Fig. A2. (A) Counterclockwise contour around a crack tip showing an element of path length ds, with unit vector n normal to the path and with stress and displacement vectors, P and u, respectively, also shown. Crack length is a. (B) An element of material experiencing two-dimensional stress. Not all three stress components need be active; often only one or two components are active. (C) Rectangular path surrounding a crack tip, used to illustrate evaluation of a J-integral.