Etude numérique statistique des champs locaux de contraintes en surface de polycristaux cubiques et hexagonaux en élasticité

Abstract : The first stages of fatigue damage in metallic polycrystalline materials are governed by local mechanical field at the grain scale. Fatigue crack initiation is often related to the emergence of persistent slip band at surface.Localization of cyclic plasticity occurs within grains of polycristals in which elastic stress field can be highly heterogeneous due to the granular microstructure and crystalline anisotropic elasticity. The main goal of this study is to evaluate and analyse statistically the stress fields in surface grains of polycristals. In this work, the elasticregime only is considered in order to study the subsequent activation of plastic slip and its variability. The possibleconsequences regarding crack formation are also addressed but in a lesser extent. The methodology is based on fullfield finite element numerical simulation of the elastic response of a set of polycrystalline aggregates in which grains orientation is chosen randomly.The two materials chosen for this study exhibit different characteristics regarding crystal elasticity and slip systems.The first one is an austenitic stainless steels 316L (face centered cubic) and the second one a titanium alloy TA6V(hexagonal close packed). The distribution of resolved shear stress within several sets of surface grains, for different classes of crystal orientation, are analysed in relation to local crystallographic configurations in order to identify those which promote - or prevent from - plastic slip activation. For TA6V, the results suggest in particular that basal slip is activated earlier than prismatic slip. In addition, the simulation strategy has been modified tomodel the presence of some sharp band-like crystallographic textures named “macrozones” whose influence has been qualitatively predicted. Plastic slip activation in TA6V is also studied experimentally. In situ tensile test using SEM has shown the earlier activation of basal slip. Experimental data has been combined with simulated distribution of resolved shear stress in order to estimate the critical resolved shear stress of basal and prismatic slip systems.
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van Truong Dang. Etude numérique statistique des champs locaux de contraintes en surface de polycristaux cubiques et hexagonaux en élasticité. Autre. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2016. Français. ⟨NNT : 2016ESMA0003⟩. ⟨tel-01319944⟩

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