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Job Openings 2009 An Energy-Conserving Two-Temperature Model of Radiation Damage in Single-Component and Binary Lennard-Jones Crystals
The 2008 International Workshop on RSPt and the Full Potential Linear Muffin-Tin Orbital Method August 25-29, 2008, researchers from SNL, LANL, UNM, Sweden, and the Netherlands, gathered in downtown Albuquerque at the Hyatt Regency for the “2008 International Workshop on RSPt and the Full Potential Linear Muffin-Tin Orbital (FP LMTO) Method”, sponsored by the Sandia National Laboratories’ Computer Science Research Institute (CSRI) and organized by Ann E Mattsson (1435). This conference was the successor to the successful FP LMTO workshop held in Belem, Brazil in November 2007.
Over the past two years, we have created a new software tool (Aleph) for the simulation of low density plasmas. Aleph is a direct simulation Monte Carlo (DSMC) plus particle-in-cell (PIC) code capable of particle representations of plasmas within arbitrary 3D geometries defined by unstructured tetrahedral meshes. We have built the software upon the foundation of existing software modules, many from the Trilinos library, such that Aleph includes dynamic load balancing and hence performs and scales well on available high performance compute resources. Our team has built Aleph as part of a larger effort aimed at neutron tube simulation, with contributions from many more people. It is anticipated that use of Aleph for neutron tube simulation will result in substantial cost savings in design and manufacturing. Secondary application areas with impact on Sandia’s mission include modeling of ion beams, arcs, and hypersonic upper-atmosphere flows. Arcs can act as sources for plasmas and ion beams, so complete modeling of such an ion beam device should include modeling of the arc discharge phenomenon. To the best of our knowledge, we are the first research group to succeed in creating a particle simulation of an arc discharge. We have built a simulation model of a hot anode vacuum arc (HAVA) that includes the key physical phenomena: high energy electrons originating from the cathode bombard the titanium anode; the anode heats up and emits neutral titanium atoms; some of the neutrals are then ionized by the incoming electrons; production of low-energy secondary electrons makes additional electron production more likely; an avalanche of electron production dramatically boosts the anode-to-cathode current (see Figure). The hot anode in our arc discharge model is simulated as a 3D heat conduction problem, with electron bombardment acting as the heat source on the surface, and the transfer of energy through the anode tracked dynamically. A vapor pressure equation is used to compute the rate of titanium particle vaporization into the void as a function of the anode surface temperature. Each computational particle carries a weighting, which signifies how many real particles each one represents. In order to reduce computational cost, this weighting can vary spatially, temporally, and according to the species type, but must be properly considered when computing the particle-induced electrostatic fields and when performing particle collisions and reactions. In addition to the variety of particle physics capabilities that are required for our HAVA model, a range of diagnostic features were also needed. Aleph features a rich array of such diagnostics for easy output of temporally- and spatially-averaged quantities of interest, such as density distributions of the various species and particle fluxes or currents through surfaces. (Contact: Paul Crozier)
Arc simulation image at t = 0.499 ms showing virtual cathode (blue), anode (red), titanium neutrals (orange), titanium ions (white), and electrons (green). Titanium ions are produced in chain reaction bursts as evidenced by the highly concentrated pockets of white dots.
Mesh-Based Simulation of Complex Material Microstructure Return to top
The interest is that the peridynamic equation of motion can be expressed in terms of this stress tensor, and the result is formally identical to the Cauchy equation of motion. This equivalence is valuable in establishing a relationship with the classical model of continuum mechanics. We also establish a variational characterization of this stress tensor field and so show uniqueness in the function space compatible with finite element approximations. A force flux, or peridynamic traction vector, can also be defined so that peridynamics can be coupled to classical continuum mechanics discretized by the finite element method. Coupling peridynamics to finite element method will allow for dramatically more efficient simulations where peridynamics is used only in regions near failure while the less costly finite element methods are used elsewhere.
Aidan Thompson (1435) has used large scale molecular dynamics (MD) simulation in a preliminary study of shock waves propagating through single crystal cadmium selenide. The high pressure generated by the shock wave causes the crystal to transform from a hexagonal wurtzite structure to a cubic rocksalt structure. In a previous experimental study by Marcus Knudson (1646) the transformation kinetics of cadmium sulfide were observed to be strongly dependent on the direction of the shock wave relative to the crystal orientation and also on the shock strength. Cadmium sulfide and cadmium selenide are closely related materials with identical crystal structures. The MD simulations have shown in detail how the atoms in these materials rearrange in different ways depending on the orientation and strength of the shock wave. The simulations were performed by impacting a long narrow block of crystal into a perfectly rigid wall at speeds ranging from 0.1 to 1 km/s. The collision causes a supersonic shock wave to propagate back through the crystal from the impact surface. The longitudinal pressure that develops in the shocked crystal increases quadratically with impact velocity. The minimum pressure required to transform the wurtzite into rocksalt was found to be higher along the a-axis of the wurtzite crystal than the c-axis. Below this transformation pressure, the wurtzite remains in an elastically compressed state for the duration of the simulation. Above the transformation pressure, the pathway to the rocksalt phase was different for the two different shock directions. For shocks along the c-direction, the rocksalt phase formed directly from the elastically compressed wurtzite, with rocksalt planes forming parallel to the original wurtzite planes. This behavior is similar to what has been observed experimentally under static loading conditions. By contrast, in the a-direction, the crystal transformed first to a face-centered tetragonal (FCT) structure, which then collapsed to form regions of rocksalt that were not aligned with the wurtzite planes. This latter situation is illustrated below, which shows a closeup snapshot of a 16 GPa shock moving from left to right along the a-axis of the wurtzite phase. The atoms are colored by their local crystal structure: elastically compressed wurtzite (gray), tetragonal (red) and rocksalt (blue). The tetragonal phase remains interspersed with rocksalt in the transformed material. These simulations provide a unique insight into the detailed atomic mechanisms by which phase transformations occur in materials under high strain rate uniaxial loading conditions. This study, supported by Campaign 2, was undertaken as part of efforts led by Clint Hall (1646) to establish at SNL a capability to treat the full science of dynamic material response: experiments, theory, and modeling and simulation. The preliminary results from Aidan’s work, performed in collaboration with Marcus Knudson, were recently presented at JOWOG32 Materials, held at LANL June 18-22, and at the APS topical meeting on Shockwave Compression in Condensed Matter, held on the Kohala Coast of Hawaii June 25-29.
(Contact: Aidan Thompson)
Staff from Department 1435, led by Stewart Silling, and Boeing Company’s Phantom Works applied Silling’s Peridynamics theory to predict the damage to laminated composite panels due to impact by hail. In this work, supported under Sandia's Umbrella CRADA with the Boeing Company, many different possible combinations of lamina properties were modeled with Sandia's EMU code, which is an implementation of the Peridynamics theory. These computational results were subjected to a statistical analysis that helped reveal which composite layups would provide maximum damage tolerance at reduced weight for fuselage materials on future aircraft such as the Boeing 787, now under development. A typical damage prediction is illustrated in Figure 1, which shows the delaminations within a laminated composite due to impact by a spherical hail particle. The coloring indicates separation (increasing from blue to red) between adjacent plies in the laminate. Continuum scale simulation of damage and failure has been dramatically advanced by Silling’s fundamentally novel, mesh-free approach to computational solid mechanics. In this approach, the conservation relations are cast as integral equations that make the theory inherently capable of simulating defect and crack development. The technique treats crack growth consistently with other forms of material deformation and failure. Peridynamics permits cracks to nucleate and grow spontaneously and unguided, providing a breakthrough in continuum simulation methods that allows EMU to model complex patterns of damage and fracture. The technique is finding increasing application to investigating, characterizing, and understanding fracture, penetration, blast, and fragmentation phenomena. It has demonstrated an amazing level of verisimilitude, reproducing well known dynamic fracture phenomena in a predictive manner. In an active collaboration, Boeing Phantom Works is supporting on-going development of the EMU code and Peridynamics theory, along with applications of EMU simulations to investigations of damage of composite structures.
Figure 1. Delamination pattern predicted by EMU.
(Contact: John Aidun ) 2006 Simulation Study of Head Impact Leading to Traumatic Brain Injury Traumatic brain injury, or TBI, is an unfortunate consequence of many civilian accident and military related scenarios. Examples include head impact sustained in sports activities and automobile accidents as well as blast wave loading from improvised explosive devices (IEDs). Depending on the extent of the damage, TBI is associated with a loss of the functional capability of the brain to perform cognitive and memory tasks, process information, and a variety motor and coordination problems. In many instances, the person involved in the event will not experience the full loss of brain function until days or weeks after the event has occurred. This suggests the existence of threshold levels and/or conditions of mechanical stress experienced by the brain that, if exceeded, lead to evolving symptoms of TBI in the days or weeks following an accident. To avoid a trial-and-error approach involving large-scale medical testing of laboratory animals to study various scenarios leading to TBI, we have developed numerical simulation models of the human head to study various impact and blast wave conditions that lead to the onset of TBI. To accomplish this task, we have recently established a collaborative effort between Sandia researchers and the Mental Illness and Neuroscience Discovery (MIND) Institute, at the University of New Mexico. This collaboration permits us to create accurate models of the various tissues and geometries of the human head as well as to conduct simulations of head impact in order to establish a correlation between the incipient levels and durations of stress and strain energy experienced by the brain and the onset of TBI. In this note, we present the results of a small study that simulates the early-time wave interactions occurring within the human head as a result of impact of an unrestrained person with the windshield of an automobile in a 30 mph head-on collision into a stationary barrier. We have conducted various simulations of this scenario over the past few years; however, the current work has been carried out with a higher fidelity head model over an extended simulation timescale. Our 3-D head model was developed by importing a segmented interpretation (displaying distinct biological materials of bone, brain, and fluid) of a CT scan from a healthy female head into the shock physics hydrocode CTH. Specific material models were created for the skull, brain, cerebral spinal fluid (CSF), and windshield glass. The simulations were run on a parallel architecture computer employing 64 processors for each simulation run. The results of the simulations demonstrate the complexities of the wave interactions that occur between the skull, brain, and CSF fluid as a result of a frontal impact with the glass windshield. These interactions lead to focused regions within the brain that experience significant levels of pressure and deviatoric (shearing) stress. In particular, the pressure waves focus roughly 30 bars of compression in the brain at the impact site (Fig. 1) and 5 bars of tension at the site opposite (contra-coup) the impact point (Fig. 2). Furthermore, our simulations predict up to 30 bars of deviatoric (shearing) stress at the interface between the brain and the ventricles that conduct the CSF fluid within the brain (Figs. 3 & 4). This interaction leads to a tearing effect on the brain tissue if the stress level is sufficiently high. The geometric complexities of the skull interior are such that there are a variety of sites that experience stress focusing, which can readily be seen in computer animations of the simulations. The significant of these results is the fact that they occur on a time scale of roughly 1-2 ms and capture the early-time wave interactions that are potentially damaging to the brain and before any coarse body motion has begun, which can lead to additional damage resulting from a “sloshing” motion of the head. The results of this study have been summarized in an article to appear in the proceedings of the 25th Army Science Conference, which will convene in November, 2006. An immediate goal of this collaborative effort is to establish a quantitative correlation between specific levels of stress/strain energy and the onset of TBI under a variety of accident conditions. This effort involves studying conditions under which accident victims experience conditions that lead to TBI, as diagnosed by medical tools such as structural and functional MRI, and conducting accurate simulations of the event. Once such correlations exist, this approach can be used to investigate mitigating strategies to minimize the conditions under which TBI occurs. Future studies are planned to investigate the occurrence of TBI as it is experienced by blast victims from improvised explosive devices (IEDs). This is a significant topic of concern for the U.S. Army and consequently, we are pursuing funding support through the Department of Defense to address this problem. October 27th Sandia Lab News Article Automated Force Field Fitting: Proof of Principle Achieved
Return to top High fidelity physics-based modeling of the electrical effects of radiation-induced defects in semiconductor devices is a major component of the QASPR project strategy for developing a robust methodology to qualify weapons systems in hostile radiation environments. Quantum density functional theory (DFT) calculations play a vital role in this strategy as many critical properties of lattice defects generated by radiation damage are not known or accessible from experiment, and must be calculated. However, conventional methods for simulating defect properties lack the accuracy needed to satisfy QASPR requirements. Peter Schultz (9235) identified the fundamental issue as the use of incorrect boundary conditions in the computational models commonly used in DFT calculations for defect systems. Over the past six months, he formulated and implemented a new, more rigorous methodology for defect simulations within DFT. This robust physics-based scheme incorporates the correct electrostatic boundary conditions, locates a fixed electronic chemical potential, and includes the bulk dielectric response. After implementing this methodology into the ASC SeqQuest DFT code, the computed formation energies and electrical defect levels for a wide variety of charged defects in silicon was calculated. The results yield remarkably accurate predictions of defect levels (<0.2 eV errors from experiment – better accuracy than might have been expected given the DFT approximation). Moreover, the method significantly reduces the computational requirements of the simulations. Use of these theoretical results in kinetic models of device response successfully filled a knowledge gap in the simulation of radiation-induced early-time transient response of electronic devices. This new methodology will be an important new capability to enable physics-based modeling within QASPR. (Contact: Peter A. Schultz) Sandia Software to Help Design New Commercial Jet
Article on Quantum DFT Calculations Accepted for Publication by "Modeling and Simulation in Materials Science and Engineering" The article by Ann, Peter Schultz (9235), Michael Desjarlais (1674), Thomas Mattsson (1674), and Kevin Leung (1116) is entitled “Designing meaningful density functional theory calculations in materials science – A primer.” Performing DFT calculations that provide useful simulations of physical phenomena and chemistry in materials (as opposed to molecules or clusters of atoms) requires careful assessment of the problem and carefully chosen manipulations of intricate computer codes. Well-constructed DFT calculations can have significant predictive capability. However, blind applications of DFT codes can give meaningless and misleading results. This article addresses issues that confront a user of DFT methods, providing practical guidance to overcoming many of the common challenges in calculation construction, and presents multiple cautionary examples of how poorly designed DFT calculations fail to give accurate predictions of material properties. At Sandia, as in the wider materials research community, quantifying the chemistry of individual atoms is becoming more critical to predicting the macroscopic response of materials. Consequently, DFT methods are playing an increasingly important role. This extends to their application to stockpile issues such as radiation effects in electronics (e.g. QASPR, ELDRS), equations of state of metals and hydrogen, including the conductivity and opacity of plasmas in the Z machine, and both neutron tube manufacture and aging. DFT is also an enabling capability for nanotechnology. Sandia has developed considerable expertise in DFT theory aimed at improving its accuracy (http://dft.sandia.gov/functionals/), in two complementary, large-scale computational codes, SeqQuest (http://dft.sandia.gov/Quest/) and Socorro (http://dft.sandia.gov/Socorro/), and
in applications in several Centers including 9200, 8700, 6100, 1800, 1600, and 1100. (Contact:
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Layout of a model polycrystal with an overlying structured mesh.
POC: Josh Robbins |
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