Issue 35

X. Liu et alii, Frattura ed Integrità Strutturale, 35 (2016) 88-97; DOI: 10.3221/IGF-ESIS.35.11 88 Focussed on Crack Paths Crack initiation characteristics and fatigue property of a high-strength steel in VHCF regime under different stress ratios Xiaolong Liu, Chengqi Sun, Youshi Hong State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China hongys@imech.ac.cn A BSTRACT . Crack initiation characteristics and fatigue property of a high-strength steel in very-high-cycle fatigue (VHCF) regime under different stress ratios were investigated in this paper. Fatigue tests were performed at stress ratios of -1, -0.5, 0.1 and 0.3 by using an ultrasonic fatigue testing machine. The difference of S-N data and the characteristics of crack initiation under different stress ratios were examined. It is shown that the magnitude of stress ratio has a substantial effect on the fatigue strength that decreases with the increase of stress ratio. However, the variation tendency of the S-N data from low-cycle fatigue to VHCF regime is similar for the four cases. SEM observations of the fracture surface indicate that fatigue crack initiates from the surface of specimen in low-cycle regime and initiates mostly from the inclusion in the interior of specimen in high-cycle and VHCF regimes, which is irrespective of the state of stress ratio. By means of Focused Ion Beam technique, the samples of crack initiation region were prepared then observed via TEM. The observations show the microscopic morphology of the crack initiation region, showing the different crack initiation characteristics and revealing the mechanism of crack initiation for different stress ratios. Moreover, the effect of inclusion size on fatigue life is discussed with the results showing that the effect of stress ratio and inclusion size on fatigue strength is well described by our proposed formula. K EYWORDS . High-strength steel; Very-high-cycle fatigue; Crack initiation characteristics; Stress ratio; Inclusion size. I NTRODUCTION n recent years, the research of very-high-cycle fatigue (VHCF) property has attracted increased interests because more and more engineering structures are required to endure 10 7 or even more fatigue cycles, and engineering alloys such as high-strength steels and titanium alloys exhibit a sharp decrease in fatigue strength between fatigue life of 10 6 and 10 9 cycles [1, 2]. Naito et al [3, 4] first reported that high-strength steels failed at the stress lower than the conventional fatigue limit and the crack initiated from non-metallic inclusions at the interior of specimen. The whole region of crack initiation and early propagation exhibits a “fish-eye” pattern and a relatively rough granular morphology is often observed surrounding the inclusion, which is called fine granular area (FGA) [5-7]. Several models have been proposed for explaining the formation mechanism of FGA, including “formation and debonding of fine granular layer” I