Delamination growth rate in composite laminates under increasing low-velocity impact energy

Impact damage, and the reduction in strength it causes, continue to drive sizing of modern composite aircraft components. Despite the morphology of delaminations at interfaces near the non-impact surface of a laminate being critical to compression after impact failure and decades of research, computationally efficient, early-stage analytical design tools for calculating interface-by-interface damage do not yet exist. This study investigates, interface-by-interface, delaminations created by impact tests on carbon fibre/epoxy laminates with different quasi-isotropic stacking sequences each obtained from standard fibre angles. Fifty-three impact tests were performed with a 75mm circular test window under a range of impact energies providing results for 48 stacking sequences. Results show that the morphology of delamination caused by impact damage at each interface depends on the fibre angle of plies bounding the interface and is independent of stacking sequence or interface location within the stacking sequence. Conversely, the extent of delamination at each interface was found to vary with the location of the interface within the stacking sequence. Rate of growth of delamination with increasing impact energy is shown to vary with the difference in ply angle at an interface and some correlation is seen with through thickness distribution of bending and shear stresses during impact. This paper provides experimental data which can inform the development of models for damage development at interfaces near the non-impact surface of composite laminates subject to an impact event.