Improving the delamination resistance and impact damage tolerance of carbon fibre-epoxy composites using multi-scale fibre toughening

This paper presents an experimental investigation of the multi-scale toughening mechanisms for improving the interlaminar fracture toughness, low-velocity impact damage resistance and compression-after-impact (CAI) strength of continuous carbon fibre-epoxy composite laminates using carbon nano- and micro-fillers spanning the nanometer to millimeter length scales. The fillers used in the epoxy matrix of the composite laminates were carbon nanofibres (CNFs) and short carbon fibres (SCFs). When used together, these two fillers yielded a less-than-additive improvement to the mode I fracture toughness. However, they created a synergistic (i.e. greater-than-additive) increase to the steady-state mode II toughness, compared to when the fillers were applied separately. Hybridising the CNFs with the SCFs also improved the impact damage resistance (up to 24%) and CAI strength (up to 29%) of the laminate at various impact energies. The key toughening mechanisms imparted by the CNFs and SCFs, when used separately or in combination, were identified through analyses and scanning electron microscopy.