Constitutive equations of creep under changing multiaxial stresses for materials with different behavior in tension and compression

A theory of creep that describes kinematic hardening of polycrystalline materials with different behavior in tension and compression is presented. A structure of the equivalent effective stress in the equation for creep deformation as well as structures of the equivalent effective stress in the describing creep hardening and of equivalent back stress in the describing thermal softening are based on three independent material characteristics obtained from uniaxial tension tests, uniaxial compression tests, and pure torsion tests for different values of constant stress at constant temperatures. A satisfactory agreement is obtained between the theoretical results and the experimental data of the creep under multiaxial loading with constant stresses for isothermal processes as well as under uniaxial nonproportional and multiaxial nonproportional loadings for both isothermal and nonisothermal conditions.