2,548 research outputs found
Cracks in rubber under tension exceed the shear wave speed
The shear wave speed is an upper limit for the speed of cracks loaded in
tension in linear elastic solids. We have discovered that in a non-linear
material, cracks in tension (Mode I) exceed this sound speed, and travel in an
intersonic range between shear and longitudinal wave speeds. The experiments
are conducted in highly stretched sheets of rubber; intersonic cracks can be
produced simply by popping a balloon.Comment: 4 pages, 5 eps figure
Dynamic Fracture in Single Crystal Silicon
We have measured the velocity of a running crack in brittle single crystal
silicon as a function of energy flow to the crack tip. The experiments are
designed to permit direct comparison with molecular dynamics simulations;
therefore the experiments provide an indirect but sensitive test of interatomic
potentials. Performing molecular dynamics simulations of brittle crack motion
at the atomic scale we find that experiments and simulations disagree showing
that interatomic potentials are not yet well understood.Comment: 4 pages, 4 figures, 19 reference
Dynamics of Epidemics
This article examines how diseases on random networks spread in time. The
disease is described by a probability distribution function for the number of
infected and recovered individuals, and the probability distribution is
described by a generating function. The time development of the disease is
obtained by iterating the generating function. In cases where the disease can
expand to an epidemic, the probability distribution function is the sum of two
parts; one which is static at long times, and another whose mean grows
exponentially. The time development of the mean number of infected individuals
is obtained analytically. When epidemics occur, the probability distributions
are very broad, and the uncertainty in the number of infected individuals at
any given time is typically larger than the mean number of infected
individuals.Comment: 4 pages and 3 figure
Dynamical stability of the crack front line
Dynamical stability of the crack front line that propagates between two
plates is studied numerically using the simple two-dimensional mass-spring
model. It is demonstrated that the straight front line is unstable for low
speed while it becomes stable for high speed. For the uniform model, the
roughness exponent in the slower speed region is fairly constant around 0.4 and
there seems to be a rough-smooth transition at a certain speed. For the
inhomogeneous case with quenched randomness, the transition is gradual.Comment: 14 pages, 7 figure
Cracks Cleave Crystals
The problem of finding what direction cracks should move is not completely
solved. A commonly accepted way to predict crack directions is by computing the
density of elastic potential energy stored well away from the crack tip, and
finding a direction of crack motion to maximize the consumption of this energy.
I provide here a specific case where this rule fails. The example is of a crack
in a crystal. It fractures along a crystal plane, rather than in the direction
normally predicted to release the most energy. Thus, a correct equation of
motion for brittle cracks must take into account both energy flows that are
described in conventional continuum theories and details of the environment
near the tip that are not.Comment: 6 page
Transillumination imaging through scattering media by use of photorefractive polymers
We demonstrate the use of a near-infrared-sensitive photorefractive polymer with high efficiency for imaging through scattering media, using an all-optical holographic time gate. Imaging through nine scattering mean free paths is performed at 800 nm with a mode-locked continuous-wave Ti:sapphire laser
Densification and preservation of ceramic nanocrystalline character by spark plasma sintering
Spark plasma sintering is a hot pressing technique where rapid heating by dc electric pulses is used simultaneously with applied pressure. Thus, spark plasma sintering is highly suitable for rapid densification of ceramic nanoparticles and preservation of the final nanostructure. A considerable portion of the shrinkage during densification of the green compact of nanoparticles in the first and intermediate stages of sintering occurs during heating by particle rearrangement by sliding and rotation. Further densification to the final stage of sintering takes place by either plastic yield or diffusional processes. Full densification in the final stage of sintering is associated with diffusional processes only. Nanoparticle sliding and rotation during heating may also lead to grain coalescence, with much faster kinetics than normal grain growth at higher temperatures. Based on existing models for particle rearrangement and sliding, the contributions of these processes in conjunction with nanoparticle properties and process parameters were highlighted
Steady-State Cracks in Viscoelastic Lattice Models
We study the steady-state motion of mode III cracks propagating on a lattice
exhibiting viscoelastic dynamics. The introduction of a Kelvin viscosity
allows for a direct comparison between lattice results and continuum
treatments. Utilizing both numerical and analytical (Wiener-Hopf) techniques,
we explore this comparison as a function of the driving displacement
and the number of transverse sites . At any , the continuum theory misses
the lattice-trapping phenomenon; this is well-known, but the introduction of
introduces some new twists. More importantly, for large even at
large , the standard two-dimensional elastodynamics approach completely
misses the -dependent velocity selection, as this selection disappears
completely in the leading order naive continuum limit of the lattice problem.Comment: 27 pages, 8 figure
Electron vortex beams in a magnetic field: A new twist on Landau levels and Aharonov-Bohm states
We examine the propagation of the recently-discovered electron vortex beams
in a longitudinal magnetic field. We consider both the Aharonov-Bohm
configuration with a single flux line and the Landau case of a uniform magnetic
field. While stationary Aharonov-Bohm modes represent Bessel beams with flux-
and vortex-dependent probability distributions, stationary Landau states
manifest themselves as non-diffracting Laguerre-Gaussian beams. Furthermore,
the Landau-state beams possess field- and vortex-dependent phases: (i) the
Zeeman phase from coupling the quantized angular momentum to the magnetic field
and (ii) the Gouy phase, known from optical Laguerre-Gaussian beams.
Remarkably, together these phases determine the structure of Landau energy
levels. This unified Zeeman-Landau-Gouy phase manifests itself in a nontrivial
evolution of images formed by various superpositions of modes. We demonstrate
that, depending on the chosen superposition, the image can rotate in a magnetic
field with either (i) Larmor, (ii) cyclotron (double-Larmor), or (iii) zero
frequency. At the same time, its centroid always follows the classical
cyclotron trajectory, in agreement with the Ehrenfest theorem. Remarkably, the
non-rotating superpositions reproduce stable multi-vortex configurations that
appear in rotating superfluids. Our results open up an avenue for the direct
electron-microscopy observation of fundamental properties of free quantum
electron states in magnetic fields.Comment: 21 pages, 10 figures, 1 table, to appear in Phys. Rev.
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