Showing papers by "Mines ParisTech published in 1999"

Structure and physicochemistry of anodic oxide films on titanium and TA6V alloy

Abstract: Anodization of titanium and its alloys is an important surface treatment, especially for adhesion applications, but is not as well studied as for aluminium alloys. This paper deals with the morphological, structural and physicochemical characterization of anodic oxide films grown on titanium and Ti–6Al–4V (TA6V) in chromic acid solution without (CA) or with (CA/HF) hydrofluoric acid addition. Several investigations methods are used: high-resolution scanning electron microscopy (HR-SEM), reflection high-energy electron diffraction (RHEED), x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), nuclear reaction analysis (NRA) and wetting angle measurements. The occurrence and morphology of the nanoporous structure for CA/HF anodization are described. The compact films grown in CA solution are amorphous and the porous films grown in the CA/HF solution are partially crystalline. The thickness and morphology of the films are described and discussed as a function of the anodizing conditions and of the composition of the underlying substrate. The composition of the film appears to be TiO2+Al2O3 (with Ti/Al atomic ratio ∽5), with incorporation of fluorine from the solution in the porous films and of small quantities of vanadium in the films that are grown. The specific role played by the Cr(VI) and F species on the film growth-and-dissolution formation process is discussed and a growth mechanism is proposed. Copyright © 1999 John Wiley & Sons, Ltd.

Some terms of reference in data fusion

Abstract: This paper discusses the needs for a concept and harmonized terms of reference in data fusion. Previously published definitions are analyzed. A new definition of the data fusion is proposed which has been set within an European working group. Several definitions and terms of reference are given which describe the information intervening in any problem of data fusion.

A multimodal registration algorithm of eye fundus images using vessels detection and Hough transform

Abstract: Image registration is a real challenge because physicians handle many images. Temporal registration is necessary in order to follow the various steps of a disease, whereas multimodal registration allows us to improve the identification of some lesions or to compare pieces of information gathered from different sources. This paper presents an algorithm for temporal and/or multimodal registration of retinal images based on point correspondence. As an example, the algorithm has been applied to the registration of fluorescein images (obtained after a fluorescein dye injection) with green images (green filter of a color image). The vascular tree is first detected in each type of images and bifurcation points are labeled with surrounding vessel orientations. An angle-based invariant is then computed in order to give a probability for two points to match. Then a Bayesian Hough transform is used to sort the transformations with their respective likelihoods. A precise affine estimate is finally computed for most likely transformations. The best transformation is chosen for registration.

A three-dimensional cellular automation-finite element model for the prediction of solidification grain structures

Abstract: A three-dimensional (3-D) model for the prediction of dendritic grain structures formed during solidification is presented. This model is built on the basis of a 3-D cellular automaton (CA) algorithm. The simulation domain is subdivided into a regular lattice of cubic cells. Using physically based rules for the simulation of nucleation and growth phenomena, a state index associated with each cell is switched from zero (liquid state) to a positive value (mushy and solid state) as solidification proceeds. Because these physical phenomena are related to the temperature field, the cell grid is superimposed to a coarser finite element (FE) mesh used for the solution of the heat flow equation. Two coupling modes between the microscopic CA and macroscopic FE calculations have been designed. In a so-called “weak” coupling mode, the temperature of each cell is simply interpolated from the temperature of the FE nodes using a unique solidification path at the macroscopic scale. In a “full” coupling mode, the enthalpy field is also interpolated from the FE nodes to the CA cells and a fraction of solid increment is computed for each mushy cell using a truncated Scheil microsegregation model. These fractions of solid increments are then fed back to the FE nodes in order to update the new temperature field, thus accounting for a more realistic release of the latent heat (i.e., the solidification path is no longer unique). Special dynamic allocation techniques have been designed in order to minimize the computation costs and memory size associated with a very large number of cells (typically 107 to 108). The potentiality of the CAFE model is demonstrated through the predictions of typical grain structures formed during the investment casting and continuous casting processes.

Ni intellectuel engagé, ni intellectuel dégagé: la double stratégie de l'attachement et du détachement

Abstract: 1. Introduction Qu’est-ce que l’engagement du sociologue dans la cite ? La question est redoutable, au moins pour deux raisons. D’abord parce qu’elle tombe au milieu d’un drame mediatique au centre duquel se trouve P. Bourdieu, drame qui remet sur le devant de la scene les intellectuels et leurs roles dans la societe francaise. Ensuite, parce que, dans sa formulation meme, la question nous replonge dans des debats surannes dont je voudrais sortir. Mais comment echapper a l’actualite, comment ...

A Lyapunov approach to control irrigation canals modeled by saint-venant equations

Abstract: This paper deals with the regulation of irrigation canals. We consider the distributed and nonlinear nature of a single reach delimited by two regulator gates and which can be described by the Saint-Venant equations. By means of a Lyapunov approach we propose a class of locally exponentially stabilizing controllers.

Methane hydrate crystallization mechanism from in‐situ particle sizing

Abstract: A new experimental setup that makes possible in-situ determinations of the population density function of the methane hydrate particles during its crystallization in a pressurized reactor is used. Thanks to this equipment, new results can be obtained, in particular concerning the granular aspects of the crystallization processes and the influence of the stirring rate. These results are discussed in the framework of a model including gas absorption, primary and secondary nucleation, crystal growth, agglomeration, and breakage. From this discussion, the relevant processes and parameters of methane hydrate crystallization can be determined and quantified.

Electric versus conventional vehicles: social costs and benefits in france

Abstract: This article compares the social costs of electric vehicles with those of conventional, thermal vehicles for typical passenger use in the Ile-de-France region (Greater Paris), a case of particular interest because nearly 80% of the electricity is generated by nuclear power plants. A four-seat electric car is compared to a new conventional car of the same make and model; for the latter both the gasoline and the diesel version are considered because almost half of new car sales in France are diesel. These results are also compared to typical existing diesel and gasoline vehicles in the current French fleet. The methodology developed by the ExternE (External Costs of Energy) Project of the European Commission is used to estimate the costs associated with atmospheric pollution due to power plants, refineries and tail pipe emissions. Our discussion of externalities is limited to air pollution thus excluding others such as costs associated with noise or accidents. Our results imply that the external costs are large and significant, even when one considers the uncertainties. If internalized by government regulations, these externalities can render the total cost of an electric vehicle more competitive with that of currently available thermal vehicles in large urban centers if the electricity is produced by sources with low pollution. However, the current generation electric vehicles are so expensive that internalization of pollution damage would not give it a very clear advantage.

Estimation of Precipitation by Kriging in the EOF Space of the Sea Level Pressure Field

Abstract: The term downscaling denotes a procedure in which local climatic information is derived from large-scale climate parameters. In this paper, the possibility of using as downscaling procedure a geostatistical interpolation technique known as kriging is explored. The authors present an example of the method by trying to reconstruct monthly winter precipitation in the Iberian Peninsula from the North Atlantic sea level pressure (SLP) field in wintertime (December‐February). The main idea consists in reducing the spatial dimension of the large-scale SLP field by means of empirical orthogonal function (EOF) analysis. Each observed SLP field is represented by a point in this low-dimensional space and this point is associated with the simultaneously observed rainfall. New values of the SLP field, for instance, those simulated by a general circulation model with modified greenhouse gas concentrations, can be represented by a new point in the EOF space. The rainfall amount to be associated to this point is estimated by kriging interpolation in the EOF space. The results obtained by this geostatistical approach are compared to the ones obtained by a simpler analog method by trying to reconstruct the observed rainfall from the SLP field in an independent period. It has been found that, generally, kriging and the analog method reproduce realistically the long-term mean, that kriging is somewhat better than the analog method in reproducing the rainfall evolution, but that, contrary to the analog method, it underestimates the variance because of the well-known smoothing effect. It is argued that there exists an intrinsic incompatibility between the estimation of the mean and replication of the variability. Finally, both methods have been also applied to daily winter rainfall. The methods are also validated by downscaling winter precipitation from SLP. It is concluded that kriging yields a better estimation of daily rainfall than the analog method, but the latter better reproduces the probability distribution of rainfall amounts and of the length of dry periods.

Cathodic hydrogen embrittlement in alloy 718

Abstract: Room temperature cathodic hydrogen embrittlement in alloy 718 was investigated by means of slow strain rate tensile tests conducted on specimens charged either prior to or during deformation. Tensile tests performed on precharged specimens at strain rates of 5×10 −7 , 5×10 −5 and 5×10 −3 s −1 suggest that hydrogen embrittlement is correlated with hydrogen segregation to moving dislocation and transport by these dislocations. Observations of 1 μm planar cleavage microfacets on fracture surfaces of specimens charged either prior to or during deformation support the idea that embrittlement occurs by strong hydrogen–deformation interactions. In light of these results, the role of the cathodic hydrogen produced by the corrosion process inside a crack during stress corrosion cracking is discussed in conjunction with the corrosion enhanced plasticity model, proposed some years ago by one of the authors. It is suggested that hydrogen transport by dislocations and localisation along active slip planes may be the controlling stage of the cracking process.

Estimating the overall properties of heterogeneous Cosserat materials

Abstract: Classical homogenization methods do not account for size effects in the effective properties of polycrystals: they do not predict the influence of grain size nor the width of shear bands in localization phenomena. The polycrystal can be regarded as an example of a heterogeneous Cosserat material. A method is proposed to estimate the effective properties of aggregates of Cosserat media. A validation of the methodology is given in the case of linear Cosserat elasticity. Some steps towards an extension of the self-consistent scheme to Cosserat materials are then presented.

Relationship between job control, work pressure and strain : studies in the USA and in The Netherlands

Abstract: The purpose of this study was to test a model of job control, work pressure and strain in two samples in the USA and in The Netherlands. According to the proposed model, work pressure has a mediating role between job control and worker strain. The present model specifies three types of job control: task or instrumental control, conceptual or resource control, and decision organization control. Task and resource control are assumed to reduce the stressor of work pressure which, in turn, is related to high strain. Organization control is assumed to be positively related to work pressure. The model was tested in two samples : one US sample of 273office employees and one Dutch sample of 958 employees in a variety of jobs. The same measure of job control was used in both samples. Resultspartially confirm the proposed model. As predicted by the proposed model, high task control was related to low work pressure, whereas high organization control was related to high work pressure. The effects of task control and ...

Low temperature nitriding of AISI 316L stainless steel and titanium in a low pressure arc discharge

Abstract: AISI 316L stainless steel (SS) and titanium nitriding were studied in a low pressure arc-assisted nitriding process where the substrate temperature and the plasma parameters are uncoupled. Lower nitriding temperature limits were explored for constant plasma parameters in Ar–N2 gas mixtures and substrates at floating potential. Nitrogen superficial concentration, layer thicknesses and X-ray diffraction analyses were performed on SS specimens nitrided at two temperatures (580 and 680 K) for different times and titanium nitriding was studied in the temperature range 750–1025 K. At low temperature, the nitriding performances are limited by a plasma–surface phenomenon that probably involves recombination of nitrogen atoms.

Recrystallization nucleation mechanism along boundaries in hot deformed Al bicrystals

Abstract: The orientation dependency of strain induced boundary migration (SIBM) has been studied on 99.995% purity aluminium bicrystals deformed in hot plane strain compression. Four bicrystal combinations were chosen to represent some typical grain boundaries in hot rolled aluminium, i.e. boundaries between a grain of S {142} orientation and adjacent grains with cube, Cu {112} , Bs {011} or another S variant orientation. The bicrystals were deformed in a channel die to true strains of ∼1.5 at 300–400°C and then annealed for short periods to allow grain boundary movement, usually by SIBM. The microstructural changes were followed by channelling contrast SEM and EBSD microtexture measurements. The stored energies of the individual grains were estimated from measurements of the average cell size and misorientation using the Read Shockley expression for the sub-boundary energies; typical values range from ∼10 to 50 kJ m−3. SIBM occurs extensively in bicrystals containing boundaries with the 40° orientation relation. Grain boundaries in other bicrystals either scarcely move or, in the case of the S/Bs combination, become nucleation sites for near-cube germs. The boundary mobilities – as deduced from the local driving forces and the migration distances – are compared with previous data from deformed polycrystals.

First direct evidence of size-dependent structural transition in nanosized nickel particles

Abstract: Finely dispersed Ni powders were obtained after chemical reduction of Ni(II) salts by activated sodium hydride in organic solvents. This process allows one to prepare particles in the nanometre range. The particles with sizes smaller than 4nm were found to be hcp whereas larger particles were fcc. To the best of our knowledge, this is the first observation that unsupported Ni powder may crystallize in the hcp system and that the phase stability of this metallic element is particle size dependent.

Morphological Scale-Space Representation with Levelings

Abstract: A morphological scale-space representation is presented based on a morphological strong filter, the levelings. The scale-properties are analysed and illustrated. From one scale to the next, details vanish, but the contours of the remaining objects are preserved sharp and perfectly localised. This paper is followed by a companion paper on pde formulations of levelings.

Compressive creep of ice containing a liquid intergranular phase: Rate-controlling processes in the dislocation creep regime

Abstract: Experiments have been conducted to investigate the effect of melt on the creep behavior of polycrystalline ice deformed in the dislocation creep regime. The transition between a mode with a stress exponent n = 3 and a mode with n < 2 is observed for both melt-free and melt-added ice samples. The large influence of the melt phase is not related to the wetting characteristics of the liquid. Owing to the large plastic anisotropy of the ice crystal, the liquid phase would attenuate the internal stress field which develops during the primary creep. The contribution to the deformation of the basal slip (the weaker slip system) would increase with the melt content.