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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 mm. 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.

The value of modelling the fatigue crack propagation process is discussed and current models are examined in the light of increasing knowledge of crack tip deformation. Elevated temperature fatigue is examined in detail as an area in which models could contribute significantly to engineering design. A model is developed which examines the role of time-dependent creep cavitation on the failure process in an interactive creep-fatigue situation.

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