ABSTRACT
First they envisage a finite region, ~a, that represents the process zone in which the actual separation occurs. They evaluate the separation energy Gt. over the region ~a by finite element computation and to that extent the explanation of their model is closely bound to their procedures. Gt. is defined by the sentence 'Suppose a crack is allowed to extend by an amount
~a under constant applied load by the quasi-static release of the cohesive forces holding the surfaces together at ~a and that the work absorbed at ~a is ~ U, then Gt. will be defined as ~ U/ ~a'. It is the essence of their argument that ~U/~a must be evaluated over some finite length, so that, when there is any plastic deformation, it will differ from the limit 0 U/oa. However, for linearly elastic brittle material Gt. degenerates to G or J, since the only dissipation, ~ U, occurs at the crack face. This use of a finite length, ~a, is consistent with the 'process zone' concept outlined in Section 2.1.
