No abstract

No abstract

The damage mechanism of SiC‐coated carbon/carbon composites subjected to cyclic loading in a combustion gas environment was investigated. At the maximum cyclic stresses <90 MPa, the hysteresis and ratchetting strain responses indicated that the damage evolutions were similar at both 1300° and 1800°C. As the applied stress was sufficient to open the coating cracks, rapid failure of the composite was observed. Because of the enhancement mechanism caused by the cyclic loading, the residual strengths of the cyclic‐loaded specimens exposed to the combustion environment were higher than those of the specimens not exposed to cyclic loading. Acoustic emission activities throughout the tensile tests of the cycled specimens showed that little damage was produced before the applied tensile load exceeded the maximum history load of the cyclic loading tests.

Under a constant stress of 50 MPa, a thermal strain with a range of 0.2% was measured on a carbon‐fiber‐reinforced SiC‐matrix composite (C/SiC) subjected to thermal cycling between 700° and 1200°C. Acoustic emission (AE) technology was implemented to assist in monitoring the occurrence of damage during testing. The monitored AE signals, together with the measured strain, were shown to have a significant dependence on temperature in a single cycle and to change periodically with repetitive temperature. In a single cycle, the cycled specimens emitted fewer acoustic emissions during heating, but as the cooling stage approached, the emission rate increased dramatically. As the cycle proceeded, the AE energy increased stepwise, whereas this stepwise increment per cycle continuously decreased until finally it nearly disappeared at 15 cycles, after which no further increase in thermal cycle creep strain was observed with a rate of zero, and the measured coating crack density reached a stable value of about 5.0 mm‐1.

No abstract