Back stress and stress relaxation behavior under biaxial loading in the high temperature environment

Tension-torsion biaxial creep-fatigue tests have been conducted with 304 stainless steel at 650°C. An overstress and back stress were analyzed under biaxial stress conditions. Under proportional biaxial loading, peak stress components were well arranged by using Von Mises equivalent stress. However, the overstress and the back stress showed a dominant anisotropy. A long-term stress relaxation behaviour was examined under proportional loading. The result suggests that a time-dependent inelastic deformation process is composed of at least two origins. One is caused from a relaxation of the overstress and the other is caused from that of the back stress. The former is much faster than the latter. The relaxation of the back stress indicates a recovery of the anisotropy.