A Fracture Mechanics Method for an Advanced Evaluation of Inclusions and the Prediction of Fatigue Life of Rolling Element Bearings

Abstract

The quantity and appearance of nonmetallic inclusions (NMIs) in terms of composition, size, and shape are strongly influenced by the process of steelmaking. Nonmetallic inclusions can have a major impact on the fatigue performance of rolling bearings. Although limited to rare cases, macroscopic inclusions may lead to unexpected premature failures. Microscopic inclusions have been recently discussed as a potential influence factor in the context of white etching cracks (WECs). Up to now, the potential effect of microscopic inclusions on fatigue life has been mostly investigated experimentally. Another option is to assess NMIs analytically by using linear elastic fracture mechanics under the assumption that NMIs can be analyzed as small cracks. The method presented here is based on orthogonal shear stress distribution and considers short crack behavior as well as the size, shape, and orientation of NMIs to determine a characteristic shear stress fatigue limit for rolling bearing steels. The calculated fatigue life, based on this approach, shows a very good correlation with life test results for different bearing sizes, for both macroscopic and microscopic inclusions. Furthermore, the results show that the standardized fatigue life calculation according to ISO 281 and ISO/TS 16281 offers a conservative approach for fatigue life prediction. Thus, the newly developed model offers the possibility of deriving material load rating factors from the microscopic rating of NMIs. Regarding WECs, the results suggest that NMIs are not the root cause of premature WEC failures. Typically, other factors, such as generation of hydrogen or electric current, are necessary conditions for the development of WECs. It is plausible that the detrimental effect of diffusible hydrogen can reduce the shear stress fatigue limit.