Effects of strain path shapes on non-proportional cyclic plasticity

The title problem was discussed to facilitate the formulation of constitutive models of cyclic plasticity under general states of loading. A series of plastic strain controlled cyclic tests was performed by applying combined axial force and torque to thin-walled tubular specimens of Type 316 stainless steel at room temperature. These tests consist of cyclic loading along uniaxial, torsional, cruciform, stellate in eight directions, square and circular plastic strain paths with a constant amplitude of equivalent plastic strain.The results of these tests showed that the strain-hardening depends markedly on the shape of the plastic strain path, and that the strain-hardening (measured by equivalent stress amplitudes) in the saturated state is significant in the order of circular, square, stellate, cruciform and proportional paths. It was also observed that these saturated values were independent of the less significant plastic strain cycles experienced in the past. Finally, the characteristic features of strain-hardening mechanisms under non-proportional loadings were discussed in some detail on the basis of the present results.