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Influence of surface roughness on rolling contact fatigue considering thermal elastohydrodynamic lubrication effects

Due to Corona it is not possible to attend this defense in person, instead attend via this link:

Time: Tue 2020-03-24 10.00

Location: Live streaming, Stockholm (Swedish)

Subject area: Solid Mechanics

Doctoral student: Carl-Magnus Everitt , Hållfasthetslära (Avd.), KTH - Royal institute of technology

Opponent: Professor Andreas Almqvist, Luleå Tekniska Universitet

Supervisor: Bo Alfredsson, Hållfasthetslära (Avd.)

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Rolling contacts are utilized in many technical applications, both in bearings and in the contacts between gear teeth. These components are subjected to high loads in many of their applications. The high loads causes very high contact pressures which may cause rolling contact fatigue, often denoted as pitting or spalling. This work focuses on the rolling contact fatigue mechanism of surface initiated pitting. Detailed simulations and experiments of rolling contacts have been performed in order to attain a better understanding of why pitting initiates and grows. The contact between two gears in a truck retarder was used as a case study. The investigated contact experienced elastohydrodynamic lubrication, EHL, conditions at which multiple pitting damages arose.

Results from numerical investigations are presented in the papers A-C. In Paper A a numerical analysis of different measured surface topographies is presented showing that the sliding direction explains why more damage initiated down in the dedendum than up in the addendum. Detailed results of single artificial defects are presented in Paper B in order to explain the difference seen in Paper A. The results show that the defects subjected to negative slip break through the lubricant more and were thus subjected to higher friction. The analysis was extended to the conditions of the pitch line in Paper C and included variations of pressure, defect sizes, choice of lubricant and operational temperatures. The results presented in Paper C shows that the underlying theories of the asperity point load mechanism also predicts damage initiation at pure rolling, which agrees with the experiments on the truck retarder.

A new imprint method for the manufacturing of well-defined micrometre high asperities in the surface of discs is presented in Paper D. The method was developed in order to enable experimental investigations of rolling contact fatigue. The manufactured asperities showed the potential to survive more than 35 million load cycles when tested in a twin-disc machine.

An experimental and numerical investigation of micrometre high artificial asperities created with the imprint method is presented in Paper E. The experimental results showed that micro-pits developed on the leading edge of the highest asperities. The comparison with numerical simulations showed that the plastic deformations occurring during run-in there caused high tensile residual stresses in this region. Rolling contact fatigue, RCF, cracks initiated behind the trailing edge of the indents in the experiments. The simulations of the continued EHL testing, showed that these loads cased high tensile stresses in this region. The conclusion was hence drawn that the micro pits were caused by the residual stresses while the EHL loads caused the initiations of the RCF cracks.