UNCERTAIN FACTORS IN ELASTIC-PLASTIC FINITE ELEMENT ANALYSIS FOR ELBOWS AND TEES

Low-cycle fatigue is one of the possible failure modes of piping systems caused by severe earthquakes. Fatigue is a type of cumulative damage during long-term operation; hence, fatigue life evaluation could be critical for the piping systems in existing plants. Considering the fatigue evaluation uncertainty, a considerable margin must be included for the seismic design of the piping systems. Low-cycle fatigue failure is thought to be associated with the equivalent strain obtained by the elastic-plastic finite element analysis. However, some uncertain factors affect the prediction of the equivalent strain of piping systems, like the actual size and material properties of pipe fittings. These properties are not always readily available in existing plants; therefore, an inevitable uncertainty remains in the analysis. Quantifying the influence of these factors is expected to provide a better understanding of the seismic margin of piping systems. In this study, we investigate the influence of model assumptions on the equivalent strain range of pipe fittings. We model elbows and equal and reducing tees as the typical pipe fittings used in piping systems. Comparing the results of the elastic-plastic analysis associated with various model assumptions, such as hardening models and structural geometries, we discuss herein how uncertain factors affect the equivalent strain prediction.