PSI - Issue 7

7

Taizo Makino et al. / Procedia Structural Integrity 7 (2017) 468–475 Taizo Makino et Al./ Structural Integrity Procedia 00 (2017) 000–000

474

2000

- 2500 -2000 - 1500 - 1000 - 500 0 500 1000 1500 2000 2500 Shear stress τ yz [MPa]

z=100μm ， x=7.5μm

0

x y z

z=0μm ， x=7.5μm

x y z

2 a

2 a

-2000

Defect

Y

Defect

- 8000 Stress in y direction σy [MPa] -6000 -4000

Y

σ y

x

Circular hole defect Soft inclusion (Bonded interface) W/O defect Soft inclusion (Separated interface)

y

Circular hole defect Soft inclusion (Bonded interface) W/O defect Soft inclusion (Separated interface)

τ yz

x

y

z

z

Middle line at x=7.5μm

Middle line at x=7.5μm

- 10000

-3

-2

- 1

0

1

2

3

-3

-2

- 1

0

1

2

3

Y/a

Y/a

(a) Changes in σ y at surface edge of defect. (b) Changes in τ yz at 0.1 mm depth along defect. Fig.5. Comparison of stress variation between circular hole defect, soft inclusion and W/O defect during rolling contact obtained from elastic FE analyses.

4. Conclusion We applied imaging technology to observation of RCF damage and attempted to observe the damage process directly to clarify the mechanism of RCF originating from stringer inclusions, i.e., sulphide inclusions. SRCL was adopted as the imaging method for successive observation of RCF damage. The RCF damage process was clarified as follows: vertical cracks are initiated at defects, horizontal cracks are initiated during vertical crack propagation, horizontal crack propagation leads to flaking failure. Both artificial defects and sulphide inclusions exhibited the same process, but different flaking lives. Therefore, FEA was performed by means of modelling a hole defect and a soft inclusion subjected to rolling contact. The FEA results revealed the difference of stress values between defects and inclusions, and the increase of stress values caused by separation of the interface of inclusion. The reason for the difference of flaking life between defects and inclusions was explained qualitatively by the above results. Acknowledgements The synchrotron radiation experiments were performed at BL46XU in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) under proposal numbers 2014A1562, 2014A1770, 2014B1602, and 2014B1890. Chen, Q., Shao, E., Zhao, D., Guo, J., Fan, Z., 1991. Measurement of the critical size of inclusions initiating contact fatigue cracks and its application in bearing steel. Wear 147, 285–294. Lewis, M.W.J., B.Tomkins, B., 2012. A fracture mechanics interpretation of rolling bearing fatigue. Proc. I. Mec. E., Part J. J. Eng. Tribol. 226, 389–405. Makino, T., Neishi, Y., Shiozawa, D., Fukuda, Y., Kajiwara, K., Nakai, Y., 2014, Evaluation of rolling contact fatigue crack path in high strength steel with artificial defects. Int J Fatigue 68,168–177. Makino, T., Neishi, Y., Shiozawa, D., Kikuchi, S., Okada, S., Kajiwara, K., Nakai, Y., 2016, Effect of defect shape on rolling contact fatigue crack initiation and propagation in high strength steel. Int J Fatigue 92,507–516. Moffat, A.J., Wright, P., Helfen, L., Baumbach, T., Johnson, G., Spearing, S.M., Sinclair, I., 2010, In situ synchrotron computed laminography of damage in carbon fibre–epoxy [90/0]s laminates, Scripta Materialia 62, 97–100. Nagao M., Hiraoka K., Unigame Y., 2005. Influence of nonmetallic inclusion size on rolling contact fatigue life in bearing steel. Sanyo Tech Report 12-1, 38–45[in Japanese]. References