The use of pure aluminium as an analogue for the history of plastic flow, in studies of ductile fracture criteria in steel compression specimens

Abstract

The plastic strain increments on the equatorial free surface of pure aluminium compression specimens are found to be identical to those for steel specimens of similar height to diameter ratio when compressed between rough platens. The results for pure aluminium specimens can be used as a complete history of the strain increments in similar steel specimens so that studies of ductile fracture criteria can be made without marking grids on the surface which fractures. A model for the effects of mild anisotropy on the relationship between the stress components and the plastic strain increments suggests that the Levy-Mises equations will give serious errors in estimates of the surface stress components in compression tests, as anisotropy develops. In any attempt to derive a criterion of ductile fracture for compression tests, based on a complete history of the strain increments and surface stress components, it will be necessary to measure the state of anisotropy and the uniaxial yield stress at various stages of compression, in order to give an accurate estimate of the surface stress components. The results suggest that improvements in metal forming lubricants and the production of certain states of anisotropy in cold heading wire should improve surface ductility in heading operations by reducing the tensile circumferential stress components. It was found that, in general, the smaller the initial height to diameter ratio of the specimen, the greater the tensile circumferential stress component.