Développement d’un banc d’essai de retrait empêché : relation entre formulation et fissurabilité des bétons au jeune âge

De récentes études ont montré la forte sensibilité des Bétons Hautes Performances (BHP) à la fissuration prématurée. Un dispositif d’essai a été conçu afin d’étudier le retrait plastique pour des sollicitations dites « empêchées ». Reposant sur le principe d’un double asservissement en force et déformation, les résultats issus de ce dispositif permettent d’estimer les évolutions durant l’essai de la contrainte de traction développée dans le béton et du degré de fluage au très jeune âge

Coupled Hydro-Mechanical Aging Of Short Flax Fiber Reinforced Composites

International audienceOne of the challenges in the widespread use of biocomposites for engineering applications is the influence of environmental conditions on their mechanical properties, particularly for a combination of aging factors such as temperature, moisture, and mechanical stresses. Thus, the purpose of this paper is to study the influence of coupled aging factors by focusing on a 100% bio-based and biodegradable composites made of flax/poly(lactic acid) with several fiber contents. The development of a specific testing setup enabled continuous in-situ measurements and allowed comparing the effects of combined aging factors to those of uncombined aging factors. It was confirmed that the aging temperature in wet conditions led to a loss of elastic properties, especially for higher fiber fractions. While creep tests in dry conditions resulted in little decrease of elastic properties, it was observed that mechanical loading of the materials combined with water immersion resulted in a strong synergistic effect on the loss of stiffness. Finally, the presence of fibers reduced environmental stress cracking mechanisms and increased the time to failure

Modelling Of Hydrothermal Aging Of Short Flax Fiber Reinforced Composites

International audienceAs a contribution to the prediction of the evolutionary behavior of biocomposites in service conditions, this study focused on the simulation of the hydrothermal aging of short natural fiber reinforced composites made by extrusion/injection molding. We endeavored to model the reversible modifications of the behavior of PLA and PLA/flax composites when immersed in water at different temperatures (20, 35 and 50 °C). A numerical model accounting for the heterogeneous mechanisms involved during aging such as water diffusion and the resulting swelling and plasticizing of polymers was implemented. Simulated data proved to be in perfect accordance with experimental results as long as no irreversible mechanism was occurring. The deviations of the simulated data from experimental results were limited at 35°C but significant at 50°C. Finally, the influence of moisture on the local elastic modulus of flax fibers was inferred thanks to the Halpin-Kardos homogenization model

Reversible and irreversible changes in physical and mechanical properties of biocomposites during hydrothermal aging

International audienceThe use of biocomposites on a daily basis for industrial products brings to light the influence of environmental factors on the evolution of mechanical properties. This aging has a major influence in the lifetime of any product based on such materials. Among biobased composites, poly(lactic acid) reinforced with plant fibers are known to be sensitive to hydrothermal aging due to the intrinsic nature of their components. Although some papers studied the influence of temperature and water absorption on such materials, so far the difference between reversible and irreversible effects of aging has been hardly studied. This distinction is the purpose of this study. Poly(lactic acid) samples reinforced with various content of flax fibers (0%, 10%, and 30%) were immersed in water at different temperature (20, 35, and 50°C) up to 51 days. The physical and chemical phenomena responsible for the changes in the mechanical properties of biocomposites were evaluated. It was observed that these changes were mainly reversible. However, irreversible effects of aging turned out to increase drastically with the amount of fiber and the aging temperature. While hydrolysis drastically deteriorated PLA for longer aging times, the lifetime of biocomposites was significantly extended (+ 230% at 50°C from 0% to 10% in fiber content) by the presence of fibers which postponed the failure of PLA

Impact on peel strength, tensile strength and shear viscosity of the addition of functionalized low density polyethylene to a thermoplastic polyurethane sheet calendered on a polyester fabric

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Instrumentation optique d'essais sur tôles : critères de validation

L'identification du comportement des matériaux métalliques et des alliages d'aluminium plats destinés à l'emboutissage est une problématique importante pour les acteurs de l'industrie, notamment automobile, qui doivent pouvoir qualifier rapidement, et de façon fiable, de nouvelles classes de matériaux que leur propose la sidérurgie. Ces qualifications nécessitent des essais de nature variée permettant de soumettre le matériau à des états de contraintes multiaxiales ainsi qu'à des trajets de chargement simples ou complexes choisis de façon à correspondre aux sollicitations subies lors de leur mise en oeuvre. L'instrumentation unidirectionnelle associée à une interprétation analytique de ces essais repose essentiellement sur l'hypothèse d'une homogénéité des contraintes dans le matériau. Bien que généralement vérifiée dans le cas de la traction simple lorsque la sollicitation à lieu dans les directions d'isotropie du matériau, ces hypothèses doivent dans tous les cas être vérifiées pour le couple classe de matériau/banc test expérimental

Comparaison de méthodes dynamiques d'analyse du comportement mécanique de matériaux

Cette étude propose une comparaison entre deux méthodes pour l'identification de propriétés mécaniques dynamiques d'un matériau. Les paramètres de comportement élastique et dissipatif sont identifiés pour des matériaux viscoélastiques modèles. L'analyse modale, en opposition à l'analyse mécanique dynamique, se distingue nettement du fait de sa rapidité et de son faible coût de mise en œuvre. De plus, nous mettons en avant le caractère non destructif de ce type d'essai, qui représente un intérêt important dans le suivi de la dégradation d'une structure