Deformation and rupture of stainless steel under cyclic, torsional creep

Abstract

Recent results from a long-term, strain-limited, cyclic creep test program upon stainless steel tubes are given. The test conditions employed were: constant temperature 500 °C, shear stress Ƭ = ± 300 MPa and shear strain limits ƴ = ± 4%. It is believed that a cyclic creep behaviour for the material has been revealed that has not been reported before in the literature. That is, the creep curves for stainless steel under repeated, shear stress reversals shows two basic square root dependencies: one upon time and the other upon cycle number. Consequently, the combined effect is such that the shear creep strain depends upon the square root of the product of cycle number and the time elapsed within that cycle. Despite extended times of cycling, with the test running into a period of over a year, no secondary or tertiary creep stages were ever observed within individual creep curves. Thus both the forward and reversed creep curves were exclusively primary in nature, within which the only visible evidence of a slow degradation of the deforming material was that the creep interval reduced successively between the imposed strain limits. However, it was found that the creep curve, when plotted within axes of cumulative creep strain and time, did recover a "pseudo-tertiary" stage. This stage concords with earlier results that showed tertiary creep to be a dominant contributor to the creep curve for this material under a steady torque. Given either the tensile ductility of the material or, a tensile creep rupture time, it is shown how final failure is predicted from the phenomenological square root law and an equivalence criterion.