This paper presents a damaging stress intensity function Kd for analyses of R-ratio effects on fatigue crack growth (FCG) in metals. 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<0.5-0.6. For a high R>0.7 it utilizes an experimentally determine correction factor, which accounts for excessive plastic dissipation in the monotonic plastic zone (MPZ).

In this paper a new interpretation and modification of the SWT function in terms of the total damaging energy density is proposed and discussed. The total damaging energy density is the sum of the damaging part of the strain energy density and complementary energy density corresponding to the first quadrant in damaging σD-εD axes. For cyclic loading with positive mean stress (σm≥0) the proposed function reduces to the original SWT formulation. For cyclic loading with negative mean stress (σm<0) the maximum stress is augmented by 1/3 of absolute value of the mean stress. The proposed approach shows a consistent correlation of the mean stress effects for both positive and negative mean stresses.