A Continuum Damage Coupled Unified Constitutive Model for Creep-Fatigue Damage Evaluation of Modified Grade 91 Tube Sheet Structures Under Flexible Loading Conditions

Abstract The Grade 91 tube-sheet structure is a critical integral part of Sodium Fast Reactors (SFR) developed by the Japan Atomic Energy Agency. Life evaluation of such a component is complex because of its geometry with the numerous holes and combined creep-fatigue loading conditions at elevated temperatures. The Sodium in the SFR tube sheets is flowed at the operating temperature varied from 650°C and 250°C for hot and cold transients. For these components, the creep-fatigue damage evaluation is commonly performed using simplified approaches such as the Stress Redistribution Locus (SRL) method or non-unified constitutive models where creep and plastic strains are not coupled. However, understanding the behavior of Grade 91 steel under fatigue, creep and creep-fatigue loading conditions and its constitutive model based on these phenomena are important to properly predict the component responses. Hence, this study developed a 3D FE model of the tube-sheet component, utilized the advanced damage coupled unified constitutive model (UCM-CDM) of Mod. Grade 91, formulated by the author earlier, to analyze the creep-fatigue strength of the tube sheet under combined flexible loading. The damage estimation using the UCM-CDM model is compared with other commonly used non-unified models, and the robustness of the UCM-CDM model is discussed.