From pore reconstruction to diffusion acceleration: Damage evolution and mechanical degradation of C/SiC composites under high-temperature and multi-loading conditions

This study focuses on the service environment of high-speed vehicles during the cruise phase. Mechanical tests with digital image correlation were conducted at room temperature and at 1200 °C under 5 kPa pressure, covering tension, compression, in-plane shear, and interlaminar shear. Combined with SEM fractography and CT-based three-dimensional reconstruction, revealing loading-path-dependent coupling between damage and diffusion. Each loading mode produced distinct crack initiation sites and pore networks, which reconfigured oxygen diffusion channels. Accordingly, the reductions in strength and modulus after the post-service condition exhibit a pronounced path dependence. Based on these observations, a Mazars damage-diffusion coupling model is developed to quantitatively capture the proposed chain mechanism. The model incorporates orthotropic constitutive behavior, mixed-mode equivalent strain, energy-based crack-band regularization, and an embedded diffusion formulation, enabling a unified reproduction of stress–strain evolution and damage-assisted diffusion acceleration along localized damage bands. The numerical results show excellent agreement with the experiments in both magnitude and spatial distribution, confirming that the chain mechanism of pore redistribution, damage amplification, and diffusion acceleration is the key driver of residual property degradation of C/SiC composites under high-temperature and low-pressure service conditions, and providing a validated predictive framework for the design of thermal protection components.