Issue34

Y. Nakai et alii, Frattura ed Integrità Strutturale, 34 (2014) 246-254; DOI: 10.3221/IGF-ESIS.34.26 247 To discuss the mechanism of RCF crack initiation under the contact surface, Grabulov et al. [3] investigated crack initiation around inclusions by a dual-beam (scanning electron microscopy (SEM)/focused ion beam (FIB)) technique. Since this method is destructive, the crack propagation behavior is difficult to observe, and synchrotron radiation micro computed tomography imaging (SRCT) has been applied for nondestructive observation [4]. Stiénon et al. [5],[6] calculated the stress field around nonmetallic inclusions in bearing steels in RCF tests using 3D shapes obtained by SRCT, which was conducted at the European Synchrotron Radiation Facility (ESRF). They used SRCT imaging for the observation of samples with flaking damage and RCF cracks [7], [8]. In these studies, samples were cut from normal size RCF specimens so that they included damaged areas, and the 3D imaging of damage before flaking provided useful information about the RCF crack initiation and propagation processes. To investigate the effect of the shape of inclusions on crack initiation, artificial defects that simulate stringer-shaped inclusions were introduced in the specimens, and the crack initiation and propagation from the artificial defects were observed. For successive SRCT imaging of the RCF process, samples must be sufficiently small to allow the transmission of X-rays, and the crosssection must be smaller than 500 μm × 500 μm. Our previous study, however, showed that the mechanism of RCF in a small sample is different from that in a bulk sample [9]. In the present study, SRCL is applied, which allows the high-resolution, nondestructive imaging of thin plates. This method provides a novel means of observing the 3D shapes of microstructures and cracks in a thin plate. A compact RCF testing machine, which enables simultaneous RCF tests and SRCL observation, was developed, and the crack initiation and propagation behaviors are discussed. Figure 1 : Rolling contact fatigue testing machine for in-situ observation. M ATERIAL AND EXPERIMENTAL PROCEDURE Material and specimen he material used in the present study was a bearing steel (modified JIS SUJ2), whose chemical composition (in mass%) was 1.00C, 0.35Si, 0.47Mn, 0.006P, 0.017-0.049S, 1.50Cr, and balance Fe. The material has intentionally contains a high concentration of sulfur to enable the observation of crack initiation from MnS inclusions. The material was forged from an ingot with 65 mm diameter, and its inclusions were intergranular with a preferential alignment along the forging direction. After spheroidizing annealing, specimens were cut from the forged bar, where the transverse crosssection of the bar corresponded to the contact surface of the specimen. The specimen was quenched at 1103 K for 0.5 h and tempered at 453 K for 2 h. The specimens used for SRCL imaging were 10 mm in width, 24 mm in length and 1 mm in thickness. The thickness of the specimens was determined to allow the transmission of X-rays with sufficient intensity for SRCL imaging. Rolling contact fatigue test The developed testing machine, which is shown in Fig. 1, is a ball-on-disk-type contact tester. In this testing machine, reciprocal sliding motion is generated by a linear guide and an eccentric cam, and then a steel ball rolls on the specimen linearly and reciprocally, unlike in a Mori-type rolling contact fatigue testing machine in which rolling is in a single direction. Ceramic balls with 6.0 mm diameter and a Young's modulus of 300 GPa were employed as contact balls, where the slide distance of the balls was 3.0 mm. Sample can be easily attached and removed from the developed machine. Fatigue tests were interrupted to conduct SRCL imaging to observe the crack initiation and propagation behaviors. T