Étude de l’influence du procédé de poinçonnage sur la tenue en fatigue à grand nombre de cycles de tôles minces ferromagnétiques

Abstract : New electrical steel grades, with improved magnetic properties, are used to build electric motors. For these steel grades, the iron losses are reduced by adjusting the chemical composition (mostly the Si content), decreasing the thickness below 0.5 mm and increasing the grain size. The punching is used to produce electric motor components because it generates important alterations of sheet edges, this work aims at elaborating a HCf fatigue design strategy for thin punched electrical steel sheets. First, the quasi-static and cyclic behavior of this electric steel was studied through monotonic and cyclic tests. The behavior model of this material, which will be used in FE simulation, is then identified. The study of the high cycle fatigue (HCF) resistance of this material is performed using smooth and notched specimen’s geometries. The effect of stress ratio, temperature (180°C) and the punching process are considered. Due to its influence on the fatigue resistance, the effect of the punching process is finely investigated. Different experimental techniques such as microscopic observations, 3D surface topography, micro‒hardness and X‒ray diffraction are combined to characterize the specimen’s edges. To dissociate the respective influences of strain hardening, residual stresses and geometrical defects induced by the punching process, and to quantify the contribution of each parameter to the HCF resistance, different specimen’s configurations were tested. A strategy allowing the identification of the critical defects, on which fatigue crack initiation occurs, was adopted. The stress distribution around defects is determined from finite element analyses (FEA) on real defect geometries. A non‒local high cycle fatigue criterion is finally used as post‒processing of FEA to consider the effect of defects and the associated stress-strain gradients in the HCF strength assessment.
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Helmi Dehmani. Étude de l’influence du procédé de poinçonnage sur la tenue en fatigue à grand nombre de cycles de tôles minces ferromagnétiques. Mécanique des matériaux [physics.class-ph]. Ecole nationale supérieure d'arts et métiers - ENSAM, 2016. Français. ⟨NNT : 2016ENAM0014⟩. ⟨tel-01380590⟩

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