Modèle viscoélastique-viscoplastique couplé avec endommagement pour les matériaux polymères semi-cristallins

Les matériaux polymères sont largement utilisés pour des applications structurelles dans le secteur automobile et leurs comportements complexes nécessitent des modèles précis pour la simulation éléments finis. Les polymères possèdent un comportement dépendant du temps et de la vitesse. La dépendance à la vitesse peut être observée par un accroissement de la rigidité et de la limite élastique en fonction de la vitesse de déformation. Le long temps nécessaire pour retrouver des contraintes nulles après sollicitation du matériau met en évidence la dépendance du temps sur le comportement. De plus, particulièrement pour les polymères chargés, le phénomène de cavitation se traduisant par la création et la croissance de micro-cavités et de microfissures conduit à un changement de volume durant la déformation. Dans ce travail, un modèle de comportement est développé pour un polymère semi-cristallin chargé de talc utilisé dans l industrie automobile. Un modèle constitutif viscoélastique-viscoplastique non-associatif avec endommagement non-local est proposé dans le but de simuler les phénomènes observés expérimentalement. Dans le modèle développé, une surface de charge non symétrique est utilisée pour prendre en compte la pression hydrostatique. La viscoplasticité non-associative couplée avec l endommagement conduit aux déformations viscoplastiques non-isochoriques caractérisées expérimentalement. Les paramètres du modèle proviennent d essais expérimentaux réalisés sous différentes conditions et a différentes vitesses de déformation. Pour ces essais, plusieurs techniques de mesure, telles que la corrélation d images et l extensommetrie optique sont utilisées pour les mesures de champs de déplacements. La bonne corrélation entre les données expérimentales et les simulations numériques mettent en évidence la précision du modèle développé afin de modéliser le comportement des matériaux polymères semi-cristallins.Polymer materials are widely used for structural applications in the automotive sector and their behaviours are complex and require accurate models for finite element simulations. Polymer materials exhibit rate and time dependent behaviours. The rate dependency can be observed by an increase of the stiffness and the yield stress at increasing strain rate. The long time to recover the zero stress after solicitation of the material highlight the time dependent behaviour. Furthermore, particularly for filled polymers, the cavitation phenomenon cause the creation and growth of micro-voids and microcracks called damage and leads to volume change during the deformation. In this work, a behaviourmodel for mineral filled semi-crystalline polymer used in automotive industry is developed. A constitutive viscoelastic-viscoplastic non-associated model coupled with nonlocal damage is proposed in order to simulate the phenomena observed experimentally. In the constitutive model, a non symmetric yield surface is used to take the hydrostatic pressure into account. The non associated viscoplasticity coupled with damage leads to the non-isochoric viscoplastic deformation characterised experimentally. The material parameters arise from experimental tests carried out under various loadings and strain rates. For these experimental tests, different measurement techniques like Digital Image Correlation and optical extensometry are used for the displacements and the strain field measurements. The good agreement between the experimental data and the numerical simulations highlights the accuracy of the developed model for polymer modelling.VALENCIENNES-Bib. électronique (596069901) / SudocSudocFranceF

Stimulated Raman adiabatic passage in Tm3+:YAG

International audienceWe report on the experimental demonstration of stimulated Raman adiabatic passage in a Tm 3+ : YAG crystal. Tm 3+ : YAG is a promising material for use in quantum information processing applications, but as yet there are few experimental investigations of coherent Raman processes in this material. We investigate the effect of inhomogeneous broadening and Rabi frequency on the transfer efficiency and the width of the two-photon spectrum. Simulations of the complete Tm 3+ : YAG system are presented along with the corresponding experimental results

Non-thermal plasma potentialities for microwave device reconfigurability

Three examples of results achieved from cooperative works with microwave and plasma research groups in Toulouse (France) are presented in this paper. They are focused on the use of few non-thermal plasmas to make a microwave device reconfigurable. The relative permittivity of such a plasma medium can be tuned from unity to negative values. This special feature appears to be very attractive, although the electromagnetic losses are significant. The use of plasmas with planar waveguides and within metamaterials is discussed. In addition, the basic principles of a scanning antenna built with a leaky wave in a plasma layer are presented

protocoles adiabatiques pour le ralentissement et le stockage de la lumière dans un cristal de Tm:YAG

Many laboratories work on the development of quantum memories, a key element of quantum information systems. The quantum memories for light in particular should play an important role in quantum communication networks over long distances. A quantum memory consists in a set of atoms on which one transposes the state of the light signal, the memory must be able to save this state for a sufficiently long time, then return it faithfully. To date, the effort has focused on protocols related to electromagnetically induced transparency (EIT), implemented in clouds of atoms cooled by laser. I work on an alternative system, the rare earth ions in crystalline matrix (TRMC). In this system, atoms are stricltly motionless. Moreover, the lifetime of quantum superposition states, carrying the information is very long at low temperatures. The inhomogeneous broadening of optical transitions, however, clearly distinguishes the TRMC from laser cooled atoms. Taking advantage of this feature, I propose an original storage protocol. I explore the theoretical and experimental sides of this new protocol.De nombreux laboratoires travaillent à la mise au point de mémoires quantiques, élément clef des systèmes d'information quantique de l'avenir. Les mémoires quantiques pour la lumière devraient en particulier jouer un rôle important dans les réseaux de télécommunications quantiques à grande distance. Constituée d'un ensemble d'atomes sur lequel on transpose l'état du signal lumineux, la mémoire doit être capable de sauvegarder cet état pendant un temps suffisamment long, puis de le restituer avec efficacité et fidélité. A ce jour, l'effort a surtout porté sur des protocoles liés à la Transparence Induite Electromagnétiquement (EIT), mis en oeuvre dans des nuages d'atomes refroidis par laser. Mon travail porte sur un système alternatif, les ions de terres rares en matrice cristalline (TRMC). Ces matériaux offrent une immobilité plus grande que les atomes froids. Par ailleurs, la durée de vie des états de superposition quantique, porteur de l'information stockée, y est très longue à basse température. L'élargissement inhomogène des transitions optiques distingue cependant nettement les TRMC des atomes refroidis par laser. Tirant parti de cette caractéristique, je propose un protocole de stockage original que j'explore sur le plan théorique et expérimental

Manger colonial ou végétal ? Les (vrais) enjeux de l'histoire de la colonisation

L'histoire de la colonisation belge reste peu enseignée dans les écoles secondaires. Il est à craindre, au vu de la considération limitée que lui accorde le Pacte d’Excellence, que cet état de fait perdure. Les tensions récurrentes que suscitent le passé colonial dans l'espace public nous rappellent pourtant à quel point il est essentiel de fournir aux élèves des outils et des connaissances sur un phénomène qui entre dynamiques globales et ancrages locaux, a façonné nos sociétés contemporaines

Damage characterization for particles filled semi-crystalline polymer

Damage evolution and characterization in semi-crystalline polymer filled with particles under various loadings is still a challenge. A specific damage characterization method using Digital Image Correlation is proposed for a wide range of strain rates considering tensile tests with hydraulic jacks as well as Hopkinson's bars. This damage measurement is obtained by using and adapting the SEE method [1] which was developed to characterize the behaviour laws at constant strain rates of polymeric materials in dynamic. To validate the characterization process, various damage measurement techniques are used under quasi-static conditions before to apply the procedure in dynamic. So, the well-known damage characterization by loss of stiffness technique under quasi-static loading is applied to a polypropylene. In addition, an in-situ tensile test, carried out in a microtomograph, is used to observe the cavitation phenomenon in real time. A good correlation is obtained between all these techniques and consequently the proposed technique is supposed suitable for measuring the ductile damage observed in semi-crystalline polymers under dynamic loading. By applying it to the semi-crystalline polymer at moderate and high speed loadings, the damage evolution is measured and it is observed that the damage evolution is not strain rate dependent but the failure strain on the contrary is strain rate dependent

Damage characterization for particles filled semi-crystalline polymer

Damage evolution and characterization in semi-crystalline polymer filled with particles under various loadings is still a challenge. A specific damage characterization method using Digital Image Correlation is proposed for a wide range of strain rates considering tensile tests with hydraulic jacks as well as Hopkinson's bars. This damage measurement is obtained by using and adapting the SEE method [1] which was developed to characterize the behaviour laws at constant strain rates of polymeric materials in dynamic. To validate the characterization process, various damage measurement techniques are used under quasi-static conditions before to apply the procedure in dynamic. So, the well-known damage characterization by loss of stiffness technique under quasi-static loading is applied to a polypropylene. In addition, an in-situ tensile test, carried out in a microtomograph, is used to observe the cavitation phenomenon in real time. A good correlation is obtained between all these techniques and consequently the proposed technique is supposed suitable for measuring the ductile damage observed in semi-crystalline polymers under dynamic loading. By applying it to the semi-crystalline polymer at moderate and high speed loadings, the damage evolution is measured and it is observed that the damage evolution is not strain rate dependent but the failure strain on the contrary is strain rate dependent