Ce travail a pour objectif d'étudier la tenue en fatigue de l'alliage de fonderie A356-T6 sous chargement multiaxial. Des essais en fatigue à 106 cycles ont été effectués pour deux rapports de chargements différents à Rσ = 0 et Rσ = -1. La première partie expérimentale est menée sur des éprouvettes issues d'une coulée en ‘V’ avec des défauts naturels de fonderie et des défauts artificiels avec une variation de la microstructure. Suite aux résultats expérimentaux, nous avons montré que la taille des défauts ainsi que la microstructure caractérisée par la SDAS, sont les principaux paramètres qui influencent la limite de fatigue de cet alliage. Par comparaison entre les résultats obtenus à Rσ = 0 et Rσ = -1, il s'avère que la contrainte moyenne joue un rôle primordial sur la sensibilité du matériau à la taille du défaut et à la SDAS. Dans la suite, des modifications ont été menées sur le critère de DSG qui consistent à introduire l’effet de la SDAS au niveau du critère. L’application de ce critère modifié dans un diagramme de Kitagawa pour les différents cas de chargement a montré que l’abattement de la limite de fatigue en fonction de la taille de défaut et de la SDAS est bien décrit. Dans la dernière partie un outil numérique a été développé permettant de simuler la limite de fatigue en partant du procédé de fonderie. Cette démarche est sous forme d’une chaîne de calcul numérique qui permet de simuler la taille de défaut et de la SDAS à partir du procédé de fonderie. Suite à cette simulation, le modèle est capable de prévoir la limite de fatigue en utilisant le critère de DSG modifié. La combinaison entre la loi de Weibull et le critère de DSG permet à la suite de la chaîne de simulation de prévoir ainsi la probabilité de rupture à chaque point de la structure. Nous avons proposé dans cette partie un moule qui permet d’élaborer des éprouvettes avec deux microstructures différentes. Dans cette étude, une deuxième campagne d’essais a été réalisée sur ces éprouvettes afin de valider la simulation numérique sur le moule proposé. Le modèle numérique prévoit raisonnablement bien les résultats expérimentaux obtenus. / This study aims to investigate the fatigue behaviour of A356-T6 aluminum alloy. Experimental fatigue tests at 106 cycles have been performed for two loading ratios: Rσ = 0 and Rσ = -1. The first experimental investigation was conducted on specimens from a ‘V’ wedge casting with natural and artificial defects which provides a variation of the microstructure. Following the experimental results, we have shown that defects characterized by their size and the microstructure characterized by SDAS, are the main parameters that control the fatigue limit. By comparing the results obtained for both loading ratios, it appears that the mean stress has an effect on the sensitivity to the defect size effect and microstructure.The DSG criterion was modified to introduce the effect of SDAS. This improved DSG criterion has been employed to predict the Kitagawa diagram for multiaxial loading for different loading cases. The simulation of the modified DSG criterion showed that the reduction of the fatigue limit with the defect size and SDAS is well described. In the last part a numerical model was developed to perform a simulation of the fatigue limit starting from the casting process. Using this numerical model, we simulated the defect size and SDAS depending on the solidification time, eventually the fatigue limit issimulated using the improved DSG criterion. With combining between Weibull law and modified DSG,we predict the probability of failure at each point of the structure. We proposed in this part a mold which let to obtain samples with two different microstructures. In this study, a second fatigue tests was carried out on these samples to validate the numerical simulation on the proposed mold. It turns out that the numerical model provides reasonably well the obtained experimental results.

In this work, the Bauschinger effect is shown to be intimately tied not only to plasticity but to damage as well. The plasticity-damage effect on the Bauschinger effect is demonstrated by employing different definitions (Bauschinger Stress Parameter, Bauschinger Effect Parameter, the Ratio of Forward-to-Reverse Yield, and the Ratio of Kinematic-to-Isotropic Hardening) for two differently processed aluminum alloys (rolled and cast) in which specimens were tested to different prestrain levels under tension and compression. Damage progression from second phase particles and inclusions that were generally equiaxed for the cast A356-T6 aluminum alloy and elongated for the rolled 7075 aluminum alloy was quantified from interrupted experiments. Observations showed that the Bauschinger effect had larger values for compression prestrains when compared to tension. The Bauschinger effect was also found to be a function of damage to particles/inclusions, dislocation/particle interaction, the work hardening rate, and the Bauschinger effect definition.

Gradientes térmicos induzidos no interior de componentes sujeitos a variações de temperatura durante o período de funcionamento podem provocar a ocorrência de tensões e deformações internas. A repetição destes ciclos térmicos pode causar a nucleação e a propagação de trincas por um processo denominado fadiga termomecânica. Este trabalho apresenta um estudo sobre o comportamento da liga de alumínio A356-T6, processada nas condições fundida e tixoextrudada, sob fadiga isotérmica e termomecânica. Foram realizados ensaios de fadiga de baixo ciclo isotérmica para as temperaturas de 120 e 280°C, e ensaios de fadiga termomecânica em-fase e fora-de-fase para a faixa de temperatura de 120 a 280°C. O material tixoextrudado apresentou melhor desempenho em fadiga nas condições isotérmica e anisotérmica (termomecânica) devido a uma microestrutura globular com menor nível de porosidade. / Thermal gradients induced in components during service under temperature changes can cause internal stresses and strains. This cyclic thermal behavior can cause crack nucleation and propagation under a process denominated thermomechanical fatigue. Permanent mold casting and tixoextruded A356-T6 aluminum alloy behavior under isothermal and thermomechanical fatigue was study in this work. Isothermal low cycle fatigue tests were performed in temperatures of 120 and 280°C. In-phase and out-of-phase thermomechanical fatigue tests were carried out in temperature range from 120 to 280°C. The tixoextruded material presented better isothermal and thermomechanical fatigue performance due to a globular microstructure and lower porosity level.

Gradientes térmicos induzidos no interior de componentes sujeitos a variações de temperatura durante o período de funcionamento podem provocar a ocorrência de tensões e deformações internas. A repetição destes ciclos térmicos pode causar a nucleação e a propagação de trincas por um processo denominado fadiga termomecânica. Este trabalho apresenta um estudo sobre o comportamento da liga de alumínio A356-T6, processada nas condições fundida e tixoextrudada, sob fadiga isotérmica e termomecânica. Foram realizados ensaios de fadiga de baixo ciclo isotérmica para as temperaturas de 120 e 280°C, e ensaios de fadiga termomecânica em-fase e fora-de-fase para a faixa de temperatura de 120 a 280°C. O material tixoextrudado apresentou melhor desempenho em fadiga nas condições isotérmica e anisotérmica (termomecânica) devido a uma microestrutura globular com menor nível de porosidade. / Thermal gradients induced in components during service under temperature changes can cause internal stresses and strains. This cyclic thermal behavior can cause crack nucleation and propagation under a process denominated thermomechanical fatigue. Permanent mold casting and tixoextruded A356-T6 aluminum alloy behavior under isothermal and thermomechanical fatigue was study in this work. Isothermal low cycle fatigue tests were performed in temperatures of 120 and 280°C. In-phase and out-of-phase thermomechanical fatigue tests were carried out in temperature range from 120 to 280°C. The tixoextruded material presented better isothermal and thermomechanical fatigue performance due to a globular microstructure and lower porosity level.

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