Prédiction de la durée de vie de structures mécanosoudées soumises à des chargements thermiques anisothermes : application aux collecteurs d'échappement en tôle

Abstract : Exhaust manifolds are classically designed in cast-iron. However, the engine performance increasing, the output gas reaches higher temperature, and other types of material like welded steel plates are considered to design exhaust manifolds. These components are subjected to complex thermomechanical loadings which must be taken into account in fatigue design. To limit the computational costs, only a few loading cycles are simulated and a fatigue criterion is used to estimate the lifetime of the structure. This study proposes a junction model combined with a fatigue criterion to assess the lifetime of a welded structure. The model is simple enough to be integrated into a computation on a complete structure and the fatigue criterion is available for anisothermal loadings.The theoretical characterization of the asymptotic states (elastic or plastic shakedown, ratchetting) is studied and adapted to anisothermal loadings in order to check whether the structure reaches a stabilized behaviour and to find the trend line of the evolution of the structure. Then isothermal low cycle fatigue tests are completed up to specimen crack initiation on plate, butt-welded plate and butt-welded plate after smoothing out the weld bead. The geometry and the microstructure of the weld have significant influence on strain localization and on the fatigue lifetime of the specimens. Finally anisothermal tests were completed on welded specimens to reproduce the typical loading seen by welds on exhaust manifolds.The stiffness of the weld is modelled thanks to additional shell elements calibrated to reproduce the deflected shapes of the welded specimens. Various fatigue criteria are developed on the base material to characterize anisothermal damage, and are then adapted to welded zones. The junction model is finally validated thanks to the simulation of a thermal shock on an exhaust manifold and the relevance of the criteria is estimated in relation to the industrial application.
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Aurélie Benoit À La Guillaume. Prédiction de la durée de vie de structures mécanosoudées soumises à des chargements thermiques anisothermes : application aux collecteurs d'échappement en tôle. Autre. Ecole Nationale Supérieure des Mines de Paris, 2012. Français. ⟨NNT : 2012ENMP0023⟩. ⟨tel-01418771⟩

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