16,364 research outputs found
A thermodynamically consistent derivation of a frictional-damage cohesive-zone model with different mode i and mode II fracture energies
The present paper deals with the derivation of an interface model characterized by macroscopic fracture energies which are different in modes I and II, the macroscopic fracture energy being the total energy dissipated per unit of fracture area. It is first shown that thermo-dynamical consistency for a model governed by a single damage variable, combined with the choice of employing an equivalent relative displacement and of a linear softening in the stress-relative displacement law, leads to the coincidence of fracture energies in modes I and II. To retrieve the experimental evidence of a greater fracture energy in mode II, a micro-structured geometry is considered at the typical point of the interface where a Representative Interface Element (RIE) characterized by a periodic arrangement of distinct inclined planes is introduced. The interaction within each of these surfaces is governed by a coupled damage-friction law. A sensitivity analysis of the correlation between micromechanical parameters and the numerically computed single-point microstructural response in mode II is reported. An assessment of the capability of the model in predicting different mixed mode fracture energies is carried out both at the single microstructural interface point level and with a structural example. For the latter a double cantilever beam with uneven bending moments has been analyzed and numerical results are compared with experimental data reported in the literature for different values of mode mixity. Š 2014 Elsevier Masson SAS. All rights reserved
Temperature dependence of the interfacial shear strength in glass reinforced polypropylene and epoxy composites
We have recently reported the development of a method which allows the measurement of IFSS over a wide temperature range [6,7]. In this paper we present data obtained using the microbond test in the temperature controlled environment of a thermo-mechanical analyser (TMA). IFSS in glass fibreâpolypropylene and glass fibre-epoxy systems in the temperature range -40°C to 150°C are presented and discussed
On the Doppler Shift and Asymmetry of Stokes Profiles of Photospheric FeI and Chromospheric MgI Lines
We analyzed the full Stokes spectra using simultaneous measurements of the
photospheric (FeI 630.15 and 630.25 nm) and chromospheric (MgI b2 517.27 nm)
lines. The data were obtained with the HAO/NSO Advanced Stokes Polarimeter,
about a near disc center sunspot region, NOAA AR 9661. We compare the
characteristics of Stokes profiles in terms of Doppler shifts and asymmetries
among the three spectral lines, which helps us to better understand the
chromospheric lines and the magnetic and flow fields in different magnetic
regions. The main results are: (1) For penumbral area observed by the
photospheric FeI lines, Doppler velocities derived from Stokes I (Vi) are very
close to those derived from linear polarization profiles (Vlp) but
significantly different from those derived from Stokes V profiles (Vzc), which
provides direct and strong evidence that the penumbral Evershed flows are
magnetized and mainly carried by the horizontal magnetic component. (2) The
rudimentary inverse Evershed effect observed by the MgI b2 line provides a
qualitative evidence on its formation height that is around or just above the
temperature minimum region. (3) Vzc and Vlp in penumbrae and Vzc in pores
generally approach their Vi observed by the chromospheric MgI line, which is
not the case for the photospheric FeI lines. (4) Outer penumbrae and pores show
similar behavior of the Stokes V asymmetries that tend to change from positive
values in the photosphere (FeI lines) to negative values in the low
chromosphere (MgI line). (5) The Stokes V profiles in plage regions are highly
asymmetric in the photosphere and more symmetric in the low chromosphere. (6)
Strong red shifts and large asymmetries are found around the magnetic polarity
inversion line within the common penumbra of the Delta spot. This study thus
emphasizes the importance of spectro-polarimetry using chromospheric lines.Comment: 10 pages, 7 figures, accepted to The Astrophysical Journa
Simulations of the Static Friction Due to Adsorbed Molecules
The static friction between crystalline surfaces separated by a molecularly
thin layer of adsorbed molecules is calculated using molecular dynamics
simulations. These molecules naturally lead to a finite static friction that is
consistent with macroscopic friction laws. Crystalline alignment, sliding
direction, and the number of adsorbed molecules are not controlled in most
experiments and are shown to have little effect on the friction. Temperature,
molecular geometry and interaction potentials can have larger effects on
friction. The observed trends in friction can be understood in terms of a
simple hard sphere model.Comment: 13 pages, 13 figure
Artificial dielectric devices for variable polarization compensation at millimeter and submillimeter wavelengths
Variable polarization compensation has been demonstrated at 100 GHz. The device consists of two interlocking V-groove artificial dielectric gratings that produce a birefringence that varies with the separation distance. A maximum retardance of 74/spl deg/ has been obtained experimentally in a silicon device, in good agreement with rigorous coupled-wave computer simulations. Further simulations predict that adding quarter wave dielectric antireflection (AR) coatings to the outer surfaces of the device can reduce the insertion loss to below 4 dB. The use of rectangular grooved gratings provides increased retardance and reduced loss. It is predicted that a coupled device with rectangular grooved gratings will be capable of maximum retardance in excess of 180/spl deg/, with low insertion loss (<0.6 dB). The sensitivity of the wave retardation as a function of mechanical separation has a peak value of 485/spl deg//mm. The design and micromachining fabrication techniques scale for operation at submillimeter wavelengths
Creep stability of the proposed AIDA mission target 65803 Didymos: I. Discrete cohesionless granular physics model
As the target of the proposed Asteroid Impact & Deflection Assessment (AIDA)
mission, the near-Earth binary asteroid 65803 Didymos represents a special
class of binary asteroids, those whose primaries are at risk of rotational
disruption. To gain a better understanding of these binary systems and to
support the AIDA mission, this paper investigates the creep stability of the
Didymos primary by representing it as a cohesionless self-gravitating granular
aggregate subject to rotational acceleration. To achieve this goal, a
soft-sphere discrete element model (SSDEM) capable of simulating granular
systems in quasi-static states is implemented and a quasi-static spin-up
procedure is carried out. We devise three critical spin limits for the
simulated aggregates to indicate their critical states triggered by reshaping
and surface shedding, internal structural deformation, and shear failure,
respectively. The failure condition and mode, and shear strength of an
aggregate can all be inferred from the three critical spin limits. The effects
of arrangement and size distribution of constituent particles, bulk density,
spin-up path, and interparticle friction are numerically explored. The results
show that the shear strength of a spinning self-gravitating aggregate depends
strongly on both its internal configuration and material parameters, while its
failure mode and mechanism are mainly affected by its internal configuration.
Additionally, this study provides some constraints on the possible physical
properties of the Didymos primary based on observational data and proposes a
plausible formation mechanism for this binary system. With a bulk density
consistent with observational uncertainty and close to the maximum density
allowed for the asteroid, the Didymos primary in certain configurations can
remain geo-statically stable without including cohesion.Comment: 66 pages, 24 figures, submitted to Icarus on 25/Aug/201
Use of Hydration Inhibitors to Improve Bond Durability of Aluminum Adhesive Joints
An investigation is conducted of the mechanisms by which nitrilotris methylene phosphonic acid (NTMP) and related compounds are adsorbed onto oxidized aluminum surfaces to inhibit hydration and increase the durability of adhesive bonds formed with inhibitor-treated panels. P - O - Al bonds are identified as the basis of adsorption, and it is found that water initially adsorbed onto the etched aluminum surfaces is displaced by the NTMP. The hydration of the NTMP-treated surfaces occurs in three stages, namely the reverisble physisorption of water, the slow dissolution of NTMP followed by rapid hydration of the freshly exposed Al2O3 to AlOOH and further hydration of the surface to Al(OH)3. Five properties of an ideal inhibitor are identified
Behavior of confined granular beds under cyclic thermal loading
We investigate the mechanical behavior of a confined granular packing of
irregular polyhedral particles under repeated heating and cooling cycles by
means of numerical simulations with the Non-Smooth Contact Dynamics method.
Assuming a homogeneous temperature distribution as well as constant temperature
rate, we study the effect of the container shape, and coefficients of thermal
expansions on the pressure buildup at the confining walls and the density
evolution. We observe that small changes in the opening angle of the
confinement can lead to a drastic peak pressure reduction. Furthermore, the
displacement fields over several thermal cycles are obtained and we discover
the formation of convection cells inside the granular material having the shape
of a torus. The root mean square of the vorticity is then calculated from the
displacement fields and a quadratic dependency on the ratio of thermal
expansion coefficients is established
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