Thermomechanical behaviour of long fibres ceramic-ceramic composites

Ceramic-ceramic composites are non brittle materials in comparison with bulk ceramics, because of the interactions between the matrix and the long fibres. That is the reason why the study of the thermomechanical behaviour of such materials implies a good knowledge of these interactions, interactions which act at various scales depending on the structure of the composite. The studies reported here, brought on four composites bi- or multi-directional (cross-weave, cross-ply, 2.5D, [0, +60, -60](n)), with SiCf- or C-f long fibres, and SiC, MAS-L or C m9atrix. Under mechanical loading, these materials exhibit a more or less complex multiple cracking of the matrix, depending on the structure of the fibre reinforcement. Hence, the macroscopic mechanical behaviour of these composites is not always controlled by the friction at the fibre/matrix interfaces, but can be controlled by the friction at yarn/matrix interfaces or ply/ply interfaces. During cyclic loading, the variation of the shape of stress/strain loops with the number of cycles applied, is due to a progressive wear of these interfaces, and at high temperature under inert atmosphere the variations of the stress/strain loops are also controlled by a release of the residual thermal stresses. In addition, for materials with radial and longitudinal residual thermal stresses in tension, an apparent stiffening during cyclic fatigue has been observed, and has been attributed to a bad closure during cycling of the cracks existing in the transverse plies.