Influence of residual stresses and mean load on the fatigue strength of case-hardened notched specimens

Case-hardened gear wheels, e.g., in vehicle transmissions are cyclically most stressed components. Their lifetime is determined by the interaction of loading, tooth flank load-capacity and tooth foot load-capacity. The latter is influenced by a multitude of parameters like mean loading or residual stress state appearing during operation. To evaluate the tooth-foot load-capacity, the knowledge of the mean stress and residual stress sensitivity of the fatigue strength of the material conditions at the sites critical for failure is necessary.The examinations on hand concerning means stress sensitivity and residual stress sensitivity usually deal with homogeneous materials. Systematic analyses for case-hardened conditions are missing. In this work case-hardened notched specimens with different residual stress states are investigated under cyclic bending. The results are analyzed using a local concept and an additional fracture mechanical approach. With the local concept it was possible to determine the crack initiation site which was partly at the surface and partly below the surface. The fracture mechanical approach was necessary to understand the crack stop behavior and to determine the site critical for failure. An uniform description of the lifetime behavior of unpeened and shot peened specimen could be achieved using a modified Haigh diagram which takes into account the local stress state at the site critical for failure.