ELASTIC-PLASTIC FRACTURE MECHANICS ( E P F M ) has developed from linear elastic fracture mechanics (LEFM) and attempts to eliminate the highly restrictive limits of that discipline so that a scientific method can be applied to structural applications for which low-strength, hightoughness materials are used. Early work by Wells (1961) was directed toward structural steels that were too tough to be characterized by LEFM. He proposed that the crack tip opening displacement (CTOD) of a blunted crack was a characteristic of the material's toughness and that it could be used as a crack-tip-characterizing parameter for materials for which LEFM was not valid. A more complete discussion of the analytical background of the CTOD method can be found in Anderson (1991). Standard methods of CTOD testing were developed in Britain (Wells 1961), and improvement has continued, leading to the recent British Standard BS 5762 (1979) and ASTM standards E 1290-89 and E 1290- 93.