No description available.

<p>The main objectives of this research are: (i) to elaborate a unified nonlinear viscoelastic model for rubber-like materials, in finite strain, accounting for material softening under deformation, and for damage induced anisotropy, (ii) to conceive, implement and test, simple, robust and efficient frictional rolling and sliding contact algorithms, in steady-state, as alternatives to existing, general purpose, contact solving strategies, (iii) to develop and verify high fidelity and computationally efficient modeling tools for isotropic and anisotropic viscoelastic objects in steady-state motion, (iv) to investigate, numerically and through experimentation, the influence of various material parameters, including material nonlinearities such at the Payne effect and the Mullins effect, as well as geometric parameters and contact surface conditions, on viscoelastic rolling resistance, and (iv) to explore, analytically and through experimentation, the conditions under which favorable mechanical synergies occur between material components and develop novel composites with improved structural performances.</p><p>A new constitutive model that unifies the behavioral characterizations of rubber-like materials in a broad range of loading regimes is proposed. The model reflects two fundamental aspects of rubber behavior in finite strain: (i) the Mullins effect, and (ii) hyper-viscoelasticity with multiple time scales, including at high strain rates. Suitable means of identifying the system's parameters from simple uniaxial extension tests are explored. A directional approach extending the model to handle softening induced anisotropy is also discussed.</p><p>Novel, simple, and yet robust and efficient algorithms for solving steady-state, frictional, rolling/sliding contact problems, in two and three dimensions are presented. These are alternatives to powerful, well established, but in particular instances, possibly `cumbersome' general-purpose numerical techniques, such as finite-element approaches based on constrained optimization. The proposed algorithms are applied to the rolling resistance of cylinders and spheres.</p><p>Two and three-dimensional boundary element formulations of isotropic, transversely isotropic, and fully orthotropic, compressible and incompressible, viscoelastic layers of finite thickness are presented, in a moving frame of reference. The proposed formulations are based on two-dimensional Fourier series expansions of relevant mechanical fields in the continuum of the layers and support any linear viscoelastic material model characterized by general frequency-domain master-curves. These modeling techniques result in a compliance matrix for the upper boundary of the layers, including the effects of steady-state motion. Such characterizations may be used as components in various problem settings to generate sequences of high fidelity solutions for varying parameters. These are applied, in combination with appropriate contact solvers, to the rolling resistance of rigid cylinders and spheres.</p><p>The problem of a viscoelastic sphere moving across a rigid surface is significantly more complicated than that of a rigid indenter on a viscoelastic plane. The additional difficulties raised by the former may explain why previous work on this topic is so sparse. A new boundary element formulation for the multi-layered viscoelastic coating of a rigid sphere is developed. The model relies on the assumption of a relatively small contact surface in order to decouple equilibrium equations in the frequency domain. It is applied in combination with an adapted rolling contact solving strategy to the rolling resistance of a coated sphere.</p><p>New modeling approaches yielding rolling resistance estimates for rigid spheres (and cylinders) on viscoelastic layers of finite thicknesses are also introduced, as lower-cost alternatives to more comprehensive solution-finding strategies, including those proposed in this work. Application examples illustrate the capabilities of the different approaches over their respective ranges of validity.</p><p>The computational tools proposed in this dissertation are verified by comparison to dynamic finite element simulations and to existing solutions in limiting cases. The dependencies of rolling resistance on problem parameters are explored. It is for instance shown that, on orthotropic layers, the dissipated power varies with the direction of motion, which suggests new ways of optimizing the level of damping in various engineering applications of very high impact. Interesting lateral viscoelastic effects resulting from material asymmetry are unveiled. These phenomena could be harnessed to achieve smooth and `invisible' guides across three-dimensional viscoelastic surfaces, and hence suggest new ways of controlling trajectories, with a broad range of potential applications.</p><p>A new experimental apparatus is designed and assembled to measure viscoelastic rolling resistance. Experiments are conducted by rolling steel balls between sheets of rubber. Principal sources of measurement error, specific to the device, are discussed. Rolling resistance predictions are obtained using the computational tools presented in this dissertation, and compared to the measurements. Interesting conclusions are drawn regarding the fundamental influence of the Payne effect on viscoelastic rolling friction.</p><p>The work presented in this dissertations finally touches on the mechanical behavior of casing-infill composite tubes, as potential new lightweight structural elements. The axial behavior of composite circular tubes is addressed analytically. The influence of material parameters and geometry on structural performances are revealed and presented in original graphical forms. It is for instance shown that significantly improved overall stiffness and capacity at yield can be obtained using a moderately soft and highly auxetic infill, which further highlights the need to develop new lightweight auxetic materials, without compromising their stiffness. It is furthermore concluded that limited mechanical synergies can be expected in metal-polymer composite tubes, within the linear range of the materials involved. This prediction is confirmed by a bending experiment conducted on an Aluminum-Urethane composite tube. The experiment however reveals unexpected and quite promising mechanical synergies under large deformations. This novel composite has a potential influence on the design and performance of lightweight protecting structures against shocks and accelerations due to impacts, which justifies that it be characterized further.</p> / Dissertation

Les composites sont devenus des matériaux courants dans l’industrie dès lors que la performance est recherchée. Les matrices polymères renforcées de fibres de verre ou de carbone sont utilisées dans l’aéronautique, l’automobile et le sport pour leurs propriétés spécifiques très élevées. Depuis peu, les fibres naturelles sont envisagées comme renforts pour les matériaux polymères pour concilier performance et écologie. Cependant, l’utilisation de pièces structurales utilisant des fibres végétales se heurte à un inconvénient majeur : leur vieillissement rapide causé par une forte sensibilité à l’humidité. Dans ce mémoire, nous avons cherché à estimer la durée de vie de ces composites à travers une caractérisation de l’endommagement en atmosphère humide du matériau, et une modélisation de son absorption d’humidité. Parmi les différentes fibres végétales disponibles, le lin a été choisi pour renforcer une matrice acrylique de la gamme « Elium ». Cette acrylique de nouvelle génération est un thermoplastique dont la polymérisation peut s’effectuer à froid par ajout d’un catalyseur au même titre que la plupart des thermodurcissables, et permet donc une bonne imprégnation des fibres, et évite tout endommagement thermique des fibres de lin. L’impact du vieillissement hygrothermique du composite sur ses propriétés mécaniques a été étudié en sollicitant le matériau à des cycles d’humidité. Ce protocole a permis de différencier deux effets du vieillissement : la plastification du composite causée par la présence de molécules entre les chaînes polymériques, et l’endommagement de la structure par fragilisation de l’interface fibre/matrice et l’oxydation de la cellulose. Le premier effet est réversible par séchage alors que le second est permanent, et est le plus dommageable pour la structure. Une loi de comportement hydromécanique a pu être déduite de ces essais, et sera utilisée en parallèle d’un modèle de diffusion par éléments finis afin de déterminer l’évolution à long terme des propriétés du composite soumis à un environnement réel. Le modèle prédit une baisse de plus de 50% du module élastique, et 60% de la contrainte à rupture après un temps d’utilisation de seulement un an. Néanmoins, le modèle utilisé se base sur de nombreuses hypothèses, notamment concernant la sensibilité de l’endommagement hydrique à la température. Une étude complémentaire est donc nécessaire afin de définir une réelle durée de vie de ces matériaux / Today, when high performance is required, composite materials are a common solution. Glass fibers or carbon fibers reinforced polymers are mostly used in aeronautic, automotive and sport industries where specific properties are needed. Recently, natural fibers have been considered as a reinforcement for polymers in order to conciliate performance and ecology. However, the production of structural parts with natural fibers face a major drawback: their fast aging caused by a high water sensitivity. In this thesis, we tried to estimate their lifetime through the characterization of the composite water damaging and the modeling of the moisture absorption. Among all the natural fibers available, flax was chosen to reinforce an acrylic matrix from the range Elium®. This new generation acrylic is a thermoplastic whose polymerization can be carried out at room temperature by adding a catalyst, like thermoset polymer are commonly produced. It allows a good fiber impregnation and avoid any thermal damage of the flax fibers during the process. The effect of the hydrothermal aging of the composite on its mechanical properties has been studied by exposing the material to moisture cycles. This protocol allowed us to distinguish two several aging effects: the composite yielding induced by the presence of water molecules into the polymer network and the structure’s damage by weakening the fiber/matrix interface and oxidizing the cellulose. The first effect is reversible by drying the composite, while the second one cause permanent decrease of mechanical properties. An hydromechanical model has been determined from these experiments and has been used with a finite element model of diffusion in order to determine the long-term evolution of the composite’s mechanical properties submitted to a reel environment. The model predicts more than 50% decrease of the composite’s stiffness, and more than 60% decrease of its strength after only one year of use. Therefore, without a proper treatment of the flax fiber for purpose of limiting its water damage, the flax/acrylic composite cannot be used as a structural part

No description available.

Equivalent, three-dimensional elastic moduli are determined from unit cell models of balanced plain weave, 2D braid, 2D triaxial braid and 4x4 twill textile composite materials consisting of interlaced or intertwined yarns. The yarn paths are modeled with undulation portions, in which one yarn passes over and under one or more yarns, and with straight portions. It is assumed that the centerline of a yarn in the undulation portions is described by the sine function, and that the cross-sectional area of a yarn and the thickness of a yarn, normal to the centerline, are uniform along the centerline. For the balanced plain weave architecture, equations for the fiber volume fraction and the cross-sectional shape of the yarn are derived for large crimp angles. It is shown that the maximum crimp angle is limited to forty-five degrees, and that limits on the ratio of the length of the undulation portion of the path to the width of the unit cell impose constraints on the fiber volume fraction and yarn packing density. For small crimp angles, approximations to the volume fraction and yarn shape equations are obtained. This assumption is used in the derivation of the geometry of the remaining architectures, and subsequent equations are obtained for the corresponding geometric parameters. For each architecture, the yarns are assumed to be transversely isotropic and a stress averaging technique based on an iso-strain assumption is used to determine the effective moduli of the unit cells. Comparisons of the effective moduli are made to other unit cell models in the literature. The micromechanical models are implemented in Fortran programs and user material subroutines for ABAQUS, called UMAT, are created out of these programs. For a balanced plain weave fabric under the small crimp angle approximation, a progressive failure model is developed to predict failure within each yarn and to degrade the material properties of the representative unit cell. Material failure is predicted by discretizing the yarns into slices and applying Tsai-Wu quadratic criterion to the on-axis strains in each slice. A stiffness and strength reduction scheme is then used to account for the change in yarn compliance. At the present time, the UMAT has only been tested as a stand-alone program with Visual Fortran 6.0, and would require further development to be used within ABAQUS on sample structural problems. / Master of Science

FRP stands for Fiber Reinforced Polymer. FRP materials have yet to be introduced inbridge construction in Sweden. Composite materials can through combined componentsand manufacturing processes be tailored to fit advanced bridge designs. FRP materials arestrong, durable and of low weight. FRP materials give the superstructure reduced weightand are therefore a suitable alternative for industrial prefabrication. This report shows thatFRP materials are possible to use in bridge construction. With the introduction of a specificEurocode we are confident that FRP materials will become a competitive alternative inbridge construction in Sweden in the future. / Broar är förenade med stora kostnader, dels för att bygga och dels för att underhålla ochreparera. FRP står för Fiber Reinforced Polymer är ett erkänt material för många andraanvändningsområden, exempelvis flyg och bilindustri. I Europa finns en mängd FRP-broar,men materialet har ännu inte introducerats i någon bro i Sverige.FRP är ett kompositmaterial som genom olika kombinationer av komponenter ochtillverkningsprocesser kan skräddarsys för den aktuella uppgiften i en konstruktion. FRPmaterialär starka, beständiga och har en låg vikt. Fördelar med FRP inom brokonstruktionär att det ger överbyggnaden en minskad egenvikt och därmed är ett lämpligt alternativ attprefabricera industriellt, då bland annat transport- och lyftbarhet gynnas samt att en högbeständighet ger minskat underhåll.Då ingen litteratur hanterar FRP i Brokonstruktion har de intervjuades åsikter varit mycketviktiga för arbetet. Litteraturstudien har legat till grund för en ökad förståelse för egenskaperutmärkande för olika typer av FRP. Intervjuer har utförts med personer som i dagslägetkommit i kontakt med materialet inom brokonstruktion. Detta har gjorts för att nå ett relevantresultat med möjlighet att kunna identifiera materialets för- respektive nackdelar samtanledningen till det låga användandet i Sverige.Rapporten visar att materialet har positiva egenskaper och är möjligt att använda vidkonstruktion av broar. Det saknas i dagsläget en specifik Eurokod som på ett enhetligt sättredovisar hur materialet ska hanteras. Med införandet av en specifik Eurokod och om en nykompetens arbetas fram inom branschen är vi övertygade om att FRP-material kommer attbli ett konkurrenskraftigt alternativ vid brokonstruktion.