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Apparent stiffening of ceramic-matrix composites induced by cyclic fatigue

1998; Elsevier BV; Volume: 18; Issue: 13 Linguagem: Inglês

10.1016/s0955-2219(98)00122-8

1873-619X

Pascal Reynaud, A. Dalmaz, C. Tallaron, D. Rouby, Gilbert Fantozzi,

Aluminum Alloys Composites Properties

This work carries on four different long-fibre-reinforced ceramic-matrix composites: 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 atmosphere, and at room temperature after a previous ageing at high temperature 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 neighbouring ply interfaces, according to the following mechanisms: (i) interfacial wear of interfaces due to to-and-fro sliding of fibres or of plies under cyclic loading, and (ii) dependence of the residual thermal stresses with the temperature of the test. Previous ageing at high temperature under vacuum can also enable in CMC some physical and chemical changes in the constituents leading for example to a slight removing of fibre/matrix interphases by oxidation. Usually, damage induced by cyclic fatigue in long-fibre-reinforced ceramic-matrix composites leads to a reduction of the tensile apparent elastic modulus as cycling proceeds. But an original macroscopic stiffening has been experimentally observed during cyclic fatigue. This phenomenon has been observed on C/C composite at room temperature, on C/SiC and on SiC/MAS-L at high temperature, and on SiC/SiC at room temperature after previous ageing 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.