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Y. Nakai et alii, Frattura ed Integrità Strutturale, 34 (2014) 246-254; DOI: 10.3221/IGF-ESIS.34.26 248 Measurement setup The measurement by SRCL imaging was carried out at the BL46XU beam line of SPring-8 (Super Photon Ring – 8 GeV), which is the brightest synchrotron radiation facility in Japan. Fig. 2 shows the apparatus used for SRCL imaging. The inclination angle of the axis in the SRCL,  , was 30°, and a 37 keV monochromatic X-ray beam was employed. In the present study, the effective voxel size in the reconstructed the 3D image was 0.74 μm. For the 3D reconstruction, a set of 720 radiographs of a specimen were recorded during 360° rotation, i.e. , in increments of 0.5°. The exposure time was 4 s for the measurements. To utilize the phase contrast effect, an X-ray area detector was set 0.30 m behind the sample. Once the 3D images were reconstructed, they were visualized using Image-J and Amira software. For the 3D representation of inclusions and cracks, a region-growing procedure was employed to segment them, where a gray value threshold between the matrix and air was employed to produce binarized 3D images. Specimen stage Synchrotron radiation X-ray 10 5 10 6 10 7 10 8 4 5 6 7 8 Number of cycles to failure, (cycles) N f Hertz stress, (GPa) p max S (mass%) 0.017 0.020 0.049 Figure 2 : SRCL imaging apparatus Figure 3 : S-N curves. E XPERIMENTAL RESULTS Fatigue life he relation between the maximum Hertz stress, p max , and the number of cycles to flaking is shown in Fig. 3, where circular, triangular, and square marks indicate the results for materials with sulfur concentrations of 0.017, 0.020, and 0.049 mass%, respectively. An increase in the sulfur concentration resulted in a decrease in the flaking life. It has been reported that the fatigue strength is not affected by the cleanliness (concentration of inclusions) of specimens [1]. The size of inclusions, however, affects the fatigue life, i.e ., larger inclusions reduce the fatigue strength [10]. For the present material, an increase in the sulfur concentration increased the number of inclusions, particularly the number of large inclusions. Figure 4 : Optical micrograph of surface. ( p max =5.39 GPa, N = 1.10×10 7 cycles). T