Skip to main content

Investigation of Microstructural Alterations in Low- and High-Speed Intermediate-Stage Wind Turbine Gearbox Bearings


Wind turbine drive train system failures have been the major source of down time and costly repairs. One of the main failure modes associated with gearbox failures is the formation of white etching cracks (WECs) in bearings. In this paper, low-speed intermediate shaft (LSIS) and high-speed intermediate shaft (HSIS) bearings are collected from failed gearboxes to examine microstructural alterations and evaluate damage. Butterfly wing formations are detected in LSIS bearings around nonmetallic inclusions at depths of up to about 260 µm from the contact surface and the measured span length ranging from 5 to 70 µm. Circumferential cracks oriented parallel to the contact surface are detected at depths greater than 300 μm. Irregular white etched cracks are revealed in HSIS bearings with random crack orientations. The crack depth is measured to be 7000 μm in the cross section perpendicular to the bearing axis. The bearing characterization revealed that microstructural alterations can occur with and without the formation of cracks. Evidence found supports the existing hypotheses that (1) white etch areas precedes crack formations and (2)  cracks are prerequistes for white etch formation. The microstructural alterations detected in LSIS and HSIS bearings are observed to have different crack morphology and could be initiated by different mechanisms. The results presented in this paper can be used to develop materials-based prognostic life prediction models. This can be achieved by linking WECs and butterfly wings qualitative observations to turbine real operating conditions, bearing design, and manufacturing process.

This is a preview of subscription content, access via your institution.

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
Fig. 10
Fig. 11
Fig. 12
Fig. 13


  1. Kotzalas, M.N., Doll, G.L.: Tribological advancements for reliable wind turbine performance. Philos. Trans. A. Math. Phys. Eng. Sci. 368, 4829–4850 (2010)

    Article  Google Scholar 

  2. Evans, M.-H.: White structure flaking (WSF) in wind turbine gearbox bearings: effects of “butterflies” and white etching cracks (WECs). Mater. Sci. Technol. 28, 22–23 (2012)

    Google Scholar 

  3. Sheng, S.: Gearbox reliability database: yesterday, today, and tomorrow (2014)

  4. Errichello, R., Budny, R., Eckert, R.: Investigations of bearing failures associated with white etching areas (WEAs) in wind turbine gearboxes. Tribol. Trans. 56, 1069–1076 (2013)

    Article  Google Scholar 

  5. Greco, A., Sheng, S., Keller, J., Erdemir, A.: Material wear and fatigue in wind turbine Systems. Wear 302, 1583–1591 (2013)

    Article  Google Scholar 

  6. Ruellan, A., Ville, F., Kleber, X., Liatard, B.: Tribological analysis of white etching crack (WEC) failures in rolling element bearings. NREL Wind Turbine Tribol. Semin. 2014 (2014)

  7. Stadler, K., Lai, J., Vegter, R.H.: A review: the dilemma with premature white etching crack (WEC) bearing failures. In: Bearing Steel Technologies: 10th Volume, Advances in Steel Technologies for Rolling Bearings, pp. 1–22. ASTM International (2015)

  8. Kruhöffer, W., Loos, J.: WEC formation in rolling bearings under mixed friction: influences and “friction energy accumulation” as indicator. Tribol. Trans. (2016). doi:10.1080/10402004.2016.1183250

  9. Lai, J., Stadler, K.: Investigation on the mechanisms of white etching crack (WEC) formation in rolling contact fatigue and identification of a root cause for bearing premature failure. Wear 364–365, 244–256 (2016)

    Article  Google Scholar 

  10. Loos, J., Bergmann, I., Goss, M.: Influence of currents from electrostatic charges on WEC formation in rolling bearings. Tribol. Trans. 2004, 865–875 (2015)

    Google Scholar 

  11. Moghaddam, S.M., Sadeghi, F.: A review of microstructural alterations around non-metallic inclusions in bearing steel during rolling contact fatigue. Tribol. Trans. 2004, 1–61 (2016)

    Google Scholar 

  12. Becker, P.C.: Microstructural changes around non-metallic inclusions caused by rolling-contact fatigue of ball-bearing steels. Metals Technol. 8, 234–243 (1981)

    Article  Google Scholar 

  13. Evans, M.H., Richardson, A.D., Wang, L., Wood, R.J.K.: Serial sectioning investigation of butterfly and white etching crack (WEC) formation in wind turbine gearbox bearings. Wear 1, 1573–1582 (2013)

    Article  Google Scholar 

  14. Grabulov, A.: Fundamentals of rolling contact fatigue (Doctoral dissertation, TU Delft, Delft University of Technology) (2010)

  15. Mobasher, S.M., Sadeghi, F., Weinzapfel, N., Liebel, A.: A Damage mechanics approach to simulate butterfly wing formation around non-metallic inclusions. ASME J. Tribol. 137, 1–13 (2014)

    Google Scholar 

  16. Voskamp, A.P.: Microstructural changes during rolling contact fatigue (1997)

  17. Bhadeshia, H.K.D.H.: Steels for bearings. Prog. Mater Sci. 57(2), 268–435 (2012)

    Article  Google Scholar 

  18. Evans, M.-H., Wang, L., Jones, H., Wood, R.J.K.: White etching crack (WEC) investigation by serial sectioning, focused ion beam and 3-D crack modelling. Tribol. Int. 65, 140–160 (2013)

    Article  Google Scholar 

  19. Hiraoka, K., Nagao, M., Isomoto, T.: Study on flaking process in bearings by white etching area generation. J. ASTM Int. 3, 1–7 (2006)

    Article  Google Scholar 

  20. Solano-Alvarez, W., Bhadeshia, H.K.D.H.: White-etching matter in bearing steel. Part II: distinguishing cause and effect in bearing steel failure. Metall. Mater. Trans. A. 45, 4916 (2014)

    Article  Google Scholar 

  21. Gould, B., Greco, A.: The influence of sliding and contact severity on the generation of white etching cracks. Tribol. Lett. 60, 29 (2015)

    Article  Google Scholar 

  22. Gould, B., Greco, A.: Investigating the Process of White Etching Crack Initiation in Bearing Steel. Tribol. Lett. 62, 26 (2016)

    Article  Google Scholar 

  23. Diederichs, A.M., Schwedt, A., Mayer, J., Dreifert, T.: Electron microscopy analysis of structural changes within white etching areas electron microscopy analysis of structural changes within white etching areas. Mater. Sci., Technol (2016)

    Google Scholar 

  24. Kang, J., Vegter, R.H., Rivera-Díaz-del-Castillo, P.E.J.: Rolling contact fatigue in martensitic 100Cr6: subsurface hardening and crack formation. Mater. Sci. Eng. A 607, 328–333 (2014)

    Article  Google Scholar 

  25. Holappa, L.E.K., Helle, A.S.: Inclusion control in high-performance steels. J. Mater. Process. Technol. 53, 177–186 (1995)

    Article  Google Scholar 

  26. Bruce, T., Rounding, E., Long, H., Dwyer-Joyce, R.S.: Characterisation of white etching crack damage in wind turbine gearbox bearings. Wear 338–339, 164–177 (2015)

    Article  Google Scholar 

  27. International Organization for Standardization, ISO/IEC 61400-4, Part 4 Des. Requir. Wind Turbine Gearboxes (2012)

  28. Chen, Q., Shao, E., Zhao, D., Guo, J., Fan, Z.: Measurement of the critical size of inclusions initiating contact fatigue cracks and its application in bearing steel. Wear 147, 285–294 (1991)

    Article  Google Scholar 

  29. J. Gegner, Tribological aspects of rolling bearing failures

  30. Ryttberg, K., Wedel, M.K., Recina, V., Dahlman, P., Nyborg, L.: The effect of cold ring rolling on the evolution of microstructure and texture in 100Cr6 steel. Mater. Sci. Eng. A 527(9), 2431–2436 (2010)

    Article  Google Scholar 

  31. Gould, B., Greco, A., Stadler, K., Xiao, X.: An analysis of premature cracking associated with microstructural alterations in an AISI 52100 failed wind turbine bearing using X-ray tomography. Mater. Des. 117, 417 (2016)

    Article  Google Scholar 

  32. Lund, T.B.: Sub-surface initiated rolling contact fatigue—influence of non-metallic inclusions, processing history, and operating conditions. J. ASTM Int. 7(5), 1–12 (2010)

    Article  Google Scholar 

  33. Evans, M.H., Richardson, A.D., Wang, L., Wood, R.J.K., Anderson, W.B.: Confirming subsurface initiation at non-metallic inclusions as one mechanism for white etching crack (WEC) formation. Tribol. Int. 75, 87–97 (2014)

    Article  Google Scholar 

  34. Solano-Alvarez, W., Duff, J., Smith, M.C., Bhadeshia, H.K.D.H.: Elucidating white-etching matter through high-strain rate tensile testing. Mater. Sci. Technol. 33, 307 (2016)

    Article  Google Scholar 

  35. Kohara, M., Kawamura, T., Egami, M.: Study on mechanism of hydrogen generation from lubricants. Tribol. Trans. 49, 53–60 (2006)

    Article  Google Scholar 

  36. A.D. Richardson, L. Wang, R.J.K. Wood, M. Evans, M. Ingram, A combined TDA and metallographic study to investigate hydrogens role in white etching crack (WEC) formation 1–4 (2016)

Download references

Author information

Authors and Affiliations


Corresponding author

Correspondence to Harpal Singh.

Additional information

This article is part of the Topical Collection on STLE Tribology Frontiers Conference 2016.

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Singh, H., Pulikollu, R.V., Hawkins, W. et al. Investigation of Microstructural Alterations in Low- and High-Speed Intermediate-Stage Wind Turbine Gearbox Bearings. Tribol Lett 65, 81 (2017).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: