Hemodynamic analysis of intracranial aneurysms using phase-contrast magnetic resonance imaging and computational fluid dynamics

Additional hemodynamic parameters are highly desirable in the clinical management of intracranial aneurysm rupture as static medical images cannot demonstrate the blood flow within aneurysms. There are two ways of obtaining the hemodynamic information—by phase-contrast magnetic resonance imaging(PCMRI) and computational fluid dynamics(CFD). In this paper, we compared PCMRI and CFD in the analysis of a stable patient's specific aneurysm.The results showed that PCMRI and CFD are in good agreement with each other. An additional CFD study of two stable and two ruptured aneurysms revealed that ruptured aneurysms have a higher statistical average blood velocity,wall shear stress, and oscillatory shear index(OSI) within the aneurysm sac compared to those of stable aneurysms.Furthermore, for ruptured aneurysms, the OSI divides the positive and negative wall shear stress divergence at the aneurysm sac.     

Calcul à la rupture en présence d'un écoulement à surface libre : construction de champs de pression approchésYield design for porous media subjected to unconfined flow: construction of approximate pressure fields

We consider the stability of a porous medium submitted to a steady-state flow with free-boundary. Assuming some hypotheses, it is possible to implement the kinematic method by using an approximate pressure field bounding the true pressure field from below. We are interested in finding such approximate pressure fields and in proving that they bound the true pressure field from below without knowing the true pressure field. We use fields which are solutions of a problem with relaxed conditions with regard to the real problem. Under a uniqueness condition of the solution of a weak formulation of the problem, such fields are lower bounds for the true pressure field. Finally, we give the example of a vertical dam. To cite this article: A. Corfdir, C. R. Mecanique 334 (2006).     

A model of film deformation and rupture under the action of thermocapillary forces

A model of the deformation of a free weightless liquid film with rims fixed at a plane contour and subject to the action of thermocapillary forces is investigated. The film free-surface temperature is assumed to be a known function of the coordinates and time. The equation describing the film thickness evolution in the long-wave approximation is of the second order with respect to time and of the fourth order with respect to the longitudinal coordinates. A model plane nonstationary problem is calculated, making it possible to estimate the lifetime of the film as a function of the rate of variation of the temperature profile curvature on the free boundary. If the characteristic time of the temperature variation is large as compared with the film’s natural oscillation period, the problem can be considered in the quasi-steady approximation, which is studied in detail for the plane and axisymmetric cases. The critical values of the temperature profile curvature, for which the film thickness on the symmetry axis vanishes, resulting in the rupture of the film, are calculated.     

On the compression process in a free-piston shock-tunnel

Preliminary results in the Marseille free-piston shock-tunnel facility are presented. The compression of the driver gas by the piston is studied experimentally for two different geometries of the end of the compression tube. Peak pressures obtained with the end of the compression tube closed, and with bursting of the diaphragm separating the high pressure from the low pressure chamber, are compared with calculated values in the cases of N₂ and He as driver gases. A phenomenon of accoustic resonance has been uncovered, generating strong pressure oscillations which, if not properly dealt with, could impair the quality of the useful flow in such a facility.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Vers une théorie énergétique de la rupture fragile

The drawbacks of the classical theory of brittle fracture, based on Griffith's criterion – a notion of critical energy release rate –, and a fracture toughness k, are numerous (think for instance the issue of crack initiation) and penalize its validity as a good model. Are all attempts at building a macroscopic theory of fracture doomed? The variety and complexity of micromechanical phenomena would suggest that this is indeed the case. We believe however that structural effects still preside over fracture and consequently propose to modify slightly Griffith theory without altering its fundamental components so that it becomes amenable to the widest range of situations. The examples presented here will demonstrate that a revisited energetic framework is a sound basis for a theory which can be used at the engineering level and which reconciles seemingly contradictory viewpoints. To cite this article: G. Francfort, J.-J. Marigo, C. R. Mecanique 330 (2002) 225–233.     

Crack tip behavior in near-surface fluid-driven fracture experiments

This Note presents experimental results for the near-tip fracture opening of fluid-driven fractures. The effect of fluid viscosity, quantified by a dimensionless parameter, was varied between tests. The tip region closely followed the classical square-root behavior from linear elastic fracture mechanics when the viscosity parameter was small. Conversely, when the viscosity parameter was of order one and the lag between the fluid-filled region and the fracture front accounted for less than 30% of the fracture, the tip region behaved according to a known intermediate asymptotic solution which results from the solid/fluid coupling. To cite this article: A.P. Bunger et al., C. R. Mecanique 333 (2005).     

Particle Velocimetry and Photoelasticity Applied to the Study of Dynamic Sliding Along Frictionally-Held Bimaterial Interfaces: Techniques and Feasibility

A laser interferometry-based technique was developed to locally measure the in-plane components of particle velocity in dynamic experiments. This technique was applied in the experimental investigation of dynamic sliding along the incoherent (frictional) interface of a Homalite–steel bimaterial structure. The bimaterial specimen was subjected to uniform compressive stress and impact-induced shear loading. The evolution of the dynamic stress field was recorded by high-speed photography in conjunction with dynamic photoelasticity. The combination of the full-field technique of photoelasticity with the local technique of velocimetry was proven to be a very powerful tool in the investigation of dynamic sliding. A relatively broad loading wave with an eye-like structure emanated from the interface. The particle velocity measurements established that sliding started behind the eye-like fringe pattern. It propagated with supershear speed with respect to Homalite. A shear Mach line originating from the sliding tip is visible in the photoelastic images. A vertical particle velocity measurement revealed the existence of a wrinkle-like pulse traveling along the bimaterial interface. The wrinkle-like pulse followed the initial shear rupture tip and propagated at a specific subshear speed.     

用电阻量测方法研究蠕变状态下的金属损伤

本文建议一种量测蠕变下金属损伤的方法。与其它方法相比,这个方法可用于进行在蠕变试验过程中的损伤测量,而无需使试件卸载或冷却。用此方法对试验数据进行加工就可得到在蠕变过程中的试验损伤曲线。对这些曲线的分析导致结论:材料破坏时的损伤是所加应力的递减函数。这一结论是以前所得理论结果的试验验证。     

Critical Stresses in Spalling and Dynamic Rupture of Metals

Results of dynamic rupture tests of a series of metals obtained using a composite Hopkinson bar and shock-wave loading of plane specimens are described. It is shown that the actual rupture strength at a strain rate of 5 · 10³ sec⁻¹ is very close to the spall strength at higher strain rates. Results of testing the same metals using a composite Hopkinson bar within a temperature range of 20–350°C are given. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 6, pp. 103–107, November–December, 2005.     

UNDERSTANDING THE MECHANICAL PROPERTIES OF SINGLE MICRO-PARTICLES AND THEIR COMPACTION BEHAVIOUR

The compaction of particulate materials to form tablets is increasingly employed as a final dosage form for functional products due to its simplicity and low cost. However, the functionality of some products may be impaired due to the high compression pressures required. The general aim of the current study is to understand the relationship between the mechanical properties of single feed particles （〈 100μm） and their compaction behaviour in order to produce tablets at low compression pressure with acceptable strength. The materials studied were pharmaceutical excipients, comprising three enteric polymer particles and three different powders in the form of agglomerates. The mechanical properties of the individual particles or agglomerates were determined by a micromanipulation technique. The samples were also compacted in cylindrical tableting dies. It was observed that there was a strong correlation between the forces required to cause the fracture of the single particles and those derived from the compaction measurements as determined using an existing analysis.