Torsional deformation and fatigue of hardened steel including mean stress and stress gradient effects

This study investigates torsional deformation and fatigue behavior of both solid and thin-walled tube specimens made of a high strength quenched and tempered spring steel. Uniaxial deformation and fatigue data of this steel were already available. Monotonic and cyclic deformation curves are predicted from the von Mises and Tresca criteria and compared to the torsional data. Three types of torsional fatigue tests involving shear mean stress as well as tensile and compressive mean stresses were also performed in order to gain a better understanding as to how to predict the behavior of the material under such loading. The stress-life (S–N) approach and strain-life curve based on commonly used failure criteria are evaluated. Failure modes and fracture surfaces were observed and are discussed with regards to the failure criteria commonly used for life predictions. The differences in cyclic deformation and fatigue behaviors between solid round specimens where a stress gradient exists, and thin-walled tube specimens where a uniform stress state is commonly assumed, are also discussed.