The objective of this work is to study the Rolling Contact Fatigue (RCF) behavior of hybrid bearings. The studied bearings are composed of Si3N4 balls rolling on steel raceways. The raceways are made out of nitrided 32CrMoV13 steel. The nitriding treatment aims at reinforcing the surface mechanical properties. As the presence of an indent on the raceway surface will dramatically decrease the fatigue life of the rolling element [1], this study focuses on the RCF behavior of pre-indented rolling element bearings. It is thus necessary to study the fatigue behavior of both the steel and the ceramic material under fatigue loading. The study presented here focuses on the fatigue behavior of nitrided 32CrMoV13 steel under rolling contact and aims at proposing a crack initiation criterion based on experimental results. Fatigue tests are performed on a bi-disks machine with indented 32CrMoV13 samples to observe the damage evolution and crack initiation stages under various indent dimensions and test conditions. In parallel simulations are performed with a semi-analytical method to accurately determine the stress history under elastic-plastic rolling contact. Semi analytical methods, classically used for the simulation of elastic contacts, have recently been extended to the consideration of plasticity [2], allowing to simulate the ball-raceway interaction in ball bearings [3] and wear or running in [4]. The main advantage of these methods is their ability to simulate the coupling between the contact conditions and the plastic behavior in reasonable computational time. Based on the experimental and simulation results, a crack initiation criterion based on the dislocation theory proposed by Tanaka and Mura [5] is proposed allowing to predict the number of cycles for crack initiation for the given material.

The Rapid Additive Forging (RAF) process is a Direct Energy Deposition (DED) Additive Manufacturing (AM) process, based on the deposition of a Titanium alloy on a substrate plate. This process has been developed for the production of Titanium parts of aeronautic components. In this study, a Finite Element (FE) numerical simulation methodology has been established to perform a fast analysis of the RAF process, including full 3D-transient thermal-metallurgical and mechanical numerical simulations. Thus, residual stresses and distortions caused by the process can be estimated. Different modelling strategies have been compared in order to find a balance between computation time and accuracy. Analyses include the effects of phase transformations in the Titanium alloy. First analyses have been performed on a simple geometry of welding wall. The influences of the material activation modelling strategy on the thermal and mechanical results have been investigated. The effects of phase transformations on residual stresses and distortions are also discussed. Then a specimen with a more complex geometry has been considered in the analysis, including the effect of different deposition paths. A full 3D simulation of the whole deposition process has been compared with several simplified computation procedures, including a reduction of the number of layers considered in the simulation.