Contribution à la caractérisation de l'endommagement des aciers de pignonnerie en fatigue de contact : méthodologie de déconvolution des mesures de diffraction X

Contribution to characterize gear steel damage in contact fatigue: new x-ray diffraction measurement and data treatment method

¹ Doctorant ENSAM Aix en Provence
² Professeur à Aix-Marseille II
³ Ingénieur au CREAS Amnéville

Abstract

In gear industry, cyclic contact leads to a damage of the superficial layers and generates residual stresses onto the work-hardened surface. The study of this stress state allows to characterize some wear mechanisms. However many measurements carried out by X-ray diffractometry reveal high lateral stress gradients in the wear track perpendicular direction which perturb the results obtained by this technique. It is therefore necessary to develop a new measurement and data treatment strategy to determine the local stress state at a given point of the material.

The method proposed in our paper is based on a step-by-step strain scanning along a fixed direction of the surface. The step size is selected in connection with the amplitude of the lateral stress gradients. At each step, the lattice strain measurement integrates the whole elementary diffraction peaks of the points belonging to the irradiated area. The acquired data result therefore from the convolution of these local peaks with the intensity of the incident beam. An accurate modeling of the true 2D-intensity distribution inside the spot has been developed for that purpose which accounts for the actual measurement conditions : shape and size of the X-ray tube focus, diameter of the incident beam collimator, diffraction angle and (ϕ, ψ) measurement direction. It first defines the true shape and size of the irradiation area. The distribution of the intensity received by the sample surface is then computed. The true local strains and stresses along the scanning direction are finally determined from the acquired peaks using an optimization through a least squares smoothing requiring a non linear function.