Issue34

Y. Nakai et alii, Frattura ed Integrità Strutturale, 34 (2014) 246-254; DOI: 10.3221/IGF-ESIS.34.26 249 Specimen with sulfur concentration of 0.020 mass% Fig. 4 shows an optical micrograph of specimen with a sulfur concentration of 0.020 mass% after N = 1.10×10 7 cycles (shortly after appearance of cracks at the rolling surface), where p max , was 5.39 GPa. It indicates that a small crack of length approximately 40 μm formed from an inclusion and propagated perpendicular to the ball-rolling direction. After the formation of the crack at the surface, successive observations by SRCL and RCF tests were conducted at SPring-8. Flaking was found to occur at N f =1.295×10 7 cycles around the site where the surface crack first appeared as shown in Fig. 5. 200 m  50 m  Rolling direction Crack Rolling direction (a) Flaking (b) Enlarged view of flaking Figure 5 : Optical micrograph of surface at flaking. ( p max = 5.39 GPa, N f = 1.295×10 7 cycles). 3D images of inclusions and cracks observed by SRCL at N = 1.10×10 7 cycles, 1.168×10 7 and 1.295×10 7 cycles are shown in Fig. 6, where (A) is a top view, (B) is a side view of the specimen, and (C) is the view from the rolling direction. Inclusions are indicated in orange. In (a) and (b), the shape of crack after N = 1.10×10 7 cycles is shown in red, and the extension of the crack from N = 1.10×10 7 cycles to 1.168×10 7 cycles is shown in white. As shown in the figure, the surface crack in Fig. 4 appears to form from a cylindrical inclusion with a length of about 30 μ m, that reaches the surface, and the crack face is perpendicular to the rolling direction. At N = 1.10×10 7 cycles, the vertical crack had propagated further than the depth of the starter inclusion. Slight growth of the crack in the depth direction was observed at N = 1.168×10 7 cycles. The shape of the flake at N f = 1.295×10 7 cycles is indicated in purple in Fig. 6 (c). It can be concluded that the flake formed around the site where the vertical crack was observed. The shape at the surface obtained by SRCL is similar to that obtained by optical microscopy as shown in Fig. 5. Since the depth of the flaking is almost the same as that of the vertical crack, the formation of the vertical crack must have affected the formation of the flake although part of the vertical crack remained at the bottom of the flake as can be seen in Fig. 5 (b). Specimen with sulfur concentration of 0.049 mass% Fig. 7 shows a SEM image of a crack initiation site at the surface ( N = 6.00×10 6 cycles, shortly after crack initiation at the surface), where p max , was 5.39 GPa. A small crack, which is perpendicular to the ball-rolling direction, can be observed. After the initiation of the crack at the surface, observation by SRCL was conducted at SPring-8. Flaking was found to occur at N =7.67×10 6 cycles at the site where the surface crack was observed as shown in Fig. 8. 3D images of inclusions and cracks observed by SRCL at N = 6.00×10 6 cycles, 6.80×10 6 cycles, 7.10×10 6 cycles, 7.50×10 6 cycles, and 7.67×10 6 cycles are shown in Fig. 9, where (A) is a top view, (B) is a side view of the specimen, and (C) is the view from the rolling direction. In these figures, red, black, blue, green, and purple indicate crack at N = 6.00×10 6 cycles, 6.80×10 6 cycles, 7.10×10 6 cycles, 7.50×10 6 cycles, and 7.67×10 6 cycles, respectively. It can be seen from these figures that the inclusions, indicated in orange are longer than those in Fig. 6. As shown in Fig. 9, a vertical crack formed from a cylindrical inclusion with a length of about 60 µ m that reaches the surface, and the crack face is perpendicular to the rolling direction. At N = 6.80×10 6 cycles, the vertical crack propagated to the deepest point of the starter inclusion without propagation along the surface. At N = 7.10×10 6 cycles, a shear-type horizontal crack, which was parallel to the rolling contact surface, formed at a depth of 35 μm from the surface. From N = 7.10×10 6 to 7.50×10 6 cycles, the vertical crack propagated in the width direction under the surface, and the horizontal shear-type crack propagated in both the width and rolling directions.