Critical Review of ASME III Plasticity Correction Factors for Fatigue Design-By-Analysis of Nuclear Power Plant Components

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, K_e. While this over conservatism was tolerable in the past, recent technical challenges arising due to consideration of environmentally assisted fatigue 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 K_e factors has since been identified as a nuclear industry priority. This paper presents a critical review of K_e factors within ASME Section III, with particular attention given to an approach proposed by Ranganath, which has recently been approved for publication as an ASME Section III Code Case. Correction factors adopted within other nuclear and nonnuclear codes and standards (C&S) were also considered. The code-based K_e factors were compared with K_e 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 finite element analysis (FEA) results, an alternative improved plasticity correction method is proposed in this paper. The need for a harmonized approach to determining K_e based on elastic–plastic FE analysis and further development of efficient plasticity correction methods for total life assessment are highlighted as desirable industry objectives.