Improving interlaminar fracture toughness of flax fiber/epoxy composites with chopped flax yarn interleaving

In this research, unidirectional flax fabrics reinforced epoxy laminates were interleaved with randomly oriented chopped flax yarns at various yarn lengths and contents. Mode I interlaminar fracture toughness of the laminates was evaluated via Double Cantilever Beam(DCB) tests. The results showed that Mode I interlaminar fracture toughness increased with the introduction of the chopped yarns. With moderate yarn length and content, the best toughening effect(31% improvement in Mode I interlaminar fracture toughness) was achieved. It was observed with the aid of Scanning Electronic Microscopy(SEM) that the introduction of the chopped yarns resulted in more tortuous in-plane crack propagation paths as well as the "trans-layer" phenomenon and fiber bridging effect of both the unidirectional yarns and the chopped yarns. These hindered the growth of the crack and led to more energy dissipation during delamination progress. Excessive yarn length or content would induce unstable crack propagation and thus weakened the toughening improvement. No remarkable change was found in the tensile properties and the Charpy impact strength for the interleaved laminates, which indicated that this interleaving method was effective on interlaminar toughening without sacrificing the comprehensive mechanical properties of the laminates.