Impact compression damages of 3D braided composites with/without axial yarns after thermo-oxidative ageing

Ageing conditions and braided structures will affect dynamic damage and failure mechanisms of 3D braided composites. Here we report the influence of thermo-oxidative ageing on impact compression behaviors of 3D braided composite with and without axial yarns, i.e., four directional (3D4d) and five directional (3D5d) braided composites. We found a significant difference in mechanical properties between epoxy resins and composites after ageing. The impact compressive strength of epoxy resins declined rapidly and then slowly, while the composites decreased continuously, with the increase of ageing days. The interfacial debonding is main degradation mechanism for both braided composites at the later ageing stage. Thermo-oxidative ageing accelerates the inner damage evolution which propagates from surface cracks. Compared with 3D4d braided composites, the 3D5d braided structure has higher strength retaining level after ageing. In addition, the 3D5d braided samples have higher resistance to impact deformation and lower crack propagation. Finite element analysis (FEA) results revealed that 3D5d composites showed higher load carrying capacity and less damages than 3D4d composites.