Mécanique de la rupture fragile en présence de plasticité : définition d'un taux de restitution d'énergie

This study aims at defining an energetic parameter which characterises progressive crack growth in a brittle elastoplastic medium. First, Francfort and Marigo's elastic fracture theory, based on a minimum energy principle, is recalled. Then, a link with the framework of generalised standard materials is exhibited and allows to introduce the new energetic contributions due to plasticity.     

Calcul numérique de la déformation mécanique d'un modèle de cellule endothéliale

Mechanical forces influence endothelial cell's (EC) morphology and functions. In this work it was proposed a numerical analysis of steady laminar flows near a modeled monolayer of elastic ECs in order to determine the local distributions of mechanical forces on the surface and inside of the cell. The numerical results showed that the flow induced mechanical stresses led to a cell deformation and that the distributions of these stresses were not uniform on cell surface. It will be interesting to study eventual correlations between the distributions of biological receptors (cytoskeleton, adhesion molecules, etc.) and that of the non-uniform mechanical forces.     

Effet du couplage thermomécanique sur la propagation d'un front de changement de phase

Calorimetric effects related to the propagation of a phase change front in a monocrystalline shape memory alloy CuZnAl were derived from thermographic data analysis. At constant load, the displacement of the front induces a creep of the sample strongly depending on thermal exchanges with the surroundings. The main role played by the thermomechanical couplings can be pointed out by reversing the heat flux at the boundary of the sample: this leads to an inversion of the front propagation way associated with a recovery of the creep strain.     

Un théorème d'existence de solutions d'un problème de shallow water

Résumé Nous présentons un théorème d'existence de solutions d'un problème de type shallow water, en formulation hauteur-vitesse (h, u). Nous obtenons une majoration de type énergie et construisons des solutions approchées respectant cette majoration. La difficulté essentielle du passage à la limite provient de l'unique majoration connue de la hauteur h qui est du type: h et hlogh bornés dans L (0T;L ¹()). Mémoire presenté par P.-L. Lions     

Étude expérimentale des courants de dérive induits par la houle au-dessus d'un fond ridé

Wave-induced current measurements have been carried out above rippled beds in a channel, using laser Doppler anemometry. Very simple formulas for the drift currents just outside the boundary layer above rippled and very rough beds in turbulent regime are proposed. The vertical profiles of drift currents in the bottom boundary layer are compared to the results of the recent model presented by Davies and Villaret.     

Proposition d'un paramètre énergétique de rupture pour les matériaux dissipatifs

We suggest an energetic fracture parameter for non elastic materials. This one is presented in a domain integral, and founded on the release rate of the total mechanical energy received by a notched solid, using a domain derivation method. This parameter is proposed for a large class of loading and materials described by internal variables. It is shown that this parameter is reduced to the Rice–Cherepanov integral when the material is either elastic or elastoplastic and submitted to proportional loading.     

Application de la méthode des approximations successives à la résolution d'un problème aux limites de l'hydrodynamique

Sans résumé Mémoire transmis par R. Finn     

Sur la nature de la transition à l'instationnaire d'un écoulement de convection naturelle en cavité différentiellement chauffée à grands écarts de température

Natural convection of air inside a rectangular cavity, differentially heated under large temperature gradients, is considered. The low Mach approximation equations are those obtained by Paolucci allowing for filtering of sound waves. Transition to unsteadiness is studied with numerical simulation, with a finite volume code based on a fractional time step method derived from projection methods used for incompressible flows. When the fluid physical properties are prescribed constants, transition to unsteadiness follows the classical scheme of a Hopf bifurcation. The transition is quite different when viscosity obeys Sutherland's law while the Prandtl number is kept constant. There is evidence of hysteresis, therefore the transition seems to be subcritical. In the vicinity of the transition, on the large amplitude branch, an intermittent solution is observed, with periodic bursts separating quasi-steady states.     

Modélisation du comportement thermique d'un outil de fraisage : approche par identification de système non entier

In this work we present an experimental apparatus devoted to the thermal characterisation of a milling tool. The experimental device used thermistors, one for each insert. Each thermistor is located at a point in the tool close to the tip of the insert. The heat flux in each insert is expressed according to the temperature at the sensor from a non-integer model. The parameters of the model are identified from transient evolutions measurements of the temperature on the sensor and on the cutting edge. An application shows the difference in the behaviour of each insert during machining from the estimated heat fluxes. To cite this article: J.-L. Battaglia et al., C. R. Mecanique 330 (2002) 857–864.     

Un opérateur de diffusion spatialement sélectif pour le transport d'un traceur passif ou actif par un écoulement de grande échelle

In a fluid flow, fields are measurable up to a cut-off scale at which they are regularized. We show that, for a smooth velocity field, this regularization adds to the advection equation a diffusive term proportional to the strain tensor. We study in two dimensions its effect on the dynamics of velocity and vorticity, and on the conservation of quadratic invariants. Vorticity and energy are still conserved, while enstrophy and tracer variance are dissipated depending on the flow topology. These properties (conservation, dissipation, spatial selectivity) suggest the use of this selective strain–diffusion operator for numerical simulations of inhomogeneous flows in the quasi-two-dimensional approximation.     

Comportement thermomécanique de l'argile de Boom

Special attention has been recently paid on temperature effects on the behaviour of deep saturated clays, in relation with nuclear deep waste storage. However, few experimental data are presently available, and existing constitutive models need to be completed. This note is aimed at completing, both experimentally and theoretically, the understanding of the effects of the overconsolidation ratio on the thermal volume changes of Boom clay (Belgium). The experimental data obtained here are in a good agreement with existing data. As a complement to existing data, they are used to develop a new elastoplastic model. The adoption of a second coupled plastic mechanism provides good simulations on a complex thermo-mechanical path.     

Endommagement d'un composite polypropylène renforcé par des fibres de verre courtes : approche expérimentale

To obtain good mechanical performance of the composite material, it is important to optimise the fibres ratio as well as the fibre/matrix interface quality which have influence on the damage. The main objective of this study is to determine the structural parameters influence on damage evolution concerning two types of polypropylene glass fibres composites. With a classical approach of damage mechanical theory which consists of load–unload tensile tests, acoustic emission permits to detect and follow damage mechanisms during loading. Fractographic analysis highlights the different assumptions and conclusions for this study.     

Une méthode mécanique pour déterminer la porosité totale d'un sol

This paper presents a mechanical method for the determination of the total porosity of soil. The presented method consists in inserting a sample of soil into a cell that can be regulated by varying the volume of the gas by increments using a piston. The porosity of the soil is deduced from the variation of the total pressure of the gaseous phase during the realization of volume increments. The method has been validated on a soil considering five samples with different porosities. The experimental results on the total porosity are in good agreement with the theoretical results over all tests.     

Étude thermodynamique de l'évolution sous chargement dynamique de structures délaminées

In order to study the evolution of damage in a delaminated structure, the assemblage of plates is considered under dynamic loading. Using a general result of the thermodynamic coupling approach, with the Hamiltonian formulation in the general case, the energy release rate associated with the propagation of the delamination is presented as a heat source.     

Influence d'un obstacle central sur la transmission d'une détonation d'un tube dans un grand volume

This work describes the mechanisms involved in the critical detonation transmission from a cylindrical tube into a half space through an annular orifice around a central obstacle. The numerical simulation reproduces faithfully the experimental observations and can be used as a predictive tool.     

Comportement thermomécanique d'un béton ordinaire en compression cyclée : expérience et modélisation

Infrared thermography observations permit to show thermal phenomena occurring on a plain concrete during a compressive test. A modelling of the thermomechanical behaviour of this materials is proposed as an extension of M. Frémond and B. Nedjar's model. It permits an interesting dialog between the experimental results and those obtained by a numerical simulation of a simple compressive test.     

Analyse mathématique de l'équation de Galbrun en écoulement uniforme

We consider Galbrun's equation, used in linear aeroacoustics. For a simple case (rigid duct with uniform flow) in the time harmonic regime, we show that an approach based on a regularized variational formulation of the problem ensures the convergence of a nodal finite-element method.     

Inversion d'un écoulement plan de Stokes – application à l'écoulement à débit moyen nul entre deux plans parallèles

We present the interest and some characteristics of the inverse transformation of a 2D Stokes flow. This method is applied to the cellular flow between two parallel plates induced by a rotating cylinder to obtain the flow around two circular cylinders in contact placed in the centre of a rotating circular cylinder.     

Interprétation de la double structure observée dans l'onde de détonation du nitrométhane gazeux

Calculations of the reaction zone of the detonation of gaseous nitromethane, either pure or diluted with oxygen, in the range of equivalence ratio Φ between 0.1 and 1.75, show that for 1.75⩾Φ⩾1.3 the chemical energy is released in two main successive reaction steps characterized by very different induction times. These results corroborate the experimental observations of two levels of cellular structures in the same range of equivalence ratios. To our knowledge this work is the first which deals with the problem of nonmonotonous chemical energy release behind the leading shock of a detonation wave.