Technical basis for revising the fatigue crack growth rates for ferritic steels in the ASME Code Section XI

ASME (米国機械学会)規格Section XIにおけるフェライト鋼の疲労き裂進展の改定の技術根拠

長谷川 邦夫; 山口 義仁 ; Udyawar, A.*

Hasegawa, Kunio; Yamaguchi, Yoshihito; Udyawar, A.*

Fatigue crack growth thresholds and reference fatigue crack growth rate da/dN curves for ferritic steels in air environment are provided in Appendix Y of the ASME Code Section XI. Therein, the thresholds and da/dN are expressed as a function of stress ratio R; however, the R is not explicit, when R is negative. The thresholds are given as constant values for negative R. In addition, the ASME provides two equations for da/dN under negative R; however, the boundary between the two equations is not technically known. Thus, the dependency of negative R on da/dN is not well understood. Herein, revised threshold 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.