No abstract

Effective prediction methods are needed by engineers to design parts used in fatigue conditions. In gears, surface fatigue damage is observed at two scales: pitting and micropitting. These two phenomena occur in response to the variation of the local loading on the contact zone between teeth. The loading is influenced by the shape of the teeth and their surface roughness, which induces stress raisers. The pertinent geometrical parameters that determine fatigue lifetime need to be identified, and in this situation improved knowledge is still required. In the current study, seven parameters are used to define a generic tooth roughness profile. The transient mechanical loading acting on this rough tooth in rolling/sliding contact with a smooth tooth is obtained using elastohydrodynamic code. The stress time history is determined by integration so that pitting and micropitting can be predicted using the Crossland fatigue criterion. The analyses were applied to results obtained for 32CrMoV13 quenched and tempered steel for about 50 variations of the generic roughness profile parameters. It was found that pitting (at the position of the maximum Hertzian shear stress) is influenced by four of the parameters, the extent to which the von Mises equivalent stress exceeds the material yield stress in the zone where micropitting occurs is influenced by two parameters, and the fatigue lifetime is influenced by three parameters.

Abstract. 2014 Due to its mechanical properties and low density, the Ti-6Al-4V titanium alloy is used in hip prostheses. But the tribological behavior of Ti-6Al-4V sliding against ultra high molecular weight polyethylene involves wear degradations. The wear behavior of (Ti-6Al-4V/UHMWPE) couple can be improved by ion implantation of nitrogen. After a characterization study to find the optimal parameters for classic ion implantation, this study presents an analysis of wear behavior in comparison with chemical composition, microhardness and adhesion work for a new implantation technique, plasma immersion ion implantation.

Nitriding is a thermo-chemical treatment to improve fatigue life of steel parts what are exposed to high cyclic loading on and close to the surface like gears for example. During the nitrogen diffusion, the precipitation of nitrides and carbides generates residual stresses and increases the superficial hardness. These residual stresses are function of the conditions of the nitriding process but also critically depend on the geometry of the steel components. Indeed, the result of the diffusion process is different for a plane geometry or a curvilinear one. In this present work, the sample is a part of a gear, composed by two teeth. Between two teeth of gear, the determination of in-depth stress gradients by classical X-rays diffraction cannot be done with a great accuracy: the spatial resolution is not sufficient because the irradiated area has the same dimension than the surface curvature of the component. Furthermore, it is very difficult to take into account the removal of matter that is required to determine in-depth stress profile because of the particular geometry. The synchrotron diffraction technique is a well-adapted method to determine such stress gradients in strongly absorbing materials due to the capability of penetration power of high energy X-rays. The removal of matter is not required and it is possible to determine an in-depth map of the stress tensor. After measurements on the ESRF ID15 beam line, stress profile has been calculated without the σ33 equal to zero hypothesis. The results are as following: compressive residual stresses were found close to the surface, and the calculated σ33 component of stress tensor is really not equal to zero. This important result seems to show the geometrical effect on stress state near non-plane surface.