Constitutive equations of creep for type 304 stainless steel under nonsteady multiaxial states of stress

In the ASME boiler and pressure vessel standard for high temperature nuclear power plants as the object, the design standard is determined assuming the existence of inelastic deformation, accordingly proper inelastic analysis must be carried out in the design of respective equipment elements. Recently, the research on the constitutive equation for inelastic creep analysis has been promoted. For the purpose of removing the difficulty in the theory of ORNL, the authors proposed the theory on the new constitutive equation for creep, which expresses the internal state of materials during creep deformation by introducing a closed curved surface in a strain space, and describes the strain hardening accompanying the progress of creep and the transient softening observed in stress variation by that method. In this study, for type 304 stainless steel at 600 deg C, the testing time and the period of stress variation were selected as 960 hours and 48 or 96 hours, respectively, and the creep test with combined tensile and torsional stresses was carried out about several typical stress variation modes. The results were compared with the analytical results according to the typical constitutive equations for creep to examine the usefulness of those equations. (Kako, I.)