Y. Nakai et alii, Frattura ed Integrità Strutturale, 34 (2014) 246-254; DOI: 10.3221/IGF-ESIS.34.26 246 Focussed on Crack Paths In situ observation of rolling contact fatigue cracks by laminography using ultrabright synchrotron radiation Y. Nakai, D. Shiozawa, S. Kikuchi, K. Sato, T. Obama Department of Mechanical Engineering, Kobe University, 1-1, Rokkodai, Nada, Kobe 657-8501, Japan T. Makino, Y. Neishi Nippon Steel & Sumitomo Metal Corporation, Japan A BSTRACT . In rolling contact fatigue (RCF), cracks usually initiate from inclusions beneath the surface and propagate to the contact surface. In the present study, synchrotron radiation computed laminography (SRCL) imaging was performed to observe flaking defects during the RCF of a high-strength steel. Specially fabricated inclusion-rich steel plate specimens were employed in the experiments. For the in situ observation of crack propagation, a compact RCF testing machine was developed, and a 4D analysis scheme was applied to the data obtained by SRCL. RCF tests were carried out near the measurement hatch of the beam line used SRCL to enable the successive observation of crack initiation and growth behaviors. Specimens before and after the occurrence of flaking were observed by SRCL, and flaking defects and cracks under the surface were successfully detected. As a result, details of the crack initiation and flaking process in RCF could be discussed. Shear-type horizontal cracks were found to initiate after the initiation and propagation of tensile-type vertical cracks along inclusions, where the face of the vertical cracks was perpendicular to the rolling direction and rolling surface. Therefore, the formation of vertical cracks is considered to affect shear-type crack formation and flaking, where the shape and length of inclusions also affect the initiation and propagation of vertical cracks. K EYWORDS . Rolling contact fatigue; Laminography; Ultra-bright synchrotron radiation; 3D imaging. I NTRODUCTION n rolling contact fatigue (RCF), cracks usually initiate from inclusions beneath the surface, and they propagate to form flakes [1, 2]. Although nonmetallic inclusions are known to have a detrimental effect on the fatigue performance of high-strength steels, the commercial production of steels with very high cleanliness is unrealistic because of the high cost. Thus, it may be possible to control the concentration and size of inclusions to obtain steels with better performance. In particulars, in bearing steels, inclusions have complex shapes and are often lined up, thus forming so-called stringers, and the effects of the shape and distribution of inclusions on RCF should be taken into account. Since phenomena occurring under the surface cannot be observed using conventional microscopes, such as optical and scanning electron microscopes, and it is difficult to observe the fracture surface of flakes because the flaking area is damaged by the rolling steel ball after its emergence, the effect of the configuration of inclusions has not yet been systematically investigated. I