Estimation of plasticity-induced crack closure in a pre-existing plastic zone

Abstract

To estimate the plasticity-induced crack closure in a pre-existing plastic zone using Newman's strip-yield model, the interactions between the crack plasticity and the pre-existing plasticity must be known. In the current study, the total plasticity of a fatigue crack propagating in a pre-existing overload plastic zone and a notch plastic zone was studied using finite element analysis. The calculated total loading-direction crack-tip plastic deformation was compared with the palstic deformation caused by two types of plasticity: (1) the pre-existing plasticity; and (2) the crack-tip plasticity without the effects of pre-existing plasticity. The results show that in an overload plastic zone, a fatigue crack cdoes not produce its own crack-tip plastic-strain field when the strain field is not as severe as the overload plastic-strain field. However, in a notch plastic zone, a fatigue crack always produces its own plastic-strain field. Simple relationships to estimate the total plastic deformation ahead of the crack-tip were proposed using the plastic deformations caused by the two types of plasticity. These relationships were used to predict the fatigue crack closure behavior in a pre-existing plastic zone by incorporating them into Newman's strip-yield model to make the predicted crack closure quantity more consistent with the published experimental results.