1,720,978 research outputs found
DEM numerical tests on dry granular specimens: the role of strain rate under evolving/unsteady conditions
No full textIn this paper, the mechanical behaviour of an ideal dry granular material under both evolving and steady conditions has been studied. Triaxial loading both constant volume and constant pressure Discrete Element (DEM) tests on periodic cells have been performed. The role played by strain rate, void ratio and imposed pressure has been analysed. The aim of the paper is to obtain a numerical data set, at present totally absent in the literature, useful for the definition of constitutive relationships capable of reproducing phase transitions (from solid-to-fluid and vice versa) taking place in granular materials. The novelty of the data set obtained derives also from the type of loading tests performed, referred as heating, cooling and cyclic, in which different strain rate histories are imposed, inspired to what locally occurs in flowing granular masses, accelerating and decelerating according to the slope inclination. The numerical results are interpreted by considering state variables (granular temperature and fabric) and the energies evolution (elastic and kinetic fluctuating) to put in evidence phase transition processes. Again, this interpretation of the results is fundamental to conceive upscaled constitutive relationships based on thermodynamic theories. The numerical results have allowed to highlight the role of the strain acceleration/deceleration, and the evolution of the directional properties of the microstructure even in the collisional regime (never described up to now). Cooling constant volume tests results provide a description, not available in the literature, of the evolving response of the material under pure collisional conditions
Three-Dimensional Constitutive Model for Dry Granular Materials Under Different Flow Regimes
No full textThe numerical simulations of granular materials, in the framework of continuum mechanics, is quite challenging since the constitutive model should be capable of reproducing the transition from solid- to fluid like regimes and vice versa. In this paper a constitutive model, valid under general three dimensional evolving conditions, and capable of describing the material response under both quasi-static and dynamic regimes is presented. The model is calibrated by employing a series of true triaxial DEM numerical simulations performed on a periodic cell. The comparison between model predictions and DEM results highlights that the capability of the constitutive relationship of taking into account the dependence of the mechanical behaviour of the dry granular material on Lode angle, strain rate, void ratio and confining pressure
Dry granular masses impacting on rigid obstacles: numerical analysis and theoretical modelling
Full text linkThe assessment of the time evolution of the impact force exerted by dry flowing masses on rigid obstacles is mandatory for the dynamic design of sheltering structures and the evaluation of the vulnerability of existing structures. In this paper, the results of an extensive numerical campaign performed by employing a discrete element method (DEM) code are presented and the role of different geometrical factors (flow length, height and front inclination) and state parameters (porosity and velocity) on the impact force–time evolution is investigated. The impact process is studied to correlate local information with the macroscopic response and a physically based force–time function, generalising the formula already introduced by the authors for the assessment of maximum impact force, in which each parameter is correlated with the previously mentioned factors, is proposed
Modelling phase transition in granular materials: From discontinuum to continuum
No full textThis work focuses on the behaviour of granular materials under unsteady, simple shear conditions and, in particular, on from solid- to fluid-like phase transition. The authors introduce a theoretical model, based on continuum mechanics, able of predicting the mechanical behaviour of granular media under both quasi-static and dynamic conditions. The model assumes a parallel scheme where confining and shear stresses are computed as the sum of two contributions: the quasi-static and the collisional one. The quasi-static contribution is obtained by employing an elastic–plastic model including the critical state concept, while the collisional one is derived from the kinetic theory of granular gases. In order to test the model under unsteady conditions, DEM numerical simulations of time evolving homogeneous shear flows have been performed by considering an assembly of frictional, deformable spheres, under constant volume conditions. Simulations have been performed by systematically changing both void ratio and shear rate. The comparison between theoretical model predictions and DEM results is done in terms of time evolution of stresses and granular temperature. Suitable initial conditions are imposed to reproduce both solid to fluid (liquefaction) and fluid to solid (solidification) phase transitions
A New Constitutive Approach for Simulating Solid- to Fluid-like Phase Transition in Dry and Saturated Granular Media
No full textThe authors focus on constitutive modelling of solid to fluid phase transition in granular soils. A new constitutive approach conceived to capture such a transition in either dry or saturated granular material is validated against DEM numerical simulations for steady conditions and discussed in the framework of the μ-e-I rheology. The approach considers granular and liquid phases separately, assuming the two phases work in parallel, as it is according to the effective stress principle in case of quasi-static conditions. The authors emphasize the importance of considering separately the two phase contributions and the limitations of the μ-e-I rheology, in particular in relation to the volumetric response of the material
Three dimensional steady-state locus for dry monodisperse granular materials: DEM numerical results and theoretical modelling
No full textIn this paper, steady-state conditions for ideal monodisperse dry granular materials are both theoretically and numerically analysed. A series of discrete element (DEM) numerical simulations have been performed on a periodic cell by imposing stress paths characterized by different Lode angles, pressures, and deviatoric strain rates. The dependence of the material response on both inertial number and loading path has been discussed in terms of void ratio, fabric, and granular temperature. DEM numerical results have been finally compared with the prediction of an already conceived model based on both kinetic and critical state theories, here suitably modified to account for three-dimensional conditions
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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