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Superplastic forming is a valuable metal working technique because of the extreme ductility that can be achieved. However, it is limited in application due to the presence of small voids that grow and coalesce during the forming process, often causing premature failure. In order to understand and control this phenomenon accurate constitutive models must be developed which account for void parameters that affect the macroscopic behavior of the material. This paper looks specifically at the effect of void size and spacing on the ductility and flow stress of viscoplastic materials. Based on the gradient-dependent theory of plasticity, a model is proposed that accounts for size effects by incorporating strain gradient terms into a continuum based constitutive equation. Both experimental testing and finite element (FE) modeling were performed on Pb–Sn, tensile specimens with small holes drilled in them in random patterns. The experimental tests indicate that a decrease in void size results in an increase in ductility. The FE results demonstrate that the gradient terms strengthen the material by diffusing the strain in areas of high strain concentration and delay failure by slowing void growth. In addition, the model predicted an increase in ductility and flow stress with decreasing void size.

Purpose – The purpose of this study to overview the trends of higher education globally, within Asia Pacific Region and specially in Pakistan. Most importantly, current study highlights the strengths and opportunities as well as weakness and threads of Pakistan’s higher education system by proposing a SWOT analysis of higher education internationalization.Design/methodology/approach – This review paper is meant to appear in conjunction with more discrete and in-depth previous literature regarding higher education internationalization. Findings – It is investigated that higher education institutions in Pakistan are in need to rethink and reshape internationalization strategies following global standard which would be useful for the smooth drive towards global knowledge society and quality of higher education can be enhanced by developing internationalized curriculum, recruiting international students and staff and establishing international research collaboration programs.Originality/value – In this study a SWOT analysis of internationalization has been proposed which illustrates the internal and external factors which are positively or negatively impacting the higher education system in Pakistan.

Using density functional theory, a series of calculations of structural and electronic properties of hydrogen vacancies in a fully hydrogenated boron nitride (fH-BN) layer were conducted. By dehydrogenating the fH-BN structure, B-terminated vacancies can be created which induce complete spin polarization around the Fermi level, irrespective of the vacancy size. On the contrary, the fH-BN structure with N-terminated vacancies can be a small-gap semiconductor, a typical spin gapless semiconductor, or a metal depending on the vacancy size. Utilizing such vacancy-induced band gap and magnetism changes, possible applications in spintronics are proposed, and a special fH-BN based quantum dot device is designed. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Different equivalent circuit models can be found in the literature to describe the electrical performance of PV panels. Among all the models, the one diode models are widely used. However the determination of the electrical parameters of these models is not an easy task. In this paper, the particle swarm optimization (PSO) method was applied to extract the equivalent circuit parameters of PV panels. The PSO method was implemented in Matlab to calculate the series and shunt resistances of two modified one-diode models used to determine the electrical performance of a photovoltaic panel. The PSO method allowed us to plot I-V curves close to those provided by the PV manufacturer and proved to be a good approach capable of working with any electrical model.

We have developed a method for measurement of temperature in glass products with the following features: (1) non-contacting, (2) real time, and (3) through-thickness. The method is based on fluorescence emission. Many glass products include Fe2O3 as an additive in various amounts. Ferric (Fe3+) and ferrous (Fe2+) ions absorb light in the ultraviolet and infrared parts of the spectrum. Absorption of light by iron ions in glass results in a predictable fluorescence emission. The emission in turn depends on the glass temperature, and this dependence is the basis for our method of measuring temperature. We have measured the fluorescence emission lifetimes in several commercial automotive glass samples over a temperature range from 25°C to 550°C (about 300–825 K). Imaging the fluorescence emission from glass samples onto a segmented photomultiplier tube provided spatially resolved measurements. A simple model that relates the temperature to the fluorescence lifetime has been developed.

The elastic crack interaction with internal defects, such as microcracks, voids and rigid inclusions, is investigated in this study for the purpose of analyzing crack propagation. The elastic stress field is obtained using linear theory of elasticity for isotropic materials. The cracks are modeled as pile-ups of edge dislocations resulting into a coupled set of integral equations, whose kernels are those of a dislocation in a medium with or without an inclusion or void. The numerical solution of these equations gives the stress intensity factors and the complete stress field in the given domain. The solution is valid for a general solid, however the propagation analysis is valid mostly for brittle materials. Among different propagation models the ones based on maximum circumferential stress and minimum strain energy density theories, are employed. A special emphasis is given to the estimation of the crack propagation direction that defines the direction of crack branching or kinking. Once a propagation direction is determined, an improved model dealing with kinked cracks must be employed to follow the propagation behavior.

Recent data indicate that the effects of light water reactor environments can significantly reduce the fatigue resistance of materials, and show that design fatigue curves may not be conservative for reactor coolant environments. Using revised fatigue curves developed by Argonne National Laboratory (ANL), the work of this paper calculates the expected probabilities of fatigue failures and associated core damage frequencies at a 40-year and 60-year plant life for a sample of components from five PWR and two BWR plants. These calculations were made possible by the development of an enhanced version of the pc-PRAISE probabilistic fracture mechanics code that has the ability to simulate the initiation of fatigue cracks followed by the linking of these cracks. Results of interim calculations subject to review are presented. Components with the highest probabilities of failure can have predicted frequencies of through-wall cracks in the order of about 5×10 −2 per year. The corresponding maximum contributions to core damage frequencies are in the order of 10 −6 per year. Components with the very high failure rates show essentially no increase in calculated core damage frequency from 40 to 60 years.

The superplastic deformation and cavitation damage characteristics of a modified aluminum alloy are investigated at a temperature range from 500 to 550°C. The baseline alloy is AA5083. Nominally this alloy contains about 4.5% Mg, 0.8% Mn, 0.2% Cr, 0.037% Si, 0.08% Fe and 0.025% Ti by weight. The experimental program consists of uniaxial tension tests and digital image analysis for measuring cavitation. The experiments reveal that evolution of damage is due to both nucleation and growth of voids. A viscoplastic model for describing deformation and damage in this alloy is developed based on a continuum mechanics framework. The model includes the effect of strain hardening, strain rate sensitivity, dynamic and static recovery, and nucleation and growth of voids. The model predictions compare well with the experimental results.

Substantial void growth in metals constitutes a problem in many industrial operations that utilize superplastic deformation. This is because of the likelihood of material failure due to such growth. Hence, there is a need to study void growth mechanisms in an effort to understand the parameters governing it. In this work, numerical and experimental studies of void growth, and the parameters that affect it, in a superplastically deforming (SPD) metal have been performed. In the numerical studies, using the finite-element method, a 1×2 sized thin plate (i.e. plane stress conditions) of a viscoplastic material with pre-existing holes has been subjected to a constant extension rate. The experimental studies were performed under similar conditions to the numerical ones and provided for qualitative comparison. The parameters affecting void growth in SPD are: m (the strain-rate sensitivity), void size (i.e. diameter) and the number (density) of existing voids. The results showed that increased m values produced strengthening and decreased the rate of void growth. In addition, larger initial void size (or, equivalently, a larger initial void fraction) had the effect of weakening the specimen through causing accelerated void growth. Finally, multiple holes had the effect of increasing the metal ductility by reducing the extent of necking and its onset. This was realized through diffusing the plastic deformation at the different hole sites and reducing the stress concentration. The numerical results were in good qualitative agreement with the experiment and suggested the need to refine existing phenomenological void growth models to include the dependence on the void fraction.

The current work develops forming-limit diagrams (FLDs) for weld materials in aluminum tailorwelded blanks (TWBs) under biaxial stretching conditions. Aluminum TWBs consist of multiple-thickness and alloy sheet materials welded together into a single, variable-thickness blank. The manufacture of TWBs and their application in automotive body panels requires their constituent weld material to deform under biaxial loading during sheet-metal stamping. The weld geometry is typically nonuniform and relatively small, causing difficulty if one attempts to determine the weld metal FLDs via traditional experimental methods. The subject work primarily relies on theoretical FLD calculation techniques using the Marciniak and Kuczynski (M-K) method. This numerical technique requires the use of material constants and levels of initial material imperfection that have been experimentally determined using unique miniature tensile specimens to isolate and characterize the weld metal. The experimental and numerical work, together with statistical analysis of the level of initial imperfection, allows generation of both an average and safe FLD. The weld metals studied in this work were produced via autogeneous gas tungsten arc welding of a 1- to 2-mm-thick 5000 series aluminum alloy sheet.

The objective of the research described in this article was to characterize and numerically describe the ductility of weld material in aluminum tailor welded blanks under uniaxial tension conditions. Aluminum tailor welded blanks consist of multiple thickness and alloy sheet materials welded together into a single, variable thickness blank. To evaluate the mechanical properties of the weld material in these tailor welded blanks, a series of tensile specimens containing varying ratios of weld and monolithic material in the gage area of the specimen were tested. These experimental results show that increasing the amount of weld in the cross-sectional area of the specimen decreases the ductility of the specimen and that the weld characteristics have a pronounced impact on ductility. Using the experimental results and classical tensile instability and necking models, a numerical model was developed to describe the ductility of the weld metal. The model involves basic material properties and an initial imperfection level in both the weld and monolithic materials. The specimens studied were produced from 1- to 2-mm AA5182-O aluminum alloy sheet material welded into blanks using an autogenous gas tungsten arc welding process.

A numerical approach has been developed to predict the probability that a fabrication flaw in a reactor pressure vessel will extend by fatigue crack growth mechanisms and become a through-wall flaw. The fracture mechanics model treats the size of the flaw, the location of the flaw, and the parameters governing the fatigue crack growth rates as stochastic variables that are described by histograms that represent their statistical distributions. A latin hypercube approach forms the basis for efficient numerical calculations of vessel failure probabilities, in particular for those cases having very low probabilities that are not readily calculated by use of more conventional Monte Carlo simulations. A second aspect of the vessel failure model evaluates the benefits of in-service inspections at prescribed inspection time intervals and with prescribed nondestructive examination capabilities (probability of detection as a function of flaw size). A third aspect of the paper evaluates flaw sizing accuracy, and the impacts of flaw acceptance criteria. For representative values of flaw detection probability, flaw sizing errors, and flaw acceptance criteria, detection capability is the most limiting factor with regard to the ability of the in-service inspections to reduce leak probabilities. However, gross sizing errors or significant relaxations of current flaw acceptance standards could negate the benefits of outstanding probability of detection capabilities.

This paper applies probabilistic fracture mechanics calculations to determine the effects of inspection on leak probabilities for piping. The approach has been to perform calculations in a structured parametric format, with the parameters selected to cover the range of pipe sizes, degradation mechanisms, operating stresses, and materials relevant to the piping systems of interest. In this paper, the calculations were intended to be generally applicable to mechanical and thermal fatigue of stainless steel piping. Specific areas of uncertainty addressed by the probabilistic calculations of this paper are the numbers of initial flaws, the distributions of flaw sizes, the crack growth rates for these initial flaws, and the probability of detection curves and inspection schedules that describe inservice inspections which are performed to detect these growing flaws. The effectiveness of an inspection strategy is quantified by the parameter ‘Factor of Improvement’, which is the relative increase in piping reliability due to a given inspection strategy as compared with the strategy of performing no inspection. The results of a systematic set of calculations are presented in this paper that address inspection effectiveness for operating stresses giving crack growth rates ranging from very low to very high. Inspection strategies are described that address three reference levels of ultrasonic inspection reliability, intervals between inspections ranging from 1 to 10 years, and both preservice and inservice inspections.

An approach has been developed that predicts leak and rupture probabilities of reactor piping in a structured parametric format. This approach applies the probabilistic fracture mechanics code pc-PRAISE (Piping Reliability Analysis Including Seismic Events) to address the mechanical and thermal fatigue life of piping. The probabilistic fracture mechanics model is applied to predict the relative effects of uniform stresses and through-thickness stress gradients on the reliability of stainless steel piping welds. Results generated using the numerical technique revealed that the calculated leak probabilities can be sensitive to the different types of stress gradients and to local stress concentrations.

The superplastic deformation characteristics of a modified aluminum alloy were investigated at temperature range from 500 to 550°C. The alloy chemical composition was 4.69% Mg, 1.56% Mn, 0.177% Cr, 0.03% Si, and 0.07% Fe by weight. Total tensile elongations as high as 750% were observed at lower strain rates. A non-isothermal constitutive equation, that includes the evolution of grain size, was established and implemented in a finite element code. Numerical simulations of the uniaxial tension specimen were performed, and good agreement between the testing and the modeling results was obtained. A pressure control algorithm for the blow forming model was developed based on an averaging scheme that considered a subset of finite elements with the highest strain rate. A pressure history was generated from the finite element simulations for a constant target strain rate of 1×10 −3 s −1 . When these pressure histories were used in tray forming experiments, good agreement was obtained between the forming and modeling results.

This paper describes probabilistic fracture mechanics calculations that simulate fatigue crack growth, flaw detection, flaw sizing accuracy, and the impacts of flaw acceptance criteria. The numerical implementation of the model is based on a Latin hypercube approach. Calculations have been performed for a range of parameters. For representative values of flaw detection probability, flaw sizing errors, and flaw acceptance criteria, detection capability is the most limiting factor with regard to the ability of the inservice inspections to reduce leak probabilities. However, gross sizing errors or significant relaxations of current flaw acceptance standards could negate the benefits of outstanding probability of detection capabilities.

International audience; The modeling of the viscoelastic stress evolution and specific volume relaxation of a bulky glass cast is presented in this article and is applied to the experimental cooling process of an inactive nuclear waste vitrification process. The concerned borosilicate glass is solidified and cooled down to ambient temperature in a stainless steel canister, and the thermomechanical response of the package is simulated. There exists a deviant compression of the liquid core due to the large glass package compared to standard tempered glass plates. The stress load development of the glass cast is finally studied for different thermal load scenarios, where the cooling process parameters or the final cooldown rates were changed, and we found a great influence of the studied cooldown rates on the maximum stress buildup at ambient temperature.

Rapid oscillations of load during deformation of superplastic materials have been reported by several researchers. In an investigation on AA7475, Ghosh and Raj developed a model to study the oscillations observed within the strain rate range from 10{sup {minus}3}s{sup {minus}1} to 5 {times} 10{sup {minus}3}s{sup {minus}1}. The oscillations in the stress-strain curve were related to changes in the grain size due to refinement by dynamic recrystallization and grain growth during deformation. Using the same alloy Hamilton et al. observed oscillations in stress-strain curves at strain rates of 5 {times} 10{sup {minus}3}s{sup {minus}1} and 10{sup {minus}2}s{sup {minus}1}. These oscillations were attributed to dynamic strain aging. Yang et al. observed serrations in the stress-strain curves of AA7475 tested at temperatures ranging from 20 to 525 C. The amplitude of the load fluctuation was related to the strain-rate sensitivity. The explanation they gave for this was that serrations are caused by continuous shifting of multiple necks from one region to another, and dynamic change in the grain structure. Li and Blackwell studied load oscillations in tensile deformation of Al-Li and Al-Mg alloys at temperatures > 500 C and strain rate < 10{sup {minus}2}s{sup {minus}1}. They also observed that the amplitude of the fluctuations more » was found to very according to the strain rate sensitivity of the material. They concluded that load serrations observed in their study are a closed-loop controlled screw-driven machine effect. They also observed that cross-head speed varied in a cyclic manner. The purpose of this study is to determine the effects of applying non-uniform strain rates on superplastic properties of AA5083 using a motor-driven system operating in an open-loop mode. « less

The applicability of the fatigue-life distribution of Birnbaum–Saunders for modeling the components of partially prestressed concrete girder bridges is argued by the agreement of the calculated distribution with a large data set comprised of many small censored samples, each put into standardized form, of the fatigue-lives of bridge components. Also, the Weibull and lognormal distributions are compared by the appropriateness of the assumptions in their derivation to the cumulative damage to bridge components. The maximum likelihood estimates of the parameters of the fatigue lives of the prestressing steel, mild steel, and concrete are found from randomly censored samples, for several censored data-sets under various stress regimes. The parameters are postulated to be log-linear functions of the maximum stress and stress regimes. Weighted least-square regression is applied to determine the unknown coefficients. To compensate for differences in the precision of the estimates obtained under each stress regime the variances are calculated by the bootstrap statistical method. This procedure gives an estimated distribution of the fatigue life with its parameters functions of the stress regime which is applicable under virtually any realistic condition.

The processing path model based on the conservation principle in the orientation space allows us to optimize processing path from a given initial state to a desired final microstructure for polycrystalline materials. This model uses texture coefficients in spherical harmonics expansion as descriptors to represent the texture state of polycrystalline materials. In this work, the effect of increasing the number of texture coefficients used in the series expansion (decided by lmax) on the prediction accuracy of texture evolution is investigated. Uber die verbesserte Vorhersage der Gefugeentwicklung mittels des Processing-Path-Modells Das Processing-Path-Modell zur Gefugeentwicklung, das in dieser Studie entwickelt wurde, basiert auf dem Erhaltungsprinzip im Orientierungsraum und ermoglicht die endgultige Mikrostruktur - aus den ursprunglichen Mikrostrukturdaten eines polykristallinen Materials - vorauszusagen. Um den Gefugestatus eines polykristallinen Materials zu beschreiben, verwendet dieses Modell zur Materialbeschreibung Gefugekoeffizienten in spharisch-harmonischer Reihenentwicklung. In dieser Arbeit wurde die Auswirkung einer Erhohung der Gefugekoeffizienten in der Reihenentwicklung (dargestellt durch lmax) auf die Voraussage des End-gefuges untersucht.

Predicting resistance to environmental degradation, especially hydrogen embrittlement (HE) has become a major concern for life assessment and risk analysis of structural materials. The microstructure of the materials plays a significant role in HE. Despite the large documentation about the subject, the contribution of hydrogen diffusion on this process stays unclear. In this work, we analyze the effects of the microstructure on hydrogen diffusion, especially the influence of grain boundaries considered as high diffusivity paths and possible sites of damage occurrence. Electrochemical permeation was simulated using finite elements method (FEM). Scale effects between the RVE (Representative Volume Element) and the size of the membrane are discussed. Domains of applicability for standard homogenization methods, especially Hashin–Shtrikman model are studied using results from microstructural based FEM. Domains of invariance of diffusion behavior and concentration profiles for grain shapes and the size of the membrane are also analyzed. Thus, the difficulty to extract diffusion properties by permeation test for heterogeneous microstructures is highlighted and discussed.

On July 31-August 2 of 2012, the U.S. Department of Energy (DOE) held a workshop entitled Grand Challenges of Advanced Computing for Energy Innovation. This workshop built on three earlier workshops that clearly identified the potential for the Department and its national laboratories to enable energy innovation. The specific goal of the workshop was to identify the key challenges that the nation must overcome to apply the full benefit of taxpayer-funded advanced computing technologies to U.S. energy innovation in the ways that the country produces, moves, stores, and uses energy. Perhaps more importantly, the workshop also developed a set of recommendations to help the Department overcome those challenges. These recommendations provide an action plan for what the Department can do in the coming years to improve the nation’s energy future.

Today, parallel processing is necessary for the solution of large systems of ordinary differential equations (ODEs) as they are obtained from large electronic circuits or from discretizing partial differential equations (PDEs). Using a fine mesh in the discretization of these problems also leads to large compute times and large storage requirements. The waveform relaxation (WR) technique, which is ideally suited for the use of multiple processors for problems with multiple time scales has been used to solve such problems on parallel processors for such large systems of ODEs. However, applying the so-called classical WR techniques to strongly coupled systems leads to non-uniform slow convergence over a window in time for which the equations are integrated. In this paper, we present a so-called optimizedWR algorithm applied to transmission line circuit problems based on the longitudinal partitioning into segments. This greatly improves the convergence for strongly coupled RLCG transmission line (TL) type circuits. The method can be applied to other similar circuits. The method is based on optimal parameters that lead to the optimal convergence of the iterations. Here, we present a practical optimized WR algorithm which is easy to use and is computationally inexpensive.

Current experimental methods are influenced by the end effects that cause non-uniform strain rates in the gauge section and material flow within the grips. A series of tension tests and finite element models confirm this for an Al-5083 alloy. Both the tests and the finite element simulations predict that the actual strain rate begins at about 60 percent of the desired strain rate and increases gradually with strain. Material flow from the grips into the gauge effectively ``slows`` the strain rate at the initial stages of the test. As the test proceeds thinning of the gauge section occurs and most of the strain occurs in the gauge section due to the relative cross-sectional areas of the grip and gauge section. Testing and models were also run comparing specimens with and without alignment holes in the grips. It was shown that alignment holes increase flow from the grips and thus introduce additional error in the tests. Further modeling was performed to evaluate the improved accuracy of specimens with increased length-to-width ratios. This work showed that a specimen with 50% reduction in the standard gauge width and double the standard gauge length (4:1 increase in length-to-width ratio) gave strain rates within 10%more » of the desired value throughout the test.« less

The Li–S batteries are becoming attractive due to the abundance of elemental sulfur and its environmental compatibility. In addition, sulfur undergoes a two-electron redox reaction with lithium and offers an extremely high theoretical specific capacity of 1675 mAh g−1 and a high energy density of 2600 Wh kg−1.1 Despite these advantages, several issues have hampered the practical use of Li–S batteries. For example, the insulating character of sulfur and lithium sulfide impede the full electrochemical utilization of sulfur. Moreover, the dissolution of lithium polysulfide inter-mediates in the electrolyte triggers undesirable shuttle reactions. Other issues such as low Coulombic efficiency, high self-discharge, and huge volume change remain unsolved. To overcome these drawbacks, several mitigation strategies have been explored by various research groups. One of the most followed routes deals with the mixing/impregnation/confinement of sulfur within highly conducting carbons. carbon nanotube-based composites have been used for high sulfur loading amount. Another approach is the use of metal oxides. In particular, TiO2 is a promising host for sulfur containment owing to its high absorption ability2. Further enhancement in the capacity and cycle life of Li–S batteries has been realized by Manthiram’s group by employing a free-standing interlayer (between the sulfur cathode and the separator) that plays the bifunctional role as a trap for mobile polysulfides and as an upper current collector for fast electron transport3. Herein, we synthesized and characterized (i) a S-HMT@CNT sulfur cathode comprised of hollow, spherical, nanostructured TiO2 and carbon nanotubes and (ii) DF-PCW interlayer comprised of highly porous carbon spheres and carbon nanotubes. The pores of TiO2 allowed for high sulfur loadings and accommodation of the volume expansion at the electrode level. The CNT component provided an overlap-ping network that improved both the electronic conductivity and mechanical strength of the S-HMT@CNT and DF-PCW. The pores in the carbon interlayer played the role of a medium that scavenged the dissolved lithium polysulfides, while improving Li-ion and electron transports. Owing to our cathode characteristics and cell design, the lithium–sulfur cell fabricated demonstrated good cycle life, high efficiency, and high rate capability. Of significance, the excellent cycling realized at the 2C and 5C rates is appealing for utilizing the lithium–sulfur batteries in high power applications

Solid oxide fuel cells (SOFCs) are high temperature fuel cells, which are being developed for large scale and distributed power systems. SOFCs promise to provide cleaner, more efficient electricity than traditional fossil fuel burning power plants. Research over the last decade has improved the design and materials used in SOFCs to increase their performance and stability for long-term operation; however, there are still challenges for SOFC researchers to overcome before SOFCs can be considered competitive with traditional fossil fuel burning and renewable power systems. In particular degradation due to contaminants in the fuel and oxidant stream is a major challenge facing SOFCs. In this paper we discuss ongoing computational and experimental research into different degradation and design issues in SOFC electrodes. We focus on contaminants in gasified coal which cause electrochemical and structural degradation in the anode, and chromium poisoning which affects the electrochemistry of the cathode. Due to the complex microstructures and multi-physics of SOFCs, multi-scale computational modeling and experimental research is needed to understand the detailed physics behind different degradation mechanisms, the local conditions within the cell which facilitate degradation, and its effects on the overall SOFC performance. We will discuss computational modeling research of SOFCs at the macro-, meso- and nano-scales which is being used to investigate the performance and degradation of SOFCs. We will also discuss the need for a multi-scale modeling framework of SOFCs, and the application of computational and multi-scale modeling to several degradation issues in SOFCs.

The development of viable nuclear energy source depends on ensuring structural materials integrity. Structural materials in nuclear reactors will operate in harsh radiation conditions coupled with high level hydrogen and helium production, as well as formation of high density of point defects and defect clusters, and thus will experience severe degradation of mechanical properties. Therefore, the main objective of this work is to develop a capability that predicts aging behavior and in-service lifetime of nuclear reactor components and, thus provide an instrumental tool for tailoring materials design and development for application in future nuclear reactor technologies. Towards this end goal, the long term effort is to develop a physically based multiscale modeling hierarchy, validated and verified, to address outstanding questions regarding the effects of irradiation on materials microstructure and mechanical properties during extended service in the fission and fusion environments. The focus of the current investigation is on modern steels for use in nuclear reactors including high strength ferritic-martensitic steels (Fe-Cr-Ni alloys). The effort is to develop a predicative capability for the influence of irradiation on mechanical behavior. Irradiation hardening is related to structural information crossing different length scales, such as composition, dislocation, and crystal orientation distribution. To predict effectivemore » hardening, the influence factors along different length scales should be considered. Therefore, a hierarchical upscaling methodology is implemented in this work in which relevant information is passed between models at three scales, namely, from molecular dynamics to dislocation dynamics to dislocation-based crystal plasticity. The molecular dynamics (MD) was used to predict the dislocation mobility in body centered cubic (bcc) Fe and its Ni and Cr alloys. The results are then passed on to dislocation dynamics to predict the critical resolved shear stress (CRSS) from the evolution of local dislocation and defects. In this report the focus is on the results obtained from large scale dislocation dynamics simulations. The effect of defect density, materials structure was investigated, and evolution laws are obtained. These results will form the bases for the development of evolution and hardening laws for a dislocation-based crystal plasticity framework. The hierarchical upscaling method being developed in this project can provide a guidance tool to evaluate performance of structural materials for next-generation nuclear reactors. Combined with other tools developed in the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program, the models developed will have more impact in improving the reliability of current reactors and affordability of new reactors.« less

A noticeable degree of inconsistent forming behaviors has been observed for the 1st generation advanced high strength steels (AHSS) in production, and they appear to be associated with the inherent microstructural-level inhomogeneities for various AHSS. This indicates that the basic material property requirements and screening methods currently used for the mild steels and high strength low alloys (HSLA) are no longer sufficient for qualifying today’s AHSS. In order to establish more relevant material acceptance criteria for AHSS, the fundamental understandings on key mechanical properties and microstructural features influencing the local formability of AHSS need to be developed. For this purpose, in this study, DP980 was selected as model steels and eight different types of DP980 sheet steels were acquired from various steel suppliers. Various experiments were then performed on the eight different DP980 steels such as chemical composition analysis, static tensile test, hole expansion test, channel forming test. Scanning electron microscope (SEM) pictures of the DP980 steels were also obtained, and image processing tools were then adopted to those SEM pictures in order to quantify their various microstructural features. The results show that all DP980 steels show large discrepancy in their performance and that the tensile properties and hole expansionmore » properties of these steels do not correlate with their local formability. According to the results up to date, it is not possible to correlate the microstructural features alone to the macroscopically measured deformation behaviors. In addition to image analysis, other experiments (i.e., nano-indentation test) are also planned to quantify the individual phase properties of the various DP steels.« less

The energetics and length scales associated with the interaction between point defects (vacancies and self-interstitial atoms) and grain boundaries in bcc Fe was explored. Molecular statics simulations were used to generate a grain boundary structure database that contained $\ensuremath{\approx}$170 grain boundaries with varying tilt and twist character. Then, vacancy and self-interstitial atom formation energies were calculated at all potential grain boundary sites within 15 \AA{} of the boundary. The present results provide detailed information about the interaction energies of vacancies and self-interstitial atoms with symmetric tilt grain boundaries in iron and the length scales involved with absorption of these point defects by grain boundaries. Both low- and high-angle grain boundaries were effective sinks for point defects, with a few low-$\ensuremath{\Sigma}$ grain boundaries (e.g., the $\ensuremath{\Sigma}3$${112}$ twin boundary) that have properties different from the rest. The formation energies depend on both the local atomic structure and the distance from the boundary center. Additionally, the effect of grain boundary energy, disorientation angle, and $\ensuremath{\Sigma}$ designation on the boundary sink strength was explored; the strongest correlation occurred between the grain boundary energy and the mean point defect formation energies. Based on point defect binding energies, interstitials have $\ensuremath{\approx}$80$%$ more grain boundary sites per area and $\ensuremath{\approx}$300$%$ greater site strength than vacancies. Last, the absorption length scale of point defects by grain boundaries is over a full lattice unit larger for interstitials than for vacancies (mean of 6--7 \AA{} versus 10--11 \AA{} for vacancies and interstitials, respectively).

Magnesium alloys have found increasing application in the transportation industry due to their low weight and high strength. However, wider application is hindered by limited ductility. Microstructural features, such as porosity, brittle eutectics, and grain size, can significantly influence the macroscopic response of a component. These features can vary widely throughout a component. Our approach to studying the microstructures influence on bulk properties begins with measuring microstructural features in different regions of a component. These measurements are used to create statistically equivalent, 3D synthetic samples of the microstructure. The synthetic microstructures are meshed using finite elements and used to simulate the response and investigate the influence of specific features. We will demonstrate how the digital microstructure samples are generated, how variations in microstructural features influence the bulk properties, and how this methodology can be used to predict component performance and optimize processing.

This paper proposes a novel architecture for high speed combined binary/decimal addition/subtraction. We start by designing a correction-free Binary Coded Decimal (BCD) digit adder which exhibits high performance. We then use the proposed BCD digit adder to create a fast multi-digit BCD adder. The resulting multi-digit BCD adder is then used to build a combined binary/decimal addition/subtraction unit. The proposed combined binary/decimal addition/subtraction unit has been functionally verified and then implemented on Xilinx FPGA using Xilinx CAD tools. Implementation results show that our design outperforms the existing designs in terms of speed and most of the existing designs in terms of area.

This report presents a distributed electrochemistry (DEC) model capable of investigating the electrochemistry and local conditions with the SOFC MEA based on the local microstructure and multi-physics. The DEC model can calculate the global current-voltage (I-V) performance of the cell as determined by the spatially varying local conditions through the thickness of the electrodes and electrolyte. The simulation tool is able to investigate the electrochemical performance based on characteristics of the electrode microstructure, such as particle size, pore size, electrolyte and electrode phase volume fractions, and triple-phase-boundary length. It can also investigate performance as affected by fuel and oxidant gas flow distributions and other environmental/experimental conditions such as temperature and fuel gas composition. The long-term objective for the DEC modeling tool is to investigate factors that cause electrode degradation and the decay of SOFC performance which decrease longevity.

This study evaluated different upscaling methods to predict thermal conductivity in loaded nuclear waste form, a heterogeneous material system. The efficiency and accuracy of these methods were compared. Thermal conductivity in loaded nuclear waste form is an important property specific to scientific researchers, in waste form Integrated performance and safety code (IPSC). The effective thermal conductivity obtained from microstructure information and local thermal conductivity of different components is critical in predicting the life and performance of waste form during storage. How the heat generated during storage is directly related to thermal conductivity, which in turn determining the mechanical deformation behavior, corrosion resistance and aging performance. Several methods, including the Taylor model, Sachs model, self-consistent model, and statistical upscaling models were developed and implemented. Due to the absence of experimental data, prediction results from finite element method (FEM) were used as reference to determine the accuracy of different upscaling models. Micrographs from different loading of nuclear waste were used in the prediction of thermal conductivity. Prediction results demonstrated that in term of efficiency, boundary models (Taylor and Sachs model) are better than self consistent model, statistical upscaling method and FEM. Balancing the computation resource and accuracy, statistical upscaling is a computational efficient method in predicting effective thermal conductivity for nuclear waste form.

Electricity production through the integration of coal gasification with solid oxide fuel cells (SOFCs) may potentially be an efficient technique for clean energy generation. However, multiple minor and trace components are naturally present in coals. These impurities in coal gas not only degrade the electrochemical performance of Ni-YSZ anode used in SOFCs, but also severely endanger the structural integrity of the Ni-YSZ anode. In this paper, effect of the trace impurity of the coal syngases on the mechanical degradation of Ni-YSZ anode was studied by using an integrated experimental/modeling approach. Phosphorus is taken as an example of impurity. Anode-support button cell was used to experimentally explore the migration of phosphorous impurity in the Ni-YSZ anode of SOFCs. X-ray mapping was used to show elemental distributions and new phase formation. The subsequent finite element stress analyses were conducted using the actual microstructure of the anode to illustrate the degradation mechanism. It was found that volume expansion induced by the Ni phase change produces high stress level such that local failure of the Ni-YSZ anode is possible under the operating conditions

Using density functional theory, we consider the adsorption of C on graphene, which gives rise to many interesting phenomena. A single-C at the bridge site shows a clearly covalent-bond feature with graphene, in which the metallic state occurs and a magnetic moment of 0.36 μ(B) was determined. For both-sided adsorption, the magnetic moment is remarkably larger than that in one-sided adsorption, and increases with concentration up to a coverage of 12.5%. High spin polarization obtained at the Fermi level indicates a high degree of passage of preferred spin, which is important for developing spin filters.

Among the high-efficiency energy conversion devices that are environmentally friendly with little or no toxic emissions, the solid oxide fuel cell (SOFC) continues to show great promise as a future power source. It has potential applications in stationary power generation and as auxiliary power units. Among various SOFC designs under development, anode-supported planar cells have shown great potential in delivering high performance at reasonable costs (Teagan et al., 2000; Chung et al. 2005). Planar SOFCs offer a significant advantage of a compact design along with higher power densities. In the meantime, they require that hermetic gas seals be incorporated for efficient and effective channeling of fuel and oxygen. Seals are the most critical components in commercializing the planar SOFC technology (Nielsen et al., 2007; Choi & Bansal, 2005; Smeacetto et al., 2008). They must adequately prevent air and fuel from leaking, effectively isolate the fuel from the oxidant, and insulate the cell from short circuit. Essentially, there are two standard methods for sealing: compressive sealing and rigid bonding (Singh, 2007; Fergus, 2005; Chou et al., 2003). In compressive sealing, a compliant, high-temperature material is captured between the two sealing surfaces and compressed with a load frame external to the stack to deliver hermetic sealing. The sealing surfaces can slide with respect to one another without disrupting the hermeticity of the seal. This technology, however, remains incomplete due to the lack of a reliable, high-temperature, sealing material (Simner & Stevenson, 2001) and the difficulty of designing the appropriate load frame under high-operating temperatures. Rigid seals rely on effective bonding of the seal material to the sealing surfaces. They offer significant advantages over compressive seals that suffer from problems of oxide scaling and chemical stability under highly reactive environments in addition to the disadvantages of incorporating an externally applied load (Singh, 2007). As a rigid seal, glass joining provides a cost-effective and relatively simple method for bonding ceramic and metal parts. However, the softening point of the glass component typically limits the maximum operating temperature to which the joint may be exposed. As discussed by Weil et al. (2004), there are a number of other key materials and processing variables that can influence the performance of glass seals, including the composition of the metal substrate against which the seal is made, the operating parameters, such as the

Stainless steels based on Fe-Cr-Ni alloys are the most popular structural materials used in reactors. High energy particle irradiation of in this kind of polycrystalline structural materials usually produces irradiation hardening and embrittlement. The development of predictive capability for the influence of irradiation on mechanical behavior is very important in materials design for next-generation reactors. Irradiation hardening is related to structural information crossing different length scale, such as composition, dislocation, crystal orientation distribution and so on. To predict the effective hardening, the influence factors along different length scales should be considered. A multiscale approach was implemented in this work to predict irradiation hardening of iron based structural materials. Three length scales are involved in this multiscale model: nanometer, micrometer and millimeter. In the microscale, molecular dynamics (MD) was utilized to predict on the edge dislocation mobility in body centered cubic (bcc) Fe and its Ni and Cr alloys. On the mesoscale, dislocation dynamics (DD) models were used to predict the critical resolved shear stress from the evolution of local dislocation and defects. In the macroscale, a viscoplastic self-consistent (VPSC) model was applied to predict the irradiation hardening in samples with changes in texture. The effects of defect density and texture more » were investigated. Simulated evolution of yield strength with irradiation agrees well with the experimental data of irradiation strengthening of stainless steel 304L, 316L and T91. This multiscale model we developed in this project can provide a guidance tool in performance evaluation of structural materials for next-generation nuclear reactors. Combining with other tools developed in the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program, the models developed will have more impact in improving the reliability of current reactors and affordability of new reactors. « less

Microstructure evolution kinetics in irradiated materials has strongly spatial correlation. For example, void and second phases prefer to nucleate and grow at pre-existing defects such as dislocations, grain boundaries, and cracks. Inhomogeneous microstructure evolution results in inhomogeneity of microstructure and thermo-mechanical properties. Therefore, the simulation capability for predicting three dimensional (3-D) microstructure evolution kinetics and its subsequent impact on material properties and performance is crucial for scientific design of advanced nuclear materials and optimal operation conditions in order to reduce uncertainty in operational and safety margins. Very recently the meso-scale phase-field (PF) method has been used to predict gas bubble evolution, void swelling, void lattice formation and void migration in irradiated materials,. Although most results of phase-field simulations are qualitative due to the lake of accurate thermodynamic and kinetic properties of defects, possible missing of important kinetic properties and processes, and the capability of current codes and computers for large time and length scale modeling, the simulations demonstrate that PF method is a promising simulation tool for predicting 3-D heterogeneous microstructure and property evolution, and providing microstructure evolution kinetics for higher scale level simulations of microstructure and property evolution such as mean field methods. This report consists of two parts. In part I, we will present a new phase-field model for predicting interstitial loop growth kinetics in irradiated materials. The effect of defect (vacancy/interstitial) generation, diffusion and recombination, sink strength, long-range elastic interaction, inhomogeneous and anisotropic mobility on microstructure evolution kinetics is taken into account in the model. The model is used to study the effect of elastic interaction on interstitial loop growth kinetics, the interstitial flux, and sink strength of interstitial loop for interstitials. In part II, we present a generic phase field model and discuss the thermodynamic and kinetic properties in phase-field models including the reaction kinetics of radiation defects and local free energy of irradiated materials. In particular, a two-sublattice thermodynamic model is suggested to describe the local free energy of alloys with irradiated defects. Fe-Cr alloy is taken as an example to explain the required thermodynamic and kinetic properties for quantitative phase-field modeling. Finally the great challenges in phase-field modeling will be discussed.

Using density functional calculations, we investigate the geometries, electronic structures and magnetic properties of hexagonal BN sheets with 3d transition metal (TM) and nonmetal atoms embedded in three types of vacancies: V(B), V(N), and V(B+N). We show that some embedded configurations, except TM atoms in V(N) vacancy, are stable in BN sheets and yield interesting phenomena. For instance, the band gaps and magnetic moments of BN sheets can be tuned depending on the embedded dopant species and vacancy type. In particular, embedment such as Cr in V(B+N), Co in V(B), and Ni in V(B) leads to half-metallic BN sheets interesting for spin filter applications. From the investigation of Mn-chain (C(Mn)) embedments, a regular 1D structure can be formed in BN sheets as an electron waveguide, a metal nanometre wire with a single atom thickness.

The goal of the "Scientific Grand Challenges - Crosscutting Technologies for Computing at the Exascale" workshop in February 2010, jointly sponsored by the U.S. Department of Energy’s Office of Advanced Scientific Computing Research and the National Nuclear Security Administration, was to identify the elements of a research and development agenda that will address these challenges and create a comprehensive exascale computing environment. This exascale computing environment will enable the science applications identified in the eight previously held Scientific Grand Challenges Workshop Series.

A Monte Carlo methodology has been developed as a means for three-dimensional (3-D) reconstruction of the microstructure of a three phase anode used in solid oxide fuel cells, based on two-point statistical functions. The salient feature of the presented reconstruction methodology is the ability to realize the 3-D microstructure from two-dimensional (2-D) scanning electron micrographs for a three phase medium extendable to n phase media. In the realization procedure different phases of the heterogeneous medium are represented by different cells, which are allowed to grow. The growth of cells, however, is controlled via several optimization parameters related to rotation, shrinkage, translation, distribution and growth rates of the cells. Indeed, the proposed realization algorithm can be categorized as a dynamic programming method and is so designed that any desired microstructure can be realized. At first an initial 2-D image is reconstructed, then the final optimization parameters are used as initial values in the initiation of the 3-D reconstruction algorithm. This paper presents a novel hybrid stochastic methodology based on the colony and kinetic algorithms to simulate the virtual microstructure. The simulation procedure involves repeated realizations, where each realization in turn consists of the nucleation and growth of cells. For each of the subsequent realizations the controlling parameters are updated by minimization of an objective function (OF) at the end of the preceding realization. The OF is defined based on two-point correlation functions derived from the simulated and real microstructures. The kinetic growth algorithm is based on the cellular automata approach, which facilitates the simulation procedure. Comparison of the two-point correlation functions from different sections of the final 3-D reconstructed microstructure with the initial real microstructure showed satisfactory agreement, which validates the proposed methodology.

A cell-level distributed electrochemistry (DEC) modeling tool has been developed to enable predicting trends in solid oxide fuel cell performance by considering the coupled and spatially varying multi-physics that occur within the tri-layer. The approach calculates the distributed electrochemistry within the electrodes, which includes the charge transfer and electric potential fields, ion transport throughout the tri-layer, and gas distributions within the composite and porous electrodes. The thickness of the electrochemically active regions within the electrodes is calculated along with the distributions of charge transfer. The DEC modeling tool can examine the overall SOFC performance based on electrode microstructural parameters, such as particle size, pore size, porosity, electrolyte- and electrode-phase volume fractions, and triple-phase-boundary length. Recent developments in electrode fabrication methods have lead to increased interest in using graded and nano-structured electrodes to improve the electrochemical performance of SOFCs. This paper demonstrates how the DEC modeling tool can be used to help design novel electrode microstructures by optimizing a graded anode for high electrochemical performance.

A comprehensive study on the optimization of simply supported partially prestressed concrete girders is presented. Using sequential quadratic programming a set of optimal geometrical dimensions, amounts of prestressing and non-prestressing steel, and spacing between shear reinforcements are obtained. The constraints used are based upon flexural stresses, fatigue stresses, crack width, ductility, initial camber, deflection due to both dead and live loads, ultimate moment capacity of the section with respect to cracking moment and factored loads, and the ultimate shear strength. Results point to the need for non-prestressing steel to obtain economical designs. Minor savings on the material occur when the section is allowed to crack. The ultimate moment capacity and the fatigue stress controlled the design in the cracked case. The tensile stress at service and the ultimate moment capacity were the binding constraints in the uncracked case. This study automates the design of partially prestressed concrete girders and provides needed design solutions to problems which are of importance to practicing engineers.

An efficient method for evaluating the safety of existing girder bridges as a function of the load and resistance parameters is presented. The bridge capacity is determined using a nonlinear finite element program in terms of the truck load which is increased until structure collapse occurs. It is determined that the reliability of fully prestressed bridges is higher than that of partially prestressed bridges, with the bridge system reliability is one-and-half to two times the girder reliability index. It is also found that the system reliability is more sensitive to accidental or local damage of the exterior girders than the interior ones.

In this paper, numerical modeling was used to understand of the effect of the geometry and mechanical properties of various components in SOFCs on the level and distribution of stresses in the stack during operating and cooling. The results of these modeling analyses will help stack designers reduce high stresses in the seals of the stack so that structural failures are prevented and high stack mechanical reliability is achieved to meet technical targets. In general, it was found that the load carrying capacity of the cathode contact layer was advantageous for reducing the transmitted loads on the cell perimeter seal under operating environments of SOFCs, but the amount of reduction depends upon the relative stiffness values of the cell, interconnect, porous media, and support structures. Comparison of a fully bonded interface to a frictionless sliding interface resulted in 30-50% less transmitted load through the perimeter seal, with the greater reductions due to stiffer contact/media/interconnect structures. These results demonstrate that the mechanical contribution of the contact layer can be substantial and warrant design consideration

This report presents a hierarchical multiscale modeling scheme based on two-way information exchange. To account for all essential phenomena in waste forms over geological time scales, the models have to span length scales from nanometer to kilometer and time scales from picoseconds to millenia. A single model cannot cover this wide range and a multi-scale approach that integrates a number of different at-scale models is called for. The approach outlined here involves integration of quantum mechanical calculations, classical molecular dynamics simulations, kinetic Monte Carlo and phase field methods at the mesoscale, and continuum models. The ultimate aim is to provide science-based input in the form of constitutive equations to integrated codes. The atomistic component of this scheme is demonstrated in the promising waste form xenotime. Density functional theory calculations have yielded valuable information about defect formation energies. This data can be used to develop interatomic potentials for molecular dynamics simulations of radiation damage. Potentials developed in the present work show a good match for the equilibrium lattice constants, elastic constants and thermal expansion of xenotime. In novel waste forms, such as xenotime, a considerable amount of data needed to validate the models is not available. Integration of multiscale modeling withmore » experimental work is essential to generate missing data needed to validate the modeling scheme and the individual models. Density functional theory can also be used to fill knowledge gaps. Key challenges lie in the areas of uncertainty quantification, verification and validation, which must be performed at each level of the multiscale model and across scales. The approach used to exchange information between different levels must also be rigorously validated. The outlook for multiscale modeling of wasteforms is quite promising.« less

The Exascale Review Panel consists of 12 scientists and engineers with experience in various aspects of high-performance computing and its application, development, and management. The Panel hear presentations by several representatives of the workshops and town meetings convened over the past few years to examine the need for exascale computation capability and the justification for a U.S. Department of Energy (DOE) program to develop such capability. This report summarizes information provided by the presenters and substantial written reports to the Panel in advance of the meeting in Washington D.C. on January 19-20, 2010. The report also summarizes the Panel's conclusions with regard to the justification of a DOE-led exascale initiative.