Fracture Toughness Gain Through Artificial Fiber Bridging in Needle-Punching Glass Fiber/Epoxy Woven Laminates

Fiber bridging is a toughness-enhancing mechanism, active in unidirectional laminates, but significantly weakened in woven laminates, due to a stronger fixation of the fibers in the interwoven yarns at the sides of the interlaminar crack. Needle-punching or felting, creating interlaminar Z-connections, is a simple way to get through-the-thickness reinforcement and drastically improve Mode I and Mode II fracture toughness of a composite laminate. When felting is batt-less (no nonwoven interleave is added), then it can be seen as creation of artificial fiber bridging. The fracture toughness increase is fully provided by connections through the crack opening, without influence of the thickness of the interleave. The paper investigates this phenomenon based on characterization of the felting microstructure in glass fiber/epoxy woven composite laminates. Analysis of the microstructure of the reinforcement establishes the relation between the felting parameters and intensity of the through-the-thickness reinforcement, characterized via the number of the pulled fibers per the punching site and the pull-out length of the fibers in the composite. These microstructural parameters are linked to the increase of the fracture toughness by 30%–40% for Mode I, II. The felting influence on the resistance to low velocity impact is analyzed as a possible use for the proposed technology.