Reference curve of fatigue crack growth for ferritic steels under negative R ratio provided by ASME Code Section XI

ASME Code Section XIに示される負の応力比におけるフェライト鋼材料の疲労亀裂進展速度線図について

長谷川 邦夫*; Mare, V.*; 山口 義仁 

Hasegawa, Kunio*; Mare, V.*; Yamaguchi, Yoshihito

Reference curves of fatigue crack growth rates for ferritic steels in air environment are provided by the ASME Code Section XI Appendix A. The fatigue crack growth rates under negative R ratio are given as da/dN vs. K. It is generally well known that the growth rates decreases with decreasing R ratios. However, the da/dN as a function of K are the same curves under R = 0, -1 and -2. In addition, the da/dN increases with decreasing R ratio for R-2. This paper converts from da/dN vs. K to da/dN vs. K, using crack closure U. It can be seen that the growth rate da/dN vs. K is better equation than da/dN vs. K from the view point of stress ratio R. Furthermore, extending crack closure U to R = -5, it can be explained that the da/dN decreases with decreasing R ratio in the range of -5 R 0. This tendency is consistent with the experimental data.