Tensile creep acceleration by superimposed cyclic torsional strain in polycrystalline metals

A theoretical model, which considers the nature of time-dependent crystallographic slip in each grain and grain interactions, is developed to study the effect of superimposed cyclic torsional strain on the tensile creep accumulation of metals. Both transient and steady creep, and active and latent hardening, are incorporated in the constitutive equations of slip systems. Under cyclic deformation, the transition of isotropic to kinematic behavior in the work hardening of crystals is also incorporated. It is pointed out that the prescribed loading condition involves combined creep in tension and relaxation in torsion. Based on Eshelby's solution of ellipsoidal inclusion, the corresponding self-consistent relations are derived to account for grain interactions. The developed theory was applied to calculate the tensile creep strains of pure aluminum under various amplitudes of torsional strain at a given tensile stress. Consistent with experimental observations, the results indicate that, even with a relatively small amplitude, tensile creep accumulation can be greatly enhanced. The nature of heterogeneous deformation in the polycrystalline aggregate is also illustrated by the different contributions of four selected constituent grains. Finally, the enhancement of tensile creep was shown to be intimately related to the rate of transition from isotropic to kinematic hardening under cyclic deformation.