18,649 research outputs found

    Molecular Dynamics Investigation of the Structural and Mechanical Properties of Off-Stoichiometric Epoxy Resins

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    We carried out molecular dynamics (MD) simulations to measure the mechanical properties of various off-stoichiometric polymers regarding amine to epoxy ratios (r) and to understand the stiffness of the polymers in terms of their structures. The aerospace-grade API-60 epoxy resin is used as an adhesive bond for assembling large-scale composite structures via the co-curing-ply bonding method. This method will produce a reliable and certifiable composite joint without additional fasteners. Calculated Young's modulus was measured from the uniaxial tension simulation with several high strain rates, and the experimental modulus was estimated by extrapolating the simulation results. We found that the stiffness was associated with molecular packing caused by chemical cross-linking. We also found that the number of network clusters gradually decreased as the ratio approached r = 1.0, which made the tighter cluster and the system much stiffer with an increase in the molecular weight and the degree of cross-linking. Structural properties such as Rg, MSD were measured to figure out the degree of stiffness with respect to the r

    Methodology for Adjusting GPRA Workforce Development Program Performance Targets for the Effects of Business Cycles

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    The U.S. Department of Labor’s Employment and Training Administration issued Training and Employment Guidance Letter (TEGL) 09-08 Change 1 on June 5, 2009. This guidance letter revises the Government Performance and Results Act (GPRA) performance measures for federal workforce development programs to take into account the effect of the recession on participants’ labor market and educational outcomes. As described in the TEGL, the performance targets of the various workforce development programs have been developed for use for the years PY2008 through PY2010. They are intended to be used for PY2009 performance target negotiations and will appear in the President’s Budget Request for FY2010. The performance targets for future program years, adjusted for unemployment rates, are driven by the economic assumptions of the President’s Budget Request for FY2010. The revised performance targets are based on analysis carried out as part of a study conducted for the U.S. Department of Labor by the W. E. Upjohn Institute for Employment Research. This working paper has two purposes. The first is to describe the methodology used to estimate the relationship between unemployment rates and workforce program performance targets. The second is to describe the procedures used to adjust the GPRA performance targets for changes in unemployment rates during the current recession and over the business cycle. The study described in this working paper is the initial phase of an ongoing analysis of the effect of economic conditions on workforce development program outcomes.performance standards, workforce programs, GPRA

    Do Athermal Amorphous Solids Exist?

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    We study the elastic theory of amorphous solids made of particles with finite range interactions in the thermodynamic limit. For the elastic theory to exist one requires all the elastic coefficients, linear and nonlinear, to attain a finite thermodynamic limit. We show that for such systems the existence of non-affine mechanical responses results in anomalous fluctuations of all the nonlinear coefficients of the elastic theory. While the shear modulus exists, the first nonlinear coefficient B_2 has anomalous fluctuations and the second nonlinear coefficient B_3 and all the higher order coefficients (which are non-zero by symmetry) diverge in the thermodynamic limit. These results put a question mark on the existence of elasticity (or solidity) of amorphous solids at finite strains, even at zero temperature. We discuss the physical meaning of these results and propose that in these systems elasticity can never be decoupled from plasticity: the nonlinear response must be very substantially plastic.Comment: 11 pages, 11 figure

    CodeNeRF: Disentangled Neural Radiance Fields for Object Categories

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    CodeNeRF is an implicit 3D neural representation that learns the variation of object shapes and textures across a category and can be trained, from a set of posed images, to synthesize novel views of unseen objects. Unlike the original NeRF, which is scene specific, CodeNeRF learns to disentangle shape and texture by learning separate embeddings. At test time, given a single unposed image of an unseen object, CodeNeRF jointly estimates camera viewpoint, and shape and appearance codes via optimization. Unseen objects can be reconstructed from a single image, and then rendered from new viewpoints or their shape and texture edited by varying the latent codes. We conduct experiments on the SRN benchmark, which show that CodeNeRF generalises well to unseen objects and achieves on-par performance with methods that require known camera pose at test time. Our results on real-world images demonstrate that CodeNeRF can bridge the sim-to-real gap. Project page: https://github.com/wayne1123/code-nerf

    Photoproduction off the nucleon revisited: Evidence for a narrow N(1688) resonance?

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    Revised analysis of Σ\Sigma beam asymmetry for the η\eta photoproduction on the free proton reveals a structure at W∼1.69W\sim 1.69 GeV. Fit of the experimental data based on the E429 solution of the SAID partial wave analysis suggests a narrow (Γ≤25\Gamma \leq 25 MeV) resonance. Possible candidates are P11,P13P_{11}, P_{13}, or D13D_{13} resonances. The result is considered in conjunction with the recent evidence for a bump-like structure at W∼1.67−1.68W\sim 1.67 - 1.68 GeV in the quasi-free η\eta photoproduction on the neutron.Comment: Contribution to the Workshop on the Physics of the Excited Nucleons NSTAR2007, Bonn, Germany, Sept. 5 - 8 2007. To be published in Eur.Phys.J.
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