The application of fracture mechanics to yielding materials

The paper begins with a restatement of the ideas underlying the choice of a hypothesis of crack opening displacement for brittle fracture initiation, in situations in which sharply notched bodies of materials with well-defined yielding properties are subject to fracture either below or at general yield. It is maintained that a displacement is preferred to a strain criterion in order to encompass fracture size effects. Slow notched bend and tension tests conducted at various temperatures on 3 in. thick, mild steel plates are described, in association with autographic measurements of crack opening displacement at fracture. The latter criterion for fracture initiation is found to be largely vindicated. Minor departures from constancy of fracture displacement at given temperatures are examined in relation to variations of notch root triaxial stress intensification with the different specimen shapes and loading conditions. The effects are found to be consistent and especially traceable to the influence of plate thickness on the maintenance of plane strain conditions.