Nowadays, bearing failure due to rolling contact fatigue (RCF) is generally a rare occurrence, and the final achieved life of rolling bearings is usually in excess of the calculated rating life. There are instances, however, where in specific applications bearings may fail prematurely. Understanding bearing failure mechanisms is fundamental for continuous improvement of bearing performance, in response to the requirement for the ever-increasing power density with modern rolling bearing applications.
The reliability of rolling bearings used in various applications obeys the weakest link principle, i.e., failure is a result of breakage of the weakest link inside the concerned system. Under poor lubrication conditions, bearing failure could be due to surface damage in the form of surface distress or wear. Otherwise, the weakest link could exist in the subsurface region. This is because the shear stress resulting from Hertzian contact reaches the maximum at a certain depth below the raceway surface. Furthermore, like most high-strength materials, bearing steels in general can suffer from lack of damage tolerance in the form of sensitivity to pre-existing material imperfections such as non-metallic inclusions as a by-product of steel-making processes.
Metallographic investigations on RCF-tested samples indicate that the so-called butterflies often develop from non-metallic inclusions. The butterfly seen from a nital-etched sample with light optical microscopy shows a pair of cracks emanating from an inclusion that are decorated with a white etching area, as shown in fig. 1. The white etching area is the altered microstructure resulting from rubbing between the crack faces [1, 2]. Under rolling contact loading, the crack may propagate and eventually break to the raceway surface, leading to surface spalling.