189 research outputs found
Roughness of stylolites: a stress-induced instability with non local interactions
We study the roughness of stylolite surfaces (i.e. natural
pressure-dissolution surfaces in sedimentary rocks) from profiler measurements
at laboratory scales. The roughness is shown to be nicely described by a
self-affine scaling invariance. At large scales, the roughness exponent is
and very different from that at small scales where
. A cross-over length scale at around mm is
well characterized and interpreted as a possible fossil stress measurement if
related to the Asaro-Tiller-Grinfeld stress-induced instability. Measurements
are consistent with a Langevin equation that describes the growth of stylolite
surfaces in a quenched disordered material with long range elastic
correlations.Comment: 4 pages, 5 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
Origin of the Universal Roughness Exponent of Brittle Fracture Surfaces: Correlated Percolation in the Damage Zone
We suggest that the observed large-scale universal roughness of brittle
fracture surfaces is due to the fracture process being a correlated percolation
process in a self-generated quadratic damage gradient. We use the quasi-static
two-dimensional fuse model as a paradigm of a fracture model. We measure for
this model, that exhibits a correlated percolation process, the correlation
length exponent nu approximately equal to 1.35 and conjecture it to be equal to
that of uncorrelated percolation, 4/3. We then show that the roughness exponent
in the fuse model is zeta = 2 nu/(1+2 nu)= 8/11. This is in accordance with the
numerical value zeta=0.75. As for three-dimensional brittle fractures, a
mean-field theory gives nu=2, leading to zeta=4/5 in full accordance with the
universally observed value zeta =0.80.Comment: 4 pages RevTeX
Anomalous roughening of wood fractured surfaces
Scaling properties of wood fractured surfaces are obtained from samples of
three different sizes. Two different woods are studied: Norway spruce and
Maritime pine. Fracture surfaces are shown to display an anomalous dynamic
scaling of the crack roughness. This anomalous scaling behavior involves the
existence of two different and independent roughness exponents. We determine
the local roughness exponents to be 0.87 for spruce and 0.88
for pine. These results are consistent with the conjecture of a universal local
roughness exponent. The global roughness exponent is different for both woods,
= 1.60 for spruce and = 1.35 for pine. We argue that the global
roughness exponent is a good index for material characterization.Comment: 7 two columns pages plus 8 ps figures, uses psfig. To appear in
Physical Review
Roughness of Interfacial Crack Front: Correlated Percolation in the Damage Zone
We show that the roughness exponent zeta of an in-plane crack front slowly
propagating along a heterogeneous interface embeded in a elastic body, is in
full agreement with a correlated percolation problem in a linear gradient. We
obtain zeta=nu/(1+nu) where nu is the correlation length critical exponent. We
develop an elastic brittle model based on both the 3D Green function in an
elastic half-space and a discrete interface of brittle fibers and find
numerically that nu=1.5, We conjecture it to be 3/2. This yields zeta=3/5. We
also obtain by direct numerical simulations zeta=0.6 in excellent agreement
with our prediction. This modelling is for the first time in close agreement
with experimental observations.Comment: 4 pages RevTeX
Distinguishing fractional and white noise in one and two dimensions
We discuss the link between uncorrelated noise and Hurst exponent for one and
two-dimensional interfaces. We show that long range correlations cannot be
observed using one-dimensional cuts through two-dimensional self-affine
surfaces whose height distributions are characterized by a Hurst exponent lower
than -1/2. In this domain, fractional and white noise are not distinguishable.
A method analysing the correlations in two dimensions is necessary. For Hurst
exponents larger than -1/2, a crossover regime leads to a systematic over
estimate of the Hurst exponent.Comment: 3 pages RevTeX, 4 Postscript figure
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
Revolving rivers in sandpiles: from continuous to intermittent flows
In a previous paper [Phys. Rev. Lett. 91, 014501 (2003)], the mechanism of
"revolving rivers" for sandpile formation is reported: as a steady stream of
dry sand is poured onto a horizontal surface, a pile forms which has a river of
sand on one side owing from the apex of the pile to the edge of the base. For
small piles the river is steady, or continuous. For larger piles, it becomes
intermittent. In this paper we establish experimentally the "dynamical phase
diagram" of the continuous and intermittent regimes, and give further details
of the piles topography, improving the previous kinematic model to describe it
and shedding further light on the mechanisms of river formation. Based on
experiments in Hele-Shaw cells, we also propose that a simple dimensionality
reduction argument can explain the transition between the continuous and
intermittent dynamics.Comment: 8 pages, 11 figures, submitted to Phys Rev
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