Interlayer toughening mechanisms of composite materials

Interlayer toughening in polymer-matrix composite materials can address the usual suspects in regard to the failure of laminated composites. Issues contributing to poor interlaminar strength and toughness can be delayed or eliminated by interleaving, which also suppresses matrix cracking, whether the root cause of delamination is isolated or synchronous. The interlayers/interleafs are considered herein as the additional design features, enabling tuning of the ply-to-ply interfacial (interlaminar) regions. A comprehensive literature review is presented for different interleaving strategies, with a special focus on nanofibrous electrospun interleafs. A road map and a series of examples are also discussed for effective incorporation of the nanofiber interlayers/interleafs into the laminated composites. Toughening mechanisms in the presence of electrospun nanofiber interleafs are shown to be effective under both in-plane and out-of-plane loading. Specifically, epoxy compatible poly(styrene-co-glycidylmethacrylate), P(St-co-GMA), and P(St-co-GMA)/MWCNT nanofibrous interlayers incorporated into carbon/epoxy laminated composites are exemplified for enhancing mechanical behavior under longitudinal and transverse tension, open-hole tension, three-point bending and end-notched flexural tests. Moreover, the working mechanism of these interlayers under in-plane loads is further elaborated by the custom design tensile tests of (0₂/90₄)_s interleaved laminates, backed-up by cracking-sound recording and analysis.