Evaluation of Multiaxial Low Cycle Creep-fatigue Life for Mod.9Cr-1Mo Steel under Non-proportional Loading

This study discusses the creep-fatigue strength for Mod.9Cr-1Mo steel at a high temperature of 823 K under multiaxial loading. Low cycle fatigue tests in various strain waveforms were performed with a hollow cylindrical specimen. The tests were conducted under a proportional loading with a fixed axial strain and a non-proportional loading with a 90-degree phase difference between axial and shear strains. The tests at different strain rates and the creep-fatigue tests at different holding times were also conducted to discuss the effects of stress relaxation and strain holding on the failure life. In this study, two types of multiaxial creep-fatigue life evaluation methods were proposed: the first method is to calculate the strain range using Manson's universal slope method with considering a non-proportional loading factor and creep damage; the second method is to calculate the fatigue damage by considering the non-proportional loading factor using the linear damage law and to calculate the creep damage from the improved ductility exhaustion law. The accuracy of the evaluation methods is much better than that of the methods used in the evaluation of actual machines such as time fraction rule. The second method proposed by the authors showed the highest evaluation accuracy. The first evaluation equation is slightly less accurate than the second, but it is useful in that the evaluation procedure is easy.