A FRACTURE CRITERION is a standard against which the expected fracture behavior of a structure can be judged. In general terms, fracture criteria are related to the three levels of fracture performance, namely plane strain, elastic plastic, or fully plastic, as shown in Figure 13.1. Although it would appear desirable always to specify fully plastic behavior, this is rarely done because it is almost always unnecessary as well as economically unfeasible in most cases. Furthermore, relying primarily on notch toughness to prevent failure rather than good robust design may lead to problems. Finally, it is unsound engineering because good design is defined as an optimization of satisfactory structural performance, safety, and economic considerations.