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Crack initiation characteristics and fatigue property of a high-strength steel in very-high-cycle\udfatigue (VHCF) regime under different stress ratios were investigated in this paper. Fatigue tests were\udperformed at stress ratios of -1, -0.5, 0.1 and 0.3 by using an ultrasonic fatigue testing machine. The difference\udof S-N data and the characteristics of crack initiation under different stress ratios were examined. It is shown\udthat the magnitude of stress ratio has a substantial effect on the fatigue strength that decreases with the increase\udof stress ratio. However, the variation tendency of the S-N data from low-cycle fatigue to VHCF regime is\udsimilar for the four cases. SEM observations of the fracture surface indicate that fatigue crack initiates from the\udsurface of specimen in low-cycle regime and initiates mostly from the inclusion in the interior of specimen in\udhigh-cycle and VHCF regimes, which is irrespective of the state of stress ratio. By means of Focused Ion Beam\udtechnique, the samples of crack initiation region were prepared then observed via TEM. The observations show\udthe microscopic morphology of the crack initiation region, showing the different crack initiation characteristics\udand revealing the mechanism of crack initiation for different stress ratios. Moreover, the effect of inclusion size\udon fatigue life is discussed with the results showing that the effect of stress ratio and inclusion size on fatigue\udstrength is well described by our proposed formula

Crack initiation characteristics and fatigue property of a high-strength steel in VHCF regime under different stress ratios