340 research outputs found
Fatigue crack propagation in a quasi one-dimensional elasto-plastic model
Fatigue crack advance induced by the application of cyclic quasistatic loads
is investigated both numerically and analytically using a lattice spring model.
The system has a quasi-one-dimensional geometry, and consists in two
symmetrical chains that are pulled apart, thus breaking springs which connect
them, and producing the advance of a crack. Quasistatic crack advance occurs as
a consequence of the plasticity included in the springs which form the chains,
and that implies a history dependent stress-strain curve for each spring. The
continuous limit of the model allows a detailed analytical treatment that gives
physical insight of the propagation mechanism. This simple model captures key
features that cause well known phenomenology in fatigue crack propagation, in
particular a Paris-like law of crack advance under cyclic loading, and the
overload retardation effect.Comment: To be published in the International Journal of Solids and Structure
Editorial
Introduction to volume 2 of Student Perspectives About Civic Engagement
Tuning spreading and avalanche-size exponents in directed percolation with modified activation probabilities
We consider the directed percolation process as a prototype of systems
displaying a nonequilibrium phase transition into an absorbing state. The model
is in a critical state when the activation probability is adjusted at some
precise value p_c. Criticality is lost as soon as the probability to activate
sites at the first attempt, p1, is changed. We show here that criticality can
be restored by "compensating" the change in p1 by an appropriate change of the
second time activation probability p2 in the opposite direction. At
compensation, we observe that the bulk exponents of the process coincide with
those of the normal directed percolation process. However, the spreading
exponents are changed, and take values that depend continuously on the pair
(p1, p2). We interpret this situation by acknowledging that the model with
modified initial probabilities has an infinite number of absorbing states.Comment: 9 pages, 11 figure
Magnetic Domain Patterns Depending on the Sweeping Rate of Magnetic Fields
The domain patterns in a thin ferromagnetic film are investigated in both
experiments and numerical simulations. Magnetic domain patterns under a zero
field are usually observed after an external magnetic field is removed. It is
demonstrated that the characteristics of the domain patterns depend on the
decreasing rate of the external field, although it can also depend on other
factors. Our numerical simulations and experiments show the following
properties of domain patterns: a sea-island structure appears when the field
decreases rapidly from the saturating field to the zero field, while a
labyrinth structure is observed for a slowly decreasing field. The mechanism of
the dependence on the field sweeping rate is discussed in terms of the concepts
of crystallization.Comment: 4 pages, 3 figure
Diffusion Anomaly in an Associating Lattice Gas Model
We investigate the relation between thermodynamic and dynamic properties of
an associating lattice gas (ALG) model. The ALG combines a two dimensional
lattice gas with particles interacting through a soft core potential and
orientational degrees of freedom. From the competition between the directional
attractive forces and the soft core potential results two liquid phases, double
criticality and density anomaly. We study the mobility of the molecules in this
model by calculating the diffusion constant at a constant temperature, . We
show that has a maximum at a density and a minimum at a
density . Between these densities the diffusivity
differs from the one expected for normal liquids. We also show that in the
pressure-temperature phase-diagram the line of extrema in diffusivity is close
to the liquid-liquid critical point and it is inside the temperature of maximum
density (TMD) line.Comment: 12 pages, 9 figure
Liquid-liquid equilibrium for monodisperse spherical particles
A system of identical particles interacting through an isotropic potential
that allows for two preferred interparticle distances is numerically studied.
When the parameters of the interaction potential are adequately chosen, the
system exhibits coexistence between two different liquid phases (in addition to
the usual liquid-gas coexistence). It is shown that this coexistence can occur
at equilibrium, namely, in the region where the liquid is thermodynamically
stable.Comment: 6 pages, 8 figures. Published versio
Surface defreezing of glasses
A glass surface may still flow below the bulk glass transition temperature,
where the underlying bulk is frozen. Assuming the existence at T=T* of a bulk
thermodynamical glass transition, we show that the glass-vapor interface is
generally wetted by a liquid layer of thickness ~ -ln(T*-T) when T--> T*.
Contrary to standard surface melting of crystals however, the integrated value
of the diffusivity across the interface remains finite for T-->T*. Difference
in shape induced by bulk and by surface flow is discussed as a possible means
of experimental detection of surface defreezing.Comment: five pages, three figure
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