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Now showing items 1 - 16 of 46577

  • Dynamic experiments to study the alpha-epsilon phase transition in cerium

    Jensen, B. J.   Cherne, F. J.   Velisavljevic, N.  

    The ability to understand and predict the response of matter at extreme conditions requires knowledge of a material's equation-of-state including the location of phase boundaries, transition kinetics, and the evolution of material strength. Cerium is a material with a complex phase diagram that continues to attract significant scientific interest. Recent dynamic experiments have provided information on the low-pressure gamma-alpha phase transition, sound speed, and Hugoniot data for the higher-pressure alpha phase, as well as the incipient shock melt transition. Despite these efforts, there are still regions of the phase diagram that are largely unexplored dynamically, including the high-pressure region below the melt boundary. Along a room temperature isotherm, diamond anvil cell data report a transition to the epsilon phase between 13 and 17 GPa. At higher temperatures, similar diamond anvil cell data show significant disagreement regarding the existence, location, and slope of the epsilon-phase boundary. In this work, double-shock loading was used to access the alpha-epsilon region of the phase diagram to obtain equation-of-state information and to determine the location of the epsilon-phase boundary for shock loading. (c) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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  • Synchronizing a 40-mm Powder Gun to an Accelerator

    Jensen, B. J.   Wright, J.   Hollander, B.   Cherne, F. J.   Tucker, T.   Owens, C. T.   Rivera, J.  

    Over the past decade, there have been significant efforts to couple gun systems to beam lines such as at the Advanced Photon Source (APS) and the Los Alamos Neutron Science Center (LANSCE) to use advanced diagnostics to study the dynamic properties of materials. Synchronizing a gun system to these beam lines is challenging and requires improved characterization of their operation and a significant reduction in the uncertainty of the system time (propellant initiation to impact). In this work, data will be presented that describes the operation of a 40-mm bore powder gun (maximum velocity 2 km/s) including details of the projectile configuration and the propellant assembly that was designed specifically to reduce the jitter in the overall system time. Measurements of breech pressure, projectile velocity, and impact times were used to develop the gun performance characteristics (LA-UR-17-27975).
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  • Jet formation in cerium metal to examine material strength

    Jensen, B. J.   Cherne, F. J.   Prime, M. B.   Fezzaa, K.   Iverson, A. J.   Carlson, C. A.   Yeager, J. D.   Ramos, K. J.   Hooks, D. E.   Cooley, J. C.   Dimonte, G.  

    Examining the evolution of material properties at extreme conditions advances our understanding of numerous high-pressure phenomena from natural events like meteorite impacts to general solid mechanics and fluid flow behavior. Recent advances in synchrotron diagnostics coupled with dynamic compression platforms have introduced new possibilities for examining in-situ, spatially resolved material response with nanosecond time resolution. In this work, we examined jet formation from a Richtmyer-Meshkov instability in cerium initially shocked into a transient, high-pressure phase, and then released to a low-pressure, higher-temperature state. Cerium's rich phase diagram allows us to study the yield stress following a shock induced solid-solid phase transition. X-ray imaging was used to obtain images of jet formation and evolution with 2-3 mu m spatial resolution. From these images, an analytic method was used to estimate the post-shock yield stress, and these results were compared to continuum calculations that incorporated an experimentally validated equation-of-state (EOS) for cerium coupled with a deviatoric strength model. Reasonable agreement was observed between the calculations and the data illustrating the sensitivity of jet formation on the yield stress values. The data and analysis shown here provide insight into material strength during dynamic loading which is expected to aid in the development of strength aware multi-phase EOS required to predict the response of matter at extreme conditions. (C) 2015 AIP Publishing LLC.
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  • Enhancing impact velocity with shock interactions in a restricting die

    Anderson, W. W.   Jensen, B. J.   Cherne, F. J.   Owens, C. T.   Ramos, K. J.   Lieber, M. A.  

    Shock compression and impact studies could benefit from the ability to increase impact velocities that can be achieved with gun systems. Single-stage guns have modest performance (0.2-2 km/s) that limits their utility for high-pressure and high-velocity studies, while more capable systems are expensive and complex. We are developing a technique that uses a low-strength sabot with a tapered die to increase the impact velocity without modifying the gun itself. Impact of the projectile with the die generates a converging shock wave in the sabot that acts to accelerate the front of the projectile, while decelerating the rear portion. Preliminary experiments using this technique have observed a velocity enhancement of up to a factor of two.
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  • A numerical study of bubble and spike velocities in shock-driven liquid metals

    Karkhanis, V.   Ramaprabhu, P.   Cherne, F. J.   Hammerberg, J. E.   Andrews, M. J.  

    We use detailed continuum hydrodynamics and molecular dynamics simulations to investigate the dynamics of ejecta that are initialized with large amplitude perturbations and non-sinusoidal shapes. Insights from the simulations are used to suggest a modified expression for the velocity associated with ejected spike structures, whereas a recently suggested model explains the observed bubble velocities. Specifically, we find the asymptotic bubble velocity prediction given by Mikaelian is in excellent agreement with the simulations, when a nonlinear correction for finite amplitudes is used in that model. In contrast, existing models can overpredict observed spike velocities if they do not include the modification of the initial spike growth rates due to nonlinearities. Instead, we find that when potential flow models are corrected with a suitable nonlinear prefactor, this leads to predictions in close agreement with our simulation data. We also propose a simple empirical expression for the nonlinear correction for spike velocities which is able to reproduce results from our simulations and published experimental and simulation data over a wide range of initial conditions and Mach numbers. We discuss extensions of these models to initial interfaces with arbitrary shapes. In particular, for non-sinusoidal shapes, the bubble and spike velocities are still predicted by these models provided we use an effective wavelength lambda(eff) which is the wavelength of an equivalent sinusoid that has the same missing area. The issues of nonlinearity, nonstandard shapes and shock Mach number addressed in this work are relevant to recent experimental campaigns involving twice-shocked targets. Published by AIP Publishing.
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  • On shock driven jetting of liquid from non-sinusoidal surfaces into a vacuum

    Cherne, F. J.   Hammerberg, J. E.   Andrews, M. J.   Karkhanis, V.   Ramaprabhu, P.  

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  • On shock driven jetting of liquid from non-sinusoidal surfaces into a vacuum

    Cherne, F. J.   Hammerberg, J. E.   Andrews, M. J.   Karkhanis, V.   Ramaprabhu, P.  

    Previous work employed Richtmyer-Meshkov theory to describe the development of spikes and bubbles from shocked sinusoidal surfaces. Here, we discuss the effects of machining different two-dimensional shaped grooves in copper and examine the resulting flow of the material after being shocked into liquid on release. For these simulations, a high performance molecular dynamics code, SPaSM, was used with machined grooves of kh(0) =3D 1 and kh(0) =3D 1/8, where 2h(0) is the peak-to-valley height of the perturbation with wavelength lambda, and k =3D 2 pi/lambda. The surface morphologies studied include a Chevron, a Fly-Cut, a Square-Wave, and a Gaussian. We describe extensions to an existing ejecta source model that better captures the mass ejected from these surfaces. We also investigate the same profiles at length scales of order 1 cm for an idealized fluid equation of state using the FLASH continuum hydrodynamics code. Our findings indicate that the resulting mass can be scaled by the missing area of a sinusoidal curve with an effective wavelength, lambda(eff), that has the same missing area. Our extended ejecta mass formula works well for all the shapes considered and captures the corresponding time evolution and total mass. (C) 2015 AIP Publishing LLC.
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  • Ejecta source model based on the nonlinear Richtmyer-Meshkov instability

    Dimonte, Guy   Terrones, Guillermo   Cherne, F. J.   Ramaprabhu, P.  

    We describe a simple algebraic model for the particulate spray that is ejected from a shocked metal surface based on the nonlinear evolution of the Richtmyer-Meshkov instability (RMI). The RMI is a shock-driven hydrodynamic instability at a material interface in which the dense and tenuous fluids penetrate each other as spikes and bubbles, respectively. In our model, the ejecta areal density is determined by the product of the post-shock metal density and the saturated bubble amplitude, which depends on both the amplitude and wavelength of the initial surface imperfections of the metal. The maximum ejecta velocity is determined by the ever-growing spikes, which are accelerated relative to the RMI growth rate by the spatial harmonics that sharpen them. The model is formulated to fit new hydrodynamics and molecular dynamics simulations of the RMI and validated by existing ejecta experiments over a wide range of material properties, shock strengths, and surface perturbations. The results are also contrasted with existing ejecta source models. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773575]
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  • THE STUDY OF HIGH-SPEED SURFACE DYNAMICS USING A PULSED PROTON BEAM

    Buttler, W. T.   Oro, D. M.   Preston, D. L.   Mikaelian, K. O.   Cherne, F. J.   Hixson, R. S.   Mariam, F. G.   Morris, C.   Stone, J. B.   Terrones, G.   Tupa, D.  

    We present experimental results supporting physics based ejecta model development, where we assume ejecta form as a special limiting case of a Richtmyer-Meshkov (RM) instability with Atwood number A = -1. We present and use data to test established RM spike and bubble growth rate theory through application of modern laser Doppler velocimetry techniques applied in a novel manner to coincidentally measure bubble and spike velocities from shocked metals. We also explore the link of ejecta formation from a solid material to its plastic flow stress at high-strain rates (10(7)/s) and high strains (700%).
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  • Measurement of the Sound Velocities behind the Shock Wave Front in Tin

    Zhernokletov, M. V.   Kovalev, A. E.   Komissarov, V. V.   Novikov, M. G.   Zocher, M. A.   Cherne, F. J.  

    Results obtained by two methods for the measurement of the sound velocity in tim samples (initial density of 7.28 g/cm(3) and impurities less than 0.085%) are presented. In the range of pressures from 30 to 150 GPa, the sound velocity is determined by the overtake method with the use of indicator liquids. The luminescence of the indicator liquids is detected by photodiode-based optical gauges. At shock compression pressures of 5-18 GPa, the sound velocity in tin is measured by the counter release method with the use of manganin-based gauges. The experimental data are compared with numerical predictions and results of other authors. The boundaries of the tin melting region on the shock adiabat are found.
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  • Study of cerium phase transitions in shock wave experiments

    Zhernokletov, M. V.   Kovalev, A. E.   Komissarov, V. V.   Novikov, M. G.   Zocher, M. A.   Cherne, F. J.  

    Cerium has a complex phase diagram that is explained by the presence of structural phase transitions. Experiments to measure the sound velocities in cerium by two methods were carried out to determine the onset of cerium melting on the Hugoniot. In the pressure range 4-37 GPa, the sound velocity in cerium samples was measured by the counter release method using manganin-based piezoresistive gauges. In the pressure range 35-140 GPa, the sound velocity in cerium was measured by the overtaking release method using carbogal and tetrachloromethane indicator liquids. The samples were loaded with plane shock wave generators using powerful explosive charges. The onset of cerium melting on the Hugoniot at a pressure of about 13 GPa has been ascertained from the measured elastic longitudinal and bulk sound velocities.
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  • Dynamic compression of cerium in the low-pressure gamma-alpha region of the phase diagram

    Jensen, B. J.   Cherne, F. J.  

    Plate impact experiments were performed to examine the dynamic response of cerium for loading paths that span the well known gamma - alpha phase transition. The anomalous nature of the gamma-phase and the large volume collapse at the gamma - alpha boundary resulted in a ramp-wave followed by a shock jump for shock loading. This structured wave provided a convenient means for locating the phase boundary and determining the volume collapse at the transition. Experiments using a preheat capability were performed to obtain equation-of-state data, to locate and determine the volume compression along the phase boundary, and to determine the location of the critical point. Experimental results show that the ramp-wave peak increased with the initial sample temperature consistent with an increase in the transition stress while the magnitude of the shock jump decreased. The data were analyzed to determine the volume compression along the boundary pointing to a critical point at 1.648 +/- 0.075 GPa. Additional experiments using a shock-release configuration were used to obtain data during release. All data were in good agreement with calculations from a multiphase equation-of-state that treats the gamma and alpha phases as a binary alloy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4732126]
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  • Predicted transport properties of liquid plutonium

    Cherne, F. J.   Baskes, M. I.   Holian, B. L.  

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  • Calculations of the Structure and Properties of Rapidly Quenched Ni/Zr Alloys

    Cherne, F. J.   Baskes, M. I.   Schwarz, R. B.   Srinivasan, S. G.  

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  • Properties of liquid nickel: A critical comparison of EAM and MEAM calculations

    Cherne, F. J.   Baskes, M. I.   Deymier, P. A.  

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  • J. F. ADE AJAYI, 1929–2014

    ADEBOYE   OLUFUNKE  

    ****
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