78 research outputs found

    Estimation of the optimal parameters for K-edge subtraction imaging with PixiRad-2/PixieIII photon counting detector on a conventional laboratory X-ray micro-tomograph

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    Photon Counting Detectors (PCDs) open new opportunities in X-ray imaging. Pixie III is a PCD using simultaneously two energy thresholds. This enables to acquire images from two distinct energy bins in a single exposure. This is particularly suited to perform K-Edge Subtraction (KES) imaging with laboratory sources. In that context, one has however to deal with an energy bin optimization: narrow energy bins leads to high KES signal, at the expense of higher noise, while wider energy bins leads to poor KES signal but better statistics. This work presents a model that aims at finding the optimal thresholds and source voltage in order to retrieve the best Contrast to Noise Ratio (CNR) for a given sample. The model also optimizes the configurations for conventional absorption modality and compares both modalities. We noticed that the input flux and the energy difference between the thresholds influence the noise on image. We included this in the model using phenomenological laws. The model is then compared to empirical optimization by experimental screening of the parameters on model materials composed of barium, iodine and water. Finally, a study of the predicted CNRs has function of the sample composition is presented as an example of usage of the model

    A new shape memory porous material made up of a single entangled NiTi wire

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    Avec leur architecture légère et leurs propriétés mécaniques potentiellement exceptionnelles, les matériaux poreux en alliages à mémoire de forme NiTi se positionnent comme des solutions intéressantes pour un large spectre d'applications, e.g. prothèses biomédicales, systèmes absorbeurs de chocs... Aujourd'hui, la plupart de ces matériaux sont produits par frittage de poudres (naturel ou SPS). Cependant, leurs propriétés mécaniques sont bien loin d’être aussi bonnes qu'attendues (peu ou pas de superélasticité ou d’effet mémoire) : ceci est en grande partie dû aux grandes difficultés pour induire des microstructures appropriées lors du frittage. Pour contourner ces problèmes, nous proposons un procédé de fabrication alternatif et original, basé sur l'auto-enchevêtrement d'un fil de NiTi, sans frittage. D’une part, les mésostructures des pelotes ainsi produites, caractérisées par microtomographie RX, sont relativement homogènes ; elles peuvent être architecturées à façon en adaptant les conditions de mise en forme. D’autre part, les propriétés thermomécaniques des pelotes peuvent être pilotées simplement par des traitements thermiques. Ces deux atouts procurent à ces matériaux de grandes possibilités. Par exemples, le comportement superélastique des pelotes en compression simple est excellent et étonnant : il combine (i) très grandes déformations recouvrables en décharge (30 à 40%), (ii) niveaux de contraintes raisonnables (5 à 10 MPa), peu dépendants de la température (0.05MPa/K) et (iii) variations de volume importantes avec des séquences complexes de consolidation et de dilatance. Le comportement ferroélastique est quant à lui très proche de celui des milieux fibreux élastoplastiques, avec un écrouissage notable en charge et une forte déformation rémanente en décharge (de l’ordre de 20%). Enfin, un simple chauffage à charge nulle permet de recouvrir totalement cette déformation par effet mémoire de forme

    Poisson's function of single-wire entangled materials : below 0 in traction and above ½ in compression

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    The mechanics of fibrous materials is both complex and intriguing. Here, we explore one class of such materials, made of the disordered entanglement of a single long fiber. This architected material is of both fundamental and technological interests. From a theoretical point of view, this system can be seen as a disordered material, akin to glasses, but with very long (possibly infinite) ranged correlations transmitted along the single fiber. From an applied point of view, this material is a potential replacement for sintered materials, because with comparable porosities, entangled structures are highly ductile while sintered powders are often intrinsically brittle. We study the mechanics of single-wire materials using a combination of experiments and simulations. Experimentally, we produce cylindrical samples by entangling a single fiber made either of NiTi, polyamide or copper. Spatial homogeneity and stress relaxation are achieved by series of thermomechanical treatments that allow to first transform the wire into a coil and then entangle the coil. The samples are subjected to cycles of compressions followed by unloading, tracking the local deformations in the samples with optical cameras and x-ray microtomography. Numerically, we employed Kirchhoff's elastic rod theory to simulate the same mechanical cycles on both numerical substitutes of the experimental samples (i.e. cylindrical samples of geometry and density comparable to the experiments) and idealized samples with a cubic and periodic geometry. We will discuss our main observations on the mechanical properties of the single-wire systems. First, stress-strain curves show a prolonged regime of non-linear elasticity and a marked hysteresis between compression loading and unloading. Also, by tracking the sample shape variations, we observe surprising non-monotonous variations of the sample volume, both in the experiments and in the simulations. At the local level, we use image analysis to follow the evolution of the local orientation of the fiber and of the number and orientation of the contacts, the latter two quantities also showing non-monotonous behaviors.  In particular, we show that this material exhibits at finite strains an unexpected variation of its Poisson's function, beyond the usual bounds: Poisson's function is above ½ in compression and below zero in tension. This material is thus reversibly compress dilatant in compression and auxetic in tension. This means that the structure expands laterally in both traction and compression and so rapidly in compression that its volume increases. This unusual variation of Poisson's function arises from the interplay between the elongation of the coiled segments that constitute the entanglement and fiber rearrangements due to steric effects.  This work illustrates that a property (Poisson's function not limited to 0 or ½), rare in bulk materials, can be readily obtained in "simple" architected materials and opens the way to developing architectures with very large, negative but also positive, Poisson's functions

    Détermination des propriétés microstructurales et physiques d'un matériau fibreux cellulosique à partir de microtomographies aux rayons X

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    Cette étude concerne la détermination des propriétés microstructurales et physiques d'un matériau fibreux cellulosique à partir de microtomographies aux rayons X. Les propriétés microstructurales telles que la porosité, la surface spécifique et l'anisotropie du matériau sont mesurées à l'aide d'outils d'analyse d'image. Les propriétés physiques, telles que la perméabilité et la conductivité thermique effective, sont estimées numériquement en s'appuyant sur les résultats de l'homogénéisation périodique. Ces estimations numériques sont comparées à des mesures expérimentales. L'influence de la microstructure sur les propriétés physiques étudiées est discutée

    Representative elementary volume assessment of three-dimensional x-ray microtomography images of heterogeneous materials: Application to limestones

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    International audienceOver the last 15 years, x-ray microtomography has become a useful technique to obtain morphological, structural, and topological information on materials. Moreover, these three-dimensional (3D) images can be used as input data to assess certain properties (e.g., permeability) or to simulate phenomena (e.g., transfer properties). In order to capture all the features of interest, high spatial resolution is required. This involves imaging small samples, raising the question of the representativity of the data sets. In this article, we (i) present a methodology to analyze the microstructural properties of complex porous media from 3D images, (ii) assess statistical representative elementary volumes (REVs) for such materials; and (iii) establish criteria to delimit these REVs. In the context of cultural heritage conservation, a statistical study was done on 30 quarry samples for three sorts of stones. We first present the principles of x-ray microtomography experiments and emphasize the care that must be taken in the 3D image segmentation steps. Results show that statistical REVs exist for these media and are reached for the image sizes studied (1300×1300×1000 voxels) for two characteristics: porosity and chord length distributions. Furthermore, the estimators used (porosity, autocorrelation function, and chord length distributions) are sufficiently sensitive to quantitatively distinguish these three porous media from each other. Lastly, this study puts forward criteria based on the above-mentioned estimators to evaluate the REVs. These criteria avoid having to repeat the statistical study for each new material studied. This is particularly relevant to quantitatively monitor the modifications in materials (weathering, deformation . . . ) or to determine the smallest 3D volume for simulation in order to reduce computing time

    Contribution à la quantification 3D de réseaux fibreux par microtomographie au rayonnement synchrotron (applications aux papiers )

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    Dans cette thèse, nous nous sommes intéressés à la quantification 3D de matériaux fibreux cellulosiques(papiers et cartons). Afin de lier les paramètres de fabrications des papiers à leurs structures et leurs propriétés d'usage, les échantillons analysés ont été imagés par microtomographie à rayonnement synchrotron. Des outils de traitement de l'image ont été développés et appliqués pour quantifier la microstructure (porosité, surface spécifique, taux de charges, orientation des fibres). Il a été démontré que les données obtenues sont représentatives d'un point de vue structural. Ainsi, les informations 3D obtenues par ces techniques peuvent être comparées aux modèles trouvés dans la littérature. Des dispositifs expérimentaux ont également été développés pour analyser in-situ l'influence de la compression et de l'humidité sur la structure des papiers. De plus, l'équation de transfert radiatif a été appliquée pour l'étude des propriétés optiques (opacité et blancheur).This thesis focuses on the quantification of cellulosic fibrous materials. ln order to link paper making variables, structures and end-use properties, samples were imaged by synchrotron microtomography. Image analysis tools were developed and applied to quantify paper structure (porosity, specifie surface misotropy, filler content). It appeared that our data are representative from a microstructural point of view. Therefore structural results were compared to the models found in the literature. Moreover, sample environments were developed to study in-situ the effect of deformation and humidity on paper structure. On the other hand, attention is focused on the prediction and the modelling of paper properties by solving in this particular case the radiative transfer equation.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF
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