On the mix-mode fracture of carbon fibre/epoxy composites interleaved with various thermoplastic veils

Abstract

This work studied the mix mode-I/II fracture behaviour of an aerospace-grade carbon fibre/epoxy composite that was interlayer-toughened by Polyamide-12 (PA), Polyphenylene-sulphide (PPS), Polyimide (PI), Polyethersulfone (PES) and Polyethylenimine (PEI) fibres. During the laminate curing process, the PA fibres melted, the PPS and PI fibres kept in their original form and the PES and PEI fibres dissolved in the epoxy matrix. This resulted in different toughening mechanisms of the veils for the mix mode-I/II fracture of the laminates, which was studied using a cracked lap-shear test. The main toughening mechanisms were observed to be plastic deformation and failure of the thermoplastic resin for the meltable PA veils, thermoplastic fibre debonding and bridging for the intact PPS and PI veils, and thermoplastic particle debonding and plastic void growth for the dissolvable PES and PEI veils. The experimental results revealed that the fibre debonding and bridging mechanism was superior for toughness enhancement, followed by the thermoplastic particle debonding and plastic void growth mechanism. For instance, interleaving the PPS and PEI veils increased the mix-mode fracture propagation energy of the laminates by 345% and 171%, respectively. However, the toughening performance of the PA and PI veils was poor, since the crack mainly propagated at the vicinity around the interlayer/laminate interface.