Determination of strain concentration factors by finite element analysis and their application to simplified elastic-plastic analysis

Abstract

ASME B&PV Code, Section III, allows simplified elastic-plastic analysis procedure when the primary plus secondary stress intensity range exceeds the allowable value. In order to reduce the conservatism included in the formula for strain concentration factor (Ke factor) used in this procedure many experimental and theoretical researches and researches by finite element analysis (FEA) have been made in many countries, and as results new formulas were introduced in French RCC-M Code and Japanese JSME Code, and recently in ASME Code Case N-779. As the maximum strain values obtained by continuous loading method and discrete loading method were equal, the latter method was adopted for research efficiency. In this research Ke factors were calculated by using FEA, compared to those factors obtained by formulas, and cumulative fatigue usage factors (CUFs) obtained by applying these Ke factors were compared to ascertain the degree of reduction in conservatism. A three-dimensional FEA model of a charging inlet nozzle attached to a reactor coolant pipe is used in this research. Ke factors by FEA were obtained by two methods - event-based and range-based methods. Event-based Ke factor was calculated for each transient event, and the bigger one of the two events for each important event pair shown in the fatigue evaluation was used for that pair. Range-based Ke factor was calculated for each important event pair considering the range of each strain component between the two extremes of the event pair. Accurate plastic property data is critical to the accuracy of the fatigue evaluation performed with Ke factors by FEA. To check the influence of the plastic property four(4) strain offset conditions including 0.2% strain offset which is used to decide the yield strength in the ASME Code were considered. The Ke factors obtained by FEA are much lower than those by ASME Code formula in the region of high primary plus secondary stress, and depending on the plastic propert...