Shear Constraint and Macroscopic Fracture Criteria for Ductile Metals

The role of material constraint in the shear fracture of ductile metals is investigated by means of a series of torsion and internal pressure tests on aluminum alloy tubing combined with analytical descriptions of the stress states at fracture. These descriptions include work hardening and, in the case of a tube under internal pressure, account for the nonaxisymmetric deformations that always precede fracture. The results, which indicate that shear fracture initiation depends on conditions at points far removed from the initiation site when the maximum shear stress vector there is directed through the interior of the body, are supported by the results of tensile tests on plates with shallow notches. These latter experiments are also used to show that tensile and shear fracture are governed by independent fracture criteria. A continuum fracture theory for ductile metals, based on the concepts of material constraint and independent fracture criteria, is proposed. The theory predicts fracture in terms of the stress state. A critical analysis of the theory is provided along with examples of fracture phenomena which the theory predicts, but which are not explained by existing theories.