The origins of failure instability in metal-working processes

Abstract

The development of pinholes in thin metal strip, the failure of aluminium compressed between opposed parallel platens and the spiral markings on hard-drawn copper tubing are discussed in relation to the cause of failure instability in technological processes. It is suggested that surface features caused initially by slip bands and at large deformations by shear bands resulting from strain concentrations dictated by the gross geometry of deformation (e.g. the geometry which is likely to be found at the edge of a tool) may initiate instability rather than “structural softening” within the shear bands, although the path of failure follows shear bands. The planes at 35° to the median plane are planes of shear instability and failure in incompressible isotropic non-work-hardening material which yields in plane stress according to the von Mises criterion, so it is not necessary to consider the orientation resulting from the mechanism of microscopic deformation to explain the direction of shear bands in rolled material.