A deviatoric version of the SWT parameter

The Smith–Watson–Topper parameter (SWT = σmaxεa) was originally suggested and is still widely used to account for the mean stress effects in fatigue life analysis. It is well recognized however, that the SWT parameter might be a non-conservative description for cyclic loads that involve relatively large compressive mean stresses that can develop in samples/components with notches after overloads. In such situations σmax tends to be small resulting in underestimating the SWT parameter. It is shown that the SWT parameter can be interpreted in terms of the sum of strain energy and complementary strain energy densities supplemented by the strain energy density associated with a mean stress in the cycle. Using this energy interpretation and its analogy with the Neuber’s rule a deviatoric version of the SWT parameter called SWTD is proposed. It is found that for positive mean stresses and moderate negative mean stresses the original SWT parameter and the proposed deviatoric SWTD parameter yield similar results. At large compressive mean stresses, the deviatoric SWTD parameter demonstrates a fairly good correlation while the original SWT parameter is unable to correlate the data. Although the proposed approach seems to be very promising, based on the limited data of this study, it should be further re-examined using more experimental data sets, in particular for multiaxial in-phase and out-of-phase loadings.