<p> The spectra of depolarized light scattered from Isotropic and anisotropic liquids were Investigated In order to obtain information on both the reorientational and collisional motions of the liquid molecules. </p> <p> The liquid spectra taken at 22°C on a Coderg spectrometer (1 cm. ^-1 slits) were decomposed into relaxational and collisional components using least squares fitting techniques. Integrated Intensities and depolarization ratios in the zero em -1 (1 cm ^-1= 3 x 10^10 hz.) frequency sjift region as well as in the less than 5 cm^-1 region are reported. </p> <p> The Isotropic liquid spectra from 5 cm ^-1 consisted of a narrow Lorentz and a broader coliisional component which was exponential at shifts > 10 cm^-1. The anisotropic spectra also consisted of relaxational and collisional components; however, the Integrated Intensity of the relaxational and the collisional components Increased by a factor of 20 and 10 respectively from the isotropic liquids. </p> <p> A simple model based on frame distortion Induced anisotropy was constructed to predict the relative Intensity ratios of the collisional component for both Isotropic and anisotropic liquids. </p> <p> The relaxation time of the Lorentz component was Interpreted for both isotropic and anisotropic liquids as being the average time between collisions. It was, found to vary as μ^((3/4)/p), where μ is the reduced mass of two colliding molecules and p the liquid density.</p> <p> The line width parameter 1/vo for all the collisional components was interpreted as being a measure of the duration of a collision and was found to vary as μ^1/2 independent of the density P. </p> / Thesis / Master of Science (MSc)

Apresentaremos uma série de estudos realizados sobre o tema anisotropias magnéticas induzidas (Kind) em 1igas amorfas. Foram usados vários tipos de tratamentos térmico, com o objetivo de obtermos uma visão amp1a do assunto. A análise teórica dos dados de Kind e de \"after-effect\" magnético (MAE) foi feita com um modelo baseado em sistemas de dois níveis (TLS). A partir da análise dos dados experimentais obtém-se um largo espectro de energias de ativação. Estas energias estão relacionadas aos tempos de re1axação, através da equação de Arrhenius: = 0 exp (E/kT), onde o pré-fator 0 é da ordem do inverso da frequência de Debye. Construiu-se um forno para tratamentos térmicos em ligas amorfas, que opera em um eletroimã de 6 kOe e desenvolveu-se os programas para análise de dados experimentais. Essas facilidades, associadas ao traçador de curvas de histerese, permitiram os seguintes estudos de anisotropias induzidas em ligas amorfas ferromagnéticas: 1. Estudo da cinética de indução de anisotropia por tratamentos térmicos na faixa de temperatura de 190 a 250 °C, para fitas amorfas \"as cast\" de composição C070.4Fe4.6 Si15B10. Com o modelo TLS pode-se obter o espectro de energias de ativação, que se apresentou na faixa de energia de 1.50 a 1.85eV, com 0 = 1.6 x 10-13s. 2. Realizou-se também o estudo da indução de anisotropia na presença de um campo magnético de 5 kOe, para amostras pré-tratadas a 400 °C por 10 minutos. Estas são de composição Co77-X Mnx Si14 B9, com X = 2 e 6 e foram submetidas a tratamentos isotérmicos a temperaturas entre 240 e 325 °C. Neste caso pode-se verificar que para as duas composiQ6es 0 pré-fator é maior ( 10-8S). Quanto ao espectro de energias de ativação, em ambos os casos, apresenta-se na faixa de energias de 1.10 a 1.55 eV. 3.Outro experimento, neste caso com amostras de composição Co67Fe4Mo1Si12B16 consistiu na aplicação, à temperatura ambiente, de tensão mecânica ( 800 MPa) na fita, enquanto se monitorava a energia de anisotropia magnética. Pudemos então observar uma variação continua dessa propriedade e posteriormente uma recuperação completa da condição inicial, com a remoção da tensão aplicada, mostrando um processo de caráter anelástico. Procuramos estudar os efeitos de tensões mecânicas sobre as propriedades magnéticas de amostras com composição Co67Fe4Mo1Si12B16. Duas linhas de trabalho foram adotadas: 4) Numa olhou-se para a isotropia induzida em amostras pré-tratadas, quando submetidas a tratamentos térmicos na faixa de temperatura de 200 a 400 °C, sob tensão mecânica de 500 MPa e posteriormente na ausência de tensão. A partir desses ensaios foi possível se separar uma componente plástica e outra anelástica na anisotropia induzida. 5) Noutra, obteve-se o comportamento do MAE, para amostras com e sem tensão mecânica aplicada, no intervalo de 300 a 500 K. Os resultados obtidos demonstram a não existência de efeitos plásticos ou anelásticos nas energias de ativação dos processos envolvidos no \"after-effect\". Por outro lado verificamos uma grande alteração na intensidade do MAE, devido à tensão. / In this thesis we will present a series of studies related to induced magnetic anisotropies (Kind) in amorphous alloys. In order to get a more general view of this theme, we used several different kinds of annealings. The theoretical analysis of the data of Kind and magnetic after-effect (MAE) was performed using a model based on two-level systems (TLS). From the analysis of the experimental data we get a large actiyation energy spectrum. These energies are related to the relaxation times, through the Arrheniu \'s expression: = 0 exp(E/kT), where the pre-factor 0 is of the order of the inverse of the Debye frequency . We constructed a furnace for thermal annealing of the amorphous alloys, which operates within an electromagnet producing 6 kOe. We also wrote, the computer programs for the analysis of the experimental data. These facilities, together with the hysteresis loop tracer permitted the followings studies of the induced anisotropies in amorphous alloys: 1) A study of the kinetics of the induced anisotropy by annealing in the temperature range from 190 to 250 °C, in as cast amorphous ribbons of composition Co70.4Fe4.6Si15B10. Using the TLS model we obtained the activation energy spectrum. It presents two peaks in the energy range from 1.50 to 1.85eV and a pre-factor 0= 1.6x10-13 s. 2) Using a 5 KOe magnetic field we studied the effects of a Field annealing treatment in samples pre-annealed at 400 °C for 10 minutes. The isothermal annealings were made in Co70-XMnXSi14B9 with x = 2 and 6, in the temperature range from 240 to 325 °C. In this case we observed for these two compositions a larger pre-factor ( 10-8s) than before. The activation-energy spectra, for the both composition, are found in the energy range from 1.10 to 1.55eV. 3) Another experiment was done using samples of Co67Fe4Mo1Si12B16. We applied a tensile stress (800MPa) to the ribbon and measured the magnetic anisotropy energy. We observed a continuous variation of this energy and, after removal of the stress, the sample recuperated its initial condition, showing a process characteristically anelastic. We studied the effects of mechanic stress on the magnetic properties of samples of composition Co67Fe4Mo1Si12B16. We worked in two directions: 4) We studied the induced anisotropy in pre-annealed samples, submitted to annealing in the range from 200 to 400 °C, under a tensile stress of 500MPa and without applied stress. From these resul ts we can separate a plastic and an anelastic component in the induced anisotropy. 5) In other, we studied the behavior of the MAE, for samples with and with and without applied tensile stress, in the range from 300 to 500 K. The results obtained show neither plastic nor anelastic effects on the activation energies of the processes involved in the MAE. On the other h~nd we can see a strong alteration in the intensity of the MAE, due the stress.

La thèse résulte d'une collaboration entre le laboratoire académique G2Elab et les entreprises Aperam Amilly et Aperam Imphy.Les matériaux magnétiques nanocristallins de type Finemet sont constitués d'une phase nanocristalline et d'une phase amorphe. Cette structure singulière leur confère des anisotropies magnéto-cristalline et magnéto-élastique évanescentes. On peut alors induire, par le biais de recuits adaptés, une anisotropie contrôlée conditionnant la forme du cycle d’Hystérésis et la perméabilité. D’un point de vue applicatif, il s’agit d’une aptitude capitale puisque les caractéristiques du circuit magnétique peuvent être adaptées pour répondre à des cahiers des charges spécifiques. La mise au point des protocoles de recuit mis en œuvre industriellement est cependant empirique.Le sujet de la thèse porte donc sur la mise au point d'un modèle capable de prédire l'amplitude K_u de l'anisotropie induite sur ces matériaux en fonction des paramètres du recuit sous champ (température T_re, champ appliqué H_re) et des caractéristiques structurales (fraction cristalline f_c, taille moyenne D des nanograins, composition y de la phase cristalline Fe1-ySiy). / This thesis is the result of a collaboration between Grenoble Electrical Engineering laboratory, Aperam Alloys Amilly and Aperam Alloys Imphy manufactories.The magnetic materials nanocrystalline Finemet are made of Fe-Si nanocrystallites embedded in a residual amorphous phase. This unconventional crystallographic structure features vanishing magnetocristalline and magnetoelastic anisotropies. As a result, it is possible to induce a cohenrent magnetic anisotropy in such material by suitable annealing treatments, allowing to control the shape of the hysteretic loop and permeability. In view of applications in electronic devices, this attract a great interest, the magnetic circuit characteristics being could be easily adapted by this way to satisfy the requirement of the regarded sensor or actuator.However, the optimization of annealing parameters (temperature, duration, amplitude of applied field…) to fit the desired properties is focused on time and resources consuming, which are based on empirical experience at present.As a result, the aim of this work is to build a model which will be able to predict the magnitude of induced anisotropy according to the field annealing parameters and the structural ones (crystalline fraction f_c, size of nanograins D, and composition of FeSi phase).

Apresentaremos uma série de estudos realizados sobre o tema anisotropias magnéticas induzidas (Kind) em 1igas amorfas. Foram usados vários tipos de tratamentos térmico, com o objetivo de obtermos uma visão amp1a do assunto. A análise teórica dos dados de Kind e de \"after-effect\" magnético (MAE) foi feita com um modelo baseado em sistemas de dois níveis (TLS). A partir da análise dos dados experimentais obtém-se um largo espectro de energias de ativação. Estas energias estão relacionadas aos tempos de re1axação, através da equação de Arrhenius: = 0 exp (E/kT), onde o pré-fator 0 é da ordem do inverso da frequência de Debye. Construiu-se um forno para tratamentos térmicos em ligas amorfas, que opera em um eletroimã de 6 kOe e desenvolveu-se os programas para análise de dados experimentais. Essas facilidades, associadas ao traçador de curvas de histerese, permitiram os seguintes estudos de anisotropias induzidas em ligas amorfas ferromagnéticas: 1. Estudo da cinética de indução de anisotropia por tratamentos térmicos na faixa de temperatura de 190 a 250 °C, para fitas amorfas \"as cast\" de composição C070.4Fe4.6 Si15B10. Com o modelo TLS pode-se obter o espectro de energias de ativação, que se apresentou na faixa de energia de 1.50 a 1.85eV, com 0 = 1.6 x 10-13s. 2. Realizou-se também o estudo da indução de anisotropia na presença de um campo magnético de 5 kOe, para amostras pré-tratadas a 400 °C por 10 minutos. Estas são de composição Co77-X Mnx Si14 B9, com X = 2 e 6 e foram submetidas a tratamentos isotérmicos a temperaturas entre 240 e 325 °C. Neste caso pode-se verificar que para as duas composiQ6es 0 pré-fator é maior ( 10-8S). Quanto ao espectro de energias de ativação, em ambos os casos, apresenta-se na faixa de energias de 1.10 a 1.55 eV. 3.Outro experimento, neste caso com amostras de composição Co67Fe4Mo1Si12B16 consistiu na aplicação, à temperatura ambiente, de tensão mecânica ( 800 MPa) na fita, enquanto se monitorava a energia de anisotropia magnética. Pudemos então observar uma variação continua dessa propriedade e posteriormente uma recuperação completa da condição inicial, com a remoção da tensão aplicada, mostrando um processo de caráter anelástico. Procuramos estudar os efeitos de tensões mecânicas sobre as propriedades magnéticas de amostras com composição Co67Fe4Mo1Si12B16. Duas linhas de trabalho foram adotadas: 4) Numa olhou-se para a isotropia induzida em amostras pré-tratadas, quando submetidas a tratamentos térmicos na faixa de temperatura de 200 a 400 °C, sob tensão mecânica de 500 MPa e posteriormente na ausência de tensão. A partir desses ensaios foi possível se separar uma componente plástica e outra anelástica na anisotropia induzida. 5) Noutra, obteve-se o comportamento do MAE, para amostras com e sem tensão mecânica aplicada, no intervalo de 300 a 500 K. Os resultados obtidos demonstram a não existência de efeitos plásticos ou anelásticos nas energias de ativação dos processos envolvidos no \"after-effect\". Por outro lado verificamos uma grande alteração na intensidade do MAE, devido à tensão. / In this thesis we will present a series of studies related to induced magnetic anisotropies (Kind) in amorphous alloys. In order to get a more general view of this theme, we used several different kinds of annealings. The theoretical analysis of the data of Kind and magnetic after-effect (MAE) was performed using a model based on two-level systems (TLS). From the analysis of the experimental data we get a large actiyation energy spectrum. These energies are related to the relaxation times, through the Arrheniu \'s expression: = 0 exp(E/kT), where the pre-factor 0 is of the order of the inverse of the Debye frequency . We constructed a furnace for thermal annealing of the amorphous alloys, which operates within an electromagnet producing 6 kOe. We also wrote, the computer programs for the analysis of the experimental data. These facilities, together with the hysteresis loop tracer permitted the followings studies of the induced anisotropies in amorphous alloys: 1) A study of the kinetics of the induced anisotropy by annealing in the temperature range from 190 to 250 °C, in as cast amorphous ribbons of composition Co70.4Fe4.6Si15B10. Using the TLS model we obtained the activation energy spectrum. It presents two peaks in the energy range from 1.50 to 1.85eV and a pre-factor 0= 1.6x10-13 s. 2) Using a 5 KOe magnetic field we studied the effects of a Field annealing treatment in samples pre-annealed at 400 °C for 10 minutes. The isothermal annealings were made in Co70-XMnXSi14B9 with x = 2 and 6, in the temperature range from 240 to 325 °C. In this case we observed for these two compositions a larger pre-factor ( 10-8s) than before. The activation-energy spectra, for the both composition, are found in the energy range from 1.10 to 1.55eV. 3) Another experiment was done using samples of Co67Fe4Mo1Si12B16. We applied a tensile stress (800MPa) to the ribbon and measured the magnetic anisotropy energy. We observed a continuous variation of this energy and, after removal of the stress, the sample recuperated its initial condition, showing a process characteristically anelastic. We studied the effects of mechanic stress on the magnetic properties of samples of composition Co67Fe4Mo1Si12B16. We worked in two directions: 4) We studied the induced anisotropy in pre-annealed samples, submitted to annealing in the range from 200 to 400 °C, under a tensile stress of 500MPa and without applied stress. From these resul ts we can separate a plastic and an anelastic component in the induced anisotropy. 5) In other, we studied the behavior of the MAE, for samples with and with and without applied tensile stress, in the range from 300 to 500 K. The results obtained show neither plastic nor anelastic effects on the activation energies of the processes involved in the MAE. On the other h~nd we can see a strong alteration in the intensity of the MAE, due the stress.

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

La mayoría de las estructuras de hormigón armado se someten a solicitaciones combinadas de esfuerzos axiles, flexión, cortante y torsión. La fisuración del hormigón, plastificación de las armaduras y otros efectos no-lineales hacen que las secciones transversales de estos elementos presenten un comportamiento anisótropo que deriva en el acoplamiento de los esfuerzos normales y tangenciales. Es decir, esfuerzos normales o momentos flectores pueden producir deformaciones de corte y vice versa. Aunque en algunas ocaciones, esta interacción es considerada de forma simplificada en el dimensionamiento de estructuras, hasta el momento no se ha realizado un análisis profundo de los efectos acoplados en secciones de forma arbitraria bajo cargas 3D generales utilizando modelos de fibras.El objetivo principal de esta tesis es generalizar el análisis de secciones de hormigón armado mediante fibras, de forma que se pueda reproducir la res-puesta no-lineal acoplada frente a esfuerzos normales y tangenciales bajo solicitaciones tridimensionales generales. De igual forma, se pretende obtener, para los esfuerzos cortantes y torsión, la misma capacidad de representación de geometrías y combinación de materiales que ofrecen los modelos de fibras para esfuerzos de flexo-compresión.La primera problemática estriba en representar adecuadamente la cinemática de la sección transversal. Con la excepción de las deformaciones normales contenidas en el plano de la sección, no existe una teoría cinemática que a priori pueda dar la distribución del resto de deformaciones o tensiones en la sección, sin dejar de satisfacer las condiciones de equilibrio interno o continuidad entre las fibras que componen la misma.Por otra parte, para materiales anisótropos, como el hormigón fisurado, en general todos los esfuerzos internos pueden estar acoplados. Además, es preciso considerar la distorsión de la sección transversal para satisfacer el equilibrio entre fibras.El problema se aborda de forma general, considerando una sección de forma y materiales cualesquiera. Se parte del problema diferencial de equilibrio de un sólido con el que se ha podido deducir un sistema de equilibrio entre fibras (equilibrio a nivel sección). Se puede demostrar que éste es complementario al problema estándar de vigas. El sistema complementario permite recuperar información tridimensional que normalmente se pierde al resolver un problema de vigas.Posteriormente, se propone una solución interna del problema complementario, en la que el alabeo y la distorsión de la sección quedan expresados como una función de las deformaciones generalizadas de una viga: deformaciones axil y cortantes, curvaturas de flexión y torsión. No son necesarios grados de libertad adicionales a nivel estructura ni hipótesis a-priori sobre la forma de los campos de deformación o tensión interna.A partir de la formulación teórica, se desarrolla un modelo de elementos finitos plano de la sección transversal. El modelo está preparado para servir como respuesta constitutiva de cualquier tipo de elemento viga en sus puntos de integración. %Se evita así la necesidad de realizar un modelo de elementos sólidos de toda la barra para estudiar la respuesta frente a una combinación general de esfuerzos normales y tangenciales.Se implementan una serie de modelos constitutivos para distintos materiales. En particular, se implementa un modelo constitutivo triaxial para hormigón fisurado, considerando la anisotropía inducida por la fisuración e incluyendo la superficie de rotura según un criterio multiaxial.La formulación seccional es validada mediante varios casos de estudio teóricos y experimentales. La respuesta no-lineal acoplada bajo diversas combinaciones de esfuerzos normales y tangenciales es reproducida con precisión, lo cual queda patente tanto en las curvas esfuerzo-deformación obtenidas como en las matrices de rigidez seccionales.Finalmente, se recopilan las conclusiones derivadas de la presente investigación y seofren recomendaciones para futuros trabajos. / Most RC structures are subjected to combined normal and tangential forces, such as bending, axial load, shear and torsion. Concrete cracking, steel yielding and other material nonlinearities produce an anisotropic sectional response that results in a coupling between the effects of normal and shear forces, i.e. normal force or bending moments may produce shear strains and vice versa. Although this interaction is sometimes taken into account, in a simplified manner, in the design of RC structures, a deep analysis of the coupling effects of RC sections using fiber models has not yet been made for arbitrary shape sections under general 3D loading.The main objective of this thesis is to generalize the fiber-like sectional analysis of reinforced concrete elements, to make it capable of considering the coupled non-linear response under tangential and normal internal forces, from a general 3D loading.Similarly, it is desired to obtain, for torque and shear forces, the same capacity and versatility in reproducing complex geometry and materials combination that fiber-like sectional representations offers for bending and stretching.The first problematic lies in finding a proper representation of the section's kinematics under such general loading. Except for in-plane normal strains, there is no single kinematical theory capable of a-priori representing the correct distribution of the others strains or stresses satisfying, at the same time, inter-fiber equilibrium and continuity. On the other hand, for rather anisotropic materials, such as cracked concrete, all internal forces are, in general, coupled. It is also required that distortion is allowed for the section's kinematics in order to guarantee satisfaction of internal equilibrium.The problem is dealt in a general form considering arbitrary shaped sections and any material behaviour. Starting from the differential equilibrium of a solid, an inter-fiber equilibrium system (equilibrium at the sectional level) was deduced. This system, which is complementary to the standard equilibrium problem of a beam-column, allows to recuperate information of the three-dimensional problem that is generally lost when solving a beam problem.Further, a solution of the equilibrium at the sectional level is proposed in which the section's warping and distortion are posed as a function of the generalized beam-column strains (axial and shear strains, bending and torsion curvatures). No additional degrees of freedom are required at the structural level nor a-priori hypotheses on the distribution of the internal strains or stresses.After the theoretical formulation, a planar finite element model for cross-sectional analysis is developed. The model can be used as a constitutive law for general beam column elements at their integration points.A series of constitutive models have been implemented for several materials. In particular, a triaxial constitutive model for cracked concrete is implemented considering crackinduced anisotropy and a multiaxial failure criterion.The sectional formulation is validated by means of various theoretical and experimental case studies. Non-linear coupled response under normal and tangential internal forces is reproduced with accuracy, as can be seen both in the predicted internal force-strain curves and in the sectional stiffness matrixes.Finally, the conclusions drawn from the current research are summarized andrecomendations for future works are given.

Ce travail de thèse a pour objectif de caractériser le comportement anisotrope des mousses acoustiques. Ces matériaux, couramment employés pour lutter contre les nuisances sonores et vibratoireset sont modélisés à l'aide du modèle de Biot. Celui-ci est basé sur le formalisme de lamécanique des milieux continus à deux champs couplés, l'un associé au solide et l'autre au fluidesaturant. Nous nous intéresserons plus particulièrement dans ce travail aux paramètres du solideet aux matériaux présentant une anisotropie (c'est-à-dire des propriétés qui varient suivant lesdirections) du squelette solide. Ici, deux types d'anisotropie sont distingués, l'anisotropie naturelledu matériau et celle induite par une action extérieure, cette dernière ayant pour principale cause lacompression statique des échantillons. Par ailleurs, trois types de symétries naturelles sont considérés : isotropie, isotropie transverse avec et sans rotation de direction principale. Celles-ci sont leplus couramment rencontrées.L'analyse expérimentale du type de symétrie des mousses se fait au moyen d'un dispositif, appelé rigidimètre, qui permet de déterminer la raideur mécanique d'échantillons cubiques de moussesous hypothèse quasi-statique. Celui-ci est couplé à une mesure au vibromètre laser à balayage, permettantde mesurer le déplacement normal des faces des cubes. Des lignes de niveaux des champsde déplacements normaux surfaciques sont ainsi obtenues. Il est alors possible de classer les différentstypes d'anisotropie en analysant ces courbes de niveaux. Ainsi, avec ces a-priori, une méthodea été élaborée pour déterminer les coefficients de Poisson à l'aide de techniques de minimisationà partir des autres constantes élastiques préalablement déterminées. Ce problème est construit àpartir d'indicateurs expérimentaux et d'indicateurs provenant d'un modèle éléments finis.L'influence de la compression statique sur les modules élastiques est ensuite étudiée. Toutd'abord, la variation du module d'Young en fonction du taux de compression est caractérisée àpartir de mesures au rigidimètre. Ensuite, la variation du module de cisaillement en fonction de lacompression statique est caractérisée par une méthode d'ondes guidées (en collaboration avec laKULeuven). Il a été montré que les variations de modules élastiques pouvaient être importantespuisqu'elles peuvent atteindre 50 %. A partir de ces déterminations expérimentales, quatre zones decomportement de la mousse ont été mises en évidence. Ces quatre zones correspondent respectivementà des effets de compression, de flambement, de densification et de réarrangement des cellules.Un modèle éléments-finis microstructural, dans lequel la cellule élémentaire est modélisée par untétrakaidécaèdre de Kelvin, est enfin proposé. Celui-ci permet de modéliser les trois premièreszones, qui correspondent aux compressions statiques usuelles dans les applications acoustiques. / This thesis deals with the characterization of the anisotropic behavior of acoustic foams. Thesematerials, which are commonly used to reduce noise and vibration, are modeled by the Biot model.This model is based on the formalism of continuum mechanics of two coupled elds, one associated to the solid, and the other one associated to the saturating fluid. This work is more particularly focused on the solid parameters and on the materials having an anisotropy (i.e. properties which vary according to directions) of the solid skeleton. Here, two types of anisotropy are distinguished,the natural anisotropy of the material, and the induced anisotropy, which is due to an external action and principally caused by a static compression samples. Furthermore, three types of natural symmetries (which are the most commonly encountered) are considered : isotropy, transverseisotropy with and without rotation of principal direction.The experimental analysis of the type of the foam symmetry is performed with the device calledrigidimeter, which allows to determine the mechanical stiness of cubic samples of foam underquasi-static assumption. This device is coupled with a laser scanning vibrometer, which measures the normal displacement of the faces of cubes. Isovalues of the displacement fields of the normal surface are obtained. According to these results, it is possible to classify the different types of anisotropy by the analysis of the isovalues of the displacement fields. Thus, with these a-priori, a method was developed to determine the Poisson's ratios with minimization techniques from theother elastic constants already determined. This problem is constructed from experimental indicators and indicators taken from a finite element model.The influence of static compression on the elastic moduli is then studied. First, the variation of Young's modulus as a function of compression ratio is characterized from the rigidimeter measurement. Then, the variation of shear modulus as a function of static compression has been characterized by a method of guided waves (in collaboration with KULeuven). It was shown that variations of elastic moduli could be important because they can reach 50 %. From these experimental determinations, four areas of behavior of the foam have been identified. These four areas correspond, respectively, to effects of compression, buckling, densification and rearrangement of cells. A microstructural finite element model, in which the unit cell is modeled by a Kelvin tetrakaidecahedron,is finally proposed. This one models successfully the first three areas, which correspond to the common static compression in the acoustic applications.

Theoretical and computational methods have been used to study nonlinear effects in the mechanical response of materials at the nano and macro scales. These methods include, acoustoelastic theory, molecular dynamics and finite element models. The nonlinear indentation response of Ni thin films of thicknesses in the nano scale was studied using molecular dynamics simulations with embedded atom method (EAM) interatomic potentials. The study included both single crystal films and films containing low angle grain boundaries perpendicular to the film surface. The simulation results for single crystal films show that as film thickness decreases, larger forces are required for similar indentation depths but the contact stress necessary to emit the first dislocation under the indenter is nearly independent of film thickness. The presence of grain boundaries in the films leads to the emission of dislocations at a lower applied stress. For a single crystal Ni thin film of a thickness of 20 nm a direct comparison of simulation and experimental results is presented, showing excellent agreement in hardness values. The effects of using different interatomic potentials and indentation rates for the simulations are also discussed. Dynamic indentation of the Ni thin film was also carried out for different frequencies. It has been found that there is a 12% increase in dislocations compared to quasi static indentation and the results are consistent with experiments. Acoustoelastic theory was used to study how nonlinear elastic properties of unidirectional graphite/epoxy (gr/ep) effect the energy flux deviation due to an applied shear stress. It was found that the quasi-transverse wave (QT) exhibits more flux deviation compared to the quasi-longitudinal (QL) or the pure transverse (PT) due to an applied shear stress. The flux shift in QT wave due to an applied shear stress is higher than that for an applied normal stress along laminate stacking direction for the same magnitude. The QT wave has energy flux deviation due to shear stress at 0o and 90o fiber orientations as compared to normal stress case where the flux deviation is zero. It was found that the energy flux shift of QT wave in gr/ep varies linearly with applied shear stress. The Finite element model of the equations of motion combined with the Newmark method in time was used to confirm the flux shift predicted by theory. / Ph. D.

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.

In an attempt to improve the quality in finite element analysis of thermoformed components, a method for predicting the thickness distribution is presented. The strain induced anisotropic material behaviour in the amorphous polymers of concern is also taken into account in the method. The method comprises of obtaining raw material data from experiments, followed by a simulation of the vacuum thermoforming process where hyperelastic material behaviour is assumed. The theory of hyperelasticity that was applied was based on the Ogden model and implemented in the FE-software LS-DYNA. Material behaviour from thermoformed prototypes is examined by experiments and implemented together with the mapped results from the thermoforming simulation in a succeeding FE-model. For the latter, the three-parameter Barlat model was suggested, giving the possibility to account for anisotropic material behaviour based on an initial plastic strain.