Overloads in ductile and brittle materials

The first part of the paper presents fatigue crack propagation experiments with single overloads at different overload ratios and specimen thickness in a very ductile austenitic steel. The results show that in the Paris regime in a ductile material, the overload effect can be explained solely in the framework of the change of the plasticity-induced crack closure. Other effects such as strain hardening, blunting, additional damage, crack deflection and branching are not significant. Whether or not this behaviour can be observed in less ductile materials and also in the threshold regime is investigated in the second part. Periodic overload experiments were performed on a relatively ductile 2124, and a more brittle 359, particle-reinforced aluminium alloy. In the Paris regime, the retardation in the 2124 reinforced alloy showed the expected behaviour for a ductile material, whereas in the 359 reinforced cast alloy, an acceleration of the mean growth rate was observed. Near the threshold the difference between the two alloys and the effect of the periodic overloads decreased.