1,514 research outputs found
Spontaneous thermal runaway as an ultimate failure mechanism of materials
The first theoretical estimate of the shear strength of a perfect crystal was
given by Frenkel [Z. Phys. 37, 572 (1926)]. He assumed that as slip occurred,
two rigid atomic rows in the crystal would move over each other along a slip
plane. Based on this simple model, Frenkel derived the ultimate shear strength
to be about one tenth of the shear modulus. Here we present a theoretical study
showing that catastrophic material failure may occur below Frenkel's ultimate
limit as a result of thermal runaway. We demonstrate that the condition for
thermal runaway to occur is controlled by only two dimensionless variables and,
based on the thermal runaway failure mechanism, we calculate the maximum shear
strength of viscoelastic materials. Moreover, during the thermal
runaway process, the magnitude of strain and temperature progressively localize
in space producing a narrow region of highly deformed material, i.e. a shear
band. We then demonstrate the relevance of this new concept for material
failure known to occur at scales ranging from nanometers to kilometers.Comment: 4 pages, 3 figures. Eq. (6) and Fig. 2a corrected; added references;
improved quality of figure
Relation Between First Arrival Time and Permeability in Self-Affine Fractures with Areas in Contact
We demonstrate that the first arrival time in dispersive processes in
self-affine fractures are governed by the same length scale characterizing the
fractures as that which controls their permeability. In one-dimensional channel
flow this length scale is the aperture of the bottle neck, i.e., the region
having the smallest aperture. In two dimensions, the concept of a bottle neck
is generalized to that of a minimal path normal to the flow. The length scale
is then the average aperture along this path. There is a linear relationship
between the first arrival time and this length scale, even when there is strong
overlap between the fracture surfaces creating areas with zero permeability. We
express the first arrival time directly in terms of the permeability.Comment: EPL (2012)
Long-Term Clustering, Scaling, and Universality in the Temporal Occurrence of Earthquakes
Scaling analysis reveals striking regularities in earthquake occurrence. The
time between any one earthquake and that following it is random, but it is
described by the same universal-probability distribution for any spatial region
and magnitude range considered. When time is expressed in rescaled units, set
by the averaged seismic activity, the self-similar nature of the process
becomes apparent. The form of the probability distribution reveals that
earthquakes tend to cluster in time, beyond the duration of aftershock
sequences. Furthermore, if aftershock sequences are analysed in an analogous
way, yet taking into account the fact that seismic activity is not constant but
decays in time, the same universal distribution is found for the rescaled time
between events.Comment: short paper, only 2 figure
Synchronization of Kuramoto Oscillators in Scale-Free Networks
In this work, we study the synchronization of coupled phase oscillators on
the underlying topology of scale-free networks. In particular, we assume that
each network's component is an oscillator and that each interacts with the
others following the Kuramoto model. We then study the onset of global phase
synchronization and fully characterize the system's dynamics. We also found
that the resynchronization time of a perturbed node decays as a power law of
its connectivity, providing a simple analytical explanation to this interesting
behavior.Comment: 7 pages and 4 eps figures, the text has been slightly modified and
new references have been included. Final version to appear in Europhysics
Letter
Nonlinear Volatility of River Flux Fluctuations
We study the spectral properties of the magnitudes of river flux increments,
the volatility. The volatility series exhibits (i) strong seasonal periodicity
and (ii) strongly power-law correlations for time scales less than one year. We
test the nonlinear properties of the river flux increment series by randomizing
its Fourier phases and find that the surrogate volatility series (i) has almost
no seasonal periodicity and (ii) is weakly correlated for time scales less than
one year. We quantify the degree of nonlinearity by measuring (i) the amplitude
of the power spectrum at the seasonal peak and (ii) the correlation power-law
exponent of the volatility series.Comment: 5 revtex pages, 6 page
A search for solar-like oscillations in the Am star HD 209625
The goal is to test the structure of hot metallic stars, and in particular
the structure of a near-surface convection zone using asteroseismic
measurements. Indeed, stellar models including a detailed treatement of the
radiative diffusion predict the existence of a near-surface convection zone in
order to correctly reproduce the anomalies in surface abundances that are
observed in Am stars. The Am star HD 209625 was observed with the Harps
spectrograph mounted on the 3.6-m telescope at the ESO La Silla Observatory
(Chile) during 9 nights in August 2005. This observing run allowed us to
collect 1243 radial velocity (RV) measurements, with a standard deviation of
1.35 m/s. The power spectrum associated with these RV measurements does not
present any excess. Therefore, either the structure of the external layers of
this star does not allow excitation of solar-like oscillations, or the
amplitudes of the oscillations remain below 20-30 cm/s (depending on their
frequency range).Comment: 5 pages, 4 figures, A&A accepte
AP-1 as a Regulator of MMP-13 in the Stromal Cell of Giant Cell Tumor of Bone
Matrix-metalloproteinase-13 (MMP-13) has been shown to be an important protease in inflammatory and neoplastic conditions of the skeletal system. In particular, the stromal cells of giant cell tumor of bone (GCT) express very high levels of MMP-13 in response to the cytokine-rich environment of the tumor. We have previously shown that MMP-13 expression in these cells is regulated, at least in part, by the RUNX2 transcription factor. In the current study, we identify the expression of the c-Fos and c-Jun elements of the AP-1 transcription factor in these cells by protein screening assays and real-time PCR. We then used siRNA gene knockdown to determine that these elements, in particular c-Jun, are upstream regulators of MMP-13 expression and activity in GCT stromal cells. We conclude that there was no synergy found between RUNX2 and AP-1 in the regulation of the MMP13 expression and that these transcription factors may be independently regulated in these cells
Probabilistic Approach to Time-Dependent Load-Transfer Models of Fracture
A probabilistic method for solving time-dependent load-transfer models of
fracture is developed. It is applicable to any rule of load redistribution,
i.e, local, hierarchical, etc. In the new method, the fluctuations are
generated during the breaking process (annealed randomness) while in the usual
method, the random lifetimes are fixed at the beginning (quenched disorder).
Both approaches are equivalent.Comment: 13 pages, 4 figures. To appear in Phys.Rev.
Power-Law Time Distribution of Large Earthquakes
We study the statistical properties of time distribution of seimicity in
California by means of a new method of analysis, the Diffusion Entropy. We find
that the distribution of time intervals between a large earthquake (the main
shock of a given seismic sequence) and the next one does not obey Poisson
statistics, as assumed by the current models. We prove that this distribution
is an inverse power law with an exponent . We propose the
Long-Range model, reproducing the main properties of the diffusion entropy and
describing the seismic triggering mechanisms induced by large earthquakes.Comment: 4 pages, 3 figures. Revised version accepted for publication. Typos
corrected, more detailed discussion on the method used, refs added. Phys.
Rev. Lett. (2003) in pres
A probabilistic approach to Zhang's sandpile model
The current literature on sandpile models mainly deals with the abelian
sandpile model (ASM) and its variants. We treat a less known - but equally
interesting - model, namely Zhang's sandpile. This model differs in two aspects
from the ASM. First, additions are not discrete, but random amounts with a
uniform distribution on an interval . Second, if a site topples - which
happens if the amount at that site is larger than a threshold value
(which is a model parameter), then it divides its entire content in equal
amounts among its neighbors. Zhang conjectured that in the infinite volume
limit, this model tends to behave like the ASM in the sense that the stationary
measure for the system in large volumes tends to be peaked narrowly around a
finite set. This belief is supported by simulations, but so far not by
analytical investigations.
We study the stationary distribution of this model in one dimension, for
several values of and . When there is only one site, exact computations
are possible. Our main result concerns the limit as the number of sites tends
to infinity, in the one-dimensional case. We find that the stationary
distribution, in the case , indeed tends to that of the ASM (up
to a scaling factor), in agreement with Zhang's conjecture. For the case ,
we provide strong evidence that the stationary expectation tends to
.Comment: 47 pages, 3 figure
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