Effect of substrate modulus on the fatigue behavior of adhesively bonded joints

The effect of the substrate material on mode-I fatigue behavior of a toughened epoxy adhesive system was examined in terms of the substrate stiffness and curing residual stress. It was found that a change in adherend material from aluminum to steel caused a reduction in the fatigue performance; i.e. the threshold energy release rate decreased and the crack growth rate increased for a given applied energy release rate. The possibility that these observations were a result of adhesive curing residual stresses was studied experimentally and analytically, but it was found that such effects were relatively small. Finite element modeling showed that the fatigue results could be explained in terms of an increase in the crack tip stresses and an enlarged plastic zone due to the greater modulus of steel compared with aluminum. The local influence of the adherend modulus proved to be much more significant than the global effect of the adherend stiffness (product of modulus and moment of inertia). The effects of adherend modulus are expected to diminish as the phase angle increases.