Effect of through-the-thickness stitching on postbuckling and impact response of composite laminates

Fiber composite materials have a very high specific strength and stiffness. This structural efficiency enables composite materials to be used in a wide variety of applications. However laminated composites have also some disadvantages such as poor inter-laminar strength, low impact resistance and poor delamination resistance. During the manufacture of fiber-reinforced composite laminates, imperfections can cause initial delamination, and impact on laminated composites by foreign objects during service can cause delamination propagation. Delamination is one of the most dominant forms of damage due to lack of reinforcement in the thickness direction. An effective method to provide this reinforcement is to use through-the-thickness stitching.; When the delaminated composite is under compression, it may undergo post-buckling. When the energy release rate is greater than critical energy release rate, the delamination will propagate in an unstable manner and finally cause catastrophic failure of the composite. In this dissertation analytical and finite element methods are proposed to investigate the effect of through-the-thickness stitching on the post-buckling of a delaminated composite. In particular the studies focus on the effect of stitching on buckling load, load-displacement behavior and energy release rate.; When a delaminated composite is subject to a low velocity impact, the crack may propagate and finally cause catastrophic failure. In this dissertation an analytical model is proposed to understand the effect of stitching on improving impact damage resistance. In particular the studies focus on the effects of stitching when the impact energy, delamination length and location, and the fracture toughness are varied.