Publications

In this report we present a model to explain the size-dependent shapes of lead nano-precipitates in aluminum. Size-dependent shape transitions, frequently observed at nanolength scales, are commonly attributed to edge energy effects. This report resolves an ambiguity in the definition and calculation of edge energies and presents an atomistic calculation of edge energies for free clusters. We also present a theory for size-dependent shapes of Pb nanoprecipitates in Al, introducing the concept of ''magic-shapes'' defined as precipitate shapes having near zero elastic strains when inserted into similarly shaped voids in the Al matrix. An algorithm for constructing a complete set of magic-shapes is presented. The experimental observations are explained by elastic strain energies and interfacial energies; edge energies play a negligible role. We replicate the experimental observations by selecting precipitates having magic-shapes and interfacial energies less than a cutoff value.

Effective elastic properties for carbon nanotube reinforced composites are obtained through a variety of micromechanics techniques. Using the in-plane elastic properties of graphene, the effective properties of carbon nanotubes are calculated utilizing a composite cylinders micromechanics technique as a first step in a two-step process. These effective properties are then used in the self-consistent and Mori-Tanaka methods to obtain effective elastic properties of composites consisting of aligned single or multi-walled carbon nanotubes embedded in a polymer matrix. Effective composite properties from these averaging methods are compared to a direct composite cylinders approach extended from the work of Hashin and Rosen (1964) and Christensen and Lo (1979). Comparisons with finite element simulations are also performed. The effects of an interphase layer between the nanotubes and the polymer matrix as result of functionalization is also investigated using a multi-layer composite cylinders approach. Finally, the modeling of the clustering of nanotubes into bundles due to interatomic forces is accomplished herein using a tessellation method in conjunction with a multi-phase Mori-Tanaka technique. In addition to aligned nanotube composites, modeling of the effective elastic properties of randomly dispersed nanotubes into a matrix is performed using the Mori-Tanaka method, and comparisons with experimental data are made. Computational micromechanical analysis of high-stiffness hollow fiber nanocomposites is performed using the finite element method. The high-stiffness hollow fibers are modeled either directly as isotropic hollow tubes or equivalent transversely isotropic effective solid cylinders with properties computed using a micromechanics based composite cylinders method. Using a representative volume element for clustered high-stiffness hollow fibers embedded in a compliant matrix with the appropriate periodic boundary conditions, the effective elastic properties are obtained from the finite element results. These effective elastic properties are compared to approximate analytical results found using micromechanics methods. The effects of an interphase layer between the high-stiffness hollow fibers and matrix to simulate imperfect load transfer and/or functionalization of the hollow fibers is also investigated and compared to a multi-layer composite cylinders approach. Finally the combined effects of clustering with fiber-matrix interphase regions are studied. The parametric studies performed herein were motivated by and used properties for single-walled carbon nanotubes embedded in an epoxy matrix, and as such are intended to serve as a guide for continuum level representations of such nanocomposites in a multi-scale modeling approach.

Abstract not provided.

Technical assessment and remodeling of existing data indicates that the Richton salt dome, located in southeastern Mississippi, appears to be a suitable site for expansion of the U.S. Strategic Petroleum Reserve. The maximum area of salt is approximately 7 square miles, at a subsurface elevation of about -2000 ft, near the top of the salt stock. Approximately 5.8 square miles of this appears suitable for cavern development, because of restrictions imposed by modeled shallow salt overhang along several sides of the dome. The detailed geometry of the overhang currently is only poorly understood. However, the large areal extent of the Richton salt mass suggests that significant design flexibility exists for a 160-million-barrel storage facility consisting of 16 ten-million-barrel caverns. The dome itself is prominently elongated from northwest to southeast. The salt stock appears to consist of two major spine features, separated by a likely boundary shear zone trending from southwest to northeast. The dome decreases in areal extent with depth, because of salt flanks that appear to dip inward at 70-80 degrees. Caprock is present at depths as shallow as 274 ft, and the shallowest salt is documented at -425 ft. A large number of existing two-dimensional seismic profiles have been acquired crossing, and in the vicinity of, the Richton salt dome. At least selected seismic profiles should be acquired, examined, potentially reprocessed, and interpreted in an effort to understand the limitations imposed by the apparent salt overhang, should the Richton site be selected for actual expansion of the Reserve.

This report summarizes the results of an effort to establish a framework for assigning and communicating technology readiness levels (TRLs) for the modeling and simulation (ModSim) capabilities at Sandia National Laboratories. This effort was undertaken as a special assignment for the Weapon Simulation and Computing (WSC) program office led by Art Hale, and lasted from January to September 2006. This report summarizes the results, conclusions, and recommendations, and is intended to help guide the program office in their decisions about the future direction of this work. The work was broken out into several distinct phases, starting with establishing the scope and definition of the assignment. These are characterized in a set of key assertions provided in the body of this report. Fundamentally, the assignment involved establishing an intellectual framework for TRL assignments to Sandia's modeling and simulation capabilities, including the development and testing of a process to conduct the assignments. To that end, we proposed a methodology for both assigning and understanding the TRLs, and outlined some of the restrictions that need to be placed on this process and the expected use of the result. One of the first assumptions we overturned was the notion of a ''static'' TRL--rather we concluded that problem context was essential in any TRL assignment, and that leads to dynamic results (i.e., a ModSim tool's readiness level depends on how it is used, and by whom). While we leveraged the classic TRL results from NASA, DoD, and Sandia's NW program, we came up with a substantially revised version of the TRL definitions, maintaining consistency with the classic level definitions and the Predictive Capability Maturity Model (PCMM) approach. In fact, we substantially leveraged the foundation the PCMM team provided, and augmented that as needed. Given the modeling and simulation TRL definitions and our proposed assignment methodology, we conducted four ''field trials'' to examine how this would work in practice. The results varied substantially, but did indicate that establishing the capability dependencies and making the TRL assignments was manageable and not particularly time consuming. The key differences arose in perceptions of how this information might be used, and what value it would have (opinions ranged from negative to positive value). The use cases and field trial results are included in this report. Taken together, the results suggest that we can make reasonably reliable TRL assignments, but that using those without the context of the information that led to those results (i.e., examining the measures suggested by the PCMM table, and extended for ModSim TRL purposes) produces an oversimplified result--that is, you cannot really boil things down to just a scalar value without losing critical information.

Experimental data for material plasticity and failure model calibration and validation were obtained from 304L stainless steel. Model calibration data were taken from smooth tension, notched tension, and compression tests. Model validation data were provided from experiments using thin-walled tube specimens subjected to path dependent combinations of internal pressure, extension, and torsion.

Sandia National Laboratories has developed high-energy all-solid-state UV sources for use in laboratory tests of the feasibility of satellite-based ozone DIAL. These sources generate 320 nm light by sum-frequency mixing the 532 nm second harmonic of an Nd:YAG laser with the 803 nm signal light derived from a self-injection-seeded image-rotating optical parametric oscillator (OPO). The OPO cavity utilizes the RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle. Two configurations were developed, one using extra-cavity sum-frequency mixing, where the sum-frequency-generation (SFG) crystal is outside the OPO cavity, and the other intra-cavity mixing, where the SFG crystal is placed inside the OPO cavity. Our goal was to obtain 200 mJ, 10 ns duration, 320 nm pulses at 10 Hz with near-IR to UV (1064 nm to 320 nm) optical conversion efficiency of 25%. To date we've obtained 190 mJ at 320 nm using extra-cavity SFG with 21% efficiency, and > 140 mJ by intra-cavity SFG with efficiency approaching 24%. While these results are encouraging, we've determined our conversion efficiency can be enhanced by replacing self-seeding at the signal wavelength of 803 nm with pulsed idler seeding at 1576 nm. By switching to idler seeding and increasing the OPO cavity dimensions to accommodate flat-top beams with diameters up to 10 mm, we expect to generate UV energies approaching 300 mJ with optical conversion efficiency approaching 25%. While our technology was originally designed to obtain high pulse energies, it can also be used to generate low-energy UV pulses with high efficiency. Numerical simulations using an idler-seeded intra-cavity SFG RISTRA OPO scaled to half its nominal dimensions yielded 560 μJ of 320 nm light from 2 mJ of 532 nm pump using an idler-seed energy of 100 μJ.

This paper reports on a novel approach to atmospheric cloud segmentation from a space based multi-spectral pushbroom satellite system. The satellite collects 15 spectral bands ranging from visible, 0.45 urn, to long wave in fared (IR), 10.7um. The images are radiometrically calibrated and have ground sample distances (GSD) of 5 meters for visible to very near IR bands and a GSD of 20 meters for near IR to long wave IR. The algorithm consists of a hybrid-classification system in the sense that supervised and unsupervised networks are used in conjunction. For performance evaluation, a series of numerical comparisons to human derived cloud borders were performed. A set of 33 scenes were selected to represent various climate zones with different land cover from around the world. The algorithm consisted of the following. Band separation was performed to find the band combinations which form significant separation between cloud and background classes. The potential bands are fed into a K-Means clustering algorithm in order to identify areas in the image which have similar centroids. Each cluster is then compared to the cloud and background prototypes using the Jeffries-Matusita distance. A minimum distance is found and each unknown cluster is assigned to their appropriate prototype. A classification rate of 88% was found when using one short wave IR band and one midwave IR band. Past investigators have reported segmentation accuracies ranging from 67% to 80%, many of which require human intervention. A sensitivity of 75% and specificity of 90% were reported as well.

In this letter, we examine the effects of discrete mobile dislocations on spinodal decomposition kinetics in lattice mismatched binary alloys. By employing a novel continuum model, we demonstrate that the effects of dislocation mobility on domain coarsening kinetics can be expressed in a unified manner through a scaling function, describing a crossover from t12 to t13 behavior. © 2005 American Institute of Physics.

During the transportation of spent nuclear fuel by truck, the possibility exists that a train could run into the spent fuel cask at a grade crossing. Sandia National Laboratories has conducted a numerical study to assess the possibility of cask breach or material release in the event of a high-speed, broadside locomotive collision. A numerical approach has the advantage over conducting a physical test as was done in the 1970s [1] in that varying parameters can be examined. For example, one of the criticisms of the 1970s test was the height of the cask. In the test, the centerline of the cask was above the main frame-rails of the locomotive. In this study the position of the cask with respect to the locomotive was varied. The response of the cask and trailer in different collision scenarios was modeled numerically with LS-DYNA [2]. The simulations were performed as a collaborative endeavor between Sandia National Laboratories (SNL), Applied Research Associates, Inc. (ARA) and Foster-Miller, Inc (FMI). ARA developed the GA-4 Spent Fuel Cask and Cask Transporter models described in this report. These models were then combined with two existing FMI heavy freight locomotive finite element models to create the overall simulation scenarios. The modeling effort, results, and conclusions are presented in this paper. Copyright © 2005 by ASME.

The physics of ballistic penetration mechanics is of great interest in penetrator and counter-measure design. The phenomenology associated with these events can be quite complex and a significant number of studies have been conducted ranging from purely experimental to 'engineering' models based on empirical and/or analytical descriptions to fully-coupled penetrator/target, thermo-mechanical numerical simulations. Until recently, however, there appears to be a paucity of numerical studies considering 'non-ideal' impacts [1]. The goal of this work is to demonstrate the SHISM algorithm implemented in the ALEGRA Multi-Material ALE (Arbitrary Lagrangian Eulerian) code [13]. The SHISM algorithm models the three-dimensional continuum solid mechanics response of the target and penetrator in a fully coupled manner. This capability allows for the study of 'non-ideal' impacts (e.g. pitch, yaw and/or obliquity of the target/penetrator pair). In this work predictions using the SHISM algorithm are compared to previously published experimental results for selected ideal and non-ideal impacts of metal penetrator-target pairs. These results show good agreement between predicted and measured maximum depth-of-penetration, DOP, for ogive-nose penetrators with striking velocities in the 0.5 to 1.5 km/s range. Ideal impact simulations demonstrate convergence in predicted DOP for the velocity range considered. A theory is advanced to explain disagreement between predicted and measured DOP at higher striking velocities. This theory postulates uncertainties in angle-of-attack for the observed discrepancies. It is noted that material models and associated parameters used here, were unmodified from those in the literature. Hence, no tuning of models was performed to match experimental data. Copyright © 2005 by ASME.

Kevlar materials make excellent body armor due to their fabric-like flexibility and ultra-high tensile strength. Carbon composites are made up from many layers of carbon AS-4 material impregnated with epoxy. Fiber orientation is bidirectional, orientated at 0° and 90°. They also have ultra-high tensile strength but can be made into relatively hard armor pieces. Once many layers are cut and assembled they can be ergonomicically shaped in a mold during the heated curing process. Kevlar and carbon composites can be used together to produce light and effective body armor. This paper will focus on computer analysis and laboratory testing of a Kevlar/carbon composite cross-section proposed for body armor development. The carbon composite is inserted between layers of Kevlar. The computer analysis was performed with a Lagrangian transversely Isotropic material model for both the Kevlar and Carbon Composite. The computer code employed is AUTODYN. Both the computer analysis and laboratory testing utilized different fragments sizes of hardened steel impacting on the armor cross-section. The steel fragments are right-circular cylinders. Laboratory testing was undertaken by firing various sizes of hardened steel fragments at square test coupons of Kevlar layers and heat cured carbon composites. The V50 velocity for the various fragment sizes was determined from the testing. This V50 data can be used to compare the body armor design with other previously designed armor systems. AUTODYN [1] computer simulations of the fragment impacts were compared to the experimental results and used to evaluate and guide the overall design process. This paper will include the detailed transversely isotropic computer simulations of the Kevlar/carbon composite cross-section as well as the experimental results and a comparison between the two. Conclusions will be drawn about the design process and the validity of current computer modeling methods for Kevlar and carbon composites. Copyright © 2005 by ASME.

For the simple geometries of Couette and Poiseuille flows, the velocity profile maintains a similar shape from continuum to free molecular flow. Therefore, modifications to the fluid viscosity and slip boundary conditions can improve the continuum based Navier-Stokes solution in the non-continuum non-equilibrium regime. In this investigation, the optimal modifications are found by a linear least-squares fit of the Navier-Stokes solution to the non-equilibrium solution obtained using the direct simulation Monte Carlo (DSMC) method. Models are then constructed for the Knudsen number dependence of the viscosity correction and the slip model from a database of DSMC solutions for Couette and Poiseuille flows of argon and nitrogen gas, with Knudsen numbers ranging from 0.01 to 10. Finally, the accuracy of the models is measured for non-equilibrium cases both in and outside the DSMC database. Flows outside the database include: combined Couette and Poiseuille flow, partial wall accommodation, helium gas, and non-zero convective acceleration. The models reproduce the velocity profiles in the DSMC database within an L2 error norm of 3% for Couette flows and 7% for Poiseuille flows. However, the errors in the model predictions outside the database are up to five times larger. Copyright © 2005 John Wiley & Sons, Ltd.

We describe the design and operation of a long-working-distance, incoherent light interference microscope that has been developed to address the growing demand for new microsystem characterization tools. The design of the new microscope is similar to that of a Linnik interference microscope and thus preserves the full working distance of the long-working-distance objectives utilized. However, in contrast to a traditional Linnik microscope, the new microscope does not rely on the use of matched objectives in the sample and the reference arms of the interferometer. An adjustable optical configuration has been devised that allows the total optical path length, wavefront curvature, and dispersion of the reference arm to be matched to the sample arm of the interferometer. The reference arm configuration can be adjusted to provide matching for 5×, 10×, and 20× long-working-distance objectives in the sample arm. In addition to retaining the full working distance of the sample arm objectives, the new design allows interference images to be acquired in situations in which intervening windows are necessary, such as occur with packaged microsystems, microfluidic devices, and cryogenic, vacuum, or environmental chamber studies of microsystem performance. The interference microscope is compatible with phase-shifting interferometry, vertical scanning interferometry, and stroboscopic measurement of dynamic processes. © 2005 Optical Society of America.

The effect of particle diameter on downward co-current gas-liquid flow through a fixed bed of particles confined within a cylindrical column is investigated. Several hydrodynamic regimes that depend strongly on the properties of the gas stream, the liquid stream, and the packed particle bed are known to exist within these systems. This experimental study focuses on characterizing the effect of wall confinement on these hydrodynamic regimes as the diameter d of the spherical particles becomes comparable to the column diameter D (or D/d becomes order-unity). The packed bed consists of polished, solid, spherical, monodisperse particles (beads) with mean diameter in the range of 0.64-2.54 cm. These diameters yield D/d values between 15 and 3.75, so this range overlaps and extends the previously investigated range for two-phase flow. Measurements of the pressure drop across the bed and across the pulses are obtained for varying gas and liquid flow rates. Copyright © 2005 by ASME.

On a single-pulse basis, the tungsten armor for the chamber walls in a laser inertial fusion energy power plant must withstand X-ray fluences of 0.4-1.2 J/cm2 with almost no mass loss, and preferably no surface changes. We have exposed preheated tungsten samples to 0.27 and 0.9 J/cm 2 X-ray fluence from the Z accelerator at Sandia National Laboratories to determine the single-shot X-ray damage threshold. Earlier focused ion beam analysis has shown that rolled powdered metal formed tungsten and tungsten alloys, will melt when exposed to 2.3 J/cm2 on Z, but not at 1.3 J/cm2. Three forms of tungsten - single-crystal (SING), chemical-vapor-deposited (CVD), and rolled powdered metal (PWM) - were exposed to fluence levels of 0.9 J/cm2 without any apparent melting. However, the CVD and PWM sample surfaces were rougher after exposure than the SING sample, which was not roughened. BUCKY (1D) calculations show a threshold of 0.5 J/cm2 for melting on Z. The present experiments indicate no melting but limited surface changes occur with polycrystalline samples (PWM and CVD) at 0.9 J/cm2 and no surface changes other than debris for samples at 0.27 J/cm2. © 2005 Elsevier B.V. All rights reserved.

Through the use of simultaneous thermogravimetry modulated beam mass spectrometry, optical microscopy, hot-stage time-lapsed microscopy, and scanning electron microscopy measurements, the physical and chemical processes that control the thermal decomposition of 1,3,5-trinitrohexahydro-s-triazine (RDX) below its melting point (160-189°C) have been identified. Two gas-phase reactions of RDX are predominant during the early stages of an experiment. One involves the loss of HONO and HNO and leads to the formation of H 2O, NO, NO 2, and oxy-s-triazine (OST) or s-triazine. The other involves the reaction of NO with RDX to form NO 2 and 1-nitroso-3,5-dinitrohexahydro-s-triazine (ONDNTA), which subsequently decompose's to form a set of products of which CH 2O and N 2O are the most abundant. Products from the gas-phase RDX decomposition reactions, such as ONDNTA, deposit on the surface of the RDX particles and lead to the development of a new set of reaction pathways that occur on the surface of the RDX particles. The initial surface reactions occur on surfaces of those RDX particles in the sample that can accumulate the greatest amount of products from the gas-phase reactions. Initial surface reactions are characterized by the formation of islands of reactivity on the RDX surface and lead to the development of an orange-colored nonvolatile residue (NVR) film on the surface of the RDX particles. The NVR film is most likely formed via the decomposition of ONDNTA on the surface of the RDX particles. The NVR film is a nonstoichiometric and dynamic material, which reacts directly with RDX and ONDNTA, and is composed of remnants from RDX and ONDNTA molecules that have reacted with the NVR. Reactions involving the NVR become dominant during the later stage of the decomposition process. The NVR reacts with RDX to form ONDNTA via abstraction of an oxygen atom from an NO 2 group. ONDNTA may undergo rapid loss of N 2 and NO 2 with the remaining portion of the molecule being incorporated into the dynamic NVR. The dynamic NVR also decomposes and leads to the formation of H 2O, CH 2O, N 2O, NH 2CHO, (CH 3) 2NCHO, (CH 3) 2NNO, C 2H 2N 2O, and (CH 3) 3N or CH 3NCH 2CH 3. The competition between reaction of the dynamic NVR with RDX and its own thermal decomposition manifests itself in a rapid increase in the rate of evolution of the NVR decomposition products as the amount of RDX remaining in the sample nears depletion. The reactions between the NVR film and RDX on the surface of the RDX particles leads to a localized environment that creates a layer of molten RDX on the surface of the particles where reactions associated with the liquid-phase decomposition of RDX may occur. The combination of these reaction processes leads to an acceleration of the reaction rate in the later stage of the decomposition process and creates an apparent reaction rate behavior that has been referred to as autocatalytic in many previous studies of RDX decomposition. A reaction scheme summarizing the reaction pathways that contribute to the decomposition of RDX below its melting point is presented. © 2005 American Chemical Society.

Inorganic nanoclusters dispersed in organic matrices are of importance to a number of emerging technologies. However, obtaining useful properties from such organic-inorganic composites often requires high concentrations of well-dispersed nanoclusters. In order to achieve this goal the chemistry of the particle surface and the matrix must be closely matched. This is based on the premise of minimization of the interfacial free energy; an excess of free energy will cause phase separation and ultimately aggregation. Thus, the optimal system is one in which the nanoclusters are stabilized by the same molecules that make up the encapsulant. Yet, the organic matrix is typically chosen for its bulk properties, and therefore may not be amenable to chemical modification. Also, the organic-inorganic interface is often critical to establishing and maintaining the desired nanocluster (and hence composite) properties, placing further constraints on proposed chemical modification. For these reasons we have adopted the use of aminefunctionalized trimethoxysilanes (ormosils) as an optical grade encapsulant. In this work, we demonstrate that ormosils can produce beneficial optical effects that are derived from interfacial phenomena, which can be maintained throughout the encapsulation process.

A 256-site microsystem comprises 4 neural recording arrays with integrated amplification and multiplexing circuitry and an implantable spike detection ASIC. The spike detector compresses the amount of neural data by 92%, increasing the total number of channels recorded wirelessly from 25 to 312. The implantable circuitry consumes 5.4mW at 3V. ©2005 IEEE.

This paper develops and investigates iterative tensor methods for solving large-scale systems of nonlinear equations. Direct tensor methods for nonlinear equations have performed especially well on small, dense problems where the Jacobian matrix at the solution is singular or ill-conditioned, which may occur when approaching turning points, for example. This research extends direct tensor methods to large-scale problems by developing three tensor-Krylov methods that base each iteration upon a linear model augmented with a limited second-order term, which provides information lacking in a (nearly) singular Jacobian. The advantage of the new tensor-Krylov methods over existing large-scale tensor methods is their ability to solve the local tensor model to a specified accuracy, which produces a more accurate tensor step. The performance of these methods in comparison to Newton-GMRES and tensor-GMRES is explored on three Navier-Stokes fluid flow problems. The numerical results provide evidence that tensor-Krylov methods are generally more robust and more efficient than Newton-GMRES on some important and difficult problems. In addition, the results show that the new tensor-Krylov methods and tensor-GMRES each perform better in certain situations. © 2005 Society for Industrial and Applied Mathematics.

A series of field tests sponsored by Sandia National Laboratories has simultaneously demonstrated the hard-rock drilling performance of different industry-supplied drag bits as well as Sandia's new Diagnostics-While-Drilling (DWD) system, which features a novel downhole tool that monitors dynamic conditions in close proximity to the bit. Drilling with both conventional and advanced ("best effort") drag bits was conducted at the GTI Catoosa Test Facility (near Tulsa, OK) in a well-characterized lithologic column that features an extended hard-rock interval of Mississippi limestone above a layer of highly abrasive Misener sandstone and an underlying section of hard Arbuckle dolomite. Output from the DWD system was closely observed during drilling and was used to make real-time decisions for adjusting the drilling parameters. This paper summarizes penetration rate and damage results for the various drag bits, shows representative DWD display data, and illustrates the application of these data for optimizing drilling performance and avoiding trouble.

Many MEMS devices are based on polysilicon because of the current availability of surface micromachining technology. However, polysilicon is not the best choice for devices where extensive sliding and/or thermal fields are applied due to its chemical, mechanical and tribological properties. In this work, we investigated the mechanical properties of three new materials for MEMS/NEMS devices: silicon carbide (SiC) from Case Western Reserve University (CWRU), ultrananocrystalline diamond (UNCD) from Argonne National Laboratory (ANL), and hydrogen-free tetrahedral amorphous carbon (ta-C) from Sandia National Laboratories (SNL). Young's modulus, characteristic strength, fracture toughness, and theoretical strength were measured for these three materials using only one testing methodology - the Membrane Deflection Experiment (MDE) developed at Northwestern University. The measured values of Young's modulus were 430GPa, 960GPa, and 800GPa for SiC, UNCD, and ta-C, repectively. Fracture toughness measurments resulted in values of 3.2, 4.5, and 6.2 MPa×m 1/2, respectively. The strengths were found to follow a Weibull distribution but their scaling was found to be controlled by different specimen size parameters. Therefore, a cross comparison of the strengths is not fully meaningful. We instead propose to compare their theoretical strengths as determined by employing Novozhilov fracture criterion. The estimated theoretical strength for SiC is 10.6GPa at a characteristic length of 58nm, for UNCD is 18.6GPa at a characteristic length of 37nm, and for ta-C is 25.4GPa at a characteristic length of 38nm. The techniques used to obtained these results as well as microscopic fractographic analyses are summarized in the article. We also highlight the importance of characterizing mechanical properties of MEMS materials by means of only one simple and accurate experimental technique.

Catastrophic failure of a Reusable Launch Vehicle (RLV) during launch poses a significant engineering problem in the context of crew escape. The explosive hazard potential of the RLV changes during the various phases of the launch. The hazard potential in the on-pad environment is characterized by release and formation of a gas phase mixture in an oxidizer rich environment, while the hazard during the in-flight phase is dominated by the boundary layer and wake flow formed around the vehicle and the interaction with the exhaust gas plume. In order to address more effectively crew escape in these explosive environments a computational analysis program was undertaken by Lockheed Martin, funded by NASA JSC, with simulations and analyses completed by Southwest Research Institute and Sandia National Laboratories. This paper presents then the details of the methodology used in this analysis, results of the study, and important conclusions that came out of the study. Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

A gas-cooled reactor may be coupled directly to turbomachinery to form a closed-Brayton-cycle (CBC) system in which the CBC working fluid serves as the reactor coolant. Such a system has the potential to be a very simple and robust space-reactor power system. Gas-cooled reactors have been built and operated in the past, but very few have been coupled directly to the turbomachinery in this fashion. In this paper we describe the option for testing such a system with a small reactor and turbomachinery at Sandia National Laboratories. Sandia currently operates the Annular Core Research Reactor (ACRR) at steady-state powers up to 4 MW and has an adjacent facility with heavy shielding in which another reactor recently operated. Sandia also has a closed-Brayton-Cycle test bed with a converted commercial turbomachinery unit that is rated for up to 30 kWe of power. It is proposed to construct a small experimental gas-cooled reactor core and attach this via ducting to the CBC turbomachinery for cooling and electricity production. Calculations suggest that such a unit could produce about 20 kWe, which would be a good power level for initial surface power units on the Moon or Mars. The intent of this experiment is to demonstrate the stable start-up and operation of such a system. Of particular interest is the effect of a negative temperature power coefficient as the initially cold Brayton gas passes through the core during startup or power changes. Sandia's dynamic model for such a system would be compared with the performance data. This paper describes the neutronics, heat transfer, and cycle dynamics of this proposed system. Safety and radiation issues are presented. The views expressed in this document are those of the author and do not necessarily reflect agreement by the government. © 2005 American Institute of Physics.

Experimental modal analysis (EMA) was carried out on a micro-machined acceleration switch to characterize the motions of the device as fabricated and to compare this with analytical results for the nominal design. Finite element analysis (FEA) of the nominal design was used for this comparison. The acceleration switch was a single-crystal silicon disc supported by four fork-shaped springs. We shook the base of the die with step sine type excitation. A Laser Doppler Velocimeter (LDV) in conjunction with a microscope was used to measure the velocities of the die at several points. The desired first three modes of the structure were identified. The fundamental natural frequency that we measured in this experiment gives an estimate of the actuation g-level for the specified stroke. The fundamental resonance and actuation g-level results from the EMA and the FEA showed large variations. The discrepancy prompted thorough dimensional measurement of the acceleration switch, which revealed discrepancies between the nominal design and tested component.