27,141 research outputs found
A mesoscale finite element simulation of intermittent plastic flow of micropillar compression under hybrid loading mode
The plastic deformation of the micropillar proceeds as a series of strain bursts, showing an intermittent plastic flow. In this work, we present a stochastic finite element method in crystal plasticity to describe the intermittent characteristic of crystal deformation under the hybrid loading mode (HLM). The microscopic boundary conditions(MBCs) using the HLM are studied and they are demonstrated to be different in various deformation periods such as loading stage, burst slip and holding stage, which occur alternatively as the plastic flow proceeds. In order to determine the MBCs, we use the Monte Carlo (MC) stochastic model to predict the amplitude of the burst displacement and then incorporate such model into our established continuum framework accounting for the characteristics of the strain burst. By implementing this continuum model into the finite element analysis, we predict the plastic flow of single crystal nickel micropillars that deform under uniaxial compression along the [2 6 9] crystalline direction. The simulation results indicate clearly visible strain bursts in the course of plastic deformation, producing a stair-case like stress-strain behavior that agrees well with experimental observations. The computational results reveal that the intermittent flow in the micrometer-scale is intensified due to the increasing amplitude of the strain burst, as well as the occurrence of successive strain bursts rather than the discrete strain bursts, with decreasing of the specimen size. In addition, the micropillar displacement in the context of burst activity predicted from our simulations is similar to the experimental observations. We demonstrate that our simulation method could provide further insights into the intermittent plastic flow characteristics such as burst time duration, micropillar velocity; plus, it is feasible to apply this method to investigate the plastic flow behaviors under complex loading conditions
Alternating Synthetic and Real Gradients for Neural Language Modeling
Training recurrent neural networks (RNNs) with backpropagation through time
(BPTT) has known drawbacks such as being difficult to capture longterm
dependencies in sequences. Successful alternatives to BPTT have not yet been
discovered. Recently, BP with synthetic gradients by a decoupled neural
interface module has been proposed to replace BPTT for training RNNs. On the
other hand, it has been shown that the representations learned with synthetic
and real gradients are different though they are functionally identical. In
this project, we explore ways of combining synthetic and real gradients with
application to neural language modeling tasks. Empirically, we demonstrate the
effectiveness of alternating training with synthetic and real gradients after
periodic warm restarts on language modeling tasks
Characteristics of oligonucleotide frequencies across genomes: Conservation versus variation, strand symmetry, and evolutionary implications
One of the objectives of evolutionary genomics is to reveal the genetic information contained in the primordial genome (called the primary genetic information in this paper, with the primordial genome defined here as the most primitive nucleic acid genome for earth’s life) by searching for primitive traits or relics remained in modern genomes. As the shorter a sequence is, the less probable it would be modified during genome evolution. For that reason, some characteristics of very short nucleotide sequences would have considerable chances to persist during billions of years of evolution. Consequently, conservation of certain genomic features of mononucleotides, dinucleotides, and higher-order oligonucleotides across various genomes may exist; some, if not all, of these features would be relics of the primary genetic information. Based on this assumption, we analyzed the pattern of frequencies of mononucleotides, dinucleotides, and higher-order oligonucleotides of the whole-genome sequences from 458 species (including archaea, bacteria, and eukaryotes). Also, we studied the phenomenon of strand symmetry in these genomes. The results show that the conservation of frequencies of some dinucleotides and higher-order oligonucleotides across genomes does exist, and that strand symmetry is a ubiquitous and explicit phenomenon that may contribute to frequency conservation. We propose a new hypothesis for the origin of strand symmetry and frequency conservation as well as for the constitution of early genomes. We conclude that the phenomena of strand symmetry and the pattern of frequency conservation would be original features of the primary genetic information
Do External Interventions Work? The Case of Trade Reform Conditions in IMF Supported Programs
Trade reform conditions are common in IMF supported programs. Of the 99 countries that had IMF programs during 1993-2003, 77 had conditions on trade reforms in their programs. Since the WTO has not been found especially effective in promoting trade openness for most developing countries, it is of great interest to see if the IMF has been more effective as it combines carrots and sticks not available to the WTO. Yet, the effectiveness of trade conditions in IMF programs has not been systematically studied. Using a unique dataset, this paper provides such an assessment. It finds that trade conditions are associated with an increase in trade openness on average, but the effect comes mostly from countries that, by some measure, have a high degree of "willingness to reform."
Collateral Damage: Exchange Controls and International Trade
While new conventional wisdom warns that developing countries should be aware of the risks of premature capital account liberalization, the costs of not removing exchange controls have received much less attention. This paper investigates the negative effects of exchange controls on trade. To minimize evasion of controls, countries often intensify inspections at the border and increase documentation requirements. Thus, the cost of conducting trade rises. The paper finds that a one standard-deviation increase in the controls on trade payment has the same negative effect on trade as an increase in tariff by about 14 percentage points. A one standard-deviation increase in the controls on FX transactions reduces trade by the same amount as a rise in tariff by 11 percentage points. Therefore, the collateral damage in terms of foregone trade is sizable.
A two-step approach to model precipitation extremes in California based on max-stable and marginal point processes
In modeling spatial extremes, the dependence structure is classically
inferred by assuming that block maxima derive from max-stable processes.
Weather stations provide daily records rather than just block maxima. The point
process approach for univariate extreme value analysis, which uses more
historical data and is preferred by some practitioners, does not adapt easily
to the spatial setting. We propose a two-step approach with a composite
likelihood that utilizes site-wise daily records in addition to block maxima.
The procedure separates the estimation of marginal parameters and dependence
parameters into two steps. The first step estimates the marginal parameters
with an independence likelihood from the point process approach using daily
records. Given the marginal parameter estimates, the second step estimates the
dependence parameters with a pairwise likelihood using block maxima. In a
simulation study, the two-step approach was found to be more efficient than the
pairwise likelihood approach using only block maxima. The method was applied to
study the effect of El Ni\~{n}o-Southern Oscillation on extreme precipitation
in California with maximum daily winter precipitation from 35 sites over 55
years. Using site-specific generalized extreme value models, the two-step
approach led to more sites detected with the El Ni\~{n}o effect, narrower
confidence intervals for return levels and tighter confidence regions for risk
measures of jointly defined events.Comment: Published at http://dx.doi.org/10.1214/14-AOAS804 in the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
- …