Bearings are employed in a number of applications under extremely demanding conditions. During long operation times, the material undergoes rolling contact fatigue where microstructural damage manifests as dark-etching regions and white-etching areas, which display different properties from the surrounding region. The aim of this study is to identify the mechanisms for such damage and to suggest models that can explain the influence of the initial microstructure and test conditions. In order to appraise the stress state in rolling contacts, two testing techniques were employed and it was examined if the testing methods could reproduce the same damage as in bearing operation. During ball-on-rod fatigue testing, microcracks were generated adjacent to inclusions and some were decorated with white-etching areas. Repetitive push tests showed a similar extent of subsurface hardening compared to the ball-on-rod tests, and allowed the strain per stress cycle to be measured. The microstructural alterations in a white-etching area were studied both on a macroscale and on an atomic-scale. The degree of stress concentration near a microcrack was calculated employing a nite element method. The microstructure, as well as the segregation behaviour of alloying elements in the white-etching area, were investigated by employing transmission electron microscopy and atom probe tomography. A nanocrystalline structure with scattered carbide particles was observed in the white-etching area. Carbon and silicon segregation was highly pronounced in some boundaries of dislocation cell structures. Models were suggested to account for the microstructural alterations during rolling contact fatigue. Carbide coarsening in dark-etching regions was modelled by considering how carbon di usion is assisted by dislocation glide. The predicted hardness evolution was consistent with experimental observation. The kinetics of carbide dissolution in white-etching areas was calculated by taking two processes into account: deformation accumulation and carbon diffusion. These models suggest that the microstructural changes during bearing operation can be controlled by tailoring the initial microstructure and managing the test conditions.

This thesis deals with the study of sudden step changes observed in model testing of standard bearing steel 100Cr6. Methodology of experiments with step change of load is proposed and applied in measurements. Three complete series of experiments were conducted, and rating life was determined for each loading trend. Based on the experiments carried out, it was found that the selected steel is able to withstand the sudden step of load changes, despite the fact that it is not material of choise for this type of load. Part of this work was to verify functionality of the innovated test stations AXMAT with modern diagnostics and a new control system, and also of the hydraulic variant of AXMAT, which allows to remotely derive dynamic loading of the test sample.

Les procédés de fabrication de pièces mécaniques par enlèvement de matière (tournage, fraisage, perçage, ...) connaissent une utilisation massive dans l’industrie aéronautique et l’automobile. Les pièces obtenues par ces procédés doivent satisfaire à des propriétés géométriques, métallurgiques et à des caractéristiques de qualité. Pour répondre à ces exigences, plusieurs essais expérimentaux basés sur le choix des conditions de coupe sont souvent nécessaires avant d’aboutir à une pièce satisfaisante. Actuellement, ces méthodes empiriques basées sur l’expérience des fabricants et des utilisateurs des outils coupants sont souvent très longues et coûteuses, donnent une large plage de choix des paramètres en fonction de leurs besoins. Toutefois, le coût très élevé d’un essai limite fondamentalement le nombre d’expériences, avoir une pièce respectant les caractéristiques souhaitées avec un coût acceptable devient une tâche difficile. / Manufacturing processes of mechanical parts by removal of material (turning, milling, drilling ...) have extensive use in aeronautic and automobile industry. The components obtained using these methods must satisfy geometric properties, metallurgical and quality characteristics. To meet these requirements, several experimental tests based on the selection of cutting conditions are often necessary before manufacturing. Currently, these empirical methods based on the experience of manufacturers and users of cutting tools (charts, diagrams with experimental findings, ...) are often very lengthy and costly. However, the high cost of a trial limits the number of experiments, so to have a deserted component with an acceptable cost is a difficult task. The importance of cutting conditions monitored by limitations is related to the type of material to be machined, since it determines the behavior of the machining.