On the R-curve behaviour of carbon/epoxy laminates with core-shell rubber nanoparticle and micro-fibre veil hybrid toughening: Carbon vs PPS veils

This paper investigates the effect of non-hybrid and hybrid toughening, via core-shell rubber (CSR) nanoparticles and non-woven micro-fibre veils, on the delamination resistance and crack migration in carbon fibre/epoxy laminates under mode-I and mode-II conditions—with an emphasis on the effect of veil fibre properties on toughening mechanisms and fracture energies. Core-shell rubber particles, varying in size from 100 nm to 3 μm, with 0–10 wt% content, are dispersed within the epoxy resin. Two non-woven veils with contrasting fibre properties (i.e. one with ∼6 mm (50 wt%) and ∼12 mm (50 wt%) short carbon fibres and another with ∼6 mm short polyphenylene sulfide (PPS) fibres) with 10 g/m2 areal weights are used to introduce hybrid toughening at the interlaminar region. Carbon fibre/epoxy laminates with five different toughening routes (i.e. CSR toughening, carbon veil toughening, PPS veil toughening, carbon veil and CSR hybrid toughening, and PPS veil and CSR hybrid toughening) are manufactured with a two-part resin using vacuum infusion and out-of-autoclave curing. Double cantilever beam (DCB) and four-point end-notch-flexure (4-ENF) specimens are tested for mode-I and mode-II fracture energies and R-curves. Fracture surfaces are investigated to characterise crack migration and energy dissipation mechanisms. The results indicate that mode-I and mode-II fracture energies are significantly enhanced (e.g. ∼275% in initiation and propagation under mode-I) with the combined core-shell rubber nanoparticle and veil toughening investigated—significantly altering micro-failure mechanisms, crack paths and R-curves. It is shown that the low modulus PPS fibre veils together with CSR particles provide stable crack growth, while high modulus carbon fibre veils with CSR particles lead to unstable crack growth.