Issue34

T. Makino et alii, Frattura ed Integrità Strutturale, 34 (2015) 334-340; DOI: 10.3221/IGF-ESIS.34.36 335 been applied to increase flaking life. Recently it was indicated that stringer-type inclusions can be the origin of RCF cracks and flaking[3], and the inclusion length influenced the RCF life[4]. In the present paper, to discuss the mechanism of the above phenomena, RCF test was conducted using specimens with artificial defects that simulate stringer-type inclusions. The effect of defect length on flaking life was evaluated using specimens with different-length defects. Synchrotron radiation micro computed tomography (SR micro CT) imaging and microscope observation were performed for RCF cracks and their fracture surface. The morphology of RCF cracks before flaking was investigated. The experimental result was discussed in terms of stress states and fracture mechanics based on the analytical result by finite element (FE) analysis. M ATERIALS , SPECIMENS , AND TEST METHODS Materials and specimens all-on-disc type RCF tests were conducted under oil lubrication. Disc specimens were made of two steels: high carbon–chromium bearing steel JIS SUJ2 and induction-hardened 0.55% carbon steel JIS S55C. Both steels were received as rolled bars. SUJ2 bar was spheroidizing-annealed, and then quenched and tempered. S55C bar was normalized and was then machined to disc shape. The ring-shaped region of the ball rolling track surface was induction- hardened and tempered. The disc specimen was 60 mm in diameter and 5 mm in thickness. The diameter of ball rolling track was 38.5 mm, the region in the vicinity of the track had a Vicker’s hardness of 700–750 HV in both specimens of SUJ2 and S55C. Artificial defects of 15 or 50  m diameter and 30–300  m length from surface in depth direction were introduced on the ball rolling track of the disc specimen by electro-discharge machining. The above length of defect is referred to as defect length in this paper. Ball specimens are commercial products made of SUJ2. The ball diameter was 9.525 mm. Test methods Three balls were rolled on one side of a disc specimen surface under oil lubrication. A vertical contact force was set so that the Hertzian stress p max was 5.22 GPa. The half-width of the contact patch a was then calculated as 0.346 mm. Tests were continued until a preset number of cycles or automatically stopped by detecting the vibration due to flaking. R OLLING C ONTACT F ATIGUE T EST R ESULT ig. 1 presents the relationship between defect length and flaking life obtained from RCF test of SUJ2 disc specimen. 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. 10 15 20 25 30 35 40 45 50 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 Surface crack length 2 c [μm] Number of cycles N [cycle] JISSUJ2, 15  mdia. defect L=  200  m L= 50  m L= 100  m Figure 1 : Relationship between defect length and flaking life obtained from RCF test of SUJ2. Figure 2 : Changes in surface length of vertical crack including defect during RCF test. B F