Compressive anisotropy effects and crack evolution mechanisms of 3D5d braided composites induced by thermo-oxidative ageing

Ageing cracks and loading directions will affect mechanical behaviors of thermo-oxidative aged 3D braided composites. Here we report ageing crack propagations, compressive responses, in-situ strain evolutions, full-field damage distributions and crack evolution mechanisms of aged three-dimensional five-directional (3D5d) braided composites along out-of-plane (OP) and in-plane (IP) loading based on X-ray micro-computed tomography and digital image correlation technique. Ageing cracks are distributed at both axial ends that corresponding to the yarn arrangement and extend to the interior with the increase in ageing time. Compressive modulus, strength, retention rate and damage mode show ageing weakening behaviors and anisotropy effects. Strain distributions exhibit structure correlation. Ageing cracks affect the surface strain and damage distributions but not internal shear modes during OP loading. Yarn arrangement affects damage evolution and accumulation. Ageing cracks change the damage evolution path in both directions. The synergistic effect of ageing crack with IP loading is greater than that with OP loading. It is expected that this work can provide a clear guideline for the structural optimization and durability design of 3D braided composites.