On the Shear Stress Parameter of Thin-walled Tubular Specimens under Torsional Loading

In the most of existing fatigue life prediction models, the maximum shear stress on a tubular specimen's outer surface is chosen as the shear stress parameter to predict fatigue lifetime for torsional loading. It has been experimentally found that the fatigue lifetime observed under fully reversed torsion is always higher than that under tension-compression for the same equivalent stress level. In this paper, an improved shear stress fatigue damage parameter based on Buch's two-parameter model was proposed. In this parameter, the influences of both shear stress gradient and cyclic strain hardening were taken into account. A serial of tensile-compressive and torsional loading tests were carried out on 2024-T4 aluminum alloy to validate the new parameter. Four σeq-Nf curves were plotted using different shear stress parameters. Compared with other shear stress parameters, the σeq-Nf curve translated from the new parameter made best coincide with the tension-compression S-N curve.