Independence of the torsional fatigue limit upon a mean shear stress

The influence of a superimposed mean shear stress on the high-cycle fatigue endurance limit of metals is a question that has been discussed for a long time but general agreement between among researchers has not yet been reached. Though seemingly a question of minor importance, the hypothesis on such effect is a critical issue in the formulation of high-cycle multiaxial fatigue criteria. The available experimental data referring to fatigue tests on steels in which a static shear (torsional) stress is superimposed to cyclic torsion have been reviewed, showing that in most cases the effect of static shear stress is negligible as long as the maximum shear stress does not exceed the static shear yield strength. The limited amount of relevant experimental data found in the literature has led the present authors to perform torsion fatigue tests on a quenched and tempered steel, commonly employed in highly stressed mechanical components, with different levels of superimposed mean shear stress. The statistical analysis of the test results shows that no significant effect of the superimposed static shear stress may be observed.