Residual Stresses Analysis Of Polycrystalline Al Alloy And Coarse-grained Fe-si Alloy By X-Ray Diffraction

The residual stress is that which remains in a body that is stationaryand at equilibrium with its surroundings. It can be very detrimental to theperformance of a material or the life of a component (hardness, staticstrength, fatigue resistance…). Alternatively, beneficial residual stressescan be introduced deliberately. Residual stresses are more difficult topredict than the in-service stresses on which they superimpose. For thisreason, it is important to have reliable methods to quantify the residualstresses and to understand the level of information they can provide.Several techniques can be used to measure residual stresses, such asXRD, hole-drilling, layer removal, sectioning, neutron diffraction,ultrasonic and magnetic methods. The X-ray Diffraction (XRD) method,which is based on the measurement of the interplanar spacing of thecrystalline material, is now commonly used for residual stresses analysisbecause its non-destructive, local, selective and quantitative determinationof deformations and stresses.However, the classical method of stress analysis by XRD (also namedthe sin2ψ method) based on certain assumptions sometimes becomesinvalid, in particular, if the grain size becomes too large in comparison tothe beam size. Therefore, the first objective of this project is to determinewhether there is a critical value of grain size for using the sin2ψ method. Inthis project, we found that the critical grain size is between111μm and153μm for using sin2ψ method to measure residual stress. When the grainsize is greater than153microns, this method was invalided.On the other hand, it is necessary to use alternative methods which are applicable to monocrystals or oligo-crystals when the grain sizebecomes large enough. Although these methods are more complex andrequire high accuracy measurements, the application of stressdetermination in coarse-grained materials is very significant for openingnew perspectives for mechanical characterization at the grain scale andtherefore allowing a better understanding of interactions between thegrains.In this project, we used the method initiated by Imura for thedetermination of strains and stresses in single crystal or oligo-crystalmaterials. And the objective of this work is to verify whether we are ableto analyze the strain and stress of a multi-crystal material by the Imuramethod with standard diffraction equipment available in LAMPA, as wellas the analysis on the reliability of this method. Through the results ofmeasurement, we could prove the feasibility of the Imura method fordetermining of strains and stresses in single crystal or oligo-crystalmaterials. However, such materials are quite difficult to be quantified theresidual stress. When the residual stress of the sample is very low, thehigher uncertainty of the residual stress will increase the complexity of theanalysis, it is necessary to use an instrument with higher precision.