Crack initiation in the VHCF regime: research on the relevant damage mechanisms at 100Cr6

At very long life spans of components under cyclic loading, failure occurs largely under the surface, for example at nonmetallic inclusions in high strength steels. This effect is predominantly observed at low stress amplitudes resulting in life times with a number of cycles to failure higher than 106. In contrast, at high stress amplitudes the failure initiates from the surface. The present research in this project aims to clarify the reasons for this change of the place of failure initiation from the surface to the volume as well as the mechanism causing the VHCF failure in the volume. In order to find the combination of stress and defect size at which the transition of failure initiation from the surface to the volume takes place, some artificial defects with varying size are introduced in the specimens´ surface. Furthermore, it will be analyzed how crack propagation at the surface differs from crack propagation inside a material. Based on the threshold values for crack initiation and the corresponding crack propagation rates it is expected that it will be possible to explain the transition of the crack initiation point to get a better understanding on VHCF failure. Additionally, fatigue test in ultra-high vacuum are used to simulate the conditions inside the material at surface defects. These tests are expected to reveal the influence of the surrounding atmosphere on the transition point and can further be used to clarify the microstructural mechanisms for internal failure in the VHCF regime. Further fatigue tests in vacuum with artificial defects are envisaged to deliver additional insights on the microstructural mechanisms.

Contact: M. Sc. D. Spriestersbach