Effect of rapid manufacturing on the performance of carbon fibre epoxy polymers

The effect of cure rate on the mechanical and fracture properties of both bulk epoxy polymers and carbon fibre-reinforced polymers (CFRP) containing reaction-induced phase-separating liquid rubber was investigated in the current work. The fracture energy of the bulk epoxy polymers increased from 229 to a maximum value of 3434 J/m². This was achieved by introducing 9 wt% phase separating rubber while keeping a curing isothermal temperature of 45 °C. A reduction in the maximum measured toughness was noted when these polymers were cured at higher isothermal temperatures and hence higher rates. The mode I propagation fracture energy for composite laminates cured using the modified bulk polymers as matrices increased from 633 to only 934 J/m². The low rise in toughness for the composite laminates is explained due to the inefficient transfer of fracture toughness from bulk polymers to fibre-reinforced polymers for toughened resins. The measured properties of the bulk polymer together with fractography studies demonstrate that this occurs due to the geometrical obstruction of the fibres preventing an unbounded fracture process zone from forming ahead of the crack tip. Increasing the weight fraction of toughening nano-modifiers in CFRP beyond a small amount does not result in further toughness gains. The results of this work underline the importance of considering material processing history when designing composite structures.