Issue34

T. Makino et alii, Frattura ed Integrità Strutturale, 34 (2015) 334-340; DOI: 10.3221/IGF-ESIS.34.36 340 C ONCLUSION CF test was conducted using specimens with artificial defects which simulate stringer-type inclusions. The effect of defect length on flaking life was evaluated using specimens with different-length defects. The following results were obtained. (1) In the case of the defect with the 15  m diameter, flaking life decreased with increasing defect length. In the case of the defect with the 50  m diameter, flaking life was almost constant from 50 to 300  m of defect length and shorter than the shortest life in the case of the 15-  m-diameter defect. (2) In a comparison of the CT image and the SEM view, the shapes of defects and the locations of the horizontal cracks were almost the same respectively, but the shapes of vertical cracks in CT images were smaller in the deeper region than those in SEM views. (3) Stress states around a circular hole during rolling contact were calculated by elastic-plastic FE analysis. Remarkably high tensile residual stress was generated at the surface edge of the defect. Defects led to higher tensile residual stress than that without defects in the region where the defect exists. The shear stress range at 0.1 mm depth on the middle line of the hole increased with increasing defect length. (4) The SIFs of the RCF cracks were calculated by elastic FE analysis. The range of mode II SIF Δ K II at the bottom of a vertical crack increased with increasing defect length . (5) Above analytical results described in (3), (4) provided valuable information for understanding the mechanism of the experimental results described in (1), (2). A CKNOWLEDGEMENTS he synchrotron radiation experiments were performed at BL19B2 in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) under proposal numbers 2011A1787, 2011B1955, 2012A1596, 2012B1306, and 2013A1307. R EFERENCES [1] Chen, Q., Shao, E., Zhao, D., Guo, J., Fan, Z., Measurement of the critical size of inclusions initiating contact fatigue cracks and its application in bearing steel, Wear, 147 (1991) 285–294. [2] Lewis, M. W. J., Tomkins, B., A fracture mechanics interpretation of rolling bearing fatigue, proceedings of the institution of mechanical engineers, Part J. J Eng. Tribol., 226(5) (2012) 389–405. [3] Nagao, M., Hiraoka, K., Unigame, Y., Influence of nonmetallic inclusion size on rolling contact fatigue life in bearing steel, Sanyo Tech Report, 12(1) (2005) 38–45 (in Japanese). [4] Neishi, Y., Makino, T., Matsui, N., Matsumoto, H., Higashida, M., Ambai, H., Influence of the Inclusion Shape on the Rolling Contact Fatigue Life of Carburized Steels, Metall. Mater. Trans. A, 44(5) (2013) 2131–2140. [5] Nakai, Y., Shiozawa, D., Fukuda, Y., Neishi, Y., Makino, T., Observation of cracks in carbon steel under contact rolling fatigue by micro CT imaging using ultrabright synchrotron radiation, 15th International conference on experimental mechanic, Paper Ref:2635, (2012) [6] Makino, T., Neishi, Y., Shiozawa, D., Fukuda, Y., Kajiwara, K., Nakai, Y., Evaluation of rolling contact fatigue crack path in high strength steel with artificial defects, Int. J. of Fatigue, 68 (2014) 168 – 177 ． R T