Amortissement physique | EPFL Graph Search

En physique, l'amortissement d'un système est une atténuation de ses mouvements par dissipation de l'énergie qui les engendre. Il peut être lié de diverses manières à la vitesse.

Le frottement entre deux solides correspond à une dissipation sous la forme de chaleur. Il est régi par la loi de Coulomb selon laquelle la force de frottement ne dépend pas de la vitesse.
Lorsque l'interface est lubrifiée l'énergie mécanique est encore transformée en chaleur mais la force de frottement devient proportionnelle à la vitesse selon la loi de la viscosité. On parle alors d'amortissement visqueux bien que cet effet linéaire apparaisse également dans des phénomènes plus ou moins éloignés. C'est l'aspect de la question essentiellement étudié dans cet article.
Un solide qui oscille dans un fluide est soumis à un tel amortissement lorsque sa vitesse est suffisamment faible pour que l'écoulement soit laminaire. À plus grande vitesse il apparaît un sillage tourbillonnaire ou turbulent qui dissip

Evaluation of the Effect of Backsheet Properties on PV Encapsulant Degradation under Ultraviolet-Damp-Heat Conditions

Djamel Eddine Mansour

The reliability of photovoltaic (PV) modules is highly determined by the durability of the polymeric components (backsheet and encapsulant). The power degradation and failure of PV modules can be caused by changes in the physical, chemical, and mechanical properties of these polymeric components during the module lifetime. In this thesis, different experimental techniques to investigate changes in PV modules polymeric components were deployed. Apart from the known techniques available in the literature, new advanced methods (e.g. Nanoindentation and Scanning Acoustic Microscopy (SAM)) were also applied and benchmarked with already established methods. PV encapsulation processes were also investigated with the aim to optimize the lamination conditions that can help to improve the reliability of PV modules.To demonstrate the effects of backsheet permeation properties on encapsulant degradation, one type of EVA was aged in glass/EVA/backsheet laminates in accelerated aging tests (up to 4000 h for Damp-Heat (DH) and up to 480 kWh/m² for Ultraviolet (UV) and UV-DH combined). Firstly, by using samples with different backsheets, the effects of different material combinations on the EVA degradation mechanisms (chemical changes) were investigated. Fourier transforminfrared spectroscopy with attenuated total reflectance (FTIR-ATR) studies identified two different EVA degradation processes. A thermal oxidation was taking place in the glass/EVA/Polyamide (PA)-based backsheet configuration after 500 h DH tests. However other EVA degradation products were observed in the glass/EVA/PET-based backsheet after 1500 h DH tests. Secondly, it has been perceived that a different material combination leads to different EVA degradation rates, by analyzing the EVA thermal and mechanical changes using Differential Scanning Calorimetry (DSC) and Nanoindentation respectively. As a consequence of these findings, the changes in the melting enthalpy and the viscoelastic properties at the EVA surface was more pronounced when PA-based backsheet was used then when PET-based backsheet was used under combined UV-DH aging tests. The comparison of three accelerated aging stress factors revealed that EVA suffers the strongest chemical and optical degradation when high UV, high temperature and high relative humidity are combined simultaneously.Additionally, in current PV module technology research, there exists a missing link between the PV material degradation and the PV performance degradation after accelerated lifetime tests. Therefore, in this research, a contribution towards this missing link was added by investigating the correlation of material degradation to power degradation. To do this, several PV modules were built and subjected to artificial aging tests. The influence of different micro-climates was investigated and changes in material properties were related to the PV performance degradation. Furthermore, a good correlation between the changes in PV performance parameters and PV module optical evolution of themodules with different material combinations was reached. Moreover, the power output and the series resistance degradation of the PV module was much more severe and stronger under this combined UV-DH aging test than the addition of both factors individually. Under these combined aging conditions, several degradation modes were observed under the visual inspection of the PV modules, including corrosion, backsheet cracking and EVA discoloration.

EPFL2021

Static and dynamic mechanical properties of nanosilica and multiwalled carbon nanotube reinforced acrylonitrile butadiene styrene composites: theoretical mechanism of nanofiller reinforcement

Thomas Keller, Harish Mohan Thettemmel

Experimental and theoretical approaches were employed to analyze the static and dynamic mechanical properties of acrylonitrile butadiene styrene (ABS) nanocomposites reinforced with multiwalled carbon nanotube (MWCNT) and nanosilica. Enhanced stiffness and tensile strength were observed for composites with 5% (wt) CNT and 3% (wt) nanosilica. Micromechanical models were highly promising to predict the experimental Young's modulus by 2% (wt) of nanofiller content, beyond which it deviated due to the nanofiller population and the reduction in interparticle distance. At the optimum content of 5% (wt) MWCNTs in the ABS matrix, enhanced storage and loss moduli and lowered damping peak were observed, which was attributed to the immobilization of the segmental polymer chains. From the DMA parameters, the dynamics of the polymer chain were investigated and the fraction of constrained regions and entanglement density were quantified. These findings demonstrated that composite with higher constrained regions and entanglements exhibited superior mechanical performance. Amongst all composites, 5% (wt) MWCNT-reinforced ABS composite showed an increment in reinforcing efficiency, entanglement density, and constrained regions by 195.6%, 115% and 55.5%, respectively, with regard to 3% (wt) nanosilica-reinforced ABS composite. The effectiveness of dispersion and interfacial adhesion of CNTs with ABS was improved by carboxyl treatment and functionalization with ABS-g-MaH compatibilizer with regard to silane-treated nanosilica/ABS composites. Combined analysis of microstructure, tensile properties and dynamic mechanical parameters such as entanglement density, effectiveness of filler, constrained volume of polymer chains and adhesion factor demonstrated the effectiveness of high aspect ratio carboxyl-treated MWCNTs as a better reinforcing agent in comparison with nanosilica in ABS matrix.

SPRINGER2021

Propriétés hors-équilibre d'une paroi adiabatique

Séverine Pache

We study the evolution of a system composed of N non-interacting particles of mass m distributed in a cylinder of length L. The cylinder is separated into two parts by an adiabatic piston of a mass M ≫ m. The length of the cylinder is a fix parameter and can be finite or infinite (in this case N is infinite). For the infinite case we carry out a perturbative analysis using Boltzmann's equation based on a development of the velocity distribution of the piston in function of a small dimensionless parameter ε = √(m/M). The non-stationary case is solved up to the order ε ;; our analysis shows that the system tends exponentially fast towards a stationary state where the piston has an average velocity V. The characteristic time scale for this relaxation is proportional to the mass of the piston (τ0 = M/A where A is the cross-section of the piston). We show that for equal pressures the collisions of the particles induce asymmetric fluctuations of the velocity of the piston which leads to a macroscopic movement of the piston in the direction of the higher temperature. In the case of the finite model a perturbative approach based on Liouville's equation (using the parameter α = 2m/(M + m)) shows that the evolution towards thermal equilibrium happens on two well separated time scales. The first relaxation step is a fast, deterministic and adiabatic evolution towards a state of mechanical equilibrium with approximately equal pressures but different temperatures. The movement of the piston is more or less damped. This damping qualitatively depends on whether the ratio R = Mgas/M between the total mass of the gas and the mass of the piston is small (R < 2) or large (R > 4). The second part of the evolution is much slower ; the typical time scales are proportional to the mass of the piston. There is a stochastic evolution including heat transfer leading to thermal equilibrium. A microscopic analysis yields the relation XM(t) = L(1/2 - ξ(at)) where the function ξ is independent of M. Using the hypothesis of homogeneity (i.e. the values of the densities, pressures and temperatures at the surface of the piston can be replaced by their respective average values) introduced in the previous analysis the observed damping does not show up. This can be explained by shock waves propagating between the piston and the walls at the extremities of the cylinder. In order to study the behaviour of the system there is hence a need to adequately describe the non-equilibrium fluids around the piston. We carry out an analysis of the infinite case, based on the perturbative approach introduced earlier. In this case the initial conditions are chosen in such a manner that the piston on average stays at the origin. It is shown that it is possible to describe the evolution of the fluids in such a way that it is coherent with the two laws of thermodynamics and the phenomenological relationships. Finally we study the case of a constant velocity of the piston in a finite cylinder. Such a condition and elastic collisions allow us to derive an explicit expression for the distribution of the fluids and hence for the hydrodynamics fields. This expression reveals the presence of shock waves between the piston and the extremities of the cylinder.

EPFL2004