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Analysis of the thermo-mechanical behaviour of the 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 viscoelasticity which led us to the definition of complex moduli and to the rules of the renowned time-temperature superposition principle (TTSP). In this context, the effect of time (i.e., rate dependence) is almost unanimously associated with viscous effects. It has however been observed that the dissipative effects associated with viscous effects may be superimposed with thermo-elastic coupling effects, indicating a high sensitivity of polymeric materials to temperature variations (thermodilatability). Indeed, because of heat diffusion, it was also noticed that these strong thermo-mechanical couplings may induce a time dependence of the material behaviour. Using traditional experimental methods of visco-analysis i.e., dynamic mechanical thermal analysis (DMTA) and via an experimental energy analysis of the behaviour using quantitive infrared techniques, the relative importance of thermoelastic heat sources compared to viscous dissipation was analysed with the increasing frequency of monochromatic cyclic tensile tests made at different ambient temperatures.

Flax retting is a major bioprocess in the cultivation and extraction cycle of flax fibres. The aim of the present study is to improve the understanding of the evolution of fibre properties and ultrastructure caused by this process at the plant cell wall scale. Initially, investigations of the mechanical performances of the flax cell walls by Atomic Force Microscopy (AFM) in Peak Force mode revealed a significant increase (+33%) in the cell wall indentation modulus with retting time. Two complementary structural studies are presented here, namely using X-Ray Diffraction (XRD) and solid state Nuclear Magnetic Resonance (NMR). An estimation of the cellulose crystallinity index by XRD measurements, confirmed by NMR, shows an increase of 8% in crystallinity with retting mainly due to the disappearance of amorphous polymer. In addition, NMR investigations show a com-paction of inaccessible cell wall polymers, combined with an increase in the relaxation times of the C4 carbon. This densification provides a structural explanation for the observed improvement in mechanical performance of the secondary wall of flax fibres during the field retting process.

Since 2004, the “Mona Lisa” painting by Leonardo da Vinci has been studied by an international research group of wood scientists and several experimental campaigns have been carried out to understand its characteristics and provide indications for its conservation. Based on the collected data, a numerical model of the wooden panel has been developed to simulate the mechanical interaction with the framing system. The main objective of this modelling work, described in this paper, is to extract as much information as possible from the experimental tests carried out and, thus, reach a sufficient level of scientific knowledge of the mechanical properties of the panel to build a predictive model. It will be used to predict the effect of modified boundary conditions and as a tool of preventive conservation.

Analysis of the thermo-mechanical behaviour of the 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 viscoelasticity, which led us to the definition of complex modules and used to identify the glass transition temperature as the so called rule of time-temperature superposition principle. In this context, the effect of time is almost unanimously associated with viscous effects. It has however been observed that the dissipative effects associated with viscous effects remains often small compared with thermomechanical coupling effects, indicating a high sensitivity of polymeric materials to temperature variations. Because of heat diffusion, it was also noticed that thermomechanical couplings induce a time dependence of the material behavior. Using traditional experimental methods of visco-analysis (DMTA) and via an experimental energy analysis of the behaviour, the goal of this research work is to restate the time-temperature equivalence rule under the Thermodynamics of Irreversible Processes, taking into account not only dissipation effects but also thermomechanical coupling effects induced by the deformation process.

Analysis 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.