Fracture surface topography and fracture mechanism in austenitic SUS316 steel plates fatigued by repeated bending

Fatigue experiments were carried out using the austenitic SUS316 steel plates (the average grain diameter is about 1.3 × 10−5 m) by repeated bending. The three-dimensional fatigue fracture surfaces were then reconstructed using stereo pairs of scanning electron micrographs by the stereo matching method. Striations were observed on the stage II fatigue fracture surface, while fine slip steps were found on the stage I fatigue fracture surface. The averaged value of the fractal dimension of stage I fracture surface was about 2.2 and was almost the same as that of stage II fracture surface when the fractal dimension was measured in the length scale range smaller than about one grain-boundary length (about 8 × 10−6 m). This may be attributed to the fact that both fracture surfaces were formed by the same mechanism, namely, slipping-off. According to the two-dimensional fractal analysis, both stage I and stage II fatigue fracture surfaces did not exhibit anisotropy in the length scale range of the fractal analysis smaller than about one grain-boundary length in the SUS316 steel. The fractal dimension of the fatigue fracture surface increased with decreasing the magnification of images when the maximum length scale of the fractal analysis was extended to the size of analyzed area. Magnification dependence of the fractal dimension was associated with large steps and ledges, which were not “typical” fractals.