where *W* is the stored energy density, *u* is the displacement, $\sigma =\u2202W/\u2202F$ the stress, *n* is the normal to the contour Γ enclosing the crack tip, and *t* is the tangent to the crack at the crack tip. Conveniently, the *J***-**integral is path independent, i.e., it does not depend on the contour Γ as long as the medium is homogeneous. This makes it extremely useful, since one can choose contours along which it is most convenient to evaluate the integrand.