Technical Basis for Revising the Fatigue Crack Growth Rates for Ferritic Steels in the ASME Code Section XI

Abstract

Fatigue crack growth thresholds ΔKth and reference fatigue crack growth rate da/dN curves for ferritic steels in air environment are provided in Appendix Y of the ASME Boiler and Pressure Vessel Code Section XI. Therein, ΔKth and da/dN are expressed as a function of stress ratio R; however, the R is not explicit, when R is negative. The thresholds ΔKth are given as constant values for negative R, and the definition of ΔKth is ambiguous. In addition, the ASME provides three equations for da/dN under negative R; however, the boundary between the three equations is not technically known. Thus, the dependency of da/dN on negative R is not well understood. Herein, revised ΔKth values for negative R are provided based on experimental data, and a new boundary between the two equations for da/dN is defined using crack opening behavior and the R dependency of da/dN based on a literature survey.