Force chains and contact network topology in packings of elongated particles

By means of contact dynamic simulations, we investigate the contact network topology and force chains in two-dimensional packings of elongated particles modeled by rounded-cap rectangles. The morphology of large packings of elongated particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and orientations of contacts between particles. We show that particle elongation affects force distributions and force/fabric anisotropy via various local structures allowed by steric exclusions and the requirement of force balance. As a result, the force distributions become increasingly broader as particles become more elongated. Interestingly, the weak force network transforms from a passive stabilizing agent with respect to strong force chains to an active force-transmitting network for the whole system. The strongest force chains are carried by side/side contacts oriented along the principal stress direction.Comment: Soumis a Physical Review

Stress-strain behavior and geometrical properties of packings of elongated particles

We present a numerical analysis of the effect of particle elongation on the quasistatic behavior of sheared granular media by means of the Contact Dynamics method. The particle shapes are rounded-cap rectangles characterized by their elongation. The macroscopic and microstructural properties of several packings subjected to biaxial compression are analyzed as a function of particle elongation. We find that the shear strength is an increasing linear function of elongation. Performing an additive decomposition of the stress tensor based on a harmonic approximation of the angular dependence of branch vectors, contact normals and forces, we show that the increasing mobilization of friction force and the associated anisotropy are key effects of particle elongation. These effects are correlated with partial nematic ordering of the particles which tend to be oriented perpendicular to the major principal stress direction and form side-to-side contacts. However, the force transmission is found to be mainly guided by cap-to-side contacts, which represent the largest fraction of contacts for the most elongated particles. Another interesting finding is that, in contrast to shear strength, the solid fraction first increases with particle elongation, but declines as the particles become more elongated. It is also remarkable that the coordination number does not follow this trend so that the packings of more elongated particles are looser but more strongly connected.Comment: Submited to Physical Review

Force transmission in a packing of pentagonal particles

We perform a detailed analysis of the contact force network in a dense confined packing of pentagonal particles simulated by means of the contact dynamics method. The effect of particle shape is evidenced by comparing the data from pentagon packing and from a packing with identical characteristics except for the circular shape of the particles. A counterintuitive finding of this work is that, under steady shearing, the pentagon packing develops a lower structural anisotropy than the disk packing. We show that this weakness is compensated by a higher force anisotropy, leading to enhanced shear strength of the pentagon packing. We revisit "strong" and "weak" force networks in the pentagon packing, but our simulation data provide also evidence for a large class of "very weak" forces carried mainly by vertex-to-edge contacts. The strong force chains are mostly composed of edge-to-edge contacts with a marked zig-zag aspect and a decreasing exponential probability distribution as in a disk packing

L’appropriation des technologies de l’Internet et ses facteurs critiques de succès : Un défi de plus pour les PME ?

Divisée en deux volets, une étude exploratoire a été réalisée dans quatre PME de la région de la Mauricie au Québec. Premièrement, à l’aide d’une représentation conceptuelle basée sur les construits de compétences de l’utilisateur (Munro et al., 1997), de sentiment d’efficacité personnelle eu égard aux technologies (Compeau, Higgins et Huff, 1999 ; Compeau et Higgins, 1995) et d’absorption cognitive (Agarwal et Karahanna, 2000), une collecte de données auprès de 30 utilisateurs d’applications de l’Internet a permis d’évaluer leur utilisation et leur appropriation en contexte de PME. Dans cette perspective de l’appropriation technologique, les résultats démontrent que les compétences et le sentiment d’efficacité personnelle par rapport à ces applications peuvent jouer un rôle important, tout en étant liés de façon significative.Deuxièmement, la réalisation d’entrevues semi-dirigées auprès des gestionnaires responsables des systèmes et des technologies de l’information a permis une analyse qualitative des facteurs contribuant ou non à encourager l’utilisation et l’appropriation des applications de l’Internet dans les PME. Il a alors été constaté que l’importance des mesures de sécurité, la consultation des futurs utilisateurs, la formulation et la diffusion d’objectifs clairs, le soutien et l’encouragement à l’utilisation de la part de l’équipe dirigeante, la disponibilité des ressources ainsi que les mécanismes de gestion des problèmes et des crises survenant inévitablement sont autant de facteurs ne trouvant pas nécessairement écho auprès des gestionnaires de PME. Ces derniers sont autant d’éléments qui risquent d’affecter le degré d’appropriation technologique des utilisateurs, et ce, de plusieurs manières. Notamment, par leurs effets, directs ou indirects, sur le niveau de compétences des utilisateurs et leur sentiment d’efficacité personnelle eu égard aux technologies.Divided into two sections, an exploratory study was carried out in four SMEs of the area of Mauricie in Quebec. Initially, using a conceptual framework based on three concepts : user competence (Munro et al., 1997), computer self-efficacy (Compeau, Higgins et Huff, 1999 ; Compeau and Higgins, 1995) and cognitive absorption (Agarwal and Karahanna, 2000), a data acquisition on 30 end-users of the Internet made it possible to evaluate their use and their appropriation in context of SME. From this point of view of the technological appropriation, the results show that competences and computer self-efficacy can play an important role, while being significantly dependant.Secondly, the realization of semi-directed interviews on the managers responsible for the systems and information technologies, allowed a qualitative analysis of the factors contributing or not to encourage the use and the appropriation of the Internet in SME. It was then noted that the importance of the security measures, the consultation of the future users, the formulation and the diffusion of clear objectives, the support and the encouragement with the use made on behalf of the executive team, the availability of resources as well as the mechanisms of management of problems and crisis inevitably occurring, are as many factors that are not necessarily taken in care by the managers of SME. These are as many elements which are likely to affect the technological level of appropriation of the users, and this by several manners. In particular, by their effects, direct or indirect, on the qualification level of the end-users and their computer self-efficacy.Un estudio exploratorio, dividido en dos apartados, ha sido realizado en cuatro PyMEs de la región de Mauricia en Quebec. En primer lugar, una colecta de datos a treinta usuarios de aplicaciones de Internet, evaluó su uso y su apropiación en el contexto de la PyME mediante una representación conceptual basada en la construcción de competencias por el usuario (Munro et al., 1997), en el sentimiento de eficacia personal frente a las tecnologías (Compeau, Higgins et Huff, 1999 ; Compeau e Higgins, 1995) y en la absorción cognoscitiva (Agarwal y Karahanna, 2000). En esta perspectiva de la apropiación tecnológica, los resultados demuestran que las competencias y el sentimiento de eficacia personal frente a estas aplicaciones pueden desempeñar un papel importante, estando relacionados de manera significativa.En segundo lugar, la realización de entrevistas semi-directivas a los responsables de los sistemas y tecnologías de la información, permitió efectuar un análisis cualitativo de los factores que contribuyen, o no, a motivar la utilización y la apropiación de las aplicaciones de Internet en las PyMEs. De esta manera, se comprobó que la importancia de las medidas de seguridad, la consulta de los futuros usuarios, la formulación y la difusión de objetivos claros, el apoyo y estímulo para su uso por parte del equipo dirigente, la disponibilidad de los recursos así como los mecanismos de gestión de los problemas y de las crisis que sobrevienen inevitablemente, son factores que no son necesariamente atendidos por los gerentes de la PyME. Sin embargo, estos últimos elementos pueden afectar el nivel de apropiación tecnológica de los usuarios de diversas maneras. En particular, por sus efectos, directos o indirectos, sobre el nivel de competencias de los usuarios y su sentimiento de eficacia personal frente a las tecnologías.In vier kleinen und mittleren Unternehmen der Region Mauricie in Québec wurde eine zweistufige explorative Studie realisiert. Die begriffliche Darstellung basiert auf den Konzepten der Nutzer-Kompetenzen (Munro et al., 1997), der Effizienz-Empfindung von Personen gegenüber Technologien (Compeau et al., 1999 ; Compeau et Higgins, 1995) und dem kognitiven Aufnahmevermögen (Agarwal et Karahanna, 2000). Diese Konzepte erlaubten in einer ersten Phase, eine Erhebung bei dreissig Nutzern von Internet-Applikationen durchzuführen und erste Auswertungen bezüglich der Anwendung und Aneignung im KMU-Kontext durchzuführen. Die Resultate belegen, dass aus der Perspektive der technologischen Aneignung, die Kompetenzen und das persönliche Effizienz-Empfinden eine wichtige Rolle spielen können und auch signifikante Beziehungen untereinander bestehen.In einer zweiten Phase wurde mittels semi-strukturierten Interviews bei Verantwortlichen von Informations-Systemen und-Technologien eine qualitative Analyse durchgeführt. Dabei wurden die Faktoren untersucht, die zur (Nicht)-Begünstigung der Anwendung und Aneignung von Internetapplikationen in KMU führen können. Es wurde festgestellt, dass folgende Punkte nicht unbedingt ausreichend Aufmerksamkeit von den KMU-Verantwortlichen erfahren : Wichtigkeit der Sicherheitsmassnahmen, Hinzuziehung der zukünftigen Anwender, die Formulierung und Bekanntmachung klarer Ziele, die Unterstützung und Förderung zur Nutzung von Anwendungen von Seite der Unternehmensführung, die Verfügbarkeit von Ressourcen sowie die Mechanismen zur Bewältigung von Problemen und unerwarteten Krisen. Die Führungsverantwortlichen sind es auch, die das Niveau der technologischen Anwendung der Nutzer beeinflussen. Namentlich durch direkte und indirekte Effekte beeinflussen sie das Kompetenzniveau der Anwender und ihr persönliches Effizienz-Empfinden gegenüber Technologien

Resonant sneutrino production at Tevatron Run II

We consider the single chargino production at Tevatron $p \bar p \to \tilde \nu_i \to \tilde \chi^{\pm}_1 l_i^{\mp}$ as induced by the resonant sneutrino production via a dominant \RPV coupling of type \l'_{ijk} L_i Q_j D_k^c. Within a supergravity model, we study the three leptons final state. The comparison with the expected background demonstrate that this signature allows to extend the sensitivity on the \susyq mass spectrum beyond the present LEP limits and to probe the relevant \RPV coupling down to values one order of magnitude smaller than the most stringent low energy indirect bounds. The trilepton signal offers also the opportunity to reconstruct the neutralino mass in a model independent way with good accuracy.Comment: 4 page

An Implicit Tensor-Mass solver on the GPU for soft bodies simulation

International audienceThe realistic and interactive simulation of deformable objects has become a challenge in Computer Graphics. In this paper, we propose a GPU implementation of the resolution of the mechanical equations, using a semi-implicit as well as an implicit integration scheme. At the contrary of the classical FEM approach, forces are directly computed at each node of the discretized objects, using the evaluation of the strain energy density of the elements. This approach allows to mix several mechanical behaviors in the same object. Results show a notable speedup of 30, especially in the case of complex scenes. Running times shows that this efficient implementation may contribute to make this model more popular for soft bodies simulations