Analyse de l’endommagement et ses conséquences sur les propriétés mécaniques

Le vieillissement thermique d’un jonc composite carbone/époxy a été étudié dans l’air entre 160 et 220 °C par tomographie X, microscopie optique et flexion trois points. La thermo-oxydation de la matrice affecte une couche superficielle trop fine pour pouvoir altérer les propriétés mécaniques. La thermolyse de la matrice est donc le principal mode de dégradation. Elle conduit à la formation de longs microcanaux parallèles aux fibres dans les régions les plus riches en matrice du cœur du jonc. Leur multiplication et leur coalescence entraînent une chute progressive du module d’élasticité et, dès que le taux de porosité atteint une valeur critique de l’ordre de 3,6 %, une chute catastrophique de la contrainte à rupture. Sur la base de ces résultats, des équations reliant l’évolution des propriétés mécaniques à des critères d’endommagement dérivés du taux de porosité ont été établies

Détermination des mécanismes de dégradation physico-chimique de vernis automobiles dans le but de prévoir leur durabilité

Sous l impact des agressions environnementales (lumière, température, eau ), les peintures de carrosseries automobiles voient leurs propriétés (couleur, brillant, ) se dégrader. L amélioration de leur tenue dans le temps est l un des enjeux importants auxquels sont confrontés les constructeurs automobiles. La compréhension des phénomènes mis en jeu nécessite un travail de recherche important. Les objectifs de cette thèse ont été d étudier les mécanismes de photovieillissement de polymères tridimensionnels (acrylique-mélamine et acrylique-uréthane) depuis l échelle moléculaire (structure chimique du polymère) jusqu à l échelle macroscopique (propriétés fonctionnelles du matériau). L étude de l influence de chacune des contraintes (lumière, eau, ) indépendamment les unes des autres a permis de comprendre le comportement du matériau dans les conditions naturelles où toutes ces contraintes se superposent. Nous avons montré que le photovieillissement des vernis mettait en jeu la dégradation des fonctions chimiques formées lors de l étape de réticulation du vernis, entraînant une densification du réseau au cours du vieillissement et menant à l apparition d un faïençage lorsqu un seuil de contraintes internes était atteint. On a également montré qu il existait une relation étroite entre le mécanisme chimique de dégradation, l obtention d un palier de dureté et l apparition du faïençage. Sur la base de comparaisons entre vieillissement naturel et vieillissement artificiel accéléré, des facteurs d accélération ont été calculés, permettant de prévoir à partir d une irradiation en conditions de vieillissement artificiel accéléré, la durée de vie des vernis.It is well known that under environmental stresses impact (light, temperature,water, ...), car paints properties (color, gloss, ...) deteriorate. Improving clearcoats performance over time is one of the most important issues that car manufacturers have to solve. This necessitates an important research activity. The main objectives of this thesis were to study the mechanisms of photoageing of crosslinked polymers (acrylic-melamine and acrylic-urethane) from the molecular (chemical structure of the polymer) to the macroscopic scale (functional properties of the material). The study of the influence of individual stress (light,water, ...) independently each from other has allowed us to understand the behavior of the material under natural conditions where all these stresses overlap. We have demonstrated that the photoageing of clearcoats leads to the degradation of the chemical function formed along with the crosslinking process. We have observed that this results in a significant network densification, leading to the appearance of cracking when a threshold of internal stresses is reached. We have also demonstrated a close relationship between the degradation mechanism, the hardness values and the time required for cracks appearance. Based on comparisons between natural and artificial ageing, acceleration factors were calculated to predict clearcoats service life from an accelerated ageing.CLERMONT FD-Bib.électronique (631139902) / SudocSudocFranceF

Investigation of radio-oxidative ageing conditions on EVA/EPDM blends: Understanding the limitations posed as to representing long term in-use exposures

International audienceThe effect of gamma-irradiation and temperature to accelerate, in a representative way, the ageing of EVA (Ethylene Vinyl Acetate)/EPDM (Ethylene Propylene Diene Monomer) blends was investigated. These blends constitute model materials of cable insulation used in Nuclear Power Plants (NPPs). Physicochemical evolutions of the blends were monitored by InfraRed (IR) spectroscopy, gel fraction measurements and Differential Scanning Calorimetry (DSC). After having pointed out the main similarities between the thermooxidative degradation (100 °C) and the radiooxidative degradation at room temperature, both stresses were combined to accelerate the ageing. It is shown that combining gamma-irradiation and high temperature, or subsequent thermooxidation after radiooxidation, accelerates the ageing but may cause distortion within the degradation mechanisms. These limitations of representativeness encountered are particularly due to an increase in chain scission reactions and formation of new crystallites in the blend

Multiscale analysis of the radiooxidative degradation of EVA/EPDM composites. ATH filler and dose rate effect

International audienceThis study is focused on the radiooxidative degradation of polymeric insulation of electric cables used in Nuclear Power Plants (NPPs). In order to investigate the degradation mechanisms of the insulation, model composites with ATH (Aluminium TriHydrate) filler and blends (without filler) based on a cross-linked mixture of EVA (Ethylene Vinyl Acetate) and EPDM (Ethylene Propylene Diene Monomer) were submitted to gamma-rays. In normal operating conditions of a NPP, the dose rate which electric cables are exposed to is around 0.1 Gy h−1. In this work, artificial accelerated ageing test process has been applied at a relatively low dose rate of 7 Gy h−1. Gamma-irradiations at higher dose rates typically used to accelerate the ageing, in the range 0.2–1 kGy h−1, were also carried out.The first part of the study is focused on irradiations performed at relatively low dose rate and is devoted to the highlighting of the radiooxidative degradation mechanisms of EVA/EPDM blend with and without ATH filler. Correlations between the evolutions of the chemical, morphological and mechanical/electrical properties of the materials occurring after the ageing process are presented. It is shown that the degradation process is governed by radical oxidation mechanism involving chain scissions leading to the formation of carboxylic acids as end-groups. One of the main effects of the ATH filler is the progressive loss of the mechanical properties of the composite upon radiooxidation whereas they are maintained in the case of the unfilled sample. Despite the oxidation of the polymer, no change in the electrical properties of the blend and of the composite could be observed.The second part of the study focuses on the dose rate effect. It is shown that one of the main consequences of an increase of the dose rate from 7 Gy h−1 to 0.2–1 kGy h−1 is a reduction of the chain scission process yield by a factor of about 20. Therefore, an important and consistent finding is that there are some limits that must not be exceeded in terms of dose rate to ensure a global satisfactorily representativeness of the accelerated ageing process

Correlation between water uptake and loss of the insulating properties of PE/ATH composites used in cables applications.

International audiencePE/ATH composites are known to lose insulating properties in use condition. The known reason for that behaviour seems to be the water uptake due to two major reasons: hydrophilic behaviour of ATH and degradation of the polymer matrix with the accumulation of oxidation products that lead to create an hydrophilic matrix. This study aims to identify the synergetic or antagonistic effects of the thermo-oxidation on the water absorption and mostly/above all to correlate the modification of the chemical structure with the loss of the functional properties

Thermooxidative degradation of crosslinked EVA/EPDM copolymers: Impact of Aluminium TriHydrate (ATH) filler incorporation

International audienceThis study focuses on the thermal oxidation of model composites of insulating materials based on a cross-linked mixture of EVA (Ethylene Vinyl Acetate) and EPDM (Ethylene Propylene Diene Monomer) highly loaded (60 wt %) with ATH (Aluminium TriHydrate) filler. A thorough analysis of the material is performed, focusing not only on the polymer but also on the ATH filler. This preliminary study is essential firstly to determine the possible influence of a large amount micrometric filler incorporation on the structure of crosslinked EVA/EPDM materials, and secondly on the thermooxidative degradation mechanisms at different levels and scales (chemical structure, microstructure and architecture, degradation profile, functional properties …). The structure of both the polymer and the filler are modified during the processing step of the composite. It is shown that the reactivity of the ATH fillers leads to the probable intercalation of some segments of the polymer into the interlayer space of the ATH, and to a much less dense polymer network in the composite compared to the unfilled material. Then, the role of the added ATH filler on the thermooxidative degradation and on the resistance of the composite to ageing is studied. After thermooxidative ageing, there is only a very limited accelerator effect of ATH on the oxidation rate of the polymer within the composite compared to the unfilled material, but significant difference between materials with or without ATH filler in terms of oxidation profile or insulating properties. Nevertheless, a noticeable effect on the mechanical properties is highlighted: the mechanical properties of the unfilled materials are retained all through the oxidation process while these properties reduce drastically in the case of filled composites. The significant loss of mechanical properties occurring in the filled composites can be explained by the combination of three factors: only 40 wt % of polymer in the composite, a low density polymer network within the composite, and de-cohesion process between the ATH filler and the polymer matrix upon thermooxidative degradation

Impact of particle size in PE/ATH composites: The relationship between the interphase and water uptake

International audienceThe impacts of particle size and polymer-filler interactions on water uptake in PE/ATH composites were characterized by combining several analytical tools to perform a multiscale analysis. SEM images and Raman confocal analysis were performed to characterize the dispersion and distribution of ATH micro- and nanoparticles, and AFM peak force analysis provided images of the interphases of the PE/ATH composites. Water uptake by gravimetry and thermoporosimetry was compared for the PE/ATH composites based on the size of the ATH micro- or nanoparticles. The water absorption was related to the specific surface area of the ATH particles and the interphases in the PE/ATH composites

De veterum ac nostrorum temporum in optimarum artium studiis liberalibusque disciplinis comparatione orationes quinque , à totidem ipsius discipulis classicis graecè & latinè utrinque habitae Lutetiae Parisiorum, in Gymnasio Calvico, 3. id. jun. 1570...

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