28,836 research outputs found
Internal dynamics and activated processes in Soft-Glassy materials
Plastic rearrangements play a crucial role in the characterization of
soft-glassy materials, such as emulsions and foams. Based on numerical
simulations of soft-glassy systems, we study the dynamics of plastic
rearrangements at the hydrodynamic scales where thermal fluctuations can be
neglected. Plastic rearrangements require an energy input, which can be either
provided by external sources, or made available through time evolution in the
coarsening dynamics, in which the total interfacial area decreases as a
consequence of the slow evolution of the dispersed phase from smaller to large
droplets/bubbles. We first demonstrate that our hydrodynamic model can
quantitatively reproduce such coarsening dynamics. Then, considering
periodically oscillating strains, we characterize the number of plastic
rearrangements as a function of the external energy-supply, and show that they
can be regarded as activated processes induced by a suitable "noise" effect.
Here we use the word noise in a broad sense, referring to the internal
non-equilibrium dynamics triggered by spatial random heterogeneities and
coarsening. Finally, by exploring the interplay between the internal
characteristic time-scale of the coarsening dynamics and the external
time-scale associated with the imposed oscillating strain, we show that the
system exhibits the phenomenon of stochastic resonance, thereby providing
further credit to the mechanical activation scenario.Comment: 21 Pages, 9 figure
X-ray scattering from stepped and kinked surfaces: An approach with the paracrystal model
A general formalism of X-ray scattering from different kinds of surface
morphologies is described. Based on a description of the surface morphology at
the atomic scale through the use of the paracrystal model and discrete
distributions of distances, the scattered intensity by non-periodic surfaces is
calculated over the whole reciprocal space. In one dimension, the scattered
intensity by a vicinal surface, the two-level model, the N-level model, the
faceted surface and the rough surface are addressed. In two dimensions, the
previous results are generalized to the kinked vicinal surface, the two-level
vicinal surface and the step meandering on a vicinal surface. The concept of
crystal truncation rod is generalized considering also the truncation of a
terrace by a step (yielding a terrace truncation rod) and a step by a kink
(yielding a step truncation rod).Comment: 33 pages, 18 figure
The Meaning of Memory Safety
We give a rigorous characterization of what it means for a programming
language to be memory safe, capturing the intuition that memory safety supports
local reasoning about state. We formalize this principle in two ways. First, we
show how a small memory-safe language validates a noninterference property: a
program can neither affect nor be affected by unreachable parts of the state.
Second, we extend separation logic, a proof system for heap-manipulating
programs, with a memory-safe variant of its frame rule. The new rule is
stronger because it applies even when parts of the program are buggy or
malicious, but also weaker because it demands a stricter form of separation
between parts of the program state. We also consider a number of pragmatically
motivated variations on memory safety and the reasoning principles they
support. As an application of our characterization, we evaluate the security of
a previously proposed dynamic monitor for memory safety of heap-allocated data.Comment: POST'18 final versio
The Gaia-ESO Survey: the chemical structure of the Galactic discs from the first internal data release
Most high-resolution spectroscopic studies of the Galactic discs were mostly
confined to objects in the solar vicinity. Here we aim at enlarging the volume
in which individual chemical abundances are used to characterise both discs,
using the first internal data release of the Gaia-ESO survey. We derive and
discuss the abundances of eight elements (Mg, Al, Si, Ca, Ti, Fe, Cr, Ni, and
Y). The trends of these elemental abundances with iron are very similar to
those in the solar neighbourhood. We find a natural division between alpha-rich
and alpha-poor stars, best seen in the bimodality of the [Mg/M] distributions
in bins of metallicity, which we attribute to thick- and thin-disc sequences,
respectively. With the possible exception of Al, the observed dispersion around
the trends is well described by the expected errors, leaving little room for
astrophysical dispersion. Using previously derived distances from Recio-Blanco
et al. (2014b), we further find that the thick-disc is more extended vertically
and is more centrally concentrated towards the inner Galaxy than the thin-disc,
which indicates a shorter scale-length. We derive the radial and vertical
gradients in metallicity, iron, four alpha-element abundances, and Al for the
two populations, taking into account the identified correlation between R_GC
and |Z|. Radial metallicity gradient is found in the thin disc. The positive
radial individual [alpha/M] gradients found are at variance from the gradients
observed in the RAVE survey. The thin disc also hosts a negative vertical
metallicity gradient, accompanied by positive individual [alpha/M] and [Al/M]
gradients. The thick-disc, presents no radial metallicity gradient, a shallower
vertical metallicity gradient than the thin-disc, an alpha-elements-to-iron
radial gradient in the opposite sense than that of the thin disc, and positive
vertical individual [alpha/M] and [Al/M] gradients.Comment: 24 pages, 10 figure
The acousto-ultrasonic approach
The nature and underlying rationale of the acousto-ultrasonic approach is reviewed, needed advanced signal analysis and evaluation methods suggested, and application potentials discussed. Acousto-ultrasonics is an NDE technique combining aspects of acoustic emission methodology with ultrasonic simulation of stress waves. This approach uses analysis of simulated stress waves for detecting and mapping variations of mechanical properties. Unlike most NDE, acousto-ultrasonics is less concerned with flaw detection than with the assessment of the collective effects of various flaws and material anomalies. Acousto-ultrasonics has been applied chiefly to laminated and filament-wound fiber reinforced composites. It has been used to assess the significant strength and toughness reducing effects that can be wrought by combinations of essentially minor flaws and diffuse flaw populations. Acousto-ultrasonics assesses integrated defect states and the resultant variations in properties such as tensile, shear, and flexural strengths and fracture resistance. Matrix cure state, porosity, fiber orientation, fiber volume fraction, fiber-matrix bonding, and interlaminar bond quality are underlying factors
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