Pesquisas e projetos envolvendo a complexa interação de maciços rochosos com estruturas de suporte de túneis evoluem constantemente. A adoção de modelos numéricos sofisticados aliada ao desenvolvimento de computadores com capacidade de processamento crescente possibilitam representações mecânicas de processos complexos. Para análises geotécnicas, considerações de resistência, deformabilidade e condutividade hidráulica do maciço são encontradas em códigos computacionais com representações próximas das encontradas nas pesquisas experimentais de vanguarda da área. Por outro lado, estes mesmos códigos atribuem com frequência ao comportamento do concreto reforçado representações simplistas, muitas vezes com base na teoria da elasticidade. Implicações econômicas e de incerteza na avaliação da segurança decorrem desta forma de representação. Esta pesquisa visa a contribuir para análises da interação entre maciço e suporte de uma escavação subterrânea. Para tanto foram elaboradas simulações numéricas considerando técnicas consagradas de avaliação mecânica de uma escavação, aliadas a uma representação do concreto reforçado considerando o marcante efeito de fissuração. Com foco no comportamento do concreto foram desenvolvidas simulações numéricas de distintas estruturas de concreto reforçado selecionadas na literatura e que contam com boa instrumentação e caracterização mecânica adequada de seus materiais. Os resultados obtidos indicam a possibilidade da avaliação concomitante das complexas características do maciço assim como do concreto reforçado. Este fato contribui para a avaliação da segurança de uma escavação subterrânea com possíveis repercussões no custo das obras. / Research and projects involving the intricate interaction between rock mass and tunnel support structures constantly evolve. The adoption of sophisticated numerical models allied to the development of computers with an increasing processing capacity allows mechanical representations of complex processes. For geotechnical analysis, considerations on rock mass strength, deformability and hydraulic conductivity are found in computational codes with representations close to those found in vanguard experimental research of this subject. However, these same codes often attribute simplistic representations to reinforced concrete behavior, frequently based on elasticity theory. Economic and uncertainty implications in safety evaluation result from this sort of representation. This research aims to contribute to analysis of interaction between rock mass and support of an underground excavation. Thus, numerical simulations were elaborated considering established techniques of mechanical evaluation of excavations, allied to a reinforced concrete representation taking into account the prominent effect of cracking. Focused on the concrete behavior, there were developed numerical simulations of distinct reinforced concrete structures selected in literature and that rely on fair instrumentation and adequate mechanical characterization of the involved materials. The results indicate the possibility of concomitant evaluation of the rock mass complex character and the reinforced concrete. This fact contributes to safety evaluation of an underground excavation with potential repercussions on project cost.

Escoamentos sem inércia de fluido elasto-viscoplástico, dentro de uma cavidade, são numericamente analisados. As soluções visam compreender a influência dos efeitos viscosos e elásticos na topologia de superfícies de escoamento. Assumindo-se que o colapso da microestrutura do material é instantâneo, o modelo mecânico é constituído pelas equações governantes de massa e momentum para fluidos incompressíveis, associado a uma equação hiperbólica para o tensor tensão extra, baseado na equação do modelo Oldroyd-B (Nassar et al, 2011). A principal característica do modelo é considerar a viscosidade e o tempo de relaxação como função da taxa de deformação, permitindo a pseudoplasticidade de viscosidade e restringindo os efeitos elásticos para as regiões não deformadas do material. As simulações numéricas são realizadas através do método de Galerkin mínimos quadrados a três campos: tensor tensão extra, pressão e velocidade. Os resultados mostram que as superfícies de escoamento do material são fortemente influenciadas pela ação combinada entre os efeitos elásticos e viscosos, estando em conformidade com a recente visualização experimental dos fluxos elasto-viscoplásticos. / Elasto viscoplastic uid ows without inertia, within a cavity, are numerically analyzed. The solutions aim to understand the in uence of viscous and elastic e ects on the topology of yield surfaces. Assuming that the collapse of the material microstructure is instantaneous, the mechanical model consists of the governing equations of mass and momentum for incompressible uids, associated with a hyperbolic equation for the extra stress tensor, based on the equation of the Oldroyd-B model (Nassar et al, 2011). The main feature of model is to consider the viscosity and the relaxation time as a function of shear rate, allowing the shear-thinning of viscosity and restricting elastic e ects for regions not deformed material. Numerical simulations are performed by the method of Galerkin Least Squares to three elds: extra stress tensor, pressure and velocity. The results show that the yield surfaces of material are strongly in uenced by the combined action between the elastic and viscous e ects, complying with the recent experimental visualization of elasto-viscoplastic ows.

Escoamentos sem inércia de fluido elasto-viscoplástico, dentro de uma cavidade, são numericamente analisados. As soluções visam compreender a influência dos efeitos viscosos e elásticos na topologia de superfícies de escoamento. Assumindo-se que o colapso da microestrutura do material é instantâneo, o modelo mecânico é constituído pelas equações governantes de massa e momentum para fluidos incompressíveis, associado a uma equação hiperbólica para o tensor tensão extra, baseado na equação do modelo Oldroyd-B (Nassar et al, 2011). A principal característica do modelo é considerar a viscosidade e o tempo de relaxação como função da taxa de deformação, permitindo a pseudoplasticidade de viscosidade e restringindo os efeitos elásticos para as regiões não deformadas do material. As simulações numéricas são realizadas através do método de Galerkin mínimos quadrados a três campos: tensor tensão extra, pressão e velocidade. Os resultados mostram que as superfícies de escoamento do material são fortemente influenciadas pela ação combinada entre os efeitos elásticos e viscosos, estando em conformidade com a recente visualização experimental dos fluxos elasto-viscoplásticos. / Elasto viscoplastic uid ows without inertia, within a cavity, are numerically analyzed. The solutions aim to understand the in uence of viscous and elastic e ects on the topology of yield surfaces. Assuming that the collapse of the material microstructure is instantaneous, the mechanical model consists of the governing equations of mass and momentum for incompressible uids, associated with a hyperbolic equation for the extra stress tensor, based on the equation of the Oldroyd-B model (Nassar et al, 2011). The main feature of model is to consider the viscosity and the relaxation time as a function of shear rate, allowing the shear-thinning of viscosity and restricting elastic e ects for regions not deformed material. Numerical simulations are performed by the method of Galerkin Least Squares to three elds: extra stress tensor, pressure and velocity. The results show that the yield surfaces of material are strongly in uenced by the combined action between the elastic and viscous e ects, complying with the recent experimental visualization of elasto-viscoplastic ows.

Les mousses fragiles font aujourd'hui partie des nouveaux matériaux très performants dont le procédé de fabrication permet un contrôle précis de la microstructure finale. De nouveaux débouchés apparaissent dans des applications structurales (absorption de choc, allègement des structures) en raison de leur excellente tenue mécanique alliée à une grande légèreté. Dans les travaux réalisés dans le cadre de cette thèse, une mousse carbone à cellules ouvertes a été étudiée dans le but d'être utilisée pour assurer la protection des capots de protection d'emballages. Le comportement mécanique en compression a été caractérisé sous des sollicitations uniaxiales et multiaxiales. La surface de charge de la mousse ainsi que son évolution au cours du chargement ont été identifiés. Les principales propriétés mécaniques ont été évaluées et certaines d'entre elles ont été corrélées avec celles prédites par le modèle micro-mécanique de Gibson et Ashby développé pour les mousses fragiles. Grâce aux observations post-mortem sous microscope électronique à balayage et en micro-tomographie aux rayons X, les mécanismes de déformation et d'absorption d'énergie ont été également caractérisés. Pour modéliser le comportement multiaxial en compression de la mousse carbone, considérée comme un milieu continu homogène et isotrope, le modèle de Deshpande et Fleck (DF) a été adopté et adapté. Ce modèle a été implanté dans le code éléments finis LS-DYNA. Il a été identifié et validé sur l'ensemble des essais triaxiaux disponibles ainsi que sur des essais d'écrasement de mini-structure. Le comportement macroscopique global, obtenu à l'aide de simulations numériques, est prédit de manière satisfaisante. Il sera amélioré par la suite pour prendre en compte certains aspects non décrits actuellement. / Due to improvements in the manufacturing process that allow a better control of their microstructure, brittle foams are now part of the new efficient materials. New markets in the field of structural applications open up thanks to their excellent mechanical properties combined with light weight.In this study, a carbon foam with open cells has been studied in order to be used as shock absorber in packagings. Its compressive mechanical behavior has been characterized under various uniaxial and multiaxial loadings. The carbon foam yield surface and its evolution during loading have been identified. The main mechanical properties have been evaluated and some of them have been correlated with those predicted by the Gibson and Ashby micromechanical model. The mechanisms of deformation and the energy absorption have been studied using post-mortem observations by scanning electron microscopy (SEM) and X-Ray microtomography.The Deshpande and Fleck model (DF) has been adopted and slightly modified to model the compressive multiaxial behavior of the carbon foam. The latter is considered as an homogeneous continuum medium. The constitutive equations have been implemented in the finite element code LS-DYNA via a Umat routine. The model parameters have been identified and the model estimations validated on available triaxial tests as well as on crushing tests made on micro-structures. Numerical simulations are relevant on predicting the global macroscopic behavior. Nevertheless, the mechanical model needs to be improved to better account for some phenomena not currently described.

Placement and compaction of the backfill behind an earth retaining wall may induce a vertical shear force at the soil-to-wall interface. This vertical shear force, or downdrag, is beneficial for the stability of the structure. A significant reduction in construction costs may result if the downdrag is accounted for during design. This potential reduction in costs is particularly interesting in the case of U.S. Army Corps of Engineers lock walls. A simplified procedure is available in the literature for estimating the downdrag force developed at the wall-backfill interface during backfilling of a retaining wall. However, finite element analyses of typical U.S. Army Corps of Engineers lock walls have shown that the magnitude of the downdrag force may decrease during operation of the lock with a rise in the water table in the backfill. They have also shown that pre- and post-construction stress paths followed by interface elements often involve simultaneous changes in shear and normal stresses and unloading-reloading. The hyperbolic formulation for interfaces (Clough and Duncan 1971) is accurate for modeling the interface response in the primary loading stage under constant normal stress. However, it has not been extended to model simultaneous changes in shear and normal stresses or unloading-reloading of the interface. The purpose of this research was to develop an interface model capable of giving accurate predictions of the interface response under field loading conditions, and to implement this model in a finite element program. In order to develop the necessary experimental data, a series of tests were performed on interfaces between concrete and two different types of sand. The tests included initial loading, staged shear, unloading-reloading, and shearing along complex stress paths. An extended hyperbolic model for interfaces was developed based on the results of the tests. The model is based on Clough and Duncan (1971) hyperbolic formulation, which has been extended to model the interface response to a variety of stress paths. Comparisons between model calculations and tests results showed that the model provides accurate estimates of the response of interfaces along complex stress paths. The extended hyperbolic model was implemented in the finite element program SOILSTRUCT-ALPHA, used by the U.S. Army Corps of Engineers for analyses of lock walls. A pilot-scale test was performed in the Instrumented Retaining Wall (IRW) at Virginia Tech that simulated construction and operation of a lock wall. SOILSTRUCT-ALPHA analyses of the IRW provided accurate estimates of the downdrag magnitude throughout inundation of the backfill. It is concluded that the extended hyperbolic model as implemented in SOILSTRUCT-ALPHA is adequate for routine analyses of lock walls. / Ph. D.

Die Arbeit widmet sich dem Fließverhalten von Stählen bei zyklisch inkrementeller Torsion. Dazu werden vergleichend Reineisen und der ferritisch-perlitische Stahl 42CrMo4N bei unterschiedlichen Verformungspfaden betrachtet. Vor allem in zyklischen Torsionsversuchen mit bleibendem Verformungsinkrement je Zyklus werden Fließkurvenverläufe und Verformungsverfestigung analysiert. Die Ergebnisse belegen den Einfluss des lamellaren Zementit auf das Fließverhalten des Stahls 42CrMo4N, während die Eigenschaften des Reineisens von der entstehenden Versetzungszellstruktur bestimmt werden. Die Richtungsabhängigkeit der Fließspannung und die Verläufe der Fließkurven unterscheiden sich für die betrachteten Werkstoffe deutlich. Fließortkurven dienen der quantitativen Beschreibung der Verfestigung. Die Vorgehensweise zu ihrer Ermittlung und ihre Abhängigkeit von den Versuchsbedingungen und den Verformungszuständen werden gezeigt. Bei Reineisen dominieren isotrope, bei dem Stahl 42CrMo4N kinematische Verfestigungsanteile das Fließverhalten. / Ferritic-pearlitic steel 42CrMo4N and pure iron under different strain paths are compared regarding their flow behaviour. Mainly in cyclic torsion tests with resulting strain increment per cycle shear stresses and strain hardening are analysed. The results show, that the cementite lamellae determine the flow behaviour of the steel 42CrMo4N, whereas the properties of pure iron are governed by the evolving dislocation cell structure. The dependency of flow stress on the strain direction is different for the two materials. Yield surfaces describe strain hardening quantitatively. The procedure for yield point detection and the dependency of subsequent yield surfaces on experimental conditions and strain states is shown. For pure iron isotropic hardening, for steel 42CrMo4N kinematic strain hardening dominates the flow behaviour.

Die Arbeit widmet sich dem Fließverhalten von Stählen bei zyklisch inkrementeller Torsion. Dazu werden vergleichend Reineisen und der ferritisch-perlitische Stahl 42CrMo4N bei unterschiedlichen Verformungspfaden betrachtet. Vor allem in zyklischen Torsionsversuchen mit bleibendem Verformungsinkrement je Zyklus werden Fließkurvenverläufe und Verformungsverfestigung analysiert. Die Ergebnisse belegen den Einfluss des lamellaren Zementit auf das Fließverhalten des Stahls 42CrMo4N, während die Eigenschaften des Reineisens von der entstehenden Versetzungszellstruktur bestimmt werden. Die Richtungsabhängigkeit der Fließspannung und die Verläufe der Fließkurven unterscheiden sich für die betrachteten Werkstoffe deutlich. Fließortkurven dienen der quantitativen Beschreibung der Verfestigung. Die Vorgehensweise zu ihrer Ermittlung und ihre Abhängigkeit von den Versuchsbedingungen und den Verformungszuständen werden gezeigt. Bei Reineisen dominieren isotrope, bei dem Stahl 42CrMo4N kinematische Verfestigungsanteile das Fließverhalten. / Ferritic-pearlitic steel 42CrMo4N and pure iron under different strain paths are compared regarding their flow behaviour. Mainly in cyclic torsion tests with resulting strain increment per cycle shear stresses and strain hardening are analysed. The results show, that the cementite lamellae determine the flow behaviour of the steel 42CrMo4N, whereas the properties of pure iron are governed by the evolving dislocation cell structure. The dependency of flow stress on the strain direction is different for the two materials. Yield surfaces describe strain hardening quantitatively. The procedure for yield point detection and the dependency of subsequent yield surfaces on experimental conditions and strain states is shown. For pure iron isotropic hardening, for steel 42CrMo4N kinematic strain hardening dominates the flow behaviour.

Si un métal est soumis à un chargement cyclique autour d'une contrainte moyenne non nulle, de la déformation plastique peut s'accumuler au cours des cycles, généralement dans la direction de la contrainte moyenne : c'est le phénomène de déformation progressive ou phénomène dit de rochet.<br />Le contexte de cette thèse était l'amélioration de la description de ce phénomène dans les structures métalliques. Pour cela, nous avons choisi d'axer nos efforts sur les modèles macroscopiques phénoménologiques qui décrivent le comportement de matériaux dans des calculs de structure.<br />Comme cela était suggéré dans plusieurs articles récents, une voie d'amélioration de ces modèles était de prendre en compte la distorsion du domaine d'élasticité, phénomène maintes fois observé expérimentalement, mais jusqu'alors négligé par soucis de simplicité. Pour introduire convenablement ce nouvel ingrédient dans la modélisation, nous nous sommes appuyés sur un dialogue entre un modèle micro-macro d'une part (plus pertinent mais aussi plus coûteux en temps de calcul) et le modèle macroscopique à construire d'autre part. Nous avons sû tirer profit des informations données par le modèle micro-macro pour constuire un modèle macroscopique capable de décrire à la fois le phénomène de rochet multiaxial et la distorsion du domaine d'élasticité prévus par le modèle micro-macro, et ce sur une large base d'"essais virtuels". Ensuite, le modèle développé a été identifié et validé avec succès sur plusieurs résultats d'essais complexes obtenus sur un élément de volume d'un matériau réel. Une validation finale du modèle sur un essai non-homogène (de structure) est en cours d'étude.

The reliability analysis of plates and shells with respect to plastic collapse or to inadaptation is formulated on the basis of limit and shakedown theorems. The loading, the material strength and the shell thickness are considered as random variables. Based on a direct definition of the limit state function, the nonlinear problems may be efficiently solved by using the First and Second Order Reliability Methods (FORM/SORM). The sensitivity analyses in FORM/SORM can be based on the sensitivities of the deterministic shakedown problem. The problem of reliability of structural systems is also handled by the application of a special barrier technique which permits to find all the design points corresponding to all the failure modes. The direct plasticity approach reduces considerably the necessary knowledge of uncertain input data, computing costs and the numerical error. / Die Zuverlässigkeitsanalyse von Platten und Schalen in Bezug auf plastischen Kollaps oder Nicht-Anpassung wird mit den Traglast- und Einspielsätzen formuliert. Die Lasten, die Werkstofffestigkeit und die Schalendicke werden als Zufallsvariablen betrachtet. Auf der Grundlage einer direkten Definition der Grenzzustandsfunktion kann die Berechnung der Versagenswahrscheinlichkeit effektiv mit den Zuverlässigkeitsmethoden erster und zweiter Ordnung (FROM/SORM) gelöst werden. Die Sensitivitätsanalysen in FORM/SORM lassen sich auf der Basis der Sensitivitäten des deterministischen Einspielproblems berechnen. Die Schwierigkeiten bei der Ermittlung der Zuverlässigkeit von strukturellen Systemen werden durch Anwendung einer speziellen Barrieremethode behoben, die es erlaubt, alle Auslegungspunkte zu allen Versagensmoden zu finden. Die Anwendung direkter Plastizitätsmethoden führt zu einer beträchtlichen Verringerung der notwendigen Kenntnis der unsicheren Eingangsdaten, des Berechnungsaufwandes und der numerischen Fehler.

Die vorliegende Dissertation beschäftigt sich mit der thermo-mechanischen Beschreibung und Bewertung von keramischen Filtern für die Metallschmelze-Filtration mithilfe der Finiten-Elemente-Methode. Infolge des zellularen Aufbaus des Werkstoffs handelt es sich um ein Mehrskalenproblem. Grundlegende Aufgaben der Arbeit waren deshalb die geometrische und mechanische Modellbildung sowie die Untersuchung verschiedener effizienzsteigernder Methoden zur Gewinnung einer akkuraten numerischen Lösung. Dabei wurden sowohl verschiedene Verfahren aus der Fachliteratur implementiert und kritisch bewertet, als auch neue Ansätze verfolgt. Die Untersuchungen konzentrierten sich auf das effektive elastische und elastisch-plastische Verhalten von Kelvin-, Weaire-Phelan- und Voronoi-Strukturen. Insbesondere die entwickelten Methoden und Werkzeuge zur automatisierten Modellbildung gestatten in einfacher Weise die Umsetzung von Parameterstudien und Optimierungsaufgaben. Aus darauf aufbauenden Sensitivitätsstudien wurden Empfehlungen hinsichtlich der geometrischen und mechanischen Modellbildung für zellulare Werkstoffe abgeleitet. Diese betreffen auch vielfach eingesetzte Methoden zur Modellreduktion für diese Werkstoffe und tragen somit zukünftig zu einer effizienteren Bewertung von Filterstrukturen bei.