1,655 research outputs found
Cracklike Dynamics at the Onset of Frictional Sliding
We propose an elasto-plastic inspired friction model which incorporates
interfacial stiffness. Steady state sliding friction is characterized by a
generic nonmonotonic behavior, including both velocity weakening and
strengthening branches. In 1D and upon the application of sideway loading, we
demonstrate the existence of transient cracklike fronts whose velocity is
independent of sound speed, which we propose to be analogous to the recently
discovered slow interfacial rupture fronts. Most importantly, the properties of
these transient inhomogeneously loaded fronts are determined by steady state
front solutions at the {\em minimum} of the sliding friction law, implying the
existence of a new velocity scale and a "forbidden gap" of rupture velocities.
We highlight the role played by interfacial stiffness and supplement our
analysis with 2D scaling arguments.Comment: 4 pages, 2 figure
Noise induced rupture process: Phase boundary and scaling of waiting time distribution
A bundle of fibers has been considered here as a model for composite
materials, where breaking of the fibers occur due to a combined influence of
applied load (stress) and external noise. Through numerical simulation and a
mean-field calculation we show that there exists a robust phase boundary
between continuous (no waiting time) and intermittent fracturing regimes. In
the intermittent regime, throughout the entire rupture process avalanches of
different sizes are produced and there is a waiting time between two
consecutive avalanches. The statistics of waiting times follows a Gamma
distribution and the avalanche distribution shows power law scaling, similar to
what have been observed in case of earthquake events and bursts in fracture
experiments. We propose a prediction scheme that can tell when the system is
expected to reach the continuous fracturing point from the intermittent phase.Comment: 6 pages, 8 figure
Dynamic instabilities of fracture under biaxial strain using a phase field model
We present a phase field model of the propagation of fracture under plane
strain. This model, based on simple physical considerations, is able to
accurately reproduce the different behavior of cracks (the principle of local
symmetry, the Griffith and Irwin criteria, and mode-I branching). In addition,
we test our model against recent experimental findings showing the presence of
oscillating cracks under bi-axial load. Our model again reproduces well
observed supercritical Hopf bifurcation, and is therefore the first simulation
which does so
Predicting the long-term impact of antiretroviral therapy scale-up on population incidence of tuberculosis.
OBJECTIVE: To investigate the impact of antiretroviral therapy (ART) on long-term population-level tuberculosis disease (TB) incidence in sub-Saharan Africa. METHODS: We used a mathematical model to consider the effect of different assumptions about life expectancy and TB risk during long-term ART under alternative scenarios for trends in population HIV incidence and ART coverage. RESULTS: All the scenarios we explored predicted that the widespread introduction of ART would initially reduce population-level TB incidence. However, many modelled scenarios projected a rebound in population-level TB incidence after around 20 years. This rebound was predicted to exceed the TB incidence present before ART scale-up if decreases in HIV incidence during the same period were not sufficiently rapid or if the protective effect of ART on TB was not sustained. Nevertheless, most scenarios predicted a reduction in the cumulative TB incidence when accompanied by a relative decline in HIV incidence of more than 10% each year. CONCLUSIONS: Despite short-term benefits of ART scale-up on population TB incidence in sub-Saharan Africa, longer-term projections raise the possibility of a rebound in TB incidence. This highlights the importance of sustaining good adherence and immunologic response to ART and, crucially, the need for effective HIV preventive interventions, including early widespread implementation of ART
Cleaved surface of i-AlPdMn quasicrystals: Influence of the local temperature elevation at the crack tip on the fracture surface roughness
Roughness of i-AlPdMn cleaved surfaces are presently analysed. From the
atomic scale to 2-3 nm, they are shown to exhibit scaling properties hiding the
cluster (0.45 nm) aperiodic structure. These properties are quantitatively
similar to those observed on various disordered materials, albeit on other
ranges of length scales. These properties are interpreted as the signature of
damage mechanisms occurring within a 2-3 nm wide zone at the crack tip. The
size of this process zone finds its origin in the local temperature elevation
at the crack tip. For the very first time, this effect is reported to be
responsible for a transition from a perfectly brittle behavior to a nanoductile
one.Comment: 8 page
Void Formation and Roughening in Slow Fracture
Slow crack propagation in ductile, and in certain brittle materials, appears
to take place via the nucleation of voids ahead of the crack tip due to plastic
yields, followed by the coalescence of these voids. Post mortem analysis of the
resulting fracture surfaces of ductile and brittle materials on the m-mm
and the nm scales respectively, reveals self-affine cracks with anomalous
scaling exponent in 3-dimensions and in
2-dimensions. In this paper we present an analytic theory based on the method
of iterated conformal maps aimed at modelling the void formation and the
fracture growth, culminating in estimates of the roughening exponents in
2-dimensions. In the simplest realization of the model we allow one void ahead
of the crack, and address the robustness of the roughening exponent. Next we
develop the theory further, to include two voids ahead of the crack. This
development necessitates generalizing the method of iterated conformal maps to
include doubly connected regions (maps from the annulus rather than the unit
circle). While mathematically and numerically feasible, we find that the
employment of the stress field as computed from elasticity theory becomes
questionable when more than one void is explicitly inserted into the material.
Thus further progress in this line of research calls for improved treatment of
the plastic dynamics.Comment: 15 pages, 20 figure
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
Crack fronts and damage in glass at the nanometer scale
We have studied the low speed fracture regime for different glassy materials
with variable but controlled length scales of heterogeneity in a carefully
mastered surrounding atmosphere. By using optical and atomic force microscopy
(AFM) techniques we tracked in real-time the crack tip propagation at the
nanometer scale on a wide velocity range (mm/s - pm/s and below). The influence
of the heterogeneities on this velocity is presented and discussed. Our
experiments reveal also -for the first time- that the crack progresses through
nucleation, growth and coalescence of nanometric damage cavities within the
amorphous phase. This may explain the large fluctuations observed in the crack
tip velocities for the smallest values. This behaviour is very similar to what
is involved, at the micrometric scale, in ductile fracture. The only difference
is very likely due to the related length scales (nanometric instead of
micrometric). Consequences of such a nano-ductile fracture mode observed at a
temperature far below the glass transition temperature in glass is finally
discussed.Comment: 12 pages, 8 figures, submitted to Journal of Physics: Condensed
Matter; Invited talk at Glass and Optical Materials Division Fall 2002
Meeting, Pittsburgh, Pa, US
Phase Transition in a Random Fragmentation Problem with Applications to Computer Science
We study a fragmentation problem where an initial object of size x is broken
into m random pieces provided x>x_0 where x_0 is an atomic cut-off.
Subsequently the fragmentation process continues for each of those daughter
pieces whose sizes are bigger than x_0. The process stops when all the
fragments have sizes smaller than x_0. We show that the fluctuation of the
total number of splitting events, characterized by the variance, generically
undergoes a nontrivial phase transition as one tunes the branching number m
through a critical value m=m_c. For m<m_c, the fluctuations are Gaussian where
as for m>m_c they are anomalously large and non-Gaussian. We apply this general
result to analyze two different search algorithms in computer science.Comment: 5 pages RevTeX, 3 figures (.eps
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
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