FAILURE MODES OF SIC-FIBER SI3N4-MATRIX COMPOSITES AT ELEVATED-TEMPERATURES/

An investigation of composite failure modes as a function of temperatu re and fiber-volume fraction was carried out in SiC-fiber-reinforced S i3N4-matrix composites fabricated in our laboratories. Mechanical test ing was carried out at temperatures from 25 degrees to 1500 degrees C. Matrix-cracking stress and ultimate strength of the composites were m easured from load-displacement curves. They were both found to decreas e with increasing temperature, but their temperature sensitivity decre ased with increasing fiber-volume fraction. The tendency for noncatast rophic failure increased with fiber-volume fraction, while the tendenc y for catastrophic failure increased with temperature. The failure mod e was demonstrated experimentally to be determined by the fiber bundle strength, S-fb, and the matrix cracking stress, sigma(c). These two p arameters, in turn, were shown to be controlled by the fiber-volume fr action, f, and the temperature. failure at various temperatures was no ncatastrophic when S-fb > sigma(c), and catastrophic when S-fb < sigma (c). Transition in composite failure mode between noncatastrophic and catastrophic failure was controlled via the variation of fiber-volume fraction and testing temperature. Catastrophic failure at high tempera tures was found to be mainly a result of fiber strength loss.