Threshold values for very high cycle fatigue failure of high-strength steels

Cracks leading to failure in very high cycle fatigue (VHCF) of high-strength steels mostly initiate at subsurface inclusions at stress intensity factors (SIF) below the classical threshold value K-th for long cracks, especially for negative stress ratios. Here, a characteristic fine granular area (FGA) can be observed at the fracture surface in the vicinity of these initiating inclusions. According to various researchers, the FGA formation might be responsible for the late initiation of a propagable long crack. This study aims to clarify the threshold values of SIF for crack initiation for VHCF failure with FGA formation at nonmetallic inclusions. For this purpose, ultrasonic fatigue tests (R=-1) with the high-strength steel 100Cr6 (SAE 52100) were carried out until an ultimate number of cycles of 10(9). In addition, very high cycle stress increase tests were performed. A serial grinding investigation of fatigued specimens acted as a technique to evaluate the distribution of unharmful inclusion in the tested volume. By the combination of these tests, a threshold value of the SIF for the VHCF by FGA formation at inclusions can be derived. This threshold value depends on the size of the crack initiating inclusion.