1,720,994 research outputs found
Development of 3D multiaxis woven preforms for composite structures : technology, modelling and optimisation
No full textLa technologie de tissage 3D a été développée comme une réponse à la faible résistance au délaminage des structures de composite stratifiées en insérant des renforts fibreux dans l’épaisseur de la structure. Pourtant, cette technologie ne permet pas de positionner des fibres dans le plan autre que dans les directions à 0° et 90°. Cela implique de faibles résistances au cisaillement dans le plan. Pour cela une technologie de tissage 3D multiaxial a été développée permettant une insertion de fils dans d’autres orientations du plan. Dans cette thèse, une nouvelle technique dédié à produire des préformes 3D multiaxial tissées avec la possibilité de contrôler l’ordre des couches est présentée. Les paramètres des fils constitutifs et de la cellule unitaire des échantillons fabriqués sont mesurés avec l’investigation de la géométrie des fils (section et trajectoire) au sein de la structure. Associé à ces développements expérimentaux, un modèle géométrique, en tant qu’outil de conception permettant de décrire les préformes élaborées, a été développé. Cette modélisation géométrique permet de générer un VER, à l’échelle mésoscopique de la structure. Afin d’évaluer l’influence de ces préformes 3D sur les caractéristiques mécaniques, une chaîne numérique par éléments finis a été mise en place afin de calculer le comportement élastique équivalent. Les résultats sur les propriétés élastiques permettent de quantifier l’apport des fils dans le plan, comparativement aux structures 3D tissées. L’influence de l’ordre des couches dans l’épaisseur est également mis en évidence sur la minimisation des contraintes interlaminaires sur les dans le composite.3D weaving technology is developed in response to the poor delamination resistance of laminated composite structures by insertion through the thickness fiber reinforcements. However, this technology is limited relatively to a possibility to insert in-plane yarns oriented other than 0° and 90°. This results in reduction of the in-plane off axis tensile properties and the in-plane shear resistance. Therefore, 3D multiaxis weaving technology has been developed in order to enable this insertion. In the thesis, a novel technique able to produce 3D multiaxis woven preforms is presented with the possibility to control the sequencing of yarn layers. The constitutive yarns and unit cell parameters of the produced samples have been measured with investigation of yarns geometry (cross section shape and path) within the structure, by analyzing the captured micrographs for the samples cross section. Predictive geometrical model has been developed. This model is indispensable design tool providing approximate estimation of the geometrical properties of the dry preforms and composites. Moreover, a geometric modeling approach is improved helping to construct an RVE of this structure as accurate as possible based on the elaborated geometrical characterization. Based on the developed RVE, a mechanical modeling has been also improved and completed using the finite element method serving firstly, to evaluate the bias yarns effect on the elastic stiffness and in-plane off-axis properties in comparison with equivalent 3D orthogonal woven composite. Secondly, it helps to investigate the influence of the in-plane layers sequence on the induced interlaminare stresses at the composite free edges
Manufacturing and charaterization of 3D warp interlock made with flax for composites materials
No full textLa caractérisation multi-échelle des préformes textiles en fibres naturelles est un moyen indispensable pour comprendre et évaluer les propriétés mécaniques et le comportement du composite. Dans cette étude, une caractérisation expérimentale multi-échelle est réalisée sur des tissus 3D interlock chaine en fibre de lin à l'échelle des fibres (micro), des mèches (méso), des tissus et des matériaux composites (macro). Les propriétés mécaniques en traction de la fibre de lin ont été déterminées en utilisant la méthodologie de l'IFBT. L'effet de la torsion a également été pris en compte dans le calcul de la rigidité de la fibre et a révélé les limites de la loi des mélanges et d'autres modèles. Des essais de traction sur des mèches sèches ont été effectués en considérant différents niveaux de torsion afin de déterminer le niveau de torsion optimal pour tisser 3D la mèche de lin. Les résultats révèlent l'importance de considérer les propriétés de la fibre et des mèches à ces échelles pour déterminer la matière première la plus adéquate pour le tissage. A l'échelle du tissu, dix sept tissus 3D interlocks chaines ont été réalisés pour comprendre l’influence des paramètres produits sur les paramètres textiles et sur le comportement mécanique. La caractérisation des structures tissées 3D interlock chaine montre le rôle prépondérant des mèches sur les propriétés structurelles et mécaniques. A l'échelle macroscopique suivante, les six structures résinées ont permis d'établir des relations entre les structures de renfort et les matériaux composites associés. Couplé à l'ensemble de ces études, une approche statistique a permis d'apporter une vision globale des paramètres produits qui influent sur les tissus 3D interlock chaine et les matériaux composites associés.The multi-scale characterisation of textile preforms made with natural fibres is a necessary method to understand and analyse the mechanical properties and behaviour of the composite. In this study, a multi-scale experimental characterisation is carried out on 3D warp interlock fabrics made with flax fibres at the fibre (micro), yarns (meso), fabric and composites (macro) scales. The mechanical tensile properties of the flax fibre were determined using the IFBT methodology. The twist effect was also taken into account in the calculation of fibre stiffness and revealed the limits of the rules of mixtures and some other models. Tensile tests on dry rovings were carried out considering different levels of twist, in order to determine the optimal twist level suited for 3D weaving process. The results reveal the importance of considering the properties of the fibre and the yarn at these scales to determine the most suitable material for weaving. At the fabric scale, seventeen 3D warp interlock fabrics were produced to understand the influence of product parameters on textile parameters and mechanical behaviour. The characterisation of 3D warp interlock woven structures shows the predominant role of yarns on structural and mechanical properties. On the following macroscopic scale, the six impregnated structures have enabled relationships to be established between the reinforcing structures and the associated composite materials. Coupled with these studies, a statistical approach provided a global vision of the product parameters that influence the 3D interlock warp interlock fabrics and the associated composite materials
Development and multi-scale analysis of hemp reinforcements for biocomposites applications
No full textCette thèse s’inscrit dans le cadre du projet européen SSUCHY (H2020-BBI) qui vise à développer des matériaux composites 100% biosourcés, en termes de renforts et de matrices. L’objectif de cette thèse est le développement et la caractérisation multi-échelle de renforts textiles en fibres de lin et de chanvre capables de répondre aux besoins des applications industrielles visées. Les travaux se focalisent d’une part sur le développement de fils et mèches aux propriétés suffisantes pour la production de renforts par tissage. Ces fils et mèches sont caractérisés sur leurs propriétés textiles et mécaniques. A l’échelle des fibres les caractéristiques sont identifiées par la méthode de l’« Impregnated Fibre Bundle Test » (IFBT). Des renforts sont élaborés par tissage à partir de différents types de fils et l’influence des paramètres de fabrication est analysée sur les propriétés textiles, de flexion à l’état sec, de traction uniaxiale, de cisaillement plan et de mise en forme lors d’essais d’emboutissage. Ces caractérisations permettent d’établir des fiches techniques et de choisir le renfort dont les paramètres de production permettront de répondre au mieux aux spécifications de la pièce composite finale. Enfin, des plaques composites sont produites avec certains des renforts développés, puis caractérisés aux moyens d’essais de traction. Les propriétés sont comparées à celles de biocomposites renforcés de fibres naturelles issues de la littérature ou déjà commercialisés.This PhD thesis is a part of the European SSUCHY project (H2020-BBI) which aims to develop 100% bio-based composite materials, in terms of reinforcements and matrices. The objective of this thesis is the development and the multi-scale characterization of textile reinforcement made of flax fibers and hemp fibers. These reinforcements have to meet the requirements of the industrial applications of SSUCHY project. First of all, the work focuses on the development of yarns and rovings with sufficient properties for the manufacturing of the reinforcements by weaving technology. For these yarns and rovings, textile and mechanical properties are studied. At the scale of the fibers, properties are obtained by the “Impregnated Fiber Bundle Test” (IFBT) method. Reinforcements are produced by weaving technology with different types of yarns. The influence of production parameters is analyzed on textiles properties, on bending properties in dry state, on uniaxial tensile properties, on plan shear properties and on shape forming capability. These characterization steps allow to build technical data sheets and to choose the reinforcement whose production parameters enables to meet the requirements of the final composite part in the best possible way. Finally, composite plates are manufactured with some of the reinforcements developed, and characterized with tensile tests. Properties of these biocomposites materials are compared with those of biocomposites reinforce by natural fibers from literature or already available on the market
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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