In the very high cycle fatigue for high strength steels, it is often reported that the fatigue crack tends to occur around the interior inclusion and a characteristic fine granular area (FGA) is formed around the inclusion. Outside the FGA, the fracture surface becomes rather flat within the fish-eye region. Remaining region outside the fish-eye is the fracture surface in the rapid crack propagation. Another typical aspect on such an interior induced fracture is the fact that the FGA formation period after starting cyclic loadings occupies a large portion of the total fatigue life. Thus, the FGA formation mechanism is one of the most important subjects in the very high cycle fatigue. From this point of view, the authors have attempted to construct the FGA formation model to interpret the fatigue fracture process inside the FGA. In this concept, polygonization is first caused around the inclusion before any micro-cracking during the long sequence of cyclic loadings. Then, micro-debondings take place at the boundaries between the polygonised layer and the matrix of the steel. The micro-debondings can also occur inside the polygonised layer. These micro-debondings can occur isolatedly and intermittently, and the number of micro-debondings tends to increase and some of them tend to coalesce to one another along the cyclic loadings. If the entire region of the FGA is fully debonded in such a way, a penny-shape crack is formed. Once the penny-shape crack is formed, it can grow continuously following the well-known Paris' law.
