Plasticity-induced crack closure under plane-strain conditions in the near-threshold regime

The change in cyclic plastic deformation at a fatigue crack tip as a function of crack extension and the variation in the crack closure are investigated. The cyclic plastic deformation is modelled by the motion of discrete dislocations. At constant applied crack driving force, the cyclic plastic deformation, that is the cyclic crack-tip-opening displacement, decreases with increasing crack extension. This is caused by an increase in stress intensity factor where the crack closes or, in other words, by a decrease in the local driving force. The crack closure stress intensity reaches a constant level after a crack extension of the order of the size of the plastic zone. Near the threshold of stress intensity this effect, namely plasticity-induced crack closure, is somewhat larger than in the Paris regime.