Application of a failure assessment diagram under rolling contact to components with hardness variable along the depth

A development of a failure assessment diagram for the evaluation of the safe working area of components subjected to rolling contact loading is here presented regarding the application to components whose hardness varies along the depth. The approach takes into account the influence of inherent defects in determining subsurface rolling contact fatigue, depending on working conditions, material properties and hardness profile along the depth. For this aim, crack propagation from inherent defects is assessed in terms of applied stress intensity factor range normalized with respect to short crack growth threshold, defect – free fatigue is assessed in terms of Dang Van stress normalized with
respect to shear fatigue limit, where the material quantities depend on hardness variable with depth. These two normalized quantities are the coordinates of the points of a “reference curve” in the failure assessment diagram, which location indicates whether and where failure is expected to occur. By analysing different combinations of loading condition, inclusion dimension and hardness profile, it was possible obtaining a design diagram of general validity, which allows a fast prediction of safety against subsurface rolling contact fatigue.