614 research outputs found
Quantitative insights into the dislocation source behavior of twin boundaries suggest a new dislocation source mechanism
Pop-in statistics from nanoindentation with spherical indenters are used to determine the stress required to activate dislocation sources in twin boundaries (TBs) in copper and its alloys. The TB source activation stress is smaller than that needed for bulk single crystals, irrespective of the indenter size, dislocation density and stacking fault energy. Because an array of pre-existing Frank partial dislocations is present at a TB, we propose that dislocation emission from the TB occurs by the Frank partials splitting into Shockley partials moving along the TB plane and perfect lattice dislocations, both of which are mobile. The proposed mechanism is supported by recent high resolution transmission electron microscopy images in deformed nanotwinned (NT) metals and may help to explain some of the superior properties of nanotwinned metals (e.g. high strength and good ductility), as well as the process of detwinning by the collective formation and motion of Shockley partial dislocations along TBs. Graphic abstract: [Figure not available: see fulltext.] © 2021, The Author(s)
ART-Owen Scrambling
We present a novel algorithm for implementing Owen-scrambling, combining the
generation and distribution of the scrambling bits in a single self-contained
compact process. We employ a context-free grammar to build a binary tree of
symbols, and equip each symbol with a scrambling code that affects all
descendant nodes. We nominate the grammar of adaptive regular tiles (ART)
derived from the repetition-avoiding Thue-Morse word, and we discuss its
potential advantages and shortcomings. Our algorithm has many advantages,
including random access to samples, fixed time complexity, GPU friendliness,
and scalability to any memory budget. Further, it provides two unique features
over known methods: it admits optimization, and it is invertible, enabling
screen-space scrambling of the high-dimensional Sobol sampler.Comment: To appear at SIGGRAPH Asia 202
High Performance Direct Gravitational N-body Simulations on Graphics Processing Units -- II: An implementation in CUDA
We present the results of gravitational direct -body simulations using the
Graphics Processing Unit (GPU) on a commercial NVIDIA GeForce 8800GTX designed
for gaming computers. The force evaluation of the -body problem is
implemented in ``Compute Unified Device Architecture'' (CUDA) using the GPU to
speed-up the calculations. We tested the implementation on three different
-body codes: two direct -body integration codes, using the 4th order
predictor-corrector Hermite integrator with block time-steps, and one
Barnes-Hut treecode, which uses a 2nd order leapfrog integration scheme. The
integration of the equations of motions for all codes is performed on the host
CPU.
We find that for particles the GPU outperforms the GRAPE-6Af, if
some softening in the force calculation is accepted. Without softening and for
very small integration time steps the GRAPE still outperforms the GPU. We
conclude that modern GPUs offer an attractive alternative to GRAPE-6Af special
purpose hardware. Using the same time-step criterion, the total energy of the
-body system was conserved better than to one in on the GPU, only
about an order of magnitude worse than obtained with GRAPE-6Af. For N \apgt
10^5 the 8800GTX outperforms the host CPU by a factor of about 100 and runs at
about the same speed as the GRAPE-6Af.Comment: Accepted for publication in New Astronom
Real-time visualization of a sparse parametric mixture model for BTF rendering
Bidirectional Texture Functions (BTF) allow high quality visualization of real world materials exhibiting complex appearance and details that can not be faithfully represented using simpler analytical or parametric representations. Accurate representations of such materials require huge amounts of data, hindering real time rendering. BTFs compress the raw original data, constituting a compromise between visual quality and rendering time. This paper presents an implementation of a state of the art BTF representation on the GPU, allowing interactive high fidelity visualization of complex geometric models textured with multiple BTFs. Scalability with respect to the geometric complexity, amount of lights and number of BTFs is also studied.Fundação para a Ciência e Tecnologi
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An explanation for the shape of nanoindentation unloading curves based on finite element simulation
Current methods for measuring hardness and modulus from nanoindentation load-displacement data are based on Sneddon`s equations for the indentation of an elastic half-space by an axially symmetric rigid punch. Recent experiments have shown that nanoindentation unloading data are distinctly curved in a manner which is not consistent with either the flat punch or the conical indenter geometries frequently used in modeling, but are more closely approximated by a parabola of revolution. Finite element simulations for conical indentation of an elastic-plastic material are presented which corroborate the experimental observations, and from which a simple explanation for the shape of the unloading curve is derived. The explanation is based on the concept of an effective indenter shape whose geometry is determined by the shape of the plastic hardness impression formed during indentation
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Cracking during nanoindentation and its use in the measurement of fracture toughness
Results of an investigation aimed at developing a technique by which the fracture toughness of a thin film or small volume can be determined in nanoindentation experiments are reported. The method is based on the radial cracking which occurs when brittle materials are deformed by a sharp indenter such as a Vickers or Berkovich diamond. In microindentation experiments, the lengths of radial cracks have been found to correlate reasonably well with fracture toughness, and a simple semi-empirical method has been developed to compute the toughness from the crack lengths. However, a problem is encountered in extending this method into the nanoindentation regime with the standard Berkovich indenter in that there are well defined loads, called cracking thresholds, below which indentation cracking does not occur in most brittle materials. We have recently found that the problems imposed by the cracking threshold can be largely overcome by using an indenter with the geometry of the comer of a cube. For the cube-corner indenter, cracking thresholds in most brittle materials are as small as 1 mN ({approximately}0.1 grams). In addition, the simple, well-developed relation between toughness and crack length used for the Vickers indenter in the microindentation regime can be used for the cube-corner indenter in the nanoindentation regime provided a different empirical constant is used
TokaMaker: An open-source time-dependent Grad-Shafranov tool for the design and modeling of axisymmetric fusion devices
In this paper, we present a new static and time-dependent MagnetoHydroDynamic
(MHD) equilibrium code, TokaMaker, for axisymmetric configurations of
magnetized plasmas, based on the well-known Grad-Shafranov equation. This code
utilizes finite element methods on an unstructured triangular grid to enable
capturing accurate machine geometry and simple mesh generation from
engineering-like descriptions of present and future devices. The new code is
designed for ease of use without sacrificing capability and speed through a
combination of Python, Fortran, and C/C++ components. A detailed description of
the numerical methods of the code, including a novel formulation of the
boundary conditions for free-boundary equilibria, and validation of the
implementation of those methods using both analytic test cases and cross-code
validation is shown. Results show expected convergence across tested polynomial
orders for analytic and cross-code test cases
Eff ect of a congregation-based intervention on uptake of HIV testing and linkage to care in pregnant women in Nigeria (Baby Shower): a cluster randomised trial
Background Few eff ective community-based interventions exist to increase HIV testing and uptake of antiretroviral
therapy (ART) in pregnant women in hard-to-reach resource-limited settings. We assessed whether delivery of an
intervention through churches, the Healthy Beginning Initiative, would increase uptake of HIV testing in pregnant
women compared with standard health facility referral.
Methods In this cluster randomised trial, we enrolled self-identifi ed pregnant women aged 18 years and older who
attended churches in southeast Nigeria. We randomised churches (clusters) to intervention or control groups,
stratifi ed by mean annual number of infant baptisms (<80 vs ≥80). The Healthy Beginning Initiative intervention
included health education and on-site laboratory testing implemented during baby showers in intervention group
churches, whereas participants in control group churches were referred to health facilities as standard. Participants
and investigators were aware of church allocation. The primary outcome was confi rmed HIV testing. This trial is
registered with ClinicalTrials.gov, identifi er number NCT 01795261.
Findings Between Jan 20, 2013, and Aug 31, 2014, we enrolled 3002 participants at 40 churches (20 per group).
1309 (79%) of 1647 women attended antenatal care in the intervention group compared with 1080 (80%) of 1355 in the
control group. 1514 women (92%) in the intervention group had an HIV test compared with 740 (55%) controls
(adjusted odds ratio 11·2, 95% CI 8·77–14·25; p<0·0001).
Interpretation Culturally adapted, community-based programmes such as the Healthy Beginning Initiative can be
eff ective in increasing HIV screening in pregnant women in resource-limited settings
Clubbing masculinities: Gender shifts in gay men's dance floor choreographies
This is an Author's Accepted Manuscript of an article published in Journal of Homosexuality, 58(5), 608-625, 2011 [copyright
Taylor & Francis], available online at: http://www.tandfonline.com/10.1080/00918369.2011.563660This article adopts an interdisciplinary approach to understanding the intersections of gender, sexuality, and dance. It examines the expressions of sexuality among gay males through culturally popular forms of club dancing. Drawing on political and musical history, I outline an account of how gay men's gendered choreographies changed throughout the 1970s, 80s, and 90s. Through a notion of “technologies of the body,” I situate these developments in relation to cultural levels of homophobia, exploring how masculine expressions are entangled with and regulated by musical structures. My driving hypothesis is that as perceptions of cultural homophobia decrease, popular choreographies of gay men's dance have become more feminine in expression. Exploring this idea in the context of the first decade of the new millennium, I present a case study of TigerHeat, one of the largest weekly gay dance club events in the United States
Similarity Relationships in Creep Contacts and Applications in Nanoindentation Tests
ABSTRACT The study of indentation responses of rate-dependent (viscoplastic or creeping) solids has generally focused on the relationship between indentation hardness and an effective strain rate, which can be defined from a similarity transformation of the governing equations. The strain rate sensitivity exponent can be determined from the slope of a log-log plot of the hardness versus effective strain rate, while determining other constitutive parameters requires a knowledge of the relationship between contact size, shape, and indentation depth. In this work, finite element simulations have shown that the effects of non-axisymmetric contact and crystallography are generally negligible. Theoretical predictions agree well with real nanoindentation measurements on amorphous selenium when tested above glass transition temperature, but deviate quite significantly for experiments on high-purity indium, coarse-grained aluminum, and nanocrystalline nickel. Such a discrepancy is likely to result from the transient creep behavior
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