Quantifying the surface roughness evolution in contacts is a crucial step in the fatigue prediction process. Surfaces are initially conditioned by the running-in process and later altered by surface fatigue. The aim of this study is to understand and predict the evolution of the micro-geometry in the first few overrolling cycles. Numerical predictions are validated by experiments. A major difficulty in understanding surface degradation is the measurement of the surface roughness evolution at the relevant scales. A twin disc micro-test rig, called μMag, was specially designed for this kind of analysis. The μMag allows the "in situ" observation of changes in the disc surface during interrupted tests, thus avoiding dismounting the specimens, which is a major cause of inaccuracy. The new method also maintains the relative position of the two discs. The precision of the measurements allows one to use the initial surface microgeometry as input for the numerical contact calculation. Thus, the plastic deformation of the surfaces can be measured during the first cycles and compared to the numerical prediction. Results show a very good agreement between numerical predictions and experimental measurements.

Lubricant starvation in rolling element bearings can have different origins, such as high speed, large size or grease lubrication. However, the overall result is a performance reduction, involving a friction increase, a bearing life decrease or both. As such it is desirable to be able to predict when starvation will occur for a certain combination of geometry, operating conditions and lubricant. Previous work used optical interferometry to predict the onset of starvation and the ensuing oil film thickness reduction. A starvation parameter was derived linking lubrication parameters (viscosity), amount of oil and operating conditions (load, speed) to predict the onset of starvation. The current paper attempts to predict the onset of starvation through friction measurements on a ball-on-disc test device. The starvation parameter values obtained from friction measurements are similar to the ones found through optical interferometry.

Rolling contacts are a major topic in scientific studies as they are highly exposed to fatigue damage. Indeed, the applied loads generate periodic stresses and the surface roughness generates stress concentrations close to the surface. The combination of these two phenomena facilitates fatigue damage. Many multi-axial criteria have been created to explain and predict this type of damage. Furthermore, a large number of papers compare the different fatigue criteria based on combined bending and torsion tests [1,2]. However, these studies are not representative of the stress gradients found in rough contacts, close to the surface. Hence the current work, which proposes a comparative study of several fatigue criteria with a combined experimental and numerical approach.