A damaging function ΔKd for analyzing FCG and R-ratios in metallic materials

•This article addresses R-ratio effect on FCG in metals.•R-ratio effects is accounted in damaging function ΔKd.•Key implication: ΔKd doesn’t involve crack closure assumption.•Formulation is independent of material and specimen geometry. This paper presents a damaging stress intensity function ΔKd for analyzing R-ratio effects on fatigue crack growth (FCG) in metallic materials under constant amplitude loading. The proposed formulation is based on the sum of strain and complementary energy and its role in FCG rate behavior in threshold and Paris region at R-ratios ranging from −2 to 0.97. It doesn’t invoke a crack closure assumption or fitting parameters for R 0.7 it utilizes an experimentally determine correction factor, which accounts for the excessive forward plastic zone (FPZ) size and its role on FCG process.