Life prediction for creep-fatigue loading conditions should be related to creep damage mechanisms. In order to examine the effect of the creep damage mode on rupture life under creep-fatigue loading, a "combined creep-fatigue loading test" was carried out on 316 stainless steel. In this method, creep loading and fatigue loading are repeated alternately. The fracture criteria under combined loading closely depend on the creep fracture modes of the static creep test. A new life prediction method which uses this new fracture criterion is proposed. The criteria are changed when the creep damage mode varies. In order to verify the adequacy of this method, fatigue tests with a tensile strain-hold wave form were carried out. It is clear that rupture life in such fatigue tests is dependent on the chosen fracture criteria.
Long-term stress relaxation behavior was investigated on NCF 800H alloy. Complicated stress relaxation behaviors such as rapid decrease of residual stress and increase of residual stress were observed. Relationship between stress relaxation behavior and microstructural changes was examined. The decrease of residual stress was caused by the reduction of creep deformation resistance due to coarsening of carbides. Minus relaxation behavior which is equivalent to the increase of residual stress was related to shrinking of testing material due to precipitation of carbides. Therefore, it is difficult to extrapolate the long-term stress relaxation curve from the result of short-term tests, and the long-term residual stress value should be predicted using aged material.
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