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The propagation of cracks in two-dimensional decagonal model quasicrystals is studied under mode I loading by means of molecular dynamics simulations. In particular, we investigate the dependence on temperature, applied load and underlying structure. The samples are endowed with an atomically sharp crack and strained by linear scaling of the displacement field. Three different regimes of propagation and discernible with increasing temperature. For low temperatures the crack velocity increases monotonically with increasing applied load. We observe that the crack follows the path of dislocations nucleated at its tip. For temperatures above 0.3?T m, where T m is the melting temperature, the crack does not remain atomically sharp but becomes blunt spontaneously. In the temperature range between 0.7?T m and 0.8?T m the quasicrystal fails by nucleation, growth and coalescence of microvoids. This gradual dislocation-free crack extension is caused by plastic deformation which is mediated by localized rearrangements comparable with the so-called shear transformation zones. These are also observed in amorphous solids. Thus, at low temperatures the crack propagates along crystallographic planes just as in periodic crystals, whereas at high temperatures a glass-like behaviour is dominant. 相似文献
3.
J. Hegseth A. Oprisan Y. Garrabos C. Lecoutre-Chabot V. S. Nikolayev D. Beysens 《The European physical journal. E, Soft matter》2008,26(4):345-353
The heating of coexisting gas and liquid phases of pure fluid through its critical point makes the fluid extremely compressible,
expandable, slows the diffusive transport, and decreases the contact angle to zero (perfect wetting by the liquid phase).
We have performed experiments on near-critical fluids in a variable volume cell in the weightlessness of an orbiting space
vehicle, to suppress buoyancy-driven flows and gravitational constraints on the liquid-gas interface. The high compressibility,
high thermal expansion, and low thermal diffusivity lead to a pronounced adiabatic heating called the piston effect. We have
directly visualized the near-critical fluid’s boundary layer response to a volume quench when the external temperature is
held constant. We have found that when the system’s temperature T is increased at a constant rate past the critical temperature T
c
, the interior of the fluid gains a higher temperature than the hot wall (overheating). This extends previous results in temperature
quenching experiments in a similarly prepared system when the gas is clearly isolated from the wall. Large elliptical wetting
film distortions are also seen during these ramps. By ray tracing through the elliptically shaped wetting film, we find very
thick wetting film on the walls. This wetting film is at least one order of magnitude thicker than films that form in the
Earth’s gravity. The thick wetting film isolates the gas bubble from the wall allowing gas overheating to occur due to the
difference in the piston effect response between gas and liquid. Remarkably, this overheating continues and actually increases
when the fluid is ramped into the single-phase supercritical phase. 相似文献
4.
N. J. Moura M. B. Ribeiro 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,67(1):101-120
This work presents an empirical study of the evolution of the personal income distribution in Brazil. Yearly samples available
from 1978 to 2005 were studied and evidence was found that the complementary cumulative distribution of personal income for
99% of the economically less favorable population is well represented by a Gompertz curve of the form G(x) = exp [exp (A-Bx)],
where x is the normalized individual income. The complementary cumulative distribution of the remaining 1% richest part of
the population is well represented by a Pareto power law distribution P(x) = βx-α. This result means that similarly to other countries, Brazil’s income distribution is characterized by a well defined two
class system. The parameters A, B, α, β were determined by a mixture of boundary conditions, normalization and fitting methods
for every year in the time span of this study. Since the Gompertz curve is characteristic of growth models, its presence here
suggests that these patterns in income distribution could be a consequence of the growth dynamics of the underlying economic
system. In addition, we found out that the percentage share of both the Gompertzian and Paretian components relative to the
total income shows an approximate cycling pattern with periods of about 4 years and whose maximum and minimum peaks in each
component alternate at about every 2 years. This finding suggests that the growth dynamics of Brazil’s economic system might
possibly follow a Goodwin-type class model dynamics based on the application of the Lotka-Volterra equation to economic growth
and cycle. 相似文献
5.
Małgorzata Klimek 《Czechoslovak Journal of Physics》2002,52(11):1247-1253
The models described by fractional order derivatives of Riemann-Liouville type in sequential form are discussed in Lagrangean
and Hamiltonian formalism. The Euler-Lagrange equations are derived using the minimum action principle. Then the methods of
generalized mechanics are applied to obtain the Hamilton’s equations. As an example free motion in fractional picture is studied.
The respective fractional differential equations are explicitly solved and it is shown that the limitα→1+ recovers classical model with linear trajectories and constant velocity.
Presented at the 11th Colloquium “Quantum Groups and Integrable Systems”, Prague, 20–22 June 2002. 相似文献
6.
We present an experimental study of the spreading dynamics of symmetric diblock copolymer droplets above and below the order-disorder
transition. Disordered diblock droplets are found to spread as a homopolymer and follow Tanner’s law (the radius grows as
R ∼ t
m , where t is time and m = 1/10 . However, droplets that are in the ordered phase are found to be frustrated by the imposed lamellar microstructure.
This frustration is likely at the root of the observed deviation from Tanner’s law: droplet spreading has a much slower power
law ( m ∼ 0.05±0.01 . We show that the different spreading dynamics can be reconciled with conventional theory if a strain-rate-dependent
viscosity is taken into account. 相似文献
7.
We discuss the stick-slip motion of an elastic block sliding along a rigid substrate. We argue that for a given external shear stress this system shows a discontinuous nonequilibrium transition from a uniform stick state to uniform sliding at some critical stress which is nothing but the Griffith threshold for crack propagation. An inhomogeneous mode of sliding occurs when the driving velocity is prescribed instead of the external stress. A transition to homogeneous sliding occurs at a critical velocity, which is related to the critical stress. We solve the elastic problem for a steady-state motion of a periodic stick-slip pattern and derive equations of motion for the tip and resticking end of the slip pulses. In the slip regions we use the linear friction law and do not assume any intrinsic instabilities even at small sliding velocities. We find that, as in many other pattern forming system, the steady-state analysis itself does not select uniquely all the internal parameters of the pattern, especially the primary wavelength. Using some plausible analogy to first-order phase transitions we discuss a soft selection mechanism. This allows to estimate internal parameters such as crack velocities, primary wavelength and relative fraction of the slip phase as functions of the driving velocity. The relevance of our results to recent experiments is discussed. 相似文献
8.
We present a simple theory of crack propagation in viscoelastic solids. We calculate the energy per unit area, G(v), to propagate a crack, as a function of the crack tip velocity v. Our study includes the non-uniform temperature distribution (flash temperature) in the vicinity of the crack tip, which has a profound influence on G(v). At very low crack tip velocities, the heat produced at the crack tip can diffuse away, resulting in very small temperature increase: in this “low-speed” regime the flash temperature effect is unimportant. However, because of the low heat conductivity of rubber-like materials, already at moderate crack tip velocities a very large temperature increase (of order of 1000 K) can occur close to the crack tip. We show that this will drastically affect the viscoelastic energy dissipation close to the crack tip, resulting in a “hot-crack” propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt, which may result in unstable crack motion, e.g. stick-slip motion or catastrophic failure, as observed in some experiments. In addition, the high crack tip temperature may result in significant thermal decomposition within the heated region, resulting in a liquid-like region in the vicinity of the crack tip. This may explain the change in surface morphology (from rough to smooth surfaces) which is observed as the crack tip velocity is increased above the instability threshold. 相似文献
9.
A. H.M. Zaheri S. Srivastava K. Tankeshwar 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(4):465-473
Expressions for zeroth, second and fourth sum rules of longitudinal and bulk
stress auto correlation functions have been derived for binary fluid mixtures.
Longitudinal and bulk viscosities of an Ar–Kr mixture have been calculated using
Mori's memory function formalism coupled with the sum rules of longitudinal and
bulk stress auto correlation functions.
The results obtained are compared with the molecular dynamics
simulation. Mass dependence of the longitudinal and bulk viscosities has
been studied for different compositions
of an isotopic mixture at different densities and temperatures. For very large mass ratio, the
longitudinal and bulk viscosities of the isotopic mixture are more dependent on mole fraction than on mass. 相似文献
10.
We have studied the roughness and the dynamics of the contact line of a viscous liquid on a disordered substrate. We have
used photolithographic techniques to obtain a controlled disorder with a correlation length ξ = 10μm. Liquids with different
viscosity were used: water and aqueous glycerol solution. We have found that the roughness W of the contact line depends neither on the viscosity nor on the velocity v of the contact line for v in the range 0.2-20μm/s. W is found to scale with the length L of the line as L
ζ with a roughness exponent ζ = 0.51±0.03. This value is similar to the one obtained with superfluid helium. In the present
experiment, we have checked that the motion of the contact line is actually overdamped, so that the phenomenological equation
first proposed by Ertas and Kardar should be relevant. However, our measurement of ζ is in disagreement with the predicted
value ζ = 0.39. We have also analyzed the avalanche-like motion of the contact line. We find that the size distribution does
not follow a power law dependence.
Received 18 April 2002 相似文献
11.
C. -L. Lee M. -C. Huang 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,64(2):257-261
Inspired by the protein folding problem, we propose a Rubik’s cube model and study its thermodynamic and kinetic behavior.
We find that the energy landscape contains a tiny funnel-like region, as the dynamics towards the native state is mostly diffusive.
In particular, from Monte Carlo simulations we observe exponential kinetics in the first-passage-time distribution towards
the native state at all temperatures considered, while the complexity of the energy landscape is exhibited through a stretched-exponential
relaxation of the energy autocorrelation function. The rollover feature in the mean first passage time, as observed in many
protein-folding dynamics studies, is captured again in our model and discussed under the statistical energy landscape approach. 相似文献
12.
P.-F. Wang S.-J. Ding J.-Y. Zhang D.W. Zhang J.-T. Wang W.W. Lee 《Applied Physics A: Materials Science & Processing》2001,72(6):721-724
Amorphous SiCOF films with high carbon concentration are prepared by PECVD (plasma-enhanced CVD) with TEOS/C4F8/O2. The dielectric constant of (α-SiCOF film is reduced to 2.6 and other electric properties are improved remarkably. The moisture
resistibility of the film is also improved. Through FTIR and XPS analyses, the chemical construction of α-SiCOF film is investigated.
The mechanism of improvements in electrical properties and stability in moisture is further discussed. It is found that the
ionic polarization and orientational polarization decrease in α-SiCOF films and contribute a lot to the reduction in dielectric
constant. In addition, because of the hydrophobicity of incorporated C-F bonds, the moisture resistibility of α-SiCOF film
is improved.
Received: 17 April 2000 / Accepted: 19 April 2000 / Published online: 23 August 2000 相似文献
13.
We have studied the effect of interface heterogeneity on fracture, at both local and global scales. The single cantilever beam adhesion test was used to investigate interfacial fracture between polycarbonate plates and an elastic/fragile epoxy adhesive. Two surface treatments were applied to a (given) polycarbonate plate giving zones of strong and weak adhesion parallel to the crack direction. Calculated fracture energies differed from those expected from a simple rule-of-mixtures. A perturbation method, proposed by Rice, was used and results compared with crack fronts observed in situ. The technique was applied successfully but the difference in values of stress intensity factor between the zones was found substantially different from the experimental value. In an attempt to explain discrepancies, specimens with discontinuous crack fronts (adhesive and/or plates severed along the strong/weak adhesion frontier) were tested. Good agreement was found with the rule-of-mixtures predictions raising questions about the role of crack front continuity in load transfer. 相似文献
14.
S. Piano J. W.A. Robinson G. Burnell M. G. Blamire 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,58(2):123-126
The physics of the π phase shift in ferromagnetic
Josephson junctions may enable a range of applications for
spin-electronic devices and quantum computing. We investigate
transitions from “0” to “π” states in Nb/Fe/Nb Josephson
junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic
dead
layer of about 1.1 nm. By fitting the characteristic voltage
oscillations with existing theoretical models we extrapolate an
exchange energy of 256 meV, a Fermi velocity of 1.98 ×105
m/s and an electron mean free path of 6.2 nm, in agreement with
other reported values. From the temperature dependence of the
ICRN product we show that its decay rate exhibits a nonmonotonic
oscillatory behavior with the Fe barrier thickness. 相似文献
15.
16.
Carvalho AJ Pereira-da-Silva MA Faria RM 《The European physical journal. E, Soft matter》2006,20(3):309-315
Self-organized block copolymer structures derived from dewetting of thin films are becoming important in nanotechnology because
of the various spontaneous and regular sub-micrometric surface patterns that may be obtained. Here, we report on the self-organization
of a poly(styrene)-b-poly(ethene-co-butene-1)-b-poly(styrene) triblock copolymer during drying of its solution over a mica
substrate. Regular submicrometric arrangements with long-range order were formed at critical polymer concentrations, consisting
of parallel ribbons and hexagonal arrays of dots (droplets). This variety of highly ordered structures is explained by the
interplay between forming mechanisms, mainly due to “fingering instabilities” at the three-phase line of the copolymer solution
during drying. The thickness of the structures was “quantized” due to the microphase separation of the block copolymer. The
formation of hexagonal patterns may be attributed to Marangoni instability at the liquid film surface prior to dewetting. 相似文献
17.
We investigate the plastic deformation and constitutive behaviour of bulk metallic glasses (BMGs). A dimensionless Deborah number DeiD = tr/ti is proposed to characterize the rate effect in BMGs, where tr is the structural relaxing characteristic time of BMGs under shear load, ti is the macroscopic imposed characteristic time of applied stress or the characteristic time of macroscopic deformation. The results demonstrate that the modified free volume model can characterize the strain rate effect in BMGs effectively. 相似文献
18.
We study the sliding friction for viscoelastic solids, e.g., rubber, on hard flat substrate surfaces. We consider first the fluctuating shear stress inside a viscoelastic solid which
results from the thermal motion of the atoms or molecules in the solid. At the nanoscale the thermal fluctuations are very
strong and give rise to stress fluctuations in the MPa-range, which is similar to the depinning stresses which typically occur
at solid-rubber interfaces, indicating the crucial importance of thermal fluctuations for rubber friction on smooth surfaces.
We develop a detailed model which takes into account the influence of thermal fluctuations on the depinning of small contact
patches (stress domains) at the rubber-substrate interface. The theory predicts that the velocity dependence of the macroscopic
shear stress has a bell-shaped form, and that the low-velocity side exhibits the same temperature dependence as the bulk viscoelastic
modulus, in qualitative agreement with experimental data. Finally, we discuss the influence of small-amplitude substrate roughness
on rubber sliding friction. 相似文献
19.
C. Albertus E. Hernández J. Nieves J. M. Verde-Velasco 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):691-694
Working in the framework of a nonrelativistic quark model we evaluate the spectra and semileptonic decay widths for the ground
state of doubly heavy Ξ and Ω baryons. We solve the three-body problem using a variational ansatz made possible by the constraints imposed by heavy-quark
spin symmetry. In order to check the dependence of our results on the inter-quark interaction, we have used five different
quark-quark potentials which include Coulomb and hyperfine terms coming from one-gluon exchange, plus a confining term. Our
results for the spectra are in good agreement with a previous calculation done using a Faddeev approach. For the semileptonic
decay our results for the total decay widths are in good agreement with the ones obtained within a relativistic quark model
in the quark-diquark approximation. 相似文献
20.
Godinho MH Trindade AC Figueirinhas JL Melo LV Brogueira P Deus AM Teixeira PI 《The European physical journal. E, Soft matter》2006,21(4):319-330
We have studied the control and manipulation of tuneable equilibrium structures in a free-standing urethane/urea elastomer
film by means of atomic force microscopy, small-angle light scattering and polarising optical microscopy. The urethane/urea
elastomer was prepared by reacting a poly(propyleneoxide)-based triisocyanate-terminated prepolymer (PU) with poly(butadienediol)
(PBDO), with a weight ratio of 60% PU/40% PBDO. An elastomer film was shear-cast onto a glass plate and allowed to cure, first
in an oven, then in air. Latent micro- and nano-periodic patterns are induced by ultra-violet (UV) irradiation of the film
and can be “developed” by applying a plane uniaxial stress or by immersing the elastomer in an appropriate solvent and then
drying it. For this elastomer we describe six pattern states, how they are related and how they can be manipulated. The morphological
features of the UV-exposed film surface can be tuned, reproducibly and reversibly, by switching the direction of the applied
mechanical field. Elastomers extracted in toluene exhibit different surface patterns depending upon the state in which they
were developed. Stress-strain data collected for the films before and after UV irradiation reveal anisotropy induced by the
shear-casting conditions and enhanced by the mechanical field. We have interpreted our results by assuming the film to consist
of a thin, stiff surface layer (“skin”) lying atop a thicker, softer substrate (“bulk”). The skin's higher stiffness is hypothesised
to be due to the more extensive cross-linking of chains located near the surface by the UV radiation. Patterns would thus
arise as a competition between the effects of bending the skin and stretching/compressing the bulk, as in the work of Cerda
and Mahadevan (Phys. Rev. Lett. 90, 074302 (2003)). We present some preliminary results of a simulation of this model using the Finite Element package ABAQUS. 相似文献