Etude viscosimétrique des effets structuraux du cation calcium sur des solutions contenant l'anion alginate

Résumé Le comportement en viscosité sous différentes vitesses de cisaillement, à différentes températures, de solutions d'alginate de sodium à des concentrations variant entre 0,05% et 1%, auxquelles on a ajouté du chlorure de calcium en quantité variable, est qualitativement toujours le même; pour des quantités croissantes de chlorure de calcium, à concentration fixe en alginate de sodium, à une phase de dépression de la viscosité succède une phase de croissance très rapide, suivie à nouveau d'une phase de décroissance. Dans la première phase les variations de la viscositè avec la vitesse de cisaillement sont représentables par un modèle répondant à la Théorie d'Eyring; dans les phases suivantes l'écoulement est représenté par une loi du type =a^b avec b<0. Cette différence dans le mode d'écoulement tiendrait à l'apparition, à partir d'une concentration en chlorure de calcium suffisante, d'une structure tridimensionelle plus ou moins stable suivant la concentration en ions calcium.Discours prononcé lors du congrès annuel des rhéologues allemands à Berlin le 9 juin, 1966.     

Resolution numérique du problème des surfaces minima

Sans résumé     

Flexure-based micromechanical testing machines

The successful design and fabrication of structures and systems at the small scale require robust methods for characterizing the mechanical behavior of materials at the same scale. In this paper we describe the design of two flexure-based micromechanical testers capable of measuring forces with an accuracy of 25 μN over a range of 1–30 N, and specimen extensions with an accuracy of 20 nm over a range of 1–5 mm. These force and displacement resolutions and ranges are required in a wide variety of material characterization applications, such as microtensile testing of micrometer-dimensioned films, foils and wires, bending of millimeter-sized beams, as well as micro-indentation. The novel feature of our machines is that they are based on the use of two compound flexures in an integrated monolithic frame: one flexure functioning as a precision guide for actuation, and the other fexure as a linear spring for force measurement. Two machines, one with a maximum load capacity of 1.5 N and the other of 30 N, have been constructed based on this concept. Details of their design, construction, and typical test results are presented in this paper.     

La dynamique des tremblements de terre vue à travers le séisme de Landers du 28 juin 1992

We study the influence of initial conditions and of friction laws on the propagation of dynamic rupture during the earthquake of 28 June 1992 in Landers, California. We model this earthquake solving the elastodynamic wave equation by a finite difference method and we model observed accelerograms in order to get a better knowledge of the dynamic rupture process of this earthquake. In our model rupture propagates spontaneously under the simultaneous control of the initial stress field and friction. We model friction by a simple slip-weakening law. Finally, we inverted the initial stress field and the friction law from the radiation produced by 1992 Landers earthquake using a trial-and-error method. The dynamic model obtained by trial-and-error inversion provides a very satisfactory fit between synthetics and strong motion data. Rupture history and duration of the Landers earthquake are in good agreement with previous kinematic inversion results, without introducing major changes in final slip distribution on the fault. The solution of the dynamic inverse problem is non-unique because this problem is intrinsically ill-posed. Two complementary mechanical models were inverted in order to model the Landers earthquake, and to reproduce the seismic data. The first model corresponds to the asperity model in which only initial stress distribution is heterogeneous. The second model is a barrier model in which the initial stress was perfectly uniform while rupture resistance was heterogeneous. To cite this article: S. Peyrat et al., C. R. Mecanique 330 (2002) 235–248.     

Computational dynamics of soft machines

Soft machine refers to a kind of mechanical system made of soft materials to complete sophisticated mis-sions, such as handling a fragile object and crawling along a narrow tunnel corner, under low cost control and actuation. Hence, soft machines have raised great challenges to compu-tational dynamics. In this review article, recent studies of the authors on the dynamic modeling, numerical simulation, and experimental validation of soft machines are summarized in the framework of multibody system dynamics. The dynamic modeling approaches are presented first for the geometric nonlinearities of coupled overall motions and large deforma-tions of a soft component, the physical nonlinearities of a soft component made of hyperelastic or elastoplastic mate-rials, and the frictional contacts/impacts of soft components, respectively. Then the computation approach is outlined for the dynamic simulation of soft machines governed by a set of differential-algebraic equations of very high dimensions, with an emphasis on the efficient computations of the non-linear elastic force vector of finite elements. The validations of the proposed approaches are given via three case stud-ies, including the locomotion of a soft quadrupedal robot, the spinning deployment of a solar sail of a spacecraft, and the deployment of a mesh reflector of a satellite antenna, as well as the corresponding experimental studies. Finally, some remarks are made for future studies.