Bondline toughening of vacuum infused composite repairs

A programme of work was undertaken to improve the toughness of the bondline in vacuum infused repairs of carbon fibre composites. The brittle resin used to infuse a repair patch was toughened by the use of fine fibre veils introduced at the bondline, between the repair patch and the substrate material, prior to the infusion process.The toughness of the repair was monitored by undertaking mode-I fracture toughness measurements utilising double cantilever beam specimens cut from plates that were manufactured to simulate the repair process but with a finite length starter crack introduced between the repair patch and the substrate. The veils used to toughen the bondline consisted of carbon fibre veils, polyester fibre veils and combinations of the two. The experiments showed that the veils did not impede the production of high quality repairs and that the bridging effects introduced by the veils resulted in a substantial increase in the measure bondline fracture toughness. The most effective veil was the combination of a double layer of polyester fibres with a central carbon fibre layer. The toughness values for initiation of the cracks were increased from 0.13 to over 0.9 kJ m−2 by the use of the bondline veils. The toughened bonds also exhibited a considerable R-curve effect. The toughening mechanism was considered using a bridging analysis to estimate the crack closure stresses induced by the bridging of the veil fibres across the crack front. The use of fine veils to enhance bond line toughness is therefore proposed as a potential route to improve the long-term durability of infused repairs for situations such as fatigue loading where bondline cracking could result in premature failure of a repair patch.