MPa the initiation of the crack in the centre of the fish eye is sometimes more complicated as is shown in figure 11. In high carbon and high strength steels, there is a transformation of the microstructure starting from the inclusion, in relation with the stress concentration and the stress field. It is difficult to understand this transformation which appears at the microscopic scale within a radius of 200 microns around an inclusion! Based on SEM observation, it can be said: -Some wings (two to four) occur around the inclusion similar to the butterfly wings observed in rolling contact fatigue. Micro cracks are observed along the boundary between the wings and the matrix,Fig 11. In comparison with the rolling contact damage it seems that a phase transformation or grain refinement occurs. . In both cases, it is reasonable to assume that the microstructure of the wings should be a nano- ferrite phase. [11] Figure 12 compares the initiation of a fatigue crack in rolling fatigue and in push pull fatigue for a same bearing steel. Indeed the features are very similar. -It is of interest to point out the relation between the wings, the oxide at the centre of the fish eye and the former austenitic grain boundaries, which is clear in figure 12. To prove this relation, a fish eye originating from a failure of high strength steel specimen was observed via electron backscatter diffraction (EBSD). To obtain a good EBSD map, the surface of the fractured specimen was polished slightly, with care taken to track the location of the particle at the center of the fish eye. The results are shown in figure 13. It is shown that the oxide inclusion, from which the initiation starts, is located at a triple point of former austenitic grains. The wings or the phase transformation appear along these grain boundaries producing internal stress and cracking along the wings for a length of about 200 microns. The orientations are colored according to the inverse pole figure color key shown in the figure. There is no indication of phase transformation from these images, but EBSD will not discern readily between bainite and martensite, so the nature of the microconstituent along the wings remains unknown. This approach proves that the initiation of a fish eye in gigacycle fatigue is not only due to PSB or polygonisation, with or without the effect of hydrogen as mentioned by Murakami [4]. The grain boundary, the interaction between the defect and grain boundary, and sometimes the phase transformation or the refining of the microstructure are involved in a complex process leading up to the formation of a crack in stage 2. 6-Conclusions When the cyclic stress is low, plasticity vanishes at the surface of the fatigue specimens, depending of the yield stress and the size of the defect. However, in low yield stress iron the initiation in VHCF is always at the surface in round and flat specimens. But, for high strength steels, the initiation around a defect occurs at the interior, even in flat specimens, due to the stress concentration in plane strain, in the gigacycle regime. In high strength steels, the stability of the microstructure and the grain boundaries are involved in the plastic deformation to explain crack initiation in the fish eye. It seems that martensite or bainite could be transformed in nanoferrite inside butterfly wings, similar to what is observed in rolling contact fatigue. This mechanism is affected by the former austenite grain boundaries.
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