Real-time monitoring of thermal cycling damage in ceramic matrix composites under a constant stress

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.