Traditional weaving technologies have been utilised over the past twenty-to-thirty years in producing woven textile components that meet engineering requirements through the interlacement of high performance yarns such as carbon, glass and Kevlar. The end performance properties and lightweight characteristics of these fabrics have been adapted within the development of both flat multilevel and shaped configurations for the composites industry. The purpose of the present research required the employment of conventional weaving technologies with limited modifications for the production of 3D woven textile preforms in a variety of truss like configurations; therefore, generating a generic procedure for all yarn combinations and strut and node dimensions for production on dissimilar jacquard looms. The ultimate driving force behind the research was to produce a truss like configuration for the aerospace industry incorporating the design criterion of solid and hollow woven counterparts. This would enable the end truss configuration to have two functions; the first being a lightweight structure by the elimination of bonding applications, through the utilisation of a fully integrated fabrication process; secondly to incorporate hollow struts for a novel storage solution.

Cette thèse s’inscrit à mi-chemin entre l’étude de la déformabilité des structures tissées et la valorisation de la fibre de lin pour des applications dans le renforcement des matériaux composites. Le premier objectif de l’étude est de caractériser expérimentalement le comportement en flexion des mèches de différentes structures constituées de fibres de lin ainsi que des tissus de différentes armures. Les travaux ont abordé aussi des paramètres tels que l’humidité relative et la composition (des mèches comélées ou en lin pure). Le deuxième objectif des travaux est d’étudier le comportement en flexion des tissus en fonction du comportement en flexion des mèches. Cette partie a commencé par la modélisation géométrique des renforts tissés dans le but de suivre l’évolution de la section du tissu qui varie dans la direction de la flexion. La modélisation mésoscopique a permis de calculer analytiquement les propriétés géométriques du tissu en particulier son moment quadratique. Les résultats obtenus ont été utilisés dans la simulation de la flexion du tissu. L’étude a permis de voir jusqu’à quel point le comportement de la mèche et le moment quadratique du tissu pilotent le comportement en flexion du tissu. D’après ces travaux, le comportement en flexion du tissu semble être approché de façon satisfaisante sur toute la gamme de longueurs envisagées à partir de ces deux grandeurs sauf pour les forts taux d’humidité où d’autres phénomènes doivent être considérés. L’étude a souligné que la différence entre deux renforts testés expérimentalement peut être anticipée numériquement. Ainsi, le concepteur de tissus sera capable d’anticiper la rigidité expérimentale du tissu pour faire des tissages adaptés à la mise en forme du renfort. Une étude paramétrique de la flexion a été également réalisée dans le but de déduire les paramètres les plus influents sur lesquels il peut jouer. / This thesis is halfway between the study of the deformability of woven structures and the use of flax fibre as reinforcement of composite materials. The first aim of the study is the experimental characterization of the bending behaviour of tows with different structures made of flax fibres and fabrics with different weaves. Parameters such as relative humidity and the composition (100% flax and commingled tows) were also considered. The second aim of the study is to link the bending behaviour of the fabric to the bending behaviour of its constituent tows. This part starts with the geometric modelling of woven fabrics in order to follow the variation of its section in the bending direction. Mesoscopic modelling allows the analytical calculation of the geometric properties of the fabric in particular its moment of inertia. The results obtained were used in the simulation of the fabrics bending to see how far the behaviour depends on the tows bending behaviour and the moment of inertia. The bending behaviour of the fabric seems to be approached satisfactorily from these two factors. This is verified within the range of lengths considered except for high humidity (in this case, other phenomena must be considered). The study pointed out that the difference between two reinforcements tested experimentally can be predicted numerically. Thus, the fabrics designer will be able to anticipate the experimental bending stiffness of the fabric in order to adapt the weaving to the shape forming. A parametric study of the bending was also achieved in order to deduce the most influential parameters of the fabric for an appropriate weaving.

La microstructure des matériaux constitue un outil essentiel pour optimiser les propriétés mécaniques des structures et ainsi améliorer leurs performances. Ce manuscrit est organisé comme suit : - Dans le chapitre 1 nous introduisons les aspects généraux de la mécanique des renforts fibreux.- Dans le chapitre 2 nous rappelons certains concepts fondamentaux concernant la mécanique des milieux continus classiques et les théories de deuxième gradient.- Dans le chapitre 3 nous nous proposons de présenter une première modélisation des renforts fibreux de composites en mettant en place des modèles numériques discrets. Dans un deuxième moment nous introduisons une modélisation continue de deuxième gradient et nous montrons que les termes d’ordre supérieur permettent une description satisfaisante des effets de flexion locale sur-cités.- Dans le chapitre 4 on particularise le cadre général de la mécanique des milieux continus introduit dans le chapitre 2 au cas particulier des milieux continus 2D. - Dans le chapitre 5 nous introduisons une hypothèse cinématique forte sur les déformations ad- missibles, en supposant que les mèches du renfort considéré sont inextensibles. Une méthode numérique permettant de montrer certaines solutions concernant le cas du bias extension test est codée en Mathematica et les résultats obtenus sont discutés. / Generalized continuum theories may be good candidates to model micro-structured materials in a more appropriate way (both in the static and dynamic regime) since they are able to account for the description of the macroscopic manifestation of the presence of microstructure in a rather simplified way. 
The present manuscript is organized as follows: In chapter 1 a general description of fibrous composite reinforcements is given, with particular attention to the introduction of standard experimental tests which are used to characterize the micro- and macro-structural mechanical properties of such materials. In chapter 2 some fundamental issues concerning classical continuum mechanical models are recalled. Moreover, second gradient continuum models are introduced and discussed by means of the Principle of Virtual Work. Since the applications targeted in this manuscript are limited to static cases, we refrain here to treat the more general case including inertia effects. In chapter 3 we start analyzing some discrete and continuum models for the description of the mechanical behavior of 2D woven composites. At this stage of the manuscript, we want to show how some discrete numerical simulations allowed us to unveil some very special deformation modes related to the effect of the local bending of fibers on the overall macroscopic deformation of fibrous composite reinforcements. Such discrete simulations showed rather clearly that microscopic bending of the fibers cannot be neglected when considering the deformation of fibrous composite reinforcements. For this reason, we subsequently introduced a continuum model which is able to account for such microstructure-related effects by means of second gradient terms appearing in the strain energy density. In chapter 4 we reduce the general continuum mechanical framework introduced in Chapter 2 to the particular case of 2D continua. We put a strong accent on the geometric interpretation of second gradient deformation measures which are seen to be directly related to the in-plane curvatures of suitable coordinate lines. Such coordinate lines will be interpreted in the next chapters are the yarns of the considered 2D woven composite, so acquiring a direct physical sense. In chapter 5 we introduce a strong kinematical hypothesis on the admissible deformations, assuming that the yarns composing the woven reinforcements are inextensible. Such assumption allows us to build-up a simplified first gradient model for the behavior of 2D woven reinforcements which is still representative of their mechanical behavior. A constrained least Action principle is proposed and the associated integral Euler-Lagrange equations are presented. A numerical method allowing to show some solutions concerning the case of bias extension test is implemented in Mathematica and the obtained results are discussed.