Self-stresses and Crack Formation by Particle Swelling in Cohesive Granular Media

We present a molecular dynamics study of force patterns, tensile strength and crack formation in a cohesive granular model where the particles are subjected to swelling or shrinkage gradients. Non-uniform particle size change generates self-equilibrated forces that lead to crack initiation as soon as strongest tensile contacts begin to fail. We find that the coarse-grained stresses are correctly predicted by an elastic model that incorporates particle size change as metric evolution. The tensile strength is found to be well below the theoretical strength as a result of inhomogeneous force transmission in granular media. The cracks propagate either inward from the edge upon shrinkage and outward from the center upon swelling

Hepatitis B Virus Impairs TLR9 Expression and Function in Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (pDCs) play a key role in detecting pathogens by producing large amounts of type I interferon (IFN) by sensing the presence of viral infections through the Toll-Like Receptor (TLR) pathway. TLR9 is a sensor of viral and bacterial DNA motifs and activates the IRF7 transcription factor which leads to type I IFN secretion by pDCs. However, during chronic hepatitis B virus (HBV) infection, pDCs display an impaired ability to secrete IFN-α following ex vivo stimulation with TLR9 ligands. Here we highlight several strategies used by HBV to block IFN-α production through a specific impairment of the TLR9 signaling. Our results show that HBV particle internalisation could inhibit TLR9- but not TLR7-mediated secretion of IFN-α by pDCs. We observed that HBV down-regulated TLR9 transcriptional activity in pDCs and B cells in which TLR9 mRNA and protein levels were reduced. HBV can interfere with TLR9 activity by blocking the MyD88-IRAK4 axis and Sendai virus targeting IRF7 to block IFN-α production. Neutralising CpG motif sequences were identified within HBV DNA genome of genotypes A to H which displayed a suppressive effect on TLR9-immune activation. Moreover, TLR9 mRNA and protein were downregulated in PBMCs from patients with HBV-associated chronic hepatitis and hepatocellular carcinoma. Thus HBV has developed several escape mechanisms to avoid TLR9 activation in both pDCs and B lymphocytes, which may in turn contribute to the establishment and/or persistence of chronic infection

Relaxation de contraintes et fissuration induites par une redistribution d'eau en milieu biphasique élastique

From a model of transport of water in elastic medium [I. Mrani, J.-C. Benet, G. Fras, Transport of water in a biconstituant elastic medium, Appl. Mech. Rev., special issue on Mechanics of swelling 48 (1995) 717-721] the risk of craking induced by the water redistribution in a cylinder sample after drying is analysed. The numerical resolution of the transport equation of water and the mechanical equilibrium equation give access to the evolution of the water content, the deformation and the stresses in the medium. One will take interest in the state of material during drying, cooling and storage phases. At the end of this study, the risks of cracking are analyzed for various configurations of drying wich determine the hydrous state evolution during storage

Microstructure et propriétés hygro-mécaniques du gel d'agar

This study concerns the measurement of the mechanical and hygroscopic properties of a biopolymer: agar gel. The morphology of the gel is shown by scanning electron microscopy using a freeze fracture technique. It is shown that the agar gel presents a diphasic morphology conserved during drying. The hygroscopic properties of the gel are established through the determination of the isothermal desorption. This study allows, in particular, to define the hygroscopic domain of the material. We study then the shrinkage of agar gel during drying, throughout determination of the variations of the contraction coefficient and the hygrometric expansion coefficient, as a function of the medium humidity. Finally, the rheological study of the gel is established. The experiments consist of the measurement of the change in Young's modulus as a function of water content, by simple compression tests. The results are then used to determine Poisson's ratio, by measuring the propagation speed of ultrasonic waves through the gel.Cette étude aborde la mesure des propriétés d'un biopolymère, le gel d'agar, au cours d'un processus de déshydratation. La morphologie du gel est mise en évidence par des observations au microscope électronique en utilisant une technique de cryofracture ; il est montré que la morphologie initiale diphasique du gel d'agar est préservée lors de l'évolution de la teneur en eau. L'étude des propriétés hygroscopiques du gel, par la détermination de l'isotherme de désorption, permet, en particulier, de situer le domaine hygroscopique du matériau. On étudie ensuite le retrait du gel d'agar, par la détermination des variations du coefficient de contraction et du coefficient d'expansion hygrométrique, en fonction de l'humidité du milieu. On aborde enfin l'étude rhéologique du gel qui présente un comportement macroscopique élastique. La variation du module de Young en fonction de la teneur en eau est déterminée par des essais de compression simple. Ce résultat est ensuite combiné à des mesures de la vitesse de propagation d'ondes ultrasonores pour déterminer la variation du coefficient de Poisson avec la teneur en eau

Optimisation du traitement thermique d'un phosphate application au phosphate du bassin gantour au maroc

Un protocole de traitement thermique d'un minerai phosphaté visant l'optimisation des dépenses de l'énergie destinée à la calcination est ici présenté. Nous avons mené une étude comparative de la calcination effectuée de deux manières différentes sur du phosphate brut. Cette méthode a été appliquée pour enrichir le phosphate sédimentaire issu d'un nouveau site du Bassin des Gantour (Maroc). Dans un premier temps, nous avons optimisé la température de calcination et le temps de séjour. En effet, les résultats montrent que le traitement à 800 °C pendant 30 min donne un minerai de meilleure concentration en phosphate. Il permet de diminuer le carbone organique et le dioxyde de carbone respectivement de 1,4 % à 0,15 % et 6,6 % à 1,8 %. Ainsi que l'augmentation de la teneur en P2O5 de 30,39 % à 34,23 %. De plus, il a été constaté un impact du refroidissement sur la distribution granulométrique. Dans un deuxième temps, il apparaît que la procédure de calcination suivie d'un refroidissement par 33 % de phosphate de même source (mélange à chaud) peut avoir d'importantes retombées sur l'économie de l'énergie et un gain de productivité tout en préservant l'environnement

Reinforcement of RC structure by carbon fibers

In recent years, rehabilitation has been the subject of extensive research due to the increased spending on building maintenance work and restoration of built works. In all cases, it is essential to carry out methods of reinforcement or maintenance of structural elements, following an inspection analysis and methodology of a correct diagnosis. This research focuses on the calculation of the necessary reinforcement sections of carbon fiber for structural elements with reinforced concrete in order to improve their load bearing capacity and rigidity. The different results obtained reveal a considerable gain in resistance and deformation capacity of reinforced sections without significant increase in the weight of the rehabilitated elements