Amorçage et propagation de fissures de fatigue dans un alliage Al-Si A319 lors d'essais in situ de fatigue oligocyclique sous tomographie synchrotron

Dans l'industrie automobile, le procédé généralement utilisé pour la fabrication des culasses est le moulage en coquille par gravité. Il est progressivement remplacé par le Procédé à Modèle Perdu (PMP) dans un but d'optimisation de la géométrie, de réduction des coûts et de contrôle de la consommation des véhicules. Cependant, les pièces automobiles en alliage d'aluminium fabriquées par PMP ont une microstructure plus grossière et plus de pores que les pièces fabriquées en moulage en coquille par gravité à des vitesses de refroidissement plus grandes. Cette microstructure a une influence majeure sur le comportement en fatigue et les mécanismes d'endommagement des alliages Al-Si. Dans ces travaux, les mécanismes d'endommagement en fatigue oligocyclique ont été étudiés lors d'essais in situ sous tomographie de synchrotron (SLS et ESRF) à la fois à température ambiante et à la température de service des culasses (250°C°) sur des échantillons prélevés dans les zones les plus critiques de la culasse (face flamme). Préalablement, des observations sous microtomographe par rayons X de laboratoire (MATEIS et ISIS4D) ont permis de sélectionner les échantillons et les zones d'observations et d'analyses au regard de la taille et de la localisation des pores en particulier. La reconstruction 3D des images permet de visualiser les différents éléments de la microstructure (Si eutectique, intermétalliques au fer (?(AlFeMnSi) et ? (AlFeSi)), intermétalliques au cuivre (Al2Cu et AlCuMgSi) à haute résolution (1,6 - 2.5 µm de taille de voxel). Le lien entre la microstructure et l'amorçage et la propagation des microfissures a ainsi pu être étudié en volume. Cette microstructure sert par ailleurs de texture naturelle pour la corrélation d'images volumiques. Grace aux champs de déplacement 3D mesurés, la localisation et le développement de la déformation sont suivis au cours du cyclage. Il est démontré que la microstructure complexe 3D du matériau et la présence des pores produit des hétérogénéités locales qui peuvent être corrélées avec l'amorçage et la propagation des fissures de fatigue

Peanut‐induced anaphylaxis in children and adolescents: Data from the European Anaphylaxis Registry

Background Peanut allergy has a rising prevalence in high-income countries, affecting 0.5%-1.4% of children. This study aimed to better understand peanut anaphylaxis in comparison to anaphylaxis to other food triggers in European children and adolescents. Methods Data was sourced from the European Anaphylaxis Registry via an online questionnaire, after in-depth review of food-induced anaphylaxis cases in a tertiary paediatric allergy centre. Results 3514 cases of food anaphylaxis were reported between July 2007 - March 2018, 56% in patients younger than 18 years. Peanut anaphylaxis was recorded in 459 children and adolescents (85% of all peanut anaphylaxis cases). Previous reactions (42% vs. 38%; p = .001), asthma comorbidity (47% vs. 35%; p < .001), relevant cofactors (29% vs. 22%; p = .004) and biphasic reactions (10% vs. 4%; p = .001) were more commonly reported in peanut anaphylaxis. Most cases were labelled as severe anaphylaxis (Ring&Messmer grade III 65% vs. 56% and grade IV 1.1% vs. 0.9%; p = .001). Self-administration of intramuscular adrenaline was low (17% vs. 15%), professional adrenaline administration was higher in non-peanut food anaphylaxis (34% vs. 26%; p = .003). Hospitalization was higher for peanut anaphylaxis (67% vs. 54%; p = .004). Conclusions The European Anaphylaxis Registry data confirmed peanut as one of the major causes of severe, potentially life-threatening allergic reactions in European children, with some characteristic features e.g., presence of asthma comorbidity and increased rate of biphasic reactions. Usage of intramuscular adrenaline as first-line treatment is low and needs to be improved. The Registry, designed as the largest database on anaphylaxis, allows continuous assessment of this condition

Influence de la microstructure dans l'endommagement par fatigue oligocyclique à 250°C d'un alliage d'aluminium obtenu par un procédé à modèle perdu

Dans les alliages Al-Si de fonderie, la microstructure a une forte influence sur le comportement en fatigue. Dans les culasses automobiles produites par le Procédé à Modèle Perdu, l'alliage Al-7Si-3Cu se compose de différentes inclusions rigides, de pores de gazage et de microretassures de grandes tailles. Afin d'étudier l'influence de cette microstructure 3D complexe sur l'amorçage et la propagation des fissures de fatigue oligocyclique à haute température (250°C), un protocole expérimental utilisant la tomographie aux rayons X, la simulation par éléments finis et la corrélation d'image volumiques (CIV) a été utilisé. La tomographie a permis de caractériser l'eutectique Al-Al2Cu, les intermétalliques au Fer et surtout la phase de silicium eutectique, ces constituants se sont avérés fournir un mouchetis naturel approprié pour la CIV. Des mesures de champs à l'échelle de la microstructure ont ainsi été effectuées au cours des essais de fatigue à 250°C. La confrontation des observations aux mesures de champs 3D a permis de mettre en évidence les relations entre les fissures et les discontinuités de déplacements et les localisations de déformations et de comprendre le scénario d'endommagement. Les pores génèrent une concentration de déformation suffisante pour amorcer puis faciliter la propagation dans leur voisinage. Les inclusions rigides et la localisation des déformations entre les pores semblent avoir une forte influence sur le chemin de propagation. La CIV a aussi permis de mesurer la déformation à rupture des différentes phases présentes au sein de l'échantillon afin de proposer une hiérarchisation de la criticité des différentes inclusions rigides.The microstructure of Al-Si cast alloys has a strong influence on the fatigue behavior. In the automotive cylinder heads produced by the Lost Foam Casting, the Al-7Si-3Cu alloy consists in various hard phases, large size gas pores and microshrinkages. To study the influence of this 3D complex microstructure on the initiation and propagation of low cycle fatigue cracks at high temperature (250°C), an experimental protocol using X-ray tomography, finite element simulation and digital volume correlation (DVC) has been used. Tomography allowed the characterization of the eutectic Al-Al2Cu, the iron intermetallics and especially the eutectic silicon phase. These constituants have proved to be a natural speckle suitable for DVC. The comparison of the observations with the 3D field measurements allowed to highlight the relationships between cracks and displacement discontinuities and strain localizations and to understand the damage mechanisms. The pores generate a sufficient strain concentration to initiate and then to facilitate the propagation in this zone. The hard phases and the strain localization between pores appear to have a strong influence on the propagation path. DVC allowed also to measure the strain level at failure of the different phases present within the sample in order to propose a hierarchy of the criticality of the different rigid inclusions

Characterisation of 3D damage mechanisms of a cast Al alloy during in‐situ high temperature low cycle fatigue tests using Synchrotron X‐ray tomography and digital volume correlation

International audienceIn the automotive industry, the Lost Foam Casting (LFC) process is gradually replacing the traditional die casting process for producing engine parts in Al alloys. The advantages of LFC are that it allows to optimize the geometry of cast parts as well as to reduce the production cost. One drawback, however, is that LFC generates a coarser microstructure and more porosity than, for example, die casting because of slower cooling rates. In this work the low cycle fatigue mechanisms of a LFC Aluminium-Silicon alloy (A319) used for making automotive cylinder heads have been studied. In situ fatigue tests monitored by synchrotron tomography have been carried out at (moderately) high temperatures representative of service conditions

Amorçage et propagation de fissures de fatigue dans un alliage Al‐Si A319 lors d'essais in situ de fatigue oligocyclique sous tomographie synchrotron

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Characterisation of 3D strain heterogeneity at the microstructure scale during Low Cycle Fatigue of an AlSi7Cu3Mg alloy at 250°C

International audience3D tomographic images of a cast AlSi7Cu3Mg alloy were obtained using synchrotron X-ray tomography during in-situ Low Cycle Fatigue tests at 250°C. While image analysis highlights the role of eutectic Si particles close to pores in damage mechanisms, high resolution digital volume correlation reveals the relationship between strain heterogeneity at the microstructural scale and hard particles failure or cracks. Monitoring strains evolution with cycles within hard particles, i.e. eutectic Si and Fe or Cu intermetallics, allows measuring their local failure strains and drawing a hierarchy of the deformation to failure. Then, a local Manson-Coffin curve per hard phase is proposed

Study of damage mechanisms in A319 aluminium alloy by X-ray tomography and Digital Volume Correlation

International audienceIn the cylinder heads produced by the Lost Foam Casting process, the microstructure consists of hard intermetallic phases and large gas and microshrinkage pores. In order to study the influence of this complex 3D microstructure on fatigue crack initiation and propagation, an experimental protocol using laboratory and synchrotron tomography, Finite Element simulation and 3D Digital Volume Correlation has been used. Tests performed at low temperatures (room temperature and 150°C) revealed the initiation of 3D cracks at large pores and a propagation along the hard inclusions towards the free surface. At temperatures characteristics of in-service conditions (above 200°C), an additional damage mechanism was observed: cracks were detected in silicon particles around the main pore that drove to failure but also in other areas of the specimen gauge length

Influence of the Casting Process in High Temperature Fatigue of A319 Aluminium Alloy Investigated By In-Situ X- Ray Tomography and Digital Volume Correlation

International audienceIn order to satisfy the economic constraints together with environmental requirements, the automotive industry has been forced to adopt a strategy of down-sizing, which has led into process modification of some engine parts like cylinder heads. Nowadays, the Lost Foam Casting process (LFC) replaces the conventional Die Casting (DC) process due to cost reduction and geometry optimization goals. However, aluminum alloy automotive parts produced by the LFC process have a coarser microstructure and more porosity defects than parts produced with conventional casting processes at faster cooling rates. In the cylinder heads produced by the LFC process, the microstructure consists of hard intermetallic phases and large gas and microshrinkage pores. In order to study the influence of this complex 3D microstructure on fatigue crack initiation and propagation, an experimental protocol using laboratory, synchrotron tomography, SEM images and 3D Digital Volume Correlation (DVC) has been used. Fatigue tests performed at temperatures characteristics of in-service conditions (250°C) revealed the initiation of 3D cracks at large pores and a propagation along the hard inclusions around the main pore that drove to failure but also in other areas of the specimen gauge length

Analysis of fatigue cracks initiation and growth in an Al-Si-Cu alloy using in-situ fatigue tests under synchrotron x-ray tomography coupled with digital volume correlation

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