Mechanism of fatigue crack initiation and propagation in the very high cycle fatigue regime of high-strength steels

Fatigue fracture surfaces of high-strength steels generated by a very high number of cycles to failure show so-called fine granular areas, for which the crack initiation and propagation mechanism is not fully understood. To clarify this mechanism of crack growth, scanning electron/ion microscopy, focused ion beam and transmission electron microscopy investigations were carried out at fracture surfaces showing this special crack part. On the basis of the results a mechanism for the formation of this area is proposed.