52 research outputs found
Dynamique d'un milieu granulaire soumis à des vibrations horizontales
La dynamique des matériaux granulaires vibrés est une question cruciale dans de nombreuses applications, notamment lors de l'opération de bourrage du ballast ferroviaire, qui consiste à rétablir l'aspect géométrique initial des voies. Nous analysons la dynamique d'une couche granulaire confinée en réponse au chargement harmonique d'une paroi en 3D. Les grains sont modélisés par des polyèdres convexes et leurs mouvements sont simulés par la méthode de Dynamique des Contacts. Le système passe par des états passifs (déchargement), actifs (chargement) et bloqué. Nous montrons qu'une expression simple de la résistance du matériau en fonction du déplacement de la paroi mobile fournit une bonne description de la dynamique. Ce travail met en évidence l'existance d'une fréquence caractéristique proche de 10 Hz pour laquelle le taux moyen de compaction est optimal
Root-reinforced sand:Kinematic response of the soil
The influence of the soil on the growth of a root system has been largely investigated. By contrast, the aim of this work is to go deep into the details of how the soil may be influenced by the root system. In particular, the root growth process and its potential to improve the soil strength is explored. Even though roots can be seen as fiber-like reinforcements, their growth changes the soil microstructure. Consequently, one of the objectives is to understand how the water content and the soil displacement fields evolve when an inclusion expands radially and axially. In particular, an investigation was carried on to characterise the deformation of the solid phase of the soil, due to the root growth. A series of in-vivo x-ray tomographies was acquired with Maize seeds growing roots into a coarse Hostun HN1.5-2 sand. Digital Image Correlation is used to calculate the soil 3D displacement fields around the growing plant roots
Vibrational dynamics of confined granular material
By means of two-dimensional contact dynamics simulations, we analyze the
vibrational dynamics of a confined granular layer in response to harmonic
forcing. We use irregular polygonal grains allowing for strong variability of
solid fraction. The system involves a jammed state separating passive (loading)
and active (unloading) states. We show that an approximate expression of the
packing resistance force as a function of the displacement of the free
retaining wall from the jamming position provides a good description of the
dynamics. We study in detail the scaling of displacements and velocities with
loading parameters. In particular, we find that, for a wide range of
frequencies, the data collapse by scaling the displacements with the inverse
square of frequency, the inverse of the force amplitude and the square of
gravity. Interestingly, compaction occurs during the extension of the packing,
followed by decompaction in the contraction phase. We show that the mean
compaction rate increases linearly with frequency up to a characteristic
frequency and then it declines in inverse proportion to frequency. The
characteristic frequency is interpreted in terms of the time required for the
relaxation of the packing through collective grain rearrangements between two
equilibrium states
Force transmission in a packing of pentagonal particles
We perform a detailed analysis of the contact force network in a dense
confined packing of pentagonal particles simulated by means of the contact
dynamics method. The effect of particle shape is evidenced by comparing the
data from pentagon packing and from a packing with identical characteristics
except for the circular shape of the particles. A counterintuitive finding of
this work is that, under steady shearing, the pentagon packing develops a lower
structural anisotropy than the disk packing. We show that this weakness is
compensated by a higher force anisotropy, leading to enhanced shear strength of
the pentagon packing. We revisit "strong" and "weak" force networks in the
pentagon packing, but our simulation data provide also evidence for a large
class of "very weak" forces carried mainly by vertex-to-edge contacts. The
strong force chains are mostly composed of edge-to-edge contacts with a marked
zig-zag aspect and a decreasing exponential probability distribution as in a
disk packing
Caracteristiques Thermoelastiques de Materiaux Composites a Fibres Courtes
INIST T 75880 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc
Modelling the mechanical effects accompanying growth and division of cells
Numerous works show that the mechanical efforts play an important role in the division and the growth of the cell on one hand, and in the organization of cells within a tissue on the other hand. The purpose of this work is to develop a mechanical model to better understand the couplings between biological effects and mechanical effects, this by being inspired by models used for cohesive granular materials. Based on a Molecular Dynamic method these last ones are adapted well to calculate the contact efforts between cells as well as their evolution. Adapted to the case of the cellular division, this model allows to describe the totality of the cycle of division while considering the mechanical effects. A growth law is also introduced, which, for a given cell, is a function of mechanical interactions with the neighbours. Hence, we can generate a population of cells from an original cell and obtain a "virtual" tissue, in which the effects of texturation and of organization are observed. Several examples are presented, one containing 7000 cells locked into a box. We show that the shape of the box leads to particular organizations, which remind the effects of texturation observed during the culture of skin
Homogenization in thermoelasticity : application to composite materials
One of the obstacles to the industrial use of metal matrix composite materials is the damage they rapidly undergo when they are subjected to cyclic thermal loadings ; local thermal stresses of high level can develop, sometimes nearby or over the elastic limit, due to the mismatch of elastic and thermal coefficients between the fibers and the matrix. For the same reasons, early cracks can appear in composites like ceramic-ceramic. Therefore, we investigate the linear thermoelastic behaviour of heterogeneous materials, taking account of the isentropic coupling term in the heat conduction equation. In the case of periodic materials, recent results, using the homogenization theory, allowed us to describe macroscopic and microscopic behaviours of such materials. This paper is concerned with the numerical simulation of this problem by a finite element method, using a multiscale approach
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