Issue34

T. Makino et alii, Frattura ed Integrità Strutturale, 34 (2015) 334-340; DOI: 10.3221/IGF-ESIS.34.36 336 Cracks were initiated at the edges of 15-  m-diameter defect in radial direction on rolling contact surface. According to the following investigation, they propagated not only in radial direction but also in depth direction along defect. These cracks are referred to as vertical cracks in this paper. The surface length of vertical crack including defect 2 c was measured during RCF test. Fig. 2 shows changes in 2 c during RCF test. Vertical cracks initiated at longer defects propagated faster. SR MICRO CT IMAGING OF CRACK INITIATION AND PROPAGATION BEHAVIOUR Inspection samples n order to discuss the mechanism of the RCF damage described in the previous section, it is of great importance to clear how the cracks observed on surface propagate in internal region and form flaking. In this section, the morphology of internal cracks initiated in disc specimens of S55C inspected by SR micro CT imaging at SPring-8 is discussed. Artificial defects introduced in disc specimens were 15  m in diameter and 50, 100, and 200  m in defect length. The RCF test stopped at an arbitrary number of cycles. Inspection samples having a square cross section of 0.5 mm × 0.5 mm with the defect were taken from the disc specimens. The dimension is regulated by the penetrative thickness for the present X-ray energy for Fe material. CT imaging method CT imaging was performed at the BL19B2 beam line of SPring-8, the most powerful synchrotron radiation facility in Japan. The measurement conditions were as follows: X-ray energy of 37 keV and sample-detector distance of 0.7 m [5]. The effective pixel size of the detector was from 0.74 to 2.8  m depending on the magnification of the beam monitor. For a 3-D reconstruction, a set of 900 radiographs of a sample were recorded at rotations over 180°, where each rotation angle was 0.2°. Slice images were reconstructed from the series of the projection images by a filtered-back projection algorithm. 20μｍ (a) L =50 μｍ, N =1×10 4 20μｍ (b) L =50 μｍ, N =1×10 6 25μｍ (c) L =100 μｍ, N =1×10 6 50μｍ 50μｍ (d) L =200 μｍ, N =1×10 6 y x z Defect Vertical crack Horizontal crack Figure 3 : CT images and SEM view of artificial defect and cracks (Defect diameter: 15 μm, Left: CT image, and Right: SEM view). CT Imaging result Images of the artificial defect and cracks shown in the left-hand side of Fig. 3 were obtained by CT scanning. Several samples were broken at liquid nitrogen temperature to observe the fracture surface of the vertical crack. SEM views of the fracture surfaces are also shown in right-hand side of Fig. 3. Shapes of RCF cracks identified from fractography were also I