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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.

The ASME Boiler and Pressure Vessel Code (ASME BPVC) Section III, Division 5, Subsection HB, Subpart B provided only one design fatigue curve for Grade 91 steel (Gr.91) at 540 °C (or 1000 °F) in 2019 and earlier versions. To overcome this disadvantage, The ASME Section III Working Group on Creep-Fatigue and Negligible Creep (WG-CFNC) had taken an action to incorporate the temperature-dependent design fatigue curves for Gr. 91 developed by Japan Society of Mechanical Engineers (JSME) into ASME BPVC Section III Division 5. As a result, the temperature dependent design fatigue curves are provided in the 2021 edition of the ASME BPVC. To clear the features of the best-fit fatigue curve equation developed by the JSME, 305 data stored in the database were analyzed. Details of the database and relationship between the best-fit fatigue curve equation and the data including the statistic values and the values of 95% and 99% lower confidence bound calculated by failure probability assessment were clarified through analysis. In addition to the best-fit fatigue curve equation, an equation for dynamic stress-strain response showing the behavior of Gr.91 steel under cyclic loading of is also provided based on the same database. Moreover, some additional available data of fatigue and creep-fatigue tests obtained in Japan are also provided for considering the creep-fatigue damage evaluation under elevated temperature condition.

This paper studies inelastic stress-strain relationship equation and creep rupture equation and creep strain equation of 99%lead-1% antimony alloy. Under the severe accident conditions, structural materials of nuclear power plants are subjected to excessive high temperature. Although it is very essential to clarify how the structure collapses under the severe accident conditions, there’re no experimental evidences of failure modes and the failure mechanisms in such high temperatures are not clarified. However, it is very difficult and expensive to perform structural tests using actual structural materials, such as austenitic stainless steels. Therefore, the authors propose to use lead alloys instead of actual structural materials. Because the strength of such alloys is much poorer than that of the actual structural materials, failure can be observed at much low temperature and by much small load. For demonstration of analogy between the failure mechanisms of lead alloy structure at low temperature and those of the actual structures at extremely elevated temperature, numerical analyses are required. The authors proposes inelastic constitutive equations of lead alloy based on a series of material tests. Nonlinear numerical analyses, e.g. finite element analyses, can be performed using the proposed equations.