The mechanical behaviour of 3D carbon/carbon composites under biaxial tensile-compressive loading

This study investigates the mechanical behaviour and damage evolution of three-dimensional (3D) carbon/carbon (C/C) composites under biaxial tension–compression loading. Initially, uniaxial tensile and compressive tests were conducted to establish the baseline mechanical properties. Subsequently, biaxial tension-compression tests at various stress ratios were performed using a cruciform specimen fixture in a biaxial testing system. The failure mechanisms were further elucidated through digital image correlation (DIC) and fractographic analysis. The results indicate that the stress state significantly alters the strength and failure characteristics of the 3D C/C composites. At a stress ratio R = −0.5, the weft-direction compressive stress enhanced the tensile strength by 35% because interfacial shear cracking was suppressed. Under R = −1, shear-dominated damage led to a 24% reduction in tensile strength. Due to the competing effects of tensile stress inhibiting fibre bending and promoting interfacial crack propagation, the biaxial compressive strength exhibits a non-monotonic trend (first decreasing and then increasing) with increasing tensile stress. Under compression-dominated loading (R = −2), the compressive failure stress increased by approximately 10%. This study provides critical experimental insights into the multiaxial mechanical behaviour of 3D C/C composites, supporting their application in complex load-bearing environments.