Fatigue d'un alliage d'aluminium moulé A357-T6 : rôle de la morphologie, de la position des défauts et application à une structure pour le calcul de la durée de vie en fatigue

Abstract : The purpose of this work is to characterize the high cycle fatigue strength of the A357-T6 (AS7G06-T6) cast aluminum alloy affected by natural casting defects (shrinkages). Six different castings have been manufactured in order to study two shrinkage types: the cavity shrinkages and the sponge shrinkages with a defect grade ranging from 2 to 4 (as defined by the ASTM E2422 standard). High cycle fatigue tests have been carried out in order to estimate the fatigue limit under positive tensile loading (R = 0.1) for each defect type at N = 2∙106 cycles. The effect of the local defect morphology has been investigated performing several fatigue tests on specimens with artificial surface defects characterized by a local morphology modification (FIB and EDM machining). The artificial defect size and morphology is representative of the natural interdendritic porosity. The local and global defect morphology of the natural defects have been studied by means of finite element simulations conducted on the real shrinkage morphology reconstructed from a μ-CT scan. Two equivalent geometries have been proposed to simplify the natural defect morphology: a sphere and an inertia - equivalent ellipsoid.The effect of the defect position on the mechanical filed evolution has also been studied by means of finite element simulations performed on the real defect geometry. The analysis of the Kitagawa-Takahashi diagram has been performedby using two different approaches for both internal and surface defects: (i) the Linear Elastic Fracture Mechanics (LEFM)(ii) the Defect Stress Gradient (DSG) fatigue criterion. An experimental campaign has been performed at a component scale. A casted A357-T6structural component has been tested with two different configurations: as received and degraded with artificial surface defects. A DSG fatigue criterion,taking into account the effect of the stress gradient at the macroscopic scale of the component, has been proposed. The model has been validated by means of finite element simulations using a data post-processing sub-routine that gives as output the critical defect size map at each point of the simulated model.
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Antonio Rotella. Fatigue d'un alliage d'aluminium moulé A357-T6 : rôle de la morphologie, de la position des défauts et application à une structure pour le calcul de la durée de vie en fatigue. Autre. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2017. Français. ⟨NNT : 2017ESMA0019⟩. ⟨tel-01692427⟩

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