The Strength of Model Composites Incorporating Silicon Carbide Fibres.

The tensile strength of a chemical vapour deposited SiC fibre (tungsten core) has been examined over a range of gauge lengths and in three states; as an unsupported single fibre, as a single fibre coated with a resin matrix and as a planar array of 10 fibres with various inter-fibre separations. The strength of the unsupported single fibre was described by a two-parameter Weibull distribution at lengths between 500mm and 50mm when fracture was found to initiate at defects on the filament surface. At 10mm gauge length a small number of high-stress failures were initiated at the tungsten core. A bi-modal two-parameter Weibull distribution has been used to describe the strength at this length. The filament has been shown to have a residual surface tensile stress and the predominance of surface initiated failures is attributed to this. When embedded in a resin matrix the filament is constrained and fracture initiates predominantly at the tungsten core rather than the fibre surface. This gives an increase in the observed mean failure stress and a decrease in the coefficient of variation. Significant matrix cracking was observed around these fractures and the shock-wave initiated by the failure was found to give extensive secondary tensile fracture of the fibre over axial distances as great as 20mm from the initial fracture. When tested as an array, at fibre spacings below 6 fibre diameters, the filament strength was comparable to the unsupported material and this is attributed to the resin 'neck' between the fibres imparting minimal surface constraint. At larger spacings the filament strength was essentially the same as for the embedded single fibre. The overall strength of the array was found to be controlled by matrix cracking effects rather than by direct load-sharing between fibres