Experimental measurements of the free surface strain increment components on high-carbon steel compression specimens were used to estimate the point at which tensile plastic instability occurred at the equatorial free surface. A number of compression tests, with various combinations of specimen geometry and platen friction conditions, were carried out to the point of macroscopic fracture on the barrelled surface, thus giving a wide variation in the history of the stress components and the strain increments at the equator. In these tests the equatorial strain to instability varied widely but the strain from instability to fracture varied only slightly. Metallographic examination of the equatorial free surface showed that there was no evidence of fracture damage in specimens prior to instability, but specimens compressed just beyond the point of instability showed abundant evidence of microscopic fracture damage. It was concluded that the start of tensile plastic instability marks the point where internal microscopic cavities begin the process of growth and coalescence by internal necking. A criterion for ductile fracture in metal-working operations is proposed which is based on an estimate of the point where tensile plastic instability begins. A number of suggested criteria for ductile fracture were examined and found to be unsatisfactory when applied to the present results. There was no evidence of local necking on the free surface of the compression specimens after instability had occurred. © 1969.
