Two-fluid model for VLBI jets. I. Homogeneous and stationary synchrotron emission simulations

In this series of papers, we develop a two-fluid model for VLBI jets. The idea is that the jet itself is non- or mildly-relativistic (electrons and protons), while the radiating blobs are relativistic electron-positron `clouds' moving on helical paths wrapped around the jet. In this work, the emphasis is on the physical description of the clouds, and not on the structure or origin of the trajectory. In the simple case where the magnetic field is uniform and homogeneous accross the cloud, and the properties of the cloud are constant, the present paper shows synthetic maps of VLBI jets in different configurations, as well as the variation of different observational parameters along the trajectory.Comment: to appear in A&A, 8 pages and 10 figure

Analyse et modélisation des mécanismes d'endommagement et de déformation en fatigue multiaxiale de matériaux composites : polyamide renforcé par des fibres courtes

The current work focuses on a new micromechanical high cycle fatigue visco-damage model for short glass fiber reinforced thermoplastic composites, namely: PA66/GF30. This material, extensively used for automotive applications, has a specific microstructure which is induced by the injection process. The multi-scale developed approach is a modified Mori-Tanaka method that includes coated reinforcements and the evolution of micro-scale damage processes. Their description is based on the experimental investigations of damage mechanisms previously performed by the team. Damage chronologies have been proposed involving three different local degradation processes: fiber-matrix interface debonding/coating degradation, matrix microcracking and fiber breakage. Their occurrence strongly depends on the microstructure. The developed model integrates these damage kinetics and accounts for the complex matrix viscoelasticity and the reinforcement orientation distributions induced by the process. Each damage mechanism is introduced through an evolution law involving local stress fields computed at the microscale. The developed constitutive law at the representative volume element scale is implemented into a C++ scientific library, SMART+, and is designed to work with Finite Element Methods. The model identification is performed via reverse engineering, taking advantage of the multiscale experimental results: in-situ SEM tests as well as quantitative and qualitative μCT investigations.Le présent travail de thèse se consacre au développement d'un nouveau modèle micromécanique pour les composites en thermoplastique renforcé par des fibres de verre courtes. L'objectif est notamment la modélisation du comportement visco-endommageable en fatigue du PA66-GF30. Ce matériau, particulièrement utilisé dans l'industrie automobile, est sujet à une microstructure spécifique issue du procédé de moulage par injection. L'approche multi-échelles développée consiste en une méthode de Mori-Tanaka modifiée, appliquée à des renforts avec enrobage et prenant en compte l'évolution de l'endommagement à l'échelle microscopique. La description des mécanismes d'endommagement se base sur une investigation expérimentale poussée préalablement menée au sein de l'équipe. Des scénarios d'endommagement ont été proposés et incluent trois processus locaux différents : la décohésion de l'interface, la microfissuration de la matrice et les ruptures de fibres. Ceux-ci sont spécialement affectés par la microstructure. L'approche développée intègre ces cinétiques d'endommagement ainsi que la viscoélasticité non-linéaire de la matrice et la distribution d'orientation des inclusions due au procédé de fabrication. Chaque mécanisme d'endommagement est modélisé par une loi d'évolution basée sur les contraintes locales calculées à l'échelle microscopique. La loi constitutive finale, à l'échelle du volume élémentaire représentatif, est implémentée dans une bibliothèque scientifique en C++, SMART+, et est conçue pour être compatible avec une analyse de structures par éléments finis. L'identification du modèle est réalisée par rétro-ingénierie, en tirant profit de résultats expérimentaux multi-échelles, dont notamment des tests in-situ au MEB ainsi qu'une analyse qualitative et quantitative par μCT

Entre textes et actions : la musique bretonne et celtique de Bretagne

Les matériaux ethnographiques recueillis in situ depuis 1989 sont à la fois des actions musiciennes et festives, des paroles et des écrits sur ces actions. Je tenterai de voir plus spécialement en quoi les paroles transcrites de musiciens régionalistes et plus particulièrement celles des bretons d’aujourd’hui, correspondent aux pratiques effectives de musique dites bretonnes et celtiques, en quoi aussi le discours des musiciens est idéologique et politique. L’imaginaire musical et plus largement culturel, qui est aujourd’hui reconstruit, prend ici d’importantes proportions. En effet, il détermine et contrôle non seulement les comportements sociaux mais aussi l’art musical lui-même : usages nouveaux d’instruments de musique types renouvelés de répertoires musicaux, styles locaux retenus par les praticiens puis régionalisés. Le problème de la référence aux hommes et aux textes face aux situations qui leur sont faites sera présenté. Il constitue un cas de ce que le sémiologue danois Hjelmslev appelle fonction sémiotique que l’on examinera ici entre texte et action.En rendant compte des enquêtes sur les processus de réappropriation des récits chantés par les musiciens bretons eux‑mêmes et du rôle de leurs associations de collectage dans le filtrage des comportements (catégories d’instruments à privilégier, manière de les apprendre...voire même sélection des répertoires à interpréter), je tenterai de déterminer le sens de cette invention artistique contemporaine en relation avec les quelques premiers résultats de nos analyses interdisciplinaires (musilinguistiques) qui concernent les formes d’expression et de contenus des systèmes poétiques et musicaux

Fatigue damage in short glass fiber reinforced PA66: Micromechanical modeling and multiscale identification approach

The paper presents a new micromechanical high cycle fatigue visco-damage model for short glass fiber reinforced thermoplastic composites, namely: PA66/GF30. This material, extensively used for automotive applications, has a specific microstructure which is induced by the injection process. The multi-scale developed approach is a modified Mori-Tanaka method that includes coated reinforcements and the evolution of micro-scale damage processes. The description of the damage processes is based on the experimental investigations of damage mechanisms previously performed by the authors and presented elsewhere [M.F. Arif et al. "In situ damage mechanisms investigation of PA66/GF30 composite: Effect of relative humidity." Composites Part B: Engineering, Volume 61: 55-65, 2014]. Damage chronologies have been proposed involving three different local degradation processes: fiber-matrix interface debonding/coating degradation, matrix microcracking and fiber breakage. Their occurrence strongly depends on the microstructure and the moisture content. The developed model integrates these damage kinetics and accounts for the complex matrix viscoelasticity and the reinforcement orientation distributions induced by the process. Each damage mechanism is introduced through an evolution law involving local stress fields computed at the microscale. The developed constitutive law at the representative volume element scale is implemented into the finite element code Abaqus using a User MATerial subroutine. The model identification is performed via reverse engineering, taking advantage of the multiscale experimental results: in-situ SEM tests as well as quantitative and qualitative μCT investigations. Experimental validation is achieved using high cycle strain controlled fatigue tests

In situ damage mechanisms investigation of PA66/GF30 composite: Effect of relative humidity

Damage mechanisms of injection molded polyamide-66/short glass fiber 30 wt% composite (PA66/GF30) were analyzed using in situ SEM mechanical tests on specimens conditioned under three relative humidity contents (RH = 0%, 50% and 100%). The validity of these in situ analyses was confirmed by Xray micro-computed tomography (lCT) observations on tensile loaded specimens. Experimental results demonstrated that relative humidity (RH) conditions influence strongly the damage level and damage mechanisms. Indeed, for specimen with RH = 0%, damage initiation occurs at significantly higher load level than those in RH = 50% and RH = 100% specimens. The higher relative humidity condition also results in higher damage level. Damage chronologies have been proposed as damage initiation in the form of fiber–matrix debonding occurs at fiber ends and more generally at locations where fibers are close to each other due to the generation of local stress concentration (for all studied RH contents), and first fiber breakages occur (RH = 0%). These debonded zones further propagate through fiber–matrix interface (for all studied RH contents), and new fiber breakages develop (RH = 0%). At high relative flexural stress, matrix microcracks appear and grow regardless the RH contents. For RH = 100%, these microcracks are also accompanied by many matrix deformation bands. Subsequently, they lead to the damage accumulation and then to the final failure.Projet FUI-DGSIS "DURAFIP

Multi-scale viscoelastic damage model of short glass fiber reinforced thermoplastics under fatigue loading

This work presents a new micromechanical fatigue damage model for reinforced thermoplastic composites. The study aims at modeling high cycle fatigue damage of a short glass fiber reinforced polyamide-66. The developed approach is a modified Mori-Tanaka method that includes coated reinforcements and microscale damage processes. The model takes into account the nonlinear matrix viscoelasticity and the damage mechanisms evolution. The latter is based on the experimental damage investigation previously performed by the authors and presented elsewhere. Damage chronologies have been proposed involving three different local processes: fiber-matrix interface debonding/coating degradation, matrix microcracking and fiber breakage. Each damage mechanism is introduced through an evolution law coupled to local stress fields computed at the microscale. The first numerical results show capability of the developed model to predict the fatigue damage accumulation of the macroscopic homogenized composite material

Investigation in situ des mécanismes d’endommagement dans un composite polyamide renforcé par des fibres courtes

Injection molded polyamide composite reinforced with short glass fibers has been widely used in automotive industry due to its high strength to weight ratio and the ability of injection process to produce complex parts. A reliable design of components made of this composite should consider the development of progressive properties degradation due to the damage. A better understanding of the damage mechanisms shall contribute to a better formulation of local damage criteria and thus to include with a higher accuracy the physical modeling of their effects to predict the overall mechanical behavior of the composite. For this purpose, in situ SEM tests were performed to observe the damage mechanisms of injection molded polyamide-66 reinforced with 30%wt of short glass fibers (PA66GF30). The observation was focused on dry as mold state (0% water content) of PA66GF30, which correspond to a relative humidity RH=0%. The specimens were subjected to a flexural load using a three-point bending micro-device and were assembled inside an environmental SEM to allow the in situ observations. Specimens were cut following two specific orientations with respect to the mold flow direction (MFD): longitudinal and transverse. Prior to observation, the surface samples were polished and metalized with gold. As the polyamide absorbs water during polishing, the samples were put after polishing inside a vacuum oven at 80 o C for 15h to ensure that the RH content goes back to zero in the whole sample

Linear polarization and composition of VLBI jets

It is shown that linear polarization data can be used to constrain the composition (normal or pair plasma) of pc-scale extragalactic jets. A simple criterion, based on synchrotron and Faraday depolarization properties, is established. It does not depend on the particle density and the length of the emitting region along the line of sight, thus eliminating two physical unknowns.Comment: 4 pages, 1 figur

Avant-propos

A partir du problématique de la cognition située, que l’on doit à des anthropologues comme Lucille A. Suchman, se précisent depuis quelques années des convergences entre l’ethnolinguistique, l’anthropologie linguistique, la sémiotique et certains cantons des sciences cognitives. Par exemple, le colloque inaugural du Centre Ferdinand de Saussure (juin 1999, Archamps‑Genève) a pris pour thème Sciences cognitives et sémiotique des cultures, associant des chercheurs comme Jerome Bruner en psychol..

Micromechanical modeling of damage and load transfer in particulate composites with partially debonded interface

A new micromechanical damage model accounting for progressive interface debonding is developed for composite materials. It consists of an original evolution law of the damage at the interface and an appropriate load transfer law at the matrix-fiber interface integrated into a generalized incremental Mori–Tanaka homogenization scheme. The interface damage evolution is driven by the interfacial stress state while the load transfer is obtained from a new model inspired by the shear lag model. Specifically, such damage evolution is supported by experimental microscopic observations for short glass fiber reinforced polyamide-66. The proposed model is validated based on numerical reference solutions provided from finite element analyses of a representative unit cell of a composite, where imperfect interfaces are represented using cohesive elements. A further comparison with experimental data proves that the proposed model is an alternative to micromechanical models involving weak interfaces in the case of spherical reinforcements. It is shown that the proposed model is able to accurately reproduce the non-linear effective response of composite materials for a broad range of reinforcement shapes, including spherical particles and matrix mechanical properties