205 research outputs found
Multiaxial Kitagawa analysis of A356-T6
Experimental Kitagawa analysis has been performed on A356-T6 containing
natural and artificial defects. Results are obtained with a load ratio of R =
-1 for three different loadings: tension, torsion and combined tension-torsion.
The critical defect size determined is 400 \pm 100 \mum in A356-T6 under
multiaxial loading. Below this value, the microstructure governs the endurance
limit mainly through Secondary Dendrite Arm Spacing (SDAS). Four theoretical
approaches are used to simulate the endurance limit characterized by a Kitagawa
relationship are compared: Murakami relationships [Y Murakami, Metal Fatigue:
Effects of Small Defects and Nonmetallic Inclusions, Elsevier, 2002.],
defect-crack equivalency via Linear Elastic Fracture Mechanics (LEFM), the
Critical Distance Method (CDM) proposed by Susmel and Taylor [L. Susmel, D.
Taylor. Eng. Fract. Mech. 75 (2008) 15.] and the gradient approach proposed by
Nadot [Y. Nadot, T. ~Billaudeau. Eng. Fract. Mech. 73 (2006) 1.]. It is shown
that the CDM and gradient methods are accurate; however fatigue data for three
loading conditions is necessary to allow accurate identification of an
endurance limit.Comment: 27 pages, 11 figure
Superdeformed rotational bands in the Mercury region; A Cranked Skyrme-Hartree-Fock-Bogoliubov study
A study of rotational properties of the ground superdeformed bands in \Hg{0},
\Hg{2}, \Hg{4}, and \Pb{4} is presented. We use the cranked
Hartree-Fock-Bogoliubov method with the {\skm} parametrization of the Skyrme
force in the particle-hole channel and a seniority interaction in the pairing
channel. An approximate particle number projection is performed by means of the
Lipkin-Nogami prescription. We analyze the proton and neutron quasiparticle
routhians in connection with the present information on about thirty presently
observed superdeformed bands in nuclei close neighbours of \Hg{2}.Comment: 26 LaTeX pages, 14 uuencoded postscript figures included, Preprint
IPN-TH 93-6
Exploring Hyperons and Hypernuclei with Lattice QCD
In this work we outline a program for lattice QCD that would provide a first
step toward understanding the strong and weak interactions of strange baryons.
The study of hypernuclear physics has provided a significant amount of
information regarding the structure and weak decays of light nuclei containing
one or two Lambda's, and Sigma's. From a theoretical standpoint, little is
known about the hyperon-nucleon interaction, which is required input for
systematic calculations of hypernuclear structure. Furthermore, the
long-standing discrepancies in the P-wave amplitudes for nonleptonic hyperon
decays remain to be understood, and their resolution is central to a better
understanding of the weak decays of hypernuclei. We present a framework that
utilizes Luscher's finite-volume techniques in lattice QCD to extract the
scattering length and effective range for Lambda-N scattering in both QCD and
partially-quenched QCD. The effective theory describing the nonleptonic decays
of hyperons using isospin symmetry alone, appropriate for lattice calculations,
is constructed.Comment: 24 pages, 7 figure
Fatigue Crack Growth Mechanisms At the Microstructure Scale in Al-Si-Mg Cast Alloys: Mechanisms in Regions II and III
The fatigue crack growth behavior in Regions 11 and III of crack growth was investigated for hypoeutectic and eutectic Al-Si-Mg cast alloys. To isolate and establish the mechanistic contributions of characteristic microstructural features (dendritic α-Al matrix, eutectic phases, Mg-Si strengthening precipitates), alloys with various Si content/morphology, grain size level, and matrix strength were studied; the effect of secondary dendrite arm spacing (SDAS) was also assessed. In Regions 11 and III of crack growth, the observed changes in the fracture surface appearance were associated with changes in crack growth mechanisms at the microstructural scale (from a linear advance predominantly through primary α-Al to a tortuous advance exclusively through AI-Si eutectic Regions). The extent of the plastic zone ahead of the crack tip was successfully used to explain the changes in growth mechanisms. The fatigue crack growth tests were conducted on compact tension specimens under constant stress ratio, R = 0.1, in ambient conditions
Improved hardness results for the guided local Hamiltonian problem
Estimating the ground state energy of a local Hamiltonian is a central problem in quantum chemistry. In order to further investigate its complexity and the potential of quantum algorithms for quantum chemistry, Gharibian and Le Gall (STOC 2022) recently introduced the guided local Hamiltonian problem (GLH), which is a variant of the local Hamiltonian problem where an approximation of a ground state (which is called a guiding state) is given as an additional input. Gharibian and Le Gall showed quantum advantage (more precisely, BQP-completeness) for GLH with 6-local Hamiltonians when the guiding state has fidelity (inverse-polynomially) close to 1/2 with a ground state. In this paper, we optimally improve both the locality and the fidelity parameter: we show that the BQP-completeness persists even with 2-local Hamiltonians, and even when the guiding state has fidelity (inverse-polynomially) close to 1 with a ground state. Moreover, we show that the BQP-completeness also holds for 2-local physically motivated Hamiltonians on a 2D square lattice or a 2D triangular lattice. Beyond the hardness of estimating the ground state energy, we also show BQP-hardness persists when considering estimating energies of excited states of these Hamiltonians instead. Those make further steps towards establishing practical quantum advantage in quantum chemistry
Random Convex Hulls and Extreme Value Statistics
In this paper we study the statistical properties of convex hulls of
random points in a plane chosen according to a given distribution. The points
may be chosen independently or they may be correlated. After a non-exhaustive
survey of the somewhat sporadic literature and diverse methods used in the
random convex hull problem, we present a unifying approach, based on the notion
of support function of a closed curve and the associated Cauchy's formulae,
that allows us to compute exactly the mean perimeter and the mean area enclosed
by the convex polygon both in case of independent as well as correlated points.
Our method demonstrates a beautiful link between the random convex hull problem
and the subject of extreme value statistics. As an example of correlated
points, we study here in detail the case when the points represent the vertices
of independent random walks. In the continuum time limit this reduces to
independent planar Brownian trajectories for which we compute exactly, for
all , the mean perimeter and the mean area of their global convex hull. Our
results have relevant applications in ecology in estimating the home range of a
herd of animals. Some of these results were announced recently in a short
communication [Phys. Rev. Lett. {\bf 103}, 140602 (2009)].Comment: 61 pages (pedagogical review); invited contribution to the special
issue of J. Stat. Phys. celebrating the 50 years of Yeshiba/Rutgers meeting
Targeted retroviral gene transfer into the rat biliary tract
The ability to induce proliferation by temporary duct ligation suggested an hypothesis that retrovirus-mediated gene transfer into cells of the biliary tract could be accomplished. The time course of histologic changes, incorporation of 3 H-thymidine and immunofluorescent staining with a monoclonal antibody to cytokeratin-19 (a marker for differentiated bile ducts) was studied in male Fischer F344 rats. A recombinant Gibbon ape leukemia virus (GALV), containing a gene encoding Escherichia coli β-galactosidase was next introduced into 24 hr obstructed bile ducts. Gene transfer was maximal when virus was exposed to the obstructed duct for 12 hr (∼0.1%). The majority of X-gal positive cells were in cytokeratin-19 negative peribiliary tissues, which had the appearance of newly forming bile ducts. The data suggest that cells targeted by retroviral infection of the obstructed rat bile duct may be a precursor of mature, fully differentiated biliary epithelium.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45547/1/11188_2006_Article_BF02374373.pd
Dust in Supernovae and Supernova Remnants I : Formation Scenarios
Supernovae are considered as prime sources of dust in space. Observations of local supernovae over the past couple of decades have detected the presence of dust in supernova ejecta. The reddening of the high redshift quasars also indicate the presence of large masses of dust in early galaxies. Considering the top heavy IMF in the early galaxies, supernovae are assumed to be the major contributor to these large amounts of dust. However, the composition and morphology of dust grains formed in a supernova ejecta is yet to be understood with clarity. Moreover, the dust masses inferred from observations in mid-infrared and submillimeter wavelength regimes differ by two orders of magnitude or more. Therefore, the mechanism responsible for the synthesis of molecules and dust in such environments plays a crucial role in studying the evolution of cosmic dust in galaxies. This review summarises our current knowledge of dust formation in supernova ejecta and tries to quantify the role of supernovae as dust producers in a galaxy.Peer reviewe
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