Abstract
It has been recognized theoretically and experimentally that the ductility of metals in tension is influenced by the volume fraction of voids and radial stress. But almost no attempts have been reported to clarify what conditions must be satisfied to induce ductile fracture. In this paper we consider three quantities, namely volume fraction of voids, radial stress and strain hardening as the factors influencing ductile fracture, and try to obtain the relation among these factors at the moment when ductile fracture occurs. Radial stress and strain hardening are each expressed in terms of the curvature of neck of specimen and strain after annealing.
Many drawn or annealed specimens of low carbon steel which were initially smooth or notched round bar were deformed and fractured in tension at room temperature. Some of specimens, interrupted deformation at various strains, were either ground by a small grinder to increase the radius of curvature of neck (R) or annealed at 700°C for 4 min in the air to remove strain hardening, and deformed again. These operations were repeated several times in some of the specimens. Fracture strains and r⁄R (r=radius of narrowest section of specimen) at various strains were measured on all specimens. The results obtained are as follows. (1) Radial stress and strain hardening give not only indirect effects upon ductile fracture through an increase of volume fraction of voids but also direct effects on the fracture condition. (2) The condition of fracture can be shown as a curved surface in three-dimensional space whose axes represent the volume fraction of voids, r⁄R, and strain after final annealing. The volume fractions of voids in various specimens were estimated from the experimental results reported previously.
