Modelling Of Hydrothermal Aging Of Short Flax Fiber Reinforced Composites

International audienceAs a contribution to the prediction of the evolutionary behavior of biocomposites in service conditions, this study focused on the simulation of the hydrothermal aging of short natural fiber reinforced composites made by extrusion/injection molding. We endeavored to model the reversible modifications of the behavior of PLA and PLA/flax composites when immersed in water at different temperatures (20, 35 and 50 °C). A numerical model accounting for the heterogeneous mechanisms involved during aging such as water diffusion and the resulting swelling and plasticizing of polymers was implemented. Simulated data proved to be in perfect accordance with experimental results as long as no irreversible mechanism was occurring. The deviations of the simulated data from experimental results were limited at 35°C but significant at 50°C. Finally, the influence of moisture on the local elastic modulus of flax fibers was inferred thanks to the Halpin-Kardos homogenization model

Isogonal moulding surfaces: A family of shapes for high node congruence in free-form structures

International audienceThe design of free-form structures is governed by structural and geometric considerations, the latter ones being closely linked to the costs of fabrication. If some construction constraints have been studied extensively, the question of the repeatability of nodes in free-form structures has rarely been addressed yet. In this paper, a family of surfaces that can be optimized regarding typical geometrical constraints and that exhibit high node congruence is proposed. They correspond to particular meshes of moulding surfaces and are called isogonal moulding surfaces by the authors. The geometrical properties of these surfaces are discussed. In particular, it is shown how to derive Edge Offset Mesh from them. It is also demonstrated that they represent all the possible meshes parallel to surfaces of revolution. Finally, the reader is introduced to some computational strategies linked to isogonal moulding surfaces

Impact on peel strength, tensile strength and shear viscosity of the addition of functionalized low density polyethylene to a thermoplastic polyurethane sheet calendered on a polyester fabric

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Effect of post curing temperature on mechanical properties of a flax fiber reinforced epoxy composite

International audienceAchieving a complete curing of biocomposites without damaging fibers is very challenging. This study assesses the impact of processing upon the mechanical properties of a unidirectional flax reinforced epoxy composite and identifies which component (resin, reinforcement or interphase) is the most sensitive to post-curing at high temperature (100, 120 or 150 °C).Post-curing temperature had a limited impact on the composite mechanical behavior excepted at 150 °C where ultimate stress and strain decreased drastically while the stabilized modulus slightly increases.Post curing is responsible of a slight decrease of the matrix tensile properties attributed to the polymer oxidation but cannot explain on its own the evolution of the composite behavior. Interfacial adhesion played a minor role in the composite behavior probably due to its intrinsic weakness. Finally, the flax fabric was highlighted to be the component most sensitive to thermal treatment thus governing the drop in the composite mechanical properties

A role of melanin-concentrating hormone producing neurons in the central regulation of paradoxical sleep

BACKGROUND: Peptidergic neurons containing the melanin-concentrating hormone (MCH) and the hypocretins (or orexins) are intermingled in the zona incerta, perifornical nucleus and lateral hypothalamic area. Both types of neurons have been implicated in the integrated regulation of energy homeostasis and body weight. Hypocretin neurons have also been involved in sleep-wake regulation and narcolepsy. We therefore sought to determine whether hypocretin and MCH neurons express Fos in association with enhanced paradoxical sleep (PS or REM sleep) during the rebound following PS deprivation. Next, we compared the effect of MCH and NaCl intracerebroventricular (ICV) administrations on sleep stage quantities to further determine whether MCH neurons play an active role in PS regulation. RESULTS: Here we show that the MCH but not the hypocretin neurons are strongly active during PS, evidenced through combined hypocretin, MCH, and Fos immunostainings in three groups of rats (PS Control, PS Deprived and PS Recovery rats). Further, we show that ICV administration of MCH induces a dose-dependant increase in PS (up to 200%) and slow wave sleep (up to 70%) quantities. CONCLUSION: These results indicate that MCH is a powerful hypnogenic factor. MCH neurons might play a key role in the state of PS via their widespread projections in the central nervous system

Contribution à l'étude de la durabilité des structures collées soumises à des efforts mécaniques après vieillissement humide.

The study presented in this manuscript deals with the durability of bonded joints in warm and humid environment. The aim of this work was to build a numerical tool able to predict the mechanical behaviour of aged bonded joints to validate their design. In order to achieve this, an important experimental study is carried out on an epoxy filled with elastomer adhesive and steel/epoxy/steel single lap joints intended for the automotive industry. A physically justified non linear diffusion model able to predict the ingress of water and relative stress and strain in the bulk adhesive, and a coupled thermohydro- mechanical behaviour law for the bulk adhesive have been built on the basis of these experimental results. These laws have been integrated in a finite element model able to create a tensile/shear load on the bonded joint before or after aging. The effect of aging on adhesive and/or interface in the assembly is then discussed to improve the understanding of experiments.Une étude sur la durabilité d'assemblages collés en milieu chaud et humide est proposée dans ce manuscrit. L'objectif a été de construire un outil numérique capable de prédire le comportement mécanique après vieillissement des assemblages collés en vue de leurs dimensionnements. A cette fin, une importante étude expérimentale a été menée sur un adhésif structural de type époxy chargé élastomère et des assemblages collés simple recouvrement acier/époxy/acier, destinés à des applications du secteur automobile. Les résultats de ces essais ont permis le développement d'un modèle de diffusion non linéaire physiquement motivé, capable de prédire la pénétration de l'eau dans l'adhésif pour une température donnée et les contraintes qui en résultent. Ces dernières ont été obtenues grâce à une loi de comportement thermohydromécanique couplée, développée spécialement pour l'adhésif structural. Toutes ces lois ont été intégrées dans un modèle éléments finis capable de simuler l'essai de traction/cisaillement sur l'assemblage avec ou sans vieillissement. L'impact du vieillissement sur la tenue de l'adhésif et/ou de l'interface dans l'assemblage a été discuté pour améliorer la compréhension des observations expérimentales

Constitutive modeling of a SEBS cast-calender: Large strain, compressibility and anisotropic damage induced by the process

International audienceThe anisotropy induced through the calendering process was investigated in cyclic and monotonic uniaxial and biaxial tests monitored using an optical device. Thin sheets of Styrene–ethylene-co-butylene–styrene (SEBS) were extruded, which resulted in significant anisotropy in the polymer: the polymer chains are principally uniaxially oriented in the same direction as the calendering direction. Cyclic uniaxial tests reveal the Mullins effect and compressibility, which are higher in the calendering direction. A successful physically based hyper-elastic model is proposed, which includes anisotropic damage in the framework of continuum damage mechanics at large strains. Two sets of experimental data (samples cut in the calendering direction and in the transverse direction) are used to validate the constitutive model. Model parameters are estimated via scripts written in Matlab®. Computational results agree rather well with experimental data, and highlight the influence of process on material properties. Model capabilities for predicting various test results (samples cut in the diagonal direction and tensile tests on a two-hole specimen) are critically discussed

Hyperelastic behavior of modified sepiolite/SEBS thermoplastic elastomers

International audienceThin elastomer films of styrene-ethylene-butylene-styrene block copolymer (SEBS) filled with sepiolite nanofibers nanocomposites were prepared by a dip-coating process. To increase the SEBS/Sepiolite elastomer performances, a new strategy of surface modification of sepiolite by SEBS polymer chains has been developed. In a first part the surface modification of sepiolite was characterized by FTIR and TGA. In a second part the mechanical properties of the filled SEBS films were assessed. Measurements of tensile properties and tear strength were carried to evaluate the impact of the sepiolite modification. These results are discussed in taking account the filler dispersion and the quality of the SEBS/sepiolite interface. The surface modification of the sepiolite nanofibers shows an interesting improvement of the tear strength without major modifications of SEBS matrix intrinsic hyperelastic behavior

Modeling natural fiber composites microstructures for predicting their water aging

International audienceThe prediction of the in-service behavior and durability of composite materials reinforced with natural fibers is a challenge when they are subjected to simultaneous multi-physical inputs, such as water exposure combined to mechanical loadings. Understanding the response of the material may require an accurate description of its microstructure at different scales. This work proposes an original approach devoted to generate realistic virtual microstructures in order to predict numerically the hygroscopic behavior of composites reinforced with natural fibers