Discontinuous fibres

General, the highest strength and stifihiess are obtained with continuous fibre reinforcement Discontinuous fibres are used only when manufacturing economics dictate the use of a process where the fibres must be in this form—for example injection moulding. 

When considering discontinuous fibres it is necessary to take account of the fibre ends. These are weak points in the composite—sites of high 

22 Stresses acting on an element of fibre embedded in a matrix. 

Predicting Of and T involves stress anafysis of the fibre-matrix composite under load. A simplified but highfy successful solution, is known as the shear lag themy (6.N.7). It assumes perfect bonding between fibres and matrix, and results in the following prediction of r, as a fimction of distance x along the fibre, measured fiom its centre (recall a = l/dy. 

Distributions along a short fibre of tensile stress tr, and interfacial shear stress T. as predicted by eqns 6.33 and 6.35. 

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In order to understand the effect of discontinuous fibres in a polymer matrix it is important to understand the reinforcing mechanism of fibres. Fibres exert their effect by restraining the deformation of the matrix as shown in Fig. 3.28. The external loading applied through the matrix is transferred to the fibres by shear at the fibre/matrix interface. The resultant stress distributions in the fibre and matrix are complex. In short fibres the tensile stress increases from zero at the ends to a value ([Pg.226]

Obtain specific properties using, for example, a self-lubricating composite insert. Discontinuous fibre reinforced thermoplastic composites can also be overmoulded onto GMT sheets. 

Over the last decade, considerable efforts have been committed to the toughening of sialons and substantial progress has been achieved using various reinforcements. According to the form of reinforcement, sialon composites can be classified as either particle reinforced, discontinuous fibre (whiskers/ short fibres) reinforced, or continuous fibre reinforced. 

Sambell, R.A.J., Briggs, A., Phillips, D.C. and Bowen, D.H., (1972a), Carbon fibre composites with ceramic and glass matrices, Part I. Discontinuous fibres , J. Mater. Sci., 7, 663-675. 

A. Kelly and K. N. Street, Creep of Discontinuous Fibre Composites II, Theory for the Steady State, Proceedings of the Royal Society, London, A328, 283-293 (1972). 

In this respect, (thermoset) plastics composites with discontinuous fibre products are already mostly used in the car body applications, where polyester/E-glass is predominating (mostly because of polyesters, economy, ease of processability and reasonable mechanical properties provided), followed by use of phenolics (when fire retardance is required, in friction linings and engine compartments), and epoxies. Replacement by carbon or aramid fibre reinforcements can reduce body mass by 40% (compared to steel) and with more added strength, but the cost is unfavourable at the moment, as mentioned previously [12, 13]. 

R. A. J. Sambell, D. H. Bowen and D. C. Phillips, Caibon Fibre Composites with Ceramic and Glass Matrices, Parti Discontinuous Fibres, 7. Mat. Sci. 7,663-675 (1972). 

K. Ogi, N. Takeda and K. M. Prewo, Fracture Process ofThermally Shocked Discontinuous Fibre-Reinforced Glass Matrix Composites Under Tensile Loading, J. Mat. Sci. 32, 6153-6162 (1997). 

Fibre-reinforced concrete is a mixture of concrete with a dispersion of discontinuous fibres. The fibres may be steel fibres, synthetic fibres (micro-synthetic or macro-synthetic), glass fibres (alkali-resistant only, AR glass fibres) or natural fibres. 

The effective fibre fraction that determines the structural reinforcing effect in any in-plane direction is usually not more than 10% by volume and the use of discontinuous fibres leads to a greater dependency on the resin for load transfer. Lower long term strength properties compared with those of continuous fibres may result. 

FRCs can be classified based on matrix and fibres. Based on fibre source, FRCs may be natural fibre reinforced and synthetic fibre reinforced. Based on fibre length, they can be continuous fibre reinforced and discontinuous fibre reinforced. But FRCs are generally classified based on matrix component. Thus according to the types of matrices stated earlier, composites are of three types (i) ceramic matrix composites (CMCs), (ii) metal matrix composites (MMCs) and (iii) organic matrix composites (OMCs). Organic matrix is subdivided into two classes, namely polymer matrix and carbon matrix. A short description of all these types of composites are discussed below. 

As the prelaminate moves and the matrix flows, the fibres can move under the influence of the matrix flow and the applied pressure. Fibres angles can shift and the process can draw continuous fibres into the new shape as the prelaminate conforms to the tool. Discontinuous fibres can slide relative to one another and accommodate stretching without drawing fibres into the tool from the prelaminate s edges. At high forming rates, the viscous drag induced by shear lag can put fibres into tension and break the fibres. 

Short fibre composites (a) tend to have low fibre concentration sheet forming is suited to continuous fibre composites or (b) long discontinuous fibre composites. 

As noted earlier in this handbook, continuous fibre composites with fibres accounting for 50% or more of the material volume present the highest properties possible. Unfortunately, continuous fibres make forming - and sheet forming in particular - difficult to control. Before specifying continuous fibre components made by sheet forming, a designer should determine whether a discontinuous fibre product could meet the need. 

Figure 5.10b shows ideal forming in a long-discontinuous fibre laminate (Lee et aL, 2008). As the flat laminate bends, the fibre ends move apart. This extension ability reduces wrinkling. New long-discontinuous fibre forms are available and since about 2001, these are called stretch-broken fibre or SBXF materials with the X replaced with a letter that represents a specific fibre. Carbon fibre material is called SBCF, for example. 

Black, S. (2008), Aligned discontinuous fibres come of age , USA Compositesworld. Available at http //www.compositesworld.com/articles/aligned-discontinuous-fibers-come-of-age. Accessed 31 May 2010. 

The reduction in tensile stress towards the ends of each fibre inevitably leads to a decrease in the tensile modulus compared with the continuous filament case. Consider a plane drawn perpendicular to the stress direction in an aligned discontinuous fibre composite (Figure 8.4), which must intercept individual filaments at random positions along their length. Hence the stress carried by the composite must be lower than that for the continuous filament case, and is dependent on the length of each fibre. Cox [9] predicted a correction factor rji for the tensile modulus in the axial direction that takes into account the finite length of the fibres so that Equation (8.3) is modified to... 

Figure 8.4 Schematic section through a discontinuous fibre composite (fibre fiaction shown very low for clarity)...

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