1,105 research outputs found
Wear and Fatigue of Railway Track Caused by Contamination, Sanding and Surface Damage
The wheel rail contact operates in an arduous
environment. Damage to the surface of either
component is possible during manufacture, installation,
or operation. The question arises as to how tolerant is
the railway wheel or section of track to surface
indentation or damage.
In this work a twin disc simulation has been used
to relate the level of surface damage (as well as the way
it is generated) to the fatigue life of the surfaces. A
related problem is the presence of solid contamination
on the track. Sand (applied for improved adhesion) or
track ballast material can cause damage to the rail and
wheel surfaces. These mechanisms have been explored
to assess the effect on contact fatigue life and wear. The
disc specimens have been either artificially damaged
(with dents and scratches) or run with particles of sand
or ballast material. The discs were then loaded and
rotated at realistic conditions of contact pressure and
controlled slip.
For normal operation of the contact, either dry or
with water lubrication, surface dents and scratches have
little effect on fatigue life. The normal plastic flow in
the rail surface layer acts to close up dents. The failure
of the disc is then by fatigue cracking across the whole
surface with no particular preference to the dent
location. Alternatively, if the contact is lubricated with
oil then this plastic flow is greatly reduced and the dents
act as stress raisers and fatigue cracks initiate from their
trailing edge.
Sand or ballast particles are crushed as they enter
the wheel/rail contact. The fragments indent the surfaces
and rapidly roughen the contact faces. The surface
indentation is relatively minor, but the presence of
particles increases the level of traction (over the wet
case) and promotes further surface plastic flow. This
can reduce the residual fatigue life of the contact.
Further, high concentrations of sand were shown to
promote a low cycle fatigue process that caused very
high wear by the spallation of material.
The twin disc simulations have shown that, under
conditions similar to that of wheel/rail operation,
surface damage is not a primary cause of fatigue failure.
However, wear is greatly accelerated by the presence of
solid contaminants and some evidence of a low cycle
fatigue process was observed for sanded contacts
The Recurrent Nature of Central Starbursts
New hydrodynamic models with feedback show that feedback driven turbulence
and subsequent relaxation can drive recurrent starbursts, though most of these
bursts fizzle due to premature, asymmetric ignition. Strong bursts are
terminated when the turbulence inflates the multiphase central disk. The period
between bursts is about twice a free-fall time onto the central disk. Transient
spirals and bars are common through the burst cycle.Comment: 7 pages + 3 figs. Conf. paper for "Starbursts: from 30 Doradus to
Lyman Break Galaxies," held at Inst. of Astronomy, Cambridge Univ., Sept.
6-10, 2004. Kluwer Academic Publishers, eds. R. de Grijs and R. M. Gonzalez
Delgado + additional materia
Microstructural evolution of Mg-7Al-2Sn Mg alloy during multi-directional impact forging
AbstractMulti-directional impact forging (MDIF) was applied to a Mg-7Al-2Sn (wt.%) Mg alloy to investigate its effect on the microstructural evolution. MDIF process exhibited high grain refinement efficiency. After MDIF 200 passes, the grain size drastically decreased to 20â”m from the initial coarse grains of ~500â”m due to dynamic recrystallization (DRX). Meanwhile, original grain boundaries remained during MDIF and large numbers of fine spherical ÎČ-Mg17Al12 particles dynamically precipitated along the original grain boundaries with high Al concentration, acting as effective pinning obstacles for the suppression of DRXed grain growth. Besides, micro-cracks nucleated during MDIF and propagated along the interface between the remained globular or cubic Al-Mn particles and Mg matrix
Stability of martensite with pulsed electric current in dual-phase steels
Softening frequently occurs in dual-phase steels under isothermal tempering of martensite. Recently, non-isothermal tempering is implemented to decrease the softening process in dual-phase steels. Here, we have discovered using high power electropulsing treatment can significantly enhance the strengthening effects via the formation of ultrafine-grained ferrite with nano-cementite particles in tempered martensitic-ferritic steels. To the best our knowledge, electropulsing treatment is a proper candidate to retard even to recovery the softening problems in the tempering of martensite in comparison with other isothermal and non-isothermal tempering methods
Analysis and simulations for a phaseâfield fracture model at finite strains based on modified invariants
Phaseâfield models have already been proven to predict complex fracture patterns for brittle fracture at small strains. In this paper we discuss a model for phaseâfield fracture at finite deformations in more detail. Among the identification of crack location and projection of crack growth the numerical stability is one of the main challenges in solid mechanics. Here we present a phaseâfield model at finite strains, which takes into account the anisotropy of damage by applying an anisotropic split of the modified invariants of the right CauchyâGreen strain tensor. We introduce a suitable weak notion of solution that also allows for a spatial and temporal discretization of the model. In this framework we study the existence of solutions and we show that the timeâdiscrete solutions converge in a weak sense to a solution of the timeâcontinuous formulation of the model. Numerical examples in two and three space dimensions illustrate the range of validity of the analytical results
Onsager coefficients of a Brownian Carnot cycle
We study a Brownian Carnot cycle introduced by T. Schmiedl and U. Seifert
[Europhys. Lett. \textbf{81}, 20003 (2008)] from a viewpoint of the linear
irreversible thermodynamics. By considering the entropy production rate of this
cycle, we can determine thermodynamic forces and fluxes of the cycle and
calculate the Onsager coefficients for general protocols, that is, arbitrary
schedules to change the potential confining the Brownian particle. We show that
these Onsager coefficients contain the information of the protocol shape and
they satisfy the tight-coupling condition irrespective of whatever protocol
shape we choose. These properties may give an explanation why the
Curzon-Ahlborn efficiency often appears in the finite-time heat engines
Improving quantum interferometry by using entanglement (to take a decision you'd better use entanglement)
We address the use of entanglement to improve the precision of generalized
quantum interferometry, i.e. of binary measurements aimed to determine whether
or not a perturbation has been applied by a given device. For the most relevant
operations in quantum optics, we evaluate the optimal detection strategy and
the ultimate bounds to the minimum detectable perturbation. Our results
indicate that entanglement-assisted strategies improve the discrimination in
comparison with conventional schemes. A concrete setup to approach performances
of the optimal strategies is also suggested.Comment: 2 fig
Mass measurements of neutron-deficient Y, Zr, and Nb isotopes and their impact on rp and Îœp nucleosynthesis processes
© 2018 The Authors. Published by Elsevier B.V. This manuscript is made available under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International licence (CC BY-NC-ND 4.0). For further details please see: https://creativecommons.org/licenses/by-nc-nd/4.0/Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of 82Zr and 84Nb were measured for the first time with an uncertainty of âŒ10 keV, and the masses of 79Y, 81Zr, and 83Nb were re-determined with a higher precision. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low α separation energies for neutron-deficient Mo and Tc isotopes, making the formation of ZrâNb cycle in the rp-process unlikely. The new proton separation energy of 83Nb was determined to be 490(400) keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the p-nucleus 84Sr relative to the neutron-deficient molybdenum isotopes in the previous Îœp-process simulations.Peer reviewe
Isotopic and spin selectivity of H_2 adsorbed in bundles of carbon nanotubes
Due to its large surface area and strongly attractive potential, a bundle of
carbon nanotubes is an ideal substrate material for gas storage. In addition,
adsorption in nanotubes can be exploited in order to separate the components of
a mixture. In this paper, we investigate the preferential adsorption of D_2
versus H_2(isotope selectivity) and of ortho versus para(spin selectivity)
molecules confined in the one-dimensional grooves and interstitial channels of
carbon nanotube bundles. We perform selectivity calculations in the low
coverage regime, neglecting interactions between adsorbate molecules. We find
substantial spin selectivity for a range of temperatures up to 100 K, and even
greater isotope selectivity for an extended range of temperatures,up to 300 K.
This isotope selectivity is consistent with recent experimental data, which
exhibit a large difference between the isosteric heats of D_2 and H_2 adsorbed
in these bundles.Comment: Paper submitted to Phys.Rev. B; 17 pages, 2 tables, 6 figure
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