199 research outputs found
Super-Arrhenius dynamics for sub-critical crack growth in disordered brittle media
Taking into account stress fluctuations due to thermal noise, we study
thermally activated irreversible crack growth in disordered media. The
influence of material disorder on sub-critical growth of a single crack in
two-dimensional brittle elastic material is described through the introduction
of a rupture threshold distribution. We derive analytical predictions for crack
growth velocity and material lifetime in agreement with direct numerical
calculations. It is claimed that crack growth process is inhibited by disorder:
velocity decreases and lifetime increases with disorder. More precisely,
lifetime is shown to follow a super-Arrhenius law, with an effective
temperature theta - theta_d, where theta is related to the thermodynamical
temperature and theta_d to the disorder variance.Comment: Submitted to Europhysics Letter
Thermal activation of rupture and slow crack growth in a model of homogenous brittle materials
Slow crack growth in a model of homogenous brittle elastic material is
described as a thermal activation process where stress fluctuations allow to
overcome a breaking threshold through a series of irreversible steps. We study
the case of a single crack in a flat sheet for which analytical predictions can
be made, and compare them with results from the equivalent problem of a 2D
spring network. Good statistical agreement is obtained for the crack growth
profile and final rupture time. The specific scaling of the energy barrier with
stress intensity factor appears as a consequence of irreversibility. In
addition, the model brings out a characteristic growth length whose physical
meaning could be tested experimentally.Comment: To be published in : Europhysics Letter
Two new topologically ordered glass phases of smectics confined in anisotropic random media
We show that smectic liquid crystals confined in_anisotropic_ porous
structures such as e.g.,_strained_ aerogel or aerosil exhibit two new glassy
phases. The strain both ensures the stability of these phases and determines
their nature. One type of strain induces an ``XY Bragg glass'', while the other
creates a novel, triaxially anisotropic ``m=1 Bragg glass''. The latter
exhibits anomalous elasticity, characterized by exponents that we calculate to
high precision. We predict the phase diagram for the system, and numerous other
experimental observables.Comment: 4 RevTeX pgs, 2 eps figures, submitted to Phys. Rev. Let
Uniaxial and biaxial soft deformations of nematic elastomers
We give a geometric interpretation of the soft elastic deformation modes of
nematic elastomers, with explicit examples, for both uniaxial and biaxial
nematic order. We show the importance of body rotations in this non-classical
elasticity and how the invariance under rotations of the reference and target
states gives soft elasticity (the Golubovic and Lubensky theorem). The role of
rotations makes the Polar Decomposition Theorem vital for decomposing general
deformations into body rotations and symmetric strains. The role of the square
roots of tensors is discussed in this context and that of finding explicit
forms for soft deformations (the approach of Olmsted).Comment: 10 pages, 10 figures, RevTex, AmsTe
Subcritical crack growth in fibrous materials
We present experiments on the slow growth of a single crack in a fax paper
sheet submitted to a constant force . We find that statistically averaged
crack growth curves can be described by only two parameters : the mean rupture
time and a characteristic growth length . We propose a model
based on a thermally activated rupture process that takes into account the
microstructure of cellulose fibers. The model is able to reproduce the shape of
the growth curve, the dependence of on as well as the effect of
temperature on the rupture time . We find that the length scale at which
rupture occurs in this model is consistently close to the diameter of cellulose
microfibrils
Averaging rheological quantities in descriptions of soft glassy materials
Many mean-field models have been introduced to describe the mechanical
behavior of glassy materials. They often rely on averages performed over
distributions of elements or states. We here underline that averaging is a more
intricate procedure in mechanics than in more classical situations such as
phase transitions in magnetic systems. This leads us to modify the predictions
of the recently proposed SGR model for soft glassy materials, for which we
suggest that the viscosity should diverge at the glass transition temperature
with an exponential form .Comment: 4 pages, Latex, 1 eps figur
Sliding Luttinger liquid phases
We study systems of coupled spin-gapped and gapless Luttinger liquids. First,
we establish the existence of a sliding Luttinger liquid phase for a system of
weakly coupled parallel quantum wires, with and without disorder. It is shown
that the coupling can {\it stabilize} a Luttinger liquid phase in the presence
of disorder. We then extend our analysis to a system of crossed Luttinger
liquids and establish the stability of a non-Fermi liquid state: the crossed
sliding Luttinger liquid phase (CSLL). In this phase the system exhibits a
finite-temperature, long-wavelength, isotropic electric conductivity that
diverges as a power law in temperature as . This two-dimensional
system has many properties of a true isotropic Luttinger liquid, though at zero
temperature it becomes anisotropic. An extension of this model to a
three-dimensional stack exhibits a much higher in-plane conductivity than the
conductivity in a perpendicular direction.Comment: Revtex, 18 pages, 8 figure
Failure time in the fiber-bundle model with thermal noise and disorder
The average time for the onset of macroscopic fractures is analytically and
numerically investigated in the fiber-bundle model with quenched disorder and
thermal noise under a constant load. We find an implicit exact expression for
the failure time in the low-temperature limit that is accurately confirmed by
direct simulations. The effect of the disorder is to lower the energy barrier.Comment: 11 pages, 6 figures; accepted for publication in Phys. Rev.
Continuum limit, Galilean invariance, and solitons in the quantum equivalent of the noisy Burgers equation
A continuum limit of the non-Hermitian spin-1/2 chain, conjectured recently
to belong to the universality class of the noisy Burgers or, equivalently,
Kardar-Parisi-Zhang equation, is obtained and analyzed. The Galilean invariance
of the Burgers equation is explicitly realized in the operator algebra. In the
quasi-classical limit we find nonlinear soliton excitations exhibiting the
dispersion relation with dynamical exponent .Comment: 12 pages, latex, no figure
Mounding Instability and Incoherent Surface Kinetics
Mounding instability in a conserved growth from vapor is analysed within the
framework of adatom kinetics on the growing surface. The analysis shows that
depending on the local structure on the surface, kinetics of adatoms may vary,
leading to disjoint regions in the sense of a continuum description. This is
manifested particularly under the conditions of instability. Mounds grow on
these disjoint regions and their lateral growth is governed by the flux of
adatoms hopping across the steps in the downward direction. Asymptotically
ln(t) dependence is expected in 1+1- dimensions. Simulation results confirm the
prediction. Growth in 2+1- dimensions is also discussed.Comment: 4 pages, 4 figure
- âŠ