Contribution to residual-stress evaluation in high-stress-gradient zones by X-ray diffraction

The non-destructive X-ray diffraction method is usually employed to evaluat e stress stales at the near surface of materials. in specific zones with hi gh-stress gradients parallel to the surface, like welding joins, areas affe cted by lasers, or ball-bearing raceways, the classical measurement method is not particularly well suited, as the irradiation spot covers regions of varying stress according to its size. This leads to inaccurate stress evalu ations. A new X-ray diffraction measurement and data treatment method is th erefore proposed. It is based on longitudinal step-by-step scanning in a fi xed direction of the surface. As the acquired data results from the convolu tion of local diffraction peaks with the incident-beam intensity, an accura te model of the true two-dimensional intensity distribution inside the spot has been developed. Firstly, the true shape and size of the irradiation ar ea is defined. The distribution of the intensity received by the sample sur face is then computed. The true local strains and stresses: along the scann ing direction are finally determined from the acquired peaks using an optim ization through a least-squares fitting by a nonlinear function.