Publications

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Significant progress has been made over the last few years in understanding properties of matter subject to strong shocks and other extreme conditions. High-accuracy multi-Mbar experiments and first-principles theoretical studies together provide detailed insights into the physics and chemistry of high energy-density matter. While comprehensive advances have been made for pure elements like deuterium, helium, and carbon, progress has been slower for equally important, albeit more challenging, materials like molecular crystals, polymers, and foams. Hydrocarbon based polymer foams are common materials and in particular they are used in designing shock- and inertial confinement fusion experiments. Depending on their initial density, foams shock to relatively higher pressure and temperature compared to shocked dense polymers/plastics. As foams and polymers are shocked, they exhibit both structural and chemical transitions. We will present experimental and theoretical results for shocked polymers in the Mbar regime. By shock impact of magnetically launched flyer plates on poly(4-methyl-1-pentene) foams, we create multi-Mbar pressures in a dense plasma mixture of hydrogen, carbon, at temperatures of several eV. Concurrently with executing experiments, we analyze the system by multi-scale simulations, from density functional theory to continuum magneto-hydrodynamics simulations. In particular, density functional theory (DFT) molecular dynamics (MD) and classical MD simulations of the principal shock Hugoniot will be presented in detail for two hydrocarbon polymers: polyethylene (PE) and poly(4-methyl-1-pentene) (PMP).

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The political dynamics associated with an election are typically a function of the interplay between political leaders and voters, as well as endogenous and exogenous factors that impact the perceptions and goals of the electorate. This paper describes an effort by Sandia National Laboratories to model the attitudes and behaviors of various political groups along with that population's primary influencers, such as government leaders. To accomplish this, Sandia National Laboratories is creating a hybrid system dynamics-cognitive model to simulate systems- and individual-level political dynamics in a hypothetical society. The model is based on well-established psychological theory, applied to both individuals and groups within the modeled society. Confidence management processes are being incorporated into the model design process to increase the utility of the tool and assess its performance. This project will enhance understanding of how political dynamics are determined in democratic society.

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The Office of Nuclear Regulatory Research (RES) at the US Nuclear Regulatory Commission (USNRC) is sponsoring work in response to a Staff Requirements Memorandum (SRM) directing an effort to establish a single human reliability analysis (HRA) method for the agency or guidance for the use of multiple methods. As part of this effort an attempt to develop a comprehensive HRA qualitative approach is being pursued. This paper presents a draft of the method's middle layer, a part of the qualitative analysis phase that links failure mechanisms to performance shaping factors. Starting with a Crew Response Tree (CRT) that has identified human failure events, analysts identify potential failure mechanisms using the mid-layer model. The mid-layer model presented in this paper traces the identification of the failure mechanisms using the Information-Diagnosis/Decision-Action (IDA) model and cognitive models from the psychological literature. Each failure mechanism is grouped according to a phase of IDA. Under each phase of IDA, the cognitive models help identify the relevant performance shaping factors for the failure mechanism. The use of IDA and cognitive models can be traced through fault trees, which provide a detailed complement to the CRT.

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A low-temperature upturn of the Coulomb drag resistivity {rho}{sub D} measured in undoped electron-hole bilayer devices, possibly manifesting from formation of a superfluid condensate or density modulated state, was recently observed. Here the effects of perpendicular and parallel magnetic fields on the drag upturn are examined. Measurements of {rho}{sub D} and drive layer resistivity {rho}{sub xx-e} as a function of temperature and magnetic field in two uEHBL devices are presented. In B{sub {perpendicular}}, the drag upturn was enhanced as the field increased up to roughly .2 T, beyond which oscillations in {rho}{sub D} and {rho}{sub xx-e}, reflecting Landau level formation, begin appearing. A small phase offset between those oscillations, which decreased at higher fields and temperatures, was also observed. In B{sub {parallel}}, the drag upturn magnitude diminished as the field increased. Above the upturn regime, both {rho}{sub D} and {rho}{sub xx-e} were enhanced by B{sub {parallel}}, the latter via decreased screening of the uniform background impurities.

Silicon is an ideal system for investigating single electron or isolated donor spins for quantum computation, due to long spin coherence times. Enhancement mode strained-silicon/silicon germanium (sSi/SiGe) devices would offer an as-yet untried path toward electron or electron/donor quantum dot systems. Thin, undoped SiGe dielectrics allow tight electrostatic confinement, as well as potential Lande g-factor engineered spin manipulation. In this talk we summarize recent progress toward sSi/SiGe enhancement mode devices on sSi on insulator, including characterization with X-ray diffraction and atomic force microscopy, as well as challenges faced and progress on integration of either top-down and bottom-up donor placement approaches in a sSi/SiGe enhancement mode structure.

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The principal design drivers in the certification of wind turbine blades are ultimate strength, fatigue resistance, adequate tip-tower clearance, and buckling resistance. Buckling resistance is typically strongly correlated to both ultimate strength and fatigue resistance. A composite shell with spar caps forms the airfoil shape of a blade and reinforcing shear webs are placed inside the blade to stiffen the blade in the flap-wise direction. The spar caps are dimensioned and the shear webs are placed so as to add stiffness to unsupported panel regions and reduce their length. The panels are not the major flap-wise load carrying element of a blade; however, they must be designed carefully to avoid buckling while minimizing blade weight. Typically, buckling resistance is evaluated by consideration of the load-deflection behavior of a blade using finite element analysis (FEA) or full-scale static testing of blades under a simulated extreme loading condition. The focus of this paper is on the use of experimental modal analysis to measure localized resonances of the blade panels. It can be shown that the resonant behavior of these panels can also provide a means to evaluate buckling resistance by means of analytical or experimental modal analysis. Further, panel resonances have use in structural health monitoring by observing changes in modal parameters associated with panel resonances, and use in improving panel laminate model parameters by correlation with test data. In recent modal testing of wind turbine blades, a set of panel modes were measured. This paper will report on the findings of these tests and accompanying numerical and analytical modeling efforts aimed at investigating the potential uses of panel resonances for blade evaluation, health monitoring, and design.

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When elastic-plastic materials, such as metals, are subjected to moderately high strain rates or dynamic loadings, the plastic stress wave trails behind the elastic wave because of its slower wave speed. Due to the inherent time-dependent nature of the plastic deformation, the elastic precursor generally loads the material to a metastable elastic state at a stres level that is higher than the static strength of the material. This metastable state gradually relaxes to the equilibrium state and the relaxation results in the so-called precursor decay. In a recent work by Asay et al. (J. Appl. Phys., 2009), the inelastic response of annealed and cold-rolled pure polycrystalline tantalum at intermediate strain rates ({approx} 106/sec) was experimentally characterized with ramp wave loading. It was found that the precursor of the annealed tantalum showed little decay over a propagation distance of 6 mm even though the peak precursor stress was well above the static strength of the mateiral. The precursor for the cold-rolled sample was more dispersive and did not exhibit the characteristics depicted by the annealed samples. In this study, a constitutive model based on the concept of dislocation motion and generation was developed to gain insights into this somewhat unusual precursor behavior, particularly for the annealed samples, and the possible underlying deformation mechanisms for tantalum. Despite its simplicity, the model worked quite well for both the annealed and cold-rolled materials. The tantalum studied here essentially exhibits strong rate sensitivity and this behavior is modeled through the low dislocation density and the strong stress dependence of the dislocation velocity. Both of these contributions may be related to the low mobility of the screw dislocations in bcc metals. This low mobility may result from its extended, three-dimensional core structure.

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An overview of the study of the effects that electrical power and signal cables introduce on the dynamic response of precision structures is presented, along with a summary of lessons learned and most significant results. This was a three-year effort conducted at the Air Force Research Laboratory, Space Vehicles Directorate to discover a set of practical approaches for updating well defined dynamical models of cableless structures where knowledge of the cable type, position, and tie-down method are known. While cables can be found on many different types of structures, the focus of this effort was on precision, low-damping, and low-first modal frequency structures. Various obstacles, classified as tangents, rat holes, and dead ends, were encountered along the way. Rather than following a strictly technical flow, the paper presents the historical, experiential progression of the project. First, methods were developed to estimate cable properties. Problems were encountered because of the flexible, highly damped nature of cables. A simple beam was used as a test article to validate experimentally derived cable properties and to refine the assumptions regarding boundary conditions. A spacecraft bus-like panel with cables attached was designed, and finite element models were developed and validated through experiment. Various paths were investigated at each stage before a consistent test and analysis methodology was developed. These twists and turns are described.

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