Fatigue analysis and testing of adhesive joints

The fatigue resistance of a single-lap aluminium adhesive joint to cyclic loading in combined shear and bending mode is investigated by nonlinear finite element analysis and crack propagation experiments. The epoxy adhesive is modelled by an elasto-plastic overlay material model. The initial cycles build up a residual stress state, leading to nearly linear material behaviour in the following cycles. Fatigue crack propagation is modelled by removing adhesive elements. Two series of experiments with one-sided cyclic load were carried out. The crack length was monitored by measuring the bending compliance around the end of the overlap with clip-gauges. The crack length is determined as a simple linear function of the measured compliance. The experiments show nearly constant rate crack growth until failure, with no appreciable crack initiation period. The rate of crack growth is proportional to the stress level to the power m = 6.2. Fatigue life results are given in the form of S-N curves for adhesive thickness of 0.1 and 0.3 mm. There is no systematic influence of the thickness of the adhesive on the fatigue life. This supports the use of a crack propagation and fatigue life criterion formulated in terms of the energy release rate.