Critical review of ASME III plasticity correction factors for fatigue design-by-analysis of nuclear power plant components

Clarkson, David M. and Bell, Christopher D. and Mackenzie, Donald; (2020) Critical review of ASME III plasticity correction factors for fatigue design-by-analysis of nuclear power plant components. In: ASME 2020 Pressure Vessels & Piping Conference. American Society of Mechanical Engineers(ASME), Virtual, Online. ISBN 9780791883815 (https://doi.org/10.1115/PVP2020-21267)

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Abstract

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 is currently being considered for inclusion 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 Ke factors were comparied 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 harmonized approach to determining Ke based on elastic-plastic FE analysis is discussed and identified as a desirable industry objective.

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