Fundamentals of Rolling Contact Fatigue

Abstract

In the mechanical industry there is a need for continuous development towards increasing performance of various types of machinery. Critical components in such machines are exposed to gradually harsher operating environments involving higher cyclic stresses and operating temperatures. Examples of such fatigue-exposed components are gears, camshafts and rolling element bearings in engines and gearboxes. The components in the bearing industry are always subject to the high cycle rolling contact fatigue, which ultimately may lead to the failure. In order to better interpret component failures, to accurately predict fatigue life under harsh operating conditions and to further develop the performance of high-strength steel products, a detailed understanding of the fatigue damage and crack initiation mechanisms (especially around non-metallic inclusions) is required. This thesis offers a significant step forward in the understanding of the role of fatigue damage between non-metallic inclusions and the hardened steel matrix during high-cycle rolling contact fatigue. Rolling contact fatigue has been studied by many researchers in the past. There is a significant knowledge present on this subject and particularly on the microstructural changes and crack initiations around non-metallic inclusions under rolling contact fatigue. There were several attempts to describe the crack initiation and growth under these fatigue conditions, but all models presented to date are based on different oversimplifying assumptions which are in several cases in a direct conflict with experimental observations. In conclusion, it is generally acknowledged that the current understanding of the crack initiation mechanism is essentially empirical and provides little guidance for fatigue researchers, steel and bearing developers and model builders. This thesis is focused on the aspects missing in the current knowledge and provides a comprehensive study on the rolling contact fatigue. A combination of state-of-the-art microscopy techniques was used to reveal the micro and nano scale fundamental studies of the microstructural changes and subsurface crack initiation. This work presents a detailed explanation of the different stages of butterfly formation and growth, including the very early stages of the damage process, at the inclusion/matrix interface using a metal physics perspective. It has been shown that early butterfly formation is the result of a rubbing interaction between the debonded oxide inclusion and the steel matrix. This interaction is the result of shear damage and repeated diffusion bonding/debonding during the stress cycle. Due to continued fatigue exposure, the butterfly crack migrates sidewise from the inclusion/matrix interface out into the steel matrix via gradual material transfer across the early formed butterfly wing and the undamaged matrix. This butterfly initiation stage constitutes the overwhelming portion of the total fatigue life. Only in the very late stage of the fatigue life, few butterflies can reach such a size that a further growth can be described using linear elastic fracture mechanics principles. These findings expand the boundaries of the current knowledge and add new understanding of the microstructural processes occurring during rolling contact fatigue. Quantitative modeling was not the primary focus in the thesis, and the findings are therefore essentially described in the qualitative manner. However these findings must be used by modelers as a basis to develop more quantitative description of the butterfly crack formation and other damage processes based on the mechanisms presented here. This will allow the development of models with higher predictive power.