A ductile material subjected to repealed rolling contact can accumulate very high levels of shear strain near the surface. At some point the material loses its integrity and fails, and this failure is manifested in the form of wear (the material detaching from the surface and producing debris) or rolling contact fatigue (initiation of micro-cracks which may subsequently propagate and branch). Models of such contacts have been developed based on ductility exhaustion [Wear 245 (2000) 204; Int. J. Fatigue 22 (2000) 205]. For wear, the material is divided into layers and each layer accumulates shear strain dependent on the stress at that depth; once a layer has accumulated a critical shear strain it is deemed to have failed. In the work presented here, the models are improved by allowing variation with depth of material properties such as ductility and shear yield stress. This reflects the statistical variation of real materials arising from the microstructure. For rolling contact fatigue, ductility exhaustion has been taken to mean initiation of a micro-crack. However, this introduces ambiguity since ductility exhaustion is also the cause of wear of material from the surface. The suggestion, here, is that the dilemma can be resolved by considering a model which has a brick wall structure. Each material brick can lose integrity and thus fail. Dependent on whether the failed material is supported by adjacent bricks which are intact, the material may detach to produce wear debris. Bricks which fail but do not detach (either at the surface or below it) behave as micro-cracks; these can interact and grow, but can also be lost if the surface material is worn away later.
展开▼
