Effects of inclusions on the very high cycle fatigue behaviour of steels

The very high cycle fatigue of high-strength steels and many other alloys is controlled by the initiation and the propagation of cracks initiated at nonmetallic inclusions. Tiny cracks are early initiated from these inclusions. They propagate very slowly in a specific zone located close to the inclusion and called the fine-grained area (FGA) or optical dark area. This area has a size of the order of that of the inclusion. The FGA is followed by the formation of conventional crack propagation area with the presence of striations in many cases. The details of initiation of these very high cycle fatigue cracks depend on the relative elastic and thermal properties of inclusions and matrix. The specificities of the FGA zone are dependent on the hydrogen segregated at the interface of the inclusions. An elasto-plastic finite element calculation is performed to determine the residual stresses in the vicinity of the inclusions and to evaluate the effect of residual stresses.