Unstable crack growth in a component subject to constant applied loads: The interaction between plasticity and geometrical effects

When a crack grows slowly by, for example, a stress corrosion mechanism, in a component that is subject to constant applied loads, it is possible that there will be a transition from slow growth to unstable nonenvironmentally-assisted growth. Earlier work has shown that, even though a material's resistance to nonenvironmentally-assisted crack growth may be only moderate, the transition should not occur unless plastic deformation traverses the ligament between the crack front and the opposite free surface, i.e. unless general yield occurs. The earlier work analyzed the model of an edge crack in a semi-infinite solid, and compared the extent of spread of plasticity with the solid width in the real situation, thereby neglecting the interaction between the plastic zone and the boundary opposite the crack in the real situation. This paper extends the scope of the earlier investigation by analyzing a model in which the interaction between plasticity and geometrical effects is accounted for in an accurate manner. Though it is shown that unstable nonenvironmentally-assisted growth can occur before general yield, such growth occurs so close to general yield that the earlier work's conclusions are essentially unchanged. The implications of the conclusions are discussed in relation to the empirical net-section stress approach for predicting failure in ductile materials.