780 research outputs found
Formability limits and process window based on fracture analysis of 5A02-O aluminium alloy in splitting spinning
Splitting spinning or rotary flow splitting is an advanced forming process for manufacturing axisymmetric integrated parts with bifurcated features and unique characteristics of high-performance and low-weight. During the process, under the kinematic effects of mandrel rotational movement and roller radial feed, plastic deformation occurs accompanied usually by undesirable fracture, which reduces the formability limit (FL). In this study, the kinematic effects on the FL of a 5A02-O aluminium alloy in the splitting spinning process were systematically investigated by finite element simulation based on a modified Lemaitre criterion and physical experiments. The results show that at a given roller feed speed or mandrel rotational speed (forming speed), the FL has a nonlinear relationship with forming speed, which increases firstly and then decreases. With the increase of forming speed, the maximum FL decreases, which appears at the larger forming speed. These variations of FL show that there exists a combined effect of the roller feed speed and mandrel rotational speed, thus a ratio between them, named as the roller feed ratio, is then used to investigate FL. It is found that there exists a critical roller feed ratio of approximately 2 mm/rev, independent of the speeds of roller and mandrel. Below this critical value, the FL increases with the roller feed ratio. While over the critical value, the FL decreases. In addition, the decrease of FL becomes more remarkable with the increase of mandrel rotational speed. Furthermore, the variations of stress triaxiality and tensile plastic strain were analyzed to see their effects on FL. The analyses show that the decrease of tensile plastic strain with the increasing roller feed ratio is dominant the increase of FL below the critical roller feed ratio value. The increase in the stress triaxiality is dominant in the decrease of FL when the roller feed ratio is over the critical value in combination with not too high forming speed, whereas both increases are dominant in the decrease of FL when it is over the critical roller feed ratio value in combination with high forming speed. Based on the kinematic effects of mandrel and roller, the process windows of the splitting spinning process were obtained to improve the FL. It is found that under the condition of the roller feed ratio within 1–2.5 mm/rev, the mandrel rotational speed within 8–100 rev/min and the roller feed speed within 0.5–4 mm/s are helpful to get high FL values. The experiments were carried out to verify the prediction on the FL and the process window. The research provides an in-depth understanding of FL and its affecting factors, and thus lays a basis for process optimization and process parameter configuration
Spin-orbit coupling in bulk GaAs
We study the spin-orbit coupling in the whole Brillouin zone for GaAs using
both the and nearest-neighbor tight-binding
models. In the -valley, the spin splitting obtained is in good
agreement with experimental data. We then further explicitly present the
coefficients of the spin splitting in GaAs and valleys. These results
are important to the realization of spintronic device and the investigation of
spin dynamics far away from equilibrium.Comment: 8 pages, 3 figures, Physica E, in pres
Plane waves with negative phase velocity in Faraday chiral mediums
The propagation of plane waves in a Faraday chiral medium is investigated.
Conditions for the phase velocity to be directed opposite to the direction of
power flow are derived for propagation in an arbitrary direction; simplified
conditions which apply to propagation parallel to the distinguished axis are
also established. These negative phase-velocity conditions are explored
numerically using a representative Faraday chiral medium, arising from the
homogenization of an isotropic chiral medium and a magnetically biased ferrite.
It is demonstrated that the phase velocity may be directed opposite to power
flow, provided that the gyrotropic parameter of the ferrite component medium is
sufficiently large compared with the corresponding nongyrotropic permeability
parameters.Comment: accepted for publication in Phys. Rev.
Solar wind pressure pulse‐driven magnetospheric vortices and their global consequences
We report the in situ observation of a plasma vortex induced by a solar wind dynamic pressure enhancement in the nightside plasma sheet using multipoint measurements from Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. The vortex has a scale of 5–10 Re and propagates several Re downtail, expanding while propagating. The features of the vortex are consistent with the prediction of the Sibeck (1990) model, and the vortex can penetrate deep (~8 Re ) in the dawn‐dusk direction and couple to field line oscillations. Global magnetohydrodynamics simulations are carried out, and it is found that the simulation and observations are consistent with each other. Data from THEMIS ground magnetometer stations indicate a poleward propagating vortex in the ionosphere, with a rotational sense consistent with the existence of the vortex observed in the magnetotail. Key Points Solar wind pressure pulse‐driven vortex was observed in the magnetosphere Simulation and ground magnetic field data confirm this tailward moving vortex The vortex can penetrate deep inside the tail plasma sheet and couple to FLRsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/107999/1/jgra51112.pd
Interatomic potentials for atomistic simulations of the Ti-Al system
Semi-empirical interatomic potentials have been developed for Al, alpha-Ti,
and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large
database of experimental as well as ab-initio data. The ab-initio calculations
were performed by the linear augmented plane wave (LAPW) method within the
density functional theory to obtain the equations of state for a number of
crystal structures of the Ti-Al system. Some of the calculated LAPW energies
were used for fitting the potentials while others for examining their quality.
The potentials correctly predict the equilibrium crystal structures of the
phases and accurately reproduce their basic lattice properties. The potentials
are applied to calculate the energies of point defects, surfaces, planar faults
in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al
system, the proposed potentials provide reasonable description of the lattice
thermal expansion, demonstrating their usefulness in the molecular dynamics or
Monte Carlo studies at high temperatures. The energy along the tetragonal
deformation path (Bain transformation) in gamma-TiAl calculated with the EAM
potential is in a fairly good agreement with LAPW calculations. Equilibrium
point defect concentrations in gamma-TiAl are studied using the EAM potential.
It is found that antisite defects strongly dominate over vacancies at all
compositions around stoichiometry, indicating that gamm-TiAl is an antisite
disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press
Efficient Recognition of Partially Visible Objects Using a Logarithmic Complexity Matching Technique
An important task in computer vision is the recognition of partially visible two-dimensional objects in a gray scale image. Recent works addressing this problem have attempted to match spatially local features from the image to features generated by models of the objects. However, many algo rithms are considerably less efficient than they might be, typ ically being O(IN) or worse, where I is the number offeatures in the image and N is the number of features in the model set. This is invariably due to the feature-matching portion of the algorithm. In this paper we discuss an algorithm that significantly improves the efficiency offeature matching. In addition, we show experimentally that our recognition algo rithm is accurate and robust. Our algorithm uses the local shape of contour segments near critical points, represented in slope angle-arclength space (θ-s space), as fundamental fea ture vectors. These feature vectors are further processed by projecting them onto a subspace in θ-s space that is obtained by applying the Karhunen-Loève expansion to all such fea tures in the set of models, yielding the final feature vectors. This allows the data needed to store the features to be re duced, while retaining nearly all information important for recognition. The heart of the algorithm is a technique for performing matching between the observed image features and the precomputed model features, which reduces the runtime complexity from O(IN) to O(I log I + I log N), where I and N are as above. The matching is performed using a tree data structure, called a kD tree, which enables multidi mensional searches to be performed in O(log) time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66975/2/10.1177_027836498900800608.pd
Simulation of the thermally induced austenitic phase transition in NiTi nanoparticles
The reverse martensitic ("austenitic") transformation upon heating of
equiatomic nickel-titanium nanoparticles with diameters between 4 and 17 nm is
analyzed by means of molecular-dynamics simulations with a semi-empirical model
potential. After constructing an appropriate order parameter to distinguish
locally between the monoclinic B19' at low and the cubic B2 structure at high
temperatures, the process of the phase transition is visualized. This shows a
heterogeneous nucleation of austenite at the surface of the particles, which
propagates to the interior by plane sliding, explaining a difference in
austenite start and end temperatures. Their absolute values and dependence on
particle diameter are obtained and related to calculations of the surface
induced size dependence of the difference in free energy between austenite and
martensite.Comment: 6 pages, 4 figures, accepted for publication in "The European
Physical Journal B
Spin dynamics in semiconductors
This article reviews the current status of spin dynamics in semiconductors
which has achieved a lot of progress in the past years due to the fast growing
field of semiconductor spintronics. The primary focus is the theoretical and
experimental developments of spin relaxation and dephasing in both spin
precession in time domain and spin diffusion and transport in spacial domain. A
fully microscopic many-body investigation on spin dynamics based on the kinetic
spin Bloch equation approach is reviewed comprehensively.Comment: a review article with 193 pages and 1103 references. To be published
in Physics Reports
Measurement of Z/gamma*+jet+X angular distributions in ppbar collisions at sqrt{s}=1.96 TeV
We present the first measurements at a hadron collider of differential cross
sections for Z+jet+X production in delta phi(Z, jet), |delta y(Z, jet)| and
|y_boost(Z, jet)|. Vector boson production in association with jets is an
excellent probe of QCD and constitutes the main background to many small cross
section processes, such as associated Higgs production. These measurements are
crucial tests of the predictions of perturbative QCD and current event
generators, which have varied success in describing the data. Using these
measurements as inputs in tuning event generators will increase the
experimental sensitivity to rare signals.Comment: Published in Physics Letters B 682 (2010), pp. 370-380. 15 pages, 6
figure
Search for the standard model Higgs boson in tau final states
We present a search for the standard model Higgs boson using hadronically
decaying tau leptons, in 1 inverse femtobarn of data collected with the D0
detector at the Fermilab Tevatron ppbar collider. We select two final states:
tau plus missing transverse energy and b jets, and tau+ tau- plus jets. These
final states are sensitive to a combination of associated W/Z boson plus Higgs
boson, vector boson fusion and gluon-gluon fusion production processes. The
observed ratio of the combined limit on the Higgs production cross section at
the 95% C.L. to the standard model expectation is 29 for a Higgs boson mass of
115 GeV.Comment: publication versio
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