Toughening mechanisms in cost-effective carbon-epoxy laminates with thermoplastic veils: Mode-I and in-situ SEM fracture characterisation

This study investigates the influence of thermoplastic non-woven veils on the interlaminar fracture energy and toughening mechanisms under mode-I dominant loading conditions in out-of-autoclave resin-infused carbon fibre-epoxy laminates. Two different non-woven micro-fibre veils, Polyetherimide (PEI) and Polyphenylene Sulfide (PPS), of low areal weight (∼10 g/m²) are introduced between carbon fibre non-crimp fabric (NCF) laminae prior to vacuum assisted resin infusion (with a low viscous two-part epoxy resin, i.e. Araldite 564/Aradur 2954). Double Cantilever Beam (DCB) specimens are tested for characterising mode-I fracture energies, and a novel miniature specimen geometry with an embedded fibre-discontinuity to trigger delamination under mode-I dominant loading is employed for in-situ SEM characterisation of micro-damage evolution and toughening mechanisms. The results obtained from DCB specimens showed that the micro-fibre thermoplastic veils considerably improved the mode-I fracture energy when compared with the untoughened fracture energy (i.e. ∼13% increase with PEI and ∼60% with the PPS veils, at the onset of crack propagation). Optical and scanning electron microscopy revealed that the veil parameters such as fibre dispersion, fibre diameter and specific surface area play an important role in enhancing fracture energy. Fracture surface micrographs indicated that progressive fibre-matrix debonding followed by fibre pull-out and fibre bridging resulted in higher fracture energy within the PPS veils. Interleaving a relatively low-cost NCF composite with thermoplastic veils could be as resistant to delamination as aerospace-grade carbon fibre-epoxy systems.