Numerical Evaluation of the Effective Elastic Properties of 2D Overlapping Random Fibre Composites

We present a numerical investigation of the elastic coefficients of random fibre composites with high contrast of properties. Here we consider a numerical study based on the generation of representative volume elements (RVEs) with overlapping random fibre network. Such a concept requires an important Monte-Carlo draw of patterns as well as an accurate determination of RVE size. In this paper, this latter is done by estimating the evolution of the standard deviation according to the number of realizations for given values of RVE size. We consider the use of an appropriate model for an automatic, reliable and fast generation of RVEs : the model with an n-order approximate geometry that allows the construction of complex overlapping fibre network. It is well-established that the morphology of the microstructure greatly affects the mechanical response of such kind material. Some morphological features, namely orientation, aspect ratio and dispersion are investigated by considering them as random variables in the design of RVEs. The results are subsequently linked to the percolation phenomenon that occurs when fibres overlap and form some pathways inside the soft phase. This phenomenon influences effective properties of heterogeneous media, particularly in the case of a high contrast of properties

Etude en rupture d'un composite à fibres végétales d'Alfa

National audienceThe behavior under monotonic loading of reinforced natural fibre composites begins to be fairly well known today. However, the fracture behavior is still poorly controlled. This work describes a numerical approach developed to simulate the propagation mechanism of a matrix crack in natural fibre reinforced composites.To this end, the fracture behavior of a REV; constituted of alfa fibre, with linear anisotropic behavior, surrounded by a matrix with non-linear viscoelastic behavior, was investigated using a finite element model. The analysis of the fracture behavior of the composite alfa fibre / epoxy resin shows that under uniaxial longitudinal or transverse load to the fibre, a crack initiated in the matrix is propagated perpendicularly to the direction of the load. Near the interface, the energy release rate decreases and this energy is higher in the presence of interfacial debonding areas generated by problems of fibre wettability. Reaching the interface, the crack is either blocked or deflected. Once deflected, the crack propagates along the interface and causes the complete debonding of the fibre.Le comportement sous chargement monotone des composites renforcés par des fibres naturelles commence à être assez bien connu aujourd'hui. Cependant, le comportement à la rupture est encore mal maîtrisé. Le présent travail décrit une approche numérique développée pour simuler le mécanisme de propagation d'une fissure matricielle à l'interface fibre végétale alfa / résine époxy dans les composites à matrice polymère. A cette fin, le comportement à la rupture d'un VER constitué d'une fibre unitaire d'alfa, de comportement linéaire anisotrope, entourée d'une matrice de comportement non linéaire viscoélastique, a été étudié à l'aide d'un modèle éléments finis. L'analyse du comportement à la rupture du composite fibre alfa/résine époxy montre que sous l'action d'un chargement uniaxial, longitudinal ou transversal par rapport à la fibre, une fissure initiée dans la matrice se propage perpendiculairement au sens de la sollicitation. Au voisinage de l'interface, le taux de restitution d'énergie diminue et ce taux est plus élevé en présence de zones de non adhésion générées par des problèmes de mouillages. Arrivée à l'interface, la fissure est soit bloquée soit déviée. Une fois déviée, la fissure se propage le long de l'interface et entraîne la décohésion de la fibre

In situ infrared absorption spectroscopy of dusty plasmas

In situ, time-resolved Fourier transform infrared spectroscopy was used to study particulate formation in rf discharges in mixtures of silane, argon, and nitrogen. The spectra were taken at a maximum rate of 20 Hz. The discharge conditions were chosen such that previous calibrations of the time evolutions of particle size and density could be used. The measurements indicate that the onset of the solid-state vibrational absorptions of the SiH and SiH2 bands only takes place after the nucleation and coagulation phase have finished; it coincides with the previously predicted start of the deposition of amorphous hydrogenated silicon on the particles. The dissociation of the silane feed gas is found to be in the range of 30%, and its time development suggests that also the large-scale dissociation of silane only starts after the coagulation phase. This is in agreement with previously observed trends for the electron temperature. If silicon partilces are grown in the plasma, and the silane flow is stopped, the Si particles stay trapped in the glow. The infrared measurements, however, show that they almost completely oxidize: the SiH/SiH2 vibrations disappear and a strong SiO vibration appears. If nitrogen gas is allowed into the plasma, the SiO vibration is replaced by a SiN vibration. © 1996 American Vacuum Societ

Estimation des coefficients d'élasticité de matériaux composites renforcés par des nanotubes de carbone multi-parois

Dans le cadre d'une approche multi-échelle, nous étudions le renforcement des matrices de matériaux composites par des nanotubes de carbone multi-parois. Nous bâtissons différents motifs de la matrice enrichie par des nanoparticules à l'aide des éléments finis puis déterminons les propriétés mécaniques du composite par homogénéisation stochastique. Dans le cadre d'une étude statistique, une base de données est construite visant à définir la pertinence de paramètres, tels le diamètre, la courbure, l'orientation, la fraction volumique, le phénomène d'agrégat et la dispersion des nanotubes

In situ infrared absorption spectroscopy of dusty plasmas

International audienceIn situ, time-resolved Fourier transform infrared spectroscopy was used to study particulate formation in rf discharges in mixtures of silane, argon, and nitrogen. The spectra were taken at a maximum rate of 20 Hz. The discharge conditions were chosen such that previous calibrations of the time evolutions of particle size and density could be used. The measurements indicate that the onset of the solid-state vibrational absorptions of the SiH and SiH 2 bands only takes place after the nucleation and coagulation phase have finished; it coincides with the previously predicted start of the deposition of amorphous hydrogenated silicon on the particles. The dissociation of the silane feed gas is found to be in the range of 30%, and its time development suggests that also the large-scale dissociation of silane only starts after the coagulation phase. This is in agreement with previously observed trends for the electron temperature. If silicon partilces are grown in the plasma, and the silane flow is stopped, the Si particles stay trapped in the glow. The infrared measurements, however, show that they almost completely oxidize: the SiH/SiH 2 vibrations disappear and a strong SiO vibration appears. If nitrogen gas is allowed into the plasma, the SiO vibration is replaced by a SiN vibration

Inhibition of Chk1 Kills Tetraploid Tumor Cells through a p53-Dependent Pathway

Tetraploidy constitutes an adaptation to stress and an intermediate step between euploidy and aneuploidy in oncogenesis. Tetraploid cells are particularly resistant against genotoxic stress including radiotherapy and chemotherapy. Here, we designed a strategy to preferentially kill tetraploid tumor cells. Depletion of checkpoint kinase-1 (Chk1) by siRNAs, transfection with dominant-negative Chk1 mutants or pharmacological Chk1 inhibition killed tetraploid colon cancer cells yet had minor effects on their diploid counterparts. Chk1 inhibition abolished the spindle assembly checkpoint and caused premature and abnormal mitoses that led to p53 activation and cell death at a higher frequency in tetraploid than in diploid cells. Similarly, abolition of the spindle checkpoint by knockdown of Bub1, BubR1 or Mad2 induced p53-dependent apoptosis of tetraploid cells. Chk1 inhibition reversed the cisplatin resistance of tetraploid cells in vitro and in vivo, in xenografted human cancers. Chk1 inhibition activated p53-regulated transcripts including Puma/BBC3 in tetraploid but not in diploid tumor cells. Altogether, our results demonstrate that, in tetraploid tumor cells, the inhibition of Chk1 sequentially triggers aberrant mitosis, p53 activation and Puma/BBC3-dependent mitochondrial apoptosis

Nucleic Acids Res

Micro (mi)RNAs are small non-coding RNAs with key regulatory functions. Recent advances in the field allowed researchers to identify their targets. However, much less is known regarding the regulation of miRNAs themselves. The accumulation of these tiny regulators can be modulated at various levels during their biogenesis from the transcription of the primary transcript (pri-miRNA) to the stability of the mature miRNA. Here, we studied the importance of the pri-miRNA secondary structure for the regulation of mature miRNA accumulation. To this end, we used the Kaposi's sarcoma herpesvirus, which encodes a cluster of 12 pre-miRNAs. Using small RNA profiling and quantitative northern blot analysis, we measured the absolute amount of each mature miRNAs in different cellular context. We found that the difference in expression between the least and most expressed viral miRNAs could be as high as 60-fold. Using high-throughput selective 2'-hydroxyl acylation analyzed by primer extension, we then determined the secondary structure of the long primary transcript. We found that highly expressed miRNAs derived from optimally structured regions within the pri-miRNA. Finally, we confirmed the importance of the local structure by swapping stem-loops or by targeted mutagenesis of selected miRNAs, which resulted in a perturbed accumulation of the mature miRNA

Low Clinical Burden of 2009 Pandemic Influenza A (H1N1) Infection during Pregnancy on the Island of La Réunion

BACKGROUND: Pregnant women have been identified as a group at risk, both for respiratory complications than for the admissions to the Intensive Care Unit (ICU) during the 2009 H1N1 influenza pandemic (pdm). The purpose of this prospective register-based cohort-study was to characterize the clinical virulence of the pdm (H1N1/09)v during pregnancy in La Réunion. METHODS/PRINCIPAL FINDINGS: Over a twelve-week pdm wave (13 July to 3 October 2009), 294 pregnant women presented with an influenza-like illness (ILI) to one of the three maternity departments of the South Reunion area, Indian Ocean. Out of these, 278 were checked by RT-PCR for influenza viruses (157 positive and 121 negative, of whom, 141 with pdm flu and 132 with ILIs of non pdm origin, 5 untyped). The median body temperature was higher in women experiencing pdm flu than in those with non pdm ILI (38.9 degrees C versus 38.3 degrees C, P<0.0001), without evidence linked to circulating viremia. Oseltamivir was given for 86% of pdm flu cases in a median time inferior than 48 hrs (range 0-7 days). The hospitalization rate for pdm flu was of 60% and not associated with underlying conditions. Six viral pneumonia and fourteen asthma attacks were observed among 84 hospitalized pdm flu cases, of whom, only one led to the ICU for an acute lung injury. No maternal death occurred during the pdm wave. None adverse pregnancy outcome was associated with pdm flu. No congenital birth defect, nor early-onset neonatal influenza infection was attributable to pdm flu exposure. CONCLUSIONS/SIGNIFICANCE: This report mitigates substantially the presumed severity of pandemic H1N1/09 influenza infection during pregnancy. The reasons for which the clinical burden of H1N1/09 influenza virus may differ worldwide raise questions about a differential local viral-strain effect and public health preparedness, notably in timely access to special care and antiviral treatments