The effects of non-proportional biaxial loading paths on ductile fracture initiation toughness are studied in this paper. To this end, the growth of a cylindrical void (hole) located in front of a mode I plane strain crack has been studied using large deformation finite element analysis (FEA). A specific microstructural feature of a steel alloy was thoroughly studied by having a single void positioned at a fixed distance from the crack tip and void that was equal to 10 times the diameter of the void. In particular, the non-proportional biaxial loading path effects on the crack tip blunting, void-growth, ligament reduction and near-tip stress fields are investigated computationally. Under small-scale yielding conditions, one proportional loading and two non-proportional loading paths are applied to the modified boundary layer (MBL) model. It is observed that the non-proportional load paths have a marked effect on the void growth, crack tip blunting and their interaction. By applying the criteria for the coalescence of the crack tip and void, the ductile fracture initiation toughness is estimated. It is shown that the ductile fracture toughness is dependent on loading paths, and the T-stress ratios. Results from this study are of relevance to ductile fracture assessment of components or pressure vessels that operate under non-proportional biaxial loading conditions.

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