118 research outputs found
Propagation of surface initiated rolling contact fatigue cracks in bearing Steel
Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Most research on rolling contact fatigue considers total life to pitting. Instead, this work studies the growth of rolling contact fatigue cracks before they develop into surface pits in an attempt to better understand crack propagation mechanisms. A triple-contact disc machine was used to perform pitting experiments on bearing steel samples under closely controlled contact conditions in mixed lubrication regime. Crack growth across the specimen surface is monitored and crack propagation rates extracted. The morphology of the generated cracks is observed by preparing sections of cracked specimens at the end of the test. It was found that crack initiation occurred very early in total life, which was attributed to high asperity stresses due to mixed lubrication regime. Total life to pitting was dominated by crack propagation. Results provide direct evidence of two distinct stages of crack growth in rolling contact fatigue: stage 1, within which cracks grow at a slow and relatively steady rate, consumed most of the total life; and stage 2, reached at a critical crack length, within which the propagation rate rapidly increases. Contact pressure and crack size were shown to be the main parameters controlling the propagation rate. Results show that crack propagation under rolling contact fatigue follows similar trends to those known to occur in classical fatigue. A log-log plot of measured crack growth rates against the product of maximum contact pressure and the square root of crack length, a parameter describing the applied stress intensity, produces a straight line for stage 2 propagation. This provides the first evidence that growth of hereby-identified stage 2 rolling contact fatigue cracks can be described by a Paris-type power law, where the rate of crack growth across the surface is proportional to the contact pressure raised to a power of approximately 7.5
Aharonov-Bohm effect and broken valley-degeneracy in graphene rings
We analyze theoretically the electronic properties of Aharonov-Bohm rings
made of graphene. We show that the combined effect of the ring confinement and
applied magnetic flux offers a controllable way to lift the orbital degeneracy
originating from the two valleys, even in the absence of intervalley
scattering. The phenomenon has observable consequences on the persistent
current circulating around the closed graphene ring, as well as on the ring
conductance. We explicitly confirm this prediction analytically for a circular
ring with a smooth boundary modelled by a space-dependent mass term in the
Dirac equation. This model describes rings with zero or weak intervalley
scattering so that the valley isospin is a good quantum number. The tunable
breaking of the valley degeneracy by the flux allows for the controlled
manipulation of valley isospins. We compare our analytical model to another
type of ring with strong intervalley scattering. For the latter case, we study
a ring of hexagonal form with lattice-terminated zigzag edges numerically. We
find for the hexagonal ring that the orbital degeneracy can still be controlled
via the flux, similar to the ring with the mass confinement.Comment: 7 pages, 7 figures, replaced with considerably extended new versio
Prediction of micropitting damage in gear teeth contacts considering the concurrent effects of surface fatigue and mild wear
The present paper studies the occurrence of micropitting damage in gear teeth contacts. An existing general micropitting model, which accounts for mixed lubrication conditions, stress history, and fatigue damage accumulation, is adapted here to deal with transient contact conditions that exist during meshing of gear teeth. The model considers the concurrent effects of surface fatigue and mild wear on the evolution of tooth surface roughness and therefore captures the complexities of damage accumulation on tooth flanks in a more realistic manner than hitherto possible. Applicability of the model to gear contact conditions is first confirmed by comparing its predictions to relevant experiments carried out on a triple-disc contact fatigue rig. Application of the model to a pair of meshing spur gears shows that under low specific oil film thickness conditions, the continuous competition between surface fatigue and mild wear determines the overall level as well as the distribution of micropitting damage along the tooth flanks. The outcome of this competition in terms of the final damage level is dependent on contact sliding speed, pressure and specific film thickness. In general, with no surface wear, micropitting damage increases with decreasing film thickness as may be expected, but when some wear is present micropitting damage may reduce as film thickness is lowered to the point where wear takes over and removes the asperity peaks and hence reduces asperity interactions. Similarly, when wear is negligible, increased sliding can increase the level of micropitting by increasing the number of asperity stress cycles, but when wear is present, an increase in sliding may lead to a reduction in micropitting due to faster removal of asperity peaks. The results suggest that an ideal situation in terms of surface damage prevention is that in which some mild wear at the start of gear pair operation adequately wears-in the tooth surfaces, thus reducing subsequent micropitting, followed by zero or negligible wear for the rest of the gear pair life. The complexities of the interaction between the contact conditions, wear and surface fatigue, as evident in the present results, mean that a full treatment of gear micropitting requires a numerical model along the lines of that applied here, and that use of overly simplified criteria may lead to misleading predictions
Three-Dimensional Dirac Electrons at the Fermi Energy in Cubic Inverse Perovskites: Ca_3PbO and its Family
The band structure of cubic inverse perovskites, Ca_3PbO and its family, are
investigated with the first-principles method. A close observation of the band
structure reveals that six equivalent Dirac electrons with a very small mass
exist on the line connecting the Gamma- and X-points, and at the symmetrically
equivalent points in the Brillouin zone. The discovered Dirac electrons are
three-dimensional and remarkably located exactly at the Fermi energy. A
tight-binding model describing the low-energy band structure is also
constructed and used to discuss the origin of the Dirac electrons in this
material. Materials related to Ca_3PbO are also studied, and some design
principles for the Dirac electrons in this series of materials are proposed.Comment: 4.2 pages, refined versio
Electronic states, Mott localization, electron-lattice coupling, and dimerization for correlated one-dimensional systems. II
We discuss physical properties of strongly correlated electron states for a
linear chain obtained with the help of the recently proposed new method
combining the exact diagonalization in the Fock space with an ab initio
readjustment of the single-particle orbitals in the correlated state. The
method extends the current discussion of the correlated states since the
properties are obtained with varying lattice spacing. The finite system of N
atoms evolves with the increasing interatomic distance from a Fermi-liquid-like
state into the Mott insulator. The criteria of the localization are discussed
in detail since the results are already convergent for N>=8. During this
process the Fermi-Dirac distribution gets smeared out, the effective band mass
increases by ~50%, and the spin-spin correlation functions reduce to those for
the Heisenberg antiferromagnet. Values of the microscopic parameters such as
the hopping and the kinetic-exchange integrals, as well as the magnitude of
both intra- and inter-atomic Coulomb and exchange interactions are calculated.
We also determine the values of various local electron-lattice couplings and
show that they are comparable to the kinetic exchange contribution in the
strong-correlation limit. The magnitudes of the dimerization and the zero-point
motion are also discussed. Our results provide a canonical example of a
tractable strongly correlated system with a precise, first-principle
description as a function of interatomic distance of a model system involving
all hopping integrals, all pair-site interactions, and the exact one-band
Wannier functions.Comment: 18 pages, REVTEX, submitted to Phys. Rev.
Impurity-assisted tunneling in graphene
The electric conductance of a strip of undoped graphene increases in the
presence of a disorder potential, which is smooth on atomic scales. The
phenomenon is attributed to impurity-assisted resonant tunneling of massless
Dirac fermions. Employing the transfer matrix approach we demonstrate the
resonant character of the conductivity enhancement in the presence of a single
impurity. We also calculate the two-terminal conductivity for the model with
one-dimensional fluctuations of disorder potential by a mapping onto a problem
of Anderson localization.Comment: 6 pages, 3 figures, final version, typos corrected, references adde
Grain Boundaries in Graphene on SiC(000) Substrate
Grain boundaries in epitaxial graphene on the SiC(000) substrate are
studied using scanning tunneling microscopy and spectroscopy. All investigated
small-angle grain boundaries show pronounced out-of-plane buckling induced by
the strain fields of constituent dislocations. The ensemble of observations
allows to determine the critical misorientation angle of buckling transition
. Periodic structures are found among the flat
large-angle grain boundaries. In particular, the observed highly ordered grain boundary is assigned to the previously
proposed lowest formation energy structural motif composed of a continuous
chain of edge-sharing alternating pentagons and heptagons. This periodic grain
boundary defect is predicted to exhibit strong valley filtering of charge
carriers thus promising the practical realization of all-electric valleytronic
devices
Ferromagnetism without flat bands in thin armchair nanoribbons
Describing by a Hubbard type of model a thin armchair graphene ribbon in the
armchair hexagon chain limit, one shows in exact terms, that even if the system
does not have flat bands at all, at low concentration a mesoscopic sample can
have ferromagnetic ground state, being metallic in the same time. The mechanism
is connected to a common effect of correlations and confinement.Comment: 37 pages, 12 figures, in press at Eur. Phys. Jour.
DNA nucleotide-specific modulation of \mu A transverse edge currents through a metallic graphene nanoribbon with a nanopore
We propose two-terminal devices for DNA sequencing which consist of a
metallic graphene nanoribbon with zigzag edges (ZGNR) and a nanopore in its
interior through which the DNA molecule is translocated. Using the
nonequilibrium Green functions combined with density functional theory, we
demonstrate that each of the four DNA nucleotides inserted into the nanopore,
whose edge carbon atoms are passivated by either hydrogen or nitrogen, will
lead to a unique change in the device conductance. Unlike other recent
biosensors based on transverse electronic transport through DNA nucleotides,
which utilize small (of the order of pA) tunneling current across a nanogap or
a nanopore yielding a poor signal-to-noise ratio, our device concept relies on
the fact that in ZGNRs local current density is peaked around the edges so that
drilling a nanopore away from the edges will not diminish the conductance.
Inserting a DNA nucleotide into the nanopore affects the charge density in the
surrounding area, thereby modulating edge conduction currents whose magnitude
is of the order of \mu A at bias voltage ~ 0.1 V. The proposed biosensor is not
limited to ZGNRs and it could be realized with other nanowires supporting
transverse edge currents, such as chiral GNRs or wires made of two-dimensional
topological insulators.Comment: 6 pages, 6 figures, PDFLaTe
Support for multiscale simulations with molecular dynamics
We present a reusable solution that supports users in combining single-scale models to create a multiscale application. Our approach applies several multiscale programming tools to allow users to compose multiscale applications using a graphical interface, and provides an easy way to execute these multiscale applications on international production infrastructures. Our solution extends the general purpose scripting approach of the GridSpace platform with simple mechanisms for accessing production resources, provided by the Application Hosting Environment (AHE). We apply our support solution to construct and execute a multiscale simulation of clay-polymer nanocomposite materials, and showcase its benefit in reducing the effort required to do a number of time-intensive user tasks. © 2013 The Authors. Published by Elsevier B.V
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