Initiation and evolution of butterflies in roller bearings due to rolling contact fatigue

This investigation entails analysis of 520 butterfly formations (BFs) and non-metallic inclusions without BFs in two RCF-tested roller bearings to elucidate the influence of inclusion characteristics and stress level they are subject to, on the initiation and growth of BFs. The results show duplex inclusions, though least frequent in the bearing steels, exhibit the highest butterfly formation rate comparing with pure sulphide/oxide inclusions. It is also found that larger inclusions and those with smaller aspect ratios are more prone to butterfly formation; BF wing length decreases with the increase of inclusion aspect ratio; BF orientation is significantly influenced by inclusion size and stress level; stress magnitude appears to either impede or facilitate BF wing growth depending on BF orientation.