5,971 research outputs found
Effect of thermal exposure, forming, and welding on high-temperature, dispersion-strengthened aluminum alloy: Al-8Fe-1V-2Si
The feasibility of applying conventional hot forming and welding methods to high temperature aluminum alloy, Al-8Fe-1V-2Si (FVS812), for structural applications and the effect of thermal exposure on mechanical properties were determined. FVS812 (AA8009) sheet exhibited good hot forming and resistance welding characteristics. It was brake formed to 90 deg bends (0.5T bend radius) at temperatures greater than or equal to 390 C (730 F), indicating the feasibility of fabricating basic shapes, such as angles and zees. Hot forming of simple contoured-flanged parts was demonstrated. Resistance spot welds with good static and fatigue strength at room and elevated temperatures were readily produced. Extended vacuum degassing during billet fabrication reduced porosity in fusion and resistance welds. However, electron beam welding was not possible because of extreme degassing during welding, and gas-tungsten-arc welds were not acceptable because of severely degraded mechanical properties. The FVS812 alloy exhibited excellent high temperature strength stability after thermal exposures up to 315 C (600 F) for 1000 h. Extended billet degassing appeared to generally improve tensile ductility, fatigue strength, and notch toughness. But the effects of billet degassing and thermal exposure on properties need to be further clarified. The manufacture of zee-stiffened, riveted, and resistance-spot-welded compression panels was demonstrated
Kinematic dynamo action in a sphere. I. Effects of differential rotation and meridional circulation on solutions with axial dipole symmetry
A sphere containing electrically conducting fluid can generate a magnetic field by dynamo action, provided the flow is sufficiently complicated and vigorous. The dynamo mechanism is thought to sustain magnetic fields in planets and stars. The kinematic dynamo problem tests steady flows for magnetic instability, but rather few dynamos have been found so far because of severe numerical difficulties. Dynamo action might, therefore, be quite unusual, at least for large-scale steady flows. We address this question by testing a two-parameter class of flows for dynamo generation of magnetic fields containing an axial dipole. The class of flows includes two completely different types of known dynamos, one dominated by differential rotation (D) and one with none. We find that 36% of the flows in seven distinct zones in parameter space act as dynamos, while the remaining 64% either fail to generate this type of magnetic field or generate fields that are too small in scale to be resolved by our numerical method. The two previously known dynamo types lie in the same zone, and it is therefore possible to change the flow continuously from one to the other without losing dynamo action. Differential rotation is found to promote large-scale axisymmetric toroidal magnetic fields, while meridional circulation (M) promotes large-scale axisymmetric poloidal fields concentrated at high latitudes near the axis. Magnetic fields resembling that of the Earth are generated by D > 0, corresponding to westward flow at the surface, and M of either sign but not zero. Very few oscillatory solutions are found
Magnetic confinement of the solar tachocline
We study the physics of the solar tachocline and related MHD instabilities.
We have performed 3-D MHD simulations of the solar radiative interior to check
whether a fossil magnetic field is able to prevent the spread of the
tachocline. Starting with a purely poloidal magnetic field and a latitudinal
shear meant to be imposed by the convection zone at the top of the radiation
zone, we have investigated the interactions between magnetic fields, rotation
and shear, using the spectral code ASH on massive parallel supercomputers. In
all cases we have explored, the fossil field diffuses outward and ends up
connecting with the convection zone, whose differential rotation is then
imprinted at latitudes above 40 deg throughout the radiative interior,
according to Ferraro's law of isorotation. Rotation remains uniform in the
lower latitude region which is contained within closed field lines. We find
that the meridional flow cannot stop the inward progression of the differential
rotation. Further, we observe the development of non-axisymmetric
magnetohydrodynamic instabilities, first due to the initial poloidal
configuration of the fossil field, and later to the toroidal field produced by
shearing the poloidal field through the differential rotation. We do not find
dynamo action as such in the radiative interior, since the mean poloidal field
is not regenerated. But the instability persists during the whole evolution,
while slowly decaying with the mean poloidal field. According to our numerical
simulations, a fossil magnetic field cannot prevent the radiative spread of the
tachocline, and thus it is unable to enforce uniform rotation in the radiation
zone. Neither can the observed thinness of that layer be invoked as a proof for
such an internal fossil magnetic field.Comment: 12 pages, 8 color figures (low res), published in A&A, october 200
Nonet Symmetry and Two-Body Decays of Charmed Mesons
The decay of charmed mesons into pseudoscalar (P) and vector (V) mesons is
studied in the context of nonet symmetry. We have found that it is badly broken
in the PP channels and in the P sector of the PV channels as expected from the
non-ideal mixing of the \eta and the \eta'. In the VV channels, it is also
found that nonet symmetry does not describe the data well. We have found that
this discrepancy cannot be attributed entirely to SU(3) breaking at the usual
level of 20--30%. At least one, or both, of nonet and SU(3) symmetry must be
very badly broken. The possibility of resolving the problem in the future is
also discussed.Comment: 9 pages, UTAPHY-HEP-
decays
Effective chiral theory of mesons is applied to study the four decay modes of
. Theoretical values of the branching ratios are in
agreement with the data. The theory predicts that the resonance plays a
dominant role in these decays. There is no new parameter in this study.Comment: 12 pages and one figur
Constant amplitude and post-overload fatigue crack growth behavior in PM aluminum alloy AA 8009
A recently developed, rapidly solidified, powder metallurgy, dispersion strengthened aluminum alloy, AA 8009, was fatigue tested at room temperature in lab air. Constant amplitude/constant delta kappa and single spike overload conditions were examined. High fatigue crack growth rates and low crack closure levels compared to typical ingot metallurgy aluminum alloys were observed. It was proposed that minimal crack roughness, crack path deflection, and limited slip reversibility, resulting from ultra-fine microstructure, were responsible for the relatively poor da/dN-delta kappa performance of AA 8009 as compared to that of typical IM aluminum alloys
Transition between nuclear and quark-gluon descriptions of hadrons and light nuclei
We provide a perspective on studies aimed at observing the transition between
hadronic and quark-gluonic descriptions of reactions involving light nuclei. We
begin by summarizing the results for relatively simple reactions such as the
pion form factor and the neutral pion transition form factor as well as that
for the nucleon and end with exclusive photoreactions in our simplest nuclei. A
particular focus will be on reactions involving the deuteron. It is noted that
a firm understanding of these issues is essential for unraveling important
structure information from processes such as deeply virtual Compton scattering
as well as deeply virtual meson production. The connection to exotic phenomena
such as color transparency will be discussed. A number of outstanding
challenges will require new experiments at modern facilities on the horizon as
well as further theoretical developments.Comment: 37 pages, 17 figures, submitted to Reports on Progress in Physic
Charged Scalar Particles and Leptonic Decay
Charged scalar particles introduced in some extensions of the standard model
can induce leptonic decay at tree level. We find that with some charged
SU(2)-singlet scalar particles, like ones introduced in Zee-type models,
leptonic decay width is always smaller than what is predicted by the standard
model, therefore they may offer a natural solution to decay puzzle. To
be more specific, we examine some Zee-type models in detail to see if at the
same time they are acceptable in particle physics, cosmology and astrophysics.
It is shown that decay data do put some constrains on these models.Comment: ICTP Report No. IC/93/31, 12 pages, Latex, one figure is not
included, it is available upon deman
Building Bridges with Boats: Preserving Community History through Intra- and Inter-Institutional Collaboration
This chapter discusses Launching through the Surf: The Dory Fleet of Pacific City, a project which documents the historical and contemporary role of dory fishers in the life of the coastal village of Pacific City, Oregon, U.S. Linfield College’s Department of Theatre and Communication Arts, its Jereld R. Nicholson Library, the Pacific City Arts Association, the Pacific City Dorymen\u27s Association, and the Linfield Center for the Northwest joined forces to engage in a collaborative college and community venture to preserve this important facet of Oregon’s history. Using ethnography as a theoretical grounding and oral history as a method, the project utilized artifacts from the dory fleet to augment interview data, and faculty/student teams created a searchable digital archive available via open access. The chapter draws on the authors’ experiences to identify a philosophy of strategic collaboration. Topics include project development and management, assessment, and the role of serendipity. In an era of value-added services where libraries need to continue to prove their worth, partnering with internal and external entities to create content is one way for academic libraries to remain relevant to agencies that do not have direct connections to higher education. This project not only developed a positive “town and gown” relationship with a regional community, it also benefited partner organizations as they sought to fulfill their missions. The project also serves as a potential model for intra- and inter-agency collaboration for all types of libraries
About one long-range contribution to K+ -> pi+ l+ l- decays
We investigate the mechanism of K+ -> pi+ l+ l- (l= e, mu) decays in which a
virtual photon is emitted either from the incoming K+ or the outgoing pi+. We
point out some inconsistencies with and between two previous calculations,
discuss the possible experimental inputs, and estimate the branching fractions.
This mechanism alone fails to explain the existing experimental data by more
than one order-of-magnitude. But it may show itself by its interference with
the leading long-range mechanism dominated by the a_1^+ and rho^0 mesons.Comment: 12 pages, RevTeX, epsf.sty, 2 embedded figure
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