The Sheet specimens of mild steel were fatigued at room temperature by plane bending stressing. The dislocation structures during the process of fatigue crack initiation and near the tips of fatigue cracks were observed using a transmission electron microscope and the dislocation density and the size of cell structure were obtained quantitatively. The relationship between the dislocation structure and the plastic deformation zone near the crack tips was examined.
The main results obtained are as follows:
(1) During the process of crack initiation, the total dislocation density ρt increased to a saturation with progress of fatigue. The dominant structure at high stress amplitude was cell structure, and that at low stress amplitude was loop patch structure. The cell width lw (or the bundle spacing ls) and the free dislocation density ρf in the cell structure decreased gradually during this process.
(2) From the observation of dislocation structure near the crack tip, the mean size of cell structure lm and the free dislocation density ρf in the cell structure were found to decrease with approaching the tip. They showed almost the same distribution in all directions from the crack tip, and this influence of decreasing lm and ρf extended to the distance of about 1000μm from the crack tip.
(3) The plastic deformation region near the crack tip determined by Vickers hardness tests was larger than the area where the mean cell size lm was found to be below about 4μm.