The Plastic Behaviour of Annealed OFHC Copper

The effect of progressive deformation in the plug-drawing process on the initial yield and subsequent plastic flow behaviour of oxygen-free high-conductivity (OFHC) copper has been investigated. This paper relates to the behaviour of the annealed material under combinations of tension, torsion and internal pressure, prior to cold-drawing.

All loads were applied under constant stress ratio, and the initial yield points were obtained by utilizing the backward extrapolation of curves of stress versus strain and creep coefficient versus stress. The Maxwell-Mises criterion described the experimental yield locus obtained under biaxial tension but not the two tension-torsion loci. The difference is attributed to a small initial anisotropy.

In post-yield behaviour, the plastic strain and strain-rate vectors were normal to the initial yield locus and remained so under increased loading. The work-hardening hypotheses of plasticity theory failed to correlate any of the combined-stress results but equivalence of the strains was established by a consideration of the coaxial stress-strain path. This method resulted in a complete correlation of the stress-strain relationship for all loading paths in all stress fields.

Primary creep of the logarithmic form ε=a In t + C was exhibited by all plastic strain components, and a correlation of the creep coefficients was established on the same basis as that used for plastic strains. Finally, a complete correlation of the stress-strain-time behaviour of this material at room temperature was established.