Effects of Tropical Climate and Language Format on Imagery Ability Among French-Creole Bilinguals

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The existence of a critical length scale in regularised friction

We study a regularisation of Coulomb's friction law on the propagation of local slip at an interface between a deformable and a rigid solid. This regularisation, which was proposed based on experimental observations, smooths the effect of a sudden jump in the contact pressure over a characteristic length scale. We apply it in numerical simulations in order to analyse its influence on the behaviour of local slip. We first show that mesh convergence in dynamic simulations is achieved without any numerical damping in the bulk and draw a convergence map with respect to the characteristic length of the friction regularisation. By varying this length scale on the example of a given slip event, we observe that there is a critical length below which the friction regularisation does not affect anymore the propagation of the interface rupture. A spectral analysis of the regularisation on a periodic variation of Coulomb's friction is conducted to confirm the existence of this critical length. The results indicate that if the characteristic length of the friction regularisation is smaller than the critical length, a slip event behaves as if it was governed by Coulomb's law. We therefore propose that there is a domain of influence of the friction regularisation depending on its characteristic length and on the frequency content of the local slip event. A byproduct of the analysis is related to the existence of a physical length scale characterising a given frictional interface. We establish that the experimental determination of this interface property may be achieved by experimentally monitoring slip pulses whose frequency content is rich enough.Comment: 21 pages, 7 figure

Simulation multi-échelles des solides par une approche couplée dynamique moléculaire/éléments finis. De la modélisation à la simulation haute performance.

Cette thèse porte sur l'étude de la simulation des solides par des méthodes de couplage multi-échelles (méthode atomique/continue ACM). Dans ce travail de thèse, nous abordons des modéles mathématiques jusqu'à la réalisation informatique dans un contexte de calculs hautes performances parallèles. Après avoir appelé le fonctionnement des méthodes numériques utilisées dans le cadre de la simulation des solides, tels que la dynamique moléculaire et la mécanique des milieux continus, nous présentons les différentes méthodes de couplage existantes utilisées pour les coupler. Nous considérons la méthode Bridging Method développée par T. Belytschko et S. Xiao qui utilise une zone de recouvrement spatiale entre les deux modèles pour réaliser le couplage des degrès de libertés. Après avoir présenté une modification améliorant la stabilité de cette solution, nous ferons une analyse spectrale d'un cas unidimensionel couplé par cette méthode. Nous présenterons ensuite les techniques informatiques qui ont été mise en place afin de réaliser l'implémentation d'un prototype. La plateforme informatique est basé sur une philosophie de composants des entités logicielles qui autorise, in fine, de coupler les codes génériques dans un environnement logiciel. Notamment on s'intéressera à l'implémentation des routines qui permettent de paralléliser le couplage. Nous présenterons enfin les résultats numériques obtenus sur des cas tests en dimension 2 et 3 pour des cas de propagations d'ondes et de propagation de fissures

Contact between representative rough surfaces

International audienceA numerical analysis of mechanical frictionless contact between rough self-affine elastic manifolds was carried out. It is shown that the lower cutoff wave number in surface spectra is a key parameter controlling the representativity of the numerical model. Using this notion we demonstrate that for representative surfaces the evolution of the real contact area with load is universal and independent of the Hurst roughness exponent. By introducing a universal law containing three constants, we extend the study of this evolution beyond the limit of infinitesimal area fractions

Dry Sliding Contact Between Rough Surfaces at the Atomistic Scale

Although, a lot is known about the factors contributing to friction, a complete physical understanding of the origins of friction is still lacking. At the macroscale several laws have long since described the relation between load (Amontons, Coulomb), apparent and real area of contact (Bowden and Tabor), and frictional forces. But it is not yet completely understood if these laws of friction extend all the way down to the atomistic level. Some current research suggests that a linear dependence of friction on the real contact area is observed at the atomistic level, but only for specific cases (indentors and rigid substrates). Because continuum models are not applicable at the atomic scale, other modeling techniques (such as molecular dynamics simulations) are necessary to elucidate the physics of friction at the small scale. We use molecular dynamics simulations to model the friction of two rough deformable surfaces, while changing the surface roughness, the sliding speed, and the applied normal load. We find that friction increases with roughness. Also all sliding cases show considerable surface flattening, reducing the friction close to zero after repetitive sliding. This questions the current view of (static) roughness at the atomistic scale, and possibly indicates that the macroscopic laws of friction break down several orders of magnitude before reaching the atomic scal

From infinitesimal to full contact between rough surfaces: Evolution of the contact area

International audienceWe carry out a statistically meaningful study on self-affine rough surfaces in elastic frictionless non-adhesive contact. We study the evolution of the true contact area under increasing squeezing pressure from zero up to full contact, which enables us to compare the numerical results both with asperity based models at light pressures and with Persson’s contact model for the entire range of pressures. A good agreement of numerical results with Persson’s model is obtained for the shape of the area-pressure curve especially near full contact, however, we obtain qualitatively different results for its derivative at light pressures. We investigate the effects of the longest and shortest wavelengths in surface spectrum, which control the surface Gaussianity and spectrum breadth (Nayak’s parameter). We revisit the influence of Nayak’s parameter, which is frequently assumed to play an important role in mechanics of rough contact

Ghost force reduction and spectral analysis of the 1D bridging method

In this paper we study the Bridging Method recently developed by T. Belytschko and S. Xiao to couple continuum mechanics with molecular dynamics. This method uses an overlap zone where both models exist and where degrees of freedom are coupled. We analyze the method in the 1D case. Firstly, we show that the spatial weighting of the potential energy leads to some appreciable numerical artifacts. Then, after presenting a modification of the time integration scheme that removes such spurious effects, we introduce the method employed to numerically measure the wave reflection rates. The limitation of these reflections constitute a major issue in all existing coupling methods for dynamics simulations. After discussing some first results achieved from numerical measurements, we present a spectral analysis that attempts to explain the behavior of the coupling zone by treating it as an independent material. The predictions of this spectral analysis are finally compared to some parametric studies issued from 1D coupling simulations

The effect of loading on surface roughness at the atomistic level

One of the key points to better understand the origins of friction is to know how two surfaces in contact adhere to one another. In this paper we present molecular dynamics (MD) simulations of two aluminium bodies in contact, exposed to a range of normal loads. The contact surfaces of both aluminium bodies have a self-affine fractal roughness, but the exact roughness varies from simulation to simulation. Both bodies are allowed to have an adhesive interaction and are fully deformable. Tracking important contact parameters (such as contact area, number of contact clusters, and contact pressure) during a simulation is challenging. We propose an algorithm (embedded within a parallel MD code) which is capable of accessing these contact statistics. As expected, our results show that contact area is increasing in proportion with applied load, and that a higher roughness reduces contact area. Contact pressure distributions are compared to theoretical models, and we show that they are shifted into the tensile regime due to the inclusion of adhesion in our mode

A Manifest-Based Framework for Organizing the Management of Personal Data at the Edge of the Network

Smart disclosure initiatives and new regulations such as GDPR allow individuals to get the control back on their data by gathering their entire digital life in a Personal Data Management Systems (PDMS). Multiple PDMS architectures exist, from centralized web hosting solutions to self-data hosting at home. These solutions strongly differ on their ability to preserve data privacy and to perform collective computations crossing data of multiple individuals (e.g., epidemiological or social studies) but none of them satisfy both objectives. The emergence of Trusted Execution Environments (TEE) changes the game. We propose a solution called Trusted PDMS, combining the TEE and PDMS properties to manage the data of each individual, and a Manifest-based framework to securely execute collective computation on top of them. We demonstrate the practicality of the solution through a real case-study being conducted over 10.000 patients in the healthcare field

Contact élastique entre des surfaces rugueuses représentatives

National audienceDans ce travail nous étudions l'évolution de l'aire réelle de contact entre des surfaces nominalement plates mais réellement rugueuses. Puis nous explicitons la loi phénoménologique proposé dans [28] pour décrire cette évolution. Enfin, nous suggérons des pistes pour le développement de la théorie du contact rugueux en partant des résultats de simulations numériques