Fatigue Crack Growth Rates for Ferritic Steels Under Negative R Ratio
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. 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 vs. Kmax to da/dN vs. ΔKI, using crack closure U. It can be obtained that the growth rates da/dN as a function of ΔKI decrease with decreasing R ratio for −2 ≤ R < 0. It can be seen that the growth rate da/dN vs. ΔKI is better equation than da/dN vs. 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 < 0. This tendency is consistent with the experimental data.