1,769 research outputs found
Dynamics of Viscoplastic Deformation in Amorphous Solids
We propose a dynamical theory of low-temperature shear deformation in
amorphous solids. Our analysis is based on molecular-dynamics simulations of a
two-dimensional, two-component noncrystalline system. These numerical
simulations reveal behavior typical of metallic glasses and other viscoplastic
materials, specifically, reversible elastic deformation at small applied
stresses, irreversible plastic deformation at larger stresses, a stress
threshold above which unbounded plastic flow occurs, and a strong dependence of
the state of the system on the history of past deformations. Microscopic
observations suggest that a dynamically complete description of the macroscopic
state of this deforming body requires specifying, in addition to stress and
strain, certain average features of a population of two-state shear
transformation zones. Our introduction of these new state variables into the
constitutive equations for this system is an extension of earlier models of
creep in metallic glasses. In the treatment presented here, we specialize to
temperatures far below the glass transition, and postulate that irreversible
motions are governed by local entropic fluctuations in the volumes of the
transformation zones. In most respects, our theory is in good quantitative
agreement with the rich variety of phenomena seen in the simulations.Comment: 16 pages, 9 figure
Thinning practices in rehabilitated mangroves: Opportunity to synergize climate change mitigation and adaptation
Mangrove trees act important roles in the coastal ecosystems, protecting community against high-tide and\ud
storms, controlling land erosion and providing fish breeding ground. In the last few decades, the massive area has\ud
devastated due to commercial shrimp and fish ponds development. To rehabilitate the coastal ecosystems, some\ud
mangrove has been planted with spacing distances of 1x1 m with minimal forest management. Those dense-spaced\ud
stands enhanced light competitions and inhibit growth. These poor quality and immature stands that reach an early\ud
climax in 10-15 years were observed in two adjacent sites near Nam Dinh and Thanh Hoa in northern Vietnam, where\ud
Kandelia candel were planted. To cultivate the resurgent stands and increase their growth, thinning mangrove is\ud
essential. Stand densities of the mangrove trees with and without the thinning practice were 17,800 and 5,200 trees ha-1,\ud
respectively. Their potential of the maximum above-ground biomass were 303 and 239 Mg ha-1, respectively. However,\ud
quality of the single tree was largely different whether or not thinning practice is conducted, as the thinned one of 46 kg\ud
tree-1 was about three times higher than the non-thinned of 17 kg tree-1. The thinning practice enhances stand biomass\ud
growth with improved growth condition in the forest, which advances carbon sequestration for the climate change\ud
mitigation. The cultivated trees also ensure the climate change adaptation of coastal protection, fishery products and\ud
bio-diversity. Synergizing mitigation and adaptation strategies with the mangrove thinning would enhance the benefits\ud
for coastal communities most vulnerable to climate change
Phase transition in inelastic disks
This letter investigates the molecular dynamics of inelastic disks without
external forcing. By introducing a new observation frame with a rescaled time,
we observe the virtual steady states converted from asymptotic energy
dissipation processes. System behavior in the thermodynamic limit is carefully
investigated. It is found that a phase transition with symmetry breaking occurs
when the magnitude of dissipation is greater than a critical value.Comment: 9 pages, 6 figure
Dynamical brittle fractures of nanocrystalline silicon using large-scale electronic structure calculations
A hybrid scheme between large-scale electronic structure calculations is
developed and applied to nanocrystalline silicon with more than 10 atoms.
Dynamical fracture processes are simulated under external loads in the [001]
direction. We shows that the fracture propagates anisotropically on the (001)
plane and reconstructed surfaces appear with asymmetric dimers. Step structures
are formed in larger systems, which is understood as the beginning of a
crossover between nanoscale and macroscale samples.Comment: 10 pages, 4 figure
Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations
We study the formation process of an oxygen torus during the 12â15 November 2012 magnetic storm, using the magnetic field and plasma wave data obtained by Van Allen Probes. We estimate the local plasma mass density (ÏL) and the local electron number density (neL) from the resonant frequencies of standing AlfvĂ©n waves and the upper hybrid resonance band. The average ion mass (M) can be calculated by M ⌠ÏL/neL under the assumption of quasi-neutrality of plasma. During the storm recovery phase, both Probe A and Probe B observe the oxygen torus at L = 3.0â4.0 and L = 3.7â4.5, respectively, on the morning side. The oxygen torus has M = 4.5â8 amu and extends around the plasmapause that is identified at LâŒ3.2â3.9. We find that during the initial phase, M is 4â7 amu throughout the plasma trough and remains at âŒ1 amu in the plasmasphere, implying that ionospheric O+ ions are supplied into the inner magnetosphere already in the initial phase of the magnetic storm. Numerical calculation under a decrease of the convection electric field reveals that some of thermal O+ ions distributed throughout the plasma trough are trapped within the expanded plasmasphere, whereas some of them drift around the plasmapause on the dawnside. This creates the oxygen torus spreading near the plasmapause, which is consistent with the Van Allen Probes observations. We conclude that the oxygen torus identified in this study favors the formation scenario of supplying O+ in the inner magnetosphere during the initial phase and subsequent drift during the recovery phase
Generative adversarial network-created brain SPECTs of cerebral ischemia are indistinguishable to scans from real patients
Deep convolutional generative adversarial networks (GAN) allow for creating images from existing databases. We applied a modified light-weight GAN (FastGAN) algorithm to cerebral blood flow SPECTs and aimed to evaluate whether this technology can generate created images close to real patients. Investigating three anatomical levels (cerebellum, CER; basal ganglia, BG; cortex, COR), 551 normal (248 CER, 174 BG, 129 COR) and 387 pathological brain SPECTs using N-isopropyl p-I-123-iodoamphetamine (I-123-IMP) were included. For the latter scans, cerebral ischemic disease comprised 291 uni- (66 CER, 116 BG, 109 COR) and 96 bilateral defect patterns (44 BG, 52 COR). Our model was trained using a three-compartment anatomical input (dataset 'A'; including CER, BG, and COR), while for dataset 'B', only one anatomical region (COR) was included. Quantitative analyses provided mean counts (MC) and left/right (LR) hemisphere ratios, which were then compared to quantification from real images. For MC, 'B' was significantly different for normal and bilateral defect patterns (P = 0.08) reached significance relative to images of real patients. With a minimum of only three anatomical compartments serving as stimuli, created cerebral SPECTs are indistinguishable to images from real patients. The applied FastGAN algorithm may allow to provide sufficient scan numbers in various clinical scenarios, e.g., for "data-hungry" deep learning technologies or in the context of orphan diseases
Phase-Space Metric for Non-Hamiltonian Systems
We consider an invariant skew-symmetric phase-space metric for
non-Hamiltonian systems. We say that the metric is an invariant if the metric
tensor field is an integral of motion. We derive the time-dependent
skew-symmetric phase-space metric that satisfies the Jacobi identity. The
example of non-Hamiltonian systems with linear friction term is considered.Comment: 12 page
Mechanical behavior of tungsten-vanadium-lanthana alloys as function of temperature
The mechanical behavior of three tungsten (W) alloys with vanadium (V) and lanthana (La2O3) additions (Wâ4%V, Wâ1%La2O3, Wâ4%Vâ1%La2O3) processed by hot isostatic pressing (HIP) have been compared with pure-W to analyze the influence of the dopants. Mechanical characterization was performed by three point bending (TPB) tests in an oxidizing air atmosphere and temperature range between 77 (immersion tests in liquid nitrogen) and 1273 K, through which the fracture toughness, flexural strength, and yield strength as function of temperature were obtained.
Results show that the V and La2O3 additions improve the mechanical properties and oxidation behavior, respectively. Furthermore, a synergistic effect of both dopants results in an extraordinary increase of the flexure strength, fracture toughness and resistance to oxidation compared to pure-W, especially at higher temperatures. In addition, a new experimental method was developed to obtain a very small notch tip radius (around 5â7 ÎŒm) and much more similar to a crack through the use of a new machined notch. The fracture toughness results were lower than those obtained with traditional machining of the notch, which can be explained with electron microscopy, observations of deformation in the rear part of the notch tip.
Finally, scanning electron microscopy (SEM) examination of the microstructure and fracture surfaces was used to determine and analyze the relationship between the macroscopic mechanical properties and the micromechanisms of failure involved, depending on the temperature and the dispersion of the alloy
Performance evaluation of a preclinical SPECT scanner with a collimator designed for medium-sized animals
BACKGROUND: Equipped with two stationary detectors, a large bore collimator for medium-sized animals has been recently introduced for dedicated preclinical single-photon emission computed tomography (SPECT) imaging. We aimed to evaluate the basic performance of the system using phantoms and healthy rabbits. METHODS: A general-purpose medium-sized animal (GP-MSA) collimator with 135âmm bore diameter and thirty-three holes of 2.5âmm diameter was installed on an ultrahigh-resolution scanner equipped with two large stationary detectors (U-SPECT5-E/CT). The sensitivity and uniformity were investigated using a point source and a cylinder phantom containing (99m)Tc-pertechnetate, respectively. Uniformity (in %) was derived using volumes of interest (VOIs) on images of the cylinder phantom and calculated as [(maximumâcount â minimumâcount)/(maximumâcount + minimumâcount) Ă 100], with lower values of % indicating superior performance. The spatial resolution and contrast-to-noise ratios (CNRs) were evaluated with images of a hot-rod Derenzo phantom using different activity concentrations. Feasibility of in vivo SPECT imaging was finally confirmed by rabbit imaging with the most commonly used clinical myocardial perfusion SPECT agent [(99m)Tc]Tc-sestamibi (dynamic acquisition with a scan time of 5âmin). RESULTS: In the performance evaluation, a sensitivity of 790âcps/MBq, a spatial resolution with the hot-rod phantom of 2.5âmm, and a uniformity of 39.2% were achieved. The CNRs of the rod size 2.5âmm were 1.37, 1.24, 1.20, and 0.85 for activity concentration of 29.2, 1.0, 0.5, and 0.1âMBq/mL, respectively. Dynamic SPECT imaging in rabbits allowed to visualize most of the thorax and to generate time-activity curves of the left myocardial wall and ventricular cavity. CONCLUSION: Preclinical U-SPECT5-E/CT equipped with a large bore collimator demonstrated adequate sensitivity and resolution for in vivo rabbit imaging. Along with its unique features of SPECT molecular functional imaging is a superior collimator technology that is applicable to medium-sized animal models and thus may promote translational research for diagnostic purposes and development of novel therapeutics
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