A fracture mechanics interpretation of rolling bearing fatigue

Abstract

In fatigue assessment of structures and components, it is now commonplace to use a defect-tolerant approach to account for the inevitable presence of surface and internal flaws, both detectable and undetectable. This approach has been applied to rolling bearings in an attempt to present contact stress limits as a function of defect size below which fatigue failure would not be expected. In essence, it is an extension to the existing rating life methods developed by Ioannides and Harris that incorporate a fatigue limit stress in the estimation of bearing life, a concept similar to that of the endurance limit in structural fatigue. Such a fracture mechanics approach to sub-surface initiated fatigue in rolling bearing steels containing non-metallic inclusions, based upon the work of Murakami, has led to an allowable contact stress limit as a function of maximum inclusion size (as estimated by extreme value analysis). An alternative, less conservative fracture mechanics based stress limit is the prevention of propagation of cracks formed on inclusions (termed ‘butterflies’) by shear (Mode II) loading. The approach has been developed by examination of butterflies formed in service in rolling element bearings manufactured in bainitic steel with maximum inclusion sizes in excess of 100 µm. The observed lack of micro-crack formation on inclusions and non-propagation of butterflies support the concept of a fatigue endurance limit that is related to the cleanliness of the bearing steel.