The effects of pore density and distribution were investigated on the fatigue crack initiation behavior in an A713 sand cast aluminum alloy plate of 12 mm thickness. The applied stress- the number of cycles to failure (S-N) curves of the samples taken from 2 mm and 5 mm from the free surface were obtained using four-point bend fatigue testing at room temperature, frequency of 20 Hz, stress ratio of 0.1, sinusoidal waveform, and in ambient air. The fatigue strengths of both, the 2 mm and 5 mm samples were 60% of the yield strength (σy=171.9 MPa) of the alloy. Optical microscopy, SEM, and EDS mapping were used to characterize pores and particles in 2 mm and 5 mm samples. The average pore sizes of the 2 mm and 5 mm samples were measured to be 10 to 14 μm, and 14 to 32 μm, respectively. The pore number densities in 5 mm and 2 mm samples were comparable, but higher number densities of non-clustered coarse pores (gas pores) were observed in 5 mm samples. The crack population found after fatigue testing showed a Weibull function of stress level. The peaks of strength distributions of fatigue weak link density of 5 mm and 2 mm samples were measured to be 0.017 mm-2 at 67.6 % σy, and 0.01027 mm-2 at 69.5% σy. Crack populations, when normalized by number densities of gas pores (non-clustered) and number densities of shrinkage pores (clustered), giving crack nucleation rate (crack/pore, mm-2), showed a good fit with the Weibull function in 2 mm and 5 mm samples. Shrinkage and gas pores could both become the main crack initiation sites (i.e. fatigue weak links) in this alloy. Higher nucleation rates of gas pores and shrinkage pores were observed in 5 mm samples compared to those rates in 2 mm samples. At high applied stresses, the 2 mm samples showed better fatigue lives than those of 5 mm samples. Fractured surfaces were analyzed using SEM and found that the main crack initiation were predominately from pores. The pores on the fractured surfaces were counted and their depth and width were measured. It was found that the cracks may not necessarily initiate from coarse pores, but sometimes from shrinkage pores (i.e. group of pores). The depth from the free surface, the width, the size, and the orientation of pores are key factors in increasing the driving force for crack initiation and subsequently those pores turn into long cracks. Moreover, the aspect ratios of pores on the main cracks were measured and found that in 5 mm samples, some pores have an aspect ratios of less than 0.7, which means that these pores are elongated in depth and have a narrow width which increase the stress concentration on the surface, thus, increasing the driving force for crack nucleation.

Este trabalho apresenta um estudo realizado com uma mistura asfáltica morna utilizando ligante aditivado com agente surfactante e borracha moída de pneus inservíveis. Um trecho experimental foi executado com a mistura asfáltica morna, localizado na SPA-248-055, Pista Oeste, entre os quilômetros 3+000 e 6+000 e comparado ao trecho de referência, localizado na mesma rodovia, entre os quilômetros 1+000 e 3+000. Foram feitos também ensaios fundamentais para caracterização do ligante modificado com borracha e com aditivo químico para mistura morna e também ensaios para verificar o comportamento mecânico da mistura de usina, de modo que possibilitou comparar seus resultados com uma mistura de referência, com produção na temperatura a quente. Os ensaios realizados para verificação do comportamento mecânico foram: resistência à tração, módulo de resiliência, fadiga e resistência à formação de trilha de roda no simulador de tráfego francês do LCPC. Comparando a mistura asfáltica morna com uma mistura de referencia, os resultados obtidos foram satisfatórios. Além destes experimentos também foram realizados ensaios de resistência ao dano por umidade induzida (DUI). No caso deste estudo, os resultados foram satisfatórios. Durante a produção da mistura asfáltica, foi realizada a análise da redução de emissões de poluentes, no momento da produção da mistura. Os resultados indicaram que há redução de poluentes como fumos totais, fumos solúveis e compostos orgânicos voláteis (VOC). Tanto o trecho experimental quanto o trecho de referência foram avaliados durante 3 anos quanto aos parâmetros de desempenho do pavimento, como irregularidade e condição de superfície. Os resultados obtidos mostram o comportamento positivo da mistura asfáltica morna, quando comparado ao pavimento do trecho de referência, ao longo de dois anos de observação. / This paper presents a study of a warm asphalt mix using binder additive with surfactant agent and tire crumb rubber. A test section was performed with the warm asphalt mixture, located in SPA-248-055, West Lane, between kilometers 3+00 to 6+000 and compared to the reference section, a hot mix section, located on the same highway, between kilometers 1 + 000 and 3 + 000. Basic tests were also made to characterize the rubber-modified binder and to verify the mechanical behavior of the plant mixture, than compared to results of a reference hot mixture. The tests performed to check the mechanical behavior were: tensile strength, resilient modulus, fatigue and rooting in French LCPC traffic simulator. Comparing the warm asphalt mixture with a mixture of reference, the results achieved were satisfactory. In addiction, in this research were also performed induced moisture damage, and check the adhesion of the binder to the aggregate. In the case of this study, the results were acceptable. During warm asphalt mix and hot mix production, analysis of reducing pollutant emissions were recorded. The results indicated that there is a reduction of pollutants such as total smoke, fumes soluble and organic volatile compounds. Both the experimental section and the reference section were evaluated for 3 years for pavement performance parameters such as roughness and surface condition. The results show a positive behavior of warm asphalt mix compared to the reference pavement section over two years of observation.

Baigiamajame magistro darbe yra atliekamas apatinių galūnių įtvarų ilgaamžiškumo tyrimas. Yra iškeliama čiurnos sąnario įtvarų patvarumo problema ir sudaroma jos sprendimo metodika. Darbe yra atliekama čiurnos sąnario biomechanikos analizė, įtvarų konstrukcijų ir medžiagų analizė, nagrinėjama ilgaamžiškumo reikšmė ir tyrimo būdai. Sudaroma tyrimo metodika. Kompiuterinio modeliavimo būdu, panaudojant SolidWorks programinę įrangą, yra sudaromas įtvaro modelis, nustatomos veikiančios apkrovos, atliekami įtempių ir poslinkių pasiskirstymo tyrimai. Atliekami modelio nuovargio bandymai. Atsižvelgiant į tyrimų rezultatus, yra sudaromi įtvarų iš aukštos temperatūros plastikų tinkamo pritaikymo nurodymai. Pateikiamos išvados ir literatūros sąrašas. / Final thesis presents the research of durability of lower limbs splints. The problem of fatigue behaviour of ankle joint splint is raised and its solution is suggested. Final thesis contains ankle joint biomechanics analysis, lower limbs splints design and material analysis. Review of durability research methods is also done. The methodology of research is arranged. Using the computer aided modeling with SolidWorks software the model of ankle joint splint is created. Load parameters are determined, research of stresses and deformations are performed. Fatigue test is also performed. Due to the results of research, the instructions for ankle joint splints made of high density polyethylene and polypropylene use and adaptation are listed. Final thesis also contains conclusions and list of literature. Size of work – 77 pages of text without attachments, 55 pictures, 9 tables and 25 bibliographical sources.

Este trabalho apresenta um estudo realizado com uma mistura asfáltica morna utilizando ligante aditivado com agente surfactante e borracha moída de pneus inservíveis. Um trecho experimental foi executado com a mistura asfáltica morna, localizado na SPA-248-055, Pista Oeste, entre os quilômetros 3+000 e 6+000 e comparado ao trecho de referência, localizado na mesma rodovia, entre os quilômetros 1+000 e 3+000. Foram feitos também ensaios fundamentais para caracterização do ligante modificado com borracha e com aditivo químico para mistura morna e também ensaios para verificar o comportamento mecânico da mistura de usina, de modo que possibilitou comparar seus resultados com uma mistura de referência, com produção na temperatura a quente. Os ensaios realizados para verificação do comportamento mecânico foram: resistência à tração, módulo de resiliência, fadiga e resistência à formação de trilha de roda no simulador de tráfego francês do LCPC. Comparando a mistura asfáltica morna com uma mistura de referencia, os resultados obtidos foram satisfatórios. Além destes experimentos também foram realizados ensaios de resistência ao dano por umidade induzida (DUI). No caso deste estudo, os resultados foram satisfatórios. Durante a produção da mistura asfáltica, foi realizada a análise da redução de emissões de poluentes, no momento da produção da mistura. Os resultados indicaram que há redução de poluentes como fumos totais, fumos solúveis e compostos orgânicos voláteis (VOC). Tanto o trecho experimental quanto o trecho de referência foram avaliados durante 3 anos quanto aos parâmetros de desempenho do pavimento, como irregularidade e condição de superfície. Os resultados obtidos mostram o comportamento positivo da mistura asfáltica morna, quando comparado ao pavimento do trecho de referência, ao longo de dois anos de observação. / This paper presents a study of a warm asphalt mix using binder additive with surfactant agent and tire crumb rubber. A test section was performed with the warm asphalt mixture, located in SPA-248-055, West Lane, between kilometers 3+00 to 6+000 and compared to the reference section, a hot mix section, located on the same highway, between kilometers 1 + 000 and 3 + 000. Basic tests were also made to characterize the rubber-modified binder and to verify the mechanical behavior of the plant mixture, than compared to results of a reference hot mixture. The tests performed to check the mechanical behavior were: tensile strength, resilient modulus, fatigue and rooting in French LCPC traffic simulator. Comparing the warm asphalt mixture with a mixture of reference, the results achieved were satisfactory. In addiction, in this research were also performed induced moisture damage, and check the adhesion of the binder to the aggregate. In the case of this study, the results were acceptable. During warm asphalt mix and hot mix production, analysis of reducing pollutant emissions were recorded. The results indicated that there is a reduction of pollutants such as total smoke, fumes soluble and organic volatile compounds. Both the experimental section and the reference section were evaluated for 3 years for pavement performance parameters such as roughness and surface condition. The results show a positive behavior of warm asphalt mix compared to the reference pavement section over two years of observation.

La fabrication additive est un procédé offrant de nouvelles opportunités aux industriels pour fabriquer des pièces complexes, sans outillage spécifique et en optimisant la matière utilisée.Cette thèse présente les propriétés mécaniques de pièces réalisées par fabrication additive et l’analyse des mécanismes d’endommagement associés, en ayant comme référence les propriétés mécaniques des procédés de fonderie et de corroyage. Ce type d’analyse est indispensable pour l’industrialisation du procédé.Les propriétés en traction et en fatigue, sur des éprouvettes en titane Ti-6Al-4V et en aluminium AlSi7Mg0,6, ont été mesurées. Les effets du procédé de fabrication, de la direction de fabrication, du post-usinage et des post-traitements thermiques ont été comparés. Les propriétés sont au moins au niveau de la fonderie.Ces résultats ont été analysés en corrélation avec les microstructures et les faciès de rupture, afin de dégager des mécanismes d’endommagement. Les critères permettant de mesurer la criticité des défauts ont été définis.Certaines de ces hypothèses ont pu être vérifiées grâce à des essais de traction in situ au micro tomographe. / Additive manufacturing offers new opportunities for industries to manufacture complex parts with no additional tooling and better optimization of the material used.This thesis is about the analysis of the mechanical properties and the damaging mechanisms of parts produced by additive manufacturing, using mechanical properties of casted and wrought parts as reference. This type of analysis is necessary in order to industrialize the process.The tensile and fatigue properties on Titanium Ti-6Al-4V and Aluminium AlSi7Mg0,6 were measured. The effects of the process, the manufacturing direction, the post-machining and the post-heat treatments were compared. Properties are at least at the level of casting.A correlation of these results with microstructures and fracture surfaces was made in order to extract the damaging mechanisms. A method to measure the criticity of the defects in a part was defined. Some of these hypotheses were verified using microtomographic in situ tensile tests.

No description available.

No description available.

L’assemblage des matériaux composites thermoplastiques tel que le PEEK est l’une des problématiques majeure de l’industrie aéronautique. Actuellement, différentes techniques sont développées pour assurer l’assemblage structural de ces matériaux, tels que : le soudage, le rivetage, le boulonnage et le collage. Les enjeux industriels majeurs sont principalement, à l’heure actuelle, la conception des structures simplifiées au maximum afin de réduire les coûts de production et la réduction des consommations énergétiques. A cet effet, l’industrie aéronautique fait fréquemment appel à l’assemblage par collage en raison de nombreux avantages qu’il offre (gain de poids, distribution régulière des contraintes, absence de trous) par rapport aux autres techniques existantes. Le PEEK (PolyEtherEtherKetone), est un matériau polymère semi-cristallin thermoplastique, à hautes performances. Ce matériau est souvent utilise dans l’industrie aéronautique principalement renforce par des fibres de carbone ou de verre. Cependant, du fait du niveau élevé de sa résistance chimique l’assemblage par collage du PEEK et de ses composites nécessitent des traitements de surfaces appropries et optimises. Or, afin d’obtenir un system collé à haute performance, la problématique scientifique et technique doit être concentrée sur la jonction entre les éléments à assembler. En effet, la qualité de cette jonction est de la plus haute importance car elle doit permettre un transfert optimal des contraintes thermomécaniques lorsque l’assemblage est soumis a ses conditions d'usage. Cette étude concerne donc, l’amélioration des propriétés mécaniques (monotones et cycliques) de l’assemblage par collage PEEK/PEEK. Dans cette optique, un traitement de surface simple de mise en œuvre est proposé. Ce traitement est le sablage, qui permet la modification topographique (morphologique) de surface. La compréhension des différents phénomènes d’interaction aux interfaces intervenant dans l’amélioration du comportement mécanique du joint de colle et qui s’inscrit dans la triptyque : « Rhéologie, Physico-chimie et topographie », est l’enjeu scientifique majeur dans cette thèse. Dans un premier temps, l’influence des paramètres du traitement tels que le temps de projection, la taille des particules, sur la morphologie de surface de différents matériaux à base de PEEK a été analysée, permettant ainsi d’établir la corrélation entre les paramètres morphologiques et les mécanismes de modification topographique de surface intervenant pendant le traitement de surface. L’un des facteurs clefs pour la compréhension des mécanismes d’interaction entre l’adhésif liquide et le substrat solide est la mouillabilité. L’analyse du comportement au mouillage en fonction des différents paramètres du traitement a été réalisée. La mouillabilité des surfaces traitées est fortement affectée par la rugosité de surface créée après ce traitement. La relation entre les paramètres morphologiques et la mouillabilité a été discutée. Enfin, l’influence des paramètres du traitement par sablage sur le comportement mécanique monotone et à long terme (essais de fatigue) sur la résistance du joint colle a été étudié à l’aide d’essais de cisaillement sur éprouvettes à simple recouvrement. Ceci a conduit, à la proposition de paramètres morphologiques surfaciques spécifiques pour l’optimisation du comportement mécanique du joint de colle des matériaux composites à matrice PEEK. / One of most problematic in the aeronautical industries is the structural joining of the high performance thermoplastic composites like PEEK composites. Actually, a lot of technologies are used for joining thermoplastic composites like welding, bolting, riveting, fastening and adhesive bonding. Due to the various advantages that characterize the adhesive bonding method, such an uniform stress distribution along the joint, weight‐light and cost reduction, makes this technique more desirable to join thermoplastic composites materials compared to the other joining techniques. PEEK (PolyEtherEtherKetone) is a semi‐crystalline thermoplastic material with high performance. This material is wildly used in aeronautical industries, principally, reinforced with carbon of glass fibres. However, its high chemical resistance makes the adhesive bonding of PEEK and its composites difficult and therefore an appropriate and optimised surface treatment is necessary. In the aim to obtain a bonded system with high performance, scientific and technical problematic should be focussed on the junction between adherents. Indeed, the quality of this junction is of utmost importance because it must allow optimum transfer of thermomechanical stresses when the assembly is subject to its terms of use. Though, at this time it is well known that thermoplastic composite materials are difficult to bond with‐out surface treatment. This study, therefore, relates to the improvement of mechanical properties (monotonic and cyclic) of the adhesive bonding system PEEK / PEEK. In this context, a surface treatment, easy to implement, is proposed. This surface treatment is sandblasting, which enables surface topographic (morphological) modifications. Understanding of various phenomena of interfaces interaction involved in the improvement of the mechanical behavior of the adhesive joint and is part of the triptych "Rheology, Physico‐chemistry and topography" is the major scientific challenge in this thesis. Initially, the influence of processing parameters such as the projection time, the particle size on surface morphology of various materials based on PEEK was analysed, thus allowing establishing the correlation between morphological parameters and modification mechanisms involved during surface treatment surface. One of the key factors for understanding the mechanisms of interaction between the liquid adhesive and the solid substrate is wettability. The analysis of the wetting behavior as a function of various parameters of the treatment was performed. The wettability of treated surfaces is strongly affected by surface roughness created after this treatment. The relationship between morphological parameters and wettability was discussed. Finally, the influence of sandblasting processing parameters on the mechanical behavior in monotoning and long‐term (fatigue tests) of the adhesive joint strength was studied, using single lap shear tests specimens. This has led to the proposal of specific surface morphological parameters for the optimization of the mechanical behavior of the adhesive joint of PEEK and its composites.

Cette thèse s’inscrit dans le cadre d’une collaboration entre le Laboratoire Génie Civil et Bâtiment (LGCB)/Laboratoire de Tribologie et Dynamique des Systèmes (LTDS) et l’entreprise Arkema, l’entreprise routière Malet, l’Agence de l’Environnement et de la Maîtrise de l’Énergie (ADEME). L’objectif de la thèse est l’étude du comportement thermomécanique des enrobés bitumineux, et spécifiquement les enrobés tièdes avec et sans agrégat d’enrobé (RAP, Reclaimed Asphalt Pavement), dans le but de prévoir leur durée de vie et d’optimiser leur dimensionnement dans un contexte de développement durable.Sept types d’enrobés bitumineux sont étudiés. Les enrobés diffèrent par leur procédé de fabrication, le pourcentage de RAP, les additifs et le pourcentage d’additif utilisé. Deux types de fabrications sont étudiées : à chaud (Hot Mix –HM) qui sert de référence et tièdes (Warm Mix –WM). Trois pourcentages de RAP sont considérés : 0%, 30% et 50%. Deux types d’additifs fournis par ARKEMA sont utilisés, ETIMA (E) et ETIMA-BIO (B), avec deux pourcentages, 0% et 0.4%. L’utilisation des enrobés tièdes contribue à réduire considérablement la consommation d’énergie, l’impact sanitaire et l’impact environnemental. D’autre part, l’ajout des RAP dans les enrobés permet d’augmenter le taux de recyclage des matières non renouvelables. Ces innovations participent à la prise en compte des enjeux du développement durable et de la préservation de l’environnement.Quatre types d’essais ont été réalisés. Ce sont : l’essai de module complexe, l’essai de fatigue, l’essai de retrait thermique empêché (TSRST) et l’essai de propagation d’ondes. Dans le domaine du comportement viscoélastique linéaire, les essais de module complexe sont réalisés sur une large gamme de températures (de -25°C à 45°C) et de fréquences (de 0.03Hz à 10Hz). Chaque test est dupliqué. Les résultats sont modélisés à l’aide du modèle analogique 2S2P1D qui a été développé au laboratoire LGCB de l’ENTPE. Ce modèle permet de modéliser correctement le comportement tridimensionnel viscoélastique linéaire des enrobés. Ensuite, le comportement en fatigue est étudié grâce à des essais de fatigue. Pour chaque matériau, quatre ou cinq éprouvettes sont testées. Quatre critères sont utilisés pour évaluer la durée de vie des matériaux. Puis, le comportement à basse température est caractérisé à l’aide de l’essai de retrait thermique empêché (TSRST). Pour chaque matériau, trois éprouvettes sont testées. Enfin, des essais de propagation d’ondes sont réalisés. La méthode de détermination du temps de vol des ondes « P » et des ondes « S » ainsi que la méthode Impact Résonance sont utilisées. Ces essais fournissent des mesures non destructives et faciles à réaliser. A partir de ces méthodes, on peut calculer les valeurs de modules et de coefficients de Poisson des matériaux.Soulignons que les essais réalisés permettent d’obtenir certain des paramètres utilisés pour le dimensionnement des chaussées. Un résultat essentiel de l’étude est la bonne performance des enrobés tièdes contenant des agrégats d’enrobés (RAP). / This PhD dissertation is part of the collaboration between the French Agency for Environment and Energy Management (ADEME), the companies Arkema and Malet and the Ecole Nationale de Travaux Publics de l’Etat (ENTPE). The aim of this dissertation is to study the thermo-mechanical behavior of asphalt mixes, specifically warm mix asphalts with and without Reclaimed Asphalt Pavement (RAP) in order to predict their fatigue life and to optimize their design by targeting sustainable development. Seven types of asphalt mixes are studied. They differ in their mixing method, RAP content, additives used and their content. Two types of mixing methods are studied: Hot Mix Asphalt (HMA), which serves as a reference, and Warm Mix Asphalt (WMA). Three RAP contents are considered: 0%, 30% and 50%. Two types of additives provided by Arkema are used : ETIMA (E) and ETIMA-BIO (B) with two contents for each of them (0% and 0.4%). The use of warm mix asphalt helps significantly reducing both energy consumption and the impact on health and environment. These innovations are significant contributions with respect to issues of sustainable development and environmental preservation. Four types of laboratory tests were performed: complex modulus tests, fatigue tests, Thermal Stress Restrained Specimen Tests (TSRST) and wave propagation tests. In the Linear ViscoElastic (LVE) behavior domain, complex modulus tests were performed on a wide range of temperatures (from -25°C to 45°C) and frequencies (from 0.03 Hz to 10 Hz). For each material, two specimens were tested. Values of complex modulus and complex Poisson’s ratio were obtained and modeled using the 2S2P1D analogies model, developed in the Laboratoire Génie Civil et Bâtiement (LGCB) of ENTPE. It was observed that this model can correctly simulate the LVE behavior of asphalt mixes. Fatigue behavior was studied by analyzing fatigue tests results. For each material, four or five specimens were tested. Four different failure criteria were used in order to evaluate fatigue life of materials. Low temperature behavior of materials was characterized using TSRST. For each material, three replicates were performed. Finally, wave propagation tests were carried out. The method to determine the time of flight of « P » and « S » waves and the impact resonance method are presented. These tests provide a non-destructive method to characterize materials, which easy to perform. Using those methods, values of complex modulus and Poisson's ratio of tested materials can be estimated. It is to be underlined that the tests performed in this PhD allow to obtaining some parameters which are required for asphalt pavement design. An important result obtained in this study is that warm mix asphalt combined with RAP and additives (ETIMA ou ETIMA-BIO) could perform as well as hot mix asphalt.

L’excellente coulabilité, les coûts de production relativement bas, et ratio poids/résistance mécanique élevé des alliages de fonderie Al-Si-Mg en font une des solutions les plus intéressantes dans le secteur automobile ainsi que dans le domaine aérospatial. Toutefois, il est bien connu que la durée de vie de ces composants moulés à grand nombre de cycles (105 < Nf < 107 cycles) est sévèrement réduite lorsque des défauts de fonderie (notamment pores et oxydes) sont débouchants et/ou subsurfaciques sont présents. Ces défauts concentrent les contraintes et peuvent considérablement réduire la période d’amorçage des fissures de fatigue en fonction de leur taille, forme et des caractéristiques microstructurales du matériau. Les défauts internes (à partir desquels les fissures peuvent amorcer et propager sans interaction avec l’air ambiant) ainsi que les défauts de surface (ceux qui sont placés à la surface et en contact direct avec l’air ambiant) vont également nuire la durée de vie des composants moulés. Toutefois, dans le cas des défauts internes, les coefficients de sécurité préconisés par les règles de conception ne font pas intervenir la distance de défaut par rapport à la surface. Le suivi de fissures de fatigue effectué à la surface d’éprouvettes macroscopiques de traction indique que la présence d’un défaut avec une taille supérieure à celle des fissures microstructuralement courtes (√A ≈ 500 μm, taille contrôlée par la SDAS) produit une remarquable réduction de la durée vie. En revanche, la durée de vie n’est pas affectée lorsqu’un défaut plus petit (√A ≈ 300 μm) est présent à la surface car l’amorçage et les premiers stades de propagation sont encore influencés par la SDAS. Les essais de fatigue en torsion pure montrent que la morphologie des surfaces de rupture est fortement influencée par le niveau de contrainte. De plus, le nombre de cycles à l’amorçage est réduit par rapport à la traction. Cet amorçage est multi-site et plusieurs fissures peuvent croitre simultanément au cours de la durée de vie d’une éprouvette, la rupture finale se produisant lors de la jonction de certaines de ces fissures. La propagation des fissures en torsion est largement influencée par la cristallographie locale et les retassures ne semblent pas être des sites de nucléation préférentiels. Les durées de vie odes échantillons macroscopiques contenant défauts artificiel internes (Øeq ≈ 2 mm) sont pratiquement similaires à celles obtenues avec un matériau de référence. L’amorçage et la propagation de fissures internes a été rarement observé lors des expériences de tomographie synchrotron. Dans les rares cas où de telles fissures ont pu être observées, le chemin de fissuration semble fortement influencé par la cristallographie alors que les fissures amorcées depuis la surface se propagent globalement en mode I. La vitesse de propagation des fissures internes est très inférieure à celle des fissures se propageant à partir de la surface. / The excellent castability, relatively low production costs, and high strength to weight ratios make Al-Si-Mg cast alloys an attractive choice for use in cheaper and lighter engineering components, in both automotive and aerospace industries. However, it is well known that High Cycle Fatigue (HCF) lives (105 < Nf < 107 cycles) of cast components are severely reduced when casting defects (notably pores and oxides) are present at the free surface or subsurface. They act as stress raisers which can considerably reduce the crack incubation period depending on their size, shape and the microstructural features of the surrounding material. Internal casting defects are of special interest to this work. The application of safety coefficients considers that all casting defects present in a component have the same deleterious effect and no attention is paid, for example, to their distance to the free surface. In other words, internal defects (corresponding to the case where the depth of the defect allows crack nucleation and propagation to essentially occur without interaction with the air environment) are considered as damaging to fatigue life as surface defects (those placed at the free surface and in contact with the air environment). Surface crack monitoring performed on uniaxial fatigue specimens indicates that the presence of a surface microshrinkage exceeding the size of microstructurally small cracks (√A ≈ 500 μm, controlled by the SDAS) readily nucleates a fatigue cracks producing steady crack propagation and remarkable reduction in the expected fatigue life. A smaller surface defect (√A ≈ 300 μm) nucleated a crack that did not reduced the expected fatigue life as in this case early stages of propagation are still nfluenced by the SDAS. Pure torsional cycling reveals that the morphology of fracture surfaces is highly influenced by the stress level. In general, torsional fatigue behaviour is described by having reduced (with respect to uniaxial testing) and multisite crack nucleation periods. Several dominant cracks can evolve simultaneously and the final failure occurs by the linkage of some of those cracks. Crack propagation is controlled by the crystallography and pores do not appear to be preferential nucleation sites. S-N curves show that macroscopic specimens containing Øeq ≈ 2 mm internal artificial defect produce similar fatigue lives to those obtained with a defect-free material. Internal crack nucleation was rarely observed during synchrotron tomography experiments; instead the fatal cracks initiated from much smaller surface defects. Tomographic images show that, in the case of internal propagation, crystallographic paths are formed while surface cracks propagate in mode I. The crack growth rate of internal cracks is much smaller than that of cracks propagating from the free surface.