Construction of probabilistic model on interior crack initiation and growth behaviors for high strength steels in very high cycle fatigue

In very high cycle fatigue for high strength steels, one can usually observe the fish-eye on the fracture surface and an inclusion is observed at the center of the fish-eye. The fine granular area (FGA) is also found in the vicinity around the inclusion at the crack initiation site on the fracture surface. If the stress cycles required to form the FGA occupies the predominant part of the final fatigue life of these steels, 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 based on the current experimental results obtained by the authors. Supposing four different probabilistic models, the FGA formation is interpreted as intermittent micro-debondings and their coalescence during the long sequence of the cyclic loadings. After the FGA was accomplished, the penny-shape crack tends to grow steadily following the well-known Paris’ law. Based on this analysis of the FGA formation, it was found that polygonization around the inclusion is more important behavior to govern the total fatigue life of the high strength steels.