Damage Evolution in Stitched and Unstitched Graphite/Epoxy Laminates Subject to Low-Velocity Impact

The response to low velocity impact loadings, which may occur during manufacturing, service and maintenance, is a key factor in the design of composite material structures. Many techniques, ranging from the adoption of tough matrices/high-strain fibers to the introduction of through-thickness reinforcement, have been proposed to improve the damage response of composite laminates subject to impact. The insertion of transverse reinforcing threads by stitching, in particular, appears very promising to restrict damage growth and to improve post-impact performance of laminates. In order to develop general models capable of addressing the issues of damage tolerance and resistance, detailed understanding of the nature and extent of damage, identification of the dominant fracture modes and assessment of the effect of stitches on the damage development are essential. In this study instrumented drop-weight tests were carried out to examine and compare the damage response of stitched and unstitched cross-ply graphite/epoxy laminates subject to low-velocity impact. The progression of damage and its relationship with impact and absorbed energy were investigated by means of an extensive series of damage observations, conducted with various techniques (X-radiography, ultrasonics, optical microscopy, deply).