Inclusion orientation dependent flaking process in rolling contact fatigue observed by laminography using ultrabright synchrotron radiation X‐ray

Abstract

The formation and propagation of cracks in rolling contact fatigue were observed by synchrotron radiation computed laminography, and the effect of stringer-type inclusion orientation was examined. For longitudinal inclusions, cracks started forming at their tips. After cracks propagated toward the rolling direction, a longitudinal crack was kinked simultaneously at both its tips, and propagated toward the direction perpendicular to the rolling direction to form lateral cracks. After kinking, horizontal cracks were formed from the deepest point of a lateral crack, leading to flaking. On the other hand, for specimens with lateral inclusions, cracks propagated to the lateral direction without the formation of longitudinal cracks. Since the propagation life of lateral cracks and that of horizontal cracks were unrelated to the inclusion orientation, the rolling contact fatigue life of specimens with longitudinal inclusions was considerably longer than that of specimens with lateral inclusions.

Observation of flaking process in rolling contact fatigue of bearing steel with spheroidal inclusions by laminography using ultrabright synchrotron radiation X‐rays

“…The material used was bearing steel (modified JIS SUJ2) similar to that used in our previous study, 61,62 and the F I G U R E 1 Orientation of inclusions in specimens. (A) The major axis of inclusion is perpendicular to the surface and perpendicular to the rolling direction, (B) the major axis of inclusion is parallel to the surface and perpendicular to the rolling direction, and (C) the major axis of inclusion is parallel to the surface and parallel to the rolling direction.…”

“…A special ball-on-disk RCF testing machine was developed as schematically illustrated in Figure 3, in which an eccentric cam with eccentric distance of 1.5 mm converts the rotary movement of the motor of 1000 rpm to the reciprocal movement of the linear stage of 3.0 mm with frequency of 33.3 Hz, the vibroconverter detects flaking of the specimen, and the proximity sensor detects the edge of the linear stage to count the number of cycles. RCF tests were performed near the experimental hatch of the BL46XU beamline of the synchrotron radiation facility of Super Photon ring 8 GeV (SPring-8), 61,62 which uses an undulator beam and provides an X-ray beam with ultrabright high spatial coherence, enabling laminography with high spatial resolution in the micrometer range. 61,62 The RCF tests were conducted at the maximum Hertzian stress p max of 5.39 GPa.…”

“…RCF tests were performed near the experimental hatch of the BL46XU beamline of the synchrotron radiation facility of Super Photon ring 8 GeV (SPring-8), 61,62 which uses an undulator beam and provides an X-ray beam with ultrabright high spatial coherence, enabling laminography with high spatial resolution in the micrometer range. 61,62 The RCF tests were conducted at the maximum Hertzian stress p max of 5.39 GPa. According to Hertzian theory, under the present experimental conditions, the depth at which the shear stress is maximum in a plane parallel to the contact surface is 67 μm.…”

“…We also examined the effect of stringer-type inclusion orientation relative to the rolling direction, as shown in Figure 1, 62 and found that the flaking formation process for stringer-shaped inclusions, which differs depending on the orientation of the inclusion relative to the rolling direction as shown in Figure 2. 41,62 In the case of specimens with inclusions whose longitudinal direction is perpendicular to the rolling direction (lateral inclusions), as shown in Figure 2A and B, cracks perpendicular to the rolling direction formed on the surface, which propagated in the internal direction, and cracks parallel to the specimen surface started to form near its deepest point, resulting in flaking.…”

“…We also examined the effect of stringer-type inclusion orientation relative to the rolling direction, as shown in Figure 1, 62 and found that the flaking formation process for stringer-shaped inclusions, which differs depending on the orientation of the inclusion relative to the rolling direction as shown in Figure 2. 41,62 In the case of specimens with inclusions whose longitudinal direction is perpendicular to the rolling direction (lateral inclusions), as shown in Figure 2A and B, cracks perpendicular to the rolling direction formed on the surface, which propagated in the internal direction, and cracks parallel to the specimen surface started to form near its deepest point, resulting in flaking. In the case of a specimen containing a longitudinal inclusion whose longitudinal direction is parallel to the rolling direction (longitudinal inclusion), as shown in Figure 2C, the crack started at the long end of the inclusion, propagated a certain distance in the rolling direction, and then kinked in the direction perpendicular to the rolling direction.…”