Fatigue behavior of X10CrNiMoV12-2-2 under the influence of mean loads and stress concentration factors in the very high cycle fatigue regime

Abstract

In this study the fatigue behavior of X10CrNiMoV12-2-2 has been investigated for different stress concentration factors (1.00 < α_k < 2.42) and load ratios from R = -1 to R = 0.7 up to 2∙109 load cycles at room temperature. The tests were performed under axial loading using ultrasonic fatigue testing setups of the type BOKU Vienna and a system developed at the Institute of Materials Science and Engineering (WKK) of TU Kaiserslautern. For cylindrical samples and specimens with low stress concentration factor (α_k = 1.09) the S-N-curves show a flat slope with no significant decrease of the fatigue strength in the VHCF-regime. A transition of crack initiation from surface to volume cracks starting at oxide inclusions of the type AlCaO or AlCaMgO can be observed at about 1∙10⁷ load cycles for load ratios from R = -1 up to 0.5. The maximum number of load cycles where sample failure occurs increases with increasing load ratio. For R = 0.5 fatigue fractures occur even beyond 2∙10⁹ load cycles. Murakami`s widely accepted √area-approach [1] shows a good correlation for a wide range of R-values over four decades of the fatigue life. Fracture surfaces for internal crack initiation show the typical fish-eye structure around the inclusion. A fine granular area can only be observed for a load ratio of R = -1. The mechanism by Grad et al. [2] can describe the FGA formation. FGAs could not be observed for higher load ratios. For specimens with high stress concentration factor (α_k = 2.42) the maximum number of load cycles where fracture occurs is about 1∙10⁶ load cycles which means no VHCF-failure can be observed. In this case, cracks are always initiated at small machining induced surface defects. In the HCF-regime the S-N-curve shows a steeper slope which is typical for notched samples.