A dislocation model for crack tip blunting in slightly predeformed low alloy steels

Previous experimental results have shown that there is a marked increase in fracture toughness in a slightly predeformed low alloy steel, namely AISI 4340 steel. This increase in toughness was attributed to an excessive blunting at the crack tip due to the presence of a high mobile dislocation density. A dislocation model is proposed to explain the crack tip blunting in predeformed low alloy steels which leads to an equation relating the fracture toughness parameter JIc to the mobile dislocation density in the vicinity of the crack and the extent of work hardening in the plastic zone. The model accurately predicts the dislocation behaviour ahead of the crack and can be used to calculate the fracture toughness in materials which exhibit considerable plasticity at the crack tip.