Threshold values for VHCF crack initiation in high-strength steelsTuesday (27.09.2016) 16:30 - 16:45 Part of:
D. Spriestersbach 1, A. Brodyanski 2, J. Lösch 2, M. Kopnarski 2, E. Kerscher 1
1 Materials Testing, University of Kaiserslautern, D-67663 Kaiserslautern, Germany
2 Institute for surface and thin film analysis GmbH, Trippstadter Straße 120, 67663 Kaiserslautern, Germany
Fatigue failure of high-strength steels still can be observed after 10^7 cycles in the very high cycle fatigue (VHCF) regime . This VHCF failure occurs at loads below the fatigue limit that can be derived for 10^7 cycles. The reason for this late failure is that the fatigue properties in the long life region are strongly affected by flaws like non-metallic inclusions inside the material . In VHCF failure cracks always initiate at subsurface inclusions. In the vicinity of the initiating inclusion a characteristic fine granular area (FGA) can be observed at the fracture surface. The FGA formation is responsible for the late initiation of a propagable long crack [3, 4].
It is still unclear whether or how different inclusion types affect the fatigue failure. Furthermore the threshold values for VHCF have to be found in order to guarantee safe fatigue design in the long life region. Our study aims to clarify the threshold values for VHCF failure with formation of the FGA at non-metallic inclusions. In this context the influence of different inclusion types on the crack initiation has to be clarified. For this purpose ultrasonic fatigue tests (R = -1) with the high-strength steel 100Cr6 were carried out until an ultimate number of cycles of 10^9. Additional very high cycle load increase test are used to refine threshold values for VHCF derived from fatigue tests. The results of mechanical testing are completed by investigating the inclusion distribution in tested specimen and evaluating the harmless inclusions. By the combination of these tests a threshold for the VHCF by FGA formation at inclusions can be derived.
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