抄録
This paper describes the results of observation of dislocation structures near the cracks having the known stress intensity factor ranges (ΔK) in fatigued bulk specimens of iron. The ΔK-values were between 80 and 140kg/mm3/2, which correspond to the crack growth rate from 0.10 to 0.08μ/cycle. A sort of cell structure was observed near the crack tips. Although the average size of the cells decreased with increasing the value of ΔK, the general feature of the cell structure did not seem to change much in spite of the difference in ΔK-value; it was characterized by the formation of highly dense dislocations in the cells just ahead of the tips and a narrow band composed of cells elongated along the direction of crack growth in front of the crack. In the vicinity of the crack sides, a clear cell structure was observed and the interior of the cells has a relatively low dislocation density. These results suggest that some of dislocations ahead of the crack tip disappear when the crack tip has passed, and, therefore, some extent of strain relaxation takes place at that moment. Some cracks were found which propagated along a cell-or grain-boundary, or penetrated into a cell. It appeared that the route of fatigue cracks in iron was not always along cell-boundaries. This work may be considered as a follow-up of the previous investigation on fatigue structures around the cracks, in which the ΔK-values were difficult to obtain.
