Fatigue Crack Tip Plasticity Associated With Overloads and Subsequent Cycling

The plastic zones associated with single overloads of cyclically loaded specimens have been mapped using electron channeling patterns. The zones are asymmetric with respect to the crack tip, and are complex in shape. Crack retardation subsequent to an overload is closely related to the size and shape of the overload zone, but has no apparent relationship to the maximum zone dimension. Following an overload, cracks try to exit from the monotonic zone by moving toward the nearest elastic-plastic boundary. The size of the overload zone is predicted by a plane strain rather than plane stress relationship. The minimum retarded growth rate corresponds to an effective stress intensity factor no greater than the threshold value for Stage II growth. This is caused by crack closure, with minimal crack tip shear strains and an absence of crack tip opening and blunting. Since the crack growth rate quickly approaches the preoverload rate once the crack crosses the overload boundary, it appears that residual stress within the overload plastic zone is the key factor in governing crack retardation.