1,670 research outputs found
Downscaling of fracture energy during brittle creep experiments
We present mode 1 brittle creep fracture experiments along fracture surfaces that contain strength heterogeneities. Our observations provide a link between smooth macroscopic time-dependent failure and intermittent microscopic stress-dependent processes. We find the large-scale response of slow-propagating subcritical cracks to be well described by an Arrhenius law that relates the fracture speed to the energy release rate. At the microscopic scale, high-resolution optical imaging of the transparent material used (PMMA) allows detailed description of the fracture front. This reveals a local competition between subcritical and critical propagation (pseudo stick-slip front advances) independently of loading rates. Moreover, we show that the local geometry of the crack front is self-affine and the local crack front velocity is power law distributed. We estimate the local fracture energy distribution by combining high-resolution measurements of the crack front geometry and an elastic line fracture model. We show that the average local fracture energy is significantly larger than the value derived from a macroscopic energy balance. This suggests that homogenization of the fracture energy is not straightforward and should be taken cautiously. Finally, we discuss the implications of our results in the context of fault mechanics
Interplay of seismic and aseismic deformations during earthquake swarms: An experimental approach
Observations of earthquake swarms and slow propagating ruptures on related faults suggest a close relation between the two phenomena. Earthquakes are the signature of fast unstable ruptures initiated on localized asperities while slow aseismic deformations are experienced on large stable segments of the fault plane. The spatial proximity and the temporal coincidence of both fault mechanical responses highlight the variability of fault rheology. However, the mechanism relating earthquakes and aseismic processes is still elusive due to the difficulty of imaging these phenomena of large spatiotemporal variability at depth. Here we present laboratory experiments that explore, in great detail, the deformation processes of heterogeneous interfaces in the brittle-creep regime. We track the evolution of an interfacial crack over 7 orders of magnitude in time and 5 orders of magnitude in space using optical and acoustic sensors. We explore the response of the system to slow transient loads and show that slow deformation episodes are systematically accompanied by acoustic emissions due to local fracture energy disorder. Features of acoustic emission activities and deformation rate distributions of our experimental system are similar to those in natural faults. On the basis of an activation energy model, we link our results to the Rate and State friction model and suggest an active role of local creep deformation in driving the seismic activity of earthquake swarms
High density QCD with static quarks
We study lattice QCD in the limit that the quark mass and chemical potential
are simultaneously made large, resulting in a controllable density of quarks
which do not move. This is similar in spirit to the quenched approximation for
zero density QCD. In this approximation we find that the deconfinement
transition seen at zero density becomes a smooth crossover at any nonzero
density, and that at low enough temperature chiral symmetry remains broken at
all densities.Comment: LaTeX, 18 pages, uses epsf.sty, postscript figures include
Influence of pore-scale disorder on viscous fingering during drainage
We study viscous fingering during drainage experiments in linear Hele-Shaw
cells filled with a random porous medium. The central zone of the cell is found
to be statistically more occupied than the average, and to have a lateral width
of 40% of the system width, irrespectively of the capillary number . A
crossover length separates lower scales where the
invader's fractal dimension is identical to capillary fingering,
and larger scales where the dimension is found to be . The lateral
width and the large scale dimension are lower than the results for Diffusion
Limited Aggregation, but can be explained in terms of Dielectric Breakdown
Model. Indeed, we show that when averaging over the quenched disorder in
capillary thresholds, an effective law relates the
average interface growth rate and the local pressure gradient.Comment: 4 pages, 4 figures, submitted to Phys Rev Letter
Angular-dependence of magnetization switching for a multi-domain dot: experiment and simulation
We have measured the in-plane angular variation of nucleation and
annihilation fields of a multi-domain magnetic single dot with a microsquid.
The dots are Fe/Mo(110) self-assembled in UHV, with sub-micron size and a
hexagonal shape. The angular variations were quantitatively reproduced by
micromagnetic simulations. Discontinuities in the variations are observed, and
shown to result from bifurcations related to the interplay of the non-uniform
magnetization state with the shape of the dot.Comment: 4 pages, 4 figures, for submission as a regular articl
Average crack-front velocity during subcritical fracture propagation in a heterogeneous medium
We study the average velocity of crack fronts during stable interfacial fracture experiments in a heterogeneous quasibrittle material under constant loading rates and during long relaxation tests. The transparency of the material (polymethylmethacrylate) allows continuous tracking of the front position and relation of its evolution to the energy release rate. Despite significant velocity fluctuations at local scales, we show that a model of independent thermally activated sites successfully reproduces the large-scale behavior of the crack front for several loading conditions
The A+B -> 0 annihilation reaction in a quenched random velocity field
Using field-theoretic renormalization group methods the long-time behaviour
of the A+B -> 0 annihilation reaction with equal initial densities n_A(0) =
n_B(0) = n_0 in a quenched random velocity field is studied. At every point (x,
y) of a d-dimensional system the velocity v is parallel or antiparallel to the
x-axis and depends on the coordinates perpendicular to the flow. Assuming that
v(y) have zero mean and short-range correlations in the y-direction we show
that the densities decay asymptotically as n(t) ~ A n_0^(1/2) t^(-(d+3)/8) for
d<3. The universal amplitude A is calculated at first order in \epsilon = 3-d.Comment: 19 pages, LaTeX using IOP-macros, 5 eps-figures. It is shown that the
amplitude of the density is universal, i.e. independent of the reaction rat
Fracture Roughness Scaling: a case study on planar cracks
Using a multi-resolution technique, we analyze large in-plane fracture fronts
moving slowly between two sintered Plexiglas plates. We find that the roughness
of the front exhibits two distinct regimes separated by a crossover length
scale . Below , we observe a multi-affine regime and the
measured roughness exponent is in
agreement with the coalescence model. Above , the fronts are
mono-affine, characterized by a roughness exponent , consistent with the fluctuating line model. We relate the
crossover length scale to fluctuations in fracture toughness and the stress
intensity factor
Controlled switching of N\'eel caps in flux-closure magnetic dots
While magnetic hysteresis usually considers magnetic domains, the switching
of the core of magnetic vortices has recently become an active topic. We
considered Bloch domain walls, which are known to display at the surface of
thin films flux-closure features called N\'eel caps. We demonstrated the
controlled switching of these caps under a magnetic field, occurring via the
propagation of a surface vortex. For this we considered flux-closure states in
elongated micron-sized dots, so that only the central domain wall can be
addressed, while domains remain unaffected.Comment: 4 pages, 3 figure
Branching annihilating random walks with parity conservation on a square lattice
Using Monte Carlo simulations we have studied the transition from an "active"
steady state to an absorbing "inactive" state for two versions of the branching
annihilating random walks with parity conservation on a square lattice. In the
first model the randomly walking particles annihilate when they meet and the
branching process creates two additional particles; in the second case we
distinguish particles and antiparticles created and annihilated in pairs. Quite
distinct critical behavior is found in the two cases, raising the question of
what determines universality in this kind of systems.Comment: 4 pages, 4 EPS figures include
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