Role of crack tip plasticity in fatigue crack growth

The effect of plasticity ahead of the crack tip as well as behind the crack tip (crack wake plasticity) on the crack tip driving force is examined using a dislocation model. The plastic zone is approximated by a superdislocation, and linear elasticity is assumed. Dislocation effects are computed using the Lin-Thomson equations. The error involved in the superdislocation approximation is shown to be small. Results indicate that the plasticity ahead of the crack tip induces a large retarding force which a crack must overcome for it to grow. The existence of a threshold in K-max for fatigue crack growth as illustrated by the unified two-parameter approach can be related to this crack growth resistance. The plasticity behind the crack tip, however, has a negligible effect on the crack tip driving force and therefore has no effect on the K-max threshold. Overload effects, underload effects and fatigue crack growth resistance with increasing K are all relatable to the internal stresses arising from the dislocations in the plastic zone. The two thresholds K-max,K-th and DeltaK(th) in the unified approach can be related to the effects of monotonic and cyclic plastic zones. Since the effects of plasticity in the wake are negligible, plasticity-induced closure due to crack wake plasticity is also negligible from the dislocation point of view. A three-zone approximation for the plastic enclave around crack tip is shown to be adequate for the description of the role of plasticity in the crack tip driving force.