Effects of temperature and of oxidation on the interfacial shear stress between fibres and matrix in ceramic-matrix composites

Abstract

Under cyclic loading, the mechanical behavior of ceramic matrix composites (SiC/SiC, SiC/ MAS-L) changes with the number of applied cycles, as shown by life-time diagrams and shape evolutions of stress/strain loops. According to these observations, a shear-lag model has been developed where the cyclic fatigue effect is attributed to an interfacial wear between fibres and matrix. At high temperature under inert atmosphere, since physical and chemical changes are inhibited, the main effect of temperature on cyclic fatigue of ceramic matrix composites is the release of radial thermal residual stresses. But under vacuum when the temperature is high, fibre/matrix interfaces can be removed due to chemical instabilities. Hence, after ageing under vacuum at high temperature, cyclic fatigue at room temperature of SiC/SiC composites exhibits an increase followed by a decrease in mechanical hysteresis. This can be explained by a decrease in the interfacial shear stress due to the previous heating at high temperature under vacuum. For such treated composites, an original stiffening effect is also observed during cyclic fatigue. This original phenomenon is attributed to a contribution of cracks in transversal yarns.