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.
