Crack-Tip-Acuity Effect on Crack Growth Initiation

For a stationary crack of an initially finite root radius in plane-strain tension, the strain fields around the smoothly blunted crack-tip are calculated for each deformation stage using the slipline field method for nonhardening plastic material. The crack growth initiation from the blunt crack-tip in small-scale yielding, with a full triaxiality ahead of the crack-line, is predicted based on the fracture criterion of the fracture strain at a characteristic distance, obtained by applying the criterion to the case of a sharp crack-tip, in which the effect of the initial root radius on the strain distribution can be neglected. The predicted toughness for mild steel is in reasonable agreement with an experiment. For other materials, the observed J-integral value at fracture initiation, consistent with the present analysis, increases linearly with the initial root radius, but its rate shows a large deviation from the model, which is discussed due to the shear band development. The analysis also predicts that the toughness of a cracked low-ductility material does not increase by making the initial crack-root profile round, and that the reduced triaxiality at the crack-tip brings about a toughness increase.