Micro-scale full-field strain measurement on a stainless steel sample elasto-plastically tested

This study deals with the characterisation of the mechanical behaviour of an austenitic stainless steel at the micrometric scale in its elasto-plastic domain. For that, the full-field “grid method”, adapted to this scale, is implementeded. The images of the grating are recorded thanks to a white-light interferometric microscope. The results show that the strain localization is in accordance with the microstructure. Some considerations about the average strain within different grains of the steel sample are made to show how the plastic deformation begins locally before the global yield point.Sans financemen

Evaluation of the Penalized Least Squares Method for Strain Computation

This work proposes an alternative procedure to smooth and differentiate experimental full-field displacement measurements to get strain fields. This one, the penalized least squares method, relies on the balance between the fidelity to original raw data and the smoothness of the reconstructed ones. To characterize its performance, a comparative study between this algorithm and two other commonly implemented strategies (the 'diffuse approximation' and the Savitzky-Golay filter) is achieved. The results obtained by the penalized least squares method are comparable in terms of quality of the reconstruction to those produced by the two other algorithms, while the proposed technique is the fastest as its computation time is totally independent from the asked amount of smoothing. Moreover, unlike both other considered methods, it is possible with this technique to perform the derivation to obtain strain maps before smoothing them (while the smoothing is normally applied to displacement maps before the differentiation) which can lead in some cases to a more effective reconstruction of the strain fields

Elasto-plastic strain field measurement at micro-scale on 316L stainless steel

This study deals with the characterisation of the mechanical behaviour of an austenitic stainless steel (316L) at the micrometric scale in its elasto-plastic domain. For that, the full-field “grid method”, adapted to this scale, is implemented. The direct interferometric photolithography is used here to produce gratings (the pitch is approximately 5 μm) on specimen surface. The gratings are digitized at every step of the tensile loading thanks to a white-light interferometric microscope. The in-plan strain fields are then deduced from these grating images (resolution is closed to 2×10-3 for a spatial resolution of bout 20 μm). The results show that the strain localisation is in accordance with the microstructure. Some considerations about the average strain within different grains of the steel sample are made to show how the plastic deformation begins locally before the global yield point σY. It was then observed that these local σY values thus determined may vary by +/-20 MPa around the macroscopic value (180 MPa).Sans financemen

Calcul des cartes de déformations par une méthode de moindres carrés pénalisés

Ce travail propose d'étudier une procédure alternative pour lisser puis dériver des mesures de champs de déplacement expérimentaux pour obtenir des champs de déformations. Celle-ci, la méthode des moindres carrés pénalisés, repose sur le calcul d'un compromis entre la fidélité aux données brutes et la nature lisse des données reconstruites. Pour caractériser ses performances, une étude comparative entre cet algorithme et deux autres stratégies couramment mises en ?uvre (l'"approximation diffuse" et le filtre Savitzky-Golay) est réalisée. Les résultats obtenus par la méthode des moindres carrés pénalisés sont comparables en termes de qualité de la reconstruction à ceux produits par les deux autres algorithmes, tandis que la technique proposée est la plus rapide et que son temps de calcul est totalement indépendant du taux de lissage demandé. En outre, contrairement à deux autres méthodes, il est possible avec cette technique d'effectuer une dérivation numérique pour obtenir les déformations avant de lisser ces dernières (tandis que le lissage est normalement appliqué aux cartes de déplacement avant la différenciation) ce qui peut conduire dans certains cas à une reconstruction plus efficace des champs de déformations

Elasto-plastic strain field measurement at micro-scale on 316L stainless steel

This study deals with the characterisation of the mechanical behaviour of an austenitic stainless steel (316L) at the micrometric scale in its elasto-plastic domain. For that, the full-field “grid method”, adapted to this scale, is implemented. The direct interferometric photolithography is used here to produce gratings (the pitch is approximately 5 μm) on specimen surface. The gratings are digitized at every step of the tensile loading thanks to a white-light interferometric microscope. The in-plan strain fields are then deduced from these grating images (resolution is closed to 2×10-3 for a spatial resolution of bout 20 μm). The results show that the strain localisation is in accordance with the microstructure. Some considerations about the average strain within different grains of the steel sample are made to show how the plastic deformation begins locally before the global yield point σY. It was then observed that these local σY values thus determined may vary by +/-20 MPa around the macroscopic value (180 MPa).Sans financemen

Identification des propriétés mécaniques d’enroulements supraconducteurs d’un aimant d’IRM

National audienceCette étude propose une méthodologie expérimentale afin d’identifier les propriétés mécanique élastiques d’anneaux supraconducteurs. Des essais de traction diamétrale associés à une technique de mesure champs de déplacements ont été effectués sur des échantillons. Les champs de déformations ont ensuite été obtenus à partir des champs de déplacements par différentiation numérique. Enfin, les quatre modules élastiques orthotropes dans le plan des enroulements ont été identifiées à l’aide la méthode des champs virtuels

Evaluation of the Penalized Least Squares Method for Strain Computation

This work proposes an alternative procedure to smooth and differentiate experimental full-field displacement measurements to get strain fields. This one, the penalized least squares method, relies on the balance between the fidelity to original raw data and the smoothness of the reconstructed ones. To characterize its performance, a comparative study between this algorithm and two other commonly implemented strategies (the 'diffuse approximation' and the Savitzky-Golay filter) is achieved. The results obtained by the penalized least squares method are comparable in terms of quality of the reconstruction to those produced by the two other algorithms, while the proposed technique is the fastest as its computation time is totally independent from the asked amount of smoothing. Moreover, unlike both other considered methods, it is possible with this technique to perform the derivation to obtain strain maps before smoothing them (while the smoothing is normally applied to displacement maps before the differentiation) which can lead in some cases to a more effective reconstruction of the strain fields

Texture evolution in Nd:YAG-laser welds of AZ31 magnesium alloy hot rolled sheets and its influence on mechanical properties

International audienceAZ31 hot rolled magnesium alloy presents a strong basal texture. Using laser beam welding (LBW) as a joining process induces high temperature gradients leading to major texture changes. EBSD was used to study the texture evolution, and tensile tests coupled with speckle interferometry were performed to understand its influence on mechanical properties. The random texture obtained in the LBW fusion zone is mainly responsible for the yield strength reduction

Analysis of Shot Peening Blast by Particle Tracking and Digital Image Correlation

Shot peening process is generally controlled by Almen intensity and coverage rate measurements. The results of the shot peening surface treatment depend on the process parameters (type of machine, nozzle, shot type, mass flow, velocity, impact angle) and on the properties of the treated parts (material behaviour, roughness). Previous studies have investigated the control of shot peening via measurement of the global flow characteristics [1, 2, 3]. Developing experimental methods to characterize a particle flux is useful to obtain the contact conditions of the shot peening process (location, velocity vector) on any interesting geometry. Using a high speed camera, this study aims at the analysis of the flux of shots ahead of the nozzle of a shot peening machine. In this study two set of process parameters are investigated: S1: P= 1 bar, D=3 kg/min S2: P= 1 bar, D=13 kg/min Quantitative evolution of the particle velocity and density in the flux is studied

Texture evolution in Nd:YAG-laser welds of AZ31 magnesium alloy hot rolled sheets and its influence on mechanical properties

International audienceAZ31 hot rolled magnesium alloy presents a strong basal texture. Using laser beam welding (LBW) as a joining process induces high temperature gradients leading to major texture changes. EBSD was used to study the texture evolution, and tensile tests coupled with speckle interferometry were performed to understand its influence on mechanical properties. The random texture obtained in the LBW fusion zone is mainly responsible for the yield strength reduction