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 evaluate stress states at the near surface of materials. In specific zones with high-stress gradients parallel to the surface, like welding joins, areas affected 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 evaluations. A new X-ray diffraction measurement and data treatment method is therefore proposed. It is based on longitudinal step-by-step scanning in a fixed direction of the surface. As the acquired data results from the convolution of local diffraction peaks with the incident-beam intensity, an accurate model of the true two-dimensional intensity distribution inside the spot has been developed. Firstly, the true shape and size of the irradiation area is defined. 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 fitting by a nonlinear function.