50 research outputs found
A Multiscale Approach for Modeling Crystalline Solids
In this paper we present a modeling approach to bridge the atomistic with
macroscopic scales in crystalline materials. The methodology combines
identification and modeling of the controlling unit processes at microscopic
level with the direct atomistic determination of fundamental material
properties. These properties are computed using a many body Force Field derived
from ab initio quantum-mechanical calculations. This approach is exercised to
describe the mechanical response of high-purity Tantalum single crystals,
including the effect of temperature and strain-rate on the hardening rate. The
resulting atomistically informed model is found to capture salient features of
the behavior of these crystals such as: the dependence of the initial yield
point on temperature and strain rate; the presence of a marked stage I of easy
glide, specially at low temperatures and high strain rates; the sharp onset of
stage II hardening and its tendency to shift towards lower strains, and
eventually disappear, as the temperature increases or the strain rate
decreases; the parabolic stage II hardening at low strain rates or high
temperatures; the stage II softening at high strain rates or low temperatures;
the trend towards saturation at high strains; the temperature and strain-rate
dependence of the saturation stress; and the orientation dependence of the
hardening rate.Comment: 25 pages, 15 figures, LaTe
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Translation, linguistic and cultural adaptation of the Argentinean Spanish Minilinguistic State Examination
Este trabajo presenta la adaptación lingüística y cultural de la versión en español argentino de una prueba breve de evaluación del lenguaje, el Minilinguistic State Examination (MLSE) diseñada para diagnosticar, clasificar y monitorear las variantes de la Afasia Progresiva Primaria. Se siguieron los lineamientos de la International Test Commission. El criterio principal fue la equivalencia de propiedades psicolingüísticas con los ítems de la versión original en inglés. Se administró a 20 participantes una primera versión (v1) con el doble de los ítems requeridos. De allí se seleccionaron aquellos con tasas de precisión entre el 80% y 95% y se elaboró una segunda versión (v2) con el número definitivo de estímulos. Esta versión se administró en 31 voluntarios. Como producto de este proceso se obtuvo la versión argentina del MLSE, la cual busca ser equivalente a las otras versiones en desarrollo (inglés, italiano y español peninsular)
Ab initio and finite-temperature molecular dynamics studies of lattice resistance in tantalum
This manuscript explores the apparent discrepancy between experimental data
and theoretical calculations of the lattice resistance of bcc tantalum. We
present the first results for the temperature dependence of the Peierls stress
in this system and the first ab initio calculation of the zero-temperature
Peierls stress to employ periodic boundary conditions, which are those best
suited to the study of metallic systems at the electron-structure level. Our ab
initio value for the Peierls stress is over five times larger than current
extrapolations of experimental lattice resistance to zero-temperature. Although
we do find that the common techniques for such extrapolation indeed tend to
underestimate the zero-temperature limit, the amount of the underestimation
which we observe is only 10-20%, leaving open the possibility that mechanisms
other than the simple Peierls stress are important in controlling the process
of low temperature slip.Comment: 12 pages and 9 figure
Synaptic Wnt signaling—a contributor to major psychiatric disorders?
Wnt signaling is a key pathway that helps organize development of the nervous system. It influences cell proliferation, cell fate, and cell migration in the developing nervous system, as well as axon guidance, dendrite development, and synapse formation. Given this wide range of roles, dysregulation of Wnt signaling could have any number of deleterious effects on neural development and thereby contribute in many different ways to the pathogenesis of neurodevelopmental disorders. Some major psychiatric disorders, including schizophrenia, bipolar disorder, and autism spectrum disorders, are coming to be understood as subtle dysregulations of nervous system development, particularly of synapse formation and maintenance. This review will therefore touch on the importance of Wnt signaling to neurodevelopment generally, while focusing on accumulating evidence for a synaptic role of Wnt signaling. These observations will be discussed in the context of current understanding of the neurodevelopmental bases of major psychiatric diseases, spotlighting schizophrenia, bipolar disorder, and autism spectrum disorder. In short, this review will focus on the potential role of synapse formation and maintenance in major psychiatric disorders and summarize evidence that defective Wnt signaling could contribute to their pathogenesis via effects on these late neural differentiation processes
Multiscale modelling of hardening in BCC crystal plasticity
The mechanical behavior of polycrystalline metals can be successfully modeled by macroscopic
theories, such as Von Mises plasticity. On the other hand, numerous studies can be performed on the atomic
scale, either by atomistic or dislocation dynamics models. The proposed model attempts to bridge those
two scales by deriving constitutive relations between slip strains, dislocation densities and resolved shear
stresses on crystallographic planes, from mechanisms of deformation playing at the level of the dislocation
line. The resulting “mesoscopic" hardening relations are controlled by dislocation self energies and
junctions strengths. Temperature and strain rate dependence result from the presence of thermally activated
mechanisms such as Peierls barriers or pair annihilation by cross slip. A set of material parameters is identified
for Tantalum by fitting the numerical stress strain curves from these tests with experimental results
gathered in the literature. These parameters prove to be in very good agreement with the values which can
be derived from molecular dynamics computations
Micro-RVE modeling of mechanistic response in porous intermetallics subject to weak and moderate impact loading
In this article we propose macroscopic (continuum) simulation schemes to predict response of porous heterogeneous material systems subjected to weak and moderate impact velocities. The proposed simulation model includes (1) an equation of state for porous solids that describes the evolution of porosity in the material as a function of shock pressure and, (2) a macroscopic rate dependent plasticity model for the porous composite that accounts for the deviatoric strength of the material at weak to moderate shock strengths. In addition, the numerical scheme employs cold-mixture theory to predict shock response of porous intermetallics. The material model is validated using gas-gun impact experiments on Ni/Al Intermolecular Reactive Composite (IRC) at 70% TMD. The proposed model is also used to understand the effect of microstructure on the material response predictions. (C) 2013 Elsevier Ltd. All rights reserved