Effect of time and thermo-mechanical couplings on polymers

National audienceAnalysis of the thermo-mechanical behaviour of polymers has been and still is the subject of many rheological studies both experimentally and theoretically. For small deformations, the modelling framework retained by rheologists is often of linear visco-elasticity, which led to the definition of complex modules and used to identify the glass transition temperature as the so called rule of time-temperature superposition. In this context, the effects of time are almost unanimously associated with viscous effects. It has also been observed that the dissipative effects associated with viscous effects are often very small compared to the coupling of sources indicating a high sensitivity of polymeric materials to temperature variations. This work is mainly focused on establishing the exact role of coupling effects, which also induce the effect of time. Using traditional experimental methods of visco-analysis (DMTA) and via an energy analysis of the behaviour, the goal of the thesis is to try to restate the time-temperature equivalence rule under the Thermodynamics of Irreversible Processes, taking into account the dissipative effects and coupling induced process deformation

Addendum to 'The equivariant spectral function of an invariant elliptic operator'

Let $M$ be a compact boundaryless Riemannian manifold, carrying an effective and isometric action of a torus $T$, and $P_0$ an invariant elliptic classical pseudodifferential operator on $M$. In this note, we strengthen asymptotics for the equivariant (or reduced) spectral function of $P_0$ derived previously, which are already sharp in the eigenvalue aspect, to become almost sharp in the isotypic aspect. In particular, this leads to hybrid equivariant $L^p$-bounds for eigenfunctions that are almost sharp in the eigenvalue and isotypic aspect.Comment: 14 pages. arXiv admin note: text overlap with arXiv:1512.0219

Effect of enthalpy-entropy compensation during sorption of water vapour in tropical woods: the case of Bubinga (Guibourtia Tessmanii J. Léonard ; G. Pellegriniana J.L.)

International audienceThe study of sorption of water vapour by a tropical wood (Bubinga) at 20°C, 38°C, 50°C and 60°C has permitted to obtain adsorption and desorption isotherms. Sigmoids have been obtained and they all show hysteresis phenomenon. From these isotherms, we were able to determine Gibbs energy, enthalpy and entropy variations. These variations were analysed and shown to be all negative for adsorption and all positive for desorption, with a great dependence to the moisture content during adsorption and desorption. Temperature had no significant effect on these parameters. Such dependence is mentioned in the scientific literature for softwood and is surely related to the availability of sorption sites. A linear relationship between enthalpy and entropy was shown to exist and the effect of enthalpy–entropy was studied for adsorption and desorption of water vapour by wood. In fact, the isokinetic temperature was determined and compared to the harmonic mean of temperatures

Projet GreenWood : Utilisation du bois vert dans la construction : Effet du séchage sur les propriétés rhéologiques et hydriques

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One-dimension visco-elastic modelling of wood in the process of formation to clarify the Hygrothermal Recovery behavior of tension wood

International audienceWood production on stem by deposit of concentric layers on its periphery are going along with the setting up of growth stress. Growth stress has two origins: (1) loading due to weight of the structure is applied progressively when the tree is growing; (2) cell maturation, which happened at the end of the deposit of a new layer, causes an expansion, called maturation deformation, which can’t happen freely due to the previous layer and lead to the creation of initial growth stress [1]. The growth stress can be released during cutting and also during hygrothermal treatment (HT), it can be called Hygrothermal Recovery (HTR) [2]

Radial Variations of Vibrational Properties of Three Tropical Woods

International audienceRadial trends of vibrational properties, including the specific dynamic modulus (E'/r) and damping coefficient (tand), were investigated for 3 tropical rainforest hardwood species, Simarouba amara, Carapa procera and Symphonia globulifera by free-free flexural vibration test. The microfibril angle (MFA) was estimated through X-Ray diffraction. Consistent patterns of radial variations were observed for all studied properties. E'/r was found to decrease from pith to bark, which is strongly related to the increasing pith-bark trend of MFA. The variation of tand along the radius can be partly explained by MFA, and partly by the gradient of extractives due to heartwood formation. The coupling effect of MFA and extractives could be separated through the analysis of log(tand) - log(E'/r) diagram. For the studied species, the extractive content putatively associated to heartwood formation generally tends to decrease the wood damping coefficient. However, this weakening effect of extractives was not observed for inner part of the heartwood, suggesting the mechanical action of extractives was reduced during their chemical ageing

Etude de la faisabilité de panneaux OSB écologiques naturellement durables

L’objectif de cette étude a été de tester la viabilité et la faisabilité de panneaux OSB contenant un mélange de lamelles de bois issues d’essences durables (robinier et cyprès) et de pin. Afin d’augmenter le caractère écologique de ces panneaux, des résines naturelles à base de lignine et de tanin ont été utilisées. Plusieurs panneaux contenant différentes proportions de mélange ont été testés d’un point de vue durabilité (termites) et mécanique (avant et après dégradation)

Biological properties of an OSB eco-product manufactured from a mixture of durable and nondurable species and natural resins

International audienceOSB panels were manufactured with a mixture of pine and cypress heartwood and lignin and tannin based resins in order to propose an eco-friendly wood composite. The resistance of OSB panels against Reticulitermes santonensis was tested according to EN 118 and EN 117 as well as in a field test. OSB made from cypress showed more resistance against the tested termite, the resistance decreased as the percentage of pine increased. The degree of attack differed according to the choice of standard procedure. The field test results revealed that in outdoor conditions and when the termites had a feeding choice, all mixtures showed the same behaviour. The decay resistance was assessed according to EN 12038, and the results revealed that the resistance increases when the percentage of cypress increases

The decreasing radial wood stiffness pattern of some tropical trees growing in the primary forest is reversed and increases when they are grown in a plantation

International audienceBackground : This study examines the radial trend in wood stiffness of tropical rainforest trees. The objective was to determine if the type of growing environment (exposed plantation or dense primary forest) would have an effect on this radial trend. Methods : The axial elastic modulus of wood samples, representing a pith to bark cross-section, of six trees from several French Guianese species (two of Eperua falcata, one of Eperua grandiflora, two of Carapa procera and one of Symphonia gloubulifera) was measured using a dynamic "forced vibration" method. Results : Primary forest trees were observed to have a decrease in wood stiffness from pith to bark, whereas plantation trees, from the same genus or species, displayed a corresponding increase in wood stiffness. Juvenile wood stiffness appears to vary depending on the environment in which the tree had grown. Conclusion : We suggest that the growth strategy of primary forest trees is to produce wood resistant to selfbuckling so that the height of the canopy may be obtained with the maximum of efficiency. In contrast, the growth strategy of the trees growing in an exposed plantation is to produce low-stiffness wood, important to provide flexibility in wind. Further experiments to study the behaviour of more species, with more individuals per species, growing across a range of physical environments, are required