[pt] A comunicação óptica por laser no espaço livre é uma área que vem despertando crescente interesse, nos últimos anos, em função da possibilidade de resolver o problema de difusão da informação, dando acesso de alta capacidade ao usuário. Sistemas ópticos sem fio oferecem rapidez na sua instalação e inicialização, além de um sistema flexível com largura de banda equivalente à da fibra óptica, em torno de 1.5 Gbps para sistemas comerciais disponíveis atualmente. O cerne da problemática que envolve as aplicações de sistemas ópticos sem fio é a propagação óptica no espaço livre. A grande diferença entre a transmissão a laser no espaço livre e na fibra óptica é a previsibilidade da atenuação da potência do sinal do laser na fibra quando comparado à atmosfera. Além da variabilidade da atenuação atmosférica devida à presença de partículas e aerossóis, um dos fenômenos que afeta a propagação de um feixe laser é a turbulência atmosférica, que ocorre mesmo em condições de alta transparência. Flutuações randômicas na temperatura do ar produzem pequenas heterogeneidades no índice de refração ao longo do caminho de propagação da luz. Essas alterações no índice de refração provocam flutuações na velocidade de fase do sinal que se propaga, causando distorção da sua frente de onda. À medida que a frente de onda se distorce e avança num meio com turbulência, ocorrem mudanças aleatórias na direção do feixe gerando flutuações na sua intensidade, contribuindo para a degradação do sinal na recepção. Nesta tese os efeitos da cintilação, decorrentes da turbulência atmosférica, foram avaliados por meio de um experimento utilizando três enlaces operando no espaço livre em três comprimentos de onda diferentes. Foi observado que o speckle gerado pela fibra óptica de alimentação dos transmissores de 780 nm and 1550 nm acentua os efeitos da cintilação. / [en] Optical laser communication in free space is an area that has been attracting increasing interest in the last years, due to its possible capacity to resolve the problem of information diffusion, giving higher capacity access to users. Wireless optical systems offer speedy installation and initialization procedures and system flexibility, with the equivalent frequency bandwidth as optical fiber systems, around 1.5 Gbps for the commercial systems available nowadays. The critical aspect involving the application of wireless optical systems is free space optical propagation. The great difference between the laser free space and optical fiber transmissions is the capacity to predict the signal power attenuation that propagates into the optical fiber, when compared to the atmosphere propagation. Besides the variability of the atmospheric attenuation due to the presence of particles and aerossois, one of the phenomena that affects laser beam propagation is atmospheric turbulence, that occurs even in high transparency atmospheric conditions. Random fluctuations in air temperature generate small inhomogenities in the refraction index throughtout the light propagation path. These changes in the refraction index cause fluctuations in the phase speed of the signal that is spread over this path, causing distortion in its wave-front. As the wave-front distorts and reaches medium with turbulence, random changes occur in the beam direction, creating fluctuations in its intensity, which contribute to the degradation of the signal reception. In this thesis the effects of the scintillation, due to atmospheric turbulence, were experimentally evaluated using three free space links with three different wavelengths. The experimental results have shown that the speckle pattern generated by the optical fiber feeding the 780 nm and 1550 nm transmitters affected the link performance at these wavelengths.

Malgré plusieurs décennies d'études expérimentales et numériques sur la tenue en fatigue d'assemblages soudés, cette problématique demeure une préoccupation principale de l'industrie automobile. En effet, de nombreux composants aux géométries complexes (e.g. berceaux) sont obtenus à partir de pièces soudées et constituent des éléments de sécurité pour lesquels aucune défaillance ne peut être acceptée. Malgré les progrès en modélisation numérique, des essais sont toujours nécessaires afin de fournir des données sur éprouvettes « simples » afin d'alimenter les modèles numériques, mais aussi de démontrer le bon dimensionnement des structures. Les procédures classiques d'essais de fatigue sont coûteuses en temps et nécessitent la destruction de plusieurs pièces. La méthode dite d'auto-échauffement permet de réduire significativement le temps d'essai et propose une approche non destructive. Cette méthode consiste à mesurer l'évolution de température en surface de la structure étudiée au cours du chargement cyclique. Cette approche permet de tirer avantage du signal thermique macroscopique afin de mettre en évidence la micro-plasticité responsable de la rupture par fatigue. Les objectifs du travail présenté sont de déterminer si un lien peut être réalisé entre les mesures de température et la tenue en fatigue d'assemblages soudés, puis d'étudier l'influence de paramètres sur la tenue en fatigue grâce à la méthode proposée. Un protocole expérimental est d'abord proposé afin de mesurer l'évolution de température des mini-structures soudées sous sollicitation cyclique. Une première analyse de la réponse thermique de simples tôles d'acier sous chargement cyclique de faible amplitude est proposée afin de valider un modèle déterministe de champ de source à la fois pour des sollicitations de traction et de flexion. Ces essais permettent par la suite de décrire correctement le comportement thermique de la matière de base des éprouvettes soudées hors du cordon de soudure. Le modèle est alors étendu à l'étude d'éprouvettes soudées, avec la prise en compte de la dissipation de la zone soudée. Une fois le champ de source identifié à partir du modèle, son évolution en fonction de l'amplitude du chargement appliqué mène à une courbe d'auto-échauffement. L'analyse de cette courbe permet une bonne estimation de la limite en fatigue de l'éprouvette soudée après seulement quelques heures d'essais. La méthode est alors appliquée à d'autres configurations de mini-structures soudées afin d'étudier l'influence du grenaillage et d'un gradient de contrainte en zone critique sur la tenue en fatigue. Finalement, la procédure d'essai est appliquée afin de déterminer la tenue en fatigue d'une pièce industrielle, un triangle de suspension mécano-soudé. À cause des mauvaises conditions aux limites thermiques sur une telle structure, une autre méthode est proposée afin de déduire une première estimation du champ de source sur toute la surface observée à partir des mesures thermiques. Cette estimation mène à l'identification de la zone critique en fatigue de la structure et à une première bonne estimation de sa limite en fatigue. / Fatigue design of weld assemblies still remains of prior concern in the automotive industry, despite several decades of experimental, theoretical and numerical body of work. Actually, many complex components (e.g. front and rear axles) are embedded thanks to welding process. Such welded assemblies are expected to be designed as high-safety parts, for which any fatigue failure is supposed to be prevented. Despite numerical modelling is more and more effective, experimental tests are still necessary in order to provide basic design data and at last to prove the design reliability. Standard fatigue tests procedures are sensibly time consuming, are usually destructive and need for several specimens in order to manage reliable results. Conversely, the so-called self-heating tests offer the opportunity to dramatically shorten the test duration and save specimens because it is a non-destructive method. It consists in measuring the temperature evolution of the structure surface during cyclic loading. Such an approach allows to take advantage of the macroscopic thermal signature of microscopic plasticity processes responsible for fatigue damage. The purpose of the present work is the determination of a correlation between thermal measurement and fatigue properties of welded structures, and then studying the influence of parameters on fatigue properties with the developed method. An experimental protocol is first proposed to measure the temperature of the tested specimen under cyclic loading. A first analysis of the thermal response of standard steel sheet specimen under low load amplitude is proposed in order to validate a deterministic heat source model for both tensile and bending tests. These tests allow us to correctly describe the thermal behavior of the base material of welded specimen away from the joint. Then, the model is extended to welded specimen in order to take into account the dissipation of the welded area. Once the dissipative heat source is identified thanks to the model, its evolution with the applied loading leads to a self-heating curve allowing an estimation of the fatigue limit of the welded specimen after only few hours of test. The method is then applied to other configurations of welded specimens in order to study the effects of shot peening and stress gradient on fatigue properties. Finally, the testing procedure is applied for the determination of an industrial component fatigue properties, a welded car wishbone. Due to the bad heat boundary condition, another analysis is proposed to estimate the heat source along the entire component. This estimation leads to the determination of the weakest area of the structure concerned by fatigue and a first good estimation of its fatigue limit.