Proposal of Negative Stress Ratio R for Fatigue Crack Growth Rates of Austenitic Stainless Steels in Air for ASME Code Section XI Based on Trend in Experimental Data

Abstract

Abstract Fatigue crack growth rates da/dN for stainless steels in air environment are provided by the ASME Code Section XI. The fatigue crack growth rates are given by da/dN = CF(ΔK)n, where CF is the fatigue crack growth rate coefficient, n is the fatigue crack growth rate exponent and ΔK is the stress intensity factor range. The coefficient CF contains a temperature parameter ST, and a scaling parameter SR, which is a function of the stress ratio R. When the stress ratio R is positive from 0 to 1, the parameter SR increases with increasing the ratio R, and da/dN increases with increasing stress ratio R. When R is less than 0, SR = 1. Accordingly, fatigue crack growth rates under negative stress ratios are independent of stress ratios. This means that a cyclic stress state with a minimum that is compressive is considered to cause the same degree of crack growth as one with the same range but a zero minimum. However, from the results of literature survey, experimental data reveal that the fatigue crack growth rates decrease with decreasing R ratios below zero, i.e. negative stress ratios. The objective of this paper is to assess fatigue crack growth rates under such negative stress ratios for stainless steels in air environment. An equation determined from trends in experimental data surveyed in this study is proposed for negative R ratios to calculate the parameter SR for the ASME Code Section XI, Appendix Y.