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Dimensionnement en fatigue en présence de singularités macroscopiques dans un thermoplastique renforcé fibres de verre courtes (PA66GF50) sous chargements uni- et multi-axiaux

Abstract : In the current environmental context, the automotive industry looks for reducing CO2 emissions while keeping vehicles power. To this aim, a solution is to lighten the vehicles. In this context, this Ph.D. thesis, supported by Solvay Engineering Plastics in collaboration with Vibracoustic, deals with fatigue resistance of injected parts in Polyamide 6.6reinforced by 50% wt. of short glass fibers, and attempts to take into account the effects of multiaxial loadings and geometrical singularities into a fatigue design methodology. The study is conducted for a given conditioning(RH50%,T80°C) representative of the atmospheric conditions experienced by motor mounts. A fully integrated numerical approach from injection simulation to lifetime prediction by a fatigue criterion, named Through Process Modelling (TPM), is used. One TPM specificity lies in the calculation of the local mechanical response at each point of the part from a multi-scale approach considering the matrix viscoelasticity and the fibers orientation due to the injection process. The fatigue criterion selected is a dissipated energy one. It is based on a “Fatigue Indicator Parameter” (FIP) assimilated to the stress-strain loop area in the stationary regime and obtained by post-treatment of mechanical fields at each point of the part.First, the matrix constitutive law is identified by reverse engineering. Then, the question of the discretization to be adopted in the thickness of an injected part is addressed on smooth samples, and the identification strategy of the fatigue criterion is defined. Subsequently, TPM is applied to a large fatigue database, specifically built for the study, and involving samples and loadings with increasing complexity. Uniaxial tension loadings are studied, firstly on smooth samples (with different fibers orientations), then on pipes, and finally on notched samples. Shear loadings on butterfly samples and torsion loadings on pipes are studied. At last, pipes under combined tension-torsion are considered. For each loading and each singularity level, attention is paid to the definition of a volume around the singularity onwhich the FIP should be spatially averaged. The definition relies on the mechanical gradients analysis. The TPM provides thus lifetimes predictions close to experimental ones in most of the studied configurations.
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Submitted on : Monday, August 2, 2021 - 1:01:32 AM
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Caroline Goutal. Dimensionnement en fatigue en présence de singularités macroscopiques dans un thermoplastique renforcé fibres de verre courtes (PA66GF50) sous chargements uni- et multi-axiaux. Autre. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2020. Français. ⟨NNT : 2020ESMA0008⟩. ⟨tel-03311679⟩

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