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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.

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The mechanical behaviors of three-dimensional angle-interlock woven composites (3DAWCs) after thermo-oxidative aging under different strain rates are imperative to retain safety in high-temperature conditions. This work presents the thermo-oxidative aging effects on strain rate sensitivity and compression failure mechanism of 3DAWCs based on the digital image correlation technique and finite element analysis. The results revealed that the compressive modulus and maximum stress of the 3DAWCs are strain rate sensitive, while the thermo-oxidative aging would reduce the strain rate sensitivity. The strain rate coefficient of the resin and composite decreased by approximately 9% and 25% after thermo-oxidative aging, respectively. The compressive damage failure modes of the 3DAWCs mainly include resin cracks, interfacial debonding, and yarn fracture. The strain rate has effects on damage initiation and brittle-ductile fracture behavior. The aging effect changes the crack propagation path, but does not affect the inclined 45° shear mode. In addition, the strain rate effects on strain localization were found and will be enhanced after thermo-oxidative aging.

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