32 research outputs found
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
Oscillatory Finite-Time Singularities in Finance, Population and Rupture
We present a simple two-dimensional dynamical system where two nonlinear
terms, exerting respectively positive feedback and reversal, compete to create
a singularity in finite time decorated by accelerating oscillations. The power
law singularity results from the increasing growth rate. The oscillations
result from the restoring mechanism. As a function of the order of the
nonlinearity of the growth rate and of the restoring term, a rich variety of
behavior is documented analytically and numerically. The dynamical behavior is
traced back fundamentally to the self-similar spiral structure of trajectories
in phase space unfolding around an unstable spiral point at the origin. The
interplay between the restoring mechanism and the nonlinear growth rate leads
to approximately log-periodic oscillations with remarkable scaling properties.
Three domains of applications are discussed: (1) the stock market with a
competition between nonlinear trend-followers and nonlinear value investors;
(2) the world human population with a competition between a
population-dependent growth rate and a nonlinear dependence on a finite
carrying capacity; (3) the failure of a material subjected to a time-varying
stress with a competition between positive geometrical feedback on the damage
variable and nonlinear healing.Comment: Latex document of 59 pages including 20 eps 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.
Clogging transition of many-particle systems flowing through bottlenecks
When a large set of discrete bodies passes through a bottleneck, the flow may become intermittent due to the development of clogs that obstruct the constriction. Clogging is observed, for instance, in colloidal suspensions, granular materials and crowd swarming, where consequences may be dramatic. Despite its ubiquity, a general framework embracing research in such a wide variety of scenarios is still lacking. We show that in systems of very different nature and scale -including sheep herds, pedestrian crowds, assemblies of grains, and colloids- the probability distribution of time lapses between the passages of consecutive bodies exhibits a power-law tail with an exponent that depends on the system condition. Consequently, we identify the transition to clogging in terms of the divergence of the average time lapse. Such a unified description allows us to put forward a qualitative clogging state diagram whose most conspicuous feature is the presence of a length scale qualitatively related to the presence of a finite size orifice. This approach helps to understand paradoxical phenomena, such as the faster-is-slower effect predicted for pedestrians evacuating a room and might become a starting point for researchers working in a wide variety of situations where clogging represents a hindrance
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