The combined creep-fatigue tests, in which the creep loading and fatigue loading are repeated alternately, were carried out on SUS 304 stainless steel up to 3000hr at 700°C. The data obtained were ...analyzed using different linear damage rules, and the relationship between the results evaluated by them and the appearance of fracture was discussed. When the creep damage was assumed to be defined as Σtc/tr (where Σtc is the time spent under creep stress, σc, in the combined creep-fatigue test, and tr is the time to rupture under σc in the creep test), the sum of creep and fatigue damage depended on σc and the degree of work hardening due to the cyclic deformation under fatigue loading. When the creep damage was assumed to be defined as Σεc/εf (where Σεc is the sum of creep strain accumulated in the combined creep-fatigue test, and εf is the true strain at fracture in the creep test), a better correlation between creep and fatigue damage was obtained with no dependence on the test variables of the combined creep-fatigue test. For higher creep damages the sum of creep and fatigue damage was less than 1 because the fracture resulted from the linkage of grain boundary cracks. The important factors of creep-fatigue interaction under the combined creep-fatigue loadings were found to be the work hardening due to the cyclic deformation under fatigue loading and the linkage of grain boundary cracks.
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The combined creep-fatigue tests, in which the stress-controlled creep loading and the strain-controlled fatigue loading are repeated alternately, were carried out on SUS 304 austenitic steel to ...investigate the effect of creep stress on the creep-fatigue interaction. The creep period under constant loading was 10hr in a combined cycle, the total strain range in fatigue loading was 1%, and the numbers of imposed fatigue-cycles (N) were 2 and 20. In the test condition with large creep damage (N=2), the total rupture time depended on creep stress. When large fatigue damage was imposed (N=20), little dependency of creep stress was observed. The fracture mode was intergranular for the test condition of N=2. The voids were observed at the grain boundaries perpendicular to the stress axis, and the cracks of a few grain-facet size were observed in the fractured specimen. The fracture seemed to result from the propagation and linkage of grain-facet cracks. For N=20, the fracture mode was transgranular. The fracture caused by the propagation of surface cracks. The linear damage rule and the strain range partitioning method were used to evaluate the results obtained from the tests. For N=20, the actual life agreed with the life predicted from both method. For N=2, the actual life was shorter than the predicted life with lowering creep stress, that is, with increasing life.
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