Alternative Approaches for ASME Code Simplified Elastic-Plastic Analysis

Subsection NB, Section III of the ASME Code provides rules for the fatigue evaluation of nuclear pressure vessel and piping components. The stress analysis in ASME code evaluation is generally based on linear elastic analysis. Simplified rules using an elastic-plastic strain correction factor, K_e, are provided in Section III to account for plastic yielding when the primary plus secondary stress intensity range exceeds the 3S_m limit. While the simplified elastic-plastic analysis rules are easy to apply and do not require nonlinear analysis, the application of the K_e correction factor can produce extremely conservative results. This paper investigates different analytical methods that are available for simplified elastic-plastic analysis and proposes an alternative method that is not overly conservative (compared to the Code K_e) and offers a more realistic approach to simplified elastic-plastic analysis. The proposed methodology is applicable for both vessel (NB-3200), core support structures (NG-3200) and piping components (NB-3600) and does not require new finite element analysis. Information in existing ASME Code stress reports should be sufficient to determine the new K_e factor. The proposed methodology is applicable to structural materials including austenitic stainless steel and nickel based alloys, carbon steel and low alloy steel. Comparison of the proposed methodology with detailed elastic-plastic finite element analysis shows that the new K_e factors are conservative but offer relief from the excessive conservatism in the Code K_e values. This paper provides the technical basis for an ASME draft Code Case for Alternative Approaches for ASME Code Simplified Elastic-plastic Analysis being pursued through the Section III ASME Code Committees.