Influence of variations in creep curve on creep behavior of a high-temperature structure

Abstract

It is one of the key issues for a high-temperature structural design guideline to evaluate the influence of variations in creep curve on the creep behavior of a high-temperature structure. In the present paper, a comparative evaluation was made to clarify such influence. The evaluation results showed that, in almost all cases of a creep behavior pattern in which creep strain accumulated during a stress cycle caused significant relaxation of the corresponding deformation-controlled stress in the following cycles, the variations in creep behavior with the creep curve were qualitatively similar to those in fundamental creep properties. On the other hand, in many cases of another creep behavior pattern in which creep strain accumulated during a cycle doesn't cause the significant stress-relaxation in the following cycles, the variations in creep behavior for earlier cycles are different from those in fundamental creep properties. Even in these cases, however, those get qualitatively similar after several cycles when their stress-time histories are stabilized. Additional consideration was given to the influence of the relationship between creep rupture life and minimum creep rate, i.e., the Monkman-Grant's relationship, on the creep damage evaluation. The consideration suggested that the Monkman-Grant's relationship be taken into account in evaluating the creep damage behavior, especially the creep damage variations. However, it was clarified that the application of the creep damage evaluation rule of ASME B.&P.V. Code Case N-47 to the “standard case”which was predicted from the average creep property would predict the creep damage on the safe side.