Ram extrusion force for a frictional plastic material: model prediction and application to cement paste

We developed a model to predict the ram extrusion force of frictional plastic materials such as cement-based pastes. The extrusion of cement-based materials has already been studied, but the interaction between shaping force and paste behaviour still have to be understood. Our model is based on the plastic frictional behaviour of cement-based materials and integrates the physical mechanisms that govern material extrusion flow and extrusion force increase. When the process starts, a pressure gradient is created in the extruder due to wall friction of the paste that is submitted to plug flow. It induces a consolidation of the material. As a result, a large increase of extrusion force appears. A Coulomb law is used to model cement-based materials, which is considered as consolidating granular media. Such modelling is compared with experimental results. Tests were carried out on extrudible cement pastes. Modelling and experimental results are in good agreement.Paper presented at the Annual European Rheology Conference, Grenoble, April 2005     

Two-dimensional modeling of an aircraft engine structural bladed disk-casing modal interaction

In modern turbo machines such as aircraft jet engines, structural contacts between the casing and bladed disk may occur through a variety of mechanisms: coincidence of vibration modes, thermal deformation of the casing, rotor imbalance due to design uncertainties to name a few. These nonlinear interactions may result in severe damage to both structures and it is important to understand the physical circumstances under which they occur. In this study, we focus on a modal coincidence during which the vibrations of each structure take the form of a k-nodal diameter traveling wave characteristic of axi-symmetric geometries. A realistic two-dimensional model of the casing and bladed disk is introduced in order to predict the occurrence of this very specific interaction phenomenon versus the rotation speed of the engine. The equations of motion are solved using an explicit time integration scheme in conjunction with the Lagrange multiplier method where friction is accounted for. This model is validated from the comparison with an analytical solution. The numerical results show that the structures may experience different kinds of behaviors (namely damped, sustained and divergent motions) mainly depending on the rotational velocity of the bladed disk.     

New formulations and valid inequalities for a bilevel pricing problem

Consider the problem of maximizing the toll revenue collected on a multi-commodity transportation network. This fits a bilevel framework where a leader sets tolls, while users respond by selecting cheapest paths to their destination. We propose novel formulations of the problem, together with valid inequalities yielding improved algorithms.     

Bilevel programming: A survey

This paper provides an introductory survey of a class of optimization problems known as bilevel programming. We motivate this class through a simple application, and then proceed with the general formulation of bilevel programs. We consider various cases (linear, linear-quadratic, nonlinear), describe their main properties and give an overview of solution approaches.Received: April 2005, Revised: May 2005, AMS classification: 90C05, 90C11, 90C20, 90C27, 90C3D, 65K05     

Hauteur des sous-schémas toriques et dualité de Legendre–Fenchel

We announce a formula for the height of a complete toric variety defined over a number field. The exact expression rests on tools from convex analysis and in particular, on the Legendre–Fenchel dual of the logarithm of the local norms of a natural section of the relevant metrized line bundle. We apply this formula to the computation of the height of projective toric curves and of toric bundles. To cite this article: J.I. Burgos Gil et al., C. R. Acad. Sci. Paris, Ser. I 347 (2009).     

Identification of the friction under high pressure between an aggregate material and steel: Experimental and modelling aspects

The interface behaviour between steel and a quasi-brittle aggregate material is characterised up to normal pressures of the magnitude of 100 MPa. This article presents a new test enabling the behaviour of the interface to be studied whilst retaining the sample’s integrity. The experimental configuration having been retained consists in sliding a cylindrical sample of the material inside a steel tube, said tube acting both as a sliding surface and containment ring. The sample is pushed on one side and faces a spring on the other. The axial compression generates the interface pressure by Poisson effect. This originality of this assembly lies in the simultaneous application of normal pressure to the interface and of its relative motion. The assembly is placed in a quasi-static testing machine. The analysis is made by means of an analytical modelling of the test. This method enables the identification of the initial contact conditions (tightening of the sample), the friction coefficient and its dependence on the pressure. Numerical simulations of the test using a finite element method enables the analytical approach to be validated as well as the set of parameters identified depending on the normal pressure regimes.     

PMMA/Zinc Oxide Nanocomposites Prepared by In‐Situ Bulk Polymerization

Summary: Dispersing surface‐modified zinc oxide nanoparticles (ZnO) in methyl methacrylate (MMA) improves the free radical bulk polymerization process as well as the thermal stability of the formed polymer. Hydroxy groups available on the ZnO surface may induce a degenerative transfer. This suppresses the gel effect, which leads to a better control of the heat evolution during the late stages of polymerization. The formation of chains having vinylidene end groups and head‐to‐head links is suppressed, which shifts the onset of thermal decomposition to the regime where decomposition occurs by random chain scission.

Thermal degradation profiles of PMMA and its composite with ZnO at 11 wt.‐%.     

AFM study of a new carrier based on PLA and salen copolymers for gene therapy

The aim of this study was to synthesize novel biodegradable charged polymers to be used in DNA complexation for genetic delivery in different diseases. A new copolymer of PLA and complexed Schiff bases was synthesized in a several steps. This copolymer will be used as a nanocarrier. Also, AFM comparative studies in tapping mode were performed; on cationic copolymer and on PLA-Schiff base copolymer, on non-oriented and oriented film and on the DNA-cationic complex. The results indicated a difference in the topology and on phase picture of AFM film with or without cationic charge.     

Nonlinear EEG Changes in a 48-Hour Cyclic Manic-Depressive Patient

This research report presents the case study of a 48-hour cyclic manicdepressive patient which further demonstrates the association between nonlinear EEG characteristics and mood variations as previously reported by Thomasson et al. (2000). The evolution of brain dynamics and mood were daily measured during a week. Global complexity of brain electrical activity was estimated by a nonlinear index (entropy) and mood modulations were evaluated by a clinical self-assessment scale (BfS'). Illustrating the concept of dynamical disease, a significant co-variation between the nonlinear EEG index and mood evolution (Spearman correlation coefficient = 0.92, p = .008) was observed. This result strengthens the previous ones and demonstrates a clear association between nonlinear brain dynamics and state of mind in psychopathology.