymer Composites mechanically or electronically operated and may allow individual yarns to be selectively controlled (jacquard loom) or control a set of yarns simultaneously (loom with shafts, as shown in Figure 2.1). The crucial point is that the lifting mechanism selects and lifts the required yarns and creates a space (the shed) into which the weft yarns are inserted at right angles to the warp (the 90" direction). The sequence in which the warp yarns are lifted controls the interlinking of the warp and weft yarns and thus the pattern that is created in the fabric (see Figure 2.2). It is this pattern that influences many of the fabric properties, such as mechanical performance, drapability, and fibre volume fraction. Therefore to manufacture a suitable 2D or 3D preform an understanding of how the required fibre architecture can be produced through the design of the correct lifting pattern is crucial in the use of this manufacturing process. / / \ Heddles Reed \ Shuttle with weft yam Figure 2.1 Illustration of conventional weaving process PLAIN TWILL SATIN WEAVE 2x2 5HS Figure 2.2 Typical 2D weave patterns