Shear failure mechanisms of C/SiC composites in high-temperature and low-pressure environments

During the service of the reusable suborbital vehicle, C/SiC composites are subjected to an extreme coupled environment, including high temperature and low-pressure. The shear performance is a key bottleneck in the design of structural reliability. In response, the residual in-plane and interlaminar shear properties of C/SiC composites are evaluated after exposure to four conditions: room temperature, 1200 °C in vacuum, 1400 °C in vacuum, and 1200 °C at 5 kPa. Combined with digital image correlation (DIC) and scanning electron microscopy (SEM), the macroscopic mechanical responses and microscopic failure mechanisms are analyzed. The results indicate that in-plane shear performance exhibits weaker temperature sensitivity than its interlaminar counterpart. When subjected to in-plane shear loading, the continuous fibers dissipate energy via fiber pulled out and bridging, whereas interlaminar shear relies on a single interface phase for bearing. Under high-temperature vacuum conditions, thermal damage accumulates, and shear performance drops markedly. Nonetheless, the silicon oxide layer formed under 1200 °C − 5 kPa facilitates uniform strain transfer, leading to a marginal improvement in both in-plane and interlaminar shear performance compared to vacuum conditions at equivalent temperatures. The microscopic mechanism reveals that a weakly oxidized atmosphere under low-pressure can retain the fiber–matrix interface bonding force, which is the core root cause for the performance recovery. This work provides data support and reference for the design of the thermal protection system of the reusable suborbital spacecraft.