Ultra-high temperature mechanical behavior and microstructural evolution of needle-punched carbon/carbon composites under time-varying thermo-mechanical coupling conditions

Carbon/carbon (C/C) composites are extensively employed in the thermal protection systems of hypersonic vehicles, and the precise acquisition of critical process information is vital for the reliable design of such vehicles. Consequently, this research introduces a high-temperature repeated loading testing protocol for needle-punched C/C composites, aimed at characterizing the mechanical behavior of re-entry vehicles in intricate thermal–mechanical coupling environments. Initially, an ultra-high-temperature speckle pattern was prepared using plasma spraying and laser etching techniques, which is suitable for the temperature range of this study (room temperature to 2000 °C). Subsequently, under time-varying temperature and load conditions, the local strain field and tensile properties were investigated. In the single-loading test, at 1500 °C, the stress–strain curve slope decreased by up to 58 %. In the cyclic loading test, at 2000 °C, the slope increased by up to 46 % with the number of cycles, while the specimen strength decreased by up to 27.1 % compared to the standard test. By examining fracture morphology and internal structure at both macroscopic and microscopic scales, the study elucidated how interfacial performance and the level of graphitization contribute to the tensile behavior. The results indicate that as the number of loading cycles increases, the stress–strain curve slope is primarily influenced by interfacial properties and carbon fiber graphitization, with each playing a dominant role at different loading stages. Additionally, tensile strength decreases with the rise in loading cycles, positively correlating with interfacial performance and inversely with carbon fiber graphitization.