The first instability of circular tubes compressed into the plastic range is axisymmetric wrinkling, which is stable. Compressed further the wrinkle amplitude grows, leading to a limit load instability followed by collapse. The two instabilities can be separated by strain levels of a few percent. This work investigates whether a tube that develops small amplitude wrinkles can be subsequently collapsed by persistent cycling. The problem was first investigated experimentally using SAF 2507 super-duplex steel tubes with D/t of 28.5. The tubes are first compressed to strain levels high enough for mild wrinkles to form and then cycled axially under stress control about a compressive mean stress. This type of cycling usually results in accumulation of compressive strain; here it is accompanied by growth of the amplitude of the initial wrinkles. The tube average strain initially grows nearly linearly with the number of cycles, but as a critical value of wrinkle amplitude is approached, wrinkling localizes, the rate of ratcheting grows exponentially and the tube collapses. Similar experiments were then performed for tubes involving axial cycling under internal pressure and the combined loads cause simultaneous ratcheting in the hoop and axial directions as well as a gradual growth of the wrinkles. The rate of ratcheting and the number of cycles to collapse depend on the initial compressive pre-strain, the internal pressure, and the stress cycle parameters all of which were varied sufficiently to generate vii a sufficient data base. Interestingly, in both the pressurized and unpressurized cases collapse was found to occur when the accumulated average strain reaches the value at which the tube localizes under monotonic compression. A custom shell model of the tube with initial axisymmetric imperfections, coupled to the Dafalias-Popov two-surface nonlinear kinematic hardening model, are presented and used to simulate the experiments performed. It is demonstrated that when suitably calibrated this modeling framework reproduces the prevalent ratcheting deformations and the evolution of wrinkling including the conditions at collapse accurately for all experiments. The calibrated model is then used to evaluate the ratcheting behavior of pipes under thermal-pressure cyclic loading histories experienced by axially restrained pipelines. / text

Dans une approche de dimensionnement en fatigue basée sur un critère multiaxial, les paramètres d’entrée ducritère (contraintes, déformations, termes énergétiques) sont généralement calculés sur un état stabilisé. Dans lesmétaux, il s’agit souvent du premier cycle, en supposant que le matériau se comporte élastiquement ou présente desprocessus de plasticité très localisés. Dans les matériaux viscoélastiques comme les polymères, l’évolutionsignificative de la raideur en début de cyclage soulève la question de la stabilisation du cycle sur lequel lesparamètres mécaniques devraient être calculés. Un enjeu majeur est donc de définir et prédire cet état stabilisé, c’està dire non seulement l’évolution de la déformation moyenne qui accompagne le cyclage mais aussi la bouclestabilisée elle-même et les contributions énergétiques pertinentes. Pour être applicable à des structures, le modèledoit conserver un formalisme aussi maniable que possible.Dans cette étude, réalisée sur les 1000 premiers cycles de la vie d’un polyéthylène, il est montré, par des essaisde recouvrance, que la contribution viscoélastique à l’évolution de la déformation moyenne est majoritaire. Unintérêt particulier est donc porté à la caractérisation expérimentale et à la modélisation macroscopique de cet aspectdu comportement.La première partie du travail est menée dans un cadre purement mécanique. L’accumulation cyclique estétudiée expérimentalement au cours des premiers 1000 cycles à force contrôlée et faible fréquence, à la températureambiante. L’influence de la fréquence et du rapport de charge sur la réponse viscoélastique est étudiée. Lacomparaison d’essais de traction et de cisaillement de type Iosipescu permet de discuter les parts volumique etdéviatorique. Un modèle viscoélastique non linéaire isotherme en petites déformations est proposé dans le cadre dela Thermodynamique des Processus Irréversibles.Dans la deuxième partie, l’étude expérimentale et théorique est étendue au cadre thermo-mécanique. Latempérature est en effet intrinsèquement couplée à la viscoélasticité dans les polymères ; cet effet peut conduire àdes auto-échauffements importants. Les mêmes essais de traction et cisaillement sont réalisés par le LMGC deMontpellier avec une métrologie différente : la mesure de champs de température et déformation au cours de l’essaipermet de calculer les différents termes de l’équation de diffusion de la chaleur et d’accéder aux sources de chaleur.Ces résultats expérimentaux sont analysés et confrontés à une extension du modèle thermo-viscoélastique dumodèle dans laquelle le couplage est introduit via la thermo-élasticité (par la déformation volumique) et via ladissipation visqueuse (sur la base du principe d’équivalence temps-température). / Fatigue design approaches based on fatigue life criteria require as an input mechanical parameters (stress orstrain components or equivalent measurements, energetic terms) usually calculated over the first cycle, assumingthat the material behaves elastically or exhibits highly localized plasticity processes. In viscous materials likepolymers, such approaches raise the question of the stabilization of the cycle over which the mechanical parametersshould be computed. The challenge is not only to predict the ratcheting strain but also the stabilized loop itself andthe relevant energetic contributions, within a formalism as simple as possible to be used for structure simulations.In this study conducted in polyethylene, a special interest is paid on the viscoelastic contribution, expected tohighly contribute in the low stress range of high-cycle fatigue. A challenge is to accurately capture two time scales,i.e. the long term scale of strain ratcheting and the short term scale of the cycle itself.In the first part, cyclic accommodation is experimentally investigated over the first thousand cycles of forcecontrolledtests at room temperature. Viscoelasticity tackles in a specific way the frequency and mean stresssensitivity which are both varied in the experiments. Viscoelasticity also questions the volume / deviator partition:tension and shear tests are compared to highlight this point.After establishing from recovery tests that viscoelasticity mainly contributes to the ratcheting strain in bothexperiments series, a small-strain non-linear viscoelastic model is proposed in the framework of Thermodynamicsof Irreversible Processes. The aim is to capture the ratcheting strain evolution and recovery kinetics, as well as thestabilized loop area and dynamic modulus. The mean strain kinetics and non-recoverable term appearunderestimated, due to the fact that inelastic strain parts are not taken into account in the model. On the other hand,the loop area and rigidity are reasonably predicted.Volume / deviatoric contributions are on two counts an open issue, because of their different contribution tothermoelastic coupling. Thermo-mechanical coupling is examined in the last part of the presentation, by couplingthe above described model to temperature and analyzing the resulting heat sources evolution.

An infinitely variable transmission (IVT) is a transmission that can smoothly and continuously vary the speed ratio between an input and output from zero to some other positive or negative ratio; they are a subset of continuously variable transmissions (CVTs), which themselves do not have the ability to produce a zero gear ratio. In this thesis, the operation, analysis, and development of a novel, highly configurable, Cam-based Infinitely Variable Transmission of the ratcheting drive type is presented. There are several categories of CVTs in existence today, including traction, belt, and ratcheting types. Drives of these types, their attributes, and associated design challenges are discussed to frame the development of the Cam-based IVT. The operation of this transmission is kinematically similar to a planetary gearset, and therefore, its operation is described with that in mind including a description of the six major components of the transmission, those being the cam, followers, carriers, planet gears, sun gears, and one way clutches. The kinematic equation describing its motion is derived based on the similarities it shares with a planetary gearset. Additionally, the equations for the cam design are developed here as the operation of the CVT is highly dependent on the shape of the cam. There are six simple inversions of this device and each inversion has special characteristics and limitations, for example, the available gear range. A method was developed to select the most suitable inversion, gearing, and follower velocity for a given application. The contact stress between the rollers and cam is the limiting stress within the transmission. A parametric study is used to quantify the relationship between this stress and the transmission parameters. Based off those results, two optimization strategies and their results are discussed. The first is an iterative brute force type numerical search and the second is a genetic algorithm. The optimization results are shown to be similar and successfully reduced the contact stress by 40%. To further improve the transmission performance, several mechanisms were developed for this unique transmission. These include a compact and lightweight differential mechanism based on a cord and pulley system to reduce the contact force on the rollers. In addition, a unique external/inverted cam topology was developed to improve the contact geometry between the rollers and said cam. A prototype was built based on both the optimization strategies and these mechanisms and is described within. Finally, a Prony brake dynamometer with cradled motor was constructed to test the transmission; the results of those tests show the Cam-based IVT to be 93% efficient at low input torque levels. / Master of Science

Os relatórios internos ajudam as organizações a manter seus sistemas de controle gerenciais atualizados e subsidiam vários propósitos, como a definição de metas e a avaliação de desempenho. A literatura prática apresenta como preocupação a quantidade de detalhes que os gestores devem fornecer aos seus superiores nesses relatórios de desempenho. Este estudo considera os relatórios dos gerentes de centros de lucro devido à posição estratégica quepossuem nas empresas e a quantidade de informações que detêm. Com base na teoria da contabilidade mental, apresenta-se como hipótese que os gerentes preferem um formato de apresentação que aumenta a utilidade do desempenho apresentado. Além disso, considera-se o frameworkdos níveis de raciocínio, pois os gerentes podem escolher um formato de apresentação pensando em uma futura decisão de seus superiores. Quando o propósito do relatório interno é a revisão metas, apresentam-se dois argumentos opostos, razão para o desenvolvimento de uma questão de pesquisa em vez de uma hipótese. O primeiro argumento é que os gerentes não seguemos pressupostos da contabilidade mental, ou seja, eles preferem apresentar seu desempenho em um formato que faz com que esse desempenho pareça menos favorável para evitar a definição de metas mais difíceis para o futuro. Como apoio a esse argumento, considera-se a evidência de que os gerentes que sentem uma pressão externa não seguem os pressupostos da contabilidade mental. O segundo argumento para o ratcheting das metas é que os gerentes seguiriam os pressupostos da contabilidade mental, ou seja, preferem apresentar seu desempenho em um formato que torne o desempenho mais favorável. Este argumento leva em consideração evidências que sugerem que o processor de definição de metas pode servir para fins de gerenciamento de impressões para gerentes. Ao se reportar para avaliação de desempenho, seguindo as previsões de contabilidade mental, elaboro a hipótese de que os gerentes preferem apresentar seu desempenho de forma a aumentar a utilidade de seu desempenho. Neste estudo, os gerentes devem apresentar relatórios internos que contenham os desvios de desempenho em relação às metas, e ganhos e perdas são os resultados das variações. Para testar tais questões e hipóteses de pesquisa, empregou-se uma abordagem experimental 2 × 2 between-subjects, em que se manipula o sinal dos desvios do desempenho em relação à meta (perdas ou ganhos) e o propósito do relatório (revisão de metas ou avaliação de desempenho). Quando os gerentes precisam reportar um ganho, tanto para propósito de revisão de metas quanto para a avaliação do desempenho, os resultados indicam que os gerentes possuem preferência similar por agregar ou desagregar um ganho combinado nessa situação. Consequentemente, não há resultados significativos para a questão de pesquisa sobre o propósito da revisão de metas e nenhuma diferença significativa para a hipótese de avaliação de desempenho. Quando os gerentes precisamreportar uma perda, tanto para propósito de revisão de metas quanto para a avaliação do desempenho, os resultados são significativos e indicam que os gerentes preferem desagregar uma perda. Desagregar uma perda mista está de acordo com os pressupostos da contabilidade mental. Considerando um relatório para a revisão de metas, esse resultado é interessante pois os indivíduos preferem apresentar seu desempenho em um formato que melhora o resultado, mesmo que um bom desempenho resulte em uma meta mais difícil para o futuro. Estas conclusões têm implicações práticas. Embora os gerentes possam usar a materialidade para justificar a informação que eles escolhem informar, os superiores devem exercer ceticismo quanto à informação de desempenho reportada. / Internal reporting helps organizations to keep their management control systems updated and subsidizes several purposes, such as target setting and performance evaluation. A concern from practitioner literature regards on how much detail managersshould providefor their superiors in these performance reports. This study considers the reporting from profit center managers due to the strategic position these managers have in firms and the amount of information they hold. Relaying on metal accounting theory, I hypothesize that managers prefer a presentation format that increases the utility of their presented performance. Additionally, I consider the levels of reasoning framework because managers can choose a presentation format thinking in a future decision by their superiors. When the purpose of the internal report is target revision, I have two opposing arguments, which makes me develop a research question instead of hypothesis. The first argument is that managers might not follow mental accounting predictions, that is, they prefer to present their performance in such a format that makes this performance looks less favorable to avoid harder targets for future. Supporting this argument, I consider evidence that managers feeling an external pressure do not follow mental accounting predictions. The second argument for target ratcheting is that managers would follow mental accounting predictions, that is, they prefer to present their performance in such a format that makes this performance looks more favorable. This argument takes into account evidence suggesting that target setting can serve an impression-management purpose for managers. When reporting for performance evaluation,following mental accounting predictions,I hypothesize that managers prefer to present their performance in a way to increase the utility of their performance. In this study, managers have to present internal reports with performance-target deviations, and gains and losses are the results of the variances. In order to test my research questions and hypothesis, I employ a 2 × 2 between-subjects experimental design, that manipulates the sign of performance-target deviations (losses or gains) and the reporting purpose (target revision or performance evaluation).When managers have to present a gain both for target revision and performance evaluation, results indicate that managers prefer either to aggregate or to disaggregate a mixed gain in this situation. Consequently, there is no significant result to the research question regarding the target revision purpose, and no significant difference for the hypothesis regarding performance evaluation.When managers have to present a loss for target review and performance evaluation, results are significant and indicate that managers prefer disaggregate a loss. Disaggregating a mixed loss is in line with metal accounting predictions. Considering a report for target revision, this result is interesting because individuals prefer to present their performance in a format that make their result looks better, even if a good performance results in a harder target for future. This conclusion has some practical implications. Although managers can use materiality to justify the information they choose to report, superiors should exert skepticism regarding the reported performance information

Os relatórios internos ajudam as organizações a manter seus sistemas de controle gerenciais atualizados e subsidiam vários propósitos, como a definição de metas e a avaliação de desempenho. A literatura prática apresenta como preocupação a quantidade de detalhes que os gestores devem fornecer aos seus superiores nesses relatórios de desempenho. Este estudo considera os relatórios dos gerentes de centros de lucro devido à posição estratégica quepossuem nas empresas e a quantidade de informações que detêm. Com base na teoria da contabilidade mental, apresenta-se como hipótese que os gerentes preferem um formato de apresentação que aumenta a utilidade do desempenho apresentado. Além disso, considera-se o frameworkdos níveis de raciocínio, pois os gerentes podem escolher um formato de apresentação pensando em uma futura decisão de seus superiores. Quando o propósito do relatório interno é a revisão metas, apresentam-se dois argumentos opostos, razão para o desenvolvimento de uma questão de pesquisa em vez de uma hipótese. O primeiro argumento é que os gerentes não seguemos pressupostos da contabilidade mental, ou seja, eles preferem apresentar seu desempenho em um formato que faz com que esse desempenho pareça menos favorável para evitar a definição de metas mais difíceis para o futuro. Como apoio a esse argumento, considera-se a evidência de que os gerentes que sentem uma pressão externa não seguem os pressupostos da contabilidade mental. O segundo argumento para o ratcheting das metas é que os gerentes seguiriam os pressupostos da contabilidade mental, ou seja, preferem apresentar seu desempenho em um formato que torne o desempenho mais favorável. Este argumento leva em consideração evidências que sugerem que o processor de definição de metas pode servir para fins de gerenciamento de impressões para gerentes. Ao se reportar para avaliação de desempenho, seguindo as previsões de contabilidade mental, elaboro a hipótese de que os gerentes preferem apresentar seu desempenho de forma a aumentar a utilidade de seu desempenho. Neste estudo, os gerentes devem apresentar relatórios internos que contenham os desvios de desempenho em relação às metas, e ganhos e perdas são os resultados das variações. Para testar tais questões e hipóteses de pesquisa, empregou-se uma abordagem experimental 2 × 2 between-subjects, em que se manipula o sinal dos desvios do desempenho em relação à meta (perdas ou ganhos) e o propósito do relatório (revisão de metas ou avaliação de desempenho). Quando os gerentes precisam reportar um ganho, tanto para propósito de revisão de metas quanto para a avaliação do desempenho, os resultados indicam que os gerentes possuem preferência similar por agregar ou desagregar um ganho combinado nessa situação. Consequentemente, não há resultados significativos para a questão de pesquisa sobre o propósito da revisão de metas e nenhuma diferença significativa para a hipótese de avaliação de desempenho. Quando os gerentes precisamreportar uma perda, tanto para propósito de revisão de metas quanto para a avaliação do desempenho, os resultados são significativos e indicam que os gerentes preferem desagregar uma perda. Desagregar uma perda mista está de acordo com os pressupostos da contabilidade mental. Considerando um relatório para a revisão de metas, esse resultado é interessante pois os indivíduos preferem apresentar seu desempenho em um formato que melhora o resultado, mesmo que um bom desempenho resulte em uma meta mais difícil para o futuro. Estas conclusões têm implicações práticas. Embora os gerentes possam usar a materialidade para justificar a informação que eles escolhem informar, os superiores devem exercer ceticismo quanto à informação de desempenho reportada. / Internal reporting helps organizations to keep their management control systems updated and subsidizes several purposes, such as target setting and performance evaluation. A concern from practitioner literature regards on how much detail managersshould providefor their superiors in these performance reports. This study considers the reporting from profit center managers due to the strategic position these managers have in firms and the amount of information they hold. Relaying on metal accounting theory, I hypothesize that managers prefer a presentation format that increases the utility of their presented performance. Additionally, I consider the levels of reasoning framework because managers can choose a presentation format thinking in a future decision by their superiors. When the purpose of the internal report is target revision, I have two opposing arguments, which makes me develop a research question instead of hypothesis. The first argument is that managers might not follow mental accounting predictions, that is, they prefer to present their performance in such a format that makes this performance looks less favorable to avoid harder targets for future. Supporting this argument, I consider evidence that managers feeling an external pressure do not follow mental accounting predictions. The second argument for target ratcheting is that managers would follow mental accounting predictions, that is, they prefer to present their performance in such a format that makes this performance looks more favorable. This argument takes into account evidence suggesting that target setting can serve an impression-management purpose for managers. When reporting for performance evaluation,following mental accounting predictions,I hypothesize that managers prefer to present their performance in a way to increase the utility of their performance. In this study, managers have to present internal reports with performance-target deviations, and gains and losses are the results of the variances. In order to test my research questions and hypothesis, I employ a 2 × 2 between-subjects experimental design, that manipulates the sign of performance-target deviations (losses or gains) and the reporting purpose (target revision or performance evaluation).When managers have to present a gain both for target revision and performance evaluation, results indicate that managers prefer either to aggregate or to disaggregate a mixed gain in this situation. Consequently, there is no significant result to the research question regarding the target revision purpose, and no significant difference for the hypothesis regarding performance evaluation.When managers have to present a loss for target review and performance evaluation, results are significant and indicate that managers prefer disaggregate a loss. Disaggregating a mixed loss is in line with metal accounting predictions. Considering a report for target revision, this result is interesting because individuals prefer to present their performance in a format that make their result looks better, even if a good performance results in a harder target for future. This conclusion has some practical implications. Although managers can use materiality to justify the information they choose to report, superiors should exert skepticism regarding the reported performance information

Long linear earthwork assets constructed in high-plasticity overconsolidated clay are known to be deteriorating due to long-term effects of wetting and drying stress cycles as a result of seasonal weather patterns. These stress cycles can lead to shallow first-time failures due to the mobilisation of post-peak strength and progressive failure. Design requirements of new earthworks and management of existing assets requires improved understanding of this critical mechanism; seasonal ratcheting. Incremental model development and validation to allow investigation of multiple inter-related strength deterioration mechanisms of cut slope behaviour in high-plasticity overconsolidated clay slopes has been presented. Initially, the mechanism of seasonal ratcheting has been considered independently and a numerical modelling approach considering unsaturated behaviour has been validated against physical modelling data. Using the validated model, the effects of slope geometry, design parameter selection and design life have been considered. Following this, an approach to allow undrained unloading of soil, stress relief, excess pore water pressure dissipation, seasonal ratcheting and progressive failure with wetting and drying boundary conditions has been considered. Hydrogeological property deterioration and the potential implications of climate change have been explored using the model. In both cases the serviceable life of cut slopes is shown to reduce significantly in the numerical analyses. Finally, a model capable of capturing hydrogeological behaviour of a real cut slope in London Clay has been developed and validated against long-term field monitored data. Using the validated model, a climate change impact assessment for the case study slope has been performed. The numerical analyses performed have indicated that seasonal ratcheting can explain shallow first-time failures in high-plasticity overconsolidated clay slopes and that the rate of deterioration of such assets will accelerate if current climate change projections are representative of future weather.

Geostructures experience repetitive load cycles, which gradually affect their long-term performance. This thesis explores the long-term response of soils subjected to mechanical load-unload, heat-cool, freeze-thaw, and atmospheric pressure oscillations. The research methodology involves new instrumented cells (oedometer, temperature-controlled triaxial chamber, and pressure-controlled drying chamber), various geophysical monitoring methods (X-ray micro-CT, NMR, S-wave, and EM-waves), and simulations using discrete element modeling. Results show that soils subjected to repetitive mechanical or environmental loading experience shear and volumetric strain accumulation and changes in saturation (during barometric pressure cycles). In all cases, soils evolve towards an asymptotic terminal void ratio; the change in void ratio is pronounced when the soil exhibits grain-displacive ice formation during freeze-thaw cycles. The initial stress obliquity defines the shear strain response, which may be either shakedown -at low stress obliquity-, or ceaseless shear strain accumulation in ratcheting mode when the maximum stress obliquity approaches failure conditions. Finally, we provide simple engineering guidelines to estimate the long-term behavior of soils subjected to repetitive mechanical or environmental loading.

Cette thèse s'inscrit dans le contexte de l’étude de l’effet de l’identification des paramètres matériau sur la prévision du comportement cyclique. Une étude de l’effet de la pré-déformation sur la réponse du matériau vis à vis le rochet est aussi présentée. Pour cela deux types d’expériences ont été menés au sein de l’équipe Mécanique du Groupe de Physique des Matériaux (GPM, UMR 6634) à l’INSA de Rouen sur un acier inoxydable de type 304L : Les essais EXP1 contenant des essais à déformation imposée axiale et multiaxiale et des essais de rochet en une et deux phases. Ces essais sont tirés de la littérature et nous ont servi de base de donnée pour la réalisation des différentes étapes de l’identification. Les essais EXP2 contenant des essais de rochet après déformation imposée.Ils sont réalisés dans le cadre de cette étude et permettent l’étude de l’effet de la pré déformation sur le phénomène de rochet. L’identification des paramètres est faite‘’automatiquement’’ avec une méthode d’optimisation en utilisant des bases de données à complexité croissante. Les paramètres identifiés sont ensuite injectés dans des simulations qui seront superposées avec les courbes expérimentales. L’ensemble des simulations est fait avec le code de calcul Zébulon avec le choix du modèle de Chaboche. Les résultats obtenus montrent l’effet de la pré déformation sur le rochet selon les histoires, les mêmes conditions à contraintes imposées peuvent mener à différents comportements cycliques : Elastique et adaptation plastique dans d’autre cas. La pré-déformation précédente peut également mener à l'anisotropie du matériau conduisant à un faible rochet sous traction compression avec une contrainte moyenne nulle !L'étude précise également l'importance de la base de données expérimentale utilisée pour l'identification de paramètres. Le choix des essais devrait étroitement être lié aux équations constitutives du modèle choisis . À savoir, la présence des essais à déformation imposée non proportionnelle peut être non appropriée si le modèle ne peut pas tenir compte explicitement d'un tel caractère. / This work has a first objective to point out the importance of the quality of the experimental data base used for the identification of the material parameters. For this goal,we will combine proportional and non proportional strain controlled as well as ratcheting tests to constitute different data bases. This work has also another objective related to an experimental study in order to see how previous strain controlled history may affect ratcheting. Such a situation may be encountered in different manufacturing processes. New tests were performed here where different strain controlled histories have been applied prior to tension-compression stress controlled tests. All tests considered here were carried out using tubular specimens made up of 304L stainless steel. In order to minimize the discrepancy of the material behavior, In this study, we consider two groups of stress/strain controlled experiments EXP1 and EXP2 performed on the 304L stainless steel. These tests have been conducted applying a sufficient number of cycles leading to the stabilization of thecyclic material response. The first set (EXP1) is taken from our previous works, it will be used in order to identify the parameters of Chaboche’s model with different combinations of strain and/or stress controlled tests. The second set (EXP2) is performed in this study; it includes tests composed of two sequences where strain controlled experiments are followed by ratcheting tests. The responses EXP2 will be simulated by Chaboche’s model with the different set of parameters identified with EXP1.The obtained results reveal the importance of such a role as according to the previous history, the same stress controlled conditions may lead to different cyclic behaviors: fully elastic in one case and plastic shakedown in anothercase. The previous pre-straining may also lead to the anisotropy of the material leading to slight ratcheting under tension-compression with zero mean stress! The study points out also the importance of the content of the experimental data base used for the parameters identification. The choice of the tests should be closely linked to the capabilities of the constitutive equations used. Namely, the presence of non proportional strain controlled tests may be not suitable if the model is not able to take into account explicitly such a character. Finally, in the industrial applications, the choice of the tests to be included for the identification process should also take into account the stress and strain states for the identification process should also take into account the stress and strain states applied to the considered components when such information is available.

In the smartphone era, instant messaging is fully embedded in our daily life. Messaging protocols must preserve the confidentiality and authenticity of sent messages both in two-party conversations and in group chats, in which the list of group members may suffer modifications over time. Hence, a precise characterization of their security is required. In this thesis, we analyze the cryptographic properties that are desirable in secure messaging protocols, particularly in asynchronous group key agreement protocols. Our main contribution is a study of the administration of a messaging group, which is a common scenario in which a subset of the group members (the administrators) are the only users allowed to modify the group structure by adding and removing group members. As we discuss, enabling secure group administration mechanisms can enhance the security of messaging protocols. For this purpose, we introduce a new primitive which extends the continuous group key agreement (CGKA) primitive to capture secure administration, which we denote by administrated CGKA (A-CGKA). The definition is followed by a correctness notion and an informal security description. We present two constructions of our A-CGKA that can be built on top of any CGKA: individual admin signatures (IAS), and dynamic group signature (DGS), both constructed using signature schemes. Furthermore, we provide a detailed overview of secure group messaging in which we discuss group evolution, efficiency, concurrency, and different adversarial models. We introduce a novel CGKA correctness definition (in the so-called propose-and-commit paradigm), followed by a security game that incorporates the correctness properties. We also survey some variants of the TreeKEM protocol and compare their security. / I de smarta telefonernas tid är direktmeddelanden en självklar del av vår vardag. Meddelandeprotokoll måste upprätthålla konfidentialitet och autenticitet för skickade meddelanden både i tvåpartskonversationer samt i gruppchatter vars medlemslistor kan förändras över tid. Därför krävs en precis karaktärisering av deras säkerhet. I detta arbete analyserar vi de kryptografiska egenskaper som är önskvärda i meddelandeprotokoll med fokus på asynkrona gruppnyckelavtalsprotokoll (group key agreement protocols). Arbetets huvudsakliga bidrag till området är en studie av administrationen av en meddelandegrupp. Detta är ett vanligt förekommande scenario där endast en delmängd av gruppmedlemmarna (administratörerna) tillåts modifiera gruppens struktur genom att lägga till och ta bort medlemmar. Som diskuteras i arbetet kan användandet av säkra gruppadministrationsmekanismer (group administration mechanisms) förbättra säkerheten för meddelandeprotokoll. I detta syfte introducerar vi en ny kryptografisk primitiv vilken uttökar den s.k. “continuous group key agreement”-primitiven (CGKA) till att även innefatta säker administration. Denna primitiv kallar vi administrated CGKA (A-CGKA), vars definition följs av en korrekthetsdefinition och en informell säkerhetsbeskrivning. Vi presenterar två konstruktioner av A-CGKA som kan byggas ovanpå vilken CGKA som helst: individual admin signatures (IAS) och dynamic group signature (DGS), som båda konstrueras via signaturscheman. Utöver detta ger vi även en detaljerad överblick över säkra gruppmeddelanden i vilken vi diskuterar gruppevolution, effektivitet, samtidighet och olika fientliga modeller. Vi introducerar en ny definition av korrekthet för CGKA (vilket följer paradigmen propose-and-commit) följt av ett s.k. “security game” som inkorporerar korrekthetsegenskaperna. Vi undersöker även varianter av TreeKEM-protokollet och jämför deras säkerhet.

Improvements in quality of life, population growth, and environmental restrictions associated with the burning of fossil fuels will accentuate the need for renewable energy and energy geo-storage. A salient characteristic of these systems is that they impose numerous cycles of effective stress, temperature, and humidity on the surrounding geomaterials. This thesis quantifies future energy consumption based on realizable scenarios and explores the behavior of geomaterials subjected to mechanical and thermal cycles in view of energy-related applications. The long-term behavior of geotechnical systems subjected to a large number of mechanical load cycles is studied with a new numerical scheme based on a hybrid finite element formulation. The numerical scheme satisfies initial conditions as well as fundamental characteristics of soil behavior, such as threshold strain, terminal density, and long-term ratcheting. Numerical results show that shallow foundations subjected to repetitive loading experience strain accumulation and stress redistribution. On the other hand, the long-term behavior of energy piles, exposed geomembranes on slopes, and jointed rock masses subjected to cyclic thermal changes is studied using a combination of numerical, analytical, and experimental methods. Results show that thermal cycles lead to the gradual accumulation of plastic displacements, which may be amplified by thermally-induced wedging in jointed rock masses. In general, cumulative effects caused by repetitive loads increase with the number of cycles, the static factor of safety, the amplitude of the cyclic excitation, and the magnitude of the cyclically-induced displacement with respect to the critical elastic displacement.