Despite significant technological progress in recent years, elastic-plastic fatigue analysis of pressure-retaining components remains a time-consuming venture. Accordingly, nuclear pressure vessel design codes such as ASME Section III provide simplified elastic-plastic analysis procedures as a practical alternative. This approach can be excessively conservative under certain conditions due to the bounding nature of the applied plasticity correction factor, Ke. Whilst this overconservatism was tolerable in the past, recent technical challenges arising due to consideration of environmentally-assisted fatigue (EAF) and design for long-term operation have posed difficulty in achieving acceptable fatigue usage based on extant Code assessment procedures for certain components. The incorporation of more accurate Ke factors has since been identified as a nuclear industry priority.
This paper presents a critical review of Ke factors within ASME Section III, with particular attention given to a recently proposed approach by Ranganath, which has recently been approved for publication as an ASME Section III Code Case. Correction factors adopted within other nuclear and non-nuclear codes and standards (C&S) were also considered. The code-based Ke factors were compared with Ke factors obtained directly from various elastic-plastic finite element (FE) models of representative plant components. The results revealed a considerable difference in conservatism between the code-based methods. Based on the elastic-plastic FE results, an alternative improved plasticity correction method was proposed. The need for a harmonised approach to determining Ke based on elastic-plastic FE analysis is discussed as a desirable industry objective.