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

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 versus Kmax. It is generally well known that the growth rates decreases with decreasing R ratios. However, the da/dN as a function of Kmax 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 versus Kmax to da/dN versus DKI, using crack closure U. It can be obtained that the growth rates da/dN as a function of DKI decrease with decreasing R ratio for -2 <= R<0. It can be seen that the growth rate da/dN versus DKI is better equation than da/dN versus Kmax 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