The Effect of Cold-Drawing on the Yield and Flow Behaviour of OFHC Copper under Combined Stress

Abstract

Tests under combinations of tension, torsion and internal pressure have been performed at constant stress ratio on oxygen-free high-conductivity (OFHC) copper, previously annealed and then subjected to progressive reductions in the plug-drawing process. The influence of cold-drawing was investigated by means of six reductions in area up to 81 per cent and this showed marked changes in the creep behaviour. Logarithmic creep no longer occurred and, in contrast to the annealed material, creep was observed at stresses well below yield when the latter is defined by the backward extrapolation method. The creep rate accelerated rapidly as the yield stress was approached and the creep strain became large compared with the instantaneous strain. Under axial tension—internal pressure, symmetrical yield loci tending progressively towards the Tresca criterion were found for reductions up to 42.4 per cent. Asymmetry developed at the largest reduction and, in general, the normality rule was obeyed in subsequent plastic strain behaviour for all tests. Under axial tension—torsion and internal pressure—torsion, almost identical elliptical yield loci were found for each reduction. These also tended towards the Tresca criterion for reductions up to 42.4 per cent and distortion was only observed at the largest reduction. Again, the normality rule was obeyed. Yield criteria derived from the theory of Edelman and Drucker were in good agreement with experiment over all yield loci. Hill's theory showed good agreement with experiment under tension—torsion, except for the largest reduction, but was inadequate for those axial tension—internal pressure tests not in agreement with the Maxwell–Mises yield criterion.