Damage mechanisms of SiC coated C/C composites subjected to cyclic loading in combustion gas environment

A SiC coated C/C composite was subjected to cyclic loading in a combustion wind tunnel at flame temperature of 1300°C. Both the ratchetting strain and hysteresis changes show that the majority of damage is produced in the first 50 cycles, and then the rate of damage accumulation gradually approaches a steady value as the cycles proceeding. Lamellar structure of the C/C composite is observed after oxidation, which is caused by the difference in reactivity between the carbon fiber and carbon matrix. The mean residual strength of survived specimens is lower than that of the as-received ones by 19%, while mean residual strength of the specimens exposed to the combustion gas environment without loading is lower by 38%. Fiber/matrix debonding of the specimen cross section is observed after cyclic loading. Acoustic emission activities throughout the tensile tests of the cycled specimens shows that little damage is produced before the applied tensile load exceeds the maximum history load of the cyclic loading tests.