La théorie économique néoclassique a historiquement évolué autour du concept d’équilibre (partiel ou général), supposé stable à long terme et indépendant des fluctuations monétaires ou réelles autour de l’équilibre même. L’attention plus récente vers le principe de dépendance au sentier et, en particulier, l’émergence du concept d’hystérèse en économie, remet en cause les propriétés de ces équilibres, notamment en ce qui concerne l’unicité, la stabilité et l’indépendance par rapport aux fluctuations. La thèse se concentre sur le modèle dit d’hystérèse « véritable », qui a ses origines dans la physique. Loin de promouvoir une approche scientifique « dure » en l’économie, il s’agit d’analyser les conséquences des discontinuités d’investissement des entreprises sur les fluctuations et sur les trajectoires de long terme. A’ travers l’approche théorique Postkeynésienne et l’approche méthodologique multi-agents, la thèse développe un modèle qui est capable de générer des fluctuations non linéaires autour d’équilibres purement transitoires, c’est à dire qui s’établissent de manière endogène à partir des sentiers d’ajustement effectivement entrepris. Dans ce cadre analytique, on retrouve renforcées les implications Postkeynésiennes de l’inégalité dans la distribution du revenu, sur l’utilisation des capacités productives existantes et sur le taux de croissance de l’économie. De surcroit, les politiques économiques de relance dites keynésiennes regagnent une place centrale sur le court ainsi que sur le long terme. / The neoclassical theory developed historically around the concept of equilibrium (partial or general), by assuming its long run stability and independence from monetary and real fluctuations. The growing emphasis on path-dependence and, particularly, on the concept of hysteresis calls into question the traditional method, by rejecting the theoretical validity of the neoclassical equilibrium and its related properties of stability. This thesis focuses on the model of “genuine” hysteresis, which first developed in the field of physics and recently extended its application to economic phenomena. Far from suggesting an appropriation of the methods that are typical of “hard” sciences, the aim is to analyse the consequences of discontinuous and hysteretic investment decisions on business cycles and long run trajectories. By relying on the Post Keynesian theory of growth and distribution, and the multi-agent methodological approach, this thesis develops a macroeconomic theoretical model that is able to generate non-linear business cycles around transitory equilibria, which are fully endogenous and historically determined according to the specific adjustment path. This theoretical framework confirms and reinforces the traditional Post Keynesian implications of income inequalities on the degree of utilization of productive capacity and on long run growth. Moreover, expansionary demand policies regain a central role in driving the economy towards the full employment of productive resources.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Vita. / Includes bibliographical references. / by Joseph Vito Minervini. / Ph.D.

A idade afeta vários fatores do sistema locomotor, incluindo as propriedades mecânicas. Os ossos são materiais viscoelásticos e uma característica destes materiais é a ocorrência de histerese quando são submetidos a ciclos de carregamento e descarregamento. Nesta pesquisa nós estudamos a tensão máxima e a energia de histerese de fêmures de ratas de diferentes idades, submetidos a ensaios de flexão em três pontos. Foram utilizados os fêmures de 90 ratas fêmeas, da raça Wistar, os animais foram divididos em três grupos de 30 animais de acordo com a idade: 25 dias (A), 65 dias (B) e 365 dias (C). Cada grupo foi subdividido em 3 lotes de 10 animais. Todos os fêmures foram submetidos a ensaio de flexão em três pontos. De cada par de ossos, o fêmur esquerdo foi submetido a carregamento até a ruptura (ensaio destrutivo) e, o fêmur direito foi submetido a carregamento até 40% (lote I), 60% (lote II) e 80% (lote III) do limite elástico obtido do fêmur contralateral e, então, descarregado. Comparando os grupos B e C, as energias de carregamento e de descarregamento aumentaram com a idade e com o nível de carregamento, mas, a energia de histerese aumentou somente com o nível de carregamento. Na comparação entre os três grupos, a tensão máxima aplicada aumentou significativamente com a idade (p 0,001) e, a perda de energia relativa foiestatisticamente igual entre todos os lotes de uma mesma idade, 25 dias (p=0,136), 65 dias (p=0,583) e 365 dias (p=0,177), mas diminuiu significativamente com a idade (p<0,001). Podemos concluir que a idade exerceu influências na tensão máxima e no comportamento viscoelástico dos fêmures de ratas, mas são necessários estudos mais profundos para compreendermos melhor este material. / Ageing affects many features of the locomotor system, including its mechanical properties. Bone is a viscoelastic material what means that it displays the phenomenon of hysteresis when submitted to loading-unloading cycles. In the present research we studied the behavior of the ultimate stress and the energy of hysteresis of femurs of rats in different ages, when tested in bending. Ninety femurs collected from female Wistar rats were used. There were three groups classified according to the animal age: 25 days (A), 65 days (B) and 365 days (C). The groups were divided into 3 subgroups (10 animals each). All the femurs were submitted to a three-point bending test. For each paired bones, the left femur was loaded until failure and the right femur was load to 40% (subgroup I), 60% (subgroup II) and 80% (subgroup II) of the yield load in relation to the opposite side femur and, then, unloaded. Comparing the groups B and C, the energies of loading and unloading increased with age and loading level, but, the energy of hysteresis increased only with loading level. The comparison among the three groups showed that the ultimate stress increased with age (p 0.001) and, the relative energy loss was equal for the subgroups belonging to the same group, A (p=0.136), B (p=0.583) and C (p=0.177), but different when the different groups were compared (p<0.001). It was concluded that ageing affects both the ultimate stress and the viscoelastic behavior of rat femurs, but further studies are necessary to deepen the knowledge in such matter.

Cette thèse porte sur la force de résistance au mouvement à faible vitesse d'un objet (sphère, cylindre ou disque) dans un empilement de grains dense et sec. Des expériences ont été menées sur deux dispositifs permettant un mouvement à vitesse contrôlée, en s’intéressant à l’influence des tailles des objets et des grains, des conditions aux limites de l’empilement, de la gravité, et d’une éventuelle vibration de l’empilement. Dans un premier dispositif expérimental permettant un mouvement vertical, nous avons consacré une partie importante de ce travail à l'étude de la forte asymétrie de la force lors de cycles de pénétration/extraction d’un cylindre horizontal et d’une sphère. Pour tenter de comprendre l'origine de cette asymétrie, différentes conditions aux limites ont été considérées : parois rigides ou souples sur les côtés ou au fond de l’empilement, surface libre ou surmontée d’un couvercle plus ou moins chargé en haut de l’empilement. La longueur caractéristique issue du champ de vitesse des grains autour d'un cylindre a été montrée comme la longueur pertinente à considérer pour le confinement latéral, ainsi que pour les déformations de la surface libre telles que la formation d’un cratère consécutif à la pénétration ou d’une bosse lors de l’extraction. Ces déformations peuvent être retrouvées par intégration d’un modèle de champ de vitesse autour de l’objet. La présence d'un couvercle chargé a par ailleurs permis de mettre en évidence une riche et complexe variété de comportements, notamment en extraction où la force ne diminue plus avec la profondeur de l’objet. Dans un second dispositif permettant le mouvement horizontal d’un gros disque intrus à l’intérieur d’une couche de petits disques photoélastiques vibrés ou non, nous avons mesuré la force globale sur l’intrus et visualisé les distributions de contraintes au sein du milieu granulaire. Au-delà des importantes fluctuations spatio-temporelles, des valeurs moyennes de force sur l’intrus et des tenseurs locaux de contraintes et taux de déformations autour de l’intrus ont pu être obtenus, dans la perspective d’établir une loi de comportement locale pour le milieu. La vibration de la couche de grains a été montré pouvoir changer considérablement la dépendance de la force avec la vitesse de l’intrus, avec un effet de fluidification du milieu. / This thesis focuses on the force of resistance to the slow motion of an object (sphere, cylinder or disk) in a dense and dry granular medium. Experiments were conducted using two devices that permitted movement at constant velocity, with the aim of studying the influence of the object size, the grain size, the boundary conditions, and vibration of the grains. Using the first device in which the object moved vertically, we studied in detail the strong asymmetry of the resistance force during cycles of penetration/withdrawal of a horizontal cylinder and a sphere. In an attempt to understand the origin of this asymmetry, we implemented different boundary conditions: rigid and deformable walls on the sides and/or the bottom of the device, and free and quasi-rigid (loaded cover) conditions at the top. Previous work has shown that the characteristic length scale of the velocity field around a moving cylinder is the relevant length for both the lateral confinement and the deformation of the free surface, such as the formation of a crater during penetration or a mound during withdrawal. These deformation fields can be found by integrating a model of the velocity field around the object. Our use of a loaded-cover boundary condition has also revealed a variety of complex behaviors, including a force that no longer decreases with the depth of the object during the withdrawal phase. The second device allows the horizontal movement of a large hard intruder into a monolayer of small photoelastic disks which could be vibrated or not. Using this, we measured the overall force on the intruder and visualized the stress distribution in the granular medium. We were able to measure not only the large spatio-temporal velocity fluctuations, but also average values of the force on the intruder and the local stress and strain rate tensors around it, which allowed us to determine a local constitutive law for the medium. Finally, we have shown that vibration of the grains significantly changes the dependence of the force on the intruder speed, indicating a fluidization effect.

Economia / Doctoral Programme in Economics / Estudos empíricos anteriores mostram que quando as decisões são tomadas num contexto de incerteza e quando existem custos de ajustamento lineares ou fixos (não convexos), as empresas não ajustam continuamente o nível de emprego de forma a acomodar choques da procura do seu produto. Consequentemente, emergem períodos de inércia o que é suficiente para produzir histerese. Nesta dissertação estuda-se a existência de histerese da dinâmica do emprego ao nível da empresa e ao nível agregado. Em primeiro lugar, efectua-se uma descrição do padrão de ajustamento do emprego a nível microeconómico e estuda-se a sua relação com três fontes de inércia: i) a existência de custos de ajustamento não convexos; ii) a existência de incerteza na dinâmica da procura agregada; iii) a possibilidade de utilização da margem intensiva de ajustamento do factor trabalho (ajustamento através do número de horas por trabalhador). Segundo, analisamos as implicações agregadas do comportamento microeconómico observado. Se ao nível microeconómico os modelos de histerese oferecem uma boa explicação para a observação empírica, ao nível macroeconómico tem-se revelado mais difícil identificar a existência de histerese na dinâmica do emprego. De facto, as séries agregadas do emprego tendem a ser mais alisadas, e por essa razão, aparentemente inconsistentes coma existência de histerese. No entanto, se tivermos em conta as diferentes propriedades da histerese fraca (histerese ao nível micro) e da histerese forte (histerese ao nível macro) e se considerarmos a existência de empresas heterogéneas, isto é, se o problema da agregação for explicitamente considerado, como deve ser na presença de custos de ajustamento não convexos, então deverá ser possível verificar a existência de sinais de histerese ao nível macroeconómico. A análise empírica foi efectuada com dados mensais de empresas industriais portuguesas ao longo de um período de 11 anos. A amostra contém informação sobre o nível de emprego e sobre o nível de horas de trabalho e sobre um conjunto de outras variáveis que podem ser utilizadas como proxies de choques. No sentido de obter uma primeira aproximação ao processo de ajustamento do emprego, efectuamos uma análise descritiva sobre a variação líquida do emprego e testamos a existência de histerese ao nível da empresa através da estimação de um modelo de resposta assimétrica do emprego, interpretado á luz do modelo de histerese Non-Ideal Relay. De forma a testar a existência de histerese na dinâmica do emprego a nível agregado, aplicamos testes construídos com base em métodos computacionais baseados no modelo de Preisach e no Linear Play Model de histerese. No sentido de comparar os resultados a nível internacional, aplicamos os testes referidos a dados agregados da OCDE e EUROSTAT de 19 países da OCDE.Concluímos que: i) existem sinais claros da existência de inércia ao nível microeconómico causada pela existência de custos de ajustamento não convexos e pela possibilidade de ajustamento através da variação do número de horas de trabalho por trabalhador; ii) os sinais de histerese que normalmente se encontram ao nível microeconómico não se desvanecessem totalmente ao nível macroeconómico; iii) as propriedades de histerese são particularmente relevantes na dinâmica do emprego das empresas pequenas; iv) encontramos evidência significativa sobre a interacção entre a flexibilidade do ajustamento do factor trabalho através da variação do número de horas de trabalho por trabalhador e a existência de histerese no emprego. Ao contrário, não encontramos evidência significativa sobre a interacção entre a existência de incerteza na dinâmica da procura agregada e a existência de histerese no emprego. Estes resultados mostram que a dinâmica do emprego a nível agregado é condicionada significativamente pela existência de um padrão de ajustamento discreto ao nível microeconómico. / Previous empirical studies have shown that when decisions are made under uncertainty and adjustment costs are fixed or linear in structure (non-convex), firms do not permanently adjust employment in order to accommodate demand shocks. Consequent to this, periods of inertia would emerge and that is sufficient to produce hysteresis. This dissertation studies the existence of hysteresis in the dynamic path of employment at the firm and aggregate level. Firstly, we describe the path of micro-level employment and we establish its relationship with three sources of inertia: i) the existence of non-convex costs of adjustment; ii) uncertainty concerning the dynamics of aggregate product demand; iii) utilization of the intensive margin of adjustment of the labor input (adjustment through hours per employee). Secondly, we analyze the aggregate implications of the observed micro behavior. If at the micro level models of hysteresis offer a good explanation for the empirical evidence, at the macro level it has been more difficult to identify the existence of hysteresis in the dynamics of employment. Aggregate series of employment tend to look smoother and, for that reason, they are apparently inconsistent with the presence of hysteresis. However, if we take into consideration the different properties of weak hysteresis (hysteresis at the micro level) and strong hysteresis (hysteresis at the macro level), and if we take into account firms heterogeneity, i.e. if the problem of aggregation is explicitly considered as it should be in the presence of non-convex costs of adjustment, it would still be possible to uncover signs of hysteresis at the macro-level. The empirical analysis was carried out with a monthly panel of Portuguese manufacturing firms spanning a period of eleven years. This dataset has information on both employment and hours of work as well as on a good set of other variables that may be taken as proxies for shocks. To obtain a first insight into the process of employment adjustment, we provide some descriptive statistics on net employment changes, and to test the existence of hysteresis at the micro level we estimate a model of employment asymmetric response with path dependence interpreted under the Non-Ideal Relay model of hysteresis. To test the existence of hysteresis in the aggregate employment dynamics, we apply tests constructed with the help of computational methods based on the Preisach Model and on the Linear Play Model of Hysteresis. To put our results in an international setting, the aggregate analysis was also made with aggregate data from OECD and EUROSTAT. We conclude that: i) there are strong signs of the existence of sources of employment inertia at micro level, caused by non-convex adjustment costs and by the adjustment of labor input through the number of hours per employee; ii) signs of hysteresis commonly found at the micro level, do not completely vanish at the macro level; iii) hysteresis properties are particularly discernible for small firms even if they are less so in the case of larger units; vi) we find strong evidence of the interrelations between the flexibility of the labor input adjustment through hours of work and the existence of aggregate employment hysteresis, but only weak evidence of the interrelations between the existence of uncertainty in the dynamics of aggregate demand and hysteresis. These findings imply that aggregate employment is significantly shaped by lumpy adjustment at the micro level.

Event-based sediment budgets were developed in a small agricultural sub-watershed using radionuclide tracers in conjunction with traditional monitoring techniques. The result of these budgets quantified the flux of material from each sediment source in the sub-watershed. The first step in the study was to quantify the net flux of material through the watershed outlet for runoff events. Results indicated that a pronounced clockwise hysteresis effect occurred during all studied events. The cause of the hysteresis effect was attributed to exhaustion of the upland (i.e., hillslopes and floodplains) sediment source. The hysteresis effect was dampened during an extreme flash flood event that caused overbank flow. This dampening was attributed to an increase in upland sediment mobilization resulting from overbank flow. Results of the event-based monitoring were compared against a previously developed sediment rating curve that assumed a power-law relationship between suspended sediment transport and water flow rates. The results indicate that the power-law relationship grossly under predicted the sediment flux over each runoff event. A tracing technique was utilized to establish the relative contributions from the uplands and the stream channel (i.e., channel banks and bed). This technique used the relationship between the naturally occurring radionuclide tracers 7Be and 210Pbxs to differentiate eroded upland soils from channel-derived sediments in the suspended sediment loads. A simple two end-member unmixing model was used to determine the relative contribution from each source. Results indicate that the upland source was the dominant contributor to the suspended load early in the runoff events, but channel contributions were more prevalent at later stages, reinforcing the conclusion drawn from the hysteresis observation. Further analysis of the results indicated that the uplands contributed the majority of the material to the suspended load because the sediment transport rate during the early stages of the event was much larger than during the later stages. Therefore, watershed managers wishing to minimize non-point source pollution resulting from erosion should first focus their efforts on reducing erosion of upland soils.

A series of tests were performed on nickel-titanium alloy wire, also known as nitinol, to determine the plausibility of designing an actuator using this wire as the method of actuation. These tests have been designed to fully characterize how the wire behaves under steady state and transient conditions allowing for a specific wire selection to be made given known actuator specifications which will result in an efficient design. The wire transient data can be used to design a controller which reduces the actuation time. The research done for the overall project covers a wide scope including wire hysteresis, nitinol transition temperature, variable wire resistance, wire actuation as a function of current and pull force, cable fabrication, and wire actuation control to optimize performance. Using these test results, a prototype actuator has been designed using nitinol wire. It has been determined that an actuator can be efficiently designed using this material.

Au cours de ce travail, nous avons décrit et mis en œuvre la fabrication de surfaces nanostructurées d’alumine par anodisation de feuilles d’aluminium de très grande pureté. Les paramètres morphologiques caractérisant la membrane d’oxyde que sont le diamètre des pores, leur profondeur et leur espacement sont finement contrôlés par les paramètres expérimentaux. Ces surfaces nanotexturées ont permis l’étude approfondie de l’interaction solide-liquide au sein des pores et de la physique de la ligne de contact devant composer avec les nano-aspérités de surface. Ces deux éléments ont pu être appréhendés par des mesures d’angles de contact à l’équilibre et d’hystérésis de mouillage. La modélisation des résultats a montré l’inadéquation des modèles classiques de CASSIE, WENZEL ou de capillarité à cette situation. L’adjonction du terme controversé de tension de ligne permet de bonnes prévisions. Nous montrons que cette interprétation n’est pas unique ; une diminution de l’énergie de surface due à la forte courbure des pores conduit à des résultats identiques. Une investigation théorique a été menée par l’intégration des forces de VAN DER WAALS. La baisse de l’énergie de surface est prévue mais dans des proportions insuffisantes. Le modèle peut être amélioré. Les mesures d’hystérésis ont dévoilé le pouvoir adhésif des surfaces nanoporeuses. À cause des forces de capillarité dans les pores, la ligne de contact ne peut jamais reculer. Les angles d’avancée ont montré que la ligne de contact a une épaisseur négligeable devant la dizaine de nanomètre. Elle peut en outre parfaitement contourner les pores, imprimant de fortes courbures à l’interface liquide-gaz à la base de la goutte. Sa forme tridimensionnelle a été abordée au travers d’un modèle numérique restant à perfectionner. / In this work, we have described and carried out the fabrication of nanostructured alumina surfaces by anodizing highly pure aluminum foils. The pore diameter, depth and spacing are finely controled through experimental parameters. These nanotextured surfaces allowed a thorough study of the solid-liquid interactions within the pores and of the contact-line constrained by the surface nanoasperities. Equilibrium contact-angle and wetting hysteresis measurements were helpful to apprehend them. Modeling the results has revealed the inability of classical CASSIE, WENZEL and capillarity models to properly match the situation. Adding the controversial line-tension term solves the problem and provides good predictions. Anyway, this interpretation is not unique. A lower surface energy within the pores due to their strong curvature yields the same modeling quality. This case has been theoretically investigated through the integration of VAN DER WAALS’ forces. A surface-energy decrease has been calculated but it is not as strong as required. The model still can be improved. Hysteresis measurements have highlighted the nanoporous surfaces are strongly adhesive. Because of the pore size, the capillarity is very marked and keeps the contact line from receding. The advancing contact angles have shown that the contact-line thickness is negligible with respect of ten nanometres. In addition, it can circumvent the pore openings which involves strong liquid–gas interface curvatures at the drop base. The three-dimensional liquid–gas interface shape has been studied with a numerical model that still has to be enhanced.

Mobile terminals allow users to access service while on the move. This unique feature has driven the rapid growth in the mobile network industry, changing it from a new technology into a massive industry in less than two decades. In this thesis, an in-depth study of the handover effects of mobile WiMAX networks is carried out. The mobile WiMAX technology is first presented as literature study and then the technologies of handovers for previous generations are introduced in detail. Further, the hard handover of the mobile WiMAX is simulated by Network Simulator-2 (NS-2). In addition, the "ping-pang" effect of handover was investigated and the call blocking and dropping probabilities are implemented using MATLAB. The goal is to find out which parameters have the significant impact on the handover performance. The results showed that the threshold and hysteresis margin of the handover should be selected by considering the tradeoff between the "ping-pang" effect and the extra interference causing to neighboring cells due to the poor quality link. The handover latency of mobile WiMAX is below 50 ms with the traveling speed of mobile station up to 20 m/s.

Cette thèse analyse les influences des interactions sociales sur les comportements économiques des ménages. Les estimations sur données ivoiriennes permettent de montrer deux types de résultats. Premièrement, la probabilité d'exercer sur un segment du marché du travail croît avec la proportion des individus du groupe qui exercent sur ce segment. Deuxièmement, la réversibilité temporelle de la consommation est rejetée pour certains types de dépenses tels que l'Alimentation et le Transport : les habitudes de consommation jouent un rôle important. De plus, les dépenses de Logement et de Transfert sont les plus sensibles à l'environnement social et donc les plus sujettes à des usages ostentatoires. Par ailleurs, un modèle théorique permet d'expliquer que la consommation ostentatoire se justifie par le besoin de reconnaissance sociale de l'individu dans la société.