Analysing the profiles and the integral widths of selected diffraction peaks provides a very efficient and non destructive method for evaluating damage mechanically induced in metallic materials. The X -ray diffraction method is used to characterize the surface hardening by residual stresses induced by shot peening process to improve fatigue properties and therefore play an important role in the lifetime of materials.
This paper reports experimental results and an investigation of the residual stresses induced by the shot peening process and their relaxation behaviour under several thermal and thermo- mechanical conditions for Ni- base superalloys (Astroloy and Inco 718). Therefore purely thermal or thermomechanical relaxation occur significantly at the surface and in near surface layers. In the first case, the relaxation mechanisms are essentially linked to thermal recovery characterized by annihilation of the unstable crystalline defects. Then the thermo- mechanicai behaviour is linked to the combined thermal and mechanical effects (microstructure, mechanical properties, local plastic strains and residual stresses).
A model is used to predict the residual stress induced by shot peening and their relaxation in the studied conditions. The model is based on some new factors such a tensorial parameter called " recovery strain" in order to describe the changes of various microscopic magnitudes with the help of half width of X -ray diffraction peaks. The predicted profile of initial residual stresses and relaxed stress values in all cases are in good agreement with the experimental distribution of residual stresses.