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.
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
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
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
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...
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 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.
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.
Balfour, Natalie; Cummins, Phillip; Pilia, Simone; Love, D
Fonte: ElsevierPublicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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.
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.
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.