Computer simulation of the microstructure and rheology of semi-solid alloys under shear

The rheological behavior of metallic alloys containing both solid and liquid phases is investigated in the low solid fraction range (<50%). This behavior depends on both the solid fraction and the shear rate. The concept of Effective Volume Fraction (EVF) is used to decorrelate the influence of these two parameters. At high shear rate the slurry behaves like a suspension of hard spheres, whereas at lower shear rate, particles tend to aggregate in clusters, entrapping liquid and thus, increasing the EVF and the viscosity. A lattice model is introduced to simulate the aggregation / break-up processes within a slurry under shear. When the steady state is reached, the entrapped liquid fraction is calculated, leading to a viscosity estimation. Simulation results for the viscosity and 3D cluster structure are in good agreement with experimental results.Comment: 30 pages, 17 figures, to be published in Acta Mate

A spring-block model for Barkhausen noise

A simple mechanical spring-block model is introduced for studying magnetization phenomena and in particularly the Barkhausen noise. The model captures and reproduces the accepted microscopic picture of domain wall movement and pinning. Computer simulations suggest that this model is able to reproduce the main characteristics of hysteresis loops and Barkhausen jumps. In the thermodynamic limit the statistics of the obtained Barkhausen jumps follows several scaling laws, in qualitative agreement with the experimental results. The simplicity of the model and the invoked mechanical analogies makes it attractive for computer simulations and pedagogical purposes.Comment: Revtex, 8 pages, 6 figure

Integrated modeling of friction stir welding of 6xxx series Al alloys: Process, microstructure and properties

International audienceCompared to most thermomechanical processing methods, friction stir welding (FSW) is a recent technique which has not yet reached full maturity. Nevertheless, owing to multiple intrinsic advantages, FSW has already replaced conventional welding methods in a variety of industrial applications especially for Al alloys. This provides the impetus for developing a methodology towards optimization, from process to performances, using the most advanced approach available in materials science and thermomechanics. The aim is to obtain a guidance both for process fine tuning and for alloy design. Integrated modeling constitutes a way to accelerate the insertion of the process, especially regarding difficult applications where for instance ductility, fracture toughness, fatigue and/or stress corrosion cracking are key issues. Hence, an integrated modeling framework devoted to the FSW of 6xxx series Al alloys has been established and applied to the 6005A and 6056 alloys. The suite of models involves an in-process temperature evolution model, a microstructure evolution model with an extension to heterogeneous precipitation, a microstructure based strength and strain hardening model, and a micro-mechanics based damage model. The presentation of each model is supplemented by the coverage of relevant recent literature. The "model chain" is assessed towards a wide range of experimental data. The final objective is to present routes for the optimization of the FSW process using both experiments and models. Now, this strategy goes well beyond the case of FSW, illustrating the potential of chain models to support a "material by design approach" from process to performances

Planning complex engineer-to-order products

The design and manufacture of complex Engineer-to-Order products is characterised by uncertain operation durations, finite capacity resources and multilevel product structures. Two scheduling methods are presented to minimise expected costs for multiple products across multiple finite capacity resources. The first sub-optimises the operations sequence, using mean operation durations, then refines the schedule by perturbation. The second method generates a schedule of start times directly by random search with an embedded simulation of candidate schedules for evaluation. The methods are compared for industrial examples

Analyse par géomatique des bilans et des flux d'azote et de phosphore dans un bassin versant agricole: le cas de la rivière Boyer.

L'essor et la concentration géographique des productions animales, dans certains bassins versant à vocation agricole, génèrent d'imposants volumes de fumiers et de lisiers qui dépassent souvent la superficie de sol disponible pour les valoriser. On assiste alors à une surfertilisation des cultures, qui provoque l'accroissement de la teneur en phosphore du sol, le risque de transfert vers l'environnement ainsi que le lessivage des nitrates. Ces nutriments, lorsqu'ils dépassent un certain niveau, sont responsables de l'eutrophisation des cours d'eau. Cette problématique de pollution diffuse est actuellement largement responsable de la détérioration de la qualité des eaux dans plusieurs régions du Québec. Ce projet de maîtrise a pour objectif d'analyser les bilans et les flux d'azote et de phosphore dans le bassin versant de la rivière Boyer, un bassin où la pollution diffuse a été constatée. Ceci permet d'estimer les stocks d'éléments nutritifs présents et de définir les zones critiques où les pertes vers l'environnement sont les plus susceptibles de se produire. Pour ce faire, nous devons considérer plusieurs données spatiales des différents facteurs qui interviennent, telles que l'occupation du sol, la topographie, la pédologie, les conditions climatiques, les différentes pratiques culturales, etc. Ces informations sont intégrées en exploitant les propriétés des Systèmes d'Information Géographique (SIG) qui tiennent compte de la localisation spatiale des données considérées. Dans le cadre du présent travail, nous exploitons le SIG IDRISI. Afin d'obtenir une représentation réaliste du mode de fonctionnement du bassin versant à l'étude, un bilan des apports et des exportations d'azote et de phosphore et des modèles de pertes annuelles d'azote et de phosphore sont établis au sein du SIG. Les pertes annuelles de phosphore particulaire et soluble peuvent être estimées en intégrant un modèle de perte de sol au SIG. Les pertes annuelles d'azote, par lessivage et ruissellement superficiel, sont estimées à l'aide d'un modèle intégré au SIG, qui est construit à partir de coefficients de pertes tirés de la littérature. Les charges annuelles de phosphore et d'azote sont cumulées sur l'ensemble du bassin versant. Le cumul se fait suivant les directions d'écoulement, qui sont déterminées grâce à un algorithme de drainage interfacé au logiciel IDRISI. Cet algorithme utilise un modèle numérique d'altitude (MNA) pour générer les directions d'écoulement suivant la plus forte pente. Une comparaison entre les valeurs des concentrations d'azote et de phosphore, issues des simulations et des valeurs mesurées en 28 points d'échantillonnage du bassin, permet de discuter des modèles développés et d'établir des relations avec certaines caractéristiques du territoire

Classical big-bounce cosmology: dynamical analysis of a homogeneous and irrotational Weyssenhoff fluid

A dynamical analysis of an effective homogeneous and irrotational Weyssenhoff fluid in general relativity is performed using the 1+3 covariant approach that enables the dynamics of the fluid to be determined without assuming any particular form for the space-time metric. The spin contributions to the field equations produce a bounce that averts an initial singularity, provided that the spin density exceeds the rate of shear. At later times, when the spin contribution can be neglected, a Weyssenhoff fluid reduces to a standard cosmological fluid in general relativity. Numerical solutions for the time evolution of the generalised scale factor in spatially-curved models are presented, some of which exhibit eternal oscillatory behaviour without any singularities. In spatially-flat models, analytical solutions for particular values of the equation-of-state parameter are derived. Although the scale factor of a Weyssenhoff fluid generically has a positive temporal curvature near a bounce, it requires unreasonable fine tuning of the equation-of-state parameter to produce a sufficiently extended period of inflation to fit the current observational data.Comment: 34 pages, 18 figure

Weyssenhoff fluid dynamics in general relativity using a 1+3 covariant approach

The Weyssenhoff fluid is a perfect fluid with spin where the spin of the matter fields is the source of torsion in an Einstein-Cartan framework. Obukhov and Korotky showed that this fluid can be described as an effective fluid with spin in general relativity. A dynamical analysis of such a fluid is performed in a gauge invariant manner using the 1+3 covariant approach. This yields the propagation and constraint equations for the set of dynamical variables. A verification of these equations is performed for the special case of irrotational flow with zero peculiar acceleration by evolving the constraints.Comment: 20 page

Traveling wave solutions in the Burridge-Knopoff model

The slider-block Burridge-Knopoff model with the Coulomb friction law is studied as an excitable medium. It is shown that in the continuum limit the system admits solutions in the form of the self-sustained shock waves traveling with constant speed which depends only on the amount of the accumulated stress in front of the wave. For a wide class of initial conditions the behavior of the system is determined by these shock waves and the dynamics of the system can be expressed in terms of their motion. The solutions in the form of the periodic wave trains and sources of counter-propagating waves are analyzed. It is argued that depending on the initial conditions the system will either tend to synchronize or exhibit chaotic spatiotemporal behavior.Comment: 12 pages (ReVTeX), 7 figures (Postscript) to be published in Phys. Rev.

Effect of electron irradiation on vortex dynamics in YBa_2Cu_3O_{7-x} single crystals

We report on drastic change of vortex dynamics with increase of quenched disorder: for rather weak disorder we found a single vortex creep regime, which we attribute to a Bragg-glass phase, while for enhanced disorder we found an increase of both the depinning current and activation energy with magnetic field, which we attribute to entangled vortex phase. We also found that introduction of additional defects always increases the depinning current, but it increases activation energy only for elastic vortex creep, while it decreases activation energy for plastic vortex creep.Comment: 4 pages, 3 figures, submited to Phys. Rev.