Reinforcing fibre forms

The reinforcing filler usually takes the form of fibres but particles (for example glass spheres) are also used. A wide range of amorphous and crystalline materials can be used as reinforcing fibres. These include glass, carbon, boron, and silica. In recent years, fibres have been produced from synthetic polymers-for example, Kevlar fibres (from aromatic polyamides) and PET fibres. The stress-strain behaviour of some typical fibres is shown in Fig. 3.2. 

Reinforcing fibres have diameters varying from 7 /im to 1(X) /im. They may be continuous or in the form of chopped strands (lengths 3 mm-50 mm). When chopped strands are used, the length to diameter ratio is called the Aspect Ratio. The properties of a short-fibre composite are very dependent on the aspect ratio - the greater the aspect ratio the greater will be the strength and stiffness of the composite. 

Table 3.3 indicates the extent to which the properties of plastics are influenced by the level of fibre content. Full details of the forms in which reinforcing fibres are available for inclusion in plastics are given in Chapter 4. 

Table 6.13 displays the strength and modulus of a 60% glass fibre reinforced resin for various fibre forms. The properties are roughly ... 

Table 6.13 Examples of properties for a 60% glass fibre reinforced resin according to tbe fibre forms...

Certain polyquinoxalines are thermo-stable with high glass transition temperatures up to 435°C,high decomposition temperatures (510-560°C) and notable oxidative resistance in air, but, on the other hand, they form highly viscous solutions that don t wet reinforcing fibres easily. 

A higher stiffness is also obtained by the incorporation of hard particles or short fibres. Particles are responsible for a 2 to 2.5 fold increase in E, while with short glass fibres a factor of 3 to 5 can be obtained. Long fibres, forming a continuous reinforcing phase, produce a much stronger effect here the fibres practically carry the whole stress, while the matrix polymer has hardly any influence on the stiffness. 

Silicon carbide has attracted considerable interest because of its good mechanical and physical properties and chemical inertness. One of the most important applications of SiC is to produce a matrix reinforced by fibres, forming ceramic-matrix composites. These composite materials exhibit much better fracture toughness than monolithic ceramics. Compared with carbon/carbon composites, fibre-reinforced SiC matrix composites possess superior oxidation resistance and mechanical properties. The Si-C-H-Cl system (e.g. methyltrichlorosilane, CH3SiCl3) has been used for SiC deposition because it is easy to produce stoichiometric SiC deposits. 

Superior mechanical properties, including impact strength and resistance to delamination, are claimed for reinforced plastics made from three-dimensional fabrics [1], Other reinforcement forms include continuous swirl mat, designed to have a minimum of fibre ends, and hybrid fabrics containing two different reinforcing fibres, for example, glass and aramid. There are many different ways in which two different fibres can be combined together. 

Commingled fibre technology involves supplying the matrix in filament form and mingling it with the reinforcing fibres (Fig. 2.4(b)). The two materials can even be cowoven in the same strands. Thus the material is available in the form of continuous roving, chopped fibres, fabrics, etc. Excellent drape qualities can be obtained with fabrics. The technique can be used for pultrusion, but not for filament winding. 

Figure 2.4 (a) Commingling by film stacking alternate layers of resin film and reinforcing fabric, (b) Cross-section of commingled reinforcement and resin, both in fibre form, before consolidation. See also Miller and Gibson [4]. O, Matrix in fibre form reinforcing fibres... 

A widely used method of laminate construction is by the contact moulding process or hand lay-up . In this, a layer of catalysed unreinforced resin, known as the gelcoat , is brushed or sprayed onto the mould and allowed to cure and then layers of reinforcing-fibre impregnated with resin are built up behind (Fig. 6.3). Variations of this include resin injection and vacuum-assisted resin injection in closed moulds. After removal of the cured moulding from the mould, the gelcoat forms the outer skin and as such performs the two functions of decoration and protection ... 

The final important form of carbon is the carbon fibre formed from polyacrylonitrile (PAN), cellulose or pitch and which is finding increasing use in fibre-reinforced composites. The corrosion of carbon fibres in composites designed for use under high temperature conditions is currently a severe limitation on their use. 

A thermoplastic film is used to form the mould for the outer surface of the cylinder. The reinforcement fibre, in the form of CSM, woven roving or similar is cut to size and laid on to the sheet. A rubber bag in the form of a cylinder with closed ends is placed on the reinforcement. Resin (catalysed and accelerated) is poured on to the reinforcement and the longitudinal edges of the sheet are brought together, forming a tube, and clamped. 

Soulat, D., Cheruet, A., Boisse, P., 2006. Simulation of continuous fibre reinforced thermoplastic forming using a shell finite element with transverse stress. Comput. Struct. 84,888-903. 

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