16,249 research outputs found
Buckling of built-up columns of pultruded fiber-reinforced polymer C-sections
This paper presents the test results of an experimental investigation to evaluate the buckling behavior of built-up columns of pultruded profiles, subjected to axial compression. Specimens are assembled by using four (off the shelf) channel shaped profiles of E-glass fiber-reinforced polymer (FRP), having similar detailing to strut members in a large FRP structure that was executed in 2009 to start the restoration of the Santa Maria Paganica church in L’Aquila, Italy. This church had partially collapsed walls and no roof after the April 6, 2009, earthquake of 6.3 magnitude. A total of six columns are characterized with two different configurations for the bolted connections joining the channel sections into a built-up strut. Test results are discussed and a comparison is made with closed-form equation predictions for flexural buckling resistance, with buckling resistance values established from both eigenvalue and geometric nonlinear finite element analyses. Results show that there is a significant role played by the end loading condition, the composite action, and imperfections. Simple closed-form equations overestimate the flexural buckling strength, whereas the resistance provided by the nonlinear analysis provides a reasonably reliable numerical approach to establishing the actual buckling behavior
Towards Quantum Cosmology without Singularities
In this paper we investigate the vanishing of cosmological singularities by
quantization. Starting from a 5d Kaluza--Klein approach we quantize, as a first
step, the non--spherical metric part and the dilaton field. These fields which
are classically singular become smooth after quantization. In addition, we
argue that the incorporation of non perturbative quantum corrections form a
dilaton potential. Technically, the procedure corresponds to the quantization
of 2d dilaton gravity and we discuss several models. From the 4d point of view
this procedure is a semiclassical approach where only the dilaton and moduli
matter fields are quantized.Comment: 9 pages, 2 figures, Latex, epsfig.sty, epsf.te
Consequence of Hawking radiation from 2d dilaton black holes
We investigate the CGHS model through numerical calculation. The behavior of
the mass function, which we introduced in our previous work as a ``local
mass'', is examined. We found that the mass function takes negative values,
which means that the amount of Hawking radiation becomes greater than the
initial mass of the black hole as in the case of the RST model.Comment: 17pages, 5 figures (three of them are attached, the other 2 figures
are available on request. Some mistakes including typographic errors have
been correcte
Universality and Scaling at the Onset of Quantum Black Hole Formation
In certain two-dimensional models, collapsing matter forms a black hole if
and only if the incoming energy flux exceeds the Hawking radiation rate. Near
the critical threshold, the black hole mass is given by a universal formula in
terms of the distance from criticality, and there exists a scaling solution
describing the formation and evaporation of an arbitrarily small black hole.Comment: 9 pages, 3 figures (uuencoded
D-Brane Probe and Closed String Tachyons
We consider a D-brane probe in unstable string background associated with
flux branes. The twist in spacetime metric reponsible for the supersymmetry
breaking is shown to manifest itself in mixing of open Wilson lines with the
phases of some adjoint matter fields, resulting in a nonlocal and
nonsupersymmetric form of Yang-Mills theory as the probe dynamics. This
provides a setup where one can study fate of a large class of unstable closed
string theories that include as a limit type 0 theories and various orbifolds
of type II and type 0 theories. We discuss the limit of orbifold
in some detail and speculate on couplings with closed string tachyons.Comment: LaTeX, 17 pages, typos fixed, references update
A theory of quantum black holes: non-perturbative corrections and no-veil conjecture
A common belief is that further quantum corrections near the singularity of a
large black hole should not substantially modify the semiclassical picture of
black hole evaporation; in particular, the outgoing spectrum of radiation
should be very close to the thermal spectrum predicted by Hawking. In this
paper we explore a possible counterexample: in the context of dilaton gravity,
we find that non-perturbative quantum corrections which are important in strong
coupling regions may completely alter the semiclassical picture, to the extent
that the presumptive space-like boundary becomes time-like, changing in this
way the causal structure of the semiclassical geometry. As a result, only a
small fraction of the total energy is radiated outside the fake event horizon;
most of the energy comes in fact at later retarded times and there is no
information loss problem. Thus we propose that this may constitute a general
characteristic of quantum black holes, that is, quantum gravity might be such
as to prevent the formation of global event horizons. We argue that this is not
unnatural from the viewpoint of quantum mechanics.Comment: 24 pages, 12 figures (not included, available by request), UTTG-22-9
Experimental observation of bias-dependent non-local Andreev reflection
We investigate transport through hybrid structures consisting of two normal
metal leads connected via tunnel barriers to one common superconducting
electrode. We find clear evidence for the occurrence of non-local Andreev
reflection and elastic cotunneling through superconductor when the separation
of the tunnel barrier is comparable to the superconducting coherence length.
The probability of the two processes is energy dependent, with elastic
cotunneling dominating at low energy and non-local Andreev reflection at higher
energies. The energy scale of the crossover is found to be the Thouless energy
of the superconductor, which indicates the phase coherence of the processes.
Our results are relevant for the realization of recently proposed entangler
devices.Comment: 4 pages, 4 figures. Accepted for publication in PR
Two Dimensional Quantum Dilaton Gravity and the Positivity of Energy
Using an argument due to Regge and Teitelboim, an expression for the ADM mass
of 2d quantum dilaton gravity is obtained. By evaluating this expression we
establish that the quantum theories which can be written as a Liouville-like
theory, have a lower bound to energy, provided there is no critical boundary.
This fact is then reconciled with the observation made earlier that the Hawking
radiation does not appear to stop. The physical picture that emerges is that of
a black hole in a bath of quantum radiation. We also evaluate the ADM mass for
the models with RST boundary conditions and find that negative values are
allowed. The Bondi mass of these models goes to zero for large retarded times,
but becomes negative at intermediate times in a manner that is consistent with
the thunderpop of RST.Comment: 16 pages, phyzzx, COLO-HEP-309. (Confusing points in previous version
clarified, discussion of ADM and Bondi masses in RST case added.
Initial singularity free quantum cosmology in two-dimensional Brans-Dicke theory
We consider two-dimensional Brans-Dicke theory to study the initial
singularity problem. It turns out that the initial curvature singularity can be
finite for a certain Brans-Dicke constant by considering the quantum
back reaction of the geometry. For , the universe starts with the
finite curvature scalar and evolves into the flat spacetime. Furthermore the
divergent gravitational coupling at the initial time can be finite effectively
with the help of quantum correction. The other type of universe is studied for
the case of .Comment: 12 pages, 4 figures, revtex, Some references are added. To be
published in Phys. Rev.
Widening use of dexamethasone implant for the treatment of macular edema
Sustained-release intravitreal 0.7 mg dexamethasone (DEX) implant is approved in Europe for the treatment of macular edema related to diabetic retinopathy, branch retinal vein occlusion, central retinal vein occlusion, and non-infectious uveitis. The implant is formulated in a biodegradable copolymer to release the active ingredient within the vitreous chamber for up to 6 months after an intravitreal injection, allowing a prolonged interval of efficacy between injections with a good safety profile. Various other ocular pathologies with inflammatory etioÂpathogeneses associated with macular edema have been treated by DEX implant, including neovascular age-related macular degeneration, Irvine–Gass syndrome, vasoproliferative retinal tumors, retinal telangiectasia, Coats’ disease, radiation maculopathy, retinitis pigmentosa, and macular edema secondary to scleral buckling and pars plana vitrectomy. We undertook a review to provide a comprehensive collection of all of the diseases that benefit from the use of the sustained-release DEX implant, alone or in combination with concomitant therapies. A MEDLINE search revealed lack of randomized controlled trials related to these indications. Therefore we included and analyzed all available studies (retrospective and prospective, comÂparative and non-comparative, randomized and nonrandomized, single center and multicenter, and case report). There are reports in the literature of the use of DEX implant across a range of macular edema-related pathologies, with their clinical experience supporting the use of DEX implant on a case-by-case basis with the aim of improving patient outcomes in many macular pathologies. As many of the reported macular pathologies are difficult to treat, a new treatÂment option that has a beneficial influence on the clinical course of the disease may be useful in clinical practice
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