Modélisation du comportement mécanique d'un alliage base nickel par une approche thermodynamique de la relaxation des milieux continus

The high-temperature behaviour of a Nickel alloy submitted to uniaxial tension, relaxation and creep, is analysed using a thermodynamics approach to non-linear relaxations. The model is essentially based on a distribution of the dissipation processes obtained by the theory of fluctuations. The agreement between experiment and simulations requires the use of four parameters. These parameters depend on the elastic properties of the alloy and also on the thermal activation necessary for mobility defects. We present a comparison with the classical model of Chaboche including a recovery term in the potential of dissipation.     

Modélisation eulerienne de l'atomisation d'un jet liquide

We propose here a new model of “atomization“ of a liquid jet based on an Eulerian approach of a “two-phase fluid”. This model permits to calculate on the one hand the liquid dispersion and on the other hand the characteristic size of the produced fragments.     

Modélisation d'un revêtement précontraint à collage par points

We determine the homogenized density of energy associated with a viscoelastic sandwich beam, non-uniformly stuck. We are then able to obtain the global dynamic behaviour of a ‘real’ viscoelastic beam with fewer degrees of freedom.     

Phénomènes rhéologiques dans les alliages à mémoire de forme

We use a simplified ID version of a 3D thermomechanical model presented by Pham, to describe relaxation and creep in shape memory alloys (SMAs) during stress-induced martensitic transformation. We show that these rheological phenomena are predicted by taking into account, in the heat propagation equation, a thermomechanical coupling associated with the phase transition (latent heat). The results of our experimental tests are in satisfactory agreement with the theoretical predictions. The role of the heat diffusion is underlined.     

Plasticité de transformation d'un acier à mémoire de forme Fe18Mn8Cr5Ni5Si

Transformation plasticity of a Fe18Mn8Cr5Ni5Si shape memory steel. This paper presents an experimental study of the transformation plasticity related to the γ→ε martensitic transformation in a Fe-18Mn-8Cr-5Ni-5Si shape memory steel. Under a weak bending stress the as-quenched austenitic samples showed a transformation plasticity by cooling at 170K with a plasticity coefficient h≈2. During heating, this transformation plasticity, related to the memory effect, is reversible, partially or completely, depending on whether the initial stress was maintained or removed at 295K during heating.     

Utilisation des Plans d'Expériences Pour la Modélisation de l'Élaboration d'un Phosphate de Calcium de Propriétés Antiseptiques à Usage Biomédical

La synthèse d'une apatite oxygénée phosphocalcique de rapport molaire Ca/P égal à 1.575 a été étudiée par un plan composite centré orthogonal. Cette étude nous a permis d'établir un modèle mathématique qui décrit l'influence des variables: pH, concentration de la solution du chlorure de calcium ([Ca ²⁺ ]), durée de maturation (D) et température (T), sur la réponse: rapport molaire Ca/P. L'exploitation de ce modèle permet de définir des conditions d'obtention d'un solide de rapport molaire Ca/P égal à 1.575.

The synthesis of phosphocalcic oxygenated apatite of Ca/P ratio equal to 1.575 was studied by an orthogonal central composite design. This study checks the effect of certain variables on the reaction, pH, concentration of the calcium chloride solution ([Ca ²⁺ ]), ripening time (D) and temperature (T) on the response, and Ca/P ratio with a mathematical model. The exploitation of this model allowed us to define conditions for obtaining a solid of Ca/P ratio equal to 1.575.     

Modélisation des temps de réponse des capteurs chimiques

Modelling consists of considering the response and recovery times of a sensor to the target chemical species, as the sum of an ‘intrinsic’ response/recovery time, independent of the experimental conditions, and of an ‘extrinsic’ one, linked to the volume of the measurement cell and to the fluid flow. The experimental response/recovery times being almost always dominated by the extrinsic part, a dissymmetry between the response and recovery signals is observed for sensors whose response is not linear with the concentration of the target chemical species.     

Description eulerienne de la mécanique d'un front de discontinuité avec apparition d'une localisation de la déformation

We study the exchanges occurring through a discontinuity front in a continuum. We focus in particular on the jump condition corresponding to the Clausius-Duhem inequality. We analyse the possibility of a localised glide on a discontinuity surface, emphasizing two different terms in the dissipation on the surface: one resulting from a possible movement of the front, the other ensuing from localisation.     

Modélisation multiphasique de matériaux renforcés par inclusions linéaires

Making use of the virtual work method, a mechanical multiphase model for reinforced materials is developed. Matrix and reinforcement are continuously distributed. In modelling the internal forces, account is taken of the ‘continuous’ and ‘curvilinearharacteristics of the matrix and reinforcements, and of their interaction. The corresponding equations of motion and associated boundary conditions are deduced for each phase by applying the virtual work principle. Such a multiphase model is a suitable framework for dealing with problems in which imperfect bonding between matrix and reinforcements should be taken into account.     

Développement et validation d'un modèle mécanique de la tête humaine

This paper presents the development and the validation of a 3D finite-element human head model intended to better understand the head injury mechanisms in case of head impact. A very realistic skull geometry has been obtained by digitalisation of a dry adult skull. Based on the established knowledge of bone mechanical properties, an elastic brittle law was attributed to the skull simulated by composite shell elements. The dynamical model response was compared with two experimental impact tests from the bibliography concerning epidural pressure and a third impact involving skull fracture. The results of the model validation appear promising and the model could be a powerful tool to predict the aggressiveness level of a head impact.     

Rôle de la précipitation sur l'oxydation d'un alliage Fe-Cr-Al dans l'air à 1100° C

A study by thermo-gravimetry of the kinetics of oxidation of the alloy Fe-22Cr-4Al was carried out at 1100°C in ambient air, for durations up to 25 hours. The morphology of oxide scales formed at high temperature and the structure of oxide films were then observed and analyzed by scanning electron microscopy. The kinetic study, performed by thermo-gravimetry, showed the existence of two stages, of which only the second corresponds to a parabolic law, with the formation of a protective coating of αalumina. The presence in the alloy of foreign elements, added or not, led to the formation of precipitates whose distribution and number were studied by the analysis of metallographic images of surfaces. The influence of a preliminary cold work before oxidation was examined. The results showed that this mechanical treatment has no influence on the nature of the precipitates, but modifies their size. It comes out from this study that the nucleation and growth of these precipitates enter in competition with the formation of the layer of alumina, by accelerating the total oxidation rate during the first stage, and modify the morphology and the adherence of the formed alumina layer. The precipitates obtained at the interface have morphologies such that the internal stresses could then be modified considerably in terms of intensities and of distribution ; their relaxation would cause the warped state of the samples observed after oxidation.     

Un bicristal mince déformé sous l'effet d'un réseau bipériodique de dislocations de misfit

The elastic field of a planar two-period network of misfit dislocations parallel to the two free surfaces of a thin bimaterial is explicitly determined from the assumption of an isotropic elasticity for each crystal. A few calculations of normal stresses enable the elastic field relaxation to be illustrated for CoSi₂/Si(111).     

Généralisation de l'identité de Germano et application à la modélisation sous-maille

In the large-eddy simulation frame, a dynamic procedure based on Germano's identity is often employed to adjust the subgrid-scale model constant. Applying an n-th degree linear operator to the Germano identity, a generalized dynamic procedure is defined to model this operator applied to the subgrid-scale tensor. This procedure leads to very satisfactory results when applied to the divergence operator in the frame of freely decaying turbulence.     

Modification de la tenue mécanique d'un dépôt type Si — C sur acier par incorporation d'une intercouche métallique

SiC based coatings were deposited on a steel substrate by PACVD from TMS-Ar mixtures. The influence of a metallic interlayer between deposit and substrate on the mechanical stability has been studied through interface analysis and several mechanical tests (scratch, indentation, microbending in a SEM). The deposit adhesion is enhanced by a tantalum interlayer and is a function of its thickness. Micro-bending tests allowed to characterize the damage evolution and to obtain experimental parameters linked with the intrinsic properties of the coatings.     

Modélisation de la durée des sons dans un système de reconnaissance automatique de la parole

A major deficiency of standard Hidden Markov Models (HMM) is that both the spectral and the prosodic features are uniformly processed. To combine more efficiently the prosodic cues with the acoustic ones, a segmental Two Level HMM has been recently studied by Suaudeau. In this paper, we present an adapted version in which the segmental processing is replaced by the classical centisecond one, and the prosodic feature is the sound duration. Experiments on a French database show that such an explicit introduction of prosodic parameters reduces recognition error rates.     

Modélisation théorique d'écoulements pulses de fluides non newtoniens en conduites viscoélastiques poreuses et anisotropes

A theoretical study concerning the pulsatile flow of an inelastic fluid through anisotropic porous viscoelastic pipes is presented. The objective is to investigate the effects of porosity, conicity in pause, anisotropy and viscoelasticity of pipe wall material for a generalized Casson fluid. An implicit difference method is used to solve the equations, and to determine the axial and radial velocity profiles, the pressure, the flow rate and the flow rate filtration distributions, the transverse rotation and the wall axial and radial displacement. This study, considered as a step in the modelling of flow in blood vessels, may also contribute to other important fields such as water desalination or gel filtration.     

Application Des Réseaux de Neurones Avec la Régularisation Bayésienne pour la Modélisation de la Synthèse de l’hydroxyapatite Élaborée à Partir du Carbonate de Calcium et de L’acide Phosphorique

We have used a new, robust model mapping technique—a Bayesian-regularized neural network—to develop a quantitative relationships model for the synthesis of the phosphocalcic hydroxyapatite by precipitation from a calcium carbonate solution and a phosphoric acid solution. This model was preformed by using a set of factors consisting on the pH of reactional medium, the Ca/P molar ratio of the reagents, reaction time, and the initial concentration of calcium. The results show that the method is robust and gives satisfied results. The Levenberg–Marquardt's algorithm implemented in the neural network Matlab's toolbox allowed a drastic improvement of the performance of the model. Very satisfactory results are then obtained by testing the validity by cross-validation technique.

We have also turned our interests to the explanatory capacities of our methodology to explore the relative contribution and/or the contribution profile of the input factors by using Garson weight portioning method.