1,720,977 research outputs found
Modeling and simulation of the impact behavior of soft polymeric-foam-based back protectors for winter sports
No full textObjectives: Winter sports are high-energy outdoor activities involving high velocities and acrobatic maneuvers, thus raising safety concerns. Specific studies on the impact mechanics of back protectors are very limited. In this study analytical and numerical models are developed to rationalize results of impact experiments and propose new design procedures for this kind of equipment. Design: Different soft-shell solutions currently available on the market are compared. In particular, the role of dynamic material constitutive properties and of environmental temperature (which affects mainly material stiffness) on energy absorption capability are evaluated. Methods: Starting from dynamic mechanical-thermal characterization of the closed-cell polymeric foams constituting the protectors, we exploited analytical modeling and Finite Element Method simulations to interpret experimental data from drop weight impact test and to characterize protectors at different temperatures and after multiple impacts. Results: The temperature and frequency dependent properties of these materials characterize their impact behavior. Modeling results are in good agreement with impact tests. Results demonstrate how ergonomic soft-shell solution provides an advantage with respect to traditional hard-shell in terms of impact protection. Moreover, it can maintain nearly unaltered its protective properties after multiple impacts on the same point. Conclusions: The coupled analytical-simulation approach here presented could be extensively used to predict the impact behavior of such equipment, starting from material characterization, allowing to save costs and time for physical prototyping and tests for design and optimization. (C) 2018 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.
2D Material Armors Showing Superior Impact Strength of Few Layers
Full text linkWe study the ballistic properties of two-dimensional (2D) materials upon the hypervelocity impacts of C60 fullerene molecules combining ab initio density functional tight binding and finite element simulations. The critical penetration energy of monolayer membranes is determined using graphene and the 2D allotrope of boron nitride as case studies. Furthermore, the energy absorption scaling laws with a variable number of layers and interlayer spacing are investigated, for homogeneous or hybrid configurations (alternated stacking of graphene and boron nitride). At the nanolevel, a synergistic interaction between the layers emerges, not observed at the micro- and macro-scale for graphene armors. This size-scale transition in the impact behavior toward higher dimensional scales is rationalized in terms of scaling of the damaged volume and material strength. An optimal number of layers, between 5 and 10, emerges demonstrating that few-layered 2D material armors possess impact strength even higher than their monolayer counterparts. These results provide fundamental understanding for the design of ultralightweight multilayer armors using enhanced 2D material-based nanocomposites
Atomistic modelling of the hypervelocity dynamics of shock-compressed graphite and impacted graphene armours
Full text linkWe present a molecular dynamics (MD) simulation study on the hypervelocity dynamics of shock compressed graphite-up to hundreds of gigapascals- and impacted multilayer graphene armours by employing the AIREBO-M potential. The Morse-type non-singular intermolecular interaction allows the usage of relatively large integration timesteps for simulating materials' response at such high strain-rate. The MD simulation results are in good agreement with the shock Hugoniot curves and with graphite-to-diamond transition obtained from both density functional theory (DFT) and experiments available in literature. We then show that thermodynamic properties of graphite from MD calculations can be used as input for a reliable equation of state to be employed in continuum simulations. Finally, we find that the size-scaling of the hypervelocity impact properties of graphene armours matches well with the DFT results and theoretical predictions of earlier studies. Our results open a concrete possibility towards accurate and fast multiscale simulation from atomistic to continuum level of shock propagation, shock-induced phase transformation, and dynamic fracture in large or hierarchical carbon systems, such as graphene-based foams and nanocomposites.
A combined experimental/numerical study on the scaling of impact strength and toughness in composite laminates for ballistic applications
Full text linkIn this paper, the impact behaviour of composite laminates is investigated, and their potential for ballistic protection
assessed, as a function of the reinforcing materials and structure for three representative fibre-reinforced
epoxy systems involving carbon, glass, and para-aramid fibre reinforcements, respectively. A multiscale coupled
experimental/numerical study on the composite material properties is performed, starting from single fibre, to
fibre bundles (yarns), to single composite ply, and finally at laminate level. Uniaxial tensile tests on single fibres
and fibre bundles are performed, and the results are used as input for non-linear Finite Element Method (FEM)
models for tensile and impact simulation on the composite laminates. Mechanical properties and energy dissipation
of the single ply and multilayer laminates under quasi-static loading are preliminarily assessed starting from
the mechanical properties of the constituents and subsequently verified numerically. FEM simulations of ballistic
impact on multilayer armours are then performed, assessing the three different composites, showing good agreement
with experimental tests in terms of impact energy absorption capabilities and deformation/failure behaviour.
As result, a generalized multiscale version of the well-known Cuniff criterion is provided as a scaling law,
which allows to assess the ballistic performance of laminated composites, starting from the tensile mechanical
properties of the fibres and fibre bundles and their volume fraction. The presented multiscale coupled experimental-
numerical characterization confirms the reliability of the predictions for full-scale laminate properties starting
from the individual constituents at the single fibre scale
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
Figure S5 from Multilayer stag beetle elytra perform better under external loading via non-symmetric bending properties
Full text linkFEM images showing the von-Mises stress distribution (unit of measure GPa) in the wing and the beetle body under a concentrated load of 0.5 N .A) real structure with void, B) elytra with no void
Effect of material elastic properties and surface roughness on grip performances of ski boot soles under wet and icy conditions
Full text linkA set of thermoplastic materials employed in soles for alpine skiing boots were characterized in terms of chemical composition, cristallinity, hardness, surface roughness, and grip. The results of friction experiments on different substrates reproducing the real environmental scenarios point out that materials provide more grip as they become softer. Moreover, higher roughness results in lower dynamic coefficient of friction (COF). Finite element simulations corroborate the experimental measures of COF and let to rationalize the role of material elasticity and surface roughness on the frictional characteristics of soles. The measure of grip on an inclined wet surface provides analogous results, indicating that COF can be used as key performance indicator in the design of ski-boot soles and of other anti-slip equipments in wet and icy environments
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