Comportement thermique macroscopique de milieux fibreux réels anisotropes : étude basée sur l'analyse d'images tridimensionnelles

Ce travail de thèse traite de la relation entre les propriétés thermiques effectives de matériaux fibreux à base de bois d’une part et leur microstructure d’autre part. La technique de prise de moyenne est utilisée, dans le cadre général du non-équilibre thermique local, pour écrire les équations de transfert macroscopique de matériaux pouvant présenter une anisotropie locale de conductivité thermique (liée ici aux propriétés intrinsèques des fibres de bois) et exprimer le tenseur de conductivité thermique effective. Le modèle est renseigThis PhD thesis treats the relation between the effective thermal properties of wood based fibrous materials and their microscopic structure. Within the general framework of thermal nonequilibrium, the volume averaging averaging technique is first used in order to express the macroscopic heat transfer equations for materials presenting local thermal conductivity anisotropy (related here to the properties of wood fibres) as well as the effective thermal conductivity tensor. Then, the association of this technique with quantitative analysis of 3D images allows predictive determination of effective thermal conductivity of real fibrous materials. In particular, tools based on mathematical morphology are implemented in order to characterize the microstructure of the fibrous network (porosity, granulometry and geometrical moments) and to quantify the anisotropy (covariograms, local orientation field). A representative elementary volume (REV) satisfying the constraints of the volume averaging technique may then be defined thanks to these informations. A segmentation method based on homotopic thinning skeletonization is also developed in order to identify each fibre. Thus, many parameters (fibres length, tortuosity, number of contact, etc.) are easily extracted from the segmented image and may provide precious information necessary for further numerical generation of fibrous structures. The general method for the computation of the effective thermal properties from an image of a fibrous material is finally validated by comparison with the experimental results.né par l’analyse quantitative d’images 3D de matériaux réels acquises par microtomographie X. Des outils issus de la morphologie mathématique sont mis en oeuvre pour caractériser finement la microstructure du réseau fibreux et en quantifier l’anisotropie locale et globale. Un volume élémentaire représentatif (VER) qui satisfait aux contraintes de la méthode de prise de moyenne peut être défini grâce à ces informations. Une méthode de segmentation basée sur un algorithme de squelettisation par amincissement homotopique est également développée afin d’identifier chaque fibre individuellement, ce qui permet d’accéder à de nombreux paramètres comme la longueur, la tortuosité ou encore le nombre de contacts. Le modèle thermique macroscopique développé ainsi que son implémentation numérique sont d’abord validés par une comparaison avec les prédictions théoriques dans des cas simples puis les valeurs des tenseurs de conductivité thermique effective, obtenus à partir des images de différents matériaux, sont finalement confrontés aux résultats expérimentaux.This PhD thesis treats the relation between the effective thermal properties of wood based fibrous materials and their microscopic structure. Within the general framework of thermal nonequilibrium, the volume averaging averaging technique is first used in order to express the macroscopic heat transfer equations for materials presenting local thermal conductivity anisotropy (related here to the properties of wood fibres) as well as the effective thermal conductivity tensor. Then, the association of this technique with quantitative analysis of 3D images allows predictive determination of effective thermal conductivity of real fibrous materials. In particular, tools based on mathematical morphology are implemented in order to characterize the microstructure of the fibrous network (porosity, granulometry and geometrical moments) and to quantify the anisotropy (covariograms, local orientation field). A representative elementary volume (REV) satisfying the constraints of the volume averaging technique may then be defined thanks to these informations. A segmentation method based on homotopic thinning skeletonization is also developed in order to identify each fibre. Thus, many parameters (fibres length, tortuosity, number of contact, etc.) are easily extracted from the segmented image and may provide precious information necessary for further numerical generation of fibrous structures. The general method for the computation of the effective thermal properties from an image of a fibrous material is finally validated by comparison with the experimental results

Prediction of effective thermal properties of fibrous material from 3D tomographic images

Fibrous composites materials are commonly used for thermal insulating purpose and cover a wide range of applications, from high temperature to building insulators. Their effective thermal properties strongly depends on the fibers arrangement, i.e. fibers orientations and number of fibers contacts., which are a consequence of the manufacturing process. Furthermore, local thermal conductivity of the fiber are often anisotropic, as it is the case in natural fibers for example. Thermal optimization of such materials is a major industrial stake and it requires tools to study the complex relation between the materials microstructure and the effective properties. In this work, we make use of tridimensionnal images of the fibrous microstructure to quantify some aspects of the microstructure, like fibers local orientations. These information are then used to compute effective properties directly on an representative elementary volume (REV) of the real structure, within the framework of the volume averaging method applied

Comportement thermique macroscopique de milieux fibreux réels anisotropes : étude basée sur l'analyse d'images tridimensionnelles.

This PhD thesis treats the relation between the effective thermal properties of wood based fibrous materials and their microscopic structure. Within the general framework of thermal non-equilibrium, the volume averaging averaging technique is first used in order to express the macroscopic heat transfer equations for materials presenting local thermal conductivity anisotropy (related here to the properties of wood fibres) as well as the effective thermal conductivity tensor. Then, the association of this technique with quantitative analysis of 3D images allows predictive determination of effective thermal conductivity of real fibrous materials. In particular, tools based on mathematical morphology are implemented in order to characterize the microstructure of the fibrous network (porosity, granulometry and geometrical moments) and to quantify the anisotropy (covariograms, local orientation field). A representative elementary volume (REV) satisfying the constraints of the volume averaging technique may then be defined thanks to these informations. A segmentation method based on homotopic thinning skeletonization is also developed in order to identify each fibre. Thus, many parameters (fibres length, tortuosity, number of contact, etc.) are easily extracted from the segmented image and may provide precious information necessary for further numerical generation of fibrous structures. The general method for the computation of the effective thermal properties from an image of a fibrous material is finally validated by comparison with the experimental results.Ce travail de thèse traite de la relation entre les propriétés thermiques effectives de matériaux fibreux à base de bois d'une part et leur microstructure d'autre part. La technique de prise de moyenne est utilisée, dans le cadre général du non-équilibre thermique local, pour écrire les équations de transfert macroscopique de matériaux pouvant présenter une anisotropie locale de conductivité thermique (liée ici aux propriétés intrinsèques des fibres de bois) et exprimer le tenseur de conductivité thermique effective. Le modèle est renseigné par l'analyse quantitative d'images 3D de matériaux réels acquises par microtomographie X. Des outils issus de la morphologie mathématique sont mis en oeuvre pour caractériser finement la microstructure du réseau fibreux et en quantifier l'anisotropie locale et globale. Un volume élémentaire représentatif (VER) qui satisfait aux contraintes de la méthode de prise de moyenne peut être défini grâce à ces informations. Une méthode de segmentation basée sur un algorithme de squelettisation par amincissement homotopique est également développée afin d'identifier chaque fibre individuellement, ce qui permet d'accéder à de nombreux paramètres comme la longueur, la tortuosité ou encore le nombre de contacts. Le modèle thermique macroscopique développé ainsi que son implémentation numérique sont d'abord validés par une comparaison avec les prédictions théoriques dans des cas simples puis les valeurs des tenseurs de conductivité thermique effective, obtenus à partir des images de différents matériaux, sont finalement confrontés aux résultats expérimentaux

Comportement thermique macroscopique de milieux fibreux réels anisotropes (étude basée sur l'analyse d'images tridimensionnelles)

BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

3D structural characterisation and deformation measurements of wood-based fibrous insulators under compression

Communication écrite + Posterabsen

Caractérisation microstructurale 3D et densification locale d'isolants fibreux cellulosiques sollicités en compression

International audienceIn this work, we focus on a thermal insulating panel made of wood fibres. The making process is a non-woven fabric process that generates a high porosity and a very lowdensity material. Thermal properties were first investigated. This paper deals with mechanical properties that are involved in the production line, transport and handling in the construction site. An insulating panel was compressed (33% and 73%) and we followed insitu its internal deformation using X-ray microtomography. The results enlighten a special behaviour that is linked to the low-density structure of the panel. In particular, the fibres are not compressed and the macroscopic deformation of the sample generates only a new organisation of the fibres in the panel. These results are confirmed by morphological measurements that were performed by 3D image analysis.On s'intéresse à des isolants thermiques à base de fibres de bois utilisés dans le bâtiment, de très forte porosité, élaborés par un procédé textile non tissé. Les propriétés thermiques ont fait l'objet des premières études et les propriétés mécaniques, intervenant lors du transport et de la manipulation, font l'objet de ce travail. Nous avons suivi in situ, grâce à la microtomographie X, les états de déformation interne d'un panneau d'isolation soumis à une sollicitation de compression transverse (33 % et 73 %). Le comportement particulier mis en lumière par l'accès à la déformation locale est relié à la très faible densité du matériau. En particulier, les fibres de bois ne sont pas individuellement mécaniquement comprimées et la déformation macroscopique du matériau n'engendre que leur réorganisation spatiale. Ces résultats sont mis en relation avec des caractéristiques morphologiques de structures évaluées par analyse d'image 3D

Microstructure et propriétés mécaniques de matériaux isolants à base de fibres de bois

On s’intéresse au comportement mécanique en compression des isolants thermiques à base de fibres de bois utilisés dans le bâtiment, de très forte porosité, élaborés par un procédé textile non tissé. Les états de la structure d'un panneau soumis à une sollicitation de compression transverse (33% et 73%) ont été suivsi en 3D par microtomographie X. Le comportement particulier mis en lumière par l'accès à la densification locale est relié à la très faible densité du matériau. En particulier, les fibres de bois ne sont pas individuellement mécaniquement comprimées et la déformation macroscopique du matériau n'engendre que leur réorganisation spatiale. Ces résultats sont mis en relation avec des caractéristiques morphologiques de structures évaluées par analyse d'image 3D.We focus on the mechanical behaviour of a thermal insulating panel made of wood fibres. The making process is a non-woven fabric process that generates a high porosity and a very low-density material. The 3D internal structure of a compressed (33% and 73%) insulating panel was followed by X-ray microtomography. The special behaviour is linked to the low-density structure of the panel. In particular, the fibres are not compressed and the macroscopic deformation of the sample only generates a new organisation of the fibres in the panel. These results are confirmed by morphological measurements that were performed by 3D image analysis

3D CHARACTERIZATION OF WOOD BASED FIBROUS MATERIALS: AN APPLICATION

Morphological characterization of wood based fibrous materials is carried out using X-ray tomography. This technique allows the non destructive observation at the scales of a fibre (microscopic scale) and of a network of fibres (mesoscopic scale). The 3D images are processed using classical tools of mathematical morphology. Measures of porosities and estimations of the size distributions of fibres and pores are carried out and compared to other results. An alternative method for the calculation of the local orientation of the fibres is also described to quantify the anisotropy of the fibres network. Finally, the individualization of the fibres is obtained from the representation of the fibrous network as a 3D skeleton, making possible further measurements on the isolated fibres

3D structural characterisation, deformation measurements and assessment of low-density wood fibreboard under compression. The use of X-ray microtomography.

 A low-density wood fibreboard has been compressed along its transversal direction. The experiment was carried out in ESRF synchrotron (ID19) and X-ray microtomographic images were recorded for each state. Stipulating that the fibreboard is a discontinuous material essentially made of air, that the compression simply re-organises the spatial distribution of the fibres and does not involve their intrinsic mechanical properties, we are able to deduce the material points density variations along the thickness of the panel. Good agreement is achieved between the macroscopic deformation of the sample and the microscopic compression rate evaluation. Then, the modifications of structural parameters are investigated by 3D image analysis. The relationship between the local structure and the behaviour of the wood fibreboard are deduced. Finally, a modelling approach allows the local densification to be predicted and confirms the initial hypotheses about the local behaviour of the material. In particular, polyester bonds are not involved. (c) 2008 Elsevier Ltd. All rights reserved

Prediction of effective thermal properties of fibrous material from 3D tomographic images

Fibrous composites materials are commonly used for thermal insulating purpose and cover a wide range of applications, from high temperature to building insulators. Their effective thermal properties strongly depends on the fibers arrangement, i.e. fibers orientations and number of fibers contacts., which are a consequence of the manufacturing process. Furthermore, local thermal conductivity of the fiber are often anisotropic, as it is the case in natural fibers for example. Thermal optimization of such materials is a major industrial stake and it requires tools to study the complex relation between the materials microstructure and the effective properties. In this work, we make use of tridimensionnal images of the fibrous microstructure to quantify some aspects of the microstructure, like fibers local orientations. These information are then used to compute effective properties directly on an representative elementary volume (REV) of the real structure, within the framework of the volume averaging method applied