Damage evolution of (0°/90°)/(±45°) woven C/SiC composites at high temperature: Analysis via in-situ X-ray CT and nnU-net segmentation

Carbon fiber-reinforced silicon carbide (C/SiC) composites exhibit significant potential for aerospace thermal structures, yet their service integrity is challenged by complex damage evolution involving multiple modes. In this study, damage evolution in (0°/90°)/(±45°) woven C/SiC composites was investigated under tensile loading at room temperature (RT) and 1200 °C using stepwise in-situ X-ray computed tomography (X-ray CT) with a thermomechanical loading stage, together with post-fracture scanning electron microscopy (SEM). A 2D nnU-Net architecture was employed to segment four internal low-density feature classes: voids, matrix cracks, tow cracks, and delamination cracks. Voids and matrix cracks were predominantly pre-existing and generally changed modestly during loading, although some near-surface voids expanded and coalesced at RT. Delamination cracks and tow cracks exhibited more pronounced load-dependent evolution. At RT, delamination cracks were observed during early loading in near-surface regions, particularly around ±45° fiber tows, and subsequently extended progressively along tow boundaries, with local contact or connection with pre-existing voids. Local crack deflection was also observed in the ±45° tow regions, where loading may introduce a shear component. The final fracture surface was irregular and tortuous, with pronounced fiber pull-out. At 1200 °C, under the present testing conditions, the specimens exhibited higher tensile strength than the specimens tested at RT. The evolution of delamination and tow cracks remained limited during the early and intermediate loading stages but increased rapidly near final failure. The final fracture surface was comparatively flatter, with less pronounced fiber pull-out. These findings relate the temperature-dependent internal damage sequence to the corresponding fracture morphology in the investigated woven architecture, providing valuable insight into the damage assessment of C/SiC composites.