23,907 research outputs found
Practical theories for service life prediction of critical aerospace structural components
A new second-order theory was developed for predicting the service lives of aerospace structural components. The predictions based on this new theory were compared with those based on the Ko first-order theory and the classical theory of service life predictions. The new theory gives very accurate service life predictions. An equivalent constant-amplitude stress cycle method was proposed for representing the random load spectrum for crack growth calculations. This method predicts the most conservative service life. The proposed use of minimum detectable crack size, instead of proof load established crack size as an initial crack size for crack growth calculations, could give a more realistic service life
(2317) meson production at RHIC
Production of (2317) mesons in relativistic heavy ion collisions at
RHIC is studied. Using the quark coalescence model, we first determine the
initial number of (2317) mesons produced during hadronization of
created quark-gluon plasma. The predicted (2317) abundance depends
sensitively on the quark structure of the (2317) meson. An
order-of-magnitude larger yield is obtained for a conventional two-quark than
for an exotic four-quark (2317) meson. To include the hadronic effect
on the (2317) meson yield, we have evaluated the absorption cross
sections of the (2317) meson by pion, rho, anti-kaon, and vector
anti-kaon in a phenomenological hadronic model. Taking into consideration the
absorption and production of (2317) mesons during the hadronic stage of
heavy ion collisions via a kinetic model, we find that the final yield of
(2317) mesons remains sensitive to its initial number produced from the
quark-gluon plasma, providing thus the possibility of studying the quark
structure of the (2317) meson and its production mechanism in
relativistic heavy ion collisions.Comment: 12 pages, 6 figure
Thermalization of quark-gluon matter by 2-to-2 and 3-to-3 elastic scatterings
Thermalization of quark-gluon matter is studied with a transport equation
that includes contributions of 2-to-2 and 3-to-3 elastic scatterings.
Thermalization time is related to the squared amplitudes for the elastic
scatterings that are calculated in perturbative QCD.Comment: LaTex, 6 pages, 3 figures, talk presented at the 19th international
conference on ultra-relativistic nucleus-nucleus collisions, Shanghai, China,
Nov. 200
Analytical and numerical studies of central galactic outflows powered by tidal disruption events -- a model for the Fermi bubbles?
Capture and tidal disruption of stars by the supermassive black hole in the
Galactic center (GC) should occur regularly. The energy released and dissipated
by this processes will affect both the ambient environment of the GC and the
Galactic halo. A single star of super-Eddington eruption generates a subsonic
out ow with an energy release of more than erg, which still is not
high enough to push shock heated gas into the halo. Only routine tidal
disruption of stars near the GC can provide enough cumulative energy to form
and maintain large scale structures like the Fermi Bubbles. The average rate of
disruption events is expected to be ~ yr, providing
the average power of energy release from the GC into the halo of dW/dt ~
3*10 erg/s, which is needed to support the Fermi Bubbles. The GC black
hole is surrounded by molecular clouds in the disk, but their overall mass and
filling factor is too low to stall the shocks from tidal disruption events
significantly. The de facto continuous energy injection on timescales of Myr
will lead to the propagation of strong shocks in a density stratified Galactic
halo and thus create elongated bubble-like features, which are symmetric to the
Galactic midplane.Comment: 11 pages, 5 figures. The title and abstract have been changed.
Accepted by Astrophysical Journa
Lymphotoxins and cytomegalovirus cooperatively induce interferon-beta, establishing host-virus détente
Tumor necrosis factor (TNF)-related cytokines regulate cell death and survival and provide strong selective pressures for viruses, such as cytomegalovirus (CMV), to evolve counterstrategies in order to persist in immune-competent hosts. Signaling by the lymphotoxin (LT)-β receptor or TNF receptor-1, but not Fas or TRAIL receptors, inhibits the cytopathicity and replication of human CMV by a nonapoptotic, reversible process that requires nuclear factor κB (NF-κB)-dependent induction of interferon-β (IFN-β). Efficient induction of IFN-β requires virus infection and LT signaling, demonstrating the need for both host and viral factors in the curtailment of viral replication without cellular elimination. LTα-deficient mice and LTβR-Fc transgenic mice were profoundly susceptible to murine CMV infection. Together, these results reveal an essential and conserved role for LTs in establishing host defense to CMV
Origins of ferromagnetism in transition-metal doped Si
We present results of the magnetic, structural and chemical characterizations of Mn<sup>+</sup>-implanted Si displaying <i>n</i>-type semiconducting behavior and ferromagnetic ordering with Curie temperature,T<sub>C</sub> well above room temperature. The temperature-dependent magnetization measured by superconducting quantum device interference (SQUID) from 5 K to 800 K was characterized by three different critical temperatures (T*<sub>C</sub>~45 K, T<sub>C1</sub>~630-650 K and T<sub>C2</sub>~805-825 K). Their origins were investigated using dynamic secondary mass ion spectroscopy (SIMS) and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy (EELS), Z-contrast STEM (scanning TEM) imaging and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn<sup>+</sup> ions, as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed T*<sub>C</sub> is attributed to the Mn<sub>4</sub>Si<sub>7</sub> precipitates identified by electron diffraction. Possible origins of and are also discussed. Our findings raise questions regarding the origin of the high ferromagnetism reported in many material systems without a careful chemical analysis
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