15,265 research outputs found
Dynamic response of phenolic resin and its carbon-nanotube composites to shock wave loading
We investigate with nonreactive molecular dynamics simulations the dynamic response of phenolic resin and its carbon-nanotube (CNT) composites to shock wave compression. For phenolic resin, our simulations yield shock states in agreement with experiments on similar polymers except the
“phase change” observed in experiments, indicating that such phase change is chemical in nature. The elastic–plastic transition is characterized by shear stress relaxation and atomic-level slip, and phenolic resin shows strong strain hardening. Shock loading of the CNT-resin composites is applied parallel or perpendicular to the CNT axis, and the composites demonstrate anisotropy in wave
propagation, yield and CNT deformation. The CNTs induce stress concentrations in the composites and may increase the yield strength. Our simulations suggest that the bulk shock response of the composites depends on the volume fraction, length ratio, impact cross-section, and geometry of the CNT components; the short CNTs in current simulations have insignificant effect on the bulk
response of resin polymer
Four dual AGN candidates observed with the VLBA
According to hierarchical structure formation models, merging galaxies are
expected to be seen in different stages of their coalescence. However,
currently there are no straightforward observational methods neither to select
nor to confirm a large number of dual active galactic nuclei (AGN) candidates.
Most attempts involve the better understanding of double-peaked narrow emission
line sources, to distinguish the objects where the emission lines originate
from narrow-line kinematics or jet-driven outflows from those which might
harbour dual AGN. We observed four such candidate sources with the Very Long
Baseline Array (VLBA) at 1.5 GHz with 10 milli-arcsecond angular
resolution where spectral profiles of AGN optical emission suggested the
existence of dual AGN. In SDSS J210449.13-000919.1 and SDSS J23044.82-093345.3,
the radio structures are aligned with the optical emission features, thus the
double-peaked emission lines might be the results of jet-driven outflows. In
the third detected source SDSS J115523.74+150756.9, the radio structure is less
extended and oriented nearly perpendicular to the position angle derived from
optical spectroscopy. The fourth source remained undetected with the VLBA but
it has been imaged with the Very Large Array at arcsec resolution a few months
before our observations, suggesting the existence of extended radio structure.
In none of the four sources did we detect two radio-emitting cores, a
convincing signature of duality.Comment: 35 pages, 3 figures, 2 tables, accepted for publication in Ap
Left-right loading dependence of shock response of (111)//(112) Cu bicrystals: Deformation and spallation
We investigate with molecular dynamics the dynamic response of Cu bicrystals with a special asymmetric grain boundary (GB), (111)//(112)〈110〉, and its dependence on the loading directions. Shock loading is applied along the GB normal either from the left or right to the GB. Due to the structure asymmetry, the bicrystals demonstrate overall strong left-right loading dependence of its shock response, including compression wave features, compression and tensile plasticity, damage characteristics (e.g., spall strength), effective wave speeds and structure changes, except that spallation remains dominated by the GB damage regardless of the loading directions. The presence or absence of transient microtwinning also depends on the loading directions
Deformation and spallation of shocked Cu bicrystals with Σ3 coherent and symmetric incoherent twin boundaries
We perform molecular dynamics simulations of Cu bicrystals with two important grain boundaries (GBs), Σ3 coherent twin boundaries (CTB), and symmetric incoherent twin boundaries (SITB) under planar shock wave loading. It is revealed that the shock response (deformation and spallation) of the Cu bicrystals strongly depends on the GB characteristics. At the shock compression stage, elastic shock wave can readily trigger GB plasticity at SITB but not at CTB. The SITB can induce considerable wave attenuation such as the elastic precursor decay via activating GB dislocations. For example, our simulations of a Cu multilayer structure with 53 SITBs (∼1.5-μm thick) demonstrate a ∼80% elastic shock decay. At the tension stage, spallation tends to occur at CTB but not at SITB due to the high mobility of SITB. The SITB region transforms into a threefold twin via a sequential partial dislocation slip mechanism, while CTB preserves its integrity before spallation. In addition, deformation twinning is a mechanism for inducing surface step during shock tension stage. The drastically different shock response of CTB and SITB could in principle be exploited for, or benefit, interface engineering and materials design
Parsec-scale jet properties of the gamma-ray quasar 3C 286
The quasar 3C~286 is one of two compact steep spectrum sources detected by
the {\it Fermi}/LAT. Here, we investigate the radio properties of the
parsec(pc)-scale jet and its (possible) association with the -ray
emission in 3C~286. The Very Long Baseline Interferometry (VLBI) images at
various frequencies reveal a one-sided core--jet structure extending to the
southwest at a projected distance of 1 kpc. The component at the jet base
showing an inverted spectrum is identified as the core, with a mean brightness
temperature of ~K. The jet bends at about 600 pc (in
projection) away from the core, from a position angle of to
. Based on the available VLBI data, we inferred the proper motion
speed of the inner jet as mas yr (), corresponding to a jet speed of about at an inclination
angle of between the jet and the line of sight of the observer. The
brightness temperature, jet speed and Lorentz factor are much lower than those
of -ray-emitting blazars, implying that the pc-scale jet in 3C~286 is
mildly relativistic. Unlike blazars in which -ray emission is in
general thought to originate from the beamed innermost jet, the location and
mechanism of -ray emission in 3C~286 may be different as indicated by
the current radio data. Multi-band spectrum fitting may offer a complementary
diagnostic clue of the -ray production mechanism in this source.Comment: 9 pages, 4 figures, accept for publication in MNRA
Shock compression and spallation of single crystal tantalum
We present molecular dynamics simulations of shock-induced plasticity and spall damage in single crystal Ta described by a recently developed embedded-atom-method (EAM) potential and a volumedependent qEAM potential. We use impact or Hugoniotstat simulations to investigate the Hugoniots, deformation and spallation. Both EAM and qEAM are accurate in predicting, e.g., the Hugoniots and γ - surfaces. Deformation and spall damage are anisotropic for Ta single crystals. Our preliminary results show that twinning is dominant for [100] and [110] shock loading, and dislocation, for [111]. Spallation initiates with void nucleation at defective sites from remnant compressional deformation or tensile plasticity. Spall strength decreases with increasing shock strength, while its rate dependence remains to be explored
Modeling of hyper-adaptability: from motor coordination to rehabilitation
Hyper-adaptability is an ability of humans and animals to adapt to large-scale changes in the nervous system or the musculoskeletal system, such as strokes and spinal cord injuries. Although this adaptation may involve similar neural processes with normal adaptation to usual environmental and body changes in daily lives, it can be fundamentally different because it requires ‘construction’ of the neural structure itself and ‘reconstitution’ of sensorimotor control rules to compensate for the changes in the nervous system. In this survey paper, we aimed to provide an overview on how the brain structure changes after brain injury and recovers through rehabilitation. Next, we demonstrated the recent approaches used to apply computational and neural network modeling to recapitulate motor control and motor learning processes. Finally, we discussed future directions to bridge the gap between conventional physiological and modeling approaches to understand the neural and computational mechanisms of hyper-adaptability and its applications to clinical rehabilitation
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