Creep of 2618 Aluminum Under Side-Steps of Tension and Torsion and Stress Reversal Predicted by a Viscous-Viscoelastic Model

Combined tension and torsion creep experiments are reported in which (A) one stress component was increased or decreased while the other remained constant; (B) one stress component was removed while the other remained constant; (C) torsion was partially or fully reversed with or without simultaneous constant tension. Among the observed features of the experimental results were the following: when one stress component was removed the creep from the other component was unaffected; upon small reductions in stress there was no recovery-type behavior; and when torsion was fully reversed, all prior strain was wiped out. A nonlinear viscous-viscoelastic model was used for which the material constants were derived from constant-stress creep and recovery data and previously reported. This model, together with certain necessary modifications, was used to compute the creep resulting from the complex stress histories described. Most features of the experimental results were predicted reasonably well by the modified theory.