X-RAY COMPUTED TOMOGRAPHY OF DAMAGE FORMATION UNDER IN-SITU LOADING

Use of X-ray Computed Tomography (XRCT) to investigate damage morphology has previously been constrained to post-test analysis of unloaded coupons. As delaminations and intra-ply cracks close when load is removed, a limit is placed on the information available for identifying mechanisms causing resin and fibre fracture. Here, a newly developed loading stage, for in-situ XRCT imaging of laminates under quasi-static impact loading, is employed to visualise the mechanisms that drive the formation of damage morphologies. Multiple X-ray CT scans taken at increasing indenter displacements reveal the evolution of damage morphology. Various laminates subject to out-of-plane, near-edge or on-edge impacts are assessed. For out-of-plane and on-edge impact, both a conventional and novel stacking sequence are considered. Favourable formation mechanisms that occur in laminates with novel sequences are highlighted. In particular, damage from out-of-plane impacts is seen to occur in two stages. The first stage is instantaneous, with multiple shear-driven intra-ply cracks and inter-ply delaminations occurring at plies with dissimilar interfaces. In the second stage, shear-driven cracking gives way to peeling of layers. This peeling is focussed at certain weaker interfaces and is driven by an intact core of material pushing through the laminate. The latter process is clearly demonstrated by testing of a laminate with stacking sequence [0(4)/90(4)](s). Results indicate that stacking sequence can be used to force the development of favourable damage morphologies that protect load carrying plies and prevent the near surface delaminations which enable sublaminate buckling driven failures.