Simplified Elastic-Plastic Modeling for Fatigue Analysis of Piping Systems in Seismic PRA

Abstract A seismic probabilistic risk assessment (PRA) of nuclear power plants necessitates the fragility evaluation of individual equipment and structures, especially piping systems, as they function as pressure boundary systems. Generally, piping-system failures are caused by elastic-plastic strain-induced fatigue. The fragility evaluation considering fatigue as a failure mode requires the determination of the elastic-plastic strain that can be obtained using finite element analysis (FEA), which is time-consuming. Thus, applying FEA to numerous piping systems in fragility evaluation is impractical. Therefore, developing a method to evaluate the elastic-plastic strain of piping systems without FEA is needed. Consequently, we developed a fragility evaluation method in this study to predict the elastic-plastic strain range by modeling the piping system as an equivalent single degree of freedom system (SDOF). The SDOF was composed of an elastic-plastic spring, damper, and mass. We evaluated the validity of the proposed method by comparatively analyzing results obtained using the method and elastic-plastic FEA. The relative displacement and strain ranges obtained from the SDOF with an elastic-plastic spring replicated the equivalent strain ranges obtained from elastic-plastic FEA. In this study, to consider the plastic deformation, we used a Ke factor and the ratio of the maximum relative displacement ranges obtained from the SDOF with elastic and elastic-plastic springs. Moreover, the discrepancy between the values of usage factor obtained from the developed method and elastic-plastic FEA necessitates improvements in the simplified fatigue analysis method.