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Transition ductile-fragile du PVC plastifié à différentes vitesses d'impact : compréhension des mécanismes et modélisation

Abstract : In this thesis, the impact resistance of two plasticized PVC skins used in the automotive industry is analyzed. Generally, these materials are compared between them by using physico-chemical techniques such as the glass transition (Tg) which does not always allow to represent the performances in the real condition of useof these materials. In order to improve the understanding of the failure of these skins and to propose a more relevant alternative to the Tg, the notion of ductile failure threshold temperature (Tdth) has been proposed (variant of the DBTT) on unscored and scored specimens. In the first step, a study of the fracture mechanisms applied to scored specimens was carried out thanks to clamped SENB tests with video monitoring, completed by a numerical simulation with Finite Elements (FE). This study allowed the identification of the calibration factor used for the measurement of the fracture toughness. It also allowed the observation that for these materials, the crack propagation starts at the apparition of the nonlinearity and not at the maximum load. This result was then used to define the parameters to be used to determine the Tdth via impact tests. These impact tests were performed using two different machines: the CEAST9350 and the High Speed Tensile Test(HSTT) machine in order to explore a wide range of impact speeds (from 2 m/s to 23 m/s). The use of a tanh function allowed to optimize the identification of the parameters of the Tdth in order to compare it for the two geometries and the two materials. The results obtained show on the one hand that the Tdth grows linearly with the impact speed and has the same slope whatever the geometry or the formulation. On the other hand, at a given impact speed, the Tdth of a scored material will always be higher than unscored’s, but this difference changes for each PVC studied. Similarly, for the same geometry, the two PVCs did not have thesame Tdth. An inversion in the performance of the materials was observed with material 1 performing better unscored than material 2, but is less competitive in the case of scored specimens. This notion is not possible to observe with Tg alone.
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Submitted on : Thursday, April 21, 2022 - 4:14:25 PM
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Clément Bertaux. Transition ductile-fragile du PVC plastifié à différentes vitesses d'impact : compréhension des mécanismes et modélisation. Matériaux. Université Paris sciences et lettres, 2021. Français. ⟨NNT : 2021UPSLM058⟩. ⟨tel-03648526⟩



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