Microscale modeling of lifetime in fiber-reinforced ceramic-matrix composites under coupled thermal-mechanical- environmental overload conditions

Under cyclic fatigue loading, stochastic overload events may occur intermittently throughout fatigue lifetime and significantly reduce the fatigue lifetime of ceramic-matrix composites (CMCs), particularly at elevated temperatures. This study develops microscale lifetime models for fiber-reinforced CMCs subjected to coupled thermal, environmental, and mechanical overload conditions. Based on dominant damage mechanisms, the microscale lifetime models are classified into two categories: Model I, an interfacial wear–driven lifetime model; and Model II, an interfacial/fiber oxidation–driven lifetime model. Three distinct stochastic overload spectra designated Case I (constant amplitude), Case II (incrementally increasing amplitude), and Case III (decrementally decreasing amplitude) are investigated. To evaluate the effect of stochastic overload stress on the degradation of fatigue lifetime, a new damage parameter of fatigue lifetime degradation rate is developed. The microscale models predict experimental fatigue lifetimes and the associated lifetime degradation rate for 2D SiC/SiC composites under these spectra at room temperature and at elevated temperatures of 750 °C, 800 °C, 1000 °C, 1100 °C, 1200 °C, and 1300 °C in both ambient air and steam environments. The coupled influences of overload sequence, stress margin, occurrence cycle number, temperature, and environmental atmosphere on the fatigue lifetime degradation rate are systematically analyzed. At a fixed peak stress, degradation rate exhibits a local maximum in the 750–800 °C range. Above 1000 °C, degradation rate peaks at 1100 °C in air and at 1300 °C in steam. Across all test conditions, Case II consistently yields the highest degradation rate, whereas Case I yields the lowest. The fatigue lifetime degradation rate parameter can serve as a basis for life management of CMC components. From a design standpoint, an engineering threshold of 10% is recommended, particularly for CMC hot-section components such as turbine shrouds, vanes, and disks.