Creep Deformation Behavior and Estimation of Creep Damage under Creep-Fatigue Condition

Abstract

Cyclic creep tests under reversed stress and combined creep-fatigue tests were carried out on SUS 304 stainless steel at 700°C in air. Creep stress was hold for 10hr per one cycle in all the tests, and the fatigue loading inserted in creep-fatigue tests was the strain controlled triangular wave shape one. The creep behavior was examined in each test and a reasonable life prediction method under creep-fatigue conditions was discussed.
When creep stress was reloaded in cyclic creep or creep-fatigue tests, a transient creep was observed in each cycle and the steady-state creep rate changed in each test as compared with the monotonic creep. These behaviors were attributed to the combined effect of the inelastic strain range for reversed stress after creep loading (Δε_cp) and that for fatigue loading (Δε_pp). Thus the steady-state creep rate was influenced by creep stress as well as by the inelastic strain ranges of Δε_cp and Δε_pp. Though the acceleration of steady-state creep rate was observed in both cyclic creep and creep-fatigue tests, there existed a certain correlation between the creep rate and the creep damage. This relation was found to be identical with that of monotonic creep data.
The rupture life of creep-fatigue tests was predictable within the factor of 2 by the linear damage rule using the above relation. The life prediction method by strain range partitioning gave also a good rersult, when based on the Δε_cp-N_cp relationship that was calculated from the relation between the steady creep rate and the rupture life mentioned above.