6,238 research outputs found
Fretting wear of Ti(CxNy) PVD coatings under variable environmental conditions
Fretting wear as a specific type of degradation is defined as an oscillatory motion at small amplitude between two nominally stationary solid bodies in mutual contact. Under external stresses the interface is being damaged by debris generation and its successive ejections outside the contact area. A potential protection against fretting damage by means of hard coatings is being offered by different surface engineering techniques. For this study TiC, TiN and TiCN hard coatings manufactured by a PVD method have been selected and tested against smooth polycrystalline alumina ball. A fretting test programme has been carried out at the frequency of 5Hz, 100N normal load, 100µm displacement amplitude and at three values of a relative humidity: 10, 50 and 90% at 295-298K temperature. It turned out that the intensity of wear process was depending not only on loading conditions but on environmental ones as well. A significant impact of RH on wear rate and friction behaviour of the coatings under investigation has been observed. Two different damage mechanisms have been identified and related to the phenomena of debris oxidation and debris adhesion to the counterbody surface. In the latter case the debris deposited onto the surface of the alumina ball lead to a change of stress distribution at the interface and as a result to accelerated wear. In this work experiments with variable relative humidity increasing from 10% to 90% within 1 a single fretting test have been completed. It follows from these experiments that there exists an intermediate value of the RH at which the friction coefficient changes rapidly. Finally a dissipated energy approach has been applied in the work in order to quantify and compare fretting wear rates of different hard coatings
Spin-Orbit and Tensor Forces in Heavy-quark Light-quark Mesons: Implications of the New Ds state at 2.32 GeV
We consider the spectroscopy of heavy-quark light-quark mesons with a simple
model based on the non-relativistic reduction of vector and scalar exchange
between fermions. Four forces are induced: the spin-orbit forces on the light
and heavy quark spins, the tensor force, and a spin-spin force. If the vector
force is Coulombic, the spin-spin force is a contact interaction, and the
tensor force and spin-orbit force on the heavy quark to order are
directly proportional. As a result, just two independent parameters
characterize these perturbations. The measurement of the masses of three p-wave
states suffices to predict the mass of the fourth. This technique is applied to
the system, where the newly discovered state at 2.32 GeV provides the
third measured level, and to the system. The mixing of the two
p-wave states is reflected in their widths and provides additional constraints.
The resulting picture is at odds with previous expectations and raises new
puzzles.Comment: 6 pages, 1 figur
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Determination of the hydrodynamic performance of marine propellers using fibre Bragg gratings
Downloading of the abstract is permitted for personal use only. A critical aspect in the design of marine propellers is their hydrodynamic performance which, when evaluated experimentally, requires a number of parameters to be monitored at the same time, i.e.The thrust and torque a propeller generates as well as the propeller shaft and vessel speed. In this investigation, three of those parameters are measured using Fibre Bragg Grating-based sensors, thus allowing for computationally derived performance values to be verified. For that purpose, open water tests were carried out where an instrumented propeller shaft was installed into a research vessel and measurements taken, evaluated and the results compared favorably with advanced computer-based simulations
Relativistic quantum plasma dispersion functions
Relativistic quantum plasma dispersion functions are defined and the
longitudinal and transverse response functions for an electron (plus positron)
gas are written in terms of them. The dispersion is separated into
Landau-damping, pair-creation and dissipationless regimes. Explicit forms are
given for the RQPDFs in the cases of a completely degenerate distribution and a
nondegenerate thermal (J\"uttner) distribution. Particular emphasis is placed
on the relation between dissipation and dispersion, with the dissipation
treated in terms of the imaginary parts of RQPDFs. Comparing the dissipation
calculated in this way with the existing treatments leads to the identification
of errors in the literature, which we correct. We also comment on a controversy
as to whether the dispersion curves in a superdense plasma pass through the
region where pair creation is allowed.Comment: 16 pages, 1 figur
Testing the Noncommutative Standard Model at a Future Photon Collider
Extensions of the Standard Model of elementary particle physics to
noncommutative geometries have been proposed as a low energy limit of string
models. Independent of this motivation, one may consider such a model as an
effective field theory with higher-dimensional operators containing an
antisymmetric rank-two background field. We study the signals of such a
Noncommutative Standard Model (NCSM) and analyze the discovery potential of a
future photon collider, considering angular distributions in fermion pair
production.Comment: 13 pages RevTeX, Feynman diagrams and figures included, references
added, typographical errors in Feynman rules corrected (all results remain
unchangend, since correct Feynman rules were used in the calculations), to
appear in Phys. Rev.
Spectroscopy of B_c Mesons in the Relativized Quark Model
We calculate the spectrum of the charm-beauty mesons using the relativized
quark model. Using the wavefunctions from this model we compute the radiative
widths of excited c\bar{b} states. The hadronic transition rates between
c\bar{b} states are estimated using the Kuang-Yan approach and are combined
with the radiative widths to give estimates of the relative branching ratios.
These results are combined with production rates at the Tevatron and the LHC to
suggest promising signals for excited B_c states. Our results are compared with
other models to gauge the reliability of the predictions and point out
differences.Comment: 15 pages, 1 fig. uses revtex4. References adde
On the Absence of an Exponential Bound in Four Dimensional Simplicial Gravity
We have studied a model which has been proposed as a regularisation for four
dimensional quantum gravity. The partition function is constructed by
performing a weighted sum over all triangulations of the four sphere. Using
numerical simulation we find that the number of such triangulations containing
simplices grows faster than exponentially with . This property ensures
that the model has no thermodynamic limit.Comment: 8 pages, 2 figure
Retinal Wave Behavior through Activity- Dependent Refractory Periods
In the developing mammalian visual system, spontaneous retinal ganglion cell (RGC) activity contributes to and drives several aspects of visual system organization. This spontaneous activity takes the form of spreading patches of synchronized bursting that slowly advance across portions of the retina. These patches are non-repeating and tile the retina in minutes. Several transmitter systems are known to be involved, but the basic mechanism underlying wave production is still not well-understood. We present a model for retinal waves that focuses on acetylcholine mediated waves but whose principles are adaptable to other developmental stages. Its assumptions are that a) spontaneous depolarizations of amacrine cells drive wave activity; b) amacrine cells are locally connected, and c) cells receiving more input during their depolarization are subsequently less responsive and have longer periods between spontaneous depolarizations. The resulting model produces waves with non-repeating borders and randomly distributed initiation points. The wave generation mechanism appears to be chaotic and does not require neural noise to produce this wave behavior. Variations in parameter settings allow the model to produce waves that are similar in size, frequency, and velocity to those observed in several species. Our results suggest that retinal wave behavior results from activity-dependent refractory periods and that the average velocity of retinal waves depends on the duration a cell is excitatory: longer periods of excitation result in slower waves. In contrast to previous studies, we find that a single layer of cells is sufficient for wave generation. The principles described here are very general and may be adaptable to the description of spontaneous wave activity in other areas of the nervous system
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