1,721,308 research outputs found
A class of auxetic three-dimensional lattices
No full textWe propose a class of auxetic three-dimensional lattice structures. The elastic microstructure can be designed to have an omnidirectional Poisson's ratio arbitrarily close to the stability limit of -1. The cubic behaviour of the periodic system has been fully characterized; the minimum and maximum Poisson's ratio and the associated principal directions are given as a function of the microstructural parameters. The initial microstructure is then modified into a body-centred cubic system that can achieve Poisson's ratio lower than -1 and that can also behave as an isotropic three-dimensional auxetic structure
Auxetic two-dimensional lattices with Poisson's ratio arbitrarily close to - 1
No full textIn this paper, we propose a class of lattice structures with macroscopic Poisson's ratio arbitrarily close to the stability limit-1. We tested experimentally the effective Poisson's ratio of the microstructured medium; the uniaxial test was performed on a thermoplastic lattice produced with a three-dimensional printing technology. A theoretical analysis of the effective properties was performed, and the expression of the macroscopic constitutive properties is given in full analytical form as a function of the constitutive properties of the elements of the lattice and on the geometry of the microstructure. The analysis was performed on three microgeometries leading to an isotropic behaviour for the cases of three-and sixfold symmetries and to a cubic behaviour for the case of fourfold symmetry
Control of elastic shear waves by periodic geometric transformation: cloaking, high reflectivity and anomalous resonances
Full text linkA doubly periodic geometric transformation is applied to the problem of out of plane shear wave propagation in a doubly periodic perforated elastic medium. The technique leads to the design of a system of radially anisotropic and inhomogeneous shells surrounding the void inclusions, that can be tuned to give the desired filtering properties. For a regular transformation, the transformed elastic system displays the same dispersion properties than the original homogeneous one, but for overlapping and unfolding transformations new filtering properties can be obtained, which include anomalous resonances at zero and finite frequencies. Low-frequency homogenisation reveals how it is possible to tune the phase and group velocity in the long-wave limit at any value, or to obtain a zero frequency band gap for Neumann boundary conditions. The dispersion properties of the medium are studied both semi-analytically by the multipole expansion and numerically by the finite element methods. Several applications are shown, including the transmission problem throughout a grating of void inclusions and an interface in a waveguide, where the capability of the proposed model is quantitatively demonstrated by computing the transmitted power flow. Finally, we gave a demonstration of defect modes in a waveguide and of tuning the transformation for the research of Dirac points
Invariance of eigenfrequencies under geometric transformation in plate structures
No full textGeometric transformation is applied here to the problem of flexural waves in thin plates. Differently from problems governed by the Helmholtz equation, the equation of motion does not retain its form. A physical interpretation is given which involves a non homogenous anisotropic plate with axial stresses. Boundary conditions are not satisfied anymore after transformation, but some possible constraints can be applied on the transformation law in order to cancel differences at the boundary of the transformed domain. As a comparison tool, we propose the eigenfrequency analysis of rectangular plates and we show the correctness of the proposed approach for different transformations and boundary conditions
Effective properties of a new auxetic triangular lattice: An analytical approach
No full textIn this article we propose a new auxetic periodic lattice with negative Poisson's ratio which tends to the limit v=-1 under particular conditions. We have studied its generation and kinematic, and we give a full description of the mechanical properties of this innovative model. Calibrating the geometrical configuration of the lattice and the mechanical properties of the constituent material we are able to have a Poisson's ratio which is arbitrarily close to -1
Dynamic characterization of a periodic microstructured flexural system with rotational inertia
Full text linkWe consider the propagation of waves in a flexural medium composed of massless beams joining a periodic array of elements, elastically supported and possessing mass and rotational inertia. The dispersion properties of the system are determined and the influence and interplay between the dynamic parameters on the structure of the pass and stop bands are analysed in detail. We highlight the existence of three special dynamic regimes corresponding to a low stiffness in the supports and/or low rotational inertia of the masses; to a high stiffness and/or high rotational inertia regime; and to a transition one where dispersion degeneracies are encountered. In the low-frequency regime, a rigorous asymptotic analysis shows that the structure approximates a continuous Rayleigh beam on an elastic foundation.This article is part of the theme issue 'Modelling of dynamic phenomena and localization in structured media (part 1)'
James H. Cone, Teologia nera de la liberazione e black power, édité par Editrice Claudiana, Torino, 1973, avec introduction de Franco Giampiccoli et traduction en italien de Bruno et Mirella Corsani,
No full textBrun M. A. James H. Cone, Teologia nera de la liberazione e black power, édité par Editrice Claudiana, Torino, 1973, avec introduction de Franco Giampiccoli et traduction en italien de Bruno et Mirella Corsani,. In: Revue d'histoire et de philosophie religieuses, 54e année n°4,1974. pp. 581-583
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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