In Part 1, a brief summary of the justification and advantages of the use of the J contour integral in elastic-plastic finite-element analysis is given. A more detailed appraisal is then made of its interpretation as an energy balance concept in the presence of irreversible plastic deformation. Numerical illustration of the points made is provided by an elastic-plastic finite-element analysis on a series of monotonically loaded centre-cracked plates of different 2 a/W ratios. With incremental plasticity it is concluded that although an energy balance interpretation can be assigned to J, it is not a release rate of energy available to propagate a fracture. In Part 2, the role of J as a crack-tip characterizing parameter is examined with the help of elastic-plastic finite-element analysis. The evidence indicates that a given value of J does not ensure a unique crack-tip environment for different cracked-body geometries. It may, however, characterize the crack tip adequately for some engineering purposes and, in particular, may be used to model the fracture behaviour of a large-scale test using a smaller but geometrically similar specimen, for a given degree of through thickness constraint.
