Fatigue failure assessment under stress gradients using small crack fatigue concepts

The present work describes a microstructurally-based methodology to assess fatigue limits in specimens under stress/strain gradients by using a single concept of modelling the growth of a small crack driven (or retarded) by a rapidly changing stress field and interacting with the microstructural barriers (e.g. grain boundaries) of the material. This description is based in what is called Microstructural Fracture Mechanics, which extend techniques successfully employed to long cracks in LEFM to cracks whose length is comparable to the size of the microstructure of the material. The methodology is successfully applied to notched components. The classical limits for crack initiation and crack propagation up to failure in notched components are clearly identified and predicted. The appearance of non-propagating cracks and their expected lengths are also estimated. Finally, a practical extension of this methodology to analyse the fatigue failure under stress/strain gradients and stress ratio variations with depth typically generated in fretting-fatigue problems is also presented.