X-ray diffraction for residual stress and cold work measurements of shot peened materials

Fatigue performance of aerospace components can be improved to some extend by surface treatment like shot peening. Shot peening is a cold work process consisting in impinging particles at high velocity onto the surface of metallic parts. The process induces a surface layer of compressive residual stresses and cold work. Shot peening is often considered as a beneficial surface treatment for fatigue life performance but is rarely taken into account at design stages. To this end, a precise measurement of the induced residual stress and cold work is needed. Numerous techniques (hole drilling, X-ray diffraction, contour, etc.) exist to measure the residual stresses but only the X-ray diffraction (XRD) allows coupling the residual stress measurement with the cold work measurement. This study is part of larger research project aiming at a better understanding of the shot peening process for the improvement of the fatigue life performance of 300M steel, aluminum alloy 7050, and nickel-based superalloy IN718 components, and at the development of process and fatigue simulation models of the shot peening process. This study primarily aims at identifying the experimental issues related to residual stress measurements. In particular, the research work involved comparing the well-known sin2 ψ method and the alternative cos α method, two XRD techniques for residual stress measurements based on two different geometries of detectors and calculation methods. Furthermore, the work shows the possibilities related to the use of the diffraction peak width for the measurement of cold work. The two XRD methods were compared, in an innovative way, through the identification of the diffracting grains from electron backscattered diffraction (EBSD) maps by considering the detector geometry and the calculation method of each method. The artificial textures produced by the XRD measurements were plotted for the first time as pole figures and X-ray elastic constant (XEC) 1/2S2 was computed and compared for each case. It results that the two XRD techniques exhibit different artificial textures and slightly different XECs were found for the different sets of diffracting grains considered during this study. Furthermore, the cold work was evaluated through the use of calibration curves relating the diffraction peaks width to the true plastic deformation for the three materials and different shot peening conditions. The measured residual stress and cold work profiles were used in the research