Validation du modèle « Visco-hyperelastic Network Unit » (VENU) pour la prédiction du comportement mécanique de polymères thermoplastiques

De nouvelles donnes économiques, telles que la réduction des réserves pétrolières mondiales, obligent nos sociétés à faire face à de nouveaux défis en terme de dépense énergétique. En particulier, les domaines des transports doivent trouver des solutions rapides afin de diminuer la consommation en essence et les émissions de CO2 des futurs véhicules. Ceci passe par l’allègement des modèles et l’utilisation de nouveaux matériaux alliant légèreté et résistance. Les matériaux composites, à renforts discontinus ou continus, se sont montrés être une alternative séduisante à ces défis. Ils permettent ainsi de développer des structures légères pour des coûts globalement équivalents aux structures métalliques. Dans cette étude, nous étudierons, au travers d’une campagne expérimentale menée sur différentes matrices thermoplastiques, la prédiction d’un modèle thermomécanique développé très récemment par Billon [1]. Le modèle “Visco-hyperelastic Network Unit” (VENU) est un modèle thermomécanique à variable d’état basé sur le modèle hyperélastique d’Edwards Vilgis [2] modifié afin de rendre compte de la mobilité des chaines de polymère tel que l’évolution de la microstructure associée au désenchevêtrement des chaînes de polymère. Ce modèle a été développé afin de rendre compte du comportement thermomécanique de matrices thermoplastiques dans leur état caoutchoutique (T>Tα). La démarche expérimentale qui s’appuie sur des mesures de champs couplés cinématique et thermique sera présentée associée à la validation du modèle pour différentes matrices thermoplastique telles que du PMMA, PET, PS et PA. L’enjeu d’un tel modèle physiquement justifié est d’améliorer la prédiction du comportement thermomécanique de la matrice afin d’affiner la modélisation du composite à renforts discontinus ou continus. [1] Billon, N., Journal of Applied Polymer Science, 125-4390-4401, 2012. [2] Edwards, S. F. and Vilgis, T., Polymer, 27: 483–492, 1986

A kinetic model for predicting the oxidative degradation of additive free polyethylene in bleach desinfected water

The chemical interactions between additive free PE and bleach were investigated by FTIR spectrophotometry and viscosimetry in molten state after immersion (for a maximum duration of one hundred days) in bleach solutions maintained at a temperature of 60 °C, a free chlorine concentration of 100 ppm, and a pH = 4, 5 or 7. It was found that the polymer undergoes a severe oxidation from the earliest days of exposure in a superficial layer of about 50–100 μm thick, almost independent of the pH value. In this layer, oxidation leads to the formation and accumulation of various carbonyl products (mostly ketones and carboxylic acids) but also, after about 2–3 weeks of exposure, to a dramatic decrease in the average molar mass due to the large predominance of chain scissions over crosslinking. It was also found that the oxidation rate is maximum at pH = 5, and of the same order of magnitude at pH = 4 and 7. Based on the equilibrium diagram giving access to the relative predominance of the three main chemical species as a function of the pH value of the bleach solution, it was assumed that oxidation is initiated by radical species coming firstly from hypochlorous acid (ClOH) and secondarily from chlorine (Cl2), given that hypochlorite ions (ClO−) are totally insoluble into the PE matrix. In addition, for explaining the surprisingly large value of the oxidized layer thickness despite the high reactivity of the involved radicals, it was assumed that ClOH and Cl2 do not decompose into radicals in the water phase, but migrate deeply into the PE matrix prior to dissociating into Cl and HO radicals and then, initiating a radical chain oxidation. The validity of the kinetic model derived from this scenario was successfully checked by comparing the numerical simulations with all the experimental data collected in this study. This model predicts the general trends of the oxidation kinetics and its dependence on the pH value, but also gives access to the transport properties of the chlorinated disinfectants and their radical species, and the rate constants of the radical attack

Linearization and implementation of venu model in small strain theory for polyamide 6.6

The so-called VENU model is a visco-hyperelastic constitutive model, designed for amorphous rubbery polymers, which is based on an original approach initially developed by N. Billon (J. Appl. Polym. Sci. 125:4390-4401, 2012) and extended by A. Maurel-Pantel et al. (Int. J. Plast. 67:102126, 2015) to three-dimensional thermomechanical framework. In the aforementioned references, the constitutive equations and thermodynamical framework are presented within large deformation theory. However, in fatigue tests of polymeric composites significant temperature gradients are noticed despite the fact that the measured strains are within the small strain theory. In addition, well established techniques and tools of micromechanics for polymeric composites are applicable in small deformation regions. These observations render important the reduction of the VENU model in the case of linear strains. Here, a method is proposed for the reduction of the VENU model to small strain theory. A proper numerical scheme is also provided, based on the so-called return-mapping algorithm. The model capabilities are illustrated by comparing numerical calculations with available experimental data for polyamide 66.project DURAFI

Estimation du comportement thermo-viscoélastique effectif des pièces composites obtenues par impression 3D-FDM

peer reviewedDans le but d’estimer le comportement effectif des pièces obtenues par le procédé de fabrication FDM pour le cas des matériaux composites à fibres courtes on a visé une méthodologie permettant la mise en place d’une procédure d'homogénéisation analytique en thermo-viscoélasticité de façon analogue à celle des matériaux élastiques linéaires ; la prise en compte de la variation des paramètres qui déterminent l’état particulier des fibres dans le filament est achevé grâce à l’introduction des fonctions de distribution obtenues via l’analyse statistique de la microstructure. La procédure d'homogénéisation a été évaluée en comparant ses prédictions aux calculs basés sur la FFT en champ complet et des résultats des essais pour des échantillons traités en autoclave, pour enlever les porosités à l'échelle des couches du filament imprimé

ESTIMATING THERMOMECHANICAL RESIDUAL STRESSES IN FDM 3D PRINTED COMPOSITE PARTS

peer reviewedWe implemented a two-step methodology to estimate the residual stresses induced by the FDM manufacturing process in 3D printed composite parts. The first step consisted in an analytical thermo-viscoelastic homogenization procedure to derive the effective behavior of the filament. The second step consisted in a coupled thermomechanical structural analysis of the part. The homogenization procedure was assessed by comparing its predictions to full-field FFT-based computations. The structural analysis was assessed by comparing its predictions to experimental results

Mean-Field Approximations in Effective Thermo-viscoelastic Behavior for Composite Parts Obtained via Fused Deposition Modeling Technology

editorial reviewedAiming to estimate the effective behavior of the parts obtained by fused deposition modeling (FDM) in the case of short fiber composite materials, the Mean-field homogenization procedure, introduced in linear elasticity, is here extended to linear thermo-viscoelasticity. The variation of the parameters describing the state of the fibers inside the printing filament is represented by introducing appropriate distribution functions obtained through the statistical analysis of the microstructure. The validation of the procedure is achieved by comparing its predictions with calculations based on full-field Fast-Fourier-Transform homogenization and experiments results from samples treated in autoclave to remove layer-scale porosities from the printed filament

Caractérisation thermomécanique de polymères thermoplastiques pour application photovoltaïque

International audienc

Caractérisation thermomécanique de polymères thermoplastiques pour application photovoltaïque

International audienc

Caractérisation thermomécanique de polymères thermoplastiques pour application photovoltaïque

International audienc