Contribution à l'utilisation des polymères à mémoire de forme pour les structures à amortissement contrôlé.

This work proposes to use shape memory polymers to control structural vibrations. These materialsexhibit a memory hysteresis which is practically associated with intrinsic damping properties whichare very high when the memory effect is strong. First, a thermomechanical characterization of theshape memory polymer of interest (tBA/PEGDMA) is performed by dynamic mechanical analysis.A rheological model based on time-temperature superposition is used to represent the viscoelasticbehavior of the polymer. Secondly, an experimental campaign is performed over a wide frequencyand temperature range, through various experimental techniques (static, modal, nanoindentation,ultrasounds, high frequency dynamic analysis, acoustic microscopy) to define the area of validity,in frequency and temperature, of the rheological model. Third, the shape memory polymeris integrated into a composite sandwich structure to highlight the awesome damping capabilitiesof the material. Finally, a damping tuning methodology by temperature control is proposed.Indeed, the power dissipation in the sandwich is related to physical properties of the tBA/PEGDMAcore which are temperaturecontrolled to optimize the damping over a given frequency range.Ces travaux de thèse proposent d'utiliser les polymères à mémoire de forme comme moyen decontrôle des vibrations des structures. Outre l'hystérésis de mémoire qui est classiquement mis enavant, ces matériaux possèdent des propriétés amortissantes intrinsèques qui sont d'autant plusintéressantes lorsque l’effet mémoire de forme est important. Dans un premier temps unecaractérisation des propriétés mécaniques du tBA/PEGDMA, polymère à mémoire de forme del'étude, est effectuée par analyse dynamique mécanique. Un modèle rhéologique basé surl'équivalence temps-température, le 2S2P1D, est utilisé pour rendre compte du comportementviscoélastique du polymère. Dans un deuxième temps, une campagne expérimentale est menée, surune large bande de fréquences et de températures, grâce à divers moyens expérimentaux (statiques,modaux, nano-indentations, ultrasons, dynamiques hautes fréquences, microscopie acoustique) afinde définir le domaine de validité, fréquentiel et thermique, du modèle rhéologique. Dans untroisième temps, le polymère à mémoire de forme est intégré à une structure composite de typesandwich pour mettre en évidence le pouvoir amortissant impressionnant du matériau. Enfin, uneméthodologie de contrôle de l'amortissement par la température est proposée. En effet, ladissipation d’énergie dans le sandwich s'avère contrôlable, la température permettant d’ajuster larigidité et le facteur de perte du tBA/PEGDMA pour un amortissement optimal sur une large bandede fréquences

Analyse multi-échelle expérimentale et numérique du comportement d'une liaison

Les vibrations des structures assemblées sont très dépendantes du comportement mécanique des liaisons. La rigidité et l'amortissement induits par ces liaisons dépendent essentiellement des conditions de contact / décollement et d'adhérence / glissement. La mesure des champs de déformations dans les zones de liaisons et à proximité de celles-ci constitue donc un enjeu important pour la mise au point de modèles prédictifs. Ce travail porte sur l'utilisation de la vélocimétrie laser en trois dimensions pour la caractérisation du comportement de ces liaisons. Un assemblage constitué de deux poutres boulonnées en plusieurs points a été testé lors d'essais vibratoires sous excitations aléatoire et harmonique. La mesure du champ de déplacement global permet d'évaluer l'évolution des paramètres modaux en fonction de l'amplitude des vibrations. Des mesures du champ de déplacement local permettent d'évaluer les conditions de contact et de frottement dans chaque assemblage. Des corrélations avec les modèles, via l'étude de l'erreur en relation de comportement, et des études de sensibilité de ces modèles aux différents paramètres sont également proposées

Wideband frequency characterization of a shape memory polymer

International audienceThis study is an experimental evaluation of the mechanical properties of shape memory polymer Veriflex R under different tests conditions. Veriflex R was chosen because of its easy accessibility and its properties similar to epoxy resins which make it very suitable for use in a wide variety of technical applications. Dynamic mechanical analysis (DMA) has been used to determine the evolution of the viscoelastic properties versus temperature and frequency under harmonic loading. The time-temperature superposition principle has been found to be valid for this material. This is illustrated here through the use of the master curves. Furthermore a modal analysis on a Veriflex R rectangular plate has been performed in order to reach higher frequencies than the DMA, and a finite element model was employed to find the viscoelastic properties of the material. A correlation between these two experimental methods allowed to highlight a disparity of results explained by the deterioration of the Veriflex R over time

Static and Dynamic Thermo Mechanical Characterization of a Bio-Compatible Shape Memory Polymer

International audienceShape memory polymers encounter a growing interest over the past ten years particularly because their eventual bio-compatibility leads to many bio-medical applications. They also present many benefits for the design of micro-adaptive systems for deployment or controlled damping materials. Indeed, the SMPs are polymeric smart materials which have the remarkable ability to recover their primary shape from a temporary one when submitted to an external stimulus. The present study deals with the synthesis and the thermo-mechanical characterization of a thermally-actuated SMP. The polymer considered hereafter is a chemically cross-linked thermoset. It is synthesized via photo polymerization (UV curing) of the monomer tert-butyl acrylate (tBA) with the crosslinking agent poly(ethylene glycol) dimethacrylate (PEGDMA) and the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA). A mechanical characterization has been performed using three kinds of tests: quasi-static tensile tests, tensile dynamic mechanical analysis (DMA) and modal tests. The Young's modulus and the Poisson ratio are determined at ambient temperature using the first technique. The DMA is used to determine the evolution of viscoelastic properties as a function of the temperature and the frequency under harmonic loading. The modal analysis is employed to identify the viscoelastic properties of the material at higher frequency. A comparison of the results obtained by these three experimental methods highlights their complementarity

Contribution to using shape memory polymers for the control of structural damping

Ces travaux de thèse proposent utiliser les polymères à mémoire de forme comme moyen de contrôle desvibrations des structures. Outre hystérésis de mémoire qui est classiquement mis en avant, ces matériauxpossèdent des propriétés amortissantes intrinsèques qui sont d'autant plus intéressantes lorsque l’effetmémoire de forme est important. Dans un premier temps une caractérisation des propriétés mécaniques dutBA/PEGDMA, polymère à mémoire de forme de l'étude, est effectuée par analyse dynamique mécanique.Un modèle rhéologique basé sur lʹéquivalence temps-température, le 2S2P1D, est utilisé pour rendre comptedu comportement viscoélastique du polymère. Dans un deuxième temps, une campagne expérimentale estmenée, sur une large bande de fréquences et de températures, grâce à divers moyens expérimentaux(statiques, modaux, nano-indentations, ultrasons, dynamiques hautes fréquences, microscopie acoustique)afin de définir le domaine de validité, fréquentiel et thermique, du modèle rhéologique. Dans un troisièmetemps, le polymère à mémoire de forme est intégré à une structure composite de type sandwich pour mettreen évidence le pouvoir amortissant impressionnant du matériau. Enfin, une méthodologie de contrôle delʹamortissement par la température est proposée. En effet, la dissipation d’énergie dans le sandwich sʹavèrecontrôlable, la température permettant d’ajuster la rigidité et le facteur de perte du tBA/PEGDMA pour unamortissement optimal sur une large bande de fréquences.This work proposes to use shape memory polymers to control structural vibrations. These materials exhibit amemory hysteresis which is practically associated with intrinsic damping properties which are very highwhen the memory effect is strong. First, a thermomechanical characterization of the shape memory polymerof interest (tBA/PEGDMA) is performed by dynamic mechanical analysis. A rheological model based on timetemperaturesuperposition is used to represent the viscoelastic behavior of the polymer. Secondly, anexperimental campaign is performed over a wide frequency and temperature range, through variousexperimental techniques (static, modal, nanoindentation, ultrasounds, high frequency dynamic analysis,acoustic microscopy) to define the area of validity, in frequency and temperature, of the rheological model.Third, the shape memory polymer is integrated into a composite sandwich structure to highlight the awesomedamping capabilities of the material. Finally, a damping tuning methodology by temperature control isproposed. Indeed, the power dissipation in the sandwich is related to physical properties of the tBA/PEGDMA core which are temperature-controlled to optimize the damping over a given frequency range

Les polymères à mémoire de forme

International audienceLes polymères à mémoire de form

Analyse de champs par vibrométrie laser sur un assemblage boulonné

International audienceAnalyse de champs par vibrométrie laser sur un assemblage boulonn

Design of an assembly for nonlinear vibration reduction

International audienceThe lightness of the space and aerospace structures causes their vulnerability to vibrations. The cold temperatures do not allow using polymer materials. Active and semi-active control using piezos embedded on the structure can be used efficiently instead of polymers but they induce energy consumption. Friction damping is less efficient but it does not depend on temperature and it is energetically passive. Unfortunately its efficiency depends on the vibration amplitude as well as on the tightening force. The damping is very low for the lowest amplitudes and the largest tightening loads and increase up to an optimal value. This optimum damping is adjustable thanks to the tightening force. The purpose of our work is to evaluate the efficiency of the control of the tightening force in bolted joints to reduce the vibration of the assembled structure. In the first part, we present a original setup and a very detailed design of experiments that highlights the optimal sets of parameter in order to get a good control of the vibrations according to the frequency and the magnitude of the load. To conclude, we propose to share experimental data with the attendees for further discussions

Etude aérodynamique instationnaire d'un rotor éolien de Savonius : mise en évidence de l'influence de la portance

In this study unsteady numerical simulations with software Star-CCM+ were conducted to understand clearly the Savonius wind turbine operation, especially by comparing static results (motionless wind turbine) and dynamic results (wind turbine in rotation). This study provides a new vision of its behavior. Indeed, the Savonius is so-called “drag” as its torque is maximum when the blades are perpendicular to the flow direction (maximum projected area). This fact is actually verfyed with a static study. However, the dynamic study shows that the maximum torque is obtained when the axis of the blade is parallel to the wind, so the Savonius wind turbine would be “lift”. This study opens new perspectives for the optimization of the Savonius rotors.Cluster éolien W4

Detection and analysis of loosening in jointed structures using acoustic emission sensors and smart bolts

International audienceIn aeronautic, automotive or civil engineering structures, the parts are assembled together by means of rivets welding points or bolted joints. The bolted joints are likely to self-loosen when the structure is subjected to vibrations [4]. As loosening can lead to serious accidents, our project focuses on the early detection of self-loosening. To achieve this goal, it is possible to integrate a sensor in a screw to monitor it, but it is impossible to instrument the thousands of screws that contains a structure. Our project therefore aims to try to overload all links with a network of some Acoustic Emission sensors well chosen and well positioned. The setup ORION has been firstly developed to perform research activity about damping and nonlinear vibrations [2]. It is made with two plates linked with three bolted joints. To facilitate the understanding of the physical phenomena, the contact between the plates is made by patches machined on the surfaces. A piezo-electric actuator (CEDRAT) has been especially developed in order to cause the loosening. A sensor (TEXYS) has been embedded into a M4 bolt head to measure the tightening loads. Acoustic Emission sensors (MISTRAS) are used to measure acoustic activity, see figure 1