Enhancing mode-I and mode-II fracture toughness of epoxy and carbon fibre reinforced epoxy composites using multi-walled carbon nanotubes

Abstract Multi-walled carbon nanotubes (MWCNTs) were added to an epoxy resin in an effort to improve the fracture toughness of bulk epoxy and also when used as matrix for carbon fibre reinforced epoxy composites (CFRPs). The incorporation of MWCNTs to bulk epoxy and CFRPs moderately increased the mode-I fracture energy, and significantly increased the mode-II fracture energy, i.e. the average mode-II fracture energy of CFRPs increased from 2026 J/m 2 to 3406 J/m 2 due to the addition of 0.5 wt% MWCNTs, and further to 5491 J/m 2 due to the addition of 1 wt% MWCNTs. The superior toughening performance of MWCNTs in mode-II fracture is attributed to two reasons: 1) increased MWCNT breaking and crack deflection mechanisms under shear load; and 2) large fracture process zone accompanied with extensive hackle markings and micro-cracks ahead of the mode-II crack tip of CFRPs, which resulted in significant number of MWCNTs contributing to toughening mechanisms.