Apparent stiffening of ceramic-matrix composites induced by cyclic fatigue

This work carries on four different long-fibre-reinforced ceramic-matrix co mposites, a cross-weave SiC/SiC, a cross-ply SiC/MAS-L, a cross-weave C/SiC and a [0, +60, -60](n) C/C laminate. Experimentally, cyclic fatigue effect has been observed at room temperature, at high temperature under inert atm osphere, and at room temperature after a previous ageing at high temperatur e under vacuum. For these four materials, the evolutions of the macroscopic mechanical behaviour with the number of cycles applied can be explained by an evolution of interfaces as well, fibre/matrix interfaces as neighbourin g ply interfaces, according to the following mechanisms: (i) interfacial we ar of interfaces due to to-and-fro sliding of fibres or of plies under cycl ic loading, and (ii) dependence of the residual thermal stresses with the t emperature of the test. Previous ageing at high temperature under vacuum ca n also enable in CMC some physical and chemical changes in the constituents leading for example to a slight removing of fibre/matrix interphases by ox idation. Usually, damage induced by cyclic fatigue in long-fibre-reinforced ceramic-matrix composites leads to a reduction of the tensile apparent ela stic modulus as cycling proceeds. But an original macroscopic stiffening ha s been experimentally observed during cyclic fatigue. This phenomenon has b een observed on CIC composite at room temperature, on C/SiC and on SiC/MAS- L at high temperature, and on SiC/SiC at room temperature after previous ag eing under vacuum at high temperature. This apparent stiffening is not well understood at present time, but appeared in materials with low interfacial shear strength and is seemingly due to incomplete closure during unloading of the cracks present in transverse yarns. (C) 1998 Elsevier Science Limit ed. All rights reserved.