Cyclic Fatigue Behaviour at Room Temperature and at High Temperature under Inert Atmosphere of a C/SiC Multilayer Composite

摘要

Cyclic fatigue at room temperature and at high temperature has been investigated on a 2.5D C/SiC multilayer CMC, at the same maximum stress level but at various temperatures from room temperature to 1500℃ under inert atmosphere. During cyclic fatigue the mechanical behaviour, exhibited by stress/strain loops, evolves with the number of cycles applied. This evolution can be quantified by 3 parameters: the mean elastic modulus, the mechanical hysteresis, the residual strain. The microstructural damages associated have also been observed. From these analyses it has been pointed out that the matrix exhibit a multiple crack at various scales: in the tows and between the tows, and that the tows are totally debonded from their surrounding matrix shell. Therefore, as shown by observations of microstructures and by the evolutions of the mechanical hysteresis during cyclic fatigue tests, the origin of the cyclic fatigue in this composite can be attributed to a progressive wear of the tow/surrounding matrix interfaces instead of the fibre/matrix interfaces. At high temperatures lower than 1000℃, this composite exhibits also an original increase of the mean elastic modulus, associated to a wide increase of the residual strain. This phenomenon is not specific to this CMC, but has been observed on other CMC like [0, +60, -60]_n C/C, [0, 90]_(12) SiC/MAS-L, or heat-treated 2D SiC/SiC. This non classical stiffening can be explained by a bad closure of cracks in the transversal yarns.