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The bearings used in various industrial machines need to have good rolling contact fatigue resistance. It is known that the fatigue resistance in clean lubricating oil is decreased when there is a nonmetallic inclusion in the bearing steel. For this reason, various studies to reduce the size and quantity of inclusions have been carried out. On the other hand, to obtain a new approach to suppress the influence of the nonmetallic inclusions, it is important to understand the influence of each nonmetallic inclusion type on the rolling contact fatigue resistance. Therefore, we have worked on evaluations of the fracture initiation and propagation on the rolling contact fatigue in bearing steels. In the previous symposium, we reported that we had created an oxide type inclusion controlled steel (OTICS) using the melting furnace in our laboratory. This steel has an excellent rolling contact fatigue life. In this paper, we will report fracture initiation and propagation on the rolling contact fatigue in OTICS and conventional steel evaluated using an ultrasonic test and an acoustic emission test. We have investigated the defects under the rolling contact surface by the ultrasonic test, with loadings applied at certain times in the thrust-type rolling contact fatigue test. During the test, we also measured the acoustic emissions generated when a fracture occurred. From the ultrasonic test, we succeeded in detecting nonmetallic inclusions with the fractures. Furthermore, the time before defects were detected in the OTICS was longer compared to conventional steel. The same tendency was observed in the acoustic emission test. Longer load times were required before acoustic emissions were detected in OTICS. According to these results, it can be considered that OTICS has a greater rolling contact fatigue life than conventional steel due to the inhibition of the fracture initiation from the nonmetallic inclusion.
The clarification of uncertain factors of rolling contact fatigue (RCF) life variation is expected to lead us to better understanding of the RCF mechanism and further improvement of the service life of bearings. The objective of this study is to clarify the effect of defect location on RCF life. Artificial cavities, pores, and drilled holes were introduced to the specimens as a flaking origin under RCF for simplification on the presumption that their physical properties and interfacial rubbing between cavity and matrix were ignorable. The RCF test resulted in flakings initiated from the pore located right below the center of the track, when a specimen included numerous pores. Their RCF lives were simply determined by fracture mechanical parameters, size of pore, and orthogonal shear stress range parallel to rolling direction. On the other hand, RCF life was prolonged when the drilled hole in a specimen was located near the contact edge, although the resultant flakings appear the same irrespective of defect location. Therefore, defect location is one of the important factors for RCF life variation. The following were found through a further verification experiment and finite-element analysis: (1) A crack initiates from a drilled hole surface because of principal tensile stress at early stages of RCF irrespective of location of the hole. (2) Both of the orthogonal shear stresses, parallel and perpendicular to rolling direction, lead to the three-dimensional propagation of crack. (3) The weakening/damaging effect from a hole near the contact edge is less than that from a hole near the center of the contact track.
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Recently, it has been generally accepted that flaking life is dependent on the size of non-metallic inclusion under proper condition in bearing use. Statistics of extreme values to predict the maximum non-metallic inclusion size and ultrasonic test to assess large non-metallic inclusions in a large volume are widely used as practical methods for the evaluation of bearing steel cleanliness. Murakami's formula is well known, which describes the relationship between non-metallic inclusion size and fatigue strength. However, the formula cannot be directly applied to rolling contact fatigue because of the difference in applied stress mode. While the equation was invented to predict the fatigue limit in principal stress mode, rolling contact fatigue is mainly caused by shear stress. It is expected that the condition of bearing use will be more critical due to the downsizing tendency in various industrial or automotive machine units. Thus the research on the flaking mechanism becomes more important from the viewpoint of global ecology because that is beneficial to the improvement in bearing life and methods for cleanliness evaluation. Due to the difficulty in experimental observation, however, the details of the flaking mechanism in rolling contact fatigue caused by non-metallic inclusion has not yet been clarified. Focusing on the relationship between non-metallic inclusion and initiation of the crack, the flaking mechanism is proposed in this paper.