Rewriting the Ratcheting Stress Limits

Abstract The 2023 Edition of ASME Section VIII, Division 2 states that the allowable limit on the primary plus secondary stress range, SPS, is the larger of 3 times the average of the S values at the highest and lowest temperatures during the operational cycle or 2 times the average of the Sy values at the highest and lowest temperatures during the operational cycle. The upcoming 2025 Edition is proposed to drop the 3S limit and replace it with a single limit of cold yield plus hot yield. This paper discusses the historical context for the 3S limit, and the technical rationale and justification for this proposed change. Historically, the Criteria Document from 1972 defined the ratcheting limit strictly in terms of yield, going so far as to state that “a calculated elastic stress range equal to twice the yield stress has a very special significance”. This document goes further to define 2Sy as the stress range limit for primary-plus-secondary membrane-plus-bending. However, at the very end of the document, it is noted that the “design” value for the pre-2007 ASME Section VIII, Division 2 is only with respect to the allowable stress, and therefore as a final step the 2Sy limit is converted to a 3S limit. It was only between the 2001 Edition and the 2004 Edition when the 3S limit was updated to have a form similar to the form that exists in the 2023 Edition, noting that the S in 3S referred to the average of the tabulated values of the material at the highest and lowest temperatures of the cycle, as well as adding 2 times the average of the Sy values at the highest and lowest temperatures during the operational cycle to the limit definition. The technical justification for this proposed change includes the historical context, where ratcheting is defined purely in terms of yield, since it is a yield-based failure mode. The justification also examines the removal of the limitation in the 2023 Edition that the yield-based criteria cannot be used when the yield-to-ultimate ratio is greater than 0.7. The justification continues to discuss the situation for materials that cyclically-soften. Finally, the justification discusses the consistency between the proposed limit format, which is appropriate for temperatures below the creep regime, and a comparable limit format for when the hot temperatures are in the creep regime.