Experimental Determination of Plastic Constraint Ahead of a Sharp Crack under Plane-Strain Conditions

摘要

An experimental method of indentifying the plastic constraint ahead of a sharp crack loaded under plane-strain conditions is proposed. The method is based on the idea that the cleavage stress-which can be measured with unnotched bars--is the peak stress developed ahead of a crack just prior to crack extension. Ways of calculating the strain, strain rate, and yield stress appropriate for the plastic region just ahead of the crack are developed. The ratio of the cleavage stress to the local yield stress indentifies the plastic constraint factor at the stress level corresponding to crack extension. Experimental results recently reported by Krafft are shown to be consistent with this interpretation. With these data, the following expression for p.c.f., the plastic constraint factor, is deduced: p.c.f. = 1 + 2K/Y, where Y is the yield stress, K the stress intensity parameter, and the numerical constant, 2, has the dimensions inches to the -1/2 power. This result offers a way of formulating K sub/Ic, the fracture toughness for crack extension by cleavage, in more basic terms and sheds some light on the metallurgical origins of K sub/Ic. (Author)