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Hermanson J.C.,U.S. Department of Agriculture | Iliopoulos A.,Center for Computational Material Science
Proceedings of the ASME Design Engineering Technical Conference | Year: 2010

Automated inverse methods for material constitutive characterization under multidimensional loading conditions has motivated the custom design, manufacturing and utilization of mechatronic loading machines. This present paper reports on the architecture of a mechatronic system capable of enforcing 6-DoF kinematic boundary conditions on deformable material specimens under testing, while at the same time measuring both the imposed kinematics and the corresponding reaction forces in a fully automated manner. This system has a recursive nature as it consists of a hexapod configuration that repeats itself six times. In addition to the architecture, we also present the historical evolution, and current status of its manufacturing implementation and the initial fielding of our system for composite material testing and characterization. Copyright © 2010 by ASME. Source

Wang T.,James Franck Institute | Vaxenburg R.,Technion - Israel Institute of Technology | Liu W.,James Franck Institute | Rupich S.M.,James Franck Institute | And 4 more authors.
ACS Nano | Year: 2015

The electronic structure of single InSb quantum dots (QDs) with diameters between 3 and 7 nm was investigated using atomic force microscopy (AFM) and scanning tunneling spectroscopy (STS). In this size regime, InSb QDs show strong quantum confinement effects which lead to discrete energy levels on both valence and conduction band states. Decrease of the QD size increases the measured band gap and the spacing between energy levels. Multiplets of equally spaced resonance peaks are observed in the tunneling spectra. There, multiplets originate from degeneracy lifting induced by QD charging. The tunneling spectra of InSb QDs are qualitatively different from those observed in the STS of other III-V materials, for example, InAs QDs, with similar band gap energy. Theoretical calculations suggest the electron tunneling occurs through the states connected with L-valley of InSb QDs rather than through states of the -valley. This observation calls for better understanding of the role of indirect valleys in strongly quantum-confined III-V nanomaterials. © 2014 American Chemical Society. Source

Michopoulos J.G.,Center for Computational Material Science | Iliopoulos A.P.,SAIC | Furukawa T.,Virginia Polytechnic Institute and State University
Proceedings of the ASME International Design Engineering Technical Conferences and Computers and Information in Engineering Conference 2009, DETC2009 | Year: 2010

The present paper reports on the progress towards the evaluation of the Mesh Free Random Grid Method (MFRGM) for the inverse constitutive characterization of composite materials. The method provides the capability for the remote (non contact) measurement of displacement and strain fields of structures bounded by flat surfaces that deform under various mechanical and generalized loading conditions. The known forward solution of an anisotropic plate with an open hole, loaded at infinity, is used to generate synthetic images MFRG. The inverse problem for determining the constitutive parameters formulated directly on the generalized constitutive law. Performance of the technique is evaluated by the usage of just one frame corresponding to one set of strain state for various amounts of noise. The evaluation is repeated by utilizing frames corresponding to different rotations of the laminate relative to the loading direction. Finally the exceedingly accurate behavior of the methodology is discussed. Copyright © 2009 by ASME. Source

Iliopoulos A.P.,SAIC | Michopoulos J.G.,Center for Computational Material Science | Lambrakos S.G.,Center for Computational Material Science | Bernstein N.,Center for Computational Material Science
Proceedings of the ASME Design Engineering Technical Conference | Year: 2010

The recent growth of General Purpose Graphic Processor Units (GPGPUs) technologies as well as the ongoing need for linking usability performance with structural materials processing and design across many length and time scales have motivated the present work. The inverse problem of determining the Lennard-Jones potential governing the fracture dynamics of atoms comprising a sheet of metal under tension, is used to examine the feasibility of utilizing efficiently GPGPU architectures. The implementation of this inverse problem under a molecular dynamics framework verifies the ability of this methodology to deliver the intended results. Subsequently, a sensitivity analysis is performed on GPGPU-enabled hardware to examine the effect of the size of the problem under consideration on the efficiency of various combinations of GPGPU and Central Processing Unit (CPU) cores. Speedup factors are determined relative to a single core CPU of a quad core processor. Copyright © 2010 by ASME. Source

News Article
Site: http://www.nrl.navy.mil/media/news-releases/

U.S. Naval Research Laboratory (NRL) research physicist, Dr. Carl Stephen Hellberg, is elected Fellow by the American Physical Society (APS) for creative and influential contributions in the fields of strongly correlated materials, quantum dots, defects, and heterostructures. Arriving at NRL in 1996 as a National Research Council (NRC) research associate, Hellberg has concentrated on researching the physics of surfaces and interfaces using density functional theory, concentrating on low-dimensional systems and the surfaces and interfaces of bulk crystals. "Dr. Hellberg is recognized for his work demonstrating the limits of strontium titanate to coherently grow beyond a few layers on silicon and how chemical substitutions at the interface can produce a better interface and more uniform thin films," said Dr. Michael Mehl, head, Center for Computational Material Science. "I am very delighted he has been elected a Fellow of the American Physical Society for his groundbreaking work in the field of computational physics." Hellberg's current research includes first principles calculations of surfaces, interfaces, and thin films. He is focusing on oxides and chalcogenides, including polarity mismatched interfaces, topological insulators, and monolayer heterostructures, with a particular interests in electrical properties and metal-insulator transitions. He also works on strongly correlated electron systems, including quantum dots, nanocrystals, and transition metal oxides. Hellberg received his undergraduate degree in physics from Princeton University in 1987, and then studied for a year at the Ludwig Maximilian University of Munich on a Fulbright Fellowship. He enrolled in graduate school at the University of Pennsylvania, where he worked with professor Eugene J. Mele, receiving his Ph.D. in 1993. He spent three years working with professor Efstratios Manousakis at Florida State University in a postdoctoral appointment developing computational techniques to study strongly correlated electrons. The APS is a non-profit membership organization working to advance and diffuse the knowledge of physics. APS Fellows are elected on the criterion of exceptional contributions to the physics enterprise that are comprised of outstanding physics research, important applications of physics, leadership in or service to physics, or significant contributions to physics education. About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.

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