18,212 research outputs found
Use of cohesive elements in fatigue analysis
Cohesive laws describe the resistance to incipient separation
of material surfaces. A cohesive finite element
is formulated on the basis of a particular cohesive
law. Cohesive elements are placed at the boundary
between adjacent standard volume finite elements
to model fatigue damage that leads to fracture at the
separation of the element boundaries per the cohesive
law. In this work, a cohesive model for fatigue
crack initiation is taken to be the irreversible loadingunloading
hysteresis that represents fatigue damage
occuring due to cyclic loads leading to the initiation of
small cracks. Various cohesive laws are reviewed and
one is selected that incorporates a hysteretic cyclic
loading that accounts for energetic dissipative mechanisms.
A mathematical representation is developed
based on an exponential effective load-separation cohesive
relationship. A three-dimensional cohesive element
is defined using this compliance relationship integrated
at four points on the mid-surface of the area
element. Implementation into finite element software
is discussed and particular attention is applied to numerical
convergence issues as the inflection point between
loading and 'unloading in the cohesive law is
encountered. A simple example of a displacementcontrolled
fatigue test is presented in a finite element
simulation. Comments are made on applications of
the method to prediction of fatigue life for engineering
structures such as pressure vessels and piping
Mentoring to reduce antisocial behaviour in childhood
The effects of social interventions need to be examined in real life situations as well as studie
Earth-like sand fluxes on Mars
Strong and sustained winds on Mars have been considered rare, on the basis of surface meteorology measurements and global circulation models, raising the question of whether the abundant dunes and evidence for wind erosion seen on the planet are a current process. Recent studies showed sand activity, but could not determine whether entire dunes were moving—implying large sand fluxes—or whether more localized and surficial changes had occurred. Here we present measurements of the migration rate of sand ripples and dune lee fronts at the Nili Patera dune field. We show that the dunes are near steady state, with their entire volumes composed of mobile sand. The dunes have unexpectedly high sand fluxes, similar, for example, to those in Victoria Valley, Antarctica, implying that rates of landscape modification on Mars and Earth are similar
Consistent Gravitationally-Coupled Spin-2 Field Theory
Inspired by the translational gauge structure of teleparallel gravity, the
theory for a fundamental massless spin-2 field is constructed. Accordingly,
instead of being represented by a symmetric second-rank tensor, the fundamental
spin-2 field is assumed to be represented by a spacetime (world) vector field
assuming values in the Lie algebra of the translation group. The flat-space
theory naturally emerges in the Fierz formalism and is found to be equivalent
to the usual metric-based theory. However, the gravitationally coupled theory,
with gravitation itself described by teleparallel gravity, is shown not to
present the consistency problems of the spin-2 theory constructed on the basis
of general relativity.Comment: 16 pages, no figures. V2: Presentation changes, including addition of
a new sub-section, aiming at clarifying the text; version accepted for
publication in Class. Quantum Grav
Relativistic general-order coupled-cluster method for high-precision calculations: Application to Al+ atomic clock
We report the implementation of a general-order relativistic coupled-cluster
method for performing high-precision calculations of atomic and molecular
properties. As a first application, the static dipole polarizabilities of the
ground and first excited states of Al+ have been determined to precisely
estimate the uncertainty associated with the BBR shift of its clock frequency
measurement. The obtained relative BBR shift is -3.66+-0.44 for the 3s^2
^1S_0^0 --> 3s3p ^3P_0^0 transition in Al+ in contrast to the value obtained in
the latest clock frequency measurement, -9+-3 [Phys. Rev. Lett. 104, 070802
(2010)]. The method developed in the present work can be employed to study a
variety of subtle effects such as fundamental symmetry violations in atoms.Comment: 4 pages, 3 tables, submitte
High-fidelity trapped-ion quantum logic using near-field microwaves
We demonstrate a two-qubit logic gate driven by near-field microwaves in a
room-temperature microfabricated ion trap. We measure a gate fidelity of
99.7(1)\%, which is above the minimum threshold required for fault-tolerant
quantum computing. The gate is applied directly to Ca "atomic clock"
qubits (coherence time ) using the microwave
magnetic field gradient produced by a trap electrode. We introduce a
dynamically-decoupled gate method, which stabilizes the qubits against
fluctuating a.c.\ Zeeman shifts and avoids the need to null the microwave
field
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