The impact behaviour of high performance fibre composites

Abstract

The use of composite materials as structural reinforcements has increased significantly throughout the years. In aircraft structures, composite materials have replaced conventional materials such as aluminium to more than 50% of the aircraft's total mass. The advantages of composite materials include a high strength-to-weight ratio, and corrosion resistance. This has attracted designers to utilise these materials to enable a more cost effective design, without compromising the aircraft's structural integrity. However, composite materials are highly vulnerable to transverse impact loading. The need to improve their impact resistance is essential to achieve a more reliable and safer design. The research presented in this PhD aims to investigate the impact performance of laminated composites, both in low and high-velocity regimes. The investigation involves impact loading on three different fibre composites, namely Carbon fibre (IM7), Glass fibre (S2-Glass), and Thermotropic Liquid Crystal Polymer (Vectran), in a thermosetting epoxy matrix. Prior to this, all three laminates were characterised to obtain their in-plane mechanical properties, as well as the Mode I and II interlaminar fracture toughness. For the Vectran composite, the Mode I translaminar (fibre tensile) response was characterised to obtain its strain energy release rate, Gc, as well as understanding the complex failure mechanisms which contribute to its fracture toughness. All experimentally obtained properties were used for numerical modelling using the non-linear explicit Finite Element Method (FEM) software, LS-Dyna. Finally, an energy-based plane-stress User MATerial (UMAT) model was developed for the Vectran composite. The UMAT was used to simulate the in-plane, as well as the low and high velocity impact response of the Vectran composite. For carbon and glass composites, the commercially available energy-based material model in LS-Dyna was utilised to numerically reproduce the experimentally obtained impact response.