Étude numérique des champs mécaniques locaux dans les agrégats polycristallins d'acier 316L sous chargement de fatigue

Abstract : Cyclic thermomechanical loadings lead to the formation of short fatigue cracks, the initiation phase of which is prominent over the total life of the component. Mechanisms related to the evolution of these cracks have a high dependence on the microstructure of the material, especially under low-amplitude loading. To study this dependence, a statistical study based on the results of finite element calculations of polycrystalline aggregates was conducted. First, the problem was treated in two dimensions, with a simplified crystal plasticity model. Analysis of the results allowed to highlight the neighboring effect in polycrystals and to reproduce the dispersion of fatigue results observed at the surface. Using a three-dimensional model and investigating different loadings, it was possible to correlate the localization structures of plastic deformation in the volume of the aggregate with the experimental observations. Also, the steps of surface intrusions and extrusions induced by fatigue loadings, preferential sites of initiation, have been reproduced and highlight the harmful effects of biaxial loadings. The problem of effects was raised, reducing the results sample that can be used by a statistical treatment. Second, the introduction of surface roughness in the simulated aggregates highlighted the competition between the microstructural parameters and geometric singularities of the free surface. This analysis has shown that there must exist a limiting surface state, from which the effect of the roughness takes over the crystallographic aspect concerning the plastic deformation localization at the surface of polycrystals. However, this effect fades quickly when depth increases and becomes almost nil under the first layer of surface grains. Finally, another three-dimensional model, both periodic and accounting for the free surface effect, called semi-periodic model has been implemented to eliminate boundary condition effects and to fully exploit the results of calculations. This method also brings new perspectives for the development of a new mean fields model dedicated to the behavior of surface of polycrystals.
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Submitted on : Friday, September 21, 2012 - 9:23:51 AM
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  • HAL Id : pastel-00732147, version 1

Citation

Yoann Guilhem. Étude numérique des champs mécaniques locaux dans les agrégats polycristallins d'acier 316L sous chargement de fatigue. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2011. Français. ⟨NNT : 2011ENMP0108⟩. ⟨pastel-00732147⟩

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