1,310 research outputs found
Dislocation Core Energies and Core Fields from First Principles
Ab initio calculations in bcc iron show that a screw dislocation
induces a short-range dilatation field in addition to the Volterra elastic
field. This core field is modeled in anisotropic elastic theory using force
dipoles. The elastic modeling thus better reproduces the atom displacements
observed in ab initio calculations. Including this core field in the
computation of the elastic energy allows deriving a core energy which converges
faster with the cell size, thus leading to a result which does not depend on
the geometry of the dislocation array used for the simulation.Comment: DOI: 10.1103/PhysRevLett.102.05550
Sliding Crack Model for Nonlinearity and Hysteresis in the Triaxial StressâStrain Curve of Rock, and Application to Antigorite Deformation
Under triaxial deviatoric loading at stresses below failure, rocks generally exhibit nonlinearity and hysteresis in the stressâstrain curve. In 1965, Walsh first explained this behavior in terms of frictional sliding along the faces of closed microcracks. The hypothesis is that crack sliding is the dominant mode of rock inelasticity at moderate compressive stresses for certain rock types. Here we extend the model of David et al. (2012, https://doi.org/10.1016/j.ijrmms.2012.02.001) to include (i) the effect of the confining stress; (ii) multiple loadâunload cycles; (iii) calculation of the dissipated strain energy upon unloadâreload; (iv) either frictional or cohesive behavior; and (v) either aligned or randomly oriented cracks. Closedâform expressions are obtained for the effective Young's modulus during loading, unloading, and reloading, as functions of the mineral's Young's modulus, the crack density, the crack friction coefficient and cohesion for the frictional and cohesive sliding models, respectively, and the crack orientation in the case of aligned cracks. The dissipated energy per cycle is quadratic and linear in stress for the frictional and cohesive models, respectively. Both models provide a good fit to a cyclic loading data set on polycrystalline antigorite, based on a compilation of literature and newly acquired data, at various pressures and temperatures. At high pressure, with increasing temperature, the model results reveal a decrease in friction coefficient and a transition from a frictionally to a cohesively controlled behavior. New measurements of fracture toughness and tensile strength provide quantitative support that inelastic behavior in antigorite is predominantly caused by shear crack sliding and propagation without dilatancy
Dislocation core field. I. Modeling in anisotropic linear elasticity theory
Aside from the Volterra field, dislocations create a core field, which can be
modeled in linear anisotropic elasticity theory with force and dislocation
dipoles. We derive an expression of the elastic energy of a dislocation taking
full account of its core field and show that no cross term exists between the
Volterra and the core fields. We also obtain the contribution of the core field
to the dislocation interaction energy with an external stress, thus showing
that dislocation can interact with a pressure. The additional force that
derives from this core field contribution is proportional to the gradient of
the applied stress. Such a supplementary force on dislocations may be important
in high stress gradient regions, such as close to a crack tip or in a
dislocation pile-up
Predicting dislocation climb: Classical modeling versus atomistic simulations
The classical modeling of dislocation climb based on a continuous description
of vacancy diffusion is compared to recent atomistic simulations of dislocation
climb in body-centered cubic iron under vacancy supersaturation [Phys. Rev.
Lett. 105 095501 (2010)]. A quantitative agreement is obtained, showing the
ability of the classical approach to describe dislocation climb. The analytical
model is then used to extrapolate dislocation climb velocities to lower
dislocation densities, in the range corresponding to experiments. This allows
testing of the validity of the pure climb creep model proposed by Kabir et al.
[Phys. Rev. Lett. 105 095501 (2010)]
Edge dislocations in crystal structures considered as traveling waves of discrete models
The static stress needed to depin a 2D edge dislocation, the lower dynamic
stress needed to keep it moving, its velocity and displacement vector profile
are calculated from first principles. We use a simplified discrete model whose
far field distortion tensor decays algebraically with distance as in the usual
elasticity. An analytical description of dislocation depinning in the strongly
overdamped case (including the effect of fluctuations) is also given. A set of
parallel edge dislocations whose centers are far from each other can depin
a given one provided , where is the average inter-dislocation
distance divided by the Burgers vector of a single dislocation. Then a limiting
dislocation density can be defined and calculated in simple cases.Comment: 10 pages, 3 eps figures, Revtex 4. Final version, corrected minor
error
Pre-main sequence stars in the Lagoon Nebula (M8)
We report the discovery of new pre-main sequence (PMS) stars in the Lagoon
Nebula (M8) at a distance of 1.25 kpc, based on intermediate resolution spectra
obtained with the Boller & Chivens spectrograph at the 6.5-m Magellan I
telescope (Las Campanas Observatory, Chile). According to the spectral types,
the presence of emission lines and the lithium 6708A absorption line, we are
able to identify 27 classical T Tauri stars, 7 weak-lined T Tauri stars and 3
PMS emission objects with spectral type G, which we include in a separated
stellar class denominated "PMS Fe/Ge class". Using near-infrared photometry
either from 2MASS or from our own previous work we derive effective
temperatures and luminosities for these stars and locate them in the
Hertzsprung-Russell diagram, in order to estimate their masses and ages. We
find that almost all of our sample stars are younger than 3 10^6 years and span
over a range of masses between 0.8 and 2.5 Msun. A cross-correlation between
our spectroscopic data and the X-ray sources detected with the Chandra ACIS
instrument is also presented.Comment: 18 pages, 15 figures, MNRAS, in pres
Giant Magnetoelectric Effect via Strain-Induced Spin-Reorientation Transitions in Ferromagnetic Films
It is shown theoretically that a giant magnetoelectric susceptibility
exceeding 10^-6 s/m may be achieved in the ferromagnetic/ferroelectric
epitaxial systems via the magnetization rotation induced by an electric field
applied to the substrate. The predicted magnetoelectric anomaly results from
the strain-driven spin-reorientation transitions in ferromagnetic films, which
take place at experimentally accessible misfit strains in CoFe2O4 and Ni films.Comment: 7 pages, 3 figure
Optical metrology alignment and impact on the measurement performance of the LISA Technology Package
Dislocation interactions mediated by grain boundaries
The dynamics of dislocation assemblies in deforming crystals indicate the
emergence of collective phenomena, intermittent fluctuations and strain
avalanches. In polycrystalline materials, the understanding of plastic
deformation mechanisms depends on grasping the role of grain boundaries on
dislocation motion. Here the interaction of dislocations and elastic, low angle
grain boundaries is studied in the framework of a discrete dislocation
representation. We allow grain boundaries to deform under the effect of
dislocation stress fields and compare the effect of such a perturbation to the
case of rigid grain boudaries. We are able to determine, both analytically and
numerically, corrections to dislocation stress fields acting on neighboring
grains, as mediated by grain boundary deformation. Finally, we discuss
conclusions and consequences for the avalanche statistics, as observed in
polycrystalline samples.Comment: 13 pages, 5 figure
Statistical approach to dislocation dynamics: From dislocation correlations to a multiple-slip continuum plasticity theory
Due to recent successes of a statistical-based nonlocal continuum crystal
plasticity theory for single-glide in explaining various aspects such as
dislocation patterning and size-dependent plasticity, several attempts have
been made to extend the theory to describe crystals with multiple slip systems
using ad-hoc assumptions. We present here a mesoscale continuum theory of
plasticity for multiple slip systems of parallel edge dislocations. We begin by
constructing the Bogolyubov-Born-Green-Yvon-Kirkwood (BBGYK) integral equations
relating different orders of dislocation correlation functions in a grand
canonical ensemble. Approximate pair correlation functions are obtained for
single-slip systems with two types of dislocations and, subsequently, for
general multiple-slip systems of both charges. The effect of the correlations
manifests itself in the form of an entropic force in addition to the external
stress and the self-consistent internal stress. Comparisons with a previous
multiple-slip theory based on phenomenological considerations shall be
discussed.Comment: 12 pages, 3 figure
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