881 research outputs found
Material weakening due to corrosion in hardened bearing steels
In rolling and ball bearings, materials are designed to withstand extremely high loads for long operation times. Most bearing steels are hardened steels with high carbon content that typically can function up to the Very High Cycle Fatigue regime (\u3e1010 load cycles). The loads under which this occurs are given by the fatigue limit, which is a parameter that does depend on the strength of the material, but is commonly seen as a fixed parameter Therefore, material weakening is playing an important role when components fail as a result of rolling contact fatigue loading. In this paper, the effects of corrosion and hydrogen generation on the material weakening of hardened ball bearing steels are described. The results of corrosion exposure tests, fatigue tests with corrosion and tests with increased hydrogen levels in steel on mechanical properties will be presented. The role of hydrogen in bearing steel will be discussed on basis of the atomistic processes that play a role in rolling contact fatigue
On generalized cluster algorithms for frustrated spin models
Standard Monte Carlo cluster algorithms have proven to be very effective for
many different spin models, however they fail for frustrated spin systems.
Recently a generalized cluster algorithm was introduced that works extremely
well for the fully frustrated Ising model on a square lattice, by placing bonds
between sites based on information from plaquettes rather than links of the
lattice. Here we study some properties of this algorithm and some variants of
it. We introduce a practical methodology for constructing a generalized cluster
algorithm for a given spin model, and investigate apply this method to some
other frustrated Ising models. We find that such algorithms work well for
simple fully frustrated Ising models in two dimensions, but appear to work
poorly or not at all for more complex models such as spin glasses.Comment: 34 pages in RevTeX. No figures included. A compressed postscript file
for the paper with figures can be obtained via anonymous ftp to
minerva.npac.syr.edu in users/paulc/papers/SCCS-527.ps.Z. Syracuse University
NPAC technical report SCCS-52
Pulling a polymer out of a potential well and the mechanical unzipping of DNA
Motivated by the experiments on DNA under torsion, we consider the problem of
pulling a polymer out of a potential well by a force applied to one of its
ends. If the force is less than a critical value, then the process is activated
and has an activation energy proportinal to the length of the chain. Above this
critical value, the process is barrierless and will occur spontaneously. We use
the Rouse model for the description of the dynamics of the peeling out and
study the average behaviour of the chain, by replacing the random noise by its
mean. The resultant mean-field equation is a nonlinear diffusion equation and
hence rather difficult to analyze. We use physical arguments to convert this in
to a moving boundary value problem, which can then be solved exactly. The
result is that the time required to pull out a polymer of segments
scales like . For models other than the Rouse, we argue that Comment: 11 pages, 6 figures. To appear in PhysicalReview
Monitoring evolved stars for binarity with the HERMES spectrograph
Binarity is often invoked to explain peculiarities that can not be explained
by the standard theory of stellar evolution. Detecting orbital motion via the
Doppler effect is the best method to test binarity when direct imaging is not
possible. However, when the orbital period exceeds the duration of a typical
observing run, monitoring often becomes problematic. Placing a high-throughput
spectrograph on a small semi- robotic telescope allowed us to carry out a
radial-velocity survey of various types of peculiar evolved stars. In this
review we highlight some findings after the first four years of observations.
Thus, we detect eccentric binaries among hot subdwarfs, barium, S stars, and
post- AGB stars with disks, which are not predicted by the standard binary
interaction theory. In disk objects, in addition, we find signs of the on-
going mass transfer to the companion, and an intriguing line splitting, which
we attribute to the scattered light of the primary.Comment: To appear in the proceedings of the conference "Setting a new
standard in the analysis of binary stars", A. Tkachenko (ed.), European
Astron. Soc. Publ. Se
Submergence of the Sidebands in the Photon-assisted Tunneling through a Quantum Dot Weakly Coupled to Luttinger Liquid Leads
We study theoretically the photon-assisted tunneling through a quantum dot
weakly coupled to Luttinger liquids (LL) leads, and find that the zero bias dc
conductance is strongly affected by the interactions in the LL leads. In
comparison with the system with Fermi liquid (FL) leads, the sideband peaks of
the dc conductance become blurring for 1/2<g<1, and finally merge into the
central peak for g<1/2, (g is the interaction parameter in the LL leads). The
sidebands are suppressed for LL leads with Coulomb interactions strong enough,
and the conductance always appears as a single peak for any strength and
frequency of the external time-dependent field. Furthermore, the quenching
effect of the central peak for the FL case does not exist for g<1/2.Comment: 9 pages, 4 figure
Ultrafast Optical Spectroscopy of Micelle-Suspended Single-Walled Carbon Nanotubes
We present results of wavelength-dependent ultrafast pump-probe experiments
on micelle-suspended single-walled carbon nanotubes. The linear absorption and
photoluminescence spectra of the samples show a number of chirality-dependent
peaks, and consequently, the pump-probe results sensitively depend on the
wavelength. In the wavelength range corresponding to the second van Hove
singularities (VHSs), we observe sub-picosecond decays, as has been seen in
previous pump-probe studies. We ascribe these ultrafast decays to intraband
carrier relaxation. On the other hand, in the wavelength range corresponding to
the first VHSs, we observe two distinct regimes in ultrafast carrier
relaxation: fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has
not been observed previously, is resonantly enhanced whenever the pump photon
energy resonates with an interband absorption peak, and we attribute it to
radiative carrier recombination. Finally, the slow component is dependent on
the pH of the solution, which suggests an important role played by H ions
surrounding the nanotubes.Comment: 6 pages, 8 figures, changed title, revised, to be published in
Applied Physics
Stability of Negative Image Equilibria in Spike-Timing Dependent Plasticity
We investigate the stability of negative image equilibria in mean synaptic
weight dynamics governed by spike-timing dependent plasticity (STDP). The
neural architecture of the model is based on the electrosensory lateral line
lobe (ELL) of mormyrid electric fish, which forms a negative image of the
reafferent signal from the fish's own electric discharge to optimize detection
of external electric fields. We derive a necessary and sufficient condition for
stability, for arbitrary postsynaptic potential functions and arbitrary
learning rules. We then apply the general result to several examples of
biological interest.Comment: 13 pages, revtex4; uses packages: graphicx, subfigure; 9 figures, 16
subfigure
Baryon number violation, baryogenesis and defects with extra dimensions
In generic models for grand unified theories(GUT), various types of baryon
number violating processes are expected when quarks and leptons propagate in
the background of GUT strings. On the other hand, in models with large extra
dimensions, the baryon number violation in the background of a string is not
trivial because it must depend on the mechanism of the proton stabilization. In
this paper we argue that cosmic strings in models with extra dimensions can
enhance the baryon number violation to a phenomenologically interesting level,
if the proton decay is suppressed by the mechanism of localized wavefunctions.
We also make some comments on baryogenesis mediated by cosmological defects. We
show at least two scenarios will be successful in this direction. One is the
scenario of leptogenesis where the required lepton number conversion is
mediated by cosmic strings, and the other is the baryogenesis from the decaying
cosmological domain wall. Both scenarios are new and have not been discussed in
the past.Comment: 20pages, latex2e, comments and references added, to appear in PR
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