Delamination in adhesively bonded CFRP joints : standard fatigue, impact-fatigue and intermittent impact

The response of adhesive joints to three fatigue regimes, namely; constant amplitude sinusoidal loading (standard fatigue, SF), cyclic in-plane impacts (impact fatigue, IF) and a combination of the two (CSIF), has been investigated. The samples used in this study were carbon fibre reinforced polymer (CFRP) lap-strap joints (LSJs) bonded with a rubber modified epoxy adhesive. It was observed that fatigue fracture at very low load amplitudes occurred in IF. Two main patterns of failure were observed in SF; cohesive failure in the adhesive, which was linked to slow fatigue crack growth behaviour, and a mixed-mode failure, involving failure in both the adhesive and the CFRP. In addition, it was observed that the transition from cohesive to mixed mode failure was accompanied by crack growth acceleration. In IF it was seen that all failure was of a mixed-mechanism nature. In the combined standard and impact fatigue it was seen that the introduction of a relatively small number of impacts between SF blocks drastically changed the dynamics of fatigue crack propagation, increasing the crack rate. A further observation was that cavitation of rubber particles in the adhesive, which is seen as evidence of active toughening, was affected by the addition of impact loading.