PSI - Issue 14

Y. Akaki et al. / Procedia Structural Integrity 14 (2019) 11–17 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Therefore, it is considered that the crack size dependency of  K  th for shear-mode fatigue cracks can be evaluated by using Eq. (4) also for the present material. Two values of  K  th measured under the H-charging condition are approximately 14 % lower than the prediction line. Taking the argument in the previous section into account, this degradation is obviously due to the influence of hydrogen. Furthermore, because of the same decreasing rate of 14% for both values of  K  th , it is suggested that  K  th has the same crack size dependency of ∆ K τ th ∝ (√ area s ) 1⁄3 also under the H-charging condition as indicated by the dotted line in Fig. 6. To verify this result, it is necessary to additionally measure the values of  K  th under the H-charging condition for shear-mode fatigue cracks with widely varying sizes. 4. Conclusions Shear-mode fatigue crack growth tests were conducted using smooth bearing steel specimens under the condition of hydrogen charging as well as that of non-charging, and the effects of hydrogen on the near-threshold shear-mode fatigue crack growth behavior and the crack size dependency of the threshold SIF range,  K  th , were investigated. The obtained results are summarized as follows: ⚫ The crack that had become a non-propagating crack at a threshold stress level under the non-charging condition restarted growth by switching to the hydrogen-charging condition at the same stress level. Hydrogen increased the fatigue crack growth rate and affected the threshold stress. ⚫ The values of  K  th measured under the non-charging condition agreed well with the prediction obtained using the equation proposed by Okazaki et al. and it had a crack size dependency of ∆ K τ th ∝ (√ area s ) 1 3 ⁄ . ⚫ The values of  K  th measured under the hydrogen-charging condition were significantly lower than the predicted values for the non-charging condition because of hydrogen. In addition, it was suggested that it had a similar crack size dependency of ∆ K τ th ∝ (√ area s ) 1 3 ⁄ . References M.-H. Evans., 2012. White structure flaking (WSF) in wind turbine gearbox bearings: effects of ‘butterflies’ and white etching cracks (WECs), Mater. Sci. Technol., 7(2), 3-22. M.-H. Evans, A.D. Richardson, L. Wang and R.J.K. Wood., 2013. Serial sectioning investigation of butterfly and white etching crack (WEC) formation in wind turbine gearbox bearings. Wear, 302(1-2), 1573-1582. Tanimoto H, Tanaka H and Sugimura J., 2011. Observation of Hydrogen Permeation into Fresh Bearing Steel Surface by Thermal Desorption Spectroscopy. JSME. A., 6(7), 291-296. Kino N and Otani K., 2003. The influence of hydrogen on rolling contact fatigue life and its improvement. JSE Review, 24(3), 289-294. Tamada K and Tanaka H., 1996. Occurrence of brittle flaking on bearings used for automotive electrical instruments and auxiliary devices. Wear, 199(12), 245-252. Fujita S, Matsuoka S, Murakami Y and Marquis G., 2010 Effect of hydrogen on Mode II fatigue crack behavior of tempered bearing steel and microstructural changes. Inter. J. F., 32(6), 943-951. Akaki Y, Matsuo T, Nishimura Y, Miyakawa S and Endo, M., 2017. Microscopic observation of shear-mode fatigue crack growth behavior under the condition of continuous hydrogen-charging. Journal of Physics: Conf. Series 843. Akaki Y, Matsuo T, Nishimura Y, Miyakawa S and Endo, M., 2017. A new testing method for investigating the shear-mode fatigue crack growth behavior in hydrogen environment. Journal of Physics: Conf. Series 842. Matsunaga H, Shomura S, Muramoto S and Endo M., 2011 Shear mode threshold for a small fatigue crack in a bearing steel. Fatigue Fract. Eng. Mater. Struct. 34 72 – 82. Okazaki S, Matsunaga H, Ueda T, Kamata H, Endo M., 2014. A practical expression for evaluating the shear-mode fatigue crack threshold in bearing steel. Theoretical and Applied Fracture Mechanics 73 161-169.