Very high cycle fatigue crack initiation: investigation of fatigue mechanisms and threshold values for 100Cr6

Abstract

High-strength steels do not show a classical fatigue limit and failure still occurs far beyond 107 cycles. The fatigue properties in this very high cycle fatigue (VHCF) region are strongly affected by non-metallic inclusions inside the material. The mechanisms responsible for this late failure are not fully understood until now. In the scope of this work the mechanisms of VHCF failure and the connected threshold values are observed in detail. Ultrasonic tension-compression fatigue tests (R = -1) with the high-strength steel 100Cr6 (AISI 52100) were carried out until an ultimate number of cycles of 109. Some additional tests were also performed with artificial surface defects in air and vacuum for comparison. Single step fatigue tests, crack propagation tests and very high cycle stress increase tests are performed to understand the fatigue behaviour for very high cycle failure. By the combination of these tests a threshold for the VHCF by fine granular area (FGA) formation at inclusions can be derived. The results of mechanical testing are completed by investigating the inclusion distribution in tested specimen and the evaluation of harmless inclusions. Comprehensive fracture mechanical investigation for the performed tests enabled the determination of a VHCF threshold value. Microstructural analyses of the crack origin with focused ion beam imaging, transmission electron microscopy and atom probe tomography are used to investigate microstructure after fatigue failure in detail. Thereby the VHCF mechanisms leading to crack initiation are revealed. Finally by combination of fatigue results and microstructural investigations a new model for VHCF crack initiation in high-strength steels is proposed.