1,720,957 research outputs found
The effect of particle dimensionality on Granular friction in laboratory shear zones
No full textTo match the boundary conditions of numerical models and to examine the effect of particle dimensionality on granular friction, we conducted laboratory experiments on rods sheared in 1-D and 2-D configurations, glass beads (3-D), and angular quartz sand (rough 3-D). The average coefficient of friction during stable sliding for 1-D, 2-D, smooth 3-D, and rough 3-D particles is 0.15, 0.3, 0.45, and 0.6, respectively. Frictional strength of 2-D layers exceeds 1-D friction by an amount associated with dilatancy and the additional contact plane in 2-D. We show that 3-D granular friction exceeds 2-D friction by the amount of interparticle friction on the out-of-plane particle contacts that do not exist in 2-D. Data from our 2-D experiments are remarkably similar to numerical results based on 2-D particle dynamic simulations. Our data indicate that application of numerical models of granular friction to tectonic faults will require computations involving rough, 3-D particles
Effect of humidity on granular friction at room temperature
No full textWe report on laboratory experiments designed to investigate the microphysical processes that result in rate- and state-dependent friction behavior. We study the effect of relative humidity (RH) (<5 to 100%) in velocity stepping tests (10-20 μm/s) and slide-hold-slide (SHS) tests (3-1000 s) on 3 mm thick layers of quartz and alumina powders sheared at 25 MPa normal stresses. Granular powders are conditioned in situ under controlled RH to create new surface area before shearing. We find a transition from velocity strengthening to velocity-weakening frictional behavior as RH increases. The transition occurs at 30-35% RH for quartz and 55-60% RH for alumina. Frictional healing is negligible at low humidity and increases with increasing RH for both materials. The coefficient of sliding friction is independent of humidity. We use normal stress vibrations in SHS tests to isolate chemically assisted healing mechanisms operative within contact junctions from compaction induced granular strengthening. We find that reorganization of granular particles influences friction but that chemically assisted mechanisms dominate. Our data show that rate- and state-dependent friction behavior for granular materials, including time-dependent healing and steady state velocity dependence, is the result of chemically assisted mechanisms that can be reduced or turned off at low humidity at room temperature in quartz and alumina
Influence of grain characteristics on the friction of granular shear zones
No full textNumerical models of granular shear show lower friction and a greater tendency for stick slip than laboratory studies designed to investigate fault mechanics. Here we report on laboratory experiments designed to reproduce the conditions of numerical models and to test the role that grain characteristics play in controlling frictional behavior. Friction and microstructural data are compared for direct shear experiments on thin layers (2-3 mm) of angular quartz sand and spherical glass beads. We study the effect of grain shape, roughness, size distribution, and comminution. In a nonfracture loading regime, sliding friction for smooth spherical particles (μ ∼ 0.45) is measurably lower than for angular particles (μ ∼ 0.6). A narrow particle size distribution (PSD) of spherical beads (105-149 μm) exhibits unstable stick-slip behavior, whereas a wide PSD of spheres (1-800 μm) and the angular gouge display stable sliding. At higher stress, where grain fracture is promoted, initially spherical particles become stable with accumulated slip, and friction increases to the level observed for angular gouge. We find that frictional strength and stability of a granular shear zone are sensitive to grain shape, PSD, and their evolution. We suggest that a low friction translation mechanism, such as grain rolling, operates in gouge composed of smooth particles. Our results show that the first-order disparities between laboratory and numerical studies of granular shear can be explained by differences in grain characteristics and loading conditions. Since natural faults predominantly contain angular gouge, we find no evidence for a fault-weakening mechanism associated with the presence of gouge
Laboratory results indicating complex and potentially unstable frictional behavior of smectite clay
No full textA central problem in explaining the apparent weakness of the San Andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake-like unstable rupture. Our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress (<30 MPa). Our data are consistent with previous work, which has focused on higher normal stress conditions (50 MPa and greater) and found only velocity strengthening. If natural fault zones contain significant smectite, one key implication of our results is that localized zones of high pore pressure, which reduce effective normal stress, could be important in controlling potential sites of earthquake nucleation. Our experiments indicate that friction of smectite is complex, and depends upon both sliding velocity and normal stress. This complexity highlights the need for detailed experiments that reflect in-situ conditions for fault gouges
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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