1,720,957 research outputs found
Stress-driven morphological instability and catastrophic failure of microdevices
No full textIn microdevices, the competition between surface energy and elastic energy could lead at the phenomenon known as stress-driven morphological instability (MI), causing an increase of Surface roughness with time. Several different mass transport mechanisms can trigger such a morphological alteration and operate simultaneously: surface and bulk diffusion, evaporation and condensation, chemical reactions. Unstable solids could eventually evolve towards crack-like surfaces thus altering mechanical, electrical and optical properties of the devices or even leading to catastrophic failures by supercritical crack propagation. In this work, a more general kinetic law is employed to estimate the onset of MI, considering the effect of the stress field on the atomic mobility. A more intuitive and straightforward approach is used to determine the stability conditions, where the rate of atomic mass motion is introduced, as a stability parameter-The critical loads and wavelengths for the onset of MI, determined as a function of material parameters alpha and beta, are compared with the limiting conditions for the supercritical crack propagation (SC) of a crack-like surface in order to asses if and under which situations catastrophic failures by SC can be observed. Two practical cases are investigated: fixed wavelength (Case I) and arbitrary rough surface with a fixed remote load (Case II). In Case I, absolute and relative threshold loads are found below which MI could never occur and a transitional wavelength over which MI would always lead to SC is introduced. In Case II, it is shown that dominant perturbation for NIT would always lead to SC given enough time for the surface to evolve towards a crack-like profile. The influence of the material properties a and on the critical parameters is also addresse
Rough contact of sliding viscoelastic layers: numerical calculations and theoretical predictions
Full text linkWe study the sliding contact of viscoelastic layers on rigid rough profiles, with two different contact configurations: constrained on the upper boundary, and uniformly loaded on the top.
Results show that layer thickness and boundary conditions affect both the average contact quantities and the viscoelastic friction. Interestingly, the latter is strongly influenced by the specific controlled parameter. Under displacement controlled conditions, large differences are observed in the frictional behavior of the two contact configuration. Conversely, in load control, the friction coefficient is almost independent of the specific boundary condition.
Finally, a comparison with a 1D version of the Persson's theory is proposed, finding a good agreement in terms of dependence of contact area and friction coefficient on the sliding velocity
Fatigue Limit Evaluation of Composite Materials by Means of TSA
No full textThermal methods represent a valid support to study in detail fatigue behavior of materials. Both the reduction of time and costs of the experimental campaign and the possibility to assess damage phenomena correlated to dissipative heat sources developed during fatigue.
The work aims to perform the fatigue characterization of GFRP composite materials by adopting a novel and rapid procedure for assessing endurance limit. The proposed method involves a multi-parameter analysis to attain several index related to the damage phenomena which fit well with those provided by the conventional procedures
Design and construction of an offshore diffuser augmented wind turbine with a high efficiency alternator
Full text linkThis paper describes the design and the assembly of a new type of a ducted wind turbine with an electric power generator adopting permanent high coercive magnets embedded in the peripheral ring of the rotor. The nominal power is 20 kW, and the maximum diameter of the external duct is 9 meters. The project has been carried out within the Marine Energy Laboratory (MEL) funded by the Italian Ministry for Education, University and Research (MIUR) and collects the most advanced technologies of naval maritime engineering and combines them with energy and turbomachinery technologies. The presence of a divergent duct enables the interception of a greater air mass flow rate, allowing the reduction of the rotor diameter and the deflections of the blades; hence, at constant tip speed ratio, a higher rotational speed compared to conventional turbines and a better efficiency of the permanent magnet power generator
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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