29,205 research outputs found
Capillary Condensation in Confined Media
We review here the physics of capillary condensation of liquids in confined
media, with a special regard to the application in nanotechnologies. The
thermodynamics of capillary condensation and thin film adsorption are first
exposed along with all the relevant notions. The focus is then shifted to the
modelling of capillary forces, to their measurements techniques (including SFA,
AFM and crack tips) and to their influence on AFM imaging techniques as well as
on the static and dynamic friction properties of solids (including granular
heaps and sliding nanocontacts). A great attention is spent in investigating
the delicate role of the surface roughness and all the difficulties involved in
the reduction of the probe size to nanometric dimensions. Another major
consequence of capillary condensation in nanosystems is the activation of
several chemical and corrosive processes that can significantly alter the
surface properties, such as dissolution/redeposition of solid materials and
stress-corrosion crack propagation.Comment: 28 pages - To appear in 2010 in the Handbook of Nanophysics - Vol 1 -
Edited by Klaus Sattler - CRC Pres
Wave Solutions
In classical continuum physics, a wave is a mechanical disturbance. Whether
the disturbance is stationary or traveling and whether it is caused by the
motion of atoms and molecules or the vibration of a lattice structure, a wave
can be understood as a specific type of solution of an appropriate mathematical
equation modeling the underlying physics. Typical models consist of partial
differential equations that exhibit certain general properties, e.g.,
hyperbolicity. This, in turn, leads to the possibility of wave solutions.
Various analytical techniques (integral transforms, complex variables,
reduction to ordinary differential equations, etc.) are available to find wave
solutions of linear partial differential equations. Furthermore, linear
hyperbolic equations with higher-order derivatives provide the mathematical
underpinning of the phenomenon of dispersion, i.e., the dependence of a wave's
phase speed on its wavenumber. For systems of nonlinear first-order hyperbolic
equations, there also exists a general theory for finding wave solutions. In
addition, nonlinear parabolic partial differential equations are sometimes said
to posses wave solutions, though they lack hyperbolicity, because it may be
possible to find solutions that translate in space with time. Unfortunately, an
all-encompassing methodology for solution of partial differential equations
with any possible combination of nonlinearities does not exist. Thus, nonlinear
wave solutions must be sought on a case-by-case basis depending on the
governing equation.Comment: 22 pages, 3 figures; to appear in the Mathematical Preliminaries and
Methods section of the Encyclopedia of Thermal Stresses, ed. R.B. Hetnarski,
Springer (2014), to appea
Rupture by damage accumulation in rocks
The deformation of rocks is associated with microcracks nucleation and
propagation, i.e. damage. The accumulation of damage and its spatial
localization lead to the creation of a macroscale discontinuity, so-called
"fault" in geological terms, and to the failure of the material, i.e. a
dramatic decrease of the mechanical properties as strength and modulus. The
damage process can be studied both statically by direct observation of thin
sections and dynamically by recording acoustic waves emitted by crack
propagation (acoustic emission). Here we first review such observations
concerning geological objects over scales ranging from the laboratory sample
scale (dm) to seismically active faults (km), including cliffs and rock masses
(Dm, hm). These observations reveal complex patterns in both space (fractal
properties of damage structures as roughness and gouge), time (clustering,
particular trends when the failure approaches) and energy domains (power-law
distributions of energy release bursts). We use a numerical model based on
progressive damage within an elastic interaction framework which allows us to
simulate these observations. This study shows that the failure in rocks can be
the result of damage accumulation
The role of chemistry in the oscillating combustion of hydrocarbons : an experimental and theoretical study
The stable operation of low-temperature combustion processes is an open challenge, due to the presence of undesired deviations from steady-state conditions: among them, oscillatory behaviors have been raising significant interest. In this work, the establishment of limit cycles during the combustion of hydrocarbons in a wellstirred reactor was analyzed to investigate the role of chemistry in such phenomena. An experimental investigation of methane oxidation in dilute conditions was carried out, thus creating quasi-isothermal conditions and decoupling kinetic effects from thermal ones. The transient evolution of the mole fractions of the major species was obtained for different dilution levels (0.0025 <= X-CH4 <= 0.025), inlet temperatures (1080K <= T <= 1190K) and equivalence ratios (0.75 <= Phi <= 1). Rate of production analysis and sensitivity analysis on a fundamental kinetic model allowed to identify the role of the dominating recombination reactions, first driving ignition, then causing extinction.
A bifurcation analysis provided further insight in the major role of these reactions for the reactor stability. One-parameter continuation allowed to identify a temperature range where a single, unstable solution exists, and where oscillations were actually observed. Multiple unstable states were identified below the upper branch, where the stable (cold) solution is preferred. The role of recombination reactions in determining the width of the unstable region could be captured, and bifurcation analysis showed that, by decreasing their strength, the unstable range was progressively reduced, up to the full disappearance of oscillations. This affected also the oxidation of heavier hydrocarbons, like ethylene. Finally, less dilute conditions were analyzed using propane as fuel: the coupling with heat exchange resulted in multiple Hopf Bifurcations, with the consequent formation of intermediate, stable regions within the instability range in agreement with the experimental observations
Failure analysis of CFRP laminates subjected to Compression After Impact: FE simulation using discrete interface elements
This paper presents a model for the numerical simulation of impact damage, permanent indentation and compression after impact (CAI) in CFRP laminates. The same model is used for the formation of damage developing during both low-velocity / low-energy impact tests and CAI tests. The different impact and CAI elementary damage types are taken into account, i.e. matrix cracking, fiber failure and interface delamination. Experimental tests and model results are compared, and this comparison is used to highlight the laminate failure scenario during residual compression tests. Finally, the impact energy effect on the residual strength is evaluated and compared to experimental results
Coronal Shock Waves, EUV waves, and their Relation to CMEs. II. Modeling MHD Shock Wave Propagation Along the Solar Surface, Using Nonlinear Geometrical Acoustics
We model the propagation of a coronal shock wave, using nonlinear geometrical
acoustics. The method is based on the Wentzel-Kramers-Brillouin (WKB) approach
and takes into account the main properties of nonlinear waves: i) dependence of
the wave front velocity on the wave amplitude, ii) nonlinear dissipation of the
wave energy, and iii) progressive increase in the duration of solitary shock
waves. We address the method in detail and present results of the modeling of
the propagation of shock-associated extreme-ultraviolet (EUV) waves as well as
Moreton waves along the solar surface in the simplest solar corona model. The
calculations reveal deceleration and lengthening of the waves. In contrast,
waves considered in the linear approximation keep their length unchanged and
slightly accelerate.Comment: 15 pages, 7 figures, accepted for publication in Solar Physic
Linear and non-linear dynamic analyses of sandwich panels with face sheet-tocore debonding
А survey of recent developments in the dynamic analysis of sandwich panels with face sheet-to-core
debonding is presented. The finite element method within the ABAQUSTM code is utilized. The emphasis
is directed to the procedures used to elaborate linear and non-linear models and to predict dynamic response
of the sandwich panels. Recently developed models are presented, which can be applied for structural
health monitoring algorithms of real-scale sandwich panels. First, various popular theories of intact
sandwich panels are briefly mentioned and a model is proposed to effectively analyse the modal dynamics
of debonded and damaged (due to impact) sandwich panels. The influence of debonding size, form and
location, and number of such damage on the modal characteristics of sandwich panels are shown. For
nonlinear analysis, models based on implicit and explicit time integration schemes are presented and dynamic
response gained with those models are discussed. Finally, questions related to debonding progression
at the face sheet-core interface when dynamic loading continues with time are briefly highlighted
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