158 research outputs found
Void Growth in BCC Metals Simulated with Molecular Dynamics using the Finnis-Sinclair Potential
The process of fracture in ductile metals involves the nucleation, growth,
and linking of voids. This process takes place both at the low rates involved
in typical engineering applications and at the high rates associated with
dynamic fracture processes such as spallation. Here we study the growth of a
void in a single crystal at high rates using molecular dynamics (MD) based on
Finnis-Sinclair interatomic potentials for the body-centred cubic (bcc) metals
V, Nb, Mo, Ta, and W. The use of the Finnis-Sinclair potential enables the
study of plasticity associated with void growth at the atomic level at room
temperature and strain rates from 10^9/s down to 10^6/s and systems as large as
128 million atoms. The atomistic systems are observed to undergo a transition
from twinning at the higher end of this range to dislocation flow at the lower
end. We analyze the simulations for the specific mechanisms of plasticity
associated with void growth as dislocation loops are punched out to accommodate
the growing void. We also analyse the process of nucleation and growth of voids
in simulations of nanocrystalline Ta expanding at different strain rates. We
comment on differences in the plasticity associated with void growth in the bcc
metals compared to earlier studies in face-centred cubic (fcc) metals.Comment: 24 pages, 12 figure
Populating the swampland: the case of U(1)^496 and E_8 x U(1)^248
For d=10 N=1 SUGRA coupled to d=10 N=1 SYM, anomaly cancellation places
severe constraints on the allowed gauge groups. Besides the ones known to
appear in string theory, only U(1)^496 and E_8 x U(1)^248 are allowed. There
are no known theories of quantum gravity that reduce in some limit to these two
last supergravity theories, and in this note I present some evidence that those
quantum theories might not exist. The first observation is that, upon
compactification, requring that the quantum theory possesses a moduli space
with finite volume typically implies the existence of singularities where the
4d gauge group is enhanced, but for these two theories that gauge enhancement
is problematic from the 10d point of view. I also point out that while these
four supergravity theories present repulson-type singularities, the known
mechanism that repairs those singularities for the first two - the non-Abelian
enhancon - is not available for the last two theories. In short, these two
supergravity theories might be too Abelian for their own good.Comment: 12 page
Stringy effects in black hole decay
We compute the low energy decay rates of near-extremal three(four) charge
black holes in five(four) dimensional N=4 string theory to sub-leading order in
the large charge approximation. This involves studying stringy corrections to
scattering amplitudes of a scalar field off a black hole. We adapt and use
recently developed techniques to compute such amplitudes as near-horizon
quantities. We then compare this with the corresponding calculation in the
microscopic configuration carrying the same charges as the black hole. We find
perfect agreement between the microscopic and macroscopic calculations; in the
cases we study, the zero energy limit of the scattering cross section is equal
to four times the Wald entropy of the black hole.Comment: 32 page
Off-Diagonal Deformations of Kerr Metrics and Black Ellipsoids in Heterotic Supergravity
Geometric methods for constructing exact solutions of motion equations with
first order corrections to the heterotic supergravity action
implying a non-trivial Yang-Mills sector and six dimensional, 6-d,
almost-K\"ahler internal spaces are studied. In 10-d spacetimes, general
parametrizations for generic off-diagonal metrics, nonlinear and linear
connections and matter sources, when the equations of motion decouple in very
general forms are considered. This allows us to construct a variety of exact
solutions when the coefficients of fundamental geometric/physical objects
depend on all higher dimensional spacetime coordinates via corresponding
classes of generating and integration functions, generalized effective sources
and integration constants. Such generalized solutions are determined by generic
off-diagonal metrics and nonlinear and/or linear connections. In particular, as
configurations which are warped/compactified to lower dimensions and for
Levi-Civita connections. The corresponding metrics can have (non) Killing
and/or Lie algebra symmetries and/or describe (1+2)-d and/or (1+3)-d domain
wall configurations, with possible warping nearly almost-K\"ahler manifolds,
with gravitational and gauge instantons for nonlinear vacuum configurations and
effective polarizations of cosmological and interaction constants encoding
string gravity effects. A series of examples of exact solutions describing
generic off-diagonal supergravity modifications to black hole/ ellipsoid and
solitonic configurations are provided and analyzed. We prove that it is
possible to reproduce the Kerr and other type black solutions in general
relativity (with certain types of string corrections) in 4-d and to generalize
the solutions to non-vacuum configurations in (super) gravity/ string theories.Comment: latex2e, 44 pages with table of content, v2 accepted to EJPC with
minor typos modifications requested by editor and referee and up-dated
reference
Critical Trapped Surfaces Formation in the Collision of Ultrarelativistic Charges in (A)dS
We study the formation of marginally trapped surfaces in the head-on
collision of two ultrarelativistic charges in space-time. The metric of
ultrarelativistic charged particles in is obtained by boosting
Reissner-Nordstr\"om space-time to the speed of light. We show that
formation of trapped surfaces on the past light cone is only possible when
charge is below certain critical - situation similar to the collision of two
ultrarelativistic charges in Minkowski space-time. This critical value depends
on the energy of colliding particles and the value of a cosmological constant.
There is richer structure of critical domains in case. In this case
already for chargeless particles there is a critical value of the cosmological
constant only below which trapped surfaces formation is possible. Appearance of
arbitrary small nonzero charge significantly changes the physical picture.
Critical effect which has been observed in the neutral case does not take place
more. If the value of the charge is not very large solution to the equation on
trapped surface exists for any values of cosmological radius and energy density
of shock waves. Increasing of the charge leads to decrease of the trapped
surface area, and at some critical point the formation of trapped surfaces of
the type mentioned above becomes impossible.Comment: 30 pages, Latex, 7 figures, Refs. added and typos correcte
A Variational Deduction of Second Gradient Poroelasticity Part I: General Theory
Second gradient theories have to be used to capture how local micro
heterogeneities macroscopically affect the behavior of a continuum. In this
paper a configurational space for a solid matrix filled by an unknown amount of
fluid is introduced. The Euler-Lagrange equations valid for second gradient
poromechanics, generalizing those due to Biot, are deduced by means of a
Lagrangian variational formulation. Starting from a generalized Clausius-Duhem
inequality, valid in the framework of second gradient theories, the existence
of a macroscopic solid skeleton Lagrangian deformation energy, depending on the
solid strain and the Lagrangian fluid mass density as well as on their
Lagrangian gradients, is proven.Comment: 20 page
Pair Production of small Black Holes in Heterotic String Theories
We study pair production of small BPS BH's in heterotic strings compactified
on tori and in the FHSV model. After recalling the identification of small BH's
in the perturbative BPS spectrum, we compute the tree-level amplitudes for
processes initiated by massless vector bosons or gravitons. We then analyze the
resulting cross sections in terms of energy and angular distributions. Finally,
we briefly comment on scenari with large extra dimensions and on
generalizations of our results to non-BPS, non-extremal and rotating BH's.Comment: 33 page
Extremal Multicenter Black Holes: Nilpotent Orbits and Tits Satake Universality Classes
Four dimensional supergravity theories whose scalar manifold is a symmetric
coset manifold U[D=4]/Hc are arranged into a finite list of Tits Satake
universality classes. Stationary solutions of these theories, spherically
symmetric or not, are identified with those of an euclidian three-dimensional
sigma-model, whose target manifold is a Lorentzian coset U[D=3]/H* and the
extremal ones are associated with H* nilpotent orbits in the K* representation
emerging from the orthogonal decomposition of the algebra U[D=3] with respect
to H*. It is shown that the classification of such orbits can always be reduced
to the Tits-Satake projection and it is a class property of the Tits Satake
universality classes. The construction procedure of Bossard et al of extremal
multicenter solutions by means of a triangular hierarchy of integrable
equations is completed and converted into a closed algorithm by means of a
general formula that provides the transition from the symmetric to the solvable
gauge. The question of the relation between H* orbits and charge orbits W of
the corresponding black holes is addressed and also reduced to the
corresponding question within the Tits Satake projection. It is conjectured
that on the vanishing locus of the Taub-NUT current the relation between
H*-orbit and W-orbit is rigid and one-to-one. All black holes emerging from
multicenter solutions associated with a given H* orbit have the same W-type.
For the S^3 model we provide a complete survey of its multicenter solutions
associated with all of the previously classified nilpotent orbits of sl(2) x
sl(2) within g[2,2]. We find a new intrinsic classification of the W-orbits of
this model that might provide a paradigm for the analogous classification in
all the other Tits Satake universality classes.Comment: 83 pages, LaTeX; v2: few misprints corrected and references adde
Modelling the human epidermis in vitro: tools for basic and applied research
Culture models of tissues and organs are valuable tools developed by basic research that help investigation of the body functions. Modelling is aimed at simplifying experimental procedures in order to better understand biological phenomena, and consequently, when sufficiently characterized, culture models can also be utilized with high potential in applied research. In skin biology and pathology, the development of cultures of keratinocytes as monolayers has allowed the elucidation of most functional and structural characteristics of the cell type. Beside the multiple great successes that have been obtained with this type of culture, this review draws attention on several neglected characteristics of monolayer cultures. The more sophisticated models created in order to reconstruct the fully differentiated epidermis have followed the monolayers. The epidermal reconstruction produces all typical layers found in vivo and thus makes the model much less simple, but only this kind of model allows the study of full differentiation in keratinocyte and production of the cornified barrier. In addition to its interest in basic research, the reconstructed epidermis is currently gaining a lot of interest for applied research, particularly as an alternative to laboratory animals in the chemical and cosmetic industry. Today several commercial providers propose reconstructed skin or epidermis, but in vitro assays on these materials are still under development. In order to be beneficial at long term, the validation of assays must be performed on a material whose availability will not be interrupted. We warn here providers and customers that the longevity of in vitro assays will be guaranteed only if these assays are done with well-described models, prepared according to published procedures, and must consider having a minimum of two independent simultaneous producers of similar material
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