A Deviatoric Stress-Strain Function for Fatigue Life Analysis

Abstract

The Smith-Watson-Topper (SWT) parameter was originally suggested and is still widely used to account for mean stress effects in fatigue life analysis. It is well recognized however, that the SWT parameter might be non-conservative for cyclic loads that involve relatively large compressive mean stresses. Such large compressive mean stresses can develop in notches after overloads. An energy interpretation of the SWT parameter is presented first. Based on analogy with the Neuber’s rule a new deviatoric formulation of the SWTD parameter 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 and non-proportional biaxial fatigue, the deviatoric SWTD parameter demonstrates a fairly good correlation to test data while the original SWT parameter results in wide scatter.