An evaluation of incremental testing to large strain in OFE copper

Abstract

OFE copper was deformed monotonically to strains in excess of 2.0 by torsion and incrementally in wiredrawing. The monotonic ‘tensile’ curve was established by testing the wiredrawn material, after different increments of strain, in uniaxial tension. We estimate that the strain rate of the wiredrawing process was ∈ ̇ = 1.0 sec −1. Stress reversal experiments, in torsion/reverse torsion and wiredrawing/compression were performed on material which had been stressed monotonically to a von Mises equivalent strain of 1.0. The strain rates in torsion of both the forward and reverse stressing were ∈ ̇ = 10 −3 sec −1. Compression, after wiredrawing to ∈ = 1.0, was done at two rates: ∈ ̇ = 1.0 sec −1 , and ∈ ̇ = 10 −3 sec −1. There was only a very slight Bauschinger effect observed when the reverse stressing was at the same strain rate as that in the forward direction. When the wiredrawing was followed with a low strain rate compressive stress reversal, a large Bauschinger effect occurred. We believe this is a manifestation of the strain rate change from that at which the wiredrawing was done. These results indicate that much of the large strain data in the literature, obtained by an incremental technique, could be in error because of different strain rates occurring in the incremental testing process.