Fatigue failure behavior of 3D stitched C/C–SiC composites in thermo-mechanical-oxidative environments

Three-dimensional stitched C/C–SiC composites exhibit excellent high-temperature resistance, ablation resistance, and out-of-plane load-bearing capability, making them promising candidates for thermal protection structures in aerospace vehicle engine nozzles and other high-temperature hot-end components. However, fatigue performance data and key parameters required for life prediction under high-temperature oxidative conditions corresponding to the typical service temperatures of such components remain limited, which restricts their engineering application and life assessment. To address this issue, a high-temperature oxidative fatigue testing system with a maximum operating temperature of 1650℃ was developed. Tension–tension fatigue tests were conducted in air at 1500℃ to characterize the fatigue life behavior and residual stiffness evolution of the material. Based on a residual stiffness degradation model, a semi-empirical residual strength degradation model, and a modified Hashin failure criterion, a thermo–mechanical–oxidative coupled fatigue progressive damage model was established. The results show that the natural logarithm of fatigue life exhibits a good linear relationship with the applied fatigue stress level under the 1500℃ oxidative environment. The fatigue life increases from only 98 cycles at a stress level of 60% to 27,004 cycles at a stress level of 6%. The residual stiffness degradation exhibits a two-stage evolution characterized by an initial rapid reduction followed by gradual degradation. The proposed progressive fatigue damage model can reasonably predict the fatigue life of open-hole specimens, and the predicted fatigue life agrees well with the experimental results, with a relative error of 33.9%. This study provides fatigue performance data for three-dimensional stitched C/C–SiC composites under a 1500℃ oxidative environment and establishes a corresponding fatigue life prediction approach, which can serve as a reference for the life assessment and structural design of thermal protection structures in aerospace vehicle engine nozzles and other high-temperature hot-end components.