Influence of Near-Surface Stress Gradients on the Very High Cycle Fatigue Behaviour of 42CrMo4 SteelTuesday (27.09.2016) 17:15 - 17:30 Part of:
Despite low loads, material failure can occur beneath the surface endurance strength at high numbers of load cycles, in the so-called Very High Cycle Fatigue (VHCF) regime. Thereby the crack initiation is often caused by non-metallic inclusions. In the research project of the German Research Council (DFG) priority program “Life - Unlimited Lifetime of Cyclically Loaded High-Performance Materials” (SPP 1466) tests at R=-1, tension and compression, are performed with variation of stress gradients and surface treatment to investigated the influence to the high cycle fatigue behavior. The experimental material of tempering steel 42CrMo4 was used with a tensile strength of 2145 MPa. Notched Specimens with different stress concentration (1, 1.23, 1.41, 1.94) were tested and combined with different depth of near surface residual stress (20 µm and 160 µm). The surface treatment leads to an increase of lifetime and the failure is caused by nonmetallic inclusions. A significant increase of the endurance limit could not be observed. For any stress concentration factor and no surface treatment no failure initiated from the volume. Only with surface treatment volume failure occurred, but for higher stress concentration (1.94) this behavior could not be observed. With increased lifetime the critical nonmetallic inclusion size decreased. Measurements on tested specimens with X-ray computed tomography could not identify larger inclusions as the critical one.