Synergistic mode II delamination toughening of composites using multi-scale carbon-based reinforcements

This paper presents an investigation into the synergistic improvements to the mode II interlaminar fracture toughness of composites using multi-scale carbon reinforcements spanning the nano to millimetre length scales. Varying volume fractions of carbon nanofibres (CNFs) and millimetre-long carbon fibre z-pins were used to increase the mode II delamination resistance of multi-scale reinforced composites. The results reveal that adding CNFs into the epoxy matrix and transversely inserting z-pins creates a synergistic, rather than additive, improvement to the mode II fracture toughness. The magnitude of the synergistic toughening effect depends on the volume contents of both the nanofibres and z-pins, with the measured synergistic improvement to the mode II toughness being up to 26% (i.e. greater than the expected additive toughening contribution from each filler type). A finite element (FE) numerical model has been developed to accurately predict the mode II fracture properties and the synergistic toughening effect exhibited by the multi-scale reinforced epoxy composites.