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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Keywords: Shear-mode fatigue crack; Bearing steel; Threshold level; Crack size dependency; Continuous hydrogen charging

1. Introduction Delamination failure of rolling element bearings, known as the white structure flaking (WSF) or the white etching crack (WEC), is a critical problem for assuring the structural integrity of automotive components, wind turbine gearboxes, etc. This failure is intimately related to a shear-mode (Mode II and Mode III) fatigue crack subjected to cyclic shear stress in the presence of heavy compressive stress. Some previous studies suggest that WSF could be associated with invasion of the hydrogen derived from decomposition of lubricant in operation ( Evans et al. (2012, 2013), Tanimoto et al. (2011), Kino et al. (2003), Tamada et al. (1996), Fujita et al. (2010)). It is known that hydrogen can readily penetrate and diffuse into material, and it facilitates fatigue crack growth to decrease fatigue lifetime. To ensure the integrity of rolling element bearings, therefore, it is necessary to investigate the effect of hydrogen on the shear-mode fatigue crack growth behavior, especially on its threshold behavior. In the previous study by Akaki et al. (2017), effects of hydrogen on the near-threshold growth behavior of a shear mode fatigue crack was investigated for a bearing steel of JIS-SUJ2. Consequently, it was shown that the threshold stress intensity factor (SIF) range for shear-mode fatigue crack growth decreased significantly by action of hydrogen. In the previous study, however, the investigation was made only for a crack with a surface length of about 900  m. It is well known that the threshold SIF range,  K th , for Mode I growth of a small fatigue crack of sub-millimeter in size has a crack size dependency, which is well characterized by a relationship of ∆ K th ∝ (√ area s ) 1 3 ⁄ , where √ area s is the square root of the area of a surface crack. Okazaki et al. (2011, 2014) investigated the crack size dependency of the threshold level,  K  th , for growth of a shear-mode small fatigue crack in a bearing steel of JIS-SUJ2 in air at room temperature and reported a similar relationship of ∆ K τ th ∝ (√ area s ) 1 3 ⁄ . This result implies that the crack size dependency would also appear in the case of the shear-mode fatigue threshold,  K  th , in the presence of hydrogen. The objective of the present paper is to investigate the crack size dependency of a shear-mode fatigue crack in a bearing steel of JIS-SUJ2 in the presence of hydrogen by measuring the threshold values of  K  th for different crack sizes.

Nomenclature a

Half the surface length of a semi-elliptical surface crack (  m)

b Depth of a semi-elliptical surface crack (  m) √ area s Square root of the area of a surface crack  a Torsional shear stress amplitude (MPa)  s

Static compressive stress applied in the axial direction of a specimen (MPa)

f

Test frequency (Hz) Number of cycles The Vickers hardness

N

HV SIF

Stress intensity factor  K  th Threshold SIF range for growth of a shear-mode fatigue crack (MPa·m 1/2 )   Torsional shear stress range,   = 2  a

2. Experimental procedures

2.1. Material and specimen

The material used in this study was a bearing steel of JIS-SUJ2, which is the same material used for balls of ball bearings. The fatigue tests were performed using two specimens with shear-mode fatigue cracks with different sizes in order to investigate the crack size dependency. The shapes and dimensions of smooth specimens are shown in Figs.1 (a) and (b). To circulate a hydrogen-charging solution during fatigue testing, a through-hole of 2 mm in diameter was