Fibr3 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 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 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.
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 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 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. …..
