Contribution à l'étude des propriétés mécaniques des structures en polymère injecté (Application aux propriétés élastiques locales et à la modélisation viscoélastique)

La thèse de doctorat présentée présente une étude expérimentale des variations locales des propriétés mécaniques des polymères suivie d'une partie traitant de la modélisation viscoélastique de ces matériaux (PEhD, PA, POM).La variation locale de caractéristiques microscopiques fait fortement évoluer les caractéristiques mécaniques, en particulier dans les zones proches de la paroi du moule. Afin de quantifier ces variations, on a étudié selon différents procédés expérimentaux l'évolution du module d'Young longitudinal dans l'épaisseur d'une éprouvette.A l'aide d'un banc d'essai de flexion vibratoire et d'un outil de traitement et de calcul numérique qui a été développé, il est possible de trouver l'évolution du module d'Young. On constate une évolution : le module augmente de +50 à +187% selon les matériaux. Afin de corréler ces résultats avec la microstructure, différentes expériences ont été menées :- des essais de calorimétrie différentielle par balayage pour mesurer une évolution du taux de cristallinité,- des essais de nanoindentation pour mesurer des variations locales du module et mettre en évidence une anisotropie. Ces essais corrèlent les résultats précédents mais présentent cependant une forte dispersion liée à la fois à l'hétérogénéité de la microstructure et au comportement viscoélastique des polymères. Une étude plus précise nécessite une meilleure modélisation du comportement viscoélastique, en particulier dans la phase de décharge. L'étude de ce problème constitue la seconde partie du travail.Les principaux modèles présentés classiquement en petites déformations ont été testés afin de valider leurs performances. A partir d'essais et de simulation des différents modèles rhéologiques linéaires et non linéaires, le caractère insuffisant de ces modèles a été mis en évidence. Une modélisation acceptable nécessite la prise en compte plus précise de la physique du matériau puis un passage micro macro.The thesis deals with an experimental study of local variations of mechanical properties of injection molded polymers, followed by a specific part dealing with the viscoelastic modelization of those materials (PEhD, PA, POM).The variation of microscopic parameters strongly influence mechanical characteristics, especially in areas close to the mould walls. The evolution of Young's modulus has been inquired using several experimental devices. With the aid of a vibrational flexural test rig and digital calculation and processing systems that have been developed, it was possible to measure the variation in local Young's modulus as a function of position in the thickness of the test piece The results obtained reveal a large variation in the local Young's modulus over the thickness of an injection moulded piece, in particular in the outer skin zone. The influence of the various operational parameters has been evaluated.In order to correlate those results with microstructure, several other experiments have been led :-differential scanning dilatometry, to measure the evolution of the cistallinity rate,- nanoindentation tests to measure the variation of the local Young's modulus and anisotropy. Those tests validate previous results but present a strong dispersion due to microstructure heterogeneity and viscoelastic behaviour of polymers.Therefore, a better modelization of the viscoelastic behaviour has to be taken into account : this is the point of the second part of the thesis. The classical models of linear and non linear viscoelasticity in small deformation have been identified on experimental traction test and simulated under several conditions (deformation rate jumps, unloading). It is obvious that the classical models are not sufficient to describe properly the viscoelastic behaviour : the physic of the material has to be taken into account.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Modeling and Control of a Redundant Tensegrity-based Manipulator

International audience<div style=""&gt<font face="arial, helvetica"&gt<span style="font-size: 13px;"&gtTensegrity-based mechanisms draw attention in particular for their deployability</span&gt</font&gt</div&gt<div style=""&gt<font face="arial, helvetica"&gt<span style="font-size: 13px;"&gtand compliance. However, task-based design and control of such systems</span&gt</font&gt</div&gt<div style=""&gt<font face="arial, helvetica"&gt<span style="font-size: 13px;"&gtare still open topics. In a previous work, a tensegrity-based manipulator was</span&gt</font&gt</div&gt<div style=""&gt<font face="arial, helvetica"&gt<span style="font-size: 13px;"&gtdesigned to respect the remote center of motion constraint encountered in medical</span&gt</font&gt</div&gt<div style=""&gt<font face="arial, helvetica"&gt<span style="font-size: 13px;"&gtapplications. Only the workspace was then analyzed. Here, we develop the kinematic</span&gt</font&gt</div&gt<div style=""&gt<font face="arial, helvetica"&gt<span style="font-size: 13px;"&gtmodel of this manipulator and then exploit it for the control. Such a manipulator</span&gt</font&gt</div&gt<div style=""&gt<font face="arial, helvetica"&gt<span style="font-size: 13px;"&gtis redundant. The use of redundancy is discussed and evaluated in simulation</span&gt</font&gt</div&gt<div style=""&gt<font face="arial, helvetica"&gt<span style="font-size: 13px;"&gtwith two control schemes using Jacobian based controllers.</span&gt</font&gt</div&g

Carbon fibre/pvc foam sandwich composite modelization for MAVs & long range drones structures

International audienceThis paper deals with the characterization, modelization and simulation of a composite structure used for MAV or generally for drones design. Composite structures are extremely difficult to simulate due to the anisotropic behaviour. The first part of the article is focused on carbon fiber/PVC foam (AIREX) sandwich composite characterization with the design of experiments method on tensile tests. This method gives equations, which describe the material mechanical behaviour (Young’s modulus, tensile strength) depending on factors values. The second part of the article deals with the improvement of mechanical simulation of an anisotropic material with the goal to get an accurate model and to generalised properties to an entire structure. Then, the macroscopic mechanical properties of the most performing sandwich will be obtained with the global composite behaviour matrix 6x6, in order to be integrated into a Finite Element Analysis software (CREO / Simulate) to simulate a fixed wing behaviour. Finally, comparisons between experiment and numerical simulation on the wing will give promising results, and simulations will reveal substantial differences between tension & compression in a flexural solicitation

Toward an MR-compatible needle holder with adaptive compliance using an active tensegrity mechanism

International audienceIntroductionThe field of MR imaging has extended from diagnosis to guidance and control in a wide variety of interventional procedures [1]. Due to the lack of space and manipulability within the MRI scanner, there is an ongoing interest for robotic assistance in MR-guided interventions. Numerous MR-compatible robots have been proposed, especially for needle manipulation.For needle insertion in the liver, patient-mounted robots are particularly interesting for the pro- vided partial compensation of the breathing motion. The needle holder must however still fulfill two contradictory requirements: it needs to be stiff during the insertion and compliant afterwards, in order to avoid organ lacerations [2]. This led to the development of needle grasping systems that allow the needle to move freely between two insertion phases [3].In this paper, an alternate approach is considered: a needle manipulation system with adaptive compliance is proposed that is based on so-called tensegrity mechanisms. Numerous challenges related to MR-compatibility and compliance control could be overcome thanks to this recent class of robots, as outlined in the following through the design of a first device for needle orientation control