266 research outputs found
Creep rupture of materials: insights from a fiber bundle model with relaxation
I adapted a model recently introduced in the context of seismic phenomena, to
study creep rupture of materials. It consists of linear elastic fibers that
interact in an equal load sharing scheme, complemented with a local
viscoelastic relaxation mechanism. The model correctly describes the three
stages of the creep process, namely an initial Andrade regime of creep
relaxation, an intermediate regime of rather constant creep rate, and a
tertiary regime of accelerated creep towards final failure of the sample. In
the tertiary regime creep rate follows the experimentally observed one over
time-to-failure dependence. The time of minimum strain rate is systematically
observed to be about 60-65 % of the time to failure, in accordance with
experimental observations. In addition, burst size statistics of breaking
events display a -3/2 power law for events close to the time of failure, and a
steeper decay for the all-time distribution. Statistics of interevent times
shows a tendency of the events to cluster temporarily. This behavior should be
observable in acoustic emission experiments
Boundary lubrication properties of materials with expansive freezing
We have performed molecular dynamics simulations of solid-solid contacts
lubricated by a model fluid displaying many of the properties of water,
particularly its expansive freezing. Near the region where expansive freezing
occurs, the lubricating film remains fluid, and the friction force decreases
linearly as the shear velocity is reduced. No sign of stick-slip motion is
observed even at the lowest velocities. We give a simple interpretation of
these results, and suggest that in general good boundary lubrication properties
will be found in the family of materials with expansive freezing.Comment: Version to appear in Phys. Rev. Let
Shaping leg muscles in Drosophila: role of ladybird, a conserved regulator of appendicular myogenesis
Legs are locomotor appendages used by a variety of evolutionarily distant vertebrates and invertebrates. The primary biological leg function, locomotion, requires the formation of a specialised appendicular musculature. Here we report evidence that ladybird, an orthologue of the Lbx1 gene recognised as a hallmark of appendicular myogenesis in vertebrates, is expressed in leg myoblasts, and regulates the shape, ultrastructure and functional properties of leg muscles in Drosophila. ladybird expression is progressively activated in myoblasts associated with the imaginal leg disc and precedes that of the founder cell marker dumbfounded. The RNAi-mediated attenuation of ladybird expression alters properties of developing myotubes, impairing their ability to grow and interact with the internal tendons and epithelial attachment sites. It also affects sarcomeric ultrastructure, resulting in reduced leg muscle performance and impaired mobility in surviving flies. The over-expression of ladybird also results in an abnormal pattern of dorsally located leg muscles, indicating different requirements for ladybird in dorsal versus ventral muscles. This differential effect is consistent with the higher level of Ladybird in ventrally located myoblasts and with positive ladybird regulation by extrinsic Wingless signalling from the ventral epithelium. In addition, ladybird expression correlates with that of FGF receptor Heartless and the read-out of FGF signalling downstream of FGF. FGF signals regulate the number of leg disc associated myoblasts and are able to accelerate myogenic differentiation by activating ladybird, leading to ectopic muscle fibre formation. A key role for ladybird in leg myogenesis is further supported by its capacity to repress vestigial and to down-regulate the vestigial-governed flight muscle developmental programme. Thus in Drosophila like in vertebrates, appendicular muscles develop from a specialised pool of myoblasts expressing ladybird/Lbx1. The ladybird/Lbx1 gene family appears as a part of an ancient genetic circuitry determining leg-specific properties of myoblasts and making an appendage adapted for locomotion
Longitudinal and transverse dissipation in a simple model for the vortex lattice with screening
Transport properties of the vortex lattice in high temperature
superconductors are studied using numerical simulations in the case in which
the non-local interactions between vortex lines are dismissed. The results
obtained for the longitudinal and transverse resistivities in the presence of
quenched disorder are compared with the results of experimental measurements
and other numerical simulations where the full interaction is considered. This
work shows that the dependence on temperature of the resistivities is well
described by the model without interactions, thus indicating that many of the
transport characteristics of the vortex structure in real materials are mainly
a consequence of the topological configuration of the vortex structure only. In
addition, for highly anisotropic samples, a regime is obtained where
longitudinal coherence is lost at temperatures where transverse coherence is
still finite. I discuss the possibility of observing this regime in real
samples.Comment: 9 pages, 7 figures included using epsf.st
Some exact results for the velocity of cracks propagating in non-linear elastic models
We analyze a piece-wise linear elastic model for the propagation of a crack
in a stripe geometry under mode III conditions, in the absence of dissipation.
The model is continuous in the propagation direction and discrete in the
perpendicular direction. The velocity of the crack is a function of the value
of the applied strain. We find analytically the value of the propagation
velocity close to the Griffith threshold, and close to the strain of uniform
breakdown. Contrary to the case of perfectly harmonic behavior up to the
fracture point, in the piece-wise linear elastic model the crack velocity is
lower than the sound velocity, reaching this limiting value at the strain of
uniform breakdown. We complement the analytical results with numerical
simulations and find excellent agreement.Comment: 9 pages, 13 figure
Mechanical versus thermodynamical melting in pressure-induced amorphization: the role of defects
We study numerically an atomistic model which is shown to exhibit a one--step
crystal--to--amorphous transition upon decompression. The amorphous phase
cannot be distinguished from the one obtained by quenching from the melt. For a
perfectly crystalline starting sample, the transition occurs at a pressure at
which a shear phonon mode destabilizes, and triggers a cascade process leading
to the amorphous state. When defects are present, the nucleation barrier is
greatly reduced and the transformation occurs very close to the extrapolation
of the melting line to low temperatures. In this last case, the transition is
not anticipated by the softening of any phonon mode. Our observations reconcile
different claims in the literature about the underlying mechanism of pressure
amorphization.Comment: 7 pages, 7 figure
Anatomical aspects of epidural and spinal analgesia
Regional anaesthesia seems to be the future of the anaesthesia in this century. The knowledge of the
anatomy of the epidural and other spinal spaces seems to play the crucial role in success of regional
anaesthesia. It's important in perioperative medicine and cancer pain treatment. Up to date there is not
too many datas considering anatomy of these compartments. Many of the results obtained by researchers
in the past are still not mentioned in the clinical textbooks. This article is an attempt to resolve this
problem.Regional anaesthesia seems to be the future of the anaesthesia in this century. The knowledge of the
anatomy of the epidural and other spinal spaces seems to play the crucial role in success of regional
anaesthesia. It's important in perioperative medicine and cancer pain treatment. Up to date there is not
too many datas considering anatomy of these compartments. Many of the results obtained by researchers
in the past are still not mentioned in the clinical textbooks. This article is an attempt to resolve this
problem
The phase diagram of high-Tc's: Influence of anisotropy and disorder
We propose a phase diagram for the vortex structure of high temperature
superconductors which incorporates the effects of anisotropy and disorder. It
is based on numerical simulations using the three-dimensional Josephson
junction array model. We support the results with an estimation of the internal
energy and configurational entropy of the system. Our results give a unified
picture of the behavior of the vortex lattice, covering from the very
anysotropic BiSrCaCuO to the less anisotropic YBaCuO, and from the first order
melting ocurring in clean samples to the continuous transitions observed in
samples with defects.Comment: 8 pages with 7 figure
Supersonic crack propagation in a class of lattice models of Mode III brittle fracture
We study a lattice model for mode III crack propagation in brittle materials
in a stripe geometry at constant applied stretching. Stiffening of the material
at large deformation produces supersonic crack propagation. For large
stretching the propagation is guided by well developed soliton waves. For low
stretching, the crack-tip velocity has a universal dependence on stretching
that can be obtained using a simple geometrical argument.Comment: 4 pages, 3 figure
Classical isotropic two body potentials generating martensitic transformations
An isotropic interaction potential for classical particles is devised in such
a way that the crystalline ground state of the system changes discontinuously
when some parameter of the potential is varied. Using this potential we model
martensitic transformations, and are able to study in detail the processes that
are usually associated with it: shape memory effect, superelasticity, as well
as many details concerning the dynamics of the transformation, particularly the
characteristics of the martensitic texture obtained as a function of parameters
affecting the transformation rate. Here we introduce the interaction potentials
and present some basic results about the transformation it describes, for the
particular case of two dimensional triangular-rombohedral and triangular-square
transformation.Comment: 13 pages, 14 figure
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