Ultra-long cycle fatigue of high-strength carbon steels part I: review and analysis of the mechanism of failure

In this study, the fatigue crack propagation from the surface and from internal inclusions is analyzed and modeled in high strength steels that show both types of crack initiation. Fatigue crack propagation lives of a crack initiated from the surface and from internal inclusions are estimated and analyzed. The high cycle fatigue life for a given crack range is estimated by: (a) defining material resistance to crack propagation as a function of crack length, and (b) assuming that the difference between the applied driving force and material resistance for crack propagation defines the effective driving force applied to the crack. If the crack growth rate as a function of this effective driving force is known for a given material, the high cycle fatigue life for a given crack length range can be estimated. The present model estimates reasonably well the fatigue life associated with crack initiated from the surface. On the other hand, the fatigue life of crack initiated from internal inclusions has an associated initiation life defined by a hydrogen assisted fatigue mechanism that represents a great part of the total fatigue life. In this case, the fatigue crack propagation life predicted by the model is a small part of the total fatigue life. The estimations and analysis made in this study, in accordance with previous observations reported in the literature, reveal that the total fatigue life associated with cracks initiated at internal inclusions is valid only if the number of cycles necessary to develop the optical dark area (ODA) by hydrogen assisted fatigue can be properly estimated.