CHARACTERISATION OF VOIDS AND THEIR INFLUENCE ON DAMAGE PROPAGATION IN RESIN INFUSED CARBON FIBRE REINFORCED POLYMERS

High-performance structural composite components have historically been most often manufactured with prepreg materials cured in an autoclave oven. To remain cost-competitive, it is generally assumed that in the future an increasing fraction of parts will be manufactured with a faster, less expensive process such as liquid composite moulding (LCM). The weakness of LCM processes is the generally higher void content compared with prepreg and autoclave processing, due to entrapment of bubbles during infusion. The effect of voids on the resulting mechanical properties is still relatively unclear, especially for heterogeneous void distributions, which is always the case in LCM. Intuitively it is expected that void parameters such as size, shape, location, clustering, and local concentration would play a role in damage development and failure, but their relative effects are currently not well understood. A better understanding of these micromechanisms could ideally give some indications as to how to supress damage formation and improve the mechanical performance.This study focuses on the formation of voids in resin transfer moulded carbon fibre reinforced polymers and their effect on damage development that ultimately leads to final failure. This was done by characterising voids in the resin of the cured laminates using X-ray computed tomography (CT), and by looking at the material failure mechanisms associated with in-plane and out-of-plane behaviour. Laminate coupons were tested under low cycle tension fatigue and low velocity out-of-plane impact/indentation. The void distribution was found to be heterogeneous with voids nucleating in plies where the fibre orientation was not parallel to the flow direction, at the ply interfaces, and along woven yarns. In out-of-plane loading, multiple cracks/delaminations were seen to interact with these void regions. As a consequence of increased loading, these delaminations grew and coalesced with their neighbours to form a single larger delamination prior to final failure. In open-hole, in-plane tension fatigue testing, larger voids resulted in delaminations initiating at the void. Transverse (out-of-plane) cracks are seen to form around smaller voids.