Effect of resin crosslink density on the impact damage resistance of laminated composites

Abstract It is generally recognized that fiber‐reinforced laminated composites are susceptible to damage resulting from low‐velocity impacts. Over recent years, many strategies have been devised to increase the fracture toughness of resin matrix materials with the aim of improving the composite's overall resistance to impact damage. One popular strategy for enhancing the fracture toughness of thermosets involves increasing the molecular weight between crosslinks, which, in turn, enhances the resins ductility. In this paper, we investigate the efficiency of this toughening approach with regard to resisting damage in composite laminates subjected to low‐velocity impacts. A mechanistic study shows that at least two distinct processes occur during an impact event. First, the laminate experiences a local failure, which resembles a Hertzian fracture process followed by subsequent delamination between the plies. Hertzian fracture occurs once at a critical threshold level initiates laminate damage through the development of a spatially configured array of matrix microcracks, which resemble that of a Hertzian cone together with radial cracks. Further damage accumulates in the laminate by inter‐ply delamination with the size of delaminated area increasing coincident with the impact load. Systematic changes in resin crosslink density show that both damage initiation and accumulation are affected. However, the maximum resistance for damage initiation occurs at a much higher crosslink density than that measured for damage accumulation.