A Modelling of the Coupled Thermodiffuso-Elastic Linear Behaviour. Application to Explosive Decompression of Polymers

A thermodiffuso-mechanical modelling is proposed in this article with the aim of describing the coupled behaviour of polymers, which are initially placed in a gaseous environment and subjected to an "explosive" decompression. Some descriptions of the elementary representative volume (ERV) are considered and detailed. The modelling is then developed from the simplest representation, which consists in describing the ERV as a homogenous mixture of polymer and gas. In the framework of generalized standard media, coupled constitutive equations are only proposed for a linear elastic behaviour. The numerical implementation of this model in ABAQUSTMled to a first qualitative study of the direct couplings. Its main results are discussed here

Numerical Modelling of the PVF

In the oil industry, polymer materials are subjected to temperature and gas pressure fluctuations that can damage them. This degradation results from couplings between mechanical, thermal and diffusion phenomena. A law modelling these couplings was implemented in the Abaqus software. By making a numerical study of these couplings, we show that it is necessary to model by an isotropic expansion coefficient the effect of CO2 diffusion on the mechanical behaviour of the PVF2. We show that this parameter is independent of the applied pressure. Moreover, with this coefficient we can predict the maximum swelling of the material when it is saturated by gas

Phénomène de porosité et instabilités ductiles dans les matériaux hybrides polyéthylène/argile

De nombreuses études actuelles traitent des mélanges de polymère avec de l’argile. Elles sont fréquemment référencées sous le nom de nanocomposites car elles se focalisent sur la dispersion à l’échelle nanométrique des feuillets d’argile. Cette faible dimension fait que les taux de charge sont peu importants. Les nanocomposites présentent potentiellement beaucoup d’intérêt face aux composites conventionnels. Toutefois beaucoup d’études ne sont pas encore concluantes. Le présent travail s’attache à décrire l’existence de phénomènes indésirables dans les mélanges polymère/argile, et cherche à comprendre l’origine de tels mécanismes. Plusieurs mélanges à base de polyéthylène, d’argile organophile et d’agents compatibilisants ont été réalisés avec des conditions de mise en œuvre variées (mélangeur interne, extrusion bivis et monovis…). Dans quelques cas, l’allongement à la rupture chute de manière catastrophique. La diminution de la ductilité semble dépendre fortement de la mise en œuvre. Les faciès de rupture révèlent l’existence de porosité. La « pseudo-fragilité » de certains mélanges pourrait être reliée à la taille des pores. Dans certains cas, les pores atteindraient des tailles critiques, éventuellement par coalescence, qui empêchent l’éprouvette dans son ensemble de supporter l’étirement même si localement le polymère reste ductile, tandis que dans d’autres cas, les dimensions des cavités sont plus faibles, de sorte que localement les chaînes peuvent se réorganiser à tout instant, et donc la striction se propage sur toute la longueur de l’éprouvette. Cette porosité de la structure ainsi mise en évidence pourrait être l’explication des résultats de perméabilité médiocres de ces échantillons. Quelques résultats complémentaires viennent montrer comment les techniques communes qui permettent de caractériser le degré d’exfoliation de l’argile (DRX, MET, Rhéologie), sont insensibles à ce genre de phénomènes

Physico-chemical and mechanical degradation of polyamide 11 induced by hydrolysis and thermal ageing

International audienceThis paper deals with the effect of both temperature and water activity on polyamide 11 physico-chemical and mechanical properties. The purpose of this work is to describe the ductile-brittle transition of polyamides during ageing in a wet environment using a mechanical behavior model. For that it is necessary to make physico-chemical analyzes (DSC, FTIR, GPC, viscosity) and mechanical tests (uniaxial tensile test and DMTA). These characterizations were carried out on safe and samples aged until 60 days under hydrothermal conditions in acid water, and under purely thermal conditions in a neutral environment. Changes in mechanical properties with ageing were observed in the polymer and were correlated to morphological changes deduced from the physicochemical characterizations. An increase of the second yield stress related to the recrystallization and a decrease of the strain at break due to the decrease of the molecular weight were observed during ageing

Investigation of the 3D crystalline network impact on the elastic properties of semi-crystalline polymers from a multi-scale modelling approach

International audienceNowadays, computational resources allow carrying out mechanical calculations on complex multi-scale materials. Finite Element (FE) calculations can especially be directly performed on microstructures of materials. This work is a first attempt to analyse the impact of the crystalline architecture at a mesoscopic scale on the macroscopic elastic properties of Semi-Crystalline Polymers (SCP). Such polymers can be considered biphasic materials, which are composed of an amorphous phase embedded in a crystalline network. The material studied here is Polyethylene (PE). Molecular Dynamics (MD) calculations are carried out on a 100% crystallized Polyethylene model to determine the elastic properties of the crystalline regions of the material. 3D mesostructures of the typical layout of the spherulitic crystalline network of Semi-Crystalline Polymers are then constructed from experimental observations. These material data and this geometrical description are then integrated in computations with the Finite Element method on elementary volumes to finally determine the macroscopic elastic properties of the material. In this work, which is a first attempt to test such a multi-scale workflow, no amorphous phase is considered. Different 3D architectures are compared demonstrating the role of the crystalline arrangement on the stiffness of the material. Three main types of mesostructures have been analysed: crystalline lamellae disposed in a complete random arrangement, crystalline lamellae disposed in a spherulite arrangement, crystalline lamellae with branches disposed in a spherulite arrangement. It appears that the 3D configuration of the lamellae, as well as the presence of branches, have an influence on the macroscopic elastic properties of the material. Then, comparisons with experimental data suggest that the macroscopic elastic properties can be represented with a purely cohesive crystalline network for crystalline degree up to about 50%. This result questions the role of the amorphous phase on the elastic properties of such systems

Quasistatic Signorini problem with Coulomb friction and coupling to adhesion

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