Abstract: With the so-called matrix method stress calculation from x-ray diffraction measurements is much easier than it used to be with older methods. The matrix method is also well suited to optimize the choice of reflections (hkl) to be measured in order to obtain the best results with least experimental effort. Pseudocodes for the stress calculation are given.
Abstract: In this study, a monotonous mechanical test is performed on a sample in Al-alloy containing about twenty-five grains. During this test, both the total and elastic intragranular strain fields are measured at the same time. The former field is observed by digital image correlation technique while the latter one is measured by X-ray diffraction. The strain fields obtained are then dedicated to identify the parameters of a crystalline constitutive model, using finite element simulation.
Abstract: Shot-peening is used to improve the lifetime of mechanical components through the introduction of compressive residual stresses (RS) in a surface layer. In this study, we investigate the impact of such a pre-stressing treatment on a single crystal nickel-based superalloy for high pressure turbine blades of engine aircrafts. In addition to conventional metallographic tools used to characterize the alloy microstructure and the zone affected by shot-peening, X-ray measurements have been performed in order to determine residual stress depth profiles.
Abstract: A diffraction spot trace method (DSTM) is very useful to measure internal stresses of materials with coarse grains. In this study, validity of the DSTM was examined using high energy synchrotron X-ray of 70 keV. The specimen was a welded plate of austenitic stainless steel (SUS316L). The thickness of the specimen was 9 mm. The improved spiral slit system was used, and PILATUS was used as an area detector. It was difficult to detect the high energy X-rays due to the reduction of counting efficiency of the PILATUS. However, the residual stress map of the welding specimen was obtained by long exposure time. The residual stresses of the welding specimen were simulated by a finite element method (FEM). The outline of the residual stress measured by the DSTM coincided with that by the FEM.
Abstract: The historical Eshelby-Kröner self-consistent model is only valid in the case when grains can be assumed similar to ellipsoids aligned preferentially along a same direction into the polycrystal. In this work, distributions of crystallites morphologies and geometrical orientations were accounted for, owing to the so-called generalized self-consistent model, in order to satisfy Hills averages principles. Different nonlinear εφψ-vs.-sin2ψ distributions were predicted in elasticity, even in the absence of crystallographic texture, in the case when several morphologies and geometrical orientations coexist within the same polycrystal.
Abstract: The effect of interlamellar spacing on monotonic behavior of C70 pearlitic steel was investigated. Tensile tests under X-ray diffraction coupled with self-consistent model have been used to identify the role of interlamellar spacing on the ferrite plasticity parameters and residual stresses. It has been established that yielding of pearlite is controlled by ferrite critical shear stresses ( τc 0α) which is higher for the smaller interlamellar spacing. Moreover, the residual stress level in ferrite is higher for the largest interlamellar spacing under the same imposed total strain. Lattice strains, measured by synchrotron X-ray diffraction, show an elastic and plastic anisotropy of ferrite crystallites and high stresses in cementite which confirm the self-consistent model calculation. Keywords: Pearlitic steel, X-ray diffraction, Synchrotron radiation, Self-consistent model, Critical shear stress, Lattice strains.
Abstract: A two-level homogenization approach is adopted in order to predict the stress state of deformed polycrystals in correlation with the microstructure. This study is devoted to complex loading paths behaviour of cubic material. A mechanical description of the grain is developed through a micro-meso transition based on elastoplastic Kröner’s model. The meso-macro transition using a self-consistent approach is applied to deduce the global behaviour. Mechanical tests and neutron diffraction measurements are used to validate and assess the model.
Abstract: This work investigates, by diffraction methods, the morphological texture influence on the residual stress analysis in polycrystals having cubic or hexagonal symmetries. Different extreme crystallite morphologies (sphere, disc and fiber, with their principal axes aligned along common directions) were considered in the present study. In a second part, crystallographic textures were accounted for, also, enabling to reflect the combined effect of the simultaneous occurrence of morphological and crystallographic textures. A stronger influence of morphological texture than that of the crystallographic texture in terms of stresses was observed. The main purpose of this work is to make the best choice of lattice planes (hkl) used for residual stress analysis, in elasticity, depending on the morphological (and crystallographic) texture of the polycristal.
Abstract: To get a quantitative estimate of residual stresses in polycrystals from XRD measurements, a micromechanical modeling is required, except in particular cases. The most widely used method is only valid for homogeneous and isotropic samples. We present here the possibility to determine residual stresses by coupling measurements with the portable INELTM Xsolo equipement with a self-consistent polycrystalline model. This methodology may take into account texture and intergranular stresses induced by thermomechanical treatments. One example obtained for titanium subjected to tensile loading illustrates the methodology.
Abstract: A new theoretical approach is developed to simulate the elastoplastic behaviour in cubic alloys during various strain-path changes. The polycristal is considered as a composite consisting of hard dislocation walls of high local dislocation density which are separated by soft regions of low local dislocation density. The improved elasto-plastic self-consistent method is applied to deduce the global behaviour of the aggregate. The model is tested by simulating the macroscopic behavior and the development of intergranular strains during different complex loads.