339 research outputs found
Springback Compensation: Fundamental Topics and Practical Application
Now that the simulation of deep drawing processes has become more reliable the virtual\ud
compensation of the forming tools has become reality. In literature, the Displacement Adjustment (DA)\ud
algorithm has proved to be most effective. In this article it is shown how the compensation factor, required for\ud
(one-step) DA depends on material, process and geometrical parameters. For this an analytical bar stretchbending\ud
model was used. A compensation factor is not required when DA is applied iteratively and the\ud
products geometrical accuracy is improved further. This was demonstrated on an industrial part. The\ud
compensation varies over the product, leading to a reduction in shape deviation of 90% and more, a result that\ud
could not have been achieved with one-step compensatio
Improvements in FE-analysis of real-life sheet metal forming
An overview will be presented of recent developments concerning the application\ud
and development of computer codes for numerical simulation of sheet metal forming\ud
processes. In this paper attention is paid to some strategies which are followed to improve the\ud
accuracy and to reduce the computation time of a finite element simulation. Special attention\ud
will be paid to the mathematical modeling of the material deformation and friction, and the\ud
effect of these models on the results of simulations. An equivalent drawbead model is\ud
developed which avoids a drastic increase of computation time without significant loss of\ud
accuracy. The real geometry of the drawbead is replaced by a line on the tool surface. When\ud
an element of the sheet metal passes this drawbead line an additional drawbead restraining\ud
force, lift force and a plastic strain are added to that element. A commonly used yield\ud
criterion for anisotropic plastic deformation is the Hill yield criterion. This description is not\ud
always sufficient to accurately describe the material behavior. This is due to the\ud
determination of material parameters by uni-axial tests only. A new yield criterion is\ud
proposed, which directly uses the experimental results at multi-axial stress states. The yield\ud
criterion is based on the pure shear point, the uni-axial point, the plane strain point and the\ud
equi-biaxial point
Charged excitons in doped extended Hubbard model systems
We show that the charge transfer excitons in a Hubbard model system including
nearest neighbor Coulomb interactions effectively attain some charge in doped
systems and become visible in photoelectron and inverse photoelectron
spectroscopies. This shows that the description of a doped system by an
extended Hubbard model differs substantially from that of a simple Hubbard
model. Longer range Coulomb interactions cause satellites in the one electron
removal and addition spectra and the appearance of spectral weight if the gap
of doped systems at energies corresponding to the excitons of the undoped
systems. The spectral weight of the satellites is proportional to the doping
times the coordination number and therefore is strongly dependent on the
dimension.Comment: 10 pages revtex, 5 figures ps figures adde
Adsorption of colloidal particles in the presence of external field
We present a new class of sequential adsorption models in which the adsorbing
particles reach the surface following an inclined direction (shadow models).
Capillary electrophoresis, adsorption in the presence of a shear or on an
inclined substrate are physical manifestations of these models. Numerical
simulations are carried out to show how the new adsorption mechanisms are
responsible for the formation of more ordered adsorbed layers and have
important implications in the kinetics, in particular modifying the jamming
limit.Comment: LaTex file, 3 figures available upon request, to appear in
Phys.Rev.Let
Switching Casimir forces with Phase Change Materials
We demonstrate here a controllable variation in the Casimir force. Changes in
the force of up to 20% at separations of ~100 nm between Au and AgInSbTe (AIST)
surfaces were achieved upon crystallization of an amorphous sample of AIST.
This material is well known for its structural transformation, which produces a
significant change in the optical properties and is exploited in optical data
storage systems. The finding paves the way to the control of forces in
nanosystems, such as micro- or nanoswitches by stimulating the phase change
transition via localized heat sources.Comment: 7 pages, 3 figures The AFM images for the inset in Fig.2 were
replaced with new ones as obtained with tips having high aspect rati
The spectral weight of the Hubbard model through cluster perturbation theory
We calculate the spectral weight of the one- and two-dimensional Hubbard
models, by performing exact diagonalizations of finite clusters and treating
inter-cluster hopping with perturbation theory. Even with relatively modest
clusters (e.g. 12 sites), the spectra thus obtained give an accurate
description of the exact results. Thus, spin-charge separation (i.e. an
extended spectral weight bounded by singularities) is clearly recognized in the
one-dimensional Hubbard model, and so is extended spectral weight in the
two-dimensional Hubbard model.Comment: 4 pages, 5 figure
Inter-site Coulomb interaction and Heisenberg exchange
Based on exact diagonalization results for small clusters we discuss the
effect of inter-site Coulomb repulsion in Mott-Hubbard or charge transfer
insulators. Whereas the exchange constant J for direct exchange is
substantially enhanced by inter-site Coulomb interaction, that for
superexchange is suppressed. The enhancement of J in the single-band models
holds up to the critical value for the charge density wave (CDW) instability,
thus opening the way for large values of J. Single-band Hubbard models with
sufficiently strong inter-site repulsion to be near a CDW instability thus may
provide `physical' realizations of t-J like models with the `unphysical'
parameter ratio J/t=1.Comment: Revtex file, 4 PRB pages, with 5 embedded ps-files. To appear in PRB,
rapid communications. Hardcopies of figures or the entire manuscript may also
be obtained by e-mail request to: [email protected]
Migraine and vascular disease biomarkers: A population-based case-control study.
Background The underpinnings of the migraine-stroke association remain uncertain, but endothelial activation is a potential mechanism. We evaluated the association of migraine and vascular disease biomarkers in a community-based population. Methods Participants (300 women, 117 men) were recruited as a part of the Dutch CAMERA 1 (Cerebral Abnormalities in Migraine, an Epidemiologic Risk Analysis) study. Participants were aged 30-60 (mean 48) years, 155 migraine had with aura (MA), 128 migraine without aura (MO), and 134 were controls with no severe headaches. Plasma concentrations of fibrinogen, Factor II, D-dimer, high sensitivity C-reactive protein (hs-CRP), and von Willebrand factor antigen were compared between groups, also stratifying by sex. Results Fibrinogen and hs-CRP were elevated in migraineurs compared to controls. In logistic regression analyses, MO and MA had increased likelihood of elevated fibrinogen, and MA had increased likelihood of elevated Factor II and hs-CRP. Fibrinogen and Factor II were associated with MA in women but not men. In the migraine subgroup, the total number of years of aura, but not headache, predicted elevated hs-CRP, and the average number of aura, but not headache, attacks predicted all biomarkers but Factor II. Conclusions Elevated vascular biomarkers were associated with migraine, particularly MA, as well as with years of aura and number of aura attacks
Ectopic Fat and Insulin Resistance: Pathophysiology and Effect of Diet and Lifestyle Interventions
The storage of triglyceride (TG) droplets in nonadipose tissues is called ectopic fat storage. Ectopic fat is associated with insulin resistance and type 2 diabetes mellitus (T2DM). Not the triglycerides per se but the accumulation of intermediates of lipid metabolism in organs, such as the liver, skeletal muscle, and heart seem to disrupt metabolic processes and impair organ function. We describe the mechanisms of ectopic fat depositions in the liver, skeletal muscle, and in and around the heart and the consequences for each organs function. In addition, we systematically reviewed the literature for the effects of diet-induced weight loss and exercise on ectopic fat depositions
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