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 usi ng a dislocation model. The plastic zone is approximated by a superdislocat ion, and linear elasticity is assumed. Dislocation effects are computed usi ng the Lin-Thomson equations. The error involved in the superdislocation ap proximation is shown to be small. Results indicate that the plasticity ahea d of the crack tip induces a large retarding force which a crack must overc ome 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, howe ver, has a negligible effect on the crack tip driving force and therefore h as no effect on the K-max threshold. Overload effects, underload effects an d fatigue crack growth resistance with increasing K are all relatable to th e 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 rel ated to the effects of monotonic and cyclic plastic zones. Since the effect s of plasticity in the wake are negligible, plasticity-induced closure due to crack wake plasticity is also negligible from the dislocation point of v iew. 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.