Mode I fracture of epoxy bonded composite joints 2. Fatigue loading

The main aims of this work were to investigate the effect of temperature on fatigue crack propagation (FCP) in bonded joints and to compare this with fracture under quasi-static loading and fatigue failure in uncracked lap joints. The fatigue tests were conducted on epoxy bonded carbon fibre reinforced polymer joints at −50°C, 22°C and 90°C and a number of techniques for determining strain energy release rate and crack propagation rate were evaluated. It was seen that temperature had a significant effect on the locus of failure and FCP, indicating that service temperature must be taken into account when designing bonded composite joints. The applicability of fracture mechanics data to the prediction of fatigue failure in uncracked lap joints was assessed by attempting to predict fatigue thresholds in two types of lap joints at three different temperatures. In most cases reasonable predictions were made, the notable exception being the overprediction of the fatigue threshold load in double lap joints tested at 90°C. This was attributed to creep in the double lap joints, which accelerated fatigue failure. It was recommended that in order to improve current prediction techniques, efforts should be made to base predictive methods on accurate physical models of the degradation and failure processes in the joints.