Simplified Elastic-Plastic Analysis Method for Piping Systems Based on Linear Time History Analysis

Abstract

Abstract A design seismic motion for design of nuclear power plant have been increased in Japan since Tohoku earthquake in 2011. Consequently, piping systems under the design seismic motion are possibly deformed plastically. Elastic-plastic FEA (Finite Element Analysis) can evaluate elastic-plastic behavior accurately. However, it takes time and cost. In this research the simplified elastic-plastic analysis method has developed. This simplified elastic-plastic analysis method can calculate an equivalent stress and stress components with considering elastic-plastic deformation based on the energy rate density equilibrium which are calculated from linear time history analysis results. Ramberg-Osgood law is defined as true stress-true strain relationship, and the hardening law is isotropic hardening. In order to validate the applicability of the simplified elastic-plastic analysis method to piping systems under seismic motion, linear and non-linear time history analyses were performed for two type of piping systems by using FEA software Abaqus2021, and simplified elastic-plastic analysis method was applied to linear time history analysis results. One of the piping systems consisted of one elbow pipe, two straight pipes and one weight. Nine sinusoidal accelerations with different frequencies were applied to this simple model in in-plane direction. The equivalent stresses at the integration point where the maximum equivalent plastic strain occur in non-linear time history analysis were compared between the simplified elastic-plastic analysis and non-linear time history analysis. At any frequency simplified elastic-plastic analysis method could predict the equivalent stresses calculated by non-linear time history FEA accurately. Another piping system simulated actual piping systems in a nuclear power plant, which include the Tee pipe and several piping supports other than elbow pipes, straight pipes and weights. The random frequency acceleration waves were applied to this model in three direction at the same time. This piping systems took into account the self-eight and the internal pressure. The equivalent stresses and stress components were compared between the simplified elastic-plastic analysis and non-linear time history analysis. As the results, the stresses calculated by the simplified elastic-plastic analysis had good agreement with the results calculated by non-linear time history analysis. Therefore, the applicability of simplified elastic-plastic analysis method to the piping systems was validated.