Influence of different non-metallic inclusion types on the crack initiation in high-strength steels in the VHCF regime

This study aims to clarify the influence of different inclusion types on the crack initiation in the very high cycle fatigue regime. For this purpose ultrasonic tension–compression fatigue tests (R = −1) with the bainitic high-strength steel 100Cr6 (SAE 52100) were carried out until an ultimate number of cycles of 109. Additionally, runout specimens were tested repeatedly with higher stress amplitudes until failure occurred. By this method threshold values for the stress intensity factors in the very high cycle fatigue regime were found to be dependent on the inclusion type. So beside the threshold value for crack propagation of long cracks inside a fish-eye, Kth, another threshold value can be found for the formation of the so called fine granular area which is characteristic for very high cycle fatigue failure. This threshold value, below which no initiation in form of a fine granular area can be observed, is further dependent on the chemical composition of the crack initiating inclusion. Thus, a true fatigue limit in the very high cycle fatigue regime depends on the inclusion size and inclusion type. Further the inclusion type with the lowest threshold value indicates an absolute threshold value for very high cycle fatigue crack initiation due to the formation of the fine granular area.