Multi-scale ageing mechanisms of 3D four directional and five directional braided composite's impact fracture behaviors under thermo-oxidative environment

Fracture behaviors of thermo-oxidative aged 3D braided composites are significantly affected by interfacial weakening and matrix degradation. Here we report the impact fracture behaviors of three-dimensional four direction (3D4d) and five directional (3D5d) braided composites under thermo-oxidative conditions. Specimens were exposed at hot air (180 degrees C) for 2, 4, 8 and 16 days, respectively. The oxidation-induced crack widths in aged specimen surface were observed and analyzed. The impact fracture behaviors and damage evolutions of braided specimens were recorded with high-speed camera. Based on the test results, we established a multi-scale finite element analyses (FEA) model to reveal interface failure, stress distribution and energy absorption of virgin and aged specimens at microstructure level. The impact fracture behaviors of both different braided composites continuously decreased with increasing ageing time. The matrix degradation and interfacial weakening are main degradation mechanisms. In addition, the FEA results show that the impact fracture properties significantly depend on the braided structures. The braided preform is the main energy absorption component and the axial yarns contribute the highest energy absorption capacity. With the insertion of axial yarns, the 3D5d braided composites exhibit a better fracture resistance after thermo-oxidative ageing.