Hardness as a tool for the estimation of the microstructural threshold

As it was described by K. Miller, fatigue resistance can be characterized by the existence of a microstructural and a mechanical threshold. The microstructural threshold is strictly related to the intrinsic microstructural properties of the material and it can be defined as the stress level needed for a microstructurally short crack (MSC) to overcome the strongest microstructural barrier, usually found to be a characteristic microstructural dimension d (grain size, pearlite colony size, bainite lath length, etc). Previous work by Chapetti provided evidence that this intrinsic threshold stress level matches the material's plain fatigue limit at a distance d from the surface. In this work, hardness is evaluated as a parameter to be included in the estimation of the microstructural threshold associated to the plain fatigue limit of metals. The proposed expression seems to work reasonably well for low and medium strength (ferritic-pearlitic and martensitic) steels and load ratio R = -1.