Une nouvelle stratégie de calcul micro/macro en mécanique des structures

A new micro-macro computational strategy is proposed for the analysis of structures which are described up to the “micro” level, such as composite structures. It is intended to replace standard homogenization techniques coupled with a local reanalysis, at least for several domains of interest. This paper describes the basis of the new approach.     

Modification de la tenue mécanique d'un dépôt type Si — C sur acier par incorporation d'une intercouche métallique

SiC based coatings were deposited on a steel substrate by PACVD from TMS-Ar mixtures. The influence of a metallic interlayer between deposit and substrate on the mechanical stability has been studied through interface analysis and several mechanical tests (scratch, indentation, microbending in a SEM). The deposit adhesion is enhanced by a tantalum interlayer and is a function of its thickness. Micro-bending tests allowed to characterize the damage evolution and to obtain experimental parameters linked with the intrinsic properties of the coatings.     

Caractérisation mécanique par flexion,nano- et macro-indentation de revêtements de SiC par PACVD micro-onde sur acier

Various SiC coatings were deposited on a steel substrate in a micro-wave plasma activated CVD device, then tested by a depth-sensing indentation instrument for measurement of their hardness and of their Young's modulus. Tests by three-point bending and by Vickers and Rockwell C indentation yielded consistent results about their rupture and adherence properties. The mechanical origin of the damage patterns induced by indentation is discussed on the basis of a numerical simulation of the testing.     

Prévision des courbes de dosage par potentiométrie bimétallique au moyen des courbes de polarisation

The knowledge of polarisation curves permits the interpretation and prediction of the form of the curves obtained by bimetallic potentiometric determination.     

Résolution du problème à deux électrons en mécanique quantique par détermination directe des orbitales naturelles

Résumé La fonction d'onde et l'énergie de l'état fondamental de l'atome d'hélium et des ions isoélectroniques sont calculées selon un procédé proposé antérieurement [10]. Les équations intégrodifférentielles relatives aux orbitales naturelles (NO) sont résolues approximativement dans une représentation matricielle utilisant les fonctions complètes de Laguerre comme base, ainsi qu'avec de simples approximations analytiques des NO. On réussit à tenir compte de 95% environ de l'énergie de corrélation. La convergence du développement naturel est telle que les NO dont le nombre quantique principal est plus petit ou égal à 4 sont les seules à apporter une contribution appreciable à l'énergie. Les contributions de l'énergie cinétique et des énergies potentielles, d'attraction nucléaire et de répulsion interélectronique à l'énergie de corrélation sont examinées. Une certaine simplification du potentiel de corrélation aboutit à une justification du principe des intégrales interélectroniques réduites.

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The wave function and the energy of the ground state of the helium atom and the corresponding isoelectronic ions are calculated by a procedure recently proposed by the author [10]. The integro-differential equations of the natural orbitals (NO) are resolved approximatively in a matrix representation using the complete Laguerre functions as its basis as well as in a simple analytical approximation for the NO. The natural expansion converges so rapidly that only those NO whose principal quantum number is 4 or less contribute significantly to the energy. This method succeeds in accounting for some 95% of the correlation energy. The kinetic, potential and interelectronic energy contributions to the correlation energy have been examined separately. By simplifying the correlation potential in a certain manner the principle of reduced electron interaction integrals is justified.

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Zusammenfassung Die Wellenfunktion und die Energie des Grundzustandes des Heliumatoms und der mit ihm isoelektronischen Ionen wird gemäß einer vom Verfasser vorgeschlagenen Methode [10] berechnet. Dazu löst man die Integrodifferentialgleichungen für die natürlichen Einelektronenfunktionen (NO) näherungsweise in einer Matrixdarstellung unter Benutzung der vollständigen Laguerre-Funktionen als Basis. In dieser Näherung erfaßt man etwa 95% der Korrelationsenergie. Die natürliche Entwicklung konvergiert so gut, daß nur die NO mit der Hauptquantenzahl kleiner oder gleich 4 wesentlich zur Energie beitragen. Die Anteile der kinetischen Energie, der potentiellen Einelektronenenergie und der Elektronenwechselwirkungsenergie an der Korrelationsenergie werden untersucht. Ausgehend von einer bestimmten Näherung für das Korrelationspotential läßt sich die Methode der reduzierten Wechselwirkungsintegrale rechtfertigen.

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L'auteur a bénéficié d'une bourse de recherches de l'OTAN qui lui a été accordée par l'intermédiaire de «Deutscher Akademischer Austauschdienst». M. Gaston Berthier, Maitre de recherches au CNRS a pris un grand intérêt à ce travail et y a beaucoup contribué. Qu'il soit remercié cordialement.     

Modélisation du comportement mécanique d'un alliage base nickel par une approche thermodynamique de la relaxation des milieux continus

The high-temperature behaviour of a Nickel alloy submitted to uniaxial tension, relaxation and creep, is analysed using a thermodynamics approach to non-linear relaxations. The model is essentially based on a distribution of the dissipation processes obtained by the theory of fluctuations. The agreement between experiment and simulations requires the use of four parameters. These parameters depend on the elastic properties of the alloy and also on the thermal activation necessary for mobility defects. We present a comparison with the classical model of Chaboche including a recovery term in the potential of dissipation.     

La mécanique des fluides numérique outil de recherche,outil industriel ?

With the help of a few examples in the field of nuelear engineering, we show that computational fluid dynamics is both a reasearch and an industrial tool.     

Préparation et structure des germanates métalliques. I

Résumé L'auteur reprend et complète la méthode de dosage microgravimétrique du germanium à l'état de germanimolybdate de tétraphénylarsonium (F _Ge=0,02137). Dans une deuxième partie, il décrit une méthode de préparation des heptagermanates alcalins et de métagermanates de métaux bivalents à partir des solutions d'oxyde GeO₂ et d'acétates métalliques. Les courbes de thermolyse et les spectres d'absorption infrarouge sont présentés. Aucun orthogermanate ne précipite des solutions aqueuses.

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Summary The author resumes and completes the method for the microgravimetric determination of germanium as the germanium molybdate of tetraphenylarsonium (F _Ge=0.02137). In a second part he describes a method for preparing some heptagermanates of divalent metals from solutions of the oxide GeO₂ and metal acetates. The thermolysis curves and the infrared absorption curves are given. No orthogermanate precipitates from aqueous solutions.

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Zusammenfassung Der Verfasser berichtet über die mikrogravimetrische Bestimmung des Germaniums als Tetraphenylarsonium-Germanium-Molybdat (F _Ge=0,02137). Im zweiten Teil der Arbeit wird die Herstellung von Alkaliheptagermanat sowie von Metagermanaten bivalenter Metalle aus Germaniumoxidlösungen und Metallacetaten beschrieben. Die Thermolysekurven und IR-Absorptionsspektren werden vorgelegt. Aus wäßrigen Lösungen fällt kein Orthogermanat aus.

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Mémoire en l'honneur du professeurA. A. Benedetti-Pichler.     

Comportement mécanique de l'acier inoxydable martensitique 17-4 PH en corrosion sous contrainte et à la fragilisation par l'hydrogéne environnemental

Age hardened martensitic stainless steels have high resistances to mechanical stress, to friction corrosion and to stress-corrosion cracking (after certain ageing heat treatments). These steels have also high mechanical toughness levels. The mechanical strength of these steels increases when performing specific heat treatments in order to promote the precipitation of hardening phases such as copper-rich ε and/or chromiumrich α′. However, it is well known that the susceptibility to stress-corrosion cracking and to hydrogen embrittlement increases when the mechanical strength of the martensitic steels is high.This paper is devoted to a study the mechanical behaviour of the 17-4 PH martensitic stainless steel with respect to stress-corrosion cracking and to embrittlement by environnemental hydrogen in different ageing conditions, i.e. ageing temperatures (200 to 650 °C) and ageing times (1 and 4 hours). The two behaviours were studied by carrying out low strain rate tensile tests (ε′ = 2.7×10⁻⁶ s⁻¹) in H₂SO₄ 1N. By using differential scanning microcalorimetry, we have identified the precipitation mechanisms of ε and α′ phases, and calculated their activation energies by applying KISSINGER'S relationship.The results obtained show that the susceptibility to stress-corrosion cracking and to hydrogen embrittlement is maximum at the optimum ageing temperature for which the strength level is maximum. The optimum ageing temperature increases when the ε phase, initially coherent, becomes non-coherent. The fracture mechanism changes from cleavage to intergranular mode, respectively. Finally, the fracture mechanism by stress-corrosion cracking is the same as that provoked by hydrogen embrittlement.     

Synthèse régiosélective par voie organométallique de pyridines, 4-picolines et 3,5-lutidines substituées en 2 par un groupe insaturé et/ou fonctionnel

A regioselective one-pot synthesis of 2-substituted pyridine derivatives from N-alkoxycarboxylpyridinium salts and α-unsaturated organozinc compounds is described. Similarly, functional alkynyl organomagnesium compounds lead to 2-alkynyl-ω-functional pyridines, from which (E)-2-alkenyl or 2-alkyl-ω-functio pyridines can be obtained by partial or complete reduction.     

Développement et validation d'un modèle mécanique de la tête humaine

This paper presents the development and the validation of a 3D finite-element human head model intended to better understand the head injury mechanisms in case of head impact. A very realistic skull geometry has been obtained by digitalisation of a dry adult skull. Based on the established knowledge of bone mechanical properties, an elastic brittle law was attributed to the skull simulated by composite shell elements. The dynamical model response was compared with two experimental impact tests from the bibliography concerning epidural pressure and a third impact involving skull fracture. The results of the model validation appear promising and the model could be a powerful tool to predict the aggressiveness level of a head impact.     

Calcul de chiralité quantitative par la méthode des moindres carrés

It is shown how the root mean square chiral index is computed without enumerating all the permutations of a set of n atoms. As an example, the quantitative measure of chirality is achieved for various helicene derivatives. It is also shown that the chiral index of a protein backbone is computable without enumeration.     

Dosage du chrome,du vanadium et du molybdène par la méthode des indicateurs radioactifs

A radiochemical method for the determination of Cr, Mo, and V by means of radioactive silver is described. Using a solution of radioactive silver nitrate, a standard curve is obtained indicating the activity of the precipitate as a function of the quantity of the metal present. Unknown concentrations of Cr, Mo, or V can be easily determined if the same experimental and geometrical conditions are reproduced.     

Étude mécanistique et structurale des résines R/F

The follow-up of the advance of the reaction of polymerization of resorcinol with formaldehyde in acid or basic medium by NMR shows that the structure of the obtained xerogel in both cases is the same one. Indeed, the junction between the aromatic nuclei is of type OCH₂OCH₂. While basing itself on the structure of the xerogel, we propose the mechanism for the polymerization reaction between resorcinol and formaldehyde.     

Interprétation des spectres d'absorption infrarouges de phosphites et d'hypophosphites métalliques

Résumé L'étude de 15 hypophosphites et de 6 phosphites métalliques en poudre, entre 300 et 3700 cm^–1 permet une interprétation des spectres infrarouges avec la considération des structures moléculaires et la comparaison avec les spectres Raman des solutions. Relativement à ce dernier point, les spectres infrarouges apparaissent beaucoup plus complexes.

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Summary The study of 15 hypophosphites and 6 metal phosphites in the form of powders, between 300 and 3700 cm^–1, allows an interpretation of the infra red spectra with consideration of the molecular structures and a comparison with the Raman spectra of the solutions. With respect to the latter point, the infra red spectra seem much more complex.

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Zusammenfassung Die Untersuchung von15 Metallhypophosphiten und 6 Metallphosphiten in Pulverform in dem Bereich zwischen 300 und 3700 cm^–1 ermöglicht eine Interpretation der Infrarotspektren in Bezug auf ihre Molekülstruktur und deren Vergleich mit den Ramanspektren ihrer Lösungen. Dieser zeigt, daß die Infrarotspektren sehr viel komplexerer Natur sind.

     

Étude par diffraction des rayons X,par résonance magnétique nucléaire 31P en phase solide et par spectrométrie infrarouge des hydroxyapatites mixtes cadmium–strontium

X-ray powder diffraction, solid-state ³¹P-MAS-NMR and IR spectroscopy of cadmium–strontium mixed hydroxyapatites. Strontium–cadmium hydroxyapatite solid solutions Cd_xSr_(10–x)(PO₄)₆(OH)₂ were prepared in aqueous medium. They were characterized by X-ray powder diffraction, ³¹P-MAS-NMR and IR spectroscopic analyses. The structural refinement, by X-ray powder pattern fitting (Rietveld method), indicates the limits of strontium–cadmium solid solutions (x≤4) and allows us to determine the cadmium repartition in the apatite cell. Through the progressive replacement of Sr²⁺ (r=1.32 Å) by the smaller cation Cd²⁺ (r=1.09 Å), all interatomic distances decrease according to the decrease of the cell parameters. However, contrary to what could be expected from the coordinance of the metallic sites S₁ (hexacoordination) and S₂ (heptacoordination) the small cadmium cation does not preferentially occupy the four sites S₁: it is almost statistically distributed over both sites, with a slight preference for S₂. The ³¹P- solid-state-NMR of the pure phases Sr₁₀(PO₄)₆(OH)₂ and Cd₁₀(PO₄)₆(OH)₂ reveals a moderate chemical shift anisotropy (≈20 ppm) corresponding to slight distortion of the phosphate tetrahedron. This observation holds also for the mixed species where the surrounding of the phosphate anion is expected to change significantly when substituting Cd for Sr. Through the progressive replacement of strontium by cadmium, the isotropic signal moves regularly to high frequency, according to the deshielding observed for Cd₁₀(PO₄)₆(OH)₂ (δ_iso=12.2 ppm) with respect to Sr₁₀(PO₄)₆(OH)₂ (δ_iso=2.9 ppm). Moreover the isotropic signal broadens considerably because of unresolved chemical shift distributions in the highly disordered material. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SAShydroxyapatite / Rietveld refinement / ³¹P-MAS-NMR / cadmium / strontium / phosphate     

Diffraction d'une onde électromagnétique plane par un biréseau métallique plan formé de cavités hémisphéroïdales

The diffraction of an electromagnetic plane wave from a bigrating of prolate spheroid cavities hollowed in a perfectly conducting metallic plate is studied using an exact differential analytic method. The theoretical treatment is carried out for any incidence and direction of polarization, but first, numerical results are given for normal incidence in view of comparison with previous results obtained in same conditions from a bigrating of hemispherical cavities. So, original efficiency curves ɛ_ff (0, 0) (or normal reflectance) versus the size of cavities are given.