Publications

Samples of nuclear grade beryllium, S65C, were diffusion bonded to a copper, chrome, zirconium alloy, C18150, using hot isostatic pressing. Metallization films of titanium, copper and gold were used to promote bonding, and inhibit the formation of undesirable beryllium copper intermetallics as well as prevent oxide formation before the bonding process commenced. The samples were bonded at 580C and 100 MPa for two hours. All of the samples formed bonds that had shear strength values that approached that of the S65C base metal. HIP bonding resulted in the formation of stoichiometric intermetallic layers between the copper and titanium films including TiCu and Ti2Cu. A much thinner intermetallic layer was also observed between the titanium film and the beryllium substrate, possibly Be2Ti. Shear specimens were used to measure the strength of the interfaces with peak strengths between 127 and 191 MPa were measured. Failure was found to occur predominately along one of the TiCu intermetallic layers. Copyright © 2009 ASM International® All rights reserved.

Abstract not provided.

The mechanisms governing D loss in ErD2 films with and without a Mo diffusion barrier on kovar substrates were studied between 200 and 600 °C via in-situ Ion Beam Analysis (IBA). Significant intermixing between kovar and Er was observed above 450°C and between kovar and ErD2 above 500 °C. The D loss mechanism in ErD2 films was found to change from intermixing between kovar and ErD2 at low temperatures (< 500 °C) to thermal decomposition at higher temperatures (> 500 °C). Diffusion between kovar and ErD2 was measured isothermally at 500 and 550 °C. An activation energy of 2.1 eV and a pre-exponential factor of 0.071 cm2/s were determined. Diffusion between the kovar components and ErD2 film was inhibited by depositing a 200 nm Mo diffusion barrier between the kovar substrate and the ErD2 film. The processing of the Mo diffusion barrier was shown to impact its performance. Intermixing between the kovar / Mo / ErD2 stack becomes significant between 500 and 550 °C with a sputter deposited Mo diffusion barrier and between 550 and 600 °C for an electron-beam evaporated Mo diffusion barrier. Copyright © 2009 ASM International® All rights reserved.

The effects of hydrogen embrittlement has been extensively researched, however, relatively little research has been devoted to the effects of internal hydrogen on the fatigue strength of structural metals. This paper examines the effect of internal hydrogen on the fatigue strength of strain-hardened type 316 stainless steel in rotating beam fatigue tests. The tensile properties and high cycle fatigue life of two type 316 stainless steel alloys were studied using thermal precharging to approximate high-pressure hydrogen exposure for long times. Tensile testing was found to be consistent with previous studies using the identical environmental condition. Hydrogen precharging increased the number of cycles to failure by 5 to 10 times compared to non-charged specimens. The basic shape of the S-N curve and the apparent fatigue limit were essentially unchanged by thermal precharging with hydrogen. Copyright © 2009 ASM International® All rights reserved.

Estimation of the uncertainty in experimental modal parameters is valuable when validation of a finite element model is performed based on modal frequencies and shapes. The uncertainty in the data is needed to establish distribution functions for the comparison to computational models. A part that is typically neglected with the experimental data is to quantify the uncertainty in the estimated parameters established from the data. This uncertainty is an important piece of the puzzle in a validation exercise. If the uncertainty in the fit is not accounted for, the uncertainty estimate of the experimental data is incomplete. This paper will explore the uncertainty of the modal parameter estimates measured from the Synthesize Modes and Correlate (SMAC) algorithm using a Monte Carlo technique. The uncertainty in the parameter fit will be determined for both analytically and experimentally determined frequency response functions. It is found that the largest uncertainty in the SMAC algorithm is within the optimization step of the fitting process. © 2009 Society for Experimental Mechanics Inc.

Traditionally modal and vibration tests have been performed separately because their classical purposes require different inputs and outputs. However, motivation exists in some instances to be able to perform a modal test on a shaker table, if the boundary conditions could be accounted for appropriately. This is especially a concern for large test articles mounted on large tables because the table has flexible dynamics in the frequency range of interest for the modal test. For the past thirty years various attempts have been made to develop a method that would allow the two tests to both be conducted on a shaker table requiring only one setup. However, in most cases the table is assumed to be rigid. When the table cannot be assumed rigid the remaining approaches usually require that all six forces and all six degrees of freedom of motion at every attachment points be measured. Most approaches neglect moments and rotation measurements. Even measuring the translational forces and accelerations is rarely done. In the method employed here, the boundary condition is constrained mathematically. However, a measure of the shaker force is required. In addition, the classical mathematical constraints to produce a fixed base result are augmented in a way that alleviates the ill conditioning that almost always results when using the classical constraint equations. The two major advances here are a method to estimate the shaker force, and improved conditioning of the constrained equations. The effect of improving the conditioning is demonstrated with a modal test of hardware on a base that is not fixed. The full process is demonstrated with a random vibration test on a simple flexible horizontal slip table with a cantilevered beam mounted as the test article. A general outline of the method proceeds as follows: 1) characterize the modes of the bare shaker table attached to the shaker; 2) mount and instrument the test article; 3) attach a portable shaker to the tip of the shaker table with a force gage and measure a specific frequency response function (FRF); 4) detach the portable shaker and run the typical random vibration test; 5) calculate transmissibilities to the tip accelerometer; 6) create acceleration/force FRFs from reciprocity by multiplying the FRF in step 3 times every transmissibility; 7) extract modal parameters from FRFs; 8) finally apply augmented constraint equations with FRFs synthesized from the modal parameters and extract the fixed base modes. © 2009 Society for Experimental Mechanics Inc.

This research utilizes a method for calculating an atomic-scale deformation gradient within the framework of continuum mechanics using atomistic simulations to examine bicrystal grain boundaries subjected to shear loading. We calculate the deformation gradient, its rotation tensor from polar decomposition, and estimates of lattice curvature and vorticity for thin equilibrium bicrystal geometries deformed at low temperature. These simulations reveal pronounced deformation fields that exist in small regions surrounding the grain boundary, and demonstrate the influence of interfacial structure on mechanical behavior for the thin models investigated. Our results also show that more profound insight is gained concerning inelastic grain boundary phenomena by analyzing the deformed structures with regard to these continuum mechanical metrics.

This work explores how the high-load limits of HCCI are affected by fuel autoignition reactivity, EGR quality/composition, and EGR unmixedness for naturally aspirated conditions. This is done for PRF80 and PRF60. The experiments were conducted in a singlecylinder HCCI research engine (0.98 liters) with a CR = 14 piston installed. By operating at successively higher engine loads, five load-limiting factors were identified for these fuels: 1) Residual-NOx-induced run-away advancement of the combustion phasing, 2) EGR-NOx- induced run-away, 3) EGR-NOx/wall-heating induced run-away 4) EGR-induced oxygen deprivation, and 5) excessive partial-burn occurrence due to EGR unmixedness. The actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the EGR quality level (where high quality refers to the absence of trace species like NO, HC and CO, i.e. simulated EGR), the level of EGR unmixedness, and the selected pressurerise rate (PRR). For a reactive fuel like PRF60, large amounts of EGR are required to control the combustion phasing. Therefore, for operation with simulated EGR, the maximum IMEP becomes limited by the available oxygen. When real EGR (with trace species) is used instead of the simulated EGR, the maximum IMEP becomes limited by EGR-NOx/wall-heating induced runaway. For the moderately reactive PRF80 operated with simulated EGR, the maximum IMEP becomes limited by residual-NOx-induced run-away. Furthermore, operation with real EGR lowers the maximum steady IMEP because of EGR-NOx-induced run-away. This is similar to PRF60. Finally, the data show that EGR/fresh-gas unmixedness can lead to a substantial reduction of the maximum stable IMEP for operation with a low PRR. This happens because the EGR unmixedness causes occasional partial-burn cycles due to excessive combustion-phasing retard for cycles that induct substantially higher-thanaverage level of EGR gases.

Analytic model validation assesses the usefulness of a model for its intended purpose. Validation of the model should be based on a blind prediction of test results, so that the predictive capability is demonstrated. However, there is certainly a place in the validation process for exercising the model in correlation and calibration before making the final blind validation prediction. In many cases, the initial model deviates from a useful state for unknown or unquantified reasons. Then model correlation exercises are performed. The term "correlation" for structural dynamics comes from the initial one to one correlation of the modes of the model with modes from a modal test. But correlation really includes more than just this initial comparison. Correlation exercises can uncover unintended errors or incorrect assumptions and simplifications in the thousands of details important to model development. Calibration, on the other hand, is designed to improve estimates on specific uncertain parameters. Sensitivity analysis is examined as a method for calibration. Sensitivity analysis is an inferential process that has definite limits, and three tools for understanding those limits are provided. In some cases, calibration does not provide sufficient model improvement, and additional correlation exercises are performed after the calibration. Often correlation and calibration are most effective when applied to subsystems of the model to isolate specific errors. © 2009 Society for Experimental Mechanics Inc.

A planar temperature imaging diagnostic has been developed and applied to an investigation of naturally occurring thermal stratification in an HCCI engine. Natural thermal stratification is critical for high-load HCCI operation because it slows the combustion heat release; however, little is known about its development or distribution. A tracer-based single-line PLIF imaging technique was selected for its good precision and simplicity. Temperature-map images were derived from the PLIF images, based on the temperature sensitivity of the fluorescence signal of the toluene tracer added to the fuel. A well premixed intake charge assured that variations in fuel/air mixture did not affect the signal. Measurements were made in a single-cylinder optically accessible HCCI research engine (displacement = 0.98 liters) at a typical 1200 rpm operating condition. Since natural thermal stratification develops prior to autoignition, all measurements were made for motored operation. Calibrations were performed in-situ, by varying the intake temperature and pressure over a wide range. Although the absolute accuracy is limited by the pressure-derived temperatures used for calibration, an uncertainty analysis shows that the precision of the diagnostic for determining temperature variations at a given condition is very good. Application of the diagnostic provided temperature-map images that showed a progressive development of natural thermal stratification in the bulk gas through the latter compression stroke and early expansion strokes. Applying a PDF analysis with corrections for measurement uncertainties provided additional quantitative results. The data show a clear trend of going from virtually no stratification at 305° CA (55° bTDC), to significant inhomogeneities at TDC. Near TDC, the images show distinct hotter and colder pockets with a turbulent structure. Images were also acquired across the charge from the mid-plane to outer boundary layer at 330° CA and TDC. They show an increase in thermal stratification and a change of its structure in the outer boundary layer, and they provide a measure of the boundary-layer thickness. Where possible, results were compared with previous fired-engine and modeling data, and good agreement was found.

Shadowgraph/schlieren imaging techniques have often been used for flow visualization of reacting and non-reacting systems. In this paper we show that high-speed shadowgraph visualization in a high-pressure chamber can also be used to identify cool-flame and high-temperature combustion regions of diesel sprays, thereby providing insight into the time sequence of diesel ignition and combustion. When coupled to simultaneous high-speed Mie-scatter imaging, chemiluminescence imaging, pressure measurement, and spatially-integrated jet luminosity measurements by photodiode, the shadowgraph visualization provides further information about spray penetration after vaporization, spatial location of ignition and high-temperature combustion, and inactive combustion regions where problematic unburned hydrocarbons exist. Examples of the joint application of high-speed diagnostics include transient non-reacting and reacting injections, as well as multiple injections. Shadowgraph and schlieren image processing steps required to account for variations of refractive index within the high-temperature combustion vessel gases are also shown.

Thin He-filled platelets bounded by an attached dislocation dipole are shown to be stable structures in fcc metals with low surface energy and high shear modulus. They can form from Griffith-like nano-cracks, grow by radial expansion of the dipole, and transition to spherical bubbles if the dislocations become detached. Formulations of gas pressures within these precipitates are tested by lattice dilation data and changes in their dimensions with age. Copyright © 2009 ASM International® All rights reserved.

Extensive experimentation over the past decade has shown that fabricated physical systems that are intended to be identical, and are nominally identical, in fact, differ from one another, and sometimes substantially. This fact makes it difficult to validate a mathematical model for any system and results in the requirement to characterize physical system behavior using the tools of uncertainty quantification. Further, because of the existence of system, component, and material uncertainty, the mathematical models of these elements sometimes seek to reflect the uncertainty. This presentation introduces some of the methods of probability and statistics, and shows how they can be applied in engineering modeling and data analysis. The ideas of randomness and some basic means for measuring and modeling it are presented. The ideas of random experiment, random variable, mean, variance and standard deviation, and probability distribution are introduced. The ideas are introduced in the framework of a practical, yet simple, example; measured data are included. This presentation is the third in a sequence of tutorial discussions on mathematical model validation. The example introduced here is also used in later presentations. © 2009 Society for Experimental Mechanics Inc.

An order-of-convergence (with respect to a path-length parameter) verification study is undertaken for an implementation of the condensed-history algorithm in a Monte Carlo electron transport code. "Condensed- history" refers to simulating the cumulative effects of the electron without modeling each individual collision. A 1992 paper by Larsen derived the expected order of convergence for a few mathematical models of this type of algorithm. We examine the order of convergence of a condensed-history algorithm based on that used in the Integrated TIGER Series (as applied to electron albedo problems) in the presence of Monte Carlo uncertainty.

Instruction-level simulation is necessary to evaluate new architectures. However, single-node simulation cannot predict the behavior of a parallel application on a supercomputer. We present a scalable simulator that couples a cycle-accurate node simulator with a supercomputer network model. Our simulator executes individual instances of IBM's Mambo PowerPC simulator on hundreds of cores. We integrated a NIC emulator into Mambo and model the network instead of fully simulating it. This decouples the individual node simulators and makes our design scalable. Our simulator runs unmodified parallel message-passing applications on hundreds of nodes. We can change network and detailed node parameters, inject network traffic directly into caches, and use different policies to decide when that is an advantage. This paper describes our simulator in detail, evaluates it, and demonstrates its scalability. We show its suitability for architecture research by evaluating the impact of cache injection on parallel application performance. Copyright 2009 ACM.

Negative valve overlap (NVO) is a valve strategy employed to retain and recompress residual burned gases to assist HCCI combustion, particularly in the difficult regime of low-load operation. NVO allows the retention of large quantities of hot residual burned gases as well as the possibility of fuel addition for combustion control purposes. Reaction of fuel injected during NVO increases charge temperature, but in addition could produce reformed fuel species that may affect main combustion phasing. The strategy holds potential for controlling and extending low-load HCCI combustion. The goal of this work is to demonstrate the feasibility of applying two-wavelength PLIF of 3-pentanone to obtain simultaneous, in-cylinder temperature and composition images during different parts of the HCCI/NVO cycle. Measurements are recorded during the intake and main compression strokes, as well as during the more challenging periods of NVO recompression and re-expansion. To improve measurement quality, effects of diagnostic uncertainty and fluorescence interference are quantified. Temperature, fuel, and EGR images are captured for a range of NVO operating conditions, including main and NVO fuel-injection timings as well total load. The results demonstrate that the diagnostic is capable of providing information useful for the study of HCCI/NVO engine operation.

Polymer foams are used as encapsulants to provide mechanical, electrical, and thermal isolation for engineered systems. In fire environments, the incident heat flux to a system or structure can cause foams to decompose. Commonly used foams, such as polyurethanes, often liquefy and flow during decomposition, and evolved gases can cause pressurization and ultimately failure of sealed containers. In systems safety and hazard analyses, numerical models are used to predict heat transfer to encapsulated objects or through structures. The thermo-mechanical response of systems involving coupled foam decomposition, liquefaction, and flow can be difficult to predict. Predicting pressurization of sealed systems is particularly challenging. To mitigate the issues caused by liquefaction and flow, hybrid polyurethane cyanate ester foams have been developed that have good adhesion and mechanical properties similar to currently used polyurethane and epoxy foams. The hybrid foam decomposes predictably during decomposition. It forms approximately 50 percent by weight char during decomposition in nitrogen. The foam does not liquefy. The charring nature of the hybrid foam has several advantages with respect to modeling heat transfer and pressurization. Those advantages are illustrated by results from recent radiant heat transfer experiments involving encapsulated objects, as well as results from numerical simulations of those experiments.

Results from an experimental study of the aerodynamic and aeroacoustic properties of a flatback version of the TU Delft DU97-W-300 airfoil are presented for a chord Reynolds number of 3 × 106. The data were gathered in the Virginia Tech Stability Wind Tunnel, which uses a special aeroacoustic test section to enable measurements of airfoil self-noise. Corrected wind tunnel aerodynamic measurements for the DU97-W-300 are compared to previous solid wall wind tunnel data and are shown to give good agreement. Aeroacoustic data are presented for the flatback airfoil, with a focus on the amplitude and frequency of noise associated with the vortex-shedding tone from the blunt trailing edge wake. The effect of a splitter plate attachment on both drag and noise is also presented. Computational Fluid Dynamics predictions of the aerodynamic properties of both the unmodified DU97-W-300 and the flatback version are compared to the experimental data.

Abstract not provided.

The discipline of mathematical model validation is increasing in importance as the value of accurate models of physical systems increases. The fundamental activity of model validation is the comparison of predictions from a mathematical model of a system to the measured behavior of the system. This discussion motivates the need for model validation and introduces some preliminary elements of model validation. This is the first in a sequence of six tutorial presentations on model validation, and will introduce five presentations to follow. © 2009 Society for Experimental Mechanics Inc.

This paper presents a basic tutorial on epistemic uncertainty quantification methods. Epistemic uncertainty, characterizing lack-of-knowledge, is often prevalent in engineering applications. However, the methods we have for analyzing and propagating epistemic uncertainty are not as nearly widely used or well-understood as methods to propagate aleatory uncertainty (e.g. inherent variability characterized by probability distributions). We examine three methods used in propagating epistemic uncertainties: interval analysis, Dempster-Shafer evidence theory, and second-order probability. We demonstrate examples of their use on a problem in structural dynamics. © 2009 Society for Experimental Mechanics Inc.

Soda-lime glass (SLG) is a potential low-cost VISAR window for use at moderate shock pressures (up to 2430 GPa) where the material remains transparent. In order for SLG to be practical as a VISAR window, the correction factor, which describes the frequency correction related to the strain dependence of the refractive index, and hence the index of refraction itself, must be characterized as a function of pressure. Characterization data are reported in this paper and compared to previous results. The present data show good agreement with those of Dandekar [J. Appl. Phys. 84, 6614 (1998)] and separate study results by Gibbons and Ahrens [J. Geophys. Res. 76, 5489 (1971)] up to 7 GPa. However, at stresses over 7 GPa, marked discrepancies are evident between the present data and that of Gibbons and Ahrens. Differences in test methods may explain these discrepancies. © 2009 American Institute of Physics.

An order-of-convergence (with respect to a path-length parameter) verification study is undertaken for an implementation of the condensed-history algorithm in a Monte Carlo electron transport code. "Condensed- history" refers to simulating the cumulative effects of the electron without modeling each individual collision. A 1992 paper by Larsen derived the expected order of convergence for a few mathematical models of this type of algorithm. We examine the order of convergence of a condensed-history algorithm based on that used in the Integrated TIGER Series (as applied to electron albedo problems) in the presence of Monte Carlo uncertainty.

The well-known "sweep" algorithm for inverting the streaming-plus-collision term in first-order deterministic radiation transport calculations has some desirable numerical properties. However, it suffers from parallel scaling issues caused by a lack of concurrency. The maximum degree of concurrency, and thus the maximum parallelism, grows more slowly than the problem size for sweeps-based solvers. We investigate a new class of parallel algorithms that involves recasting the streaming-plus-collision problem in prefix form and solving via cyclic reduction. This method, although computationally more expensive at low levels of parallelism than the sweep algorithm, offers better theoretical scalability properties. Previous work has demonstrated this approach for one-dimensional calculations; we show how to extend it to multidimensional calculations. Notably, for multiple dimensions it appears that this approach is limited to long-characteristics discretizations; other discretizations cannot be cast in prefix form. We implement two variants of the algorithm within the radlib/SCEPTRE transport code library at Sandia National Laboratories and show results on two different massively parallel systems. Both the "forward" and "symmetric" solvers behave similarly, scaling well to larger degrees of parallelism then sweeps-based solvers. We do observe some issues at the highest levels of parallelism (relative to the system size) and discuss possible causes. We conclude that this approach shows good potential for future parallel systems, but the parallel scalability will depend heavily on the architecture of the communication networks of these systems.

By comparing the frequency deviations of the TE and TM modes of identically designed silicon microdisk-resonators across a wafer, we demonstrate that layer thickness non-uniformity is the dominant cause of fabrication-induced microdisk-resonator frequency deviation. © 2009 Optical Society of America.