Crack Size Dependency of Shear-mode Fatigue Threshold in Bearing Steel Subjected to Continuous Hydrogen Charging

Premature delamination failure, characterized by the white structure flaking (WSF) or the white etching crack (WEC), often occurs in rolling element bearings and it deteriorates the durability of bearing substantially. It is known that this failure is caused by shear-mode (Mode II and Mode III) crack growth in conjunction with evolution and invasion of hydrogen into material during operation. To ensure the structural integrity associated with rolling element bearing, it is important to clarify the effect of hydrogen on the shear-mode fatigue crack growth behavior near the threshold level.
In our previous study, the effect of hydrogen on the shear-mode fatigue crack growth behavior in a bearing steel of JIS SUJ2 was examined near the threshold level. Consequently, it was shown that the threshold stress intensity factor (SIF) range for shear-mode fatigue crack growth decreased significantly by action of hydrogen. However, the investigation was made only for a crack with a surface length of about 900 mm. To thoroughly understand the critical condition for delamination failure, it is important to investigate the crack size dependency of the threshold level for a shear-mode small fatigue crack in the presence of hydrogen. In the present study, correspondingly, the threshold SIF ranges for a shear-mode crack with different length were additionally measured in the same material by using a novel technique that enables continuous charging of hydrogen in a specimen during long-term fatigue test. Then, a clear reduction in crack growth rate and a crack size dependency of the threshold SIF range were observed under the environmental condition of continuous hydrogen charging.