Le matériau composite agrégataire énergétique étudié a un comportement viscoélastique endommageable sensible à la pression de confinement et à la température. Ces travaux concernent la modélisation de l'anisotropie induite par endommagement avec deux objectifs principaux. Dans un premier temps, le caractère anisotrope de l'endommagement est mis en évidence expérimentalement. Des essais alternant tension et compression permettant d'observer l'effet unilatéral d'endommagement. Ensuite, un modèle de comportement est développé pour le matériau d'étude. Des modèles pertinents sont tout d'abord comparés. Le modèle le plus approprié est ensuite amélioré par l'ajout de mécanismes d'endommagement, d'effectivité du dommage et d'un mécanisme de plasticité. Les données expérimentales sont utilisées pour identifier les paramètres du modèle. Ce dernier a été ensuite implémenté dans un logiciel de calcul aux éléments finis (Abaqus / standard) sous la forme d'une procédure Fortran (UMAT). Différents types de chargements sont simulés et confrontés aux résultats expérimentaux. / An explosive aggregate material exhibits a visco-elastic behaviour with damage, internal friction and sensitivity to the confining pressure and temperature. This thesis focuses on the anisotropic elastic damage with unilateral effect. The first aim of this study is to highlight experimentally the anisotropic nature of the damage. Then, a new model is proposed for the studied material. This is achieved using a comparison of some relevant models in order to select the most appropriate among them. The selected model is then improved by adding unilateral effect mechanisms and plasticity. Experimental data is used to characterize the material behaviour and to determine the parameters of improved model. This model has been implemented in the finite element software (Abaqus / Standard) using Fortran procedure (UMAT) and then tested for different loads and compared with experimental results.

Les applications pour lesquelles des élastomères sont soumis à des sollicitations cycliques sont nombreuses. Des charges sont généralement utilisées afin d'améliorer leurs propriétés, cependant, elles induisent également un adoucissement important de la contrainte lors de sollicitations cycliques. A ce jour, les phénomènes physiques conduisant à l’apparition de cet adoucissement ne sont pas clairement établis et sa modélisation demeure une difficulté majeure.Afin d'étudier l'adoucissement, des élastomères chargés sont soumis à des chargements cycliques. Des méthodes de caractérisations originales sont proposées afin de quantifier les effets de l'intensité du chargement et du nombre de cycles. Pour faire le lien avec la microstructure du matériau, plusieurs mélanges de compositions différentes sont utilisés. Des chargements non proportionnels de traction uniaxiale et biaxiale sont appliqués afin de mettre en évidence l'anisotropie induite par l'adoucissement. Ces données expérimentales non conventionnelles sont utilisées afin de définir un critère général pour l'activation de l'adoucissement Mullins. Une loi de comportement fondée sur une analyse approfondie des données expérimentales est proposée. La modélisation est basée sur une approche directionnelle. L'adoucissement Mullins est modélisé en utilisant le concept d'amplification de la déformation et son activation est pilotée par un critère directionnel. La capacité du modèle à prédire les réponses d'un matériau ayant subit un historique de chargement non proportionnel est validée / Rubber-like materials are submitted to cyclic loading conditions in various applications. Fillers are always incorporated within rubber compounds. They improve the mechanical properties but induce a significant stress-softening under cyclic loadings. The physical source of the softening is not yet established and its modeling remains a challenge. For a better understanding of the softening, filled rubbers are submitted to cyclic loadings. In order to quantify the effects of the loading intensity and the number of cycles, original methods are proposed to characterize the softening. To study the influence of the material microstructure on the softening, compounds with various compositions are considered.Non proportional tensile tests including uniaxial and biaxial loading paths are applied in order to highlight the softening induced anisotropy. Such unconventional experimental data are used to provide a general criterion for the softening activation. A constitutive modeling grounded on a thorough analysis of experimental data is proposed. The model is based on a directional approach. The Mullins softening is accounted for by the strain amplification concept and is activated by a directional criterion. The model ability to predict non proportional softened material responses is demonstrated

The ability to adjust structural response to external loading and ensure structural safety and serviceability is a characteristic of Smart Systems. The key to achieving this is through the development and implementation of smart materials. An example of a smart material is a Shape Memory Alloy (SMA). Reinforced concrete structures are designed to sustain severe damage and permanent displacement during strong earthquakes, while maintaining their integrity, and safeguarding against loss of life. The design philosophy of dissipating the energy of major earthquakes leads to significant strains in the steel reinforcement and, consequently, damage in the plastic hinge zones. Most of the steel strain is permanent, thus leading to large residual deformations that can render the structure unserviceable after the earthquake. Alternative reinforcing materials such as superelastic SMAs offer strain recovery upon unloading, which may result in improved post-earthquake recovery. Shape Memory Alloys have the ability to dissipate energy through repeated cycling without significant degradation or permanent deformation. Superelastic SMAs possess stable hysteretic behavior over a certain range of temperature, where its shape is recoverable upon removal of load. Alternatively, Martensite SMAs also possess the ability to recover its shape through heating. Both types of SMA demonstrate promise in civil infrastructure applications, specifically in seismic-resistant design and retrofit of structures. The primary objective of this research is to investigate experimentally the performance of concrete beams and shear walls reinforced with superelastic SMAs in plastic hinge regions. Furthermore, this research program involves complementary numerical studies and the development of a proposed hysteretic constitutive model for superelastic SMAs applicable for nonlinear finite element analysis. The model considers the unique characteristics of the cyclic response of superelastic materials.

This paper deals with the problem of abdominal aortic aneurysms (AAA), taking into account the possibility of using mechanical tests of aortic tissues for improvement of level of their constitutive models. First part of thesis deals with the introduction into the problem, description of the structure of the wall of the healthy aorta, its main components and the degenerative changes which lead to formation of AAA. This is followed by a brief excursion into constitutive modeling, which focuses closely on the description of the models used to describe the mechanical behavior of soft tissues. The theoretical part is then supplemented by a narrower selection of constitutive models used for modeling aortic wall and intraluminal thrombus, together with published results, which are reviewed and discussed at the end of this section. The main part of this thesis is devoted to tests of mechanical properties of arterial tissues. First, the methodology is presented together with the description of the customizations of the laboratory equipments together with the test rig. In addition, attention is focused on the results of mechanical tests of intraluminal thrombus, where the results of both uniaxial tensile tests and equbiaxial testing are presented. Also the influence of distance ILT from the lumen on the mechanical properties of the thrombus is examined. Another area of interest is the investigation of the effect of elastase on the chnage of mechanical properties of pig aorta. In this case, porcine aortas are experimentally tested only by biaxial testing, and the time of elastase action to alter the mechanical properties is analyzed so that the resulting tissue has a similar stress-strain response as aneurysmal tissue. Finally, the results of experimental measurements, limitations and other possible ways of research are summarized.

Over the last few decades, rechargeable lithium-ion batteries have been extensively used in portable instruments due to their high energy density and low self-discharge rate. Recently, lithium-ion batteries have emerged as the most promising candidate for electric vehicles and stationary energy storage. However, the maximum energy that lithium-ion batteries can store decreases as they are used because of various irreversible degradation mechanisms. Lithium-ion batteries are complex systems to understand, and various processes and their interactions are responsible for the degradation over time. The mechanical integrity and stability of the electrode layers inside the battery highly influence the battery performance, which makes it a necessity to characterize the mechanical behavior of electrode active layers for mesoscopic and macroscopic level modeling. In papers 1 and 2, the macroscopic mechanical behavior of active layers in the electrodes is investigated using U-shape bending tests. The active layers are porous and a different tensile and compressive behavior is captured by performing tests on single side coated dry specimens. The experiments reveal that the active layer is stiffer in compression as compared to tension. The compressive stiffness increases with bending strain whereas the tensile stiffness is almost independent of the bending strain. A very low value of modulus of the active layer (1-5 GPa) is measured in comparison to the metal foils (70-110 GPa) and the active particles (50-200 GPa) which shows that the electrode properties are governed majorly by the binders present in the active layers.  The time-dependent and hysteresis effects are also captured by the method which circumvents the flaws of many other test methods presented in the literature.   In paper 3, we present a multiscale homogenization method that couples mechanics and electrochemistry at the particle, electrode, and battery scales. The active materials of lithium-ion battery electrodes exhibit volume change during lithium intercalation or deintercalation. A lithium concentration gradient develops inside particles, as well as inside the active layer. The developed stress due to deformations further affects solid diffusion.  We utilized models that have already been developed to couple particle and electrode layer levels. The mechanical coupling between the electrode and the battery level is achieved by homogenization of the layered battery using three-dimensional laminate theory.  By application of the model, we demonstrate that the stresses on all considered scales can be predicted from the homogenized model. It is furthermore demonstrated that the effects of external battery loadings like battery stacks, casings, and external pressure can be captured by the model. The model can also capture the effect of various electrochemical cycling rates and mechanical parameters like layer thicknesses, stiffnesses, and swelling properties. The presented multi-scale model is fast, accurate and the efficiency of the method is demonstrated by comparisons to detailed finite element computations where each layer is individually modeled.

[pt] Desenvolveu-se estudo teórico e numérico para simular o comportamento tensão-deformação de solos e rochas. Procurou-se estabelecer modelagem constitutiva apta a representar as peculiaridades inerentes ao comportamento destes materiais sob grandes deformações e degradação da estrutura. Dentro do contexto geotécnico brasileiro, o objetivo da pesquisa foi, também, investigar uma nova abordagem constitutiva para modelar o comportamento de solos moles, solos residuais e rochas sedimentares. O trabalho está dividido nos seguintes tópicos: revisão bibliográfica; estudo de medidas de deformações e taxas objetivas de tensões; definição e desenvolvimento dos modelos constitutivos a serem testados; definição dos algoritmos de retorno para integração das equações constitutivas; implementação em elementos finitos; simulação do comportamento observado em ensaios de laboratório. Os resultados da pesquisa indicam que o comportamento viscoso da argila mole do Sarapuí pode ser reproduzido corretamente adotando-se modelo constitutivo viscoplástico. A abordagem de solo estruturado está condizente com o comportamento do arenito de Vila Velha. Por fim, para modelar o comportamento de solos residuais dentro de um novo quadro constitutivo, foi proposta uma separação das deformações irreversíveis. / [en] A theoretical and numerical study was developed to simulate the stressstrain behavior of soils and rocks, formulating constitutive models able to catch the peculiarities inherent to the behavior of these materials under large strains and structure degradation. Within the Brazilian geotechnical context, the objective of the research was also to investigate constitutive approaches to model the behavior of soft soils, residual soils and sedimentary rocks. The work is divided into the following topics: literature review; study of strain measurements and objective stress rates; definition and development of the constitutive models to be tested; definition of the return mapping algorithms for integrating the constitutive equations; finite element implementation; and simulation of the behavior observed in laboratory tests. Results indicate that the viscous behavior of the Sarapuí soft clay can be correctly reproduced by adopting a viscoplastic constitutive model. The structured soil approach appears to be consistent with the behavior of Vila Velha sandstone. Finally, a decomposition of irreversible strains was proposed to model the behavior of residual soils within a novel constitutive framework.

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<p>Controllable continuous feeding of biomass feedstock in a biorefinery is critical to upscaling current ethanol conversion techniques to a commercial scale. Mechanical pretreatment of biomass feedstock performed using a compression feed screw (CFS) improves the ethanol yield but is subject to flowability issues, especially the plugging of biomass. The mechanical behavior, and hence, the flowability of biomass feedstock, is strongly affected by several factors, including preparation method, moisture content, physical composition, and particle size distribution. In addition, the current design of CFS is guided by limited experimentation and even fewer theoretical correlations. This thesis aims at developing computational methods to model the flow of densified feedstock in a CFS and experimental techniques to characterize the mechanical properties required for the model. We adopted a modified Drucker-Prager Cap constitutive (mDPC) law for milled corn stover (a widely used feedstock for bioethanol production) to model the material’s rate-independent bulk behavior in a CFS. The mDPC elastoplastic law captures the frictional shear and permanent volumetric changes in corn stover using a continuous porosity-dependent yield surface. The parameters of the mDPC model are calibrated using a unified set of single-ended die compaction and multiple shear failure tests. In addition, we quantified the changes in the mDPC parameters with moisture content up to the water-holding capacity of corn stover particles. A Coupled Eulerian-Lagrangian Finite Element Method model developed for the CFS geometry predicts the deformation of the material using the calibrated mDPC parameters. We model the interaction between the material and the CFS surface using a Coulomb wall friction coefficient calibrated using the Janssen-Walker method for a punch and die system. A laboratory-scale compression feed screw is designed and fabricated to characterize the flow of dense granular materials in collaboration with undergraduate students in the School of Mechanical Engineering. FEM model predictions of feeding torque and mass flow rate are validated against the laboratory-scale feeder for microcrystalline cellulose Avicel PH-200 and milled corn stover. The model predictions agree with the experiments for Avicel PH-200 but have a higher error in the case of corn stover. Some physical effects, such as shear hardening and particle erosion observed in milled corn stover, are not captured using the current implementation of the mDPC model, which explains the different model accuracies for both materials. The continuum model is used to uncover material density distribution, torque, and pressure inside the CFS, otherwise challenging through experiments. The FEM model showed a significantly higher sensitivity of the feeder performance to two material properties, namely the hydrostatic yield stress and the wall friction coefficient. The characterized variation of material properties with moisture content and the effect of each material property on the feeder performance provide strategies to engineer the feedstock for better flowability. Further, the continuum model offers a method to study design changes before manufacturing the equipment. Finally, we propose the possibility of a reduced-order analytical model based on the critical material properties and the material deformation mechanism demonstrated by the FEM model.</p>

The apoptosis and dysfunction of vascular smooth muscle cells in the human descending thoracic aorta is often associated with cardiovascular diseases like aortic dissection and aneurysms. Knowledge of the mechanical effects of contractile smooth muscle cells plays a crucial role in the understanding these potentially lethal conditions. Located in the medial layer, vascular smooth muscle cells are arranged in the so-called herringbone pattern. In regards to the mechanics of the aorta, the consequences of this type of anisotropy have not been fully discussed in literature so far. In this end, a novel hyperelastic constitutive law is proposed which accounts for the dispersive, transmural alignment of vascular smooth muscle cells and their characteristic length-tension behaviour. The model is calibrated with experimental data and is applied to the simulation of an aortic ring under in vivo conditions. By approximating the geometry of the aorta as a layered, thick-walled cylinder, the corresponding quasistatic, mechanical boundary value problem is solved semi-analytically. It is shown that the herringbone pattern induces shear deformation and equalises the normal stress gradients in the aortic wall. Since arterial vessels are able to actively adapt and alter the alignment and activity of smooth muscle cells, the existence of the herringbone pattern is in accordance with Fung's principle of optimal operation.