Tensile dynamic behaviour of unidirectional glass fibre-reinforced thermoset matrix composites

The aim of this study is to determine the tensile behaviour of unidirectional glass fibre-reinforced polyester matrix composites at high strain rate (≈ 10 -1). Different orientations of the composite are tested with an hydraulic tensile machine which covers the range from static strain rate to 50 s-1. It is shown that difficulties might appear with solicitations of high velocity. The loading of the specimen becomes very complex. In this study, some improvements have been made to limit these effects. The geometry of the tests specimens is first optimised by a numerical simulation in order to obtain uniformly distributed stress and strain rate in the measurement zone. Another improvement is to choose an appropriate damping joint for limiting disturber loading. Displacement and force versus time signals obtained experimentally are free of oscillations. The longitudinal (0°) and transversal (90°) orientation of fibres in the composite are tested. For both orientation, it is observed that the mechanical properties (maximum stress, elastic modulus and failure strain) depend on the strain rate. Effect of the rate dependence on 0° specimens is moderate : the maximum stress increases slightly. For 90° specimens, the strain rate effect is much more significant on the maximum stress and the yield stress

Intercomparison between MRI and stereovision/PIV measurements for diagnosis of aorta pathologies

International audienceThe objective of the study is to validate the ability of magnetic resonance imaging (MRI) to perform measurements of deformations and internal flow of an aortic phantom with aneurysm. An in-vitro measurement experimental device to simulate blood flow was developed. It was tested within MRI and within stereovision and particle image velocimetry (PIV) devices. The measurement results are well correlated and can be used to establish a preoperative diagnosis based on computed parietal stresses with a finite element model (FEM)

Comparison of Flow Measurement by 4D Flow Magnetic Resonance Imaging and by Particles Image Velocimetry on Phantom of Abdominal Aortic Aneurysm

International audiencePredicting the rupture of Aortic Aneurysms is a complex problem that interests, from several decades, many researchers. The works on this issue are very complex, involving both the study of mechanical behavior of the artery as the flow of blood. The Magnetic Resonance Imaging (MRI) technique allows to obtain anatomic information of the arteries, than the flow inside thereof. The goal of this study is an inter comparison betweenflow data from MRI and those obtained by Particle Image Velocimetry (PIV). An experimental device simulating hemodynamic circulation is used. Initially in order to validate the device, the flow in a cylindrical glass tube is measured by these two techniques and then compared to a theoretical model. Secondly, the flow in a phantom in silicone, with an axisymmetric aneurysm, is evaluated with 4D flow MRI sequences and the easurements are compared with those obtained by PIV with good agreement. The ability of the MRI technique to measure the flow thus makes an essential device for the study of cardiovascular diseas

Effective orifice area assessment in BI-leaflet mechanical valves by MRI,

International audienceEffective orifice area (EOA) and mean pressure gradientare routine echocardiographic measurements of valvularprosthesis function in both the mitral and aortic position.EOA computation is based on the continuity equationand is used to describe the “effective” orifice formed bythe flow traversing a geometrical orifice. In the case ofbi-leaflet biomechanical valvular prosthesis (BMHV),the orifice of the valve is divided into 3 zones. BMHVperformance assessment by MRI have been described inthe in-vitro setting in [1] using the maximum pressuregradient. Additionally, a method using jet layer has beenused to detect EOA dynamically [2]. 4D-PC flowsequences enable measurement of three velocitycomponents by MRI with a temporal resolutionaveraging (~ 30 msec). In patients with a BMHV,despite the metallic blurring due to the valve structure,MRI examination enhances both anatomical andhemodynamic diagnosis. Aim of this work is to assessthe capacity of MRI to measure BMHV EOA by 4D-PCflowMRI, using simple computational fluid dynamic(CFD) simulation for validation

Phospholipid membranes decorated by cholesterol-based oligonucleotides as soft hybrid nanostructures

DNA monomers and oligomers are currently showing great promise as building blocks for supramolecular arrays that can self-assemble in a fashion preprogrammed by the base pairing code. The design and build-up of hybrid DNA/amphiphilic self-assemblies can expand the range of possible architectures and enhance the selectivity toward a well-specified geometry. We report on the self-assembly properties in aqueous solution of a cholesteryl?tetraethylenglycol single stranded 18-mer oligonucleotide (ON1TEG-Chol) and on its spontaneous insertion in fluid phospholipid membranes. Up to 500 units of these lipophilic ss-oligonucleotides can be incorporated in the outer leaflet of 350 Å radius POPC vesicle. The insertion and hybridization with the complementary oligonucleotide are monitored through light scattering as an increase of hydrodynamic thickness, which is interpreted in terms of average distance between anchoring sites. The conformation of the ss-oligonucleotidic portion is strongly dependent on surface coverage, passing from a quasi-random coil to a more rigid configuration, as concentration increases. Interestingly, conformational details affect in a straightforward fashion the hybridization kinetics. Liposomes with single- and double-strand decorations remain stable within the experimental time window (about one week). The structure represents an example of successful and stable amphiphile/DNA supramolecular hybrid, where a DNA guest is held in a membrane by hydrophobic interactions. The lipophilic oligonucleotide under investigation is therefore a suitable building block that can effectively serve as a hydrophobic anchor in the fluid bilayer to assemble supramolecular constructs based on the DNA digital code

Fasting leptin and appetite responses induced by a 4-day 65%-energy-restricted diet.

OBJECTIVE: Animal studies show that the leptin decline after acute severe caloric restriction is a peripheral signal to increase food intake. However, most human studies have failed to observe such a relationship. We studied the acute effects of severe caloric restriction on the association between serum leptin concentrations and subjective appetite. SUBJECTS: A total of 44 healthy adult men (aged: 43 +/- 5 years; BMI: 27.3 +/- 3.2 kg/m(2)). MEASUREMENTS: Fasting serum leptin concentrations and self-perceived appetite levels were measured during a 4-day diet containing 36% of the estimated energy requirements. Appetite levels were assessed with a 10-point Likert scale, reflecting hunger, fullness, desire to eat, prospective consumption and total appetite. RESULTS: After the 4-day energy deficit, fasting leptin concentrations decreased by 39.4% (95% CI: -43.6; -34.9%). This decline was associated with an increase in fasting hunger (r = -0.42; P < 0.01), desire to eat (r = -0.39; P < 0.05) and total appetite (r = -0.38; P < 0.05). Furthermore, the association between fasting leptin concentrations and fasting appetite levels became stronger during the energy restriction period (for total appetite: day 0 r = -0.15, P = 0.32; day 2 r = -0.31, P =< 0.05; day 4 r = -0.41, P < 0.01). CONCLUSIONS: The acute proportional reduction in fasting leptin after 4-day energy restriction is associated with an increase in self-perceived appetite. Additionally, the inverse association between proportional fasting leptin concentrations and self-perceived appetite response becomes stronger as energy restriction is prolonged. These findings suggest that leptin has an instrumental role in restoring energy balance in humans through the expression of appetite

Polymer-decorated tethered membranes under good- and poor-solvent conditions

We study tethered membranes grafted by polymer chains on one side. Mean-field and scaling arguments predicting a spontaneous curvature are compared to the results of lattice-based Monte Carlo simulations using the Bond Fluctuation Model, which are carried out for various grafting densities and chain lengths. We show that already slightly overlapping chains bend the membrane significantly. This proves the entropic origin for the bending stiffness, which is of order kT . To understand the membrane curvature under conditions of very small bending stiffness we apply a geometrical model which takes into account the state of chains at the overlap threshold. Applying a thermal solvent model for the grafted chains, we demonstrate that the bending direction of the membrane can be triggered by variation of the solvent quality. This indicates that polymer-decorated membranes may serve as switchable nanoscale devices