Crack Initiation Mechanisms and Threshold Values of Very High Cycle Fatigue Failure of High Strength Steels

Fatigue failure of high-strength steels still occurs beyond 107 cycles in the very high cycle fatigue (VHCF) regime [1], . The reason for this late failure is that the fatigue properties in the long life region are strongly affected by flaws like non-meta llic inclusions inside the materia l [2], . In the case of VHCF failure a characteristic fine granular area (FGA) which is responsible for subsequent crack init iation can be observed in the vicinity of the inclusion on the fracture surface [3]. It is still unclear how diffe rent inclusion types affect the initiation mechanis m. Our study aims to clarify the influence of different inclusion types on the crack initiat ion in the very high cycle fatigue regime. Additionally, the threshold value for crack in itiation as a result of FGA formation shall be revealed. For this purpose ultrasonic fatigue tests (R = -1) of the high-strength steel 100Cr6 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 an inclusion/flaw type dependent threshold value for the formation of the FGA was derived from fracture surfaces and stress amplitudes. The lowest threshold value indicates an absolute threshold value for VHCF crack initiation due to FGA formation.