47 research outputs found
Failure time and critical behaviour of fracture precursors in heterogeneous materials
The acoustic emission of fracture precursors, and the failure time of samples
of heterogeneous materials (wood, fiberglass) are studied as a function of the
load features and geometry. It is shown that in these materials the failure
time is predicted with a good accuracy by a model of microcrack nucleation
proposed by Pomeau. We find that the time interval between events
(precursors) and the energy are power law distributed and that
the exponents of these power laws depend on the load history and on the
material. In contrast, the cumulated acoustic energy presents a critical
divergency near the breaking time which is % E\sim \left( \frac{\tau
-t}\tau \right) ^{-\gamma }. The positive exponent is independent,
within error bars, on all the experimental parameters.Comment: to be published on European Physical Journa
Unjamming a granular hopper by vibration
We present an experimental study of the outflow of a hopper continuously
vibrated by a piezoelectric device. Outpouring of grains can be achieved for
apertures much below the usual jamming limit observed for non vibrated hoppers.
Granular flow persists down to the physical limit of one grain diameter, a
limit reached for a finite vibration amplitude. For the smaller orifices, we
observe an intermittent regime characterized by alternated periods of flow and
blockage. Vibrations do not significantly modify the flow rates both in the
continuous and the intermittent regime. The analysis of the statistical
features of the flowing regime shows that the flow time significantly increases
with the vibration amplitude. However, at low vibration amplitude and small
orifice sizes, the jamming time distribution displays an anomalous statistics
Role of vibrations in the jamming and unjamming of grains discharging from a silo
We present experimental results of the jamming of non-cohesive particles
discharged from a flat bottomed silo subjected to vertical vibration. When the
exit orifice is only a few grain diameter wide, the flow can be arrested due to
the formation of blocking arches. Hence, an external excitation is needed to
resume the flow. The use of a continuous gentle vibration is a usual technique
to ease the flow in such situations. Even though jamming is less frequent, it
is still an issue in vibrated silos. There are, in principle, two possible
mechanisms through which vibrations may facilitate the flow: (i) a decrease in
the probability of the formation of blocking arches, and (ii) the breakage of
blocking arches once they have been formed. By measuring the time intervals
inside an avalanche during which no particles flow through the outlet, we are
able to estimate the probability of breaking a blocking arch by vibrations. The
result agrees with the prediction of a bivariate probabilistic model in which
the formation of blocking arches is equally probable in vibrated and
non-vibrated silos. This indicates that the second aforementioned mechanism is
the main responsible for improving the flowability in gently vibrated silos
Time dependence of breakdown in a global fiber-bundle model with continuous damage
A time-dependent global fiber-bundle model of fracture with continuous damage
is formulated in terms of a set of coupled non-linear differential equations. A
first integral of this set is analytically obtained. The time evolution of the
system is studied by applying a discrete probabilistic method. Several results
are discussed emphasizing their differences with the standard time-dependent
model. The results obtained show that with this simple model a variety of
experimental observations can be qualitatively reproduced.Comment: APS style, two columns, 4 figures. To appear in Phys. Rev.
Statistical properties of acoustic emission signals from metal cutting processes
Acoustic Emission (AE) data from single point turning machining are analysed
in this paper in order to gain a greater insight of the signal statistical
properties for Tool Condition Monitoring (TCM) applications. A statistical
analysis of the time series data amplitude and root mean square (RMS) value at
various tool wear levels are performed, �nding that ageing features can
be revealed in all cases from the observed experimental histograms. In
particular, AE data amplitudes are shown to be distributed with a power-law
behaviour above a cross-over value. An analytic model for the RMS values
probability density function (pdf) is obtained resorting to the Jaynes' maximum
entropy principle (MEp); novel technique of constraining the modelling function
under few fractional moments, instead of a greater amount of ordinary moments,
leads to well-tailored functions for experimental histograms.Comment: 16 pages, 7 figure
The effects of grain shape and frustration in a granular column near jamming
We investigate the full phase diagram of a column of grains near jamming, as
a function of varying levels of frustration. Frustration is modelled by the
effect of two opposing fields on a grain, due respectively to grains above and
below it. The resulting four dynamical regimes (ballistic, logarithmic,
activated and glassy) are characterised by means of the jamming time of
zero-temperature dynamics, and of the statistics of attractors reached by the
latter. Shape effects are most pronounced in the cases of strong and weak
frustration, and essentially disappear around a mean-field point.Comment: 17 pages, 19 figure
Fracture model with variable range of interaction
We introduce a fiber bundle model where the interaction among fibers is
modeled by an adjustable stress-transfer function which can interpolate between
the two limiting cases of load redistribution, the global and the local load
sharing schemes. By varying the range of interaction several features of the
model are numerically studied and a crossover from mean field to short range
behavior is obtained. The properties of the two regimes and the emergence of
the crossover in between are explored by numerically studying the dependence of
the ultimate strength of the material on the system size, the distribution of
avalanches of breakings, and of the cluster sizes of broken fibers. Finally, we
analyze the moments of the cluster size distributions to accurately determine
the value at which the crossover is observed.Comment: 8 pages, 8 figures. Two columns revtex format. Final version to be
published in Phys. Rev.
Cognition and schizophrenia: from neurocognition to social cognition
Los déficit neurocognitivos en la esquizofrenia
han sido descritos desde las primeras
descripciones del trastorno. Su influencia en
la funcionalidad y en la calidad de vida ha
sido puesta de manifiesto en múltiples estudios.
La iniciativa Measurement and Treatment
Research to Improve Cognition in Schizophrenia
(MATRICS) del National Institute of
Mental Health (NIMH) de Estados Unidos fue
puesta en marcha para impulsar el desarrollo
de una batería cognitiva de consenso que
pudiera ser empleada en ensayos clínicos de
fármacos para mejorar la neurocognición en
la esquizofrenia. Aunque en el momento de
consensuar los diferentes dominios cognitivos
que deberían ser incluidos en dicha batería, la
denominada cognición social no cumplía con
los requisitos para ser incluida, se decidió finalmente
incluir este dominio dada la importante
relación con la funcionalidad que presentaba.
Estudios posteriores han demostrado
el acierto de incluir dicho dominio cognitivo,
dada la relevancia que la cognición social ha
demostrado en relación a la funcionalidad y
calidad de vida de los pacientes con esquizofrenia;
bien como variable per se, o bien como
variable mediadora entre la neurocognición y
la funcionalidad
Statistical properties of microcracking in polyurethane foams under tensile test, influence of temperature and density
We report tensile failure experiments on polyurethane (PU) foams. Experiments
have been performed by imposing a constant strain rate. We work on
heterogeneous materials for whom the failure does not occur suddenly and can
develop as a multistep process through a succession of microcracks that end at
pores. The acoustic energy and the waiting times between acoustic events follow
power-law distributions. This remains true while the foam density is varied.
However, experiments at low temperatures (PU foams more brittle) have not
yielded power-laws for the waiting times. The cumulative acoustic energy has no
power law divergence at the proximity of the failure point which is
qualitatively in agreement with other experiments done at imposed strain. We
notice a plateau in cumulative acoustic energy that seems to occur when a
single crack starts to propagate