Página 13 dos resultados de 251 itens digitais encontrados em 0.007 segundos

A network model for granular statics with impenetrability constraints

Ariyawansa, K. A.; Berlyand, Leonid; Panchenko, Alexander
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 11/09/2006 Português
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We study quasi-static deformation of dense granular packings. The packing is deformed by imposing external boundary conditions, which model engineering experiments such as shear and compression. We propose a two-dimensional network model of such deformations. The model takes into account elastic interparticle interactions and incorporates geometric impenetrability constraints. The effects of friction are neglected. In our model, a granular packing is represented by a spring-lattice network, whereby the particle centers correspond to vertices of the network, and interparticle contacts correspond to the edges. When the network deforms, each spring either preserves its length (this corresponds to a solid-like contact), or expands (this represents a broken contact). We prove that under certain geometric conditions on the network, in the energy-minimizing configuration there are at least two non-stretched springs attached to each node, which means that every particle has at least two solid-like contacts. This eliminates micro-avalanches as a mechanism for structural weakening in small shear deformation.

Smooth Initial Conditions from Weak Gravity

Greene, Brian; Hinterbichler, Kurt; Judes, Simon; Parikh, Maulik K.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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CMB measurements reveal an unnaturally smooth early universe. We propose a mechanism to make this smoothness natural by weakening the strength of gravity at early times, and therefore altering which initial conditions have low entropy.; Comment: 14 pages, 5 figures. Minor changes, version appearing in PLB

Strongly nonequilibrium flux flow in the presence of perforating submicron holes

Babic, D.; Bentner, J.; Suergers, C.; Strunk, C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 01/06/2005 Português
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We report on the effects of perforating submicron holes on the vortex dynamics of amorphous Nb0.7Ge0.3 microbridges in the strongly nonequilibrium mixed state, when vortex properties change substantially. In contrast to the weak nonequilibrium - when the presence of holes may result in either an increase (close to Tc) or a decrease (well below Tc) of the dissipation, in the strong nonequilibrium an enhanced dissipation is observed irrespectively of the bath temperature. Close to Tc this enhancement is similar to that in the weak nonequilibrium, but corresponds to vortices shrunk due to the Larkin-Ovchinnikov mechanism. At low temperatures the enhancement is a consequence of a weakening of the flux pinning by the holes in a regime where electron heating dominates the superconducting properties.; Comment: 6 pages, 5 figures

Size-induced acoustic hardening and optic softening of Ramn phonons

Sun, Chang Q; Pan, L. K.; Li, C. M.; Li, S.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/06/2005 Português
Relevância na Pesquisa
16.66101%
It has been puzzling that the Raman optical modes shift to lower frequency (or termed as optical mode softening) associated with creation of Raman acoustic modes that shift to higher energy (or called as acoustic hardening) upon nanosolid formation and size reduction. Understandings of the mechanism behind the size-induced acoustic hardening and optic softening have been quite controversial. On the basis of the recent bond order-length-strength (BOLS) correlation [Phys. Rev. B 69 045105 (2004)], we show that the optical softening arises from atomic cohesive energy weakening of surface atoms and the acoustic mode hardening is predominated by intergrain interaction. Agreement between predictions and observations has been reached for Si, CdS, InP, TiO2, CeO2, and SnO2 nanostructures with elucidation of vibration frequency of the corresponding isolated dimers. Findings further evidence the impact of bond order loss to low-dimensional systems and the essentiality of the BOLS correlation in describing the behavior of nanostructures.; Comment: 14 pages, 2 figures

Morphological Fractions of Galaxies in WINGS Clusters: revisiting the Morphology-Density Paradigm

Fasano, G.; Poggianti, B. M.; Bettoni, D.; Donofrio, M.; Dressler, A.; Vulcani, B.; Moretti, A.; Gullieuszik, M.; Fritz, J.; Omizzolo, A.; Cava, A.; Couch, W. J.; Ramella, M.; Biviano, A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
16.66101%
We present the Morphology-Density and Morphology-Radius relations (T-Sigma and T-R, respectively) obtained from the WINGS database of galaxies in nearby clusters. Aiming to achieve the best statistics, we exploit the whole sample of galaxies brighter than MV=-19.5 (5,504 objects), stacking up the 76 clusters of the WINGS survey altogether. Using this global cluster sample, we find that the T-Sigma relation holds only in the inner cluster regions (R<1/3xR200), while the T-R relation keeps almost unchanged over the whole range of local density. A couple of tests and two sets of numerical simulations support the robustness of these results against the effects of the limited cluster area coverage of the WINGS imaging. The above mentioned results hold for all cluster masses (X-ray luminosity and velocity dispersion) and all galaxy stellar masses (M). The strength of the T-Sigma relation (where present) increases with increasing M, while this effect is not found for the T-R relation. Noticeably, the absence/presence of subclustering determines the presence/absence of the T-Sigma relation outside the inner cluster regions, leading us to the general conclusion that the link between morphology and local density is preserved just in dynamically evolved regions. We hypothesize that some mechanism of morphological broadening/redistribution operates in the intermediate/outer regions of substructured (non relaxed) clusters...

Fenton chemistry at aqueous interfaces

Enami, Shinichi; Sakamoto, Yosuke; Colussi, Agustín J.
Fonte: National Academy of Sciences Publicador: National Academy of Sciences
Tipo: Article; PeerReviewed Formato: application/pdf; application/pdf
Publicado em 14/01/2014 Português
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In a fundamental process throughout nature, reduced iron unleashes the oxidative power of hydrogen peroxide into reactive intermediates. However, notwithstanding much work, the mechanism by which Fe^(2+) catalyzes H_2O_2 oxidations and the identity of the participating intermediates remain controversial. Here we report the prompt formation of O=Fe^(IV)CI_3^− and chloride-bridged di-iron O=Fe^(IV)·CI·FeIICI_4^− and O=Fe^(IV)·CI·Fe^(III)CI_5^− ferryl species, in addition to Fe^(III)CI_4^−, on the surface of aqueous FeCI_2 microjets exposed to gaseous H_2O_2 or O_3 beams for <50 μs. The unambiguous identification of such species in situ via online electrospray mass spectrometry let us investigate their individual dependences on Fe^(2+), H_2O_2, O_3, and H^+ concentrations, and their responses to tert-butanol (an ·OH scavenger) and DMSO (an O-atom acceptor) cosolutes. We found that (i) mass spectra are not affected by excess tert-butanol, i.e., the detected species are primary products whose formation does not involve ·OH radicals, and (ii) the di-iron ferryls, but not O=Fe^(IV)CI_3^−, can be fully quenched by DMSO under present conditions. We infer that interfacial Fe(H_2O)_n^(2+) ions react with H_2O_2 and O_3 >10^3 times faster than Fe(H_2O)_6^(2+) in bulk water via a process that favors inner-sphere two-electron O-atom over outer-sphere one-electron transfers. The higher reactivity of di-iron ferryls vs. O=Fe^(IV)CI_3^− as O-atom donors implicates the electronic coupling of mixed-valence iron centers in the weakening of the Fe^(IV)–O bond in poly-iron ferryl species.

The 2013 Okhotsk deep-focus earthquake: Rupture beyond the metastable olivine wedge and thermally controlled rise time near the edge of a slab

Meng, Lingsen; Ampuero, Jean-Paul; Bürgmann, Roland
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Article; PeerReviewed Formato: application/pdf; application/pdf; application/pdf
Publicado em 16/06/2014 Português
Relevância na Pesquisa
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The 2013 M8.3 Okhotsk earthquake involves two primary mechanisms of deep-focus earthquake rupture, mineral phase transformation of olivine to spinel and thermal shear instability. Backprojection imaging of broadband seismograms recorded by the North American and European networks indicates bilateral rupture toward NE and SSE. The rupture paths of the NE segment and other regional M7 earthquakes are confined in narrow regions along the slab contours, consistent with the phase transformation mechanism. However, the SSE rupture propagates a long distance across the slab and aftershocks are distributed across a ~60 km wide zone, beyond the plausible thickness of the metastable olivine wedge, favoring thermal shear weakening. While the NE rupture is only visible at high frequencies, the SSE rupture is consistently observed across a broad-frequency range. This frequency-dependent rupture mode can be explained by lateral variations of rise time controlled by thermal thinning of the slab near its northern end.

Scaling differences between large interplate and intraplate earthquakes

Scholz, C. H.; Aviles, C. A.; Wesnousky, S. G.
Fonte: Seismological Society of America Publicador: Seismological Society of America
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /02/1986 Português
Relevância na Pesquisa
16.66101%
A study of large intraplate earthquakes with well-determined source parameters shows that these earthquakes obey a scaling law similar to large interplate earthquakes, in which M_0 ∝ L^2 or u = αL, where L is rupture length and u is slip. In contrast to interplate earthquakes, for which α ≈ 1 × 10^(−5), for for the intraplate events α ≈ 6 × 10^(−5), which implies that these earthquakes have stress drops about 6 times higher than interplate events. This result is independent of focal mechanism type. This implies that intraplate faults have a higher frictional strength than do plate boundaries, and hence that faults are velocity or slip weakening in their behavior. This factor may be important in producing the concentrated deformation that creates and maintains plate boundaries.

Localization of intraplate deformation through fluid-assisted faulting in the lower-crust: The Flinders Ranges, South Australia

Balfour, Natalie; Cummins, Phillip; Pilia, Simone; Love, D
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
16.66101%
In this paper we present a hypothesis for localized, intraplate deformation in the continental crust of south-central Australia that involves fluid-assisted reactivation of faults in the mid- to lower crust. Using data from a temporary seismometer deployment in the Flinders Ranges, we show that earthquakes, relocated in a 3D velocity model, cluster in elongated low vp /. vs anomalies that extend to depths exceeding 20. km, and are aligned with the axis of the Flinders Ranges. In the northern Flinders Ranges these low vp /. vs anomalies can be interpreted as fractured Neoproterozoic to Cambrian sediments that separate two cratonic blocks, the Gawler Craton to the west and the Curnamona Province in the east. Previous studies of Helium isotopes in springs to the north of the area provide evidence of mantle-derived fluids that may influence faulting at depth. Our focal mechanism and stress inversion results show a regionally compressive stress field that provides no evidence for stress concentration. We also argue that mechanisms for localized faulting such as thermal weakening and isostatic rebound also fail to account for the occurrence of earthquakes at mid- to lower crustal depth in this area of high heat flow and that the focused seismicity can only be explained by high pore fluid pressure in the lower crust.

Rheology of calcite - quartz aggregates deformed to large strain in torsion

Rybacki, Erik; Paterson, Mervyn; Wirth, Richard; Dresen, Georg
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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High-temperature, high-pressure torsion experiments have been conducted on Solnhofen limestone and synthetic calcite-quartz aggregates to investigate the evolution of mechanical strength and microstructure to large strains. Hot isostatic pressing of powders containing 1-30 wt% quartz particles produced synthetic two-phase aggregates containing trace amounts of wollastonite. The experiments were performed at constant twist rate, 300 MPa confining pressure, and temperatures ranging from 900 to 1300 K in the stability fields of calcite plus quartz and wollastonite plus carbon dioxide. The mechanical data from torsion tests of most samples show a pronounced peak stress at shear strains <1 and subsequent weakening. For most samples, a steady state stress is reached only at shear strains >5. A distinct shape and lattice preferred orientation developed in Solnhofen limestone and in the calcite matrix of the synthetic aggregates. However, at large shear strains, flattened porphyroclasts were almost entirely consumed by recrystallization. Elongation parallel to the specimen axis at temperature below 1100 K is possibly due to rotation of minerals into a preferred crystallographic orientation. Stress exponent and activation energy for creep of the calcite-quartz aggregates increase substantially with increasing quartz content. The strength of the two-phase aggregates increases with quartz fraction up to 20 wt% by a factor of 2-5 depending on temperature and finite strain. Continuum models underestimate particle strengthening of the calcite-quartz aggregates. An alternative microphysical mechanism for the observed strengthening may be related to the reduced mobility of dislocation through diffusion of silicon into dislocation cores.

The Wall Street walk when blockholders compete for flows

Dasgupta, Amil; Piacentino, Giorgia
Fonte: Blackwell Publishing for the American Finance Association Publicador: Blackwell Publishing for the American Finance Association
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em //2015 Português
Relevância na Pesquisa
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Effective monitoring by equity blockholders is important for good corporate governance. A prominent theoretical literature argues that the threat of block sale (“exit”) can be an effective governance mechanism. Many blockholders are money managers. We show that when money managers compete for investor capital, the threat of exit loses credibility, weakening its governance role. Money managers with more skin in the game will govern more successfully using exit. Allowing funds to engage in activist measures (“voice”) does not alter our qualitative results. Our results link widely prevalent incentives in the ever-expanding money management industry to the nature of corporate governance.