Effects of Mode-I and Mode-II interlaminar fracture toughness on the energy absorption of CFRP twill/weave composite box sections

Mode-I and Mode-II interlaminar crack growth affect the failure modes of the progressive crushing of composite box structures. These failure modes which are known as lamina bending, brittle fracture, transverse shearing and local buckling contribute to specific energy absorption (SEA) of composite box. In this regard, the effect of laminate lay-up of the composite crush box was sought by studying their effects on Mode-I and Mode-II interlaminar fracture toughness. The double cantilever beam (DCB), three-point-end-notched flexure (3ENF) and axial crush box specimens were fabricated from carbon/epoxy twill–weave fabrics of [0]4, [45]4 and [0/45]2 and they were tested under quasi-static condition to determine the interlaminar fracture toughness in Mode-I (GIC), Mode-II (GIIC) and SEA of each lay-up. It was shown that interlaminar crack propagation in Mode-I and Mode-II contributes significantly on the type of the progressive crushing mode and SEA. The interfaces of 0/45 and 0/0 have higher Mode-I and Mode-II interlaminar fracture toughness and as a result the crushed box with these lay-ups showed a higher energy absorption capability in comparison with crush box lay-up of [45]4. An analytical solution was proposed to predict the mean crushing force for each failure mode. The crushing process of composite boxes was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental results.