4,462 research outputs found
On a discrete-to-continuum convergence result for a two dimensional brittle material in the small displacement regime
We consider a two-dimensional atomic mass spring system and show that in the
small displacement regime the corresponding discrete energies can be related to
a continuum Griffith energy functional in the sense of Gamma-convergence. We
also analyze the continuum problem for a rectangular bar under tensile boundary
conditions and find that depending on the boundary loading the minimizers are
either homogeneous elastic deformations or configurations that are completely
cracked generically along a crystallographic line. As applications we discuss
cleavage properties of strained crystals and an effective continuum fracture
energy for magnets
Onset of Propagation of Planar Cracks in Heterogeneous Media
The dynamics of planar crack fronts in hetergeneous media near the critical
load for onset of crack motion are investigated both analytically and by
numerical simulations. Elasticity of the solid leads to long range stress
transfer along the crack front which is non-monotonic in time due to the
elastic waves in the medium. In the quasistatic limit with instantaneous stress
transfer, the crack front exhibits dynamic critical phenomenon, with a second
order like transition from a pinned to a moving phase as the applied load is
increased through a critical value. At criticality, the crack-front is
self-affine, with a roughness exponent . The dynamic
exponent is found to be equal to and the correlation length
exponent . These results are in good agreement with those
obtained from an epsilon expansion. Sound-travel time delays in the stress
transfer do not change the static exponents but the dynamic exponent
becomes exactly one. Real elastic waves, however, lead to overshoots in the
stresses above their eventual static value when one part of the crack front
moves forward. Simplified models of these stress overshoots are used to show
that overshoots are relevant at the depinning transition leading to a decrease
in the critical load and an apparent jump in the velocity of the crack front
directly to a non-zero value. In finite systems, the velocity also shows
hysteretic behaviour as a function of the loading. These results suggest a
first order like transition. Possible implications for real tensile cracks are
discussed.Comment: 51 pages + 20 figur
How river rocks round: resolving the shape-size paradox
River-bed sediments display two universal downstream trends: fining, in which
particle size decreases; and rounding, where pebble shapes evolve toward
ellipsoids. Rounding is known to result from transport-induced abrasion;
however many researchers argue that the contribution of abrasion to downstream
fining is negligible. This presents a paradox: downstream shape change
indicates substantial abrasion, while size change apparently rules it out. Here
we use laboratory experiments and numerical modeling to show quantitatively
that pebble abrasion is a curvature-driven flow problem. As a consequence,
abrasion occurs in two well-separated phases: first, pebble edges rapidly round
without any change in axis dimensions until the shape becomes entirely convex;
and second, axis dimensions are then slowly reduced while the particle remains
convex. Explicit study of pebble shape evolution helps resolve the shape-size
paradox by reconciling discrepancies between laboratory and field studies, and
enhances our ability to decipher the transport history of a river rock.Comment: 11 pages, 5 figure
On microscopic origins of generalized gradient structures
Classical gradient systems have a linear relation between rates and driving
forces. In generalized gradient systems we allow for arbitrary relations
derived from general non-quadratic dissipation potentials. This paper describes
two natural origins for these structures.
A first microscopic origin of generalized gradient structures is given by the
theory of large-deviation principles. While Markovian diffusion processes lead
to classical gradient structures, Poissonian jump processes give rise to
cosh-type dissipation potentials.
A second origin arises via a new form of convergence, that we call
EDP-convergence. Even when starting with classical gradient systems, where the
dissipation potential is a quadratic functional of the rate, we may obtain a
generalized gradient system in the evolutionary -limit. As examples we
treat (i) the limit of a diffusion equation having a thin layer of low
diffusivity, which leads to a membrane model, and (ii) the limit of diffusion
over a high barrier, which gives a reaction-diffusion system.Comment: Keywords: Generalized gradient structure, gradient system,
evolutionary \Gamma-convergence, energy-dissipation principle, variational
evolution, relative entropy, large-deviation principl
Statistical mechanics and dynamics of solvable models with long-range interactions
The two-body potential of systems with long-range interactions decays at
large distances as , with , where is the
space dimension. Examples are: gravitational systems, two-dimensional
hydrodynamics, two-dimensional elasticity, charged and dipolar systems.
Although such systems can be made extensive, they are intrinsically non
additive. Moreover, the space of accessible macroscopic thermodynamic
parameters might be non convex. The violation of these two basic properties is
at the origin of ensemble inequivalence, which implies that specific heat can
be negative in the microcanonical ensemble and temperature jumps can appear at
microcanonical first order phase transitions. The lack of convexity implies
that ergodicity may be generically broken. We present here a comprehensive
review of the recent advances on the statistical mechanics and
out-of-equilibrium dynamics of systems with long-range interactions. The core
of the review consists in the detailed presentation of the concept of ensemble
inequivalence, as exemplified by the exact solution, in the microcanonical and
canonical ensembles, of mean-field type models. Relaxation towards
thermodynamic equilibrium can be extremely slow and quasi-stationary states may
be present. The understanding of such unusual relaxation process is obtained by
the introduction of an appropriate kinetic theory based on the Vlasov equation.Comment: 118 pages, review paper, added references, slight change of conten
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