8 research outputs found
Mechanical Behavior of a Triaxially Braided Textile Composite at High Temperature
RÉSUMÉ
L'utilisation de composites à matrice polymère (CMP) dans l'industrie aéronautique est aujourd'hui une réalité incontournable, tel qu'illustré par les matériaux utilisés dans le Boeing 787 ou le C-series de Bombardier. Ces matériaux, qui possèdent un rapport de rigidité/masse avantageux ainsi qu'une grande flexibilité de propriétés, représentent d'excellents candidats pour des applications de haute performance. En particulier, les composites textiles offrent une grande diversité mécanique résultant du choix de matériau pour les fibres et la matrice, ainsi que de l'agencement de l'architecture textile. Cependant, les CMP sont habituellement restreints à des applications à température ambiante, à cause des faibles propriétés mécaniques de la matrice à haute température.
Le développement de matrices polymère possédant une meilleure résistance à la température crée de nouvelles opportunités pour les CMP, notamment pour les moteurs d'avions. Cependant, le comportement mécanique de la matrice varie en fonction de la température et du temps. De ce fait, les CMP peuvent exhiber un comportement viscoélastique à haute température et être sujets au vieillissement physique et chimique en présence d'un environnement oxydant. Les interactions entre le comportement de la matrice et l'architecture du composite textile doivent être analysés avant de concevoir des pièces avec ces matériaux.
L'objectif principal de cette thèse était de comprendre l'influence de la température, de la viscoélasticité et du vieillissement sur les propriétés mécaniques d'un composite textile tressé. Le matériau étudié est un composite tressé carbone/MVK10 dont les torons sont orientés à 0°/±60°.
La première étape consistait à développer un cadre d'homogénéisation analytique et numérique afin de prédire le comportement viscoélastique de textiles tissés et tressés. Des simulations ont été réalisées pour plusieurs fractions volumiques et rapports de contraste, en utilisant le modèle numérique comme base de vérification pour les modèles analytiques. L'étude a démontré que le modèle Mori-Tanaka tenant compte de l'ondulation des torons était le plus approprié pour évaluer les propriétés de composites textiles viscoélastiques.
Lors de la deuxième étape, l'influence de la température sur l'endommagement du textile tressé carbone/MVK a été étudiée dans deux directions afin de tenir compte de l'anisotropie du matériau. Une série de tests de traction fut réalisée sur des échantillons de composite à température ambiante et à température de service. Une technique de réplication fut utilisée afin de surveiller l'évolution des fissures à la surface du matériau. Par la suite, la distribution des contraintes dans les torons a été évaluée à l'aide de modèles analytiques et numériques. Une nouvelle approche de maillage a été utilisée pour obtenir un Volume Élémentaire Représentatif de la structure. L'étude a permis de réaliser qu'à haute température, les propriétés élastiques transverses et de cisaillement des torons diminuent, engendrant une redistribution des contraintes au sein de l'architecture textile. Cette redistribution affecte les modes de rupture du composite ainsi que la progression de l'endommagement. L'étude a aussi montré le potentiel des modèles analytiques et numériques à expliquer les modes de ruptures au sein de l'architecture textile.----------ABSTRACT
Polymer Matrix Composite Materials (PMCMs) have gained increasing attention from the aerospace industry over the past several years, as illustrated by their increased use in aircraft, like the Boeing Dreamliner or the Bombardier C-series. Their high strength to weight ratio, combined with the possibility to tailor their properties, provides great opportunities for the replacement of traditional materials, such as steel and aluminum.
Textile composites can yield a broad range of mechanical properties, depending on matrix and fibres properties, fibres orientation and distribution within the composite, and the yarns volume fraction. In particular, carbon fibres provide high tensile modulus and strength, when compared to other reinforcing fibres. The yarns interlacing provides additional impact resistance. However, the use of these materials is usually restricted to low temperature applications due to the matrix properties degradation with temperature.
The development of new temperature resilient polyimide matrices paved the way for new applications for PMCMs, especially in the gas turbine industry. In high temperature oxidative environments, the matrix properties degradation can significantly affect the composite's overall mechanical behaviour, even if the fibres can safely withstand much higher temperatures. Of particular interest are structural components exposed to temperatures of up to 300°C under permanent regimes. The literature has shown that PMCMs behaviour at elevated temperatures depends on the reinforcement specific architecture as well as the matrix' thermal stability.
The work presented in this thesis aimed at understanding the influence of viscoelasticity, temperature and aging on the mechanical behaviour of a textile composite using experimental, analytical and numerical tools. The studied material was a triaxially braided composite with fibres in the 0°/±60° directions. The yarns were made of carbon fibres, embedded in an MVK10 temperature resistant polyimide matrix.
The first step consisted in developing analytical and numerical frameworks to predict viscoelastic behaviour in textile composites. Simulations were performed for both braided and woven textile architectures, at different stiffness contrasts and yarns volume fractions. The analytical framework accuracy was verified with the help of the numerical simulations. An important finding of this study was that the analytical framework, combined with the Mori-Tanaka model, leads to relatively accurate predictions for both the permanent and transient parts. Therefore, the authors believe that the Mori-Tanaka model with an adjusted aspect ratio to take into account yarn curvature is reliable for predicting viscoelastic behaviour in textile composites. The textile composite that was studied in this project did not display viscoelastic behaviour, due to the high yarn volume fraction. However, the framework remains relevant for higher temperature applications or lower yarn volume fractions.
The second step was to investigate the temperature effect on the tensile behavior of the carbon/MVK10 triaxially braided composite material studied in this project. To achieve this goal, a series of room and high temperature tensile tests on both matrix and composite samples were performed
Idiopathic Chylopericardium: A case Report and Breif Review of Literature
Chylopericardium is the accumulation of chylous fluid in the pericardial space. Many etiologies can be found and the treatment depends on the etiology and remains unclear. This rare entity needs some tests to determine its causes. Diagnosis is based on many tools; the pericardial biopsy puncture and lymphoscintigraphy are the most important ones. The treatment requires low fat diet, lipid lowering therapy and surgery in some cases. We here report the first case in the IBN ROCHD university hospital’s Cardiology department of Casablanca, Morocco. It deals with a 32-year-old patient who consulted for dyspnea exacerbation for 18 months, becoming at rest. Chest X-ray and echocardiography were performed discovering pericardial effusion. Pericardiocentesis was performed to confirm the nature of liquid and lymphoscintigraphy showed no abnormal communication between the thoracic duct and pericardium
Recommended from our members
A Successful National and Multipartner Approach to Increase Immunization Coverage: The Democratic Republic of Congo Mashako Plan 2018-2020.
BackgroundThe immunization system in the Democratic Republic of the Congo faces many challenges, including persistent large-scale outbreaks of polio, measles, and yellow fever; a large number of unvaccinated children for all antigens; minimal and delayed funding; and poor use of immunization data at all levels. In response, the Expanded Programme on Immunization within the Ministry of Health (MOH) collaborated with global partners to develop a revitalization strategy for the routine immunization (RI) system called the Mashako Plan.Mashako plan design and developmentThe Mashako Plan aimed to increase full immunization coverage in children aged 12-23 months by 15 percentage points overall in 9 of 26 provinces within 18 months of implementation. In 2018, we conducted a diagnostic review and identified gaps in coordination, service delivery, vaccine availability, real-time monitoring, and evaluation as key areas for intervention to improve the RI system. Five interventions were then implemented in the 9 identified provinces.DiscussionAccording to the 2020 vaccine coverage survey, full immunization coverage increased to 56.4%, and Penta3/DTP3 increased to 71.1% across the Mashako Plan provinces; the initial objective of the plan was reached and additional improvements in key service delivery indicators had been achieved. Increases in immunization sessions held per month, national stock of pentavalent vaccine, and supervision visits conducted demonstrate that simple, measurable changes at all levels can quickly improve immunization systems. Despite short-term improvements in all indicators tracked, challenges remain in vaccine availability, regular funding of immunization activities, systematic provision of immunization services, and ensuring long-term sustainability.ConclusionsStrong commitment of MOH staff combined with partner involvement enabled the improvement of the entire system. A simple set of interventions and indicators focused the energy of managers on discrete actions to improve outcomes. Further exploration of the results is necessary to determine the long-term impact and generate all-level engagement for sustainable success in all provinces
Recommended from our members
Strengthening immunization programs through innovative sub-national public-private partnerships in selected provinces in the Democratic Republic of the Congo.
BACKGROUND: Low immunization coverage rates in the Democratic Republic of Congo (DRC) have been reflective of challenges with vaccine access, support and delivery in the country. Motivated by measles and vaccine-derived polio virus (VDPV) outbreaks in 2016-17 and low vaccination rates, the provinces of Haut Lomami and Tanganyika were identified as pilot locations for an innovative approach focused on establishing a consortium of partners supporting local government. This approach was formalized through Memorandums of Understanding (MoUs) between the Bill and Melinda Gates Foundation and Provincial governments in 2018. A third province, Lualaba, established an MoU in 2021. MOU IMPLEMENTATION: These MoUs were 5-year partnerships designed to aid provinces in meeting four key objectives: 80 % immunization coverage, management/elimination of polio/cVDPV outbreaks, improvement of vaccine accessibility, and transfer of immunization service management to provincial leadership. OUTCOMES: During the MoU period, Haut-Lomami saw an increase in full immunization coverage, from 35.7 % (MICS 2018) to 88.9 % (VCS 2021-22), the highest in country. A sharp drop in percentage of zero-dose children was observed in the 3 provinces, confirming improved access to immunization services. Tanganyika saw initial improvement in full immunization coverage, followed by a drop in the VCS 2021-22 due to COVID-19 and healthcare worker strikes. Coverage improved in Tanganyika in the 2023 VCS. The 3 provinces increased their financial contributions to routine immunization and are now the top contributing provinces. While no cVDPV cases were recorded in 2020 and 2021, cVDPV1 and cVDPV2 outbreaks are afflicting the 3 provinces since 2022. CONCLUSIONS: Ultimately, the provincial MoUs were successful in bolstering provincial autonomy and capacity building with the biggest success being a drop in zero-dose children. While not all objectives have been met, the MoU approach served as an innovative program for key aspects of strengthening routine immunization in the DRC