Tesis sobre el tema "Viscoplastic modeling"

The deformation behavior of unsaturated soil has been the subject of numerous experimental and theoretical investigations. However, this phenomenon is not fully understood. Problems, such as the adoption of the proper stress variables, reduction of suction inducing collapse, suction effect on soil stiffness, rate dependency and air trapped within the soil under rainfall infiltration still need additional studies. In the present studies, an elasto-viscoplastic model for unsaturated soil is used based on two stress variables: 1) the skeleton stress is adopted as the stress variable; 2) suction is incorporated into the constitutive model to describe the collapse behavior. In addition, to investigate the multiphase behavior of unsaturated soil, a three-phase coupled model has been proposed based on the Theory of Porous Media (TPM) and finite deformation theory. Van Genuchten type of equation is employed as a constitutive equation between the saturation and the suction. Three-dimensional multiphase simulations are carried out to reproduce the behavior of unsaturated soil during monotonic loading triaxial tests under drained and undrained conditions for water and air. Compared with experimental results and the simulated results, it is seen that the proposed formulation is very suitable to describe the mechanical behaviors of unsaturated soil. Cyclic behavior of unsaturated soil has attracted much attention during the past few years. An elasto-viscoplastic cyclic model for saturated soil is extended for modeling of unsaturated soil. Based on finite deformation theory, three-dimensional multiphase analyses for unsaturated soil under cyclic loading are presented. The simulations are verified with cyclic triaxial tests on unsaturated silty clay under undrained for water and air conditions. It shows that the proposed multiphase formulation can be used to simulate the behaviors of unsaturated soil under cyclic loading. The high expansiveness of bentonite is another significant problem in unsaturated soil mechanics. In this research, an elasto-viscoplastic model for unsaturated expansive soil has been developed. An evolutional equation is adopted for describing the absorption of water into interlayer of clay platelets. In addition, the internal compaction effect caused by swelling of clay unit is expressed with the expansion of overconsolidation boundary surface and static yield surface. Based on the model, one-dimensional finite element analysis is conducted to study the development of swelling pressure. Compared with experimental results and simulated results, it is found that the proposed model can reproduce the effects of dry density and initial water content on swelling behavior. Using the proposed swelling model, two-dimensional swelling behaviors of the waste barrier are simulated.
Kyoto University (京都大学)
0048
新制・課程博士
博士(工学)
甲第13775号
工博第2879号
新制||工||1425(附属図書館)
25991
UT51-2008-C691
京都大学大学院工学研究科社会基盤工学専攻
(主査)教授 岡 二三生, 教授 松岡 俊文, 准教授 木元 小百合
学位規則第4条第1項該当

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2003.
Includes bibliographical references (p. 251-258).
Glassy polymers, such as polystyrene (PS), poly(methyl methacrylate) (PMMA) and polycarbonate (PC), are common engineering polymers that have found uses in consumer products ranging from portable computers and optical lenses, to automotive components and appliance housings. PMMA and PS are typically considered to be brittle polymers, since they fail in a brittle manner under low triaxiality conditions, such as under uniaxial tension. Polycarbonate is considered to be a more ductile polymer than PMMA and PS, since it will deform plastically under uniaxial tension. However, PC does exhibit brittle behavior under certain loading conditions, such as low temperatures, high strain rates, or highly (tensile) triaxial stress states. A technique used for reducing the brittleness (increasing the fracture toughness) of glassy polymers is rubber-toughening. The technology of rubber-toughening, which involves blending a small volume fraction (5-20%) of rubber particles with the homopolymer, has been used commercially since the 1940s, and has been of major importance to the plastics industry. The technology of rubber-toughening is qualitatively well understood, but quantitative tools to study the material response are still at an early stage of development. The purpose of this thesis is to develop numerical tools to investigate the mechanical behavior of rubber-toughened glassy polymers, with emphasis on rubber-toughened PC. To this end, several tools are developed.
(cont.) Three-dimensional micromechanical models of the heterogeneous microstructure are developed to study the effects of particle volume fraction on the underlying elastic visco-plastic deformation mechanisms in the material, and how these mechanisms influence the macroscopic [continuum-level] response of the material. A continuum-level constitutive model is developed for the homogenized large-strain elastic-viscoplastic behavior of the material. The model is calibrated against micromechanical modeling results for rubber-toughened polycarbonate. The constitutive model is used to study boundary value problems such as notched tensile bars, where a multi-scale modeling approach enables assessment of failure due to local stress and strain levels in the material. The results are compared to experimental studies to establish correlations between the continuum-level response of the material, and observed failure mechanisms in the material.
by Mats Danielsson.
Ph.D.

Dans cette thèse certains problèmes liés au comportement thermo-viscoplastiques des alliages métalliques sont analysés. Ainsi, la réponse des métaux pour un large spectre de vitesses de déformation et en températures a été étudiée. Ceci est d'un intérêt majeur dans de nombreuses applications industrielles. L'optimisation des matériaux dans le but de supporter de plus en plus de sollicitations extrêmes est un problème très actuel. Au niveau modélisation la validation du modèle Rusineck-Klepaczko (RK) a été étendu à différents alliages sans dépendance avec la déformation plastique dans la définition du volume thermiquement activé. De plus, une extension du modèle MRK a été faite dans le but de décrire le comportement des matériaux présentant une dépendance en déformation dans la description de la sensibilité à la vitesse de déformation. Ce modèle a été utilisé pour décrire les matériaux avec une sensibilité négative à la vitesse de déformation et présentant un fort effet du drainage visqueux. En complément une loi de comportement a été développée pour décrire le comportement des matériaux avec une transformation de phase de type martensitique. Ces lois de comportement ont été implémentées dans un code éléments finis. Grâce à ces outils des études précises et fines ont été menées au niveau des instabilités thermo-viscoplastiques notamment sous chargement dynamique. Deux études particulières ont été faites, l'une sur le processus d'expansion des anneaux et une seconde sur la traction dynamique. Une étude complète a été menée sur le couplage entre la plasticité locale et le phénomène lié à la propagation des ondes élastiques générées par l'impact du chargement. En complément des essais 3D de type perforation ont été réalisés dans le but de valider les approches analytiques. Divers matériaux ont été utilisés comme : l'acier ES, un alliage d'aluminium 2024-T3, un acier AISI304 et un acier de type TRIP 1000. L'ensemble des matériaux ayant été caractérisé au préalable via des essais simples. Le but de ces études de perforation étant d'étudier leur réponse sous impact dynamique et de valider les outils analytiques et numériques
In this doctoral Thesis the thermo-viscoplastic behaviour of metallic alloys used for structural protection purposes has been analyzed. The study includes the proposition of advanced constitutive relations and their integration into numerical models. These numerical models are validated for impact problems within the low-intermediate range of impact velocities (until 85 m/s). The advanced constitutive relations derived are based on the Rusinek-Klepaczko model whose validity is extended to metallic alloys showing dependence on plastic strain on the volume thermally activated. In addition the constitutive relations developped allow describing macroscopically viscous drag effects at high strain rates, negative strain rate sensitivity and martensitic transformation phenomena. Implementation of previous constitutive relations has been conducted into the FE code ABAQUS/Explicit. Thus, development of numerical models for the simulation of ring expansion test and conventional dynamic tension test has allowed analyzing the formation of plastic instabilities. In this analysis the effects of strain rate sensitivity, strain hardening and plastic wave propagation have been considered. Finally, it has been examined the impact behaviour of metallic alloys widely used for structural protection purposes: the mild steel ES, the aluminium alloy 2024-T3, the steel AISI 304 and the steel TRIP 1000. For that goal conventional characterization tests as well as impact tests have been conducted. Numerical models based on the constitutive relations derived have been developped in order to simulate the impact tests. These numerical models offered a suitable description of the perforation process in terms of ballistic limit and the associated failure mode of the target

The prediction of motion of slowly moving landslides, also referred to as creeping slopes, is important for the reduction of landslide hazards. Such continuous and slowly moving landslides do not represent the usual stability problems of geotechnical analysis because these slopes are neither still nor ruptured but they move. For proper modeling of the motion of landslides, it is essential to develop improved techniques that integrate appropriate modeling of geological materials involved, laboratory and field tests, and verifications using computational methods. This dissertation focusses attention on the development of such an appropriate model for the time-dependent behavior of creeping landslides. Based on field observations it is proposed that the phenomenon of creeping landslides can be considered as involving the motion of a large mass of soil over a parent (fixed) mass with pronounced shear deformations occuring in a thin layer between the moving mass and the parent mass. The thin layer is refered to as interface zone while the overlying mass is refered to as solid body. The generalized Hierarchical Single Surface (HiSS) series of plasticity models are adopted to characterize the solid body. The interface zone is modeled using the specialization of the HiSS models for conditions occuring in the thin layer. Time dependency is introduced in constitutive models by adopting Perzyna's elastoviscoplastic formulation. The parameters for the HiSS and interface models are determined from laboratory tests on soils obtained from an actual slowly moving landslide at Villarbeney in Switzerland. Triaxial tests along various stress paths and oedemeter tests are conducted for the solid body. New analytical solutions are derived for prediction of oedometer tests. A general procedure for determination of viscous parameters is developed and techniques to process raw creep test data are proposed. Novel and representative simple shear interface tests are conducted to find parameters for the interface model. Special techniques for experimental analysis have been developed. A modified interface model to simulate the observed phenomenon of only compaction under shear is proposed. The parameters for the constitutive models are verified by numerically backpredicting experimental tests. An existing finite element code has been modified to incorporate various aspects of the small strain elastoviscoplastic formulation. Field measurements in the form of inclinometer profiles at various borehole locations on Villarbeney landslide are available. These inclinometer profiles are predicted using the proposed model. A comparison of the field measurements and the results from finite element analysis shows that such a model can be successfully used for predicting the behavior of slowly moving landslides.

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Bachelors
Engineering and Computer Science
Aerospace Engineering

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 37-39).
Aluminum 1235-H18 foils with sub-micron grain dimensions are often used as current collectors in Li-ion batteries. Due to their contribution to the structural integrity of batteries under impact loading, their plastic and fracture response is investigated in detail. Using a novel micro-tensile testing device with a piezoelectric actuator, dogbone specimens with a 1.25 mm wide and 5.7 mm long gage section are tested for three different in-plane material orientations and for strain rates ranging from 10-5/s to 10-2/s. It was found that the stress at a proof strain of 2% increased by about 25% from 160MPa to 200MPa within this range of strain rates. Furthermore, pronounced inplane anisotropy is observed as reflected by Lankford ratios variations from 0.2 to 1.5 .A material model is proposed which borrows elements of the anisotropic Yld2000-2d plasticity model and integrates these into a basic viscoplasticity framework that assumes the multiplicative decomposition of the equivalent stress into a strain and strain rate dependent contributions. The an isotropic fracture response is characterized for a strain rate of 10-3 /s using notched tension and Hasek punch experiments. It is found that a simple stress state independent version of the anisotropic MMC fracture initiation model provides a reasonable approximation of the observed experimental results.
by Colin Bonatti.
S.M.

O presente trabalho tem como objetivo a caracterização experimental e modelagem constitutiva do comportamento de metais CFC (Cúbicos de Face Centrada) policristalinos quando submetidos a altas taxas de deformação. O material empregado no desenvolvimento do trabalho é uma liga de alumínio comercialmente pura: o alumínio AA1050. No âmbito da presente investigação, os experimentos são conduzidos à temperatura ambiente. O desenvolvimento experimental tem por objetivo evidenciar as principais características constitutivas que descrevem o comportamento macroscópico desta classe de metais quando submetidos a processos de deformação envolvendo altas taxas de deformação: (i) o endurecimento induzido pela deformação; (ii) o endurecimento induzido pela taxa de deformação; e (iii) a sensibilidade instantânea em relação à taxa de deformação. Para a caracterização de cada uma destes aspectos constitutivos, são realizados experimentos específicos utilizando equipamentos desenvolvidos, em sua maioria, no contexto da presente investigação. De forma geral, os experimentos consistem em ensaios de compressão envolvendo uma ampla faixa de taxas de deformação, variando desde condições quasi-estáticas a taxas na ordem de 104 s−1. Os resultados experimentais, juntamente com evidências experimentais macro e microscópicas disponíveis na literatura, dão suporte ao desenvolvimento de um modelo constitutivo elasto-viscoplástico. A formulação constitutiva segue uma abordagem semi-física, na qual a escolha das variáveis inelásticas e proposição de suas regras de evolução são qualitativamente guiadas por considerações metalúrgicas baseadas no acúmulo e organização de discordâncias O modelo proposto, embora consista em uma abordagem simplificada quando comparado a modelos de base física, é capaz de representar separadamente cada uma das características constitutivas destacadas anteriormente. Com base nos resultados experimentais aqui obtidos, o modelo elasto-viscoplástico proposto é então ajustado e posteriormente validado. Na sequência é desenvolvida a formulação numérica relacionada ao modelo proposto. A abordagem como um todo é inserida em um contexto de deformações finitas seguindo uma descrição Lagrangiana Total. O desenvolvimento numérico descreve o procedimento utilizado para solução de problemas de equilíbrio não lineares seguindo uma formulação incremental implícita empregando o método dos elementos finitos. Em um contexto local, é utilizado um esquema de integração implícito seguindo um mapeamento exponencial. A linearização das equações de mapeamento de retorno possibilita a derivação analítica do módulo tangente consistente. O modelo constitutivo, bem como o procedimento numérico, são utilizados para a solução de problemas numéricos clássicos como: ensaio de compressão em condições de deformações homogêneas, e compressão envolvendo contato com atrito. As simulações numéricas avaliam tanto a capacidade constitutiva do modelo proposto em descrever o comportamento de estruturas quando deformadas sob condições envolvendo elevadas taxas de deformação, quanto à eficiência do procedimento numérico a partir de análises de convergência Em conclusão, com o procedimento experimental adotado é possível evidenciar as principais características macroscópicas inerentes ao comportamento de metais quando submetidos a processos de deformação envolvendo altas velocidades. Além disso, com base nos resultados analíticos e numéricos, observa-se que o modelo constitutivo proposto é capaz de reproduzir de forma satisfatória os comportamentos evidenciados experimentalmente.
The present work aims at performing the experimental characterization and constitutive modeling associated with the mechanical behavior of polycrystalline FCC (Face Centered Cubic) metals when subjected to high strain-rate deformations. The material to be employed in the experiments is a commercially pure aluminum alloy: aluminum AA1050. Within the present investigation context, experiments are performed at room temperatures. The primary objective of the laboratory experiments is to assess the main constitutive features associated with the macroscopic mechanical behavior observed for FCC metals subjected to high strain-rate deformation processes: (i) strain-hardening; (ii) strain-rate-hardening; and (iii) instantaneous rate-sensitivity. In order to characterize each constitutive feature, experiments using equipments specifically devised to achieve the objectives are performed. The laboratory investigation consists of compression tests involving a wide strain-rate range, from quasi-static conditions to strain-rates of the order of 104 s−1. Experimental results together with micro and macroscopic experimental evidences available in the literature give support to the development of a elastic-viscoplastic model. The stress-strain formulation follows a semi-physical approach, in which inelastic variables and their evolution equations are qualitatively motivated by metallurgical considerations based on the storage and arrangement of dislocations. Although its simplified nature when compared to physically-based models, the proposed model is capable of representing separately each one of the constitutive features highlighted early In addition, in analogy to the stress-strain proposition, a model describing the material hardness evolution in terms of strain and strain-rate histories is also provided. Based on the obtained experimental results, the proposed elastic-viscoplastic and hardness evolution models are adjusted and then validated. The corresponding stress-strain numerical formulation is developed in a subsequent step. The approach as a whole is integrated into finite strain framework following a Total Lagrangian description. The procedure employed to solve nonlinear equilibrium problem follows an implicit incremental formulation implemented in the context of the finite element method. At a local level, an implicit integration scheme based on an exponential mapping is adopted. From linearization of return mapping equations, an analytical consistent tangent modulus is obtained. Both constitutive model and numerical approach are employed to simulated classical problems: a compression test involving homogeneous deformation and a compression test involving contact and frictional conditions. Numerical simulations evaluate the constitutive capabilities associated with the proposed model when predicting the structural behavior at high strain-rate loadings. Furthermore, numerical efficiency and robustness related to the present procedure are also assessed by means of convergence analysis. While the adopted experimental procedure gave fundamental evidences of the main macroscopic features inherent in the metallic material behavior when subjected to high strain-rate deformations, the analytical and numerical results demonstrated that the proposed constitutive model is able to suitably reproduce the observed behavior.

De nombreuses manifestations macroscopiques de vieillissement dans les alliages dilués de zirconium et dans le domaine de température 20°C-600°C ont été reportés dans la littérature. Les phénomènes de vieillissement peuvent être associés à l'interaction dislocations-atomes de soluté. Toutefois, l'influence systématique des éléments d'addition et la caractérisation précise du domaine de vieillissement en termes de température et de vitesse de déformation restent toutefois mal appréhendées à ce jour .....

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 185-193).
Amorphous polymers are important engineering materials; however, their nonlinear, strongly temperature- and rate-dependent elastic-viscoplastic behavior is still not very well understood, and is modeled by existing constitutive theories with varying degrees of success. There is no generally agreed upon theory to model the large-deformation, thermo-mechanically coupled response of these materials in a temperature range which spans their glass transition temperature. Such a theory is crucial for the development of a numerical capability for the simulation and design of important polymer processing operations, and also for predicting the relationship between processing methods and the subsequent mechanical properties of polymeric products. We have developed a large-deformation thermo-mechanically coupled elastic-viscoplastic theory for thermoplastic amorphous polymers and shape memory polymers which spans their glass transition temperature. The theory has been specialized to represent the major features of the thermo-mechanical response of three technologically important thermoplastic amorphous polymers - a cyclo-olefin polymer (Zeonex-690R), polycarbonate, poly(methyl methacrylate) and a representative thermoset shape memory polymer - in a temperature range from room temperature to approximately 40 C above the glass transition temperature of each material, in a strain-rate range of ~ 10-4 to 101 s-1, and compressive true strains exceeding 100%. Our theory has been implemented in the finite element program ABAQUS. In order to validate the predictive capability of our constitutive theory, we have performed a variety of macro- and micro-scale validation experiments involving complex inhomogeneous deformations and thermal processing cycles. By comparing some key features, such as the experimentally-measured deformed shapes and the load-displacement curves from various validation experiments against corresponding results from numerical simulations, we show that our theory is capable of reasonably accurately reproducing the results obtained in the validation experiments.
by Vikas Srivastava.
Ph.D.

A nonlinear finite element model of the hot and cold rolling processes has been developed for flat rolling stock with rectangular cross section. This model can be used to analyze the flat rolling of cold and hot steel rectangular strips under a series of different parameters, providing the rolling designer with a tool that he can use to understand the behavior of the steel as it flows through the different passes. The models developed, take into account all of the non-linearities present in the rolling problem: material, geometric, boundary, and heat transfer. A coupled thermal-mechanical analysis approach is used to account for the coupling between the mechanical and thermal phenomena resulting from the pressure-dependent thermal contact resistance between the steel slab and the steel rolls. The model predicts the equivalent stress, equivalent plastic strain, maximum strain rate, equivalent total strain, slab temperature increase, increase in roll temperature, strip length increase, slab thickness % reduction (draft), and stripâ s velocity increase, for both the cold and hot rolling processes. The FE model results are an improvement over the results obtained through the classical theory of rolling. The model also demonstrates the role that contact, plastic heat generation and friction generated heat plays in the rolling process. The analysis performed shows that the steel in cold rolling can be accurately modeled using the elastic-plastic (solid Prandtl-Reuss) formulation, with a von Mises yield surface, the Praguer kinematic hardening rule, and the Ramberg-Osgood hardening material model. The FE models also demonstrate that the steel in hot rolling can be modeled using the rigid-viscoplastic (flow Levy-Mises) formulation, with a von Mises yield surface, and Shidaâ s material model for high temperature steel where the flow stress is a function of the strain, strain rate, and the temperature. Other important contributions of this work are the demonstration that in cold rolling, plane sections do not remain plane as the classic theory of rolling assumes. As a consequence, the actual displacements, velocity, and stress distributions in the workpiece are compared to and shown to be an improvement over the distributions derived from the classical theory. Finally, the stress distribution in the rolls during the cold rolling process is found, and shown to be analogous to the stress distribution of the Hertz contact problem.
Master of Science

Au cours des conflits récents, les véhicules ont été sujet à des attaques impliquant une grande quantité d'explosif enterré. En général, les planchers sont visés, et leurs déformations absorbent une partie de l'énergie. Le véhicule subit également une impulsion, transmise par la détonation, qui peut blesser gravement les occupants. L'intensité de l'impulsion dépend principalement de trois paramètres: le degré de saturation du sol, la nature du sol dans lequel l'explosif est enterré, et la profondeur d'enfouissement de la charge. Les simulations numériques doivent reproduire le processus intégral de l'explosion, incluant l'initiale interaction entre la charge et le sol, l'expansion des produits de détonation, la propagation de l'onde de choc jusqu'au plancher du véhicule, et la grande déformation des projections de sol. L'information mécanique transmise jusqu'au niveau du plancher doit être suffisamment précise car les effets sur le véhicule constituent l'objectif des simulations. Développer un modèle constitutif de sol est par conséquent délicat.%délicat. un défi. tâche difficile. Le modèle de sol est visco-plastique avec un cap écrouissable, une surface de rupture pour limiter les contraintes de cisaillement et un cut off contre les contraintes excessives de tension. La surface de charge est entièrement lisse afin que les incréments de déformation plastique soient continus. Un mécanisme de rigidification est présenté pour tenir compte de la compression de l'air et des très grandes contraintes dans l'environnement immédiat de la charge. Le taux d'humidité du sol est également pris en compte. Deux séries d'essais ont été réalisés à DGA TT pour servir de référence pour les simulations numériques. Une première campagne a mis en jeu de petites quantités d'explosif à plusieurs profondeurs d'enfouissement. Les pressions aériennes ont été mesurées à plusieurs hauteurs et comparées aux simulations. Un moyen d'essai a été utilisé lors de la seconde campagne d'essais pour mesurer à la fois la déformation d'une plaque représentant un plancher de véhicule, mais aussi l'impulsion transmise par l'explosion enfouie. Un certain nombre de taux d'humidité, profondeurs d'enfouissement, épaisseurs de plaques et gardes au sol a été étudié. Au cours des essais, le sol STANAG, composé de sable et de graviers, défini dans l'AEP 55 relatif au STANAG 4569, a été utilisé. Des simulations Eulériennes ont été réalisées et sont en accord avec les résultats d'essais. Le rôle des trois parties de la surface de charge, des mécanismes de rigidification élastique et d'écrouissage plastique a été évalué. La masse volumique initiale du sol et la position initiale du cap sont les paramètres ayant la plus grande influence sur les pressions aériennes. Quant à l'impulsion, elle est principalement contrôlée par le degré de saturation du sol
In recent conflicts, vehicles have been facing underbelly attacks involving a large quantity of buried explosive. A part of the energy is absorbed by the deformation of the belly. Still the vehicle is subjected to the impulse transmitted by the detonation which may severely injure occupants. The intensity of the impulse is highly dependent on three main parameters which are the degree of saturation of the soil, the nature of the soil in which the explosive is buried and the depth of burial of the charge. Computer simulations should follow the complete process of the explosion, including the early interaction of the charge with the soil, the expansion of the detonation products, the propagation of the shock wave up to the vehicle floor, and the large deformation of the soil projections. The mechanical information transmitted up to the floor level should be sufficiently accurate because the impact on the vehicle is the key target of the simulations. Developing a constitutive soil model is therefore a challenging task. The soil model is visco-plastic with a hardening cap surface, a failure surface to limit shear stresses and a cut off surface against excessive tension. The entire yield surface is smooth so that the plastic strain increments are continuous. A stiffening mechanism is introduced to account for air compression and accommodate the huge stresses in the immediate neighborhood of the charge. The water content of the soil is also taken into account. Two sets of experiments were carried out at DGA TT to serve as a reference for computer simulations. A first campaign involved small quantity of explosive at several depths of burial. The aerial pressures were measured at various heights and compared to simulations. The second set of experiments used a test rig to measure both the deformation of a floor simulating plate and the impulse transmitted by the buried explosion. A range of soil water contents, depths of burial of the explosive, plate thicknesses and stand off distances has been explored. For all the experiments, the sandy gravel STANAG soil defined in AEP 55 of STANAG 4569 has been used. Eulerian simulations were run and fit with these experiments. The roles of the three parts of the yield surface, of the elastic stiffening and plastic hardening mechanisms could be assessed. The initial soil density and the initial cap position were found to have the largest influence on aerial pressures. As for the impulse transmitted to the metallic plate, it was observed to be mostly controlled by the degree of saturation of the soil

A maioria dos líquidos encontrados na natureza são não newtonianos e o estudo do seu comportamento reológico tem uma importância significante em diferentes áreas da engenharia. Entre eles, existe uma classe de fluidos que exibem pequena deformação aparente quando sujeitos a um nível de tensões inferior a uma tensão limite de escoamento, referido como comportamento viscoplástico. Nesta classe de materiais, alguns apresentam também comportamento elástico quando submetidos a baixas taxas de cisalhamento. A presente Tese tem como objetivo o estudo numérico de escoamentos bidimensionais em regime permanente de fluidos elasto-viscoplásticos através de uma expansão-contração planar. O modelo mecânico é definido pelas equações de conservação de massa e de balanço de momentum acopladas ao modelo elasto-viscoplástico proposto nesta Tese. Esta modelagem é aproximada por um método de elementos finitos multi-campos estabilizado baseado na metodologia de Galerkin mínimos-quadrados que possui como variáveis primais os campos de tensão extra polimérica, velocidade e pressão. As condições de compatibilidade entre os sub-espaços de elementos finitos para tensão extra-velocidade e velocidade-pressão são violadas, permitindo assim a utilização de interpolações de igual ordem. O método estabilizado foi implementado no código de elementos finitos para fluidos não newtonianos em desenvolvimento no Laboratório de Mecânica dos Fluidos Aplicada e Computacional (LAMAC) da Universidade Federal do Rio Grande do Sul. Nesta Tese é adotada uma metodologia alternativa para a definição das zonas rígidas do escoamento como sendo a posição onde a taxa de cisalhamento é igual a um valor dado pela relação de parâmetros reológicos do fluido, especificamente a tensão limite de escoamento e a viscosidade newtoniana para baixas taxas de cisalhamento. Nas simulações numéricas realizadas, o tempo de relaxação adimensional, o número de salto, o coeficiente power-law, a vazão adimensional e a massa específica adimensional são variados de forma a avaliar de que modo influenciam na dinâmica de escoamentos elastoviscoplásticos. Os resultados obtidos estão qualitativamente de acordo com a literatura, atestando a estabilidade da formulação empregada.
Non-Newtonian fluids are the majority of liquids found in nature and the study of their rheological behavior has a significant importance on different areas of engineering. Among them, there is a class of materials that exhibits little apparent deformation when subjected to a stress level behind an yield stress, referenced as viscoplastic material. In this class of materials, some fluids also exhibit elastic behavior at low shear rates. The present work aimed to a numerical study of two-dimensional steady state laminar flows of elasto-viscoplastic fluids through a planar expansion-contraction cavity. The mechanical model was defined by the mass conservation and momentum balance equations coupled to the elasto-viscoplastic model porposed in this work. This modeling has been approximated by a stabilized multi-field finite element method based on the Galerkin least-squares methodology, having as primal variables the elastic extra-stress component, velocity and pressure fields. In this way, the compatibility conditions between the extra-stress-velocity and pressure-velocity (Babuška- Brezzi condition) finite element subspaces are violated, allowing to use equal-order finite element interpolations. The stabilized method has been implemented in the finite element code for non-Newtonian fluids under development at the Laboratory of Applied and Computational Fluid Mechanics (LAMAC) of the Federal University of Rio Grande do Sul. An alternative methodology is adopted to define the yield surface as the position where the strain rate is equal to a value given by the relation of the rheological parameters of the fluid, namely the yield stress and the viscosity at low shear rates. In the performed numerical simulations, the non-dimensional relaxation time, the jump number, the power-law coefficient, the non-dimensional flow rate, and the non-dimensional density are varied in order to evaluate their influence on the elasto-viscoplastic fluid dynamics. All results found are in qualitatively accordance with the affine literature, and attesting the good stability features of the formulation.

The first part of the thesis describes the concepts of viscoplasticity as a continuous plasticity theory highlighting different kinds of yield functions, plastic potentials and visocplastic constitutive laws.
A 2-dimensional elasto-viscoplastic finite element model for stress/stability analysis of mining excavations has been developed for use on microcomputers. An iterative explicit time stepping scheme is implemented. The program uses automatic time-step calculator based on equations giving a limit on the time step in an attempt to prevent numerical instability when common forms of isotropic yield functions and plastic potentials are used in the viscoplastic solution. When the input data are read parallel to the analysis undertaken the user can simulate compound behaviour by stopping the analysis, examining the results graphically and restarting it again and possibly implementing a certain decision in the subsequent appended input. This also imposes no limit on the number of time stations at which instantaneous changes like elements cut, elements backfilled, loads added or simply outputs are required. The program is equipped with graphical pre- and post- processors.

A maioria dos líquidos encontrados na natureza são não-Newtonianos e o estudo do seu comportamento tem uma importância significante em diferentes áreas da engenharia. Entre eles, uma larga classe de materiais que exibem pequena ou nenhuma deformação quando sujeitos a um nível de tensões inferiores a uma tensão limite de escoamento – chamado de comportamento viscoplástico. A presente Dissertação tem como objetivo o estudo numérico de escoamentos bidimensionais em regime permanente de fluidos viscoplásticos não-lineares em uma cavidade forçada. O modelo mecânico é definido pelas equações de conservação de massa e de balanço de momentum acopladas ao modelo viscoplástico recentemente introduzido por Souza Mendes e Dutra – SMD – e é aproximado por um método de elementos finitos multi-campos estabilizado baseado na metodologia de Galerkin mínimos-quadrados que possui como variáveis primais os campos de tensão-extra, velocidade e pressão. As condições de compatibilidade entre os subespaços de elementos finitos para tensão-extra-velocidade e velocidade-pressão são violadas, permitindo assim a utilização de interpolações de igual ordem. O método estabilizado foi implementado no código de elementos finitos para fluidos não-Newtonianos em desenvolvimento no Laboratório de Mecânica dos Fluidos Aplicada e Computacional (LAMAC) da UFRGS. Em diversos trabalhos encontrados na literatura, a superfície de escoamento do material é definida como a região onde o módulo da tensão-extra é igual à tensão limite de escoamento. É mostrado nesta Dissertação que esta metodologia pode conduzir à alguns erros, dado ao grande aumento experimentado pela taxa de cisalhamento em uma pequena faixa de tensões próximas à tensão limite de escoamento. Assim, foi adotada outra metodologia, definindo a superfície de escoamento como a linha onde a taxa de cisalhamento é igual a um valor dado pela relação de parâmetros reológicos do fluido, especificamente a tensão limite de escoamento e a viscosidade Newtoniana para baixas taxas de cisalhamento. Nas simulações numéricas realizadas, o número de salto, J, o coeficiente de power-law, n, e a vazão adimensional, U*, são variados de forma a avaliar de que modo influenciam na dinâmica de escoamentos viscoplásticos. Os resultados obtidos estão de acordo com a literatura e atestam a estabilidade da formulação empregada.
Non-Newtonian fluids are the majority of liquids found on the nature and the study of their behavior has a significant importance on different areas of engineering. Among them, there is a wide class of materials that exhibits little or no deformation when subjected to a stress level behind an apparent yield stress – called the viscoplastic behavior. The present thesis aimed to a numerical study of two dimensional steady state laminar flows of non-linear viscoplastic fluids in a lid-driven cavity. The mechanical model was defined by the mass conservation and momentum balance equations coupled to the recently introduced Souza Mendes and Dutra – SMD – viscoplastic model and has been approximated by a stabilized multi-field finite element method based on the Galerkin least-squares methodology, having as primal variables the extra-stress, velocity and pressure fields. In this way, the compatibility conditions between the extra-stressvelocity and pressure-velocity (Babuška-Brezzi condition) finite element subspaces are violated, allowing to use equal-order finite element interpolations. The stabilized method has been implemented in the finite element code for non-Newtonian fluids under development at the Laboratory of Applied and Computational Fluid Mechanics (LAMAC) of UFRGS. In several works found on the literature, the yield surface of the material is defined as the region where the stress modulus is equal to the yield stress. Is shown in this work that this methodology can lead to some errors, due to the large strain rate increasing in a small range of values of stress on the vicinity of the yield stress. Therefore, it was adopted another approach, defining the yield surface as the line where the strain rate is equal to a value given by the relation of the rheological parameters of the fluid, namely the yield stress and the viscosity at low shear rates. In the performed numerical simulations, the jump number, J, the the power-law coefficient, n,and the non-dimensional flow rate, U*, are ranged in order to evaluate how they the influence on the viscoplastic fluid dynamics have been investigated. All results found were in accordance with the affine literature and attests the good stability features of the formulation.

A major instrumented geosynthetic reinforced approach embankment was constructed to 5.5 m elevation above ground, with prefabricated vertical drains, over a soft compressible clay deposit at Leneghan, Newcastle, Australia in May 1995. The field monitoring of settlements for over six years shows that the embankment manifests significant creep. The instrumentation, field performance and the finite element analyses for predicting the long-term performance of this embankment are described in this thesis. The maximum settlement of 1.1 m was observed one year after the completion of construction. However, the embankment continued to settle at a rate of 0.4 mm/day for the next 5 years. The horizontal displacements of 0.09-0.14 m at various locations and the maximum reinforcement strains of 0.67% were recorded. A numerical model was developed to perform a fully coupled large deformation elasto-viscoplastic finite element analysis for this performance prediction based on creep model proposed by Kutter and Sathialingam (1992). The foundation soil was modelled with creep material behaviour using six noded linear strain triangular elements. A well-documented case history ??? Sackville embankment, New Brunswick, Canada was analysed using this model as a benchmark problem and the model was found to predict all the behaviour characteristics reasonably well. The results obtained from finite element analysis using this model are shown to be in reasonable agreement with the observed performance of Leneghans embankment in terms of settlements, horizontal displacements, excess pore pressures and geosynthetic strains. But, the prediction of settlements was less than satisfactory beyond April 1999. Finite element analyses were performed to study the sensitivity of this embankment behaviour on the variation of hydraulic conductivity values and geosynthetic reinforcement properties. This sensitivity study indicated that the kv variation, the kh/kv ratio and the nominal values of geosynthetic properties adopted in the benchmark analysis are reasonable enough for the long-term behaviour prediction.

ONDRAF est responsable, depuis 1980, de la gestion de sûreté des déchets radioactifs en Belgique. Pour garantir la sureté et la faisabilité du stockage géologique des déchets radioactifs, un laboratoire de recherche souterrain (URL) a été construit à Mol, en Belgique, dans la formation de l'argile de Boom. Pendant la construction et l’exploration, les performances de la barrière naturelle vont changer à long terme à cause de la convergence des galeries dans le temps, l'évolution de l'interaction entre l'argile de Boom et le revêtement en béton de la galerie, etc.Le présent travail de thèse vise à mieux comprendre le comportement à long terme de l'argile de Boom à l'aide d'essais de fluage oedométrique, d'essais à taux de déformation constant (CRS) et d'essais de fluage triaxial. De plus, les mécanismes microstructuraux liés au fluage et à l’effet du taux de déformation sont étudiés par la porosimétrie à intrusion de mercure (MIP), aidant à mieux comprendre le comportement mécanique.Le fluage en compression et en expansion obtenu à partir des essais oedométriques dépend fortement du niveau de contrainte et du nombre de cycles de déchargement/rechargement. D'après les paramètres de compressibilité et de fluage déterminés, on constate que le fluage en compression est accéléré sous un niveau de contrainte élevé pendant le chargement, tandis que le fluage en expansion atteint son maximum à un faible niveau de contrainte pendant le déchargement. Les résultats du MIP réalisés à différents niveaux des essais de fluage oedométrique montrent un changement de microstructure, passant d'une distribution unimodale pendant le chargement à une distribution bimodale pendant le déchargement, mettant en évidence le réarrangement des particules d'argile en structure dense et lâche, respectivement.Les tests CRS montrent que l'argile de Boom présente un comportement dépendant du taux de déformation. Ce comportement a été déduit de la relation unique entre (σ_v^',ε_v,ε ̇_v) à travers les courbes de compression à différents taux de déformation et mis en évidence par le concept d'isotach dans lequel les courbes se déplacent vers le haut avec l'augmentation du taux de déformation. Un bon accord entre les essais du CRS et les essais de fluage de l'oedomètre est obtenu.En outre, la déformation axiale (ε_1) et la déformation radiale (ε_3) mesurées lors des essais de fluage triaxiaux sont significativement affectées avec l'augmentation de la contrainte déviatrice constante appliquée (q_fluage), l'incrément du niveau de contrainte (∆SL), et la pression effective de confinement. La déformation volumétrique (ε_v) a montré une transition de comportement de la contraction à la dilatation et vice versa à des niveaux plus élevés de q_fluage. Pour l'argile de Boom, un seuil de fluage entre 20% et 40% de q_max sous une contrainte de confinement effective (σ_3^') est déterminé. Un fluage secondaire est identifié sous σ_3^'=4.5 MPa et à 90% de q_max, tandis qu’un fluage primaire pour tous les autres niveaux de q_fluage est observé. Une relation entre le taux de déformation de fluage axial (ε ̇_1) le comportement de changement de volume actuel est établie.Enfin, une nouvelle loi de comportement EVP, ACC-2 EVP, qui est une extension de l'ACC-2 EP, est formulée sur la base de la théorie NSFS et de la relation unique de taux de déformation-contrainte-déformation viscoplastique. La loi de comportement EVP proposée nécessite trois paramètres visqueux α,ε ̇_(v,ref)^vp,et p ̅_(c0,ref)^', qui peuvent être facilement déterminés à partir de la relation linéaire entre σ_y^' et ε ̇_v^vp obtenue à partir d'une série d’essai CRS. La loi de comportement ACC-2 EVP est capable de décrire divers comportements viscoplastiques, notamment les effets de taux et le fluage drainé. Un bon accord est obtenu entre les simulations et les mesures, montrant la performance de la loi de comportement développée
The Belgian National Agency for Radioactive Waste and enriched Fissile Material (ONDRAF/NIRAS) is responsible, since 1980, for the safe management of radioactive waste in Belgium. To study and to demonstrate the feasibility of geological disposal of radioactive waste, an underground research laboratory (URL) was built in the Boom Clay Formation in Mol, Belgium. During the repository construction and exploration, the performance of the natural barrier will change in the long-term, because of the convergence of galleries over time, the evolution of the interaction between Boom Clay and the gallery concrete lining, etc.The present PhD work aims at better understanding the long-term behaviour of Boom Clay through oedometer creep tests, constant rate of strain (CRS) tests, and triaxial creep tests. Moreover, the microstructural mechanisms related to creep and strain rate effects are investigated through mercury intrusion porosimetry (MIP), helping better understand the time-dependent mechanical behavior.The compressive and expansive creep obtained from oedometer tests are strongly dependent on the stress level and the number of unloading/reloading cycles. From the compressibility and creep parameters determined for Boom Clay, it is found that the compressive creep is accelerated under high stress level during loading while the expansive creep reached its maximum at low stress level during unloading. The MIP results performed at different stages of the oedometer creep tests show a microstructure change from unimodal pattern during loading to a bimodal pattern during unloading, highlighting the rearrangement of the clay particles in dense and loose structure, respectively.The CRS tests highlight the strain rate-dependent behaviour of the Boom Clay. This behaviour was deduced from the unique relationship between (σ_v^',ε_v,ε ̇_v) through the compression curves at different strain rates and evidenced by the isotach concept in which the curves move upward with the increase of strain rate. The good agreement between the CRS tests and the IL oedometer creep tests indicates the capability of CRS in determining the compressibility parameters, and also the creep parameter (secondary deformation coefficient) based on the σ_p^'-ε ̇_v relationship.In addition, the axial strain (ε_1) and the radial strain (ε_3) measured during triaxial creep tests are significantly affected by the increases of the deviator stress (q_creep), the increment of the stress level (∆SL), and the effective confining pressure. The volumetric strain (ε_v) showed a transition in behaviour from contraction to dilatancy and vice versa at higher levels of constant q_creep. From triaxial tests on Boom Clay, a creep threshold between 20% and 40% of q_max under effective confining stress (σ_3^') is determined; two creep phases are identified, a secondary creep phase under σ_3^'=4.5 MPa at 90% of q_max, and primary creep for all other q_creep levels; and a relationship between the axial creep strain rate (ε ̇_1) and the current volume change behaviour is established.Finally, a new elasto-viscoplastic (EVP) model, ACC-2 EVP, which is an extension of the ACC-2 elasto-plastic (EP), is developed based on the nonstationary flow surface (NSFS) theory and the unique stress-strain-viscoplastic strain rate concept. The proposed EVP model requires three viscous parameters α,ε ̇_(v,ref)^vp,and p ̅_(c0,ref)^', which can be easily determined from the linear relation between σ_y^' and ε ̇_v^vp obtained from a set of CRS tests. The ACC-2 EVP model is able to describe various viscoplastic behaviours, including rate effects and drained creep. Good agreement between simulations and measurements was obtained, showing the performance of the model

The flow of thickened mine tailings within a tailings storage facility is a complex interaction between unconstrained viscoplastic free surface flow and possible coarse particle settling within the flow depth. The broad focus of this work is developing a robust framework for modelling tailings beach flows. Modelling tailings flow evolution in three dimensions within a storage facility will ultimately provide greater understanding of beach slope formation, as well as the ability to optimize deposition sequencing. This thesis focuses on the first step of developing a tailings model considering the transport and settling of mono-sized coarse particles within two dimensional (length and depth) laminar viscoplastic carrier fluid sheet flow. The 2D model consists of a semi-implicit finite difference shallow water sheet flow model for predicting the viscoplastic flow depth and discharge down the beach. The coarse particle transport and hindered settling within the flow are predicted using a scalar transport model. The scalar transport and shallow water flow model are coupled together using coarse particle rheology augmentation. Two key novel advancements were made through the model development. The first is coupling the coarse particle rheology augmentation within the free surface flow to the coarse particle hindered settling behavior with depth. This coupling allows for the rheology augmentation due to the coarse solid fraction to be incorporated seamlessly into both the fluid flow solver and the particle settling model. The second advancement is expanding the rheology augmentation and hindered settling coupling to particle flows beyond the Stoke?s flow regime. Ultimately, the 2D model results are compared against Spelay?s (2007) laminar settling experimental measurements for oil sand thickened tailings (TT) and composite tailings (CT) slurries, along with Spelay?s 1D settling model. The 2D model provides improved prediction of the particle concentration profiles within the fluid flow compared to the 1D model. The 2D model is also able to predict the increase in flow depth due to the particle accumulation on the bed, as well as the downslope particle transport and settling behavior.

An increased environmental awareness, legal demands and the large part of total costs attributable to fuel cost are all incentives for the automotive industry to reduce fuel consumption. The optimal driveline to enable this reduction depends on the operational conditions and the available infrastructure. Moreover, special care is needed when developing the driveline isolators, since the demands on noise, vibration and harshness (NVH) are the same regardless of driveline. To this end, computer aided calculations can be used in order to evaluate a large number of configurations. However, these calculations are only, at best, as good as the material models employed. In the foreseeable future, rubber with reinforcing fillers will be used in vibration isolators in order to obtain the desired properties of these components. However, the stiffness and damping of rubber with reinforcing fillers are highly non-linear functions, and the available material models in commercial software and in the literature are often insufficient. Therefore, a finite strain viscoplastic material model is derived in the time domain and implemented as a user defined material model in Abaqus Explicit. The model captures the strain amplitude and frequency dependency of the storage and loss modulus for a carbon black filled natural rubber. The model is accurate over a wide range of shear strain amplitudes and frequencies, 0.2-50 % and 0.5-20 Hz, respectively, using only 5 material parameters. In addition, the model correctly captures the response from bimodal excitations. The implementation in Abaqus Explicit enables component characteristics to be evaluated early in the development phase, with material parameters derived from simple test specimens. The improved accuracy of simulations of these components can aid engineers develop more optimized solutions faster than with conventional methods.
En ökad miljömedvetenhet, juridiska krav och den stora delen av de totala kostnaderna som kan hänföras till bränslekostnader är alla incitament för fordonsindustrin att minska bränsleförbrukningen. Den optimala drivlinan för att möjliggöra denna minskning beror på driftförhållanden och den tillgängliga infrastrukturen. Dessutom ställs höga krav på utvecklingen av drivlineisolatorer, eftersom kraven på buller och vibrationer (NVH) är desamma oavsett drivlina. För detta ändamål kan datorstödda beräkningar användas för att utvärdera ett stort antal konfigurationer. Dessa beräkningar är, i bästa fall, endast så bra som de använda materialmodellerna. Inom en överskådlig framtid kommer gummi med förstärkande fyllmedel användas i vibrationsisolatorer för att erhålla de önskade egenskaperna hos dessa komponenter. Men styvheten och dämpningen i gummi med förstärkande fyllmedel är kraftigt icke-linjära funktioner, och de tillgängliga materialmodellerna i kommersiella programvaror och i litteraturen är ofta otillräckliga. Därför är en viskoplastisk materialmodell för finita deformationer framtagen i tidsdomänen och implementeras som ett användardefinierat material i Abaqus Explicit. Modellen fångar töjningsamplitud- och frekvensberoendet av lagrings- och förlustmodulen för ett kimröksfyllt naturgummi. Den är korrekt över ett brett intervall av skjuvtöjningsamplituder och frekvenser, 0.2-50% respektive 0.5-20 Hz, och kräver endast 5 materialparametrar. Dessutom fångar modeller responsen från bimodala excitationer. Implementeringen i Abaqus Explicit gör att komponentegenskaper kan utvärderas tidigt i utvecklingsfasen, med materialparametrar som härrör från enkla provkroppar. Den förbättrade noggrannheten i simuleringar av dessa komponenter kan hjälpa ingenjörer att utveckla mer optimerade lösningar snabbare än med konventionella metoder.

QC 20160406

Mise au point d'un modele thermomecanique du laminage a chaud, destine a fournir la geometrie du metal, les cartes de temperature, d'ecrouissage, de contraintes et d'endommagement qu'il a subi ainsi que les efforts et couples de laminage

L’utilisation de composites à matrice thermoplastique renforcée par fibres courtes (TRFC) connait une forte croissance pour une large gamme d’applications industrielles pour des conditions de chargement extrêmes (e.g. pare-chocs d’automobiles). Il est donc indispensable de développer des modèles de comportement des TRFC tenant compte des spécificités du matériau pour une large gamme de vitesse de déformation. Toutefois, le comportement de ces composites est complexe. Cette complexité est due, en premier lieu, au comportement viscoélastique (VE)-viscoplastique (VP) de la matrice avec une sensibilité à la pression. A cela s’ajoute les caractéristiques complexes du renfort en termes de distributions d’orientation des fibres courtes. De plus, le comportement de ces composites est affecté par des phénomènes d’endommagement coexistants (e.g. endommagement de la matrice et décohésion l’interface fibre/matrice). Dans ce travail, un modèle permettant la prise en compte de l’ensemble de ces phénomènes est proposé. Sa formulation est basée sur la décomposition du matériau en un milieu matriciel et plusieurs milieux de fibres, sur la base d’une décomposition additive du potentiel thermodynamique. Cette approche permet une implémentation simplifiée avec une résolution successive (mais non indépendante) du comportement de chaque milieu. Un avantage immédiat est la possibilité de prendre en compte tout type de comportement matriciel et tout type d’orientation. L’interface fibre/matrice, siège de la transmission de l’effort est modélisée par un transfert par cisaillement, avec sur une hypothèse locale d’iso-déformation dans la direction de la fibre. L’endommagement ductile de la matrice est pris en compte par un modèle d’endommagement anisotrope. La dégradation de l’interface fibre/matrice est décrite par un modèle de décohésion initiée en pointe de fibres. Un critère de rupture se basant sur le taux maximal de vide crée par décohésion est enfin introduit. La caractérisation du modèle est basée sur des campagnes d’essais quasi-statiques et dynamiques pour le cas de polypropylène pur et renforcé par fibres courtes de verre, à différents angles de chargement par rapport à la direction d’injection. Ces essais sont complétés par des observations au microtomographe permettant la caractérisation des distributions d’orientation locale des fibres. Des observations au MEB ont enfin permis de constater une éventuelle influence de la vitesse de sollicitation sur les mécanismes d’endommagement
Short fibre-reinforced composites are commonly used in a variety of engineering applications, including automotive and aerospace industry. Today, their use is progressively extended to parts possibly subjected to severe loading conditions (e.g. crash...), characterised by high strain rates. Therefore, an efficient modelling that takes into account material’s specificities at a large strain rate range is needed. A constitutive model of viscous behaviour of short-fibre reinforced composites (SFRC) where complex distributions of fibre orientations are taken into account is proposed in this work. The approach considered for the computation of composite macroscopic behavior is based on an additive decomposition of the state potential. The SFRC is assimilated to an assembly of several fibre media embedded in a polymeric matrix medium. One of the main assets of this approach is the possibility to model reinforcement with complex distributions of fibre orientations. Moreover, this decomposition allows the implementation of complex behaviour laws coupled with damage models. The polymeric matrix behaviour is typically strain-rate sensitive, i.e. viscoelastic-viscoplastic. This property has to be taken into account when the modelling of the composite behaviour over a large range of strain rate is intended. Therefore, a viscoelastic constitutive model, based on generalised Maxwell model, and a viscoplastic correction scheme, based on an overstress approach, are implemented for matrix material. The developed constitutive model is then coupled to two damage laws. The first one is introduced in the framework of Continuum Damage Mechanics in order to model the anisotropic ductile damage behaviour of the matrix material. The second one deals with fibre/matrix interfacial degradation through an interfacial debonding law. In order to identify the parameters involved in the present model, experimental tests are performed (case of polypropylene reinforced with short glass fibres). Microcomputed tomography is used for the characterisation of the fibres distribution of orientation. The efficiency of the proposed model is demonstrated by comparisons between numerical and experimental responses in different loading conditions, including dynamic loadings

Computational investigation of the stability of natural slopes within a coupled hydro-mechanical approach becomes more and more attractive due to an increasing number of slope movements caused by heavy rainfall events. Seasonal cycles of summer drying and winter-spring wetting affect the stability and serviceability of natural and man-made slopes, threatening lives and property worldwide. Heavy rainfall will increase the soil moisture content and lead to a reduction in suction and a consequent weakening of the soil. As a result, the slope may fail either in a diffused or in a localized pattern. The localization process is a classical mode of mechanical instability and is described as the concentration of shear strains in narrow bands within the soil strata. As it is well-known, the width of those bands cannot be properly computed using the standard continuum mechanical approach with unstable materials (in the sense of Drucker). In the framework of the standard finite element method the computation of shear band phenomena reveals a strong mesh dependency of the numerical solution and the objectivity of the computational results is blown. Each mesh refinement results in an overall different outcome and the width of the localization band has the size of the element of the adopted mesh. In the literature two main categories of methodologies can be found for regularizing this problem. One is to simulate the formation and propagation of such discontinuities via suitable enrichment functions and is the concept of the so-called extended finite elements. The other solution is to exploit enhanced continuum theories. These theories contain an internal length scale which is an additional material parameter related to the shear band width and removes the spurious mesh sensitivity of the numerical simulation results. The present work is focused on the second category. In this context, viscoplasticity and non-local theories are adopted to effectively study strain localization mechanism, assuming the soil as a multiphase porous medium. This means that the localization analysis is considered as a fully coupled hydro-mechanical problem with the material consisting of an elasto-viscoplastic skeleton and open pores filled with incompressible liquid water and compressible gas. Both regularization techniques are physically sound. Rate dependency is experimentally motivated as the mechanical response of granular materials seems to be rapid, but not instantaneous and a viscoplastic constitutive model is able to reproduce creep and relaxation processes. The physical interpretation of the non-local theory stems from the fact that no real material is an ideal continuous medium and the evolution of the microstructure at one point influences the surrounding points when irreversible strains take place. Two types of viscoplastic models are applied; the Perzyna and the Duvaut-Lions. The former is extended with respect to the non-local integral approach. In the first part of this work, the generalized effective stress is limited by the Drucker-Prager yield criterion. The models are implemented and numerically validated in the finite element code Comes-geo and further verified by simulating an experimental plane-strain biaxial test and a benchmark slope failure problem. This work is in essence aimed at answering key questions, such as: what are the most significant influential factors in the development of strain localization for each regularization technique and what is the role of suction and drainage conditions; is the viscosity of the soil affected by the presence of water and how does this fact influence the regularizing capabilities of the method; how do the internal lengths introduced by viscosity and non-locality interact with each other and under which circumstances is the one method preferable to the other? The second half of this work comprises the extension to viscoplasticity of the existing advanced elastoplastic constitutive model for unsaturated sands developed by Buscarnera and Nova. This model allows for hydraulic bonding and debonding effects and therefore the main mechanisms of unsaturated materials are captured. The elastoplastic version of the model is first implemented in the finite element code Comes-geo and validated with results from the literature. The model is then extended to viscoplasticity and is also implemented and validated in the code. The proposed viscoplastic formulation, as further novelty, is enhanced with the dependence of the constitutive parameters on the relative density. To validate the elastoplastic model in the finite element code Comes-geo a series of tests are simulated: triaxial shear tests at different suction and net confining pressure, oedometric tests with drying and wetting paths and triaxial and plane-strain compression tests in drained and undrained conditions. The viscoplastic model is also validated through oedometric tests, creep tests and triaxial compression tests on different sand densities. The onset of shear strain localization is then studied and the finite element results are compared with the results of a theoretical stability analysis. The verification of the viscoplastic model is finalized with the simulation of a triaxial compression test. The main motivation for the present work stems from these premises and this thesis presents an efficient tool to simulate strain localization with regularization techniques, which do not need to increase the number of the state variables of the numerical model, taking into account fully coupled hydro-mechanical analysis and using advanced constitutive model for unsaturated sands. This dissertation has been performed for the first two years in the University of Padova (Italy) and for the last year at Baugrund Dresden (Germany), geotechnical design and construction company within the Marie Curie Initial Training Network project MuMoLaDe (Multiscale Modelling of Landslide and Debris flow, 7th Framework Programme of the European Union, project n. 289911, http://www.mumolade.com/). This practice is in the scope of linking academia and industry and the transfer of knowledge. In this framework, the advanced elastoplastic and elasto-viscoplastic models for unsaturated soils are implemented and validated in PLAXIS commercial finite element analysis software. The validation included single element tests of drained and undrained triaxial loading and triaxial tests at different suction levels. The implementation is verified and the effectiveness of the model is displayed by numerical simulations of a partially saturated slope failure of the laboratory scale.
Lo studio numerico della stabilità dei pendii con un approccio idro-meccanico accoppiato è sempre più importante e necessario nella pratica ingegneristica a causa del continuo aumento dei fenomeni franosi, che determinano perdite di vite umane e danni all’ambiente naturale e costruito. In genere, tali fenomeni sono provocati da eventi metereologici violenti, che determinano la riduzione delle forze capillari intergranulari e della resistenza del materiale costituente i pendii. Inoltre, anche cicli stagionali di essicazione estiva e imbibizione invernale e primaverile favoriscono l’instabilità di pendii naturali e artificiali. Il collasso dei pendii avviene con un meccanismo diffuso oppure localizzato; in quest’ultimo caso avviene mediante la formazione di zone di ampiezza limitata dette bande di taglio in cui si concentrano le deformazioni del materiale. Quando si simula numericamente il collasso causato dalla formazione di bande di taglio e si utilizza la meccanica classica del continuo di Cauchy con un modello costitutivo per materiale instabile nel senso di Drucker, è noto che la larghezza delle bande di taglio non può essere calcolata né definita nel modello. In particolare, lo studio numerico di fenomeni di localizzazione delle deformazioni con il metodo degli elementi finiti evidenzia una forte dipendenza della soluzione numerica e della larghezza delle bande di taglio dalla mesh utilizzata, con perdita della soluzione all’infittimento della mesh. Infatti, accade che la larghezza delle bande di taglio risulta fissata dalla dimensione dell’elemento finito utilizzato, tendendo a zero al tendere a zero della dimensione dell’elemento finito. Di conseguenza, per poter simulare in modo obiettivo fenomeni di localizzazione delle deformazioni, è necessario modificare (o regolarizzare) il continuo di Cauchy. In letteratura si trovano due classi principali di regolarizzazione a cui ricorrere per risolvere questo problema. Il primo propone di simulare la formazione e la propagazione delle bande di taglio utilizzando campi discontinui di spostamento, dando origine al metodo degli elementi finiti estesi (extended finite element method). Il secondo propone di arricchire il modello continuo a livello cinematico oppure a livello costitutivo (enhanced continuum theories), in modo da fornire al modello una lunghezza di scala interna da cui dipende la larghezza della banda di taglio, eliminando in questo modo il problema della mesh dipendenza citata in precedenza. Questa tesi di dottorato è svolta scegliendo di arricchire il continuo di Cauchy a livello costitutivo e individua nella viscoplasticità locale e non-locale l’approccio utile allo studio della localizzazione delle deformazioni nei suoli. Inoltre, per tenere conto delle interazioni fra la parte solida e quella fluida, i suoli sono analizzati come mezzi porosi multifase. Di conseguenza, l’analisi della localizzazione delle deformazioni nei suoli è compiuta considerando il materiale dei pendii come un mezzo poroso costituito da uno scheletro solido elasto-viscoplastico e pori contenenti acqua liquida e aria umida. La scelta delle due tecniche di regolarizzazione sopra citate è stata dettata dal loro significato fisico, in quanto la risposta meccanica dei materiali granulari non è istantanea a causa dei processi viscosi o di rilassamento. Inoltre, l’introduzione della teoria non-locale deriva dalla considerazione che nessun materiale reale è un continuo nel senso matematico, a causa dell’evoluzione della microstruttura in un punto materiale che influenza i punti del suo intorno quando si sviluppano deformazioni irreversibili. In questa tesi sono utilizzati due approcci viscoplastici: quello di Perzyna e quello di Duvaut-Lions. Il primo è stato esteso con l’approccio non-locale. Nella prima parte di questo lavoro di tesi i modelli viscoplastici sono sviluppati utilizzando il criterio di snervamento di Drucker-Prager, implementati nel codice agli elementi finiti Comes-geo, validati e verificati numericamente simulando test sperimentali di compressione biassiale in stato piano di deformazione e un test numerico di collasso di un pendio. Questa parte del lavoro ha lo scopo di rispondere alle seguenti domande-chiave sulla localizzazione delle deformazioni nei mezzi porosi viscoplastici: quali sono i fattori più importanti che influenzano lo sviluppo della bande di taglio e qual è il ruolo delle pressioni capillari e delle condizioni di drenaggio; se la viscosità dei suoli è influenzata dalla presenza dell’acqua e se influenza le proprietà di regolarizzazione del metodo locale o non-locale; come interagiscono la lunghezza di scala interna indotta dalla viscosità e quella introdotta dalla non-località; quando un metodo è preferibile ad un altro. La seconda parte di questo lavoro di tesi ha per oggetto lo sviluppo di un nuovo e avanzato modello viscoplastico a partire dal modello costitutivo elasto-plastico per le sabbie parzialmente sature proposto da Buscarnera e Nova. Questo modello permette di simulare i principali meccanismi del comportamento micromeccanico di materiali parzialmente saturi, ovvero la coesione idraulica (hydraulic bounding) e gli effetti di decoesione (debonding) nelle sabbie. Questa parte del lavoro è stata svolta in due fasi; nella prima fase si è discretizzato e implementato il modello elasto-plastico nel codice agli elementi finiti Comes-Geo, successivamente validato utilizzando risultati di letteratura. In particolare sono state simulate: test di taglio a vari valori di pressione capillare (suzione) e pressione di confinamento, test di compressione edometrica con percorsi di desaturazione e saturazione e test di compressione drenata e non drenata in condizione di stato piano di deformazione. Nella seconda fase, questo modello è stato esteso alla viscoplasticità, in modo da tener conto del comportamento viscoso delle sabbie, implementato nel codice Comes-Geo e validato simulando prove di laboratorio di letteratura. La formulazione viscoplastica proposta è stata poi arricchita formulando la dipendenza dei parametri costitutivi dalla densità relativa. Il modello viscoplastico di base e quello arricchito sono stati validati simulando prove di compressione edometrica, prove di viscosità e prove di compressione triassiale a vari valori di densità. Inoltre, con il modello arricchito, è stata simulata una prova di localizzazione su sabbie dense e sciolte e i risultati numerici ottenuti sono stati confrontati con i risultati dell’analisi di stabilità. Con questa tesi si è sviluppato uno strumento numerico efficiente per la simulazione della localizzazione delle deformazioni con metodi regolarizzati che non necessitano di aumentare il numero di variabili di stato e che utilizza modelli costitutivi avanzati considerando il comportamento idro-meccanico accoppiato delle sabbie parzialmente sature. Questo lavoro è stato svolto presso l’Università degli Studi di Padova (Italia) durante i primi due anni di dottorato e presso l’unità di ricerca e sviluppo della società di progettazione e costruzione Baugrund Dresden (Germania) durante il terzo anno di dottorato; l’attività è stata svolta all’interno del progetto Marie Curie Initial Training Network MuMoLaDe (Multiscale Modelling of Landslide and Debris flow), 7th Framework Programme of the European Union, progetto n. 289911, http://www.mumolade.com/. I modelli costitutivi avanzati elasto-plastico e viscoplastici sono stati inoltre implementati e validati anche nel codice commerciale agli elementi finiti Plaxis durante la collaborazione con Baugrund Dresden.

Nous présentons une contribution autour de la modélisation des écoulements viscoplastiques. En vue d'applications réalistes telle que la simulation numérique des coulées de lave volcanique, le travail se concentre particulièrement sur les fluides complexes dont la rhéologie dépend fortement de grandeurs physiques telle que la température ou la concentration en particule. Nous développons un nouvel algorithme de résolution numérique des équations de Herschel-Bulkley combinant une méthode de Lagrangien augmenté à paramètre d'augmentation variable, une méthode des caractéristiques d'ordre 2 et une adaptation de maillage automatique. Sur des problèmes stationnaires ou en évolution tel que le problème test de la cavité entraînée, il apporte une solution efficace pour garantir à la fois une précision numérique élevée et un temps de calcul raisonnable. Cet algorithme est ensuite étendue et adapté au cas des rhéologies non-isothermes et aux suspensions. Concernant la simulation numérique des coulées de lave volcanique, nous détaillons une méthode de réduction par analyse asymptotique des équations de Herschel-Bulkley pour des écoulements de faible épaisseur sur une topographie arbitraire. Elle permet alors de décrire ces écoulements tridimensionnels de fluides viscoplastiques à surface libre par des équations bidimensionnelles surfaciques. Cette approche est ensuite étendue au cas non-isotherme en y ajoutant l'équation de la chaleur et des dépendances thermiques sur la rhéologie. Par intégration verticale de l'équation de la chaleur, on retrouve un modèle bidimensionnel. Le modèle non-isotherme est validé sur une expérience de dôme réalisée en laboratoire et une simulation numérique est réalisée autour d'une coulée qui a eu lieu sur le volcan du Piton de la Fournaise à la Réunion, en décembre 2010. La comparaison donne des résultats qui sont de notre point de vue satisfaisants et encourageants
We present a contribution about modeling of viscoplastic flows. For realistic applications such as numerical simulation of volcanic lava flows, the work focuses particularly on complex fluids whose rheology strongly depends on physical quantities such as temperature or the particle concentration. We develop a new numerical resolution algorithm of Herschel-Bulkley's equations combining an augmented Lagrangian method with variable augmentation parameter, a second order characteristic method and an auto-adaptive mesh procedure. On stationary or evolving problems as the lid-driven cavity flow benchmark, it provides an effective solution to ensure both a high numerical accuracy within a reasonable computing time. This algorithm is then extended and adapted to the case of non-isothermal rheological and suspensions. On the numerical simulation of volcanic lava flows, we describe a method of reducing by asymptotic analysis of the Herschel-Bulkley's equations for thin flows on arbitrary topography. It allows to describe the three-dimensional flows of viscoplastic fluid with free surface by bidimensional surface equations. This approach is then extended to the non-isothermal case by adding the heat equation and thermal dependencies on rheology. By vertical integration of the heat equation, a two-dimensional model is maintained . The non-isothermal model is validated on a laboratory experiment of dome and a numerical simulation is performed on a December 2010 Piton de la Fournaise lava flow from La Réunion island. In our view, the comparison gives satisfactory and encouraging results

Le procédé d’estampage à chaud est considéré comme l'un des procédés les plus prometteurs de micro-Réplication pour l'élaboration des micro-Composants ou nano-Composants avec des matériaux polymériques dans différents domaines d'application . La mémoire consiste à caractériser les propriétés physiques des polymères thermoplastiques amorphes (PS, PMMA et PC) sur une large plage de température, ainsi que la modélisation physique, la simulation numérique et vérification expérimentale du procédé d’estampage à chaud.[...]Les propriétés visco élastiques des polymères retenus ont été caractérisées avec les essais décompression dynamiques. Le module de conservation, le module de perte et le facteur d'amortissement de polymère PMMA à partir de la température ambiante jusqu'à légèrement au-Dessus de Tg ont été obtenues. Le comportement visco élastique des polymères a été décrit par un modèle de Maxwell Généralisé et un bon accord a été observé. La simulation numérique des étapes du remplissage de procédé d’estampage à chaud a été réalisée enprenant compte des propriétés visco élastiques de polymère. Les effets de la température et dela pression de compression sur l'exactitude de réplication dans le procédé d’estampage à chaud ont été étudiés.[...]Un nouveau moule de compression complet, y compris le système de chauffage, le système de refroidissement et le système de vide a été développé dans notre groupe de recherche. Les dispositifs micro fluidiques avec la dimension de la cavité : environ 200 μm, 100 μm et 50 μmen PS, PMMA et PC plaque (épaisseur de 2 mm) ont été élaborés par le procédé d’estampage à chaud. Les effets des paramètres du procédé, tels que l’entrefer imposé, la température décompression, la matière compressée et la dimension de micro cavité, sur l’exactitude de réplication du procédé d’estampage à chaud ont été étudiés
Hot embossing process is considered as one of the most promising micro replication processes for the elaboration of micro or nano components with polymeric materials invarious application fields. The thesis consists to characterize the physical properties of widelyused amorphous thermoplastic polymers (PS, PMMA and PC) over a large temperature range,along with the physical modelling, numerical simulation and experimental verification of thehot embossing process.[...] The polymers’ viscoelastic properties have been characterized with the dynamical compression tests. The storage modulus, loss modulus and damping factor of PMMA polymerfrom ambient temperature to lightly above Tg have been obtained. The viscoelastic behaviourof polymer has been described by a proposed Generalized Maxwell model and a good agreement has been observed. The numerical simulation of filling stage of hot embossing process has been achieved by taking into account of polymer’s viscoelastic properties. Theeffect of compression temperature and pressure on the replication accuracy in hot embossing process has been investigated in the simulation.[...] A new complete micro compression mould tools, including heating system, cooling system and vacuum system have been developed in our research group. The microfluidic devices with the cavity dimension eq. to about 200 μm, 100 μm and 50 μm in PS, PMMA and PC plate(thickness eq. to 2 mm) have been elaborated by the hot embossing process. The effects of the processing parameters, such as the compressive gap imposed, compression temperature, embossed material and die cavity dimensions, on the replication accuracy of hot embossing process have been investigated

Les renforts à base de fibres de lin sont aujourd’hui une alternative capable de concurrencer les fibres synthétiques conventionnelles puisqu’ils sont écologiques, économiques et présentent des propriétés mécaniques intéressantes. Cependant, leur inconvénient majeur est leur absorption d’eau potentiellement importante qui affecte leurs propriétés mécaniques. Ce projet de recherche propose d’étudier le comportement hygroscopique et le couplage hygro-mécanique dans les composites lin / époxy. Cette étude repose sur une approche expérimentale multi-échelle et une modélisation du comportement visco-élasto-plastique avec prise en compte du couplage hygro-mécanique des composites renforcés par des fibres de lin. Les cinétiques de diffusion dans l’époxy et dans le composite ont été modélisées par une loi de type Langmuir et Fick respectivement. Les coefficients d’hygro-expansion des composites et des fils élémentaires qui constituent le renfort tissu ont été déterminés expérimentalement. Une étude de l’influence du conditionnement jusqu’à saturation à différentes humidités relatives sur le comportement mécanique dans les trois directions du stratifié a également été menée. Cette étude a montré l’existence d’une teneur en eau optimale pour laquelle les propriétés mécaniques sont optimales. L’émergence d’un comportement à deux régions linéaires a été mise en évidence et attribuée à la présence d’hétérogénéités locales au sein du renfort tissu. Des essais de fluage / recouvrance et de relaxation / effacement ont permis de mettre en place un modèle visco-élasto-plastique avec prise en compte du couplage hygro-mécanique. Ce modèle offre de bonnes capacités de prédiction et permettra de prévoir le comportement des structures composites renforcés par des fibres de lin en atmosphère humide
Flax fibre reinforcements are nowadays an alternative able to compete with conventional synthetic fibres since they are ecological, economic and have interesting mechanical properties. However, their major drawback is their potentially significant water absorption which affects their mechanical properties. This research project proposes to study the hygroscopic behaviour and hygro-mechanical coupling in flax / epoxy composites. This study is based on a multi-scale experimental approach. A modelling of visco-elasto-plastic behaviour taking into account the hygro-mechanical coupling within flax /epoxy composites is established. The diffusion kinetics in composites were modelled by a Fick law. However, the diffusion kinetics in epoxy were modelled by a Langmuir law. The hygro-expansion coefficients of the composites and the elementary yarns that constitute the fabric reinforcement were determined experimentally. A study of the influence of conditioning until saturation at different relative humidities on the mechanical behaviour in the three main directions of the laminates was conducted. This study showed the existence of an optimal water content for which the mechanical properties are maximum. The emergence of a two-linear-region behaviour was pointed out and attributed to the presence of local heterogeneities within the fabric reinforcement. Creep / recovery and stress relaxation tests were exploited in order to develop a visco-elastoplastic model with consideration of the hygro-mechanical coupling. This model offers good predictive capabilities and could be used to predict the behaviour of flax fibres reinforced composite structures in humid atmospheres

Time- and rate-dependent materials such as polymers, bituminous materials, and soft materials clearly display all four fundamental responses (i.e. viscoelasticity, viscoplasticity, viscodamage, and healing) where contribution of each response strongly depends on the temperature and loading conditions. This study proposes a new general thermodynamic-based framework to specifically derive thermo-viscoelastic, thermo-viscoplastic, thermo-viscodamage, and micro-damage healing constitutive models for bituminous materials and asphalt mixes. The developed thermodynamic-based framework is general and can be applied for constitutive modeling of different materials such as bituminous materials, soft materials, polymers, and biomaterials. This framework is build on the basis of assuming a form for the Helmohelotz free energy function (i.e. knowing how the material stores energy) and a form for the rate of entropy production (i.e. knowing how the material dissipates energy). However, the focus in this work is placed on constitutive modeling of bituminous materials and asphalt mixes. A viscoplastic softening model is proposed to model the distinct viscoplastic softening response of asphalt mixes subjected to cyclic loading conditions. A systematic procedure for identification of the constitutive model parameters based on optimized experimental effort is proposed. It is shown that this procedure is simple and straightforward and yields unique values for the model material parameters. Subsequently, the proposed model is validated against an extensive experimental data including creep, creep-recovery, repeated creep-recovery, dynamic modulus, constant strain rate, cyclic stress controlled, and cyclic strain controlled tests in both tension and compression and over a wide range of temperatures, stress levels, strain rates, loading/unloading periods, loading frequencies, and confinement levels. It is shown that the model is capable of predicting time-, rate-, and temperature-dependent of asphalt mixes subjected to different loading conditions.

Yes
An established statistical mechanical theory of amorphous polymer deformation has been incorporated as a plastic mechanism into a constitutive model and applied to a range of polymer mechanical deformations. The temperature and rate dependence of the tensile yield of PVC, as reported in early studies, has been modeled to high levels of accuracy. Tensile experiments on PET reported here are analyzed similarly and good accuracy is also achieved. The frequently observed increase in the gradient of the plot of yield stress against logarithm of strain rate is an inherent feature of the constitutive model. The form of temperature dependence of the yield that is predicted by the model is found to give an accurate representation. The constitutive model is developed in two-dimensional form and implemented as a user-defined subroutine in the finite element package ABAQUS. This analysis is applied to the tensile experiments on PET, in some of which strain is localized in the form of shear bands and necks. These deformations are modeled with partial success, though adiabatic heating of the instability causes inaccuracies for this isothermal implementation of the model. The plastic mechanism has advantages over the Eyring process, is equally tractable,and presents no particular difficulties in implementation with finite elements.
F. Boutenel acknowledges an Erasmus Programme Scholarship

The main objective of this study is to develop and validate a framework for microstructural modeling of asphalt composite materials using a coupled thermo-viscoelastic, thermo-viscoplastic, and thermo-viscodamage constitutive model. In addition, the dissertation presents methods that can be used to capture and represent the two-dimensional (2D) and three-dimensional (3D) microstructure of asphalt concrete. The 2D representative volume elements (RVEs) of asphalt concrete were generated based on planar X-ray Computed Tomography (CT) images. The 2D RVE consists of three phases: aggregate, matrix, and interfacial transmission zone (ITZ). The 3D microstructures of stone matrix asphalt (SMA) and dense-graded asphalt (DGA) concrete were reconstructed from slices of 2D X-ray CT images; each image consists of the matrix and aggregate phases. The matrix and ITZ were considered thermo-viscoelastic, thermo-viscoplastic, and thermo-viscodamaged materials, while the aggregate is considered to be a linear, isotropic elastic material. The 2D RVEs were used to study the effects of variation in aggregate shape, distribution, volume fraction, ITZ strength, strain rate, and temperature on the degradation and micro-damage patterns in asphalt concrete. Moreover, the effects of loading rate, temperature, and loading type on the thermo-mechanical response of the 2D and 3D microstructures of asphalt concrete were investigated. Finally, the model parameters for Fine Aggregate Mixture (FAM) and full asphalt mixture were determined based on the analysis of repeated creep recovery tests and constant strain rate tests. These material parameters in the model were used to simulate the response of FAM and full asphalt mixture, and the results were compared with the responses of the corresponding experimental tests. The microstructural modeling presented in this dissertation provides the ability to link the microstructure properties with the macroscopic response. This modeling combines nonlinear constitutive model, finite element analysis, and the unique capabilities of X-ray CT in capturing the material microstructure. The modeling results can be used to provide guidelines for designing microstructures of asphalt concrete that can achieve the desired macroscopic behavior. Additionally, it can be helpful to perform 'virtual testing' of asphalt concrete, saving numerous resources used in conducting real experimental tests.

There is a growing need to better understand the relationship between time, strain rate, and temperature on the load-deformation behaviour of clay soils in engineering applications. These applications may include: infrastructure constructed in northern regions where climate change is a growing concern; disposal of nuclear waste; and, industrial structures, such as furnaces, foundries, and refrigeration plants. Temperature variations may induce changes in internal pressure in the soil, swelling and shrinkage, and affect the mechanical properties of the soil. This thesis presents thermal numerical modeling for two instrumented field sites in northern Manitoba. Thermal conductivity testing on samples from these sites and field data are used to calibrate these thermal numerical models. Various boundary conditions are examined. The capabilities of the models are evaluated to determine if the models adequately simulate and predict changes in temperature in geotechnical structures. A discussion is presented on the strengths and weaknesses in the models and the predictive capabilities of the models. The thesis then shifts into understanding the concepts of thermoplasticity and viscoplasticity and the mathematics relating these concepts. Mathematical models that describe these concepts are examined and compared with traditional soil mechanics approaches. The concepts of thermoplasticity and viscoplasticity are combined in an encompassing elastic thermo-viscoplastic (ETVP) model using a semi-empirical framework. A sensitivity analysis is used to evaluate quantitatively the response of the model. The model is then validated qualitatively against published laboratory data. Applications of the ETVP model are discussed.

The purpose of this work was to develop a practical method for constitutive modelling of polyethylene, based on a phenomenological approach, which can be applied for structural analysis. Polyethylene (PE) is increasingly used as a structural material, for example in pipes installed by trenchless methods where relatively low stiffness of PE reduces the required installation forces, chemical inertness makes it applicable for corrosive environments, and adequate strength allows to use it for sewer, gas and water lines. Polyethylene exhibits time-dependent constitutive behaviour, which is also dependent on the applied stress level resulting in nonlinear stress-strain relationships. Nonlinear viscoelastic theory has been well established and a variety of modelling approaches have been derived from it. In order to be able to realistically utilize the nonlinear modelling approaches in design, a simple method is needed for finding the constitutive formulation for a specific polyethylene type. In this study, time-dependent constitutive relationships for polymers are investigated for polyethylene materials. Creep tests on seven polyethylene materials were conducted and the experimental results indicate strong nonlinear viscoelasticity in the material responses. Creep tests on seven materials were conducted for 24 hours for modelling purposes. However, creep tests up to fourteen days were performed on one material to study long-term creep behaviour. Multiple-stepped creep tests were also investigated. Constant rate (load and strain rate) tensile tests were conducted on two of the seven polyethylene materials. A practical approach to nonlinear viscoelastic modelling utilizing both multi-Kelvin element theory and power law functions to model creep compliance is presented. Creep tests are used to determine material parameters and models are generated for four different polyethylene materials. The corroboration of the models is achieved by comparisons with the results of different tensile creep tests, with one dimensional step loading test results and with test results from load and displacement rate loading.

Random Glass Mat Thermoplastic (GMT) composites are increasingly being used by the automotive industry for manufacturing semi-structural components. The polypropylene based materials are characterized by superior strength, impact resistance and toughness. Since polymers and their composites are inherently viscoelastic, i.e. their mechanical properties are dependent on time and temperature. They creep under constant mechanical loading and the creep rate is accelerated at elevated temperatures. In typical automotive operating conditions, the temperature of the polymer composite part can reach as high as 80°C. Currently, the only known report in the open literature on the creep response of commercially available GMT materials offers data for up to 24 MPa at room temperature. In order to design and use these materials confidently, it is necessary to quantify the creep behaviour of GMT for the range of stresses and temperatures expected in service. The primary objective of this proposed research is to characterize and model the creep behaviour of the GMT composites under thermo-mechanical loads. In addition, tensile testing has been performed to study the variability in mechanical properties. The thermo-physical properties of the polypropylene matrix including crystallinity level, transitions and the variation of the stiffness with temperature have also been determined. In this work, the creep of a long fibre GMT composite has been investigated for a relatively wide range of stresses from 5 to 80 MPa and temperatures from 25 to 90°C. The higher limit for stress is approximately 90% of the nominal tensile strength of the material. A Design of Experiments (ANOVA) statistical method was applied to determine the effects of stress and temperature in the random mat material which is known for wild experimental scatter. Two sets of creep tests were conducted. First, preliminary short-term creep tests consisting of 30 minutes creep followed by recovery were carried out over a wide range of stresses and temperatures. These tests were carried out to determine the linear viscoelastic region of the material. From these tests, the material was found to be linear viscoelastic up-to 20 MPa at room temperature and considerable non-linearities were observed with both stress and temperature. Using Time-Temperature superposition (TTS) a long term master curve for creep compliance for up-to 185 years at room temperature has been obtained. Further, viscoplastic strains were developed in these tests indicating the need for a non-linear viscoelastic viscoplastic constitutive model. The second set of creep tests was performed to develop a general non-linear viscoelastic viscoplastic constitutive model. Long term creep-recovery tests consisting of 1 day creep followed by recovery has been conducted over the stress range between 20 and 70 MPa at four temperatures: 25°C, 40°C, 60°C and 80°C. Findley’s model, which is the reduced form of the Schapery non-linear viscoelastic model, was found to be sufficient to model the viscoelastic behaviour. The viscoplastic strains were modeled using the Zapas and Crissman viscoplastic model. A parameter estimation method which isolates the viscoelastic component from the viscoplastic part of the non-linear model has been developed. The non-linear parameters in the Findley’s non-linear viscoelastic model have been found to be dependent on both stress and temperature and have been modeled as a product of functions of stress and temperature. The viscoplastic behaviour for temperatures up to 40°C was similar indicating similar damage mechanisms. Moreover, the development of viscoplastic strains at 20 and 30 MPa were similar over all the entire temperature range considered implying similar damage mechanisms. It is further recommended that the material should not be used at temperature greater than 60°C at stresses over 50 MPa. To further study the viscoplastic behaviour of continuous fibre glass mat thermoplastic composite at room temperature, multiple creep-recovery experiments of increasing durations between 1 and 24 hours have been conducted on a single specimen. The purpose of these tests was to experimentally and numerically decouple the viscoplastic strains from total creep response. This enabled the characterization of the evolution of viscoplastic strains as a function of time, stress and loading cycles and also to co-relate the development of viscoplastic strains with progression of failure mechanisms such as interfacial debonding and matrix cracking which were captured in-situ. A viscoplastic model developed from partial data analysis, as proposed by Nordin, had excellent agreement with experimental results for all stresses and times considered. Furthermore, the viscoplastic strain development is accelerated with increasing number of cycles at higher stress levels. These tests further validate the technique proposed for numerical separation of viscoplastic strains employed in obtaining the non-linear viscoelastic viscoplastic model parameters. These tests also indicate that the viscoelastic strains during creep are affected by the previous viscoplastic strain history. Finally, the developed comprehensive model has been verified with three test cases. In all cases, the model predictions agreed very well with experimental results.

The present work is aimed at the extraction of material’s yield and creep parameters using cantilevers by conducting constant deflection rate and constant load tests respectively. Under the assumption of Euler-Bernoulli’s beam theory, stress and strain components are considered along beam length only. A stress and strain gradient throughout the cantilever makes bending rich in information as stress-strain rate-strain at every location of the beam corresponds to a single uniaxial test. By extracting these stress-strain rate strain information from multiple locations of the cantilever throughout deformation, high throughput feature of bending is utilized by extracting all the material yield and creep parameters from a single cantilever. The position dependent strain is measured using digital image correlation (DIC) in this work. The estimation of stress in a cantilever during non-linear deformation (i.e., non-linear dependence of stress on strain and/or strain rate), however, needs numerical methods which solve for stress distribution throughout the cantilever using equilibrium equations based on material’s constitutive behavior, beam geometry, loading and boundary conditions. Such numerical methods are developed to gain understanding of the stress redistribution which is found to be transient in nature and finally saturates when permanent (plastic or creep) strain are large (∼3-4%) such that elastic strain are negligible. The influence of material’s yield and creep parameters on stress redistribution profile and associated timelines is studied and limitations of the existing analytical expressions for saturated stress profile are discussed. The numerical method is also utilized to estimate the effect of limitations on strain accuracy measured by DIC on the extracted parameters. Therefore, the present work aims at developing cantilever bending combined with DIC for strain measurement as an alternative testing technique to extract material’s yield and creep parameters from a single cantilever. The procedure for extraction of yield parameters like yield strength and strain hardening exponent is established for a beam of tension-compression symmetric, strain rateinsensitive material using the concept of the ‘invariant point’. These points are identified using numerical methods as the unique locations at every cross-section of the cantilever, one each in tensile and compressive regions, where stress remains almost unchanged during stress redistribution. The hardening exponent and yield strength are measured from a single cantilever with better statistics and thereby improved reliability using strain measured at the invariant points in tensile and compressive regions within an accuracy of 99.5% and 90% respectively for pure copper and aluminium alloys. The procedures for measuring strain rate sensitivity for a tension-compression symmetric, strain-rate sensitive material and yield parameters for a tension-compression asymmetric, strain-rate insensitive material are also proposed and the challenges are highlighted with the understanding that bending has the potential to measure yield parameters for these latter systems as well in future. The timelines associated with stress saturation under creep deformation have been quantified using numerical methods in terms of a parameter stress saturation time (SST). Based on the recommendations obtained from SST, loading profile for T22 boiler steel is redesigned in the form of small loading steps due to which stress gets sufficient time to relax during loading itself and does not exceed yield strength during redistribution. Thus, creep parameters can be extracted at loads as high as yield strength, which is not possible otherwise. This makes testing faster and thereby efficient because creep rates are higher at high load and steady state is achieved faster. In contrast, a high SST at low loads has been identified to explain the misinterpretation of experimental data in terms of mechanism shift at low loads for P91 steel. The numerical method is further developed to include primary creep response at loads above yield which holds relevance to room-temperature creep response of an hcp system, i.e., titanium alloy Ti-6Al. In case of Ti-6Al, it is found from uniaxial creep tests conducted above yield that prior plastic deformation does not affect creep behaviour which implies that plasticity affects only the initial stress distribution. The invariant points which remain invariant to stress redistribution even under the combined effect of creep and plastic deformation are identified based on the numerical methods. The strain at these locations in tension as well as in compression, measured using DIC, are utilized to extract equivalent primary creep response for Ti-6Al using a single cantilever. Therefore, the present work aims at establishing bending as the testing technique to measure yield and creep parameters for a range of materials (FCC, BCC, HCP) and testing conditions (RT-600oC) utilizing minimum sample volume with reduced testing and better statistics.
ARDB 0242, IMPRINT 0009