Interlayer modulus mismatch and shear-lag effect dominated failure mechanisms in UHTCs-C/C composites under multidirectional loading

The mechanical stability control of ultra-high temperature ceramic-modified carbon/carbon composites (UHTCs-C/C) during high-temperature synthesis remains a challenge for thermal protection systems. Mechanical degradation mechanisms of C/C–ZrC–SiC (CZS) composites fabricated via reactive melt infiltration (RMI) at 1800–2100 °C were investigated using digital image correlation (DIC) analysis of mechanical behaviors under compressive (single-stress) and three-point bending (combined-stress) loading conditions with a full-field strain measurement system. Notably, specimens treated at 2100 °C exhibited 27 % lower compressive strength and approximately 48 % reduced flexural strength compared to those processed at 1800 °C, alongside a progressively pronounced pseudo-ductile failure trend. Further investigations of strain responses revealed that elevated processing temperatures under single-stress conditions triggered periodic layered strain distributions, attributed to aggravated interlaminar modulus mismatch. Under combined-stress conditions, higher processing temperature gradually increased the proportion of shear strain, a phenomenon ascribed to shear-lag effect intensified by interlaminar modulus mismatch, which exacerbated the uneven distribution of stress. These findings provide novel insights for enhancing the mechanical performance of UHTCs-C/C composites.