Mémoire organisationnelle adaptative en vue d'une classification automatique pour la capitalisation d'informations

International audienceUne mémoire organisationnelle est un espace de capitalisation d’informations diverses circulant au sein d’une entreprise. L’objectif de nos travaux est d’assister les usagers dans le classement de ces informations au sein de cette mémoire organisationnelle tout en limitant leur contribution. Pour ce faire, nous proposons un méta-modèle de mémoire organisationnelle sur lequel nous souhaitons faire reposer un processus de classification automatique

Organizational memory: A model based on a heterogeneous network and an automatic information integration process

Organizational memory is a space where various information circulating in a company are capitalized. From the users' point of view, an organizational memory, which can be seen as an information system component, is very important since it stores the “shared knowledge” of the organization. But, at the same time, the cost of this knowledge is relatively high since users' participation, i.e. to integrate/maintain... the memory is important. The aim of our work is to model an organizational memory through a heterogeneous network on which is based an automatic information integration process to assist users in this task while limiting their effort. We developed a prototype and evaluated through an experiment its ability to integrate new information into an organizational memory based on the proposed model

Micromechanics of an epoxy matrix for fiber reinforced composites : experiments and physics-based modelling

Given the multi-scale nature of the damage and deformation mechanisms in fiber reinforced composites, bottom-up approaches are believed to be the more efficient strategy to develop generic models for the predictions of the mechanical behaviour of large structures. These analyses start at the microscopic scale, where reliable models for the constituents are essential input ingredients. This thesis is dedicated to the characterization, understanding and modelling of the fracture and deformation mechanisms of the RTM6 epoxy resin. A first part of the thesis aimed at unravelling the origin of crack initiation in epoxy resins. It provided a physical explanation of the pressure dependence of the fracture stress and strain in highly cross-linked epoxy resins, unifying the occurrence of fracture under a wide range of stress triaxialities based on a single mechanism. A second part of the thesis was devoted to the microscale characterization of RTM6 in a unidirectional composite, based on in situ experimental tests providing quantitative pictures of the strain field at the microscale. The comparison between experimental results and finite element analyses both at the macro- and microscale enabled a critical assessment of the validity of macroscopic properties at the microscopic scale. Finally, a novel modelling approach based on the shear transformation zones framework was developed for epoxy resins. The proposed approach offers an alternative to complex phenomenological constitutive models, reducing the number of parameters from more than twenty to seven, while bridging the gap between atomistic and continuum mechanics approaches by providing the missing ingredients for seamless scale transition.(FSA - Sciences de l'ingénieur) -- UCL, 201

Nanomechanical characterisation of unidirectional fibre reinforced composites at the fibre matrix level

The use of bottom-up multiscale methods has become the privileged approach to predict the deformation and failure of fibre-reinforced polymer composites. The development of accurate computational multi-scale models relies on the proper description of the individual components of the composite ply. The determination of the local properties of these constituents is challenging, dependent on the specific local curing conditions and generation of heterogeneities, and accurate data are scarce for the interfaces and interphases [1]. The matrix is usually described based on continuum models, yet this may lead to inaccurate prediction of the local strain field around fibres. These challenges limit the accuracy of composite model predictions, even for unidirectional (UD) composites loaded in transverse compression, where the macroscopic deformation response is dictated by the matrix [2]. In this study, we propose a combined experimental and numerical approach to characterise the constituents of a carbon fibre-reinforced UD composite. The measurement and prediction of the matrix response at the fibre/matrix level is of particular interest, as matrix size effects may exist at this scale. Nanoindentation is used to determine the properties of the matrix in confined volumes in-between fibres. Care is taking to deconvolute artefacts resulting from the test procedure and the data treatment on possible size effects. Transverse compression tests on UD specimens are conducted inside a scanning electron microscope (SEM) allowing the use of micro digital image correlation (DIC). The objective is to quantify the local strain field with an accuracy as small as a few tens of nano-metres. The DIC strain maps are confronted with FEA results using a model enriched by the nanoindentation measurements

Ancrage territorial du modèle contractuel rizicole sénégalais : une perspective géographique et relationnelle

International audienceTo meet the challenges of African agriculture, particularly food self-sufficiency, job creation and wealth, the agricultural contract model is being promoted by some governments and international donors in the framework of a performative discourse on the virtues of partnerships between agro-industries and producers. However, agricultural contracts give rise to contradictory interpretations. Some see it as a model that would have spillover effects on the rural economy, while others see it as a strategy of predation of agricultural resources, which is harmful to local communities. On both sides, analyses remain focused on ethical and sometimes ideological issues. The operating conditions of the contracts and their impacts in terms of development are little addressed. Our contribution, based on sociological surveys conducted in Senegal, aims at objectively characterizing the forms of interaction (positive or negative) between agricultural contracts and the development of the territories where the rice sector is located. On the basis of these issues of articulation between agricultural development and territorial development, we are proposing a methodological approach to carry out this type.of diagnosis. At the scale of the regions dominated by rice growing, we highlight that the territorial trajectories are contrasted in relation to the interaction between the agricultural model and territorial development. This diagnosis is interpreted through the knowledge of the systems of actors, in particular with the help of two indicators, namely centrality, which gives an indication of the issues of territorial governance, and the density of social relations, which provides information on the importance of the agricultural contract in relation to other modes of relationship building. With regard to the insertion of agricultural contracts, the structure of the networks of actors makes it possible to identify a centralized mode of governance at the level of the national agricultural bank and a few agro-industrial enterprises. Nevertheless, informal relationships and traditional local structures (families, friends, neighbors) persist and function in parallel with the agricultural contract, calling into question the political discourse regarding the transformative potential of agricultural contracts.Pour relever les défis de l’agriculture africaine, notamment de l’autosuffisance alimentaire, de la création d’emploi et de richesse, le modèle du contrat agricole est promu par certains États et bailleurs defonds internationaux dans le cadre d’un discours performatif sur les vertus des partenariats entre agro-industries et producteurs. Toutefois, les contrats agricoles suscitent des interprétations contradictoires. Les uns en font un modèle qui aurait des effets d’entraînement sur l’économie rurale, tandis que les autres y voient des stratégies de prédation des ressources agricoles, qui sont néfastes pour les communautés locales. D’un côté comme de l’autre, les analyses restent centrées sur des questions éthiques et parfois idéologiques. Les conditions de fonctionnement des contrats et leurs impacts en termes de développement sont peu abordés. Notre contribution, basée sur des enquêtes sociologiques menées au Sénégal, vise à caractériser objectivement les formes d’interaction (positive ou négative) entre contrats agricoles et développement des territoires qui abritent une filière rizicole. C’est à partir de ces enjeux de croisement entre développement agricole et développement territorial que nous développons une démarche méthodologique permettant de réaliser ce type de diagnostic. À l’échelle des régions dominées par la riziculture, nous mettons en exergue des trajectoires territoriales contrastées par rapport à l’interaction entre modèle agricole et développement territorial. Ce diagnostic est interprété par la connaissance des systèmes d’acteurs, notamment à l’aide de deux indicateurs, à savoir : la centralité, qui donne une indication sur les questions de gouvernance territoriale, et la densité des liens, qui renseigne sur l’importance du contrat agricole par rapport à d’autres modes de mise en relation. Concernant l’insertion des contrats agricoles, la structure des réseaux d’acteurs permet d’identifier un mode de gouvernance centralisé autour de la banque agricole nationale et de quelques industriels. Mais on note aussi une persistance des rapports informels et des structures communautaires (familles, amis, voisins) qui fonctionnent en parallèle du contrat agricole et qui remettent en cause le discours politique par rapport au potentiel transformateur des contrats agricoles

Multi-scale characterization and modelling of the transverse compression response of unidirectional carbon fiber reinforced epoxy

Thick unidirectional fiber reinforced RTM6 epoxy resin specimens were processed by resin transfer molding and tested in transverse compression. Homogenized response from micromechanical analyses on representative volume elements, using a constitutive model for the matrix identified and validated on bulk RTM6 specimens, were assessed towards the experimental results. Large discrepancies were observed in terms of failure initiation and non-linearity in the stress-strain response. Regarding the interfaces, the use of frictional cohesive elements greatly enhanced the damage tolerance of the RVEs to interface damage. Regarding the matrix behavior, in situ tests on UD specimens and microscale digital image correlation highlighted significant gaps between the numerical and experimental strain fields. Excessive strain localization in the simulations is believed to be the main reason for these differences. This study raises fundamental questions on the degree of confidence that can be granted to constitutive models validated at the macroscale to predict strain field at the microscale

Mechanical characterization and modeling of the deformation and failure of the highly crosslinked RTM6 epoxy resin

The nonlinear deformation and fracture of RTM6 epoxy resin is characterized as a function of strain rate and temperature under various loading conditions involving uniaxial tension, notched tension, uniaxial compression, torsion, and shear. The parameters of the hardening law depend on the strain-rate and temperature. The pressure-dependency and hardening law, as well as four different phenomenological failure criteria, are identified using a subset of the experimental results. Detailed fractography analysis provides insight into the competition between shear yielding and maximum principal stress driven brittle failure. The constitutive model and a stress-triaxiality dependent effective plastic strain based failure criterion are readily introduced in the standard version of Abaqus, without the need for coding user subroutines, and can thus be directly used as an input in multi-scale modeling of fibre-reinforced composite material. The model is successfully validated against data not used for the identification and through the full simulation of the crack propagation process in the V-notched beam shear test

INFLUENCE OF CRYSTALLIZATION CONDITIONS ON THE NANO- /MICRO- BEHAVIOR OF CARBON FIBER-REINFORCED PEEK COMPOSITE

In the context of the energy transition, the transportation sector faces the double challenge of producing lighter but higher-performance structural parts while improving their recyclability. Thermoset-based composite materials allow the manufacturing of light structures with excellent mechanical properties, but are very hardly recyclable and can only be processed via liquid molding techniques (e.g. vacuum infusion) or prepreg consolidation. Moreover, high-rate composite processing is impossible with such matrices: they require a curing step, which often lasts a few hours at high temperatures. Transitioning from thermoset to thermoplastic polymer matrix composites overcomes these shortcomings. However, this involves understanding how processing conditions influence the microstructure of the thermoplastic matrix and the mechanical performances of the composite. Among thermoplastic polymers, semi-crystalline polymers like polyetheretherketone (PEEK) offer superior mechanical properties. The performances of PEEK-based composites are related to the amount and characteristics of the crystalline phase, which depend on the processing conditions. In this work, chemico-physical characterization techniques are combined with nano-/micro- mechanical tests to link the polymer microstructure to the mechanical response at the fiber/matrix level. The crystalline phase morphology is assessed through atomic force microscopy (AFM), scanning electron microscopy (SEM) as well as polarized optical microscopy (POM). The mechanical properties of the inter-/intra spherulitic and trans-crystallization zones are evaluated via nanoindentation (NI) tests. The knowledge of the crystal formation is used to guide the NI mapping, allowing identification of spherulite rich/poor regions around the fibers. The deformation and damage mechanisms occurring in the matrix at the micro-scale during transverse compression tests are studied using nano-digital image correlation (DIC)

A shear transformation zone model to predict the deformation and failure of glassy polymers in fibre reinforced composites

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