14 C.Soutis Proaress in Aerospace Sciences 41 (2005)143-151 56m in diameter and consist of small crystallites of carbon.The rong are weak 的R Fabrics can be wo d plane Young's modulus parallel to the a-axis is 1000 GPa 30GPa.Alignment of the basal plane s of knitting machine.to fibre oSopocsibtke.withcertaing stiff fibre red to the shape of the ity eventu ally speak the cosoPoiasinwoembi.orabopnkmc congated o the fibre during manufacturet setting (epo associated the manufac ropylene. Nylon 6.6 of the is also important since it affects the transverse and shear size to aid bonding to the specified matrix.Wherea propert is pr have a thir tial ide out of gth is.i and a core with important.The aim of the materia pith based exhibit s to a systen alan se the properties erties an lead to imp ved lamina or laminat of cous those of the composites PopcTchcanmpornanticidofibre-malnxnicr proc has t nsile strength (4.5GPa)and in strain to fracture （more than2 PAN- based hi .Thes strongly bonded to the matrix if their high strength an e6is are to high strength(HS.with a modulus of around 230 GPa the interfa A weak interface results in a low stifnes of 4.5 GPa and strength but hi h resistance to fracture.whereas gh-strengr rong intertace pro values of%before fracture.The tensile stresstrain er The selec tion of the ate fibr nd ntal very much on the military aircraft by the characteristics of the interface.In these cases.the oth and high str are desirab relationship betv ween properties and interface charac high-fibre modulus stie nsive experimental evidence ren ector dishes,antennas and their supporting struc are required tun Thermop tic materials are b ovings are the hasic in hich fibr supplicd.a roving being the number of strandsor currently used are the ermosetting epoxics.The matri bundles of filaments wound into a package or creel,the material is the Achilles'heel of the mposite system and ing up the tent5–6 mm in diameter and consist of small crystallites of ‘turbostratic’ graphite, one of the allotropic forms of carbon. The graphite structure consists of hexagonal layers, in which the bonding is covalent and strong (
525 kJ/mol) and there are weak van der Waal forces (o10 kJ/mol) between the layers [1,2]. This means that the basic crystal units are highly anisotropic; the in￾plane Young’s modulus parallel to the a-axis is approximately 1000 GPa and the Young’s modulus parallel to the c-axis normal to the basal planes is only 30 GPa. Alignment of the basal plane parallel to the fibre axis gives stiff fibres, which, because of the relatively low density of around 2 mg/m3 , have extremely high values of specific stiffness (
200 GPa/((mg/m3 )). Imperfections in alignment, introduced during the manufacturing process, result in complex-shaped voids elongated parallel to the fibre axis. These act as stress raisers and points of weakness leading to a reduction in strength properties. Other sources of weakness, which are often associated with the manufacturing method, include surface pits and macro-crystallites. The arrange￾ment of the layer planes in the cross-section of the fibre is also important since it affects the transverse and shear properties of the fibre. Thus, for example, the normal polyacrylonitrile-based (PAN-based) Type I carbon fibres have a thin skin of circumferential layer planes and a core withrandom crystallites. In contrast, some mesophase pith-based fibres exhibit radially oriented layer structures. These different structures result in some significant differences in the properties of the fibres and, of course, those of the composites. Refinements in fibre process technology over the past 20 years have led to considerable improvements in tensile strength(
4.5 GPa) and in strain to fracture (more than 2%) for PAN-based fibres. These can now be supplied in three basic forms, high modulus (HM,
380 GPa), intermediate modulus (IM,
290 GPa) and high strength (HS, with a modulus of around 230 GPa and tensile strengthof 4.5 GPa). The more recent developments of the high-strength fibres have led to what are known as high-strain fibres, which have strain values of 2% before fracture. The tensile stress–strain response is elastic up to failure and a large amount of energy is released when the fibres break in a brittle manner. The selection of the appropriate fibre depends very muchon the application. For military aircrafts, both high modulus and high strength are desirable. Satellite applications, in contrast, benefit from use of high-fibre modulus improving stability and stiffness for reflector dishes, antennas and their supporting struc￾tures. Rovings are the basic forms in which fibres are supplied, a roving being the number of strands or bundles of filaments wound into a package or creel, the lengthof the roving being up to several kilometres, depending on the package size. Rovings or tows can be woven into fabrics, and a range of fabric constructions are available commercially, suchas plain weave, twills and various satin weave styles, woven witha choice of roving or tow size depending on the weight or density of fabric required. Fabrics can be woven withdifferent kinds of fibre, for example, carbon in the weft and glass in the warp direction, and this increases the range of properties available to the designer. One advantage of fabrics for reinforcing purposes is their ability to drape or conform to curved surfaces without wrinkling. It is now possible, withcertain types of knitting machine, to produce fibre performs tailored to the shape of the eventual component. Generally speaking, however, the more highly convoluted each filament becomes, as at crossover points in woven fabrics, or as loops in knitted fabrics, the lower its reinforcing ability. The fibres are surface treated during manufacture to prepare adhesion with the polymer matrix, whether thermosetting (epoxy, polyester, phenolic, polyimide resins) or thermoplastic (polypropylene, Nylon 6.6, PMMA, PEEK). The fibre surface is roughened by chemical etching and then coated with an appropriate size to aid bonding to the specified matrix. Whereas composite strengthis primarily a function of fibre properties, the ability of the matrix to both support the fibres and provide out-of-plane strength is, in many situations, equally important. The aim of the material supplier is to provide a system witha balanced set of properties. While improvements in fibre and matrix properties can lead to improved lamina or laminate properties, the all-important field of fibre–matrix inter￾face must not be neglected. The load acting on the matrix has to be transferred to the reinforcement via the interface. Thus, fibres must be strongly bonded to the matrix if their high strength and stiffness are to be imparted to the composite. The fracture behaviour is also dependent on the strength of the interface. A weak interface results in a low stiffness and strength but high resistance to fracture, whereas a strong interface produces high stiffness and strength but often a low resistance to fracture, i.e., brittle behaviour. Conflict therefore exists and the designer must select the material most nearly meeting his requirements. Other properties of a composite, suchas resistance to creep, fatigue and environmental degradation, are also affected by the characteristics of the interface. In these cases, the relationship between properties and interface character￾istics are generally complex and analytical/numerical models supported by extensive experimental evidence are required. Thermoplastic materials are becoming more available; however, the more conventional matrix materials currently used are thermosetting epoxies. The matrix material is the Achilles’ heel of the composite system and limits the fibre from exhibiting its full potential in terms of laminate properties. The matrix performs a number ARTICLE IN PRESS 144 C. Soutis / Progress in Aerospace Sciences 41 (2005) 143–151