Développement d'expérimentations pour la caractérisation des milieux fibreux : Des matériaux manufacturés aux tissus biologiques

Qualifiés sous le terme générique de « milieux fibreux anisotropes », les matériaux auxquels je me suis interessés au cours de mes recherches sont soit d'origine synthétique noyés dans une matrice (composite) ou non (fibres et fils de nylon, carbone, verre, composites ...), soit d'origine biologique (veines, artères et anévrismes, os, diaphragme ...). L'essentiel de mon travail s'est concentré sur la compréhension aux échelles meso et microscopiques des déformations de matériaux et structure sous charges, ainsi que la caractérisation du comportement mécanique macroscopique par le développement dispositifs expérimentaux originaux. Le fil conducteur de mon manuscrit de HDR est la "Mise au point d’expérimentations pour milieux fibreux : des matériaux manufacturés aux milieux biologiques". L'objectif de mon projet de recherche est de comprendre les mécanismes de déformation et de fonctionnement des tissus biologiques fibreux (au cours de la croissance ou lors de perçage par exemple) dans le but de les réparer notamment en développant des substituts de remplacement à base de fibres

Anisotropic Mechanical Behavior of Nonwoven Geotextiles Stressed by Uniaxial Tension

The complex behavior of nonwoven structures can be studied by tensile tests. To understand the influence of mesoscopic phenomena on macroscopic behavior, the whole thermomechanical behavior was considered. During uniaxial tensile tests three parameters were measured: the load, the strain, and the temperature on the surface of the specimen. Mesoscopic phenomena corresponding to a rise of temperature are said to be dissipative. Two nonwoven structures were considered: a needlepunched and a thermo-bonded fabric. The needlepunched structure was quite anisotropic in terms of the strain field as well as the temperature. The dissipative phenomena involved were the anelastic extension of filaments and their failure. The thermo-bonded structure was more isotropic but was also a locked structure. The mechanical behavior was similar in the two directions and a rise of temperature was noticed during the whole test as a consequence of the failure of the bonded points, the anelastic extension, and the failure of filaments

Comportement dynamique sous différentes températures des polypropylènes recyclés

National audienceIn order to satisfy the different customer requirements such as use temperature and speed, recyclers introduce additives and/or fillers to the regenerated polypropylene. However, this addition can change significantly the mechanical properties as well as the deformation mechanisms. Moreover, the properties of the regenerated material depend necessarily on the waste deposit origins and on the chemical constituents. The objective of this paper is to understand the influence of adjuvants, fillers and/or contaminants on the dynamic behavior of the regenerated polypropylene under different temperatures. We considered a first sample of regenerated PP which contains three references with three different fluidity indices (grades). To determine the variability of the mechanical properties over a one-year period, different samples of regenerated PP grade 10 from the same recycling process were collected every three months and studied. The chemical constituents of the recycled polypropylene were determined by physico-chemical analysis in a previous study [1]. Dynamic compression tests on Hopkinson bars are then performed at different temperatures, below and above the glass transition temperature. This experimentation allows Comptes Rendus des JNC 21-Bordeaux INP-1-3 juillet 2019 2 identifying the sensitivity of the dynamic behavior to the temperature. The results obtained are finally compared with those of a non-regenerated (Virgin) PP in order to explain the combined effect of additives and recycling.Pour répondre à des cahiers des charges très variables en termes de conditions d'usage telles que la température et la vitesse, les recycleurs de polypropylène PP sont amenés à ajouter des adjuvants et/ou des charges. Toutefois, l'ajout de certains éléments peut modifier considérablement les propriétés mécaniques ainsi que les mécanismes de déformation. De plus, les propriétés du matériau recyclé dépendent forcément de l'origine des dépôts des déchets. L'objectif de cette étude est de comprendre l'influence des adjuvants et des charges et/ou contaminants sur le comportement dynamique sous différentes températures des polypropylènes recyclés. Nous avons considéré un premier lot de PP régénérés comportant trois références qui correspondent à trois indices de fluidité (grades) différents. Afin de déterminer la variabilité des propriétés mécaniques sur une durée d'une année, différents lots de PP régénérés de grade 10 issus du même procédé de recyclage ont été prélevés tous les trois mois et étudiés. Les constituants chimiques des polypropylènes recyclés ont été déterminés par analyses physico-chimiques lors d'une précédente étude [1]. Des essais de compression dynamique sur des barres d'Hopkinson sont ensuite réalisés à différentes températures, en dessous et au-dessus de la température de transition vitreuse, permettant ainsi d'identifier la sensibilité du comportement à la température lors de sollicitations sévères de type choc. Les résultats obtenus sont finalement comparés à ceux d'un PP non régénéré pour expliquer l'effet combiné des additifs et du recyclage

Auxetic behavior in fibrous soft tissues

National audienceAuxetic materials have a negative Poisson’s ratio: when stretched they becomethicker perpendicularly to the loading direction, and when compressed they becomethinner perpendicularly to the loading direction. The presentation is firstly aimed atproviding a brief literature review on auxetic behavior in biological tissues.Current knowledge about the causes of auxetic behavior in soft tissues and themechanical models which can reproduce it will be emphasized. Then, a fibrousstructure which includes both fibre corrugation and cross linking will be introduced. Itwill be shown that this microstructure leads to an auxetic behavior and that thecharacteristics of this fibrous structure can be considered as at least one of the causesof auxetism. Finally, some perspectives for further research will be proposed

Auxetic behavior in fibrous soft tissues

National audienceAuxetic materials have a negative Poisson’s ratio: when stretched they becomethicker perpendicularly to the loading direction, and when compressed they becomethinner perpendicularly to the loading direction. The presentation is firstly aimed atproviding a brief literature review on auxetic behavior in biological tissues.Current knowledge about the causes of auxetic behavior in soft tissues and themechanical models which can reproduce it will be emphasized. Then, a fibrousstructure which includes both fibre corrugation and cross linking will be introduced. Itwill be shown that this microstructure leads to an auxetic behavior and that thecharacteristics of this fibrous structure can be considered as at least one of the causesof auxetism. Finally, some perspectives for further research will be proposed

Transverse compression behavior of textile rovings: finite element simulation and experimental study

International audienceResults from finite element simulation of the transverse compression of Polyamide 6.6 rovings made of 40 filaments are compared in this paper against experimental data. The finite element simulation, using a finite strain beam model to represent each filament of the roving, focuses on the modeling of contact-friction interactions between filaments. Experimental tests, consisting in crushing rovings between two rigid planes, varying the twist and the tensile force, are reproduced by simulation. Experimental and simulation results are compared with a good agreement. The simulation studies the influence of the roving twist, the applied tensile force, and the friction coefficient on the transverse compression behavior. The occurrence of plateaus in the transverse compression curve is highlighted and discussed

Experimental evaluation of transverse friction between fibers

International audienceThe mechanical behavior of fibrous assemblies, particularly the transverse behavior of fiber yarns and multifilament strands depends, inter alia, on the fiber-to-fiber contact and sliding. All studies on inter fiber friction presented in the literature focus on cases where the relative inter fiber motion occurs either in longitudinal-to-longitudinal or longitudinal-to-transverse direction. The transverse-to-transverse inter fiber friction plays an important role as far as the mechanical transverse behavior of fiber yarns and multifilament strands are concerned. In order to evaluate the case named here ‘transverse friction, the present study proposes an original method and an associated model analyzing the transversal component of friction between two oblique crossing fibers. A relatively simple and original statistical approach has also been developed to evaluate the confidence interval of friction coefficient. The first results show a relative stability of the transverse friction. This coefficient is also significantly inferior to the longitudinal one