A Method for Materials Selection in Reliability-Based Design Diagram of Structural Components under Creep-Fatigue Damage Mode
Kazunari FUJIYAMA
Abstract:A new method for materials selection in reliability-based design was applied to the high temperature structural components under creep-fatigue damage mode. The creep-fatigue interaction law was formulated in terms of creep time fraction (creep damage) and fatigue cycle fraction (fatigue damage) using pure creep and pure fatigue material properties referring NIMS creep and fatigue datasheets for ferritic heat resistant steels such as 1Cr-1Mo-0.25V steel forging, 12Cr-1Mo-1W-0.3V steel bar and 2.25Cr-1Mo steel plate. The statistical distribution under the creep-fatigue damage mode was fitted well with the two-dimensional log-normal distribution function but the scatter bands of creep-fatigue data were considerably larger compared with the cases using pure creep and pure fatigue damage distributions. Once the ratio of creep damage to fatigue damage was fixed, the confidence limit could be restricted within more reasonable range for design application. The diagram was expressed by the relationship between creep stress vs. fatigue strain range for those three materials and the apparent superiority of 12Cr-1Mo-1W-0.3V steel bar to other materials was demonstrated. Thus this diagram was proved to be an effective tool for decision making of materials selection and structural reliability-based design even at the very initial stage of design process, that is, the front-loading design of components. Key Words:Reliability-based design, Materials selection, Two-dimensional log-normal distribution, Creep-fatigue interaction, Creep damage, Fatigue damage