MA (Moe) Khaleel, Ph.D., P.E. is a Laboratory Fellow and the Director of the Computational Sciences and Mathematics Division at Pacific Northwest National Laboratory. His research interests are in multi-scale modeling of materials, multi-physics modeling of electrochemical device, carbon capture technologies and metal forming processes. Moe is a member of the Washington State Academy of Sciences, a fellow of the American Society of Mechanical Engineers, a fellow of the American Society of Civil Engineers, and a fellow of the American Association for the Advancement of Science. He is also an adjunct Professor in the School of Mechanical and Materials Engineering at Washington State University.
Abstract: We present a pore-scale model of a solid oxide fuel cell (SOFC) cathode. Volatile chromium species are known to migrate from the current collector of the SOFC into the cathode where over time they decrease the voltage output of the fuel cell. A pore-scale model is used to investigate the reactive transport of chromium species in the cathode and to study the driving forces of chromium poisoning. A multi-scale modeling approach is proposed which uses a cell level model of the cathode, air channel and current collector to determine the boundary conditions for a pore-scale model of a section of the cathode. The pore-scale model uses a discrete representation of the cathode to explicitly model the surface reactions of oxygen and chromium with the cathode material. The pore-scale model is used to study the reaction mechanisms of chromium by considering the effects of reaction rates, diffusion coefficients, chromium vaporization, and oxygen consumption on chromium's deposition in the cathode. The study shows that chromium poisoning is most significantly affected by the chromium reaction rates in the cathode and that the reaction rates are a function of the local current density in the cathode. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: 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 recon structed, 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 (C) 2010 Acta Materialia Inc Published by Elsevier Ltd All rights reserved
Abstract: This paper examines the electrochemical and direct internal steam-methane reforming performance of the solid oxide fuel cell when subjected to pressurization. Pressurized operation boosts the Nernst potential and decreases the activation polarization, both of which serve to increase cell voltage and power while lowering the heat load and operating temperature. A model considering the activation polarization in both the fuel and the air electrodes was adopted to address this effect on the electrochemical performance. The pressurized methane conversion kinetics and the increase in equilibrium methane concentration are considered in a new rate expression. The models were then applied in simulations to predict how the distributions of direct internal reforming rate, temperature, and current density are effected within stacks operating at elevated pressure. A generic 10 cm counter-flow stack model was created and used for the simulations of pressurized operation. The predictions showed improved thermal and electrical performance with increased operating pressure. The average and maximum cell temperatures decreased by 3% (20 degrees C) while the cell voltage increased by 9% as the operating pressure was increased from 1 to 10 atm. (C) 2010 Published by Elsevier B.V.
Abstract: Creep deformation of glass sealant used in Solid Oxide Fuel Cell (SOFC) under operating temperature is not negligible. The goal of the study is to develop a creep model to capture the creep behavior of glass ceramic materials at high temperature and to investigate the effect of creep of glass ceramic sealant materials on stresses in glass seal and on the various interfaces of glass seal with other layers. The creep models were incorporated into SOFC-MP and Mentat FC, and finite element analyses were performed to quantify the stresses in various parts. The stress in glass seals were released due to its creep behavior during the operating environments.
Abstract: High-temperature ferritic alloys are potential candidates as interconnect (IC) materials and spacers due to their low cost and coefficient of thermal expansion (CTE) compatibility with other components for most of the solid oxide fuel cells (SOFCs). However, creep deformation becomes relevant for a material when the operating temperature exceeds or even is less than half of its melting temperature (in degrees of Kelvin). The operating temperatures for most of the SOFCs under development are around 1,073 K. With around 1,800 K of the melting temperature for most stainless steel (SS), possible creep deformation of ferritic IC under the typical cell operating temperature should not be neglected. In this paper, the effects of IC creep behaviour on stack geometry change and the stress redistribution of different cell components are predicted and summarised. The goal of the study is to investigate the performance of the fuel cell stack by obtaining the changes in fuel- and air-channel geometry due to creep of the ferritic SS IC, therefore indicating possible changes in SOFC performance under long-term operations. The ferritic IC creep model was incorporated into software SOFC-MP and Mentat-FC, and finite element analyses (FEAs) were performed to quantify the deformed configuration of the SOFC stack under the long-term steady-state operating temperature. It was found that the creep behaviour of the ferritic SS IC contributes to narrowing of both the fuel- and the air-flow channels. In addition, stress re-distribution of the cell components suggests the need for a compliant sealing material that also relaxes at operating temperature.
Abstract: A full statistical analysis of the microstructure of glass-ceramic solid oxide fuel cell (SOFC) seal material, G18, is performed to calculate elastic properties. Predictions are made for samples aged for 4h and 1000 h, giving different crystallinity levels. Microstructure of the glass-ceramic G18 is characterized by correlation function for each individual phase. Predicted results are compared with the Voigt and Reuss bounds in this study. The weak contrast analysis results in elastic modulus predictions between the upper and lower bounds but closer to the upper bound. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: In this paper, we examine the ultimate ductility and failure modes of a TRIP (TRansformation-Induced Plasticity) 800 steel with an advanced micromechanics-based finite element analysis. The representative volume element (RVE) for the TRIP800 under examination is developed based on an actual microstructure obtained from scanning electron microscopy (SEM). The evolution of retained austenite during deformation process and the mechanical properties of the constituent phases of the TRIP800 steel are obtained from the synchrotron-based in-situ high-energy Xray diffraction (HEXRD) experiments and a self-consistent (SC) model. The ductile failure of the TRIP800 under different loading conditions is predicted in the form of plastic strain localization without any prescribed failure criteria for the individual phases. Comparisons of the computational results with experimental measurements suggest that the microstructure-based finite element analysis can well capture the overall macroscopic behavior of the TRIP800 steel under different loading conditions. The methodology described in this study may be extended for studying the ultimate ductile failure mechanisms of TRIP steels as well as the effects of the various processing parameters on the macroscopic behaviors of TRIP steels.
Abstract: An integrated experimental/modeling approach was utilized to assess the structural integrity of Ni-yttriastabilized zirconia (YSZ) porous anode supports during the solid oxide fuel cell (SOFC) operation on coal gas containing trace amounts of phosphorus impurities. Phosphorus was chosen as a typical impurity exhibiting strong interactions with the nickel followed by second phase formation. Tests were performed using Ni-YSZ anode-supported button cells exposed to 0.5-10 ppm of phosphine in synthetic coal gas at 700-800 degrees C. The extent of Ni-P interactions was determined by a post-test scanning electron microscopy (SEM) analysis. Severe damage to the anode support due to nickel phosphide phase formation and extensive crystal coalescence was revealed, resulting in electric percolation loss. The subsequent finite element stress analyses were conducted using the actual anode support microstructures to assist in degradation mechanism explanation. Volume expansion induced by the Ni phase alteration was found to produce high stress levels such that local failure of the Ni-YSZ anode became possible under the operating conditions. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: The migration mechanisms and the corresponding activation energies of Cr-vacancy (Cr-V) clusters and Cr interstitials in alpha-Fe have been investigated using the dimer and the nudged elastic-band methods. Dimer searches are employed to find the possible transition states of these defects and the lowest-energy paths are used to determine the energy barriers for migration. A substitutional Cr atom can migrate to a nearest-neighbor vacancy through an energy barrier of 0.56 eV but this simple mechanism alone is unlikely to lead to the long-distance migration of Cr unless there is a supersaturated concentration of vacancies in the system. The Cr-vacancy clusters can lead to long-distance migration of a Cr atom that is accomplished by Fe and Cr atoms successively jumping to nearest-neighbor vacancy positions, defined as a self-vacancy-assisted migration mechanism, with the migration energies ranging from 0.64 to 0.89 eV. In addition, a mixed Cr-Fe dumbbell interstitial can easily migrate through Fe lattices, with the migration energy barrier of 0.17, which is lower than that of the Fe-Fe interstitial. The on-site rotation of the Cr-Fe interstitial and Cr atom hopping from one site to another are believed to comprise the dominant migration mechanism. The calculated binding energies of Cr-V clusters are strongly dependent on the size of clusters and the concentration of Cr atoms in clusters.
Abstract: Fission product accumulation and gas bubble microstructure evolution in nuclear fuels strongly influence their thermo-mechanical properties such as thermal conductivity, gas release, volume swelling and cracking, and hence fuel performance. In this paper, a general phase-field model is developed to predict gas bubble formation and evolution. Important materials processes and thermodynamic properties including the generation of gas atoms and vacancies, sinks for vacancies and gas atoms, elastic interaction among defects, gas re-solution, and inhomogeneity of elasticity and diffusivity are accounted for in the model. The results demonstrate the potential applications of the phase-field method in investigating: 1) heterogeneous nucleation of gas bubbles at defects; 2) effect of elastic interaction, inhomogeneity of material properties, and gas re-solution on gas bubble microstructures; and 3) effective properties from the output of phase-field simulations such as distribution of defects, gas bubbles, and stress fields.
Abstract: This study utilizes nanoindentation to investigate and measure creep properties of a barium calcium alumino-silicate glass-ceramic used for solid oxide fuel cell seals (SOFCs). Samples of the glass-ceramic seal material were aged for 5, 50, and 100 h to obtain different degrees of crystallinity. Instrumented nanoindentation was performed on the samples with different aging times at different temperatures to investigate the strain rate sensitivity during inelastic deformation. The temperature dependent behavior is important since SOFCs operate at high temperatures (800-1000 degrees C). Results show that the samples with higher crystallinity were more resistant to creep, and the creep compliance tended to decrease with increasing temperature, especially with further aged samples. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: High operating temperatures of solid oxide fuel cells (SOFCs) require that the sealant must function at a high temperature between 600 degrees C and 900 degrees C and in the oxidizing and reducing environments of fuel and air. This paper describes tests to investigate the temporal evolution of the volume fraction of ceramic phases, the evolution of micro-damage, and the self-healing behavior of the glass-ceramic sealant used in SOFCs. It was found that after the initial sintering process, further crystallization of the glass-ceramic sealant does not stop, but slows down and reduces the residual glass content while boosting the ceramic crystalline content. Under a long-term operating environment, distinct fibrous and needle-like crystals in the amorphous phase disappeared, and smeared/diffused phase boundaries between the glass phase and ceramic phase were observed. Meanwhile, the micro-damage was induced by the cooling down process from the operating temperature to room temperature, which can potentially degrade the mechanical properties of the glass/ceramic sealant. The glass/ceramic sealant exhibited self-healing upon reheating to the SOFC operating temperature, which can restore the mechanical performance of the glass/ceramic sealant.
Abstract: Statistical correlation function, including two-point function, is one of the popular methods to digitize microstructure quantitatively. This paper investigated how to represent statistical correlations using layered fast spherical harmonics expansion. A set of spherical harmonics coefficients may be used to represent the corresponding microstructures. It is applied to represent carbon nanotube composite microstructures to demonstrate how efficiently and precisely the harmonics coefficients will characterize the microstructure. This microstructure representation methodology will dramatically improve the computational efficiencies for future works in microstructure reconstruction and property prediction. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Transformation-induced plasticity (TRIP) steel is a typical representative of first generation advanced high-strength steel, which exhibits a combination of high strength and excellent ductility due to its multiphase microstructure. In this article, we study the crack propagation behavior and fracture resistance of a TRIP 800 steel using a microstructure-based finite element method with the various phase properties characterized by in-situ high-energy X-ray diffraction (HEXRD) technique. Uniaxial tensile tests on the notched TRIP 800 sheet specimens were also conducted, and the experimentally measured tensile properties and R curves (resistance curves) were used to calibrate the modeling parameters and to validate the overall modeling results. The comparison between the simulated and experimentally measured results suggests that the micromechanics based modeling procedure can well capture the overall complex crack propagation behaviors and the fracture resistance of TRIP steels. The methodology adopted here may be used to estimate the fracture resistance of various multiphase materials.
Abstract: Advanced high strength steels (AHSS) are performance-based steel grades and their global material properties can be achieved with various steel chemistries and manufacturing processes, leading to various microstructures. In this paper, we investigate the influence of the manufacturing process and the resulting microstructure difference on the overall mechanical properties, as well as the local formability behaviors of AHSS. For this purpose, we first examined the basic material properties and the transformation kinetics of three different commercial transformation induced plasticity, (TRIP) 800 steels under different testing temperatures. The experimental results show that the mechanical and microstructural properties of the TRIP 800 steels significantly), depend on the thermomechanical processing parameters employed in making these steels. Next, we examined the local formability of two commercial dual phase (DP) 980 steels which exhibit noticeably, different formability, during the stamping process. Microstructure-based finite element analysis are carried out to simulate the localized deformation process with the two DP 980 microstructures, and the results suggest that the possible reason for the difference in formability lies in the morphology of the hard martensite phase in the DP microstructure. The results of this study suggest that a set of updated material acceptance and screening criteria is needed to better quantify and ensure the manufacturability of AHSS. [DOI: 10.1115/1.3183778]
Abstract: A microstructure design framework for multiscale modeling of wear resistance in bioimplant materials is presented here. The increase in service lifetime of arthroplasty depends on whether we can predict wear resistance and microstructure evolution of a bioimplant material made from ultra high molecular weight polyethylene during processing. Experimental results show that the anisotropy introduced during deformation increases wear resistance in desired directions. After uniaxial compression, wear resistance along the direction, perpendicular to compression direction, increased 3.3 times. Micromechanical models are used to predict microstructure evolution and the improvement in wear resistance during processing. Predicted results agree well with the experimental data. These models may guide the materials designer to optimize processing to achieve better wear behavior along desired directions. [DOI: 10.1115/1.3183786]
Abstract: With the issuing of the recent Environmental Protection Agency (EPA) report to Congress (EPA, 2007), the issue of data center energy consumption is now a matter of intense industry focus. Key findings of the EPA report a re that most data centers do not know what their energy efficiency is, and an equal number do not even know how to go about measuring such energy efficiency. Knowledge of a data center's energy efficiency is now considered a competitive advantage by many world-class companies. With funding from the Department of Energy, the Pacific Northwest National Laboratory (PNNL) has established its Energy Smart Data Center Test Bed (ESDC-TB), which is housed in a mixed-use facility. In the latest phase of this effort, PNNL has undertaken a major effort to instrument its ESDC-TB TB, and the supporting facility, in order to quantify the energy efficiency of information technology (IT) equipment and infrastructure Under Test (ITUT), in real-time. In addition, PNNL has created a comprehensive software tool, FRED, to acquire, reduce, and display the power consumption of the various parts of the ESDC-TB and the supporting facility in real-time. FRED will soon have the capability to calculate and display the real-time energy efficiency of ITUT-the energy efficiency metric of choice is The Green Grid's Data Center Infrastructure Efficiency (DCiE). This paper describes these capabilities in detail and also describes how to quantify the ITUT's share of the total power consumption for major subsystems such as its facility chiller plant.
Abstract: The aim of this study is to improving microstructure and mechanical properties of the weldable gas pipeline steel using laboratory mill. To achieve the required microstructure and mechanical properties of thermo mechanically processed HSLA steels, it is necessary to have an idea about the role of composition and process parameters. The large numbers of parameters obtained during the production process in the plant were systematically changed to optimize the strength and toughness properties. The optimized parameters were used for the production of the API X60/X70 steel. However, the controlled cooling after rolling should result in transformed products that provide excellent combination of strength and toughness. The coiling at an appropriate temperature have the advantage of the precipitation strengthening, giving further rise to the high yield strength and also improvement in toughness of the steel. The coiling temperature is a decisive parameter because it determines the beginning of the formation of fine precipitations. Therefore, four different laboratory cooling systems were used, in this study to simulate the rolling conditions of a real industrial Thermomechanically controlled process, as close as possible and to check the possibilities of improving the mechanical properties of the welded pipeline steel.
Abstract: Ductile failure of metals is often treated as the result of void nucleation, growth and coalescence. Various criteria have been proposed to capture this failure mechanism for various materials. In this study, ductile failure of dual phase steels is predicted in the form of plastic strain localization resulting from the incompatible deformation between the harder martensite phase and the softer ferrite matrix. Microstructure-level inhomogeneity serves as the initial imperfection triggering the instability in the form of plastic strain localization during the deformation process. Failure modes and ultimate ductility of two dual phase steels are analyzed using finite element analyses based on the actual steel microstructures. The plastic work hardening properties for the constituent phases are determined by the in-situ synchrotron-based high-energy X-ray diffraction technique. Under different loading conditions, different failure modes and ultimate ductility are predicted in the form of plastic strain localization. It is found that the local failure mode and ultimate ductility of dual phase steels are closely related to the stress state in the material. Under plane stress condition with free lateral boundary, one dominant shear band develops and leads to final failure of the material. However, if the lateral boundary is constrained, splitting failure perpendicular to the loading direction is predicted with much reduced ductility. On the other hand, under plane strain loading condition, commonly observed necking phenomenon is predicted which leads to the final failure of the material. These predictions are in reasonably good agreement with experimental observations. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: In this paper, we examine the key factors influencing ductile failure of various grades of dual phase (DP) steels using the microstructure-based modeling approach. Various microstructure-based finite element models are generated based on the actual microstructures of DP steels with different martensite volume fractions. These models are, then, used to investigate the influence of ductility of the constituent ferrite phase and also the influence of voids introduced in the ferrite phase on the overall ductility of DP steels. It is found that with volume fraction of martensite in the microstructure less than 15%, the overall ductility of the DP steels strongly depends on the ductility of the ferrite matrix, hence pre-existing micro-voids in the microstructure significantly reduce the overall ductility of the steel. When the volume fraction of martensite is above 15%, the pre-existing voids in the ferrite matrix does not significantly reduce the overall ductility of the DP steels, and the overall ductility is more influenced by the mechanical property disparity between the two phases. The applicability of the phase inhomogeneity driven ductile failure of DIP steels is then discussed based on the obtained computational results for various grades of DP steels, and the experimentally obtained scanning electron microscopy (SEM) pictures of the corresponding grades of DP steels near fracture surface are used as evidence for result validations. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: In this paper, we present an integrated experimental and modeling methodology in predicting the life of coated and uncoated metallic interconnect (IC) for solid oxide fuel cell (SOFC) applications. The ultimate goal is to provide cell designer and manufacture with a predictive methodology such that the life of the IC system can be managed and optimized through different coating thickness to meet the overall cell designed life. Crofer 22 APU is used as the example IC material system. The life of coated and uncoated Crofer 22 APU under isothermal cooling was predicted by comparing the predicted interfacial strength and the interfacial stresses induced by the cooling process from the operating temperature to room temperature, together with the measured oxide scale growth kinetics. It was found that the interfacial strength between the oxide scale and the Crofer 22 APU substrate decreases with the growth of the oxide scale, and that the interfacial strength for the oxide scale/spinel coating interface is much higher than that of the oxide scale/Crofer 22 APU substrate interface. As expected, the predicted life of the coated Crofer 22 APU is significantly longer than that of the uncoated Crofer 22 APU. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: This paper discusses experimental determination of solid oxide fuel cell (SOFC) glass-ceramic seal material properties and seal/interconnect interfacial properties to support development and optimization of SOFC designs through modeling. Material property experiments such as dynamic resonance, dilatometry, flexure, creep, tensile, and shear tests were performed on PNNL's glass-ceramic sealant material, designated as G18, to obtain property data essential to constitutive and numerical model development. Characterization methods for the physical, mechanical, and interfacial properties of the sealing material, results, and their application to the constitutive implementation in SOFC stack modeling are described. Published by Elsevier B.V.
Abstract: The effects of the mechanical properties of the martensite phase on the failure mode and ductility of dual-phase (DP) steels are investigated using a micromechanics-based finite element method. Actual microstructures of DP steels obtained from scanning electron microscopy (SEM) are used as representative volume elements (RVEs) in the finite element calculations. Ductile failure of the RVE is predicted as plastic strain localization during the deformation process. Systematic computations are conducted on the RVE to quantitatively evaluate the influence of the martensite mechanical properties and volume fraction on the macroscopic mechanical properties of DP steels. These properties include the ultimate tensile strength (UTS), ultimate ductility, and failure modes. The computational results show that, as the strength and volume fraction of the martensite phase increase, the UTS of DP steels increases, but the UTS strain and failure strain decrease. In addition, shear-dominant failure modes usually develop for DP steels with lower martensite strengths, whereas split failure modes typically develop for DP steels with higher martensite strengths. The methodology and data presented in this article can be used to tailor DP steel design for its intended purposes and desired properties.
Abstract: In this study, the effective elastic properties and coefficients of thermal expansion (CTE) of a glass-ceramic were predicted using homogenization techniques. Using G18, a glass-ceramic solid oxide fuel cell (SOFC) sealant as an initial reference material, the effectiveness of different homogenization models was investigated for a two-phase glass-ceramic. The elastic properties and CTEs of the G18 amorphous phase are currently unknown. Thus, estimated values were used as an input to the models. The predictive model offers accurate macroscopic values on both the elastic modulus and the CTE of glass-ceramic materials, providing the estimated amorphous values are reasonable. This model can be used in designing glass-ceramic SOFC seal materials for its specific operation conditions. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: A probabilistic-based component design methodology is developed for a solid oxide fuel cell (SOFC) stack. This method takes into account the randomness in SOFC material properties as well as the stresses arising from different manufacturing and operating conditions. The purpose of this work is to provide the SOFC designers a design methodology so that the desired level of component reliability can be achieved with deterministic design functions using an equivalent safety factor to account for the uncertainties in material properties and structural stresses. Multiphysics-based finite element analyses were used to predict the electrochemical and thermal mechanical responses of SOFC stacks with different geometric variations and under different operating conditions. Failures in the anode and the seal were used as design examples. The predicted maximum principal stresses in the anode and the seal were compared with the experimentally determined strength characteristics for the anode and the seal, respectively. Component failure probabilities for the current design were then calculated under different operating conditions. It was found that anode failure probability is very low under all conditions examined. The seal failure probability is relatively high, particularly for high fuel utilization rate under low average cell temperature. Next, the procedures for calculating the equivalent safety factors for the anode and seal were demonstrated so that a uniform failure probability of the anode and seal can be achieved. Analysis procedures were also included for non-normal distributed random variables so that more realistic distributions of strength and stress can be analyzed using the proposed design methodology.
Abstract: Due to mismatch of the coefficients of thermal expansion of various layers in the positive/electrolyte/negative (PEN) structures of solid oxide fuel cells (SOFC), thermal stresses and warpage on the PEN are unavoidable due to the temperature changes from the stress-free sintering temperature to room temperature during the PEN manufacturing process. In the meantime, additional mechanical stresses will also be created by mechanical flattening during the stack assembly process. In order to ensure the structural integrity of the cell and stack of SOFC, it is necessary to develop failure criteria for SOFC PEN structures based on the initial flaws occurred during cell sintering and stack assembly. In this paper, the global relationship between the critical energy release rate and critical curvature and maximum displacement of the warped cells caused by the temperature changes as well as mechanical flattening process is established so that possible failure of SOFC PEN structures may be predicted deterministically by the measurement of the curvature and displacement of the warped cells. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: We Study the ultimate ductility and failure modes of a commercial transformation-induced plasticity (TRIP) 800 steel under different loading conditions with an advanced microstructure-based finite-element analysis. The representative Volume element (RVE) for the TRIP 800 under examination is developed based oil an actual microstructure obtained from scanning electron microscopy. The ductile failure of the TRIP 800 Under different loading conditions is predicted in (lie form of plastic strain localization without any prescribed failure criteria for the individual phases. This indicates that the microstructure-level inhomogeneity of the various constituent phases can be the key factor influencing the final ductility of the TRIP 800 Steel Under different loading, conditions. Comparisons of the computational results With experimental measurements suggest that the microstructure-based modeling approach accurately captures the overall macroscopic behavior of the TRIP 800 steel Under different loading and boundary conditions. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Abstract: High temperature ferritic alloys are potential candidates as interconnect (IC) materials and spacers due to their low cost and CTE compatibility with other SOFC components for most of the solid oxide fuel cells (SOFC) under development in the SECA program. Possible creep deformation of IC under the typical cell operating temperature should not be neglected. In this paper, the effects of interconnect creep behavior on stack geometry change and stress redistribution of different cell components are predicted and summarized. The goal Of the Study is to investigate the performance of the fuel cell stack by obtaining the fuel and air channel geometry changes due to creep of the ferritic stainless steel interconnect. therefore indicating possible SOPC performance change under long term operations. IC creep models were incorporated into SOFC-MP and Mentat FC, and Finite element analyses were performed to quantify the deformed configuration of the SOFC stack Under the long term steady state operating temperature. It is found that creep behavior of the terrific stainless steel IC contributes to narrowing of both the fuel and the air flow channels. In addition. stress re-distribution of the cell components suggests the need for a compliant sealing material that also relaxes at Operating temperature.
Abstract: An extension of the classical phenomenological dislocation theory U.F. Kocks and H. Mecking, Prog. Mater. Sci. 48 (2003) p. 171, Y. Estrin, J. Mater. Processing Technol. 80-81 (1998) p. 33 is proposed to develop a viscoplastic constitutive equation for steels undergoing (') martensitic phase transformation. Such a class of metallic material exhibits an additional inelastic strain resulting from the phase transformation itself and from the plastic accommodation in parent (austenite) and product (martensite) phases due to different sources of internal stresses. This inelastic strain, known as the transformation-induced plasticity (TRIP) strain, enhances ductility at an appropriate strength level due to the typical properties of martensite. The principal features of martensitic transformation at different scales are discussed and a macroscopic model derived from microscopic considerations. The material is considered as a combination of two viscoplastic phases, where the martensitic one is considered as a strengthening phase with evolving volume fraction. The methodology consists of two parts: a combination of two kinetics laws, which describe the material response at a given microstructure with the corresponding evolution equations of the appropriate internal variables and provide the constitutive equation of the two phases; a viscoplastic self-consistent homogenization technique that provides the constitutive equation of the two-phase composite material. The model could be regarded as a semi-phenomenological approach with sufficient link between microstructure and overall properties, and therefore with good predictive capabilities. Its simplicity allows a modular structure for its implementation in metal forming codes.
Abstract: We Study the temperature dependent Young's modulus for the glass/ceramic seal material used in solid oxide fuel cells (SOFCs). With longer heat treatment or aging time during operation, further devitrification may reduce the residual glass content in the seal material while boosting the ceramic crystalline content. In the meantime, micro-voids induced by the cooling process from the high operating temperature to room temperature can potentially degrade the mechanical properties of the glass/ceramic sealant. Upon reheating to the SOFC operating temperature, possible self-healing phenomenon may occur in the glass/ceramic sealant which can potentially restore some of its mechanical properties. A phenomenological model is developed to model the temperature dependent Young's modulus of glass/ceramic seal considering the combined effects of aging, micro-voids, and possible self-healing. An aging time-dependent crystalline content model is first developed to describe the increase of the crystalline content due to the continuing devitrification under high operating temperature. A continuum damage mechanics (CDM) model is then adapted to model the effects of both cooling induced micro-voids and reheating induced self-healing. This model is applied to model the glass-ceramic G18, a candidate SOFC seal material previously developed at PNNL Experimentally determined temperature-dependent Young's modulus is used to validate the model predictions. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Multiple-step ultrasonic spray pyrolysis was developed to produce a gradient porous lanthanum strontium manganite (LSM) cathode on yttria-stabilized zirconia (YSZ) electrolyte for use in intermediate temperature solid oxide fuel cells (IT-SOFCs). The effect of solvent and precursor type on the morphology and compositional homogeneity of the LSM film was first identified. The LSM film prepared from organometallic precursor and organic solvent showed a homogeneous crack-free microstructure before and after hear treatment as opposed to aqueous solution. With respect to the effect of processing parameters, increasing the temperature and solution flow rate in the specific range of 520-580 degrees C leads to change the microstructure from a dense to a highly porous structure. Using a dilute organic solution a nanocrystal line thin layer was first deposited at 520 degrees C and Solution flow rate of 0.73 ml/min on YSZ surface: then, three gradient porous layers were sprayed from concentrated Solution at higher temperatures (540-580 degrees C) and Solution flow rates (1.13-1.58 ml/min) to form a gradient porous LSM cathode film with similar to 30 mu m thickness. The microstructure, phase crystallinity and compositional homogeneity or the fabricated films were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive analysis of X-ray (EDX). Results showed that the spray pyrolized gradient film fabricated in the temperature range of 520-580 degrees C is composed of highly crystalline LSM phase which can remove the need for subsequent hear treatment. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Thermal stress and warpage of the PEN are unavoidable due to the temperature change from the stress-free sintering temperature to room temperature and mismatch of the coefficient of thermal expansion (CTE) of various layers in the PEN structures of solid oxide fuel cells (SOFC) during the PEN manufacturing process. In the meantime, additional mechanical stresses will also be created by mechanical flattening during the stack assembly process. The porous nature of anode and cathode in the PEN structures determines presence of the initial flaws and crack on the interfaces of anode/electrolyte/cathode and in the interior of the materials. The sintering/assembling induced stresses may cause the fracture failure of PEN structure. Therefore, fracture failure criteria for SOFC PEN structures is developed in order to ensure the structural integrity of the cell and stack of SOFC. In this paper, the fracture criteria based on the relationship between the critical energy release rate and critical curvature and maximum displacement of the warped cells caused by the temperature changes as well as mechanical flattening process is established so that possible failure of SOFC PEN structures may be predicted deterministically by the measurement of the curvature and displacement of the warped cells.
Abstract: Compared to other advanced high-strength steels, transformation-induced plasticity (TRIP) steels exhibit better ductility at a given strength level and can be used to produce complicated automotive parts. This enhanced formability comes from the transformation of retained austenite to martensite during plastic deformation. In this study, as a first step in predicting optimum processing parameters in TRIP steel productions, a micromechanical finite element model is developed based on the actual microstructure of a TRIP 800 steel. The method uses a microstructure-based representative volume element (RVE) to capture the complex deformation behavior of TRIP steels. The mechanical properties of the constituent phases of the TRIP 800 steel and the fitting parameters describing the martensite transformation kinetics are determined using the synchrotron-based in-situ high-energy X-ray diffraction (HEXRD) experiments performed under a uniaxial tensile deformation. The experimental results suggest that the HEXRD technique provides a powerful tool for characterizing the phase transformation behavior and the microstress developed due to the phase-to-phase interaction of TRIP steels during deformation. The computational results suggest that the response of the RVE well represents the overall macroscopic behavior of the TRIP 800 steel under deformation. The methodology described in this study may be extended for studying the effects of the various processing parameters on the macroscopic behaviors of TRIP steels.
Abstract: This paper presents the limit analyses of instantaneous crack propagation angle in the classical linear-elastic Hertzian field. The formation and propagation of Hertzian cone crack have been studied extensively in the open literature. Many experiments have been carried out on soda-lime glass under static loading. The observed final Hertzian cone angle is usually around 22, and it tends to increase with increasing indentation velocity. The goal of this paper is to use the crack tip stress field derived in linear elastic fracture mechanics (LEFM) to bound the instantaneous surface ring crack extension angles in the Hertzian field. The upper and lower bounds for the instantaneous crack extension angle are expressed in terms of the ratios of the mode I and mode II stress intensity factors at the crack tip field. It is shown that under a pure mode I dominant stress field, instantaneous crack growth would follow the original surface crack direction; and under a perfect mode II stress field, the instantaneous crack extension angle has a limiting angle of 19.5 degrees. The actual instantaneous crack extension angle will fall between these two limit states depending on the ratio of K-I/K-II at the crack tip.
Abstract: This paper describes the fabrication process for the thin cast-in-place laminate glazing systems to be used in cars of the future to achieve the weight reduction goals of FreedomCAR. The primary objective of the project is to reduce vehicle weight, improve fuel economy, and reduce vehicle emissions through the use of structurally reliable, high acoustic performance, and lightweight glazing systems with low manufacturing costs. Energy savings come from reducing weight by using thinner glazing: prior studies at Pacific Northwest National Laboratory (PNNL) have demonstrated a potential of 30% weight reductions compared with standard glazing system. Energy savings will also come from reducing interior heat loads; that, in turn, will reduce the demand for air conditioning. The evaluation of alternative glazing concepts seeks to improve acoustical performance such that reduced interior noise levels can be achieved while maintaining glazing at minimal thickness and weight levels. The most important factor in utilizing laminated glazing systems as vehicle side glass is its advantage in cost savings for material and manufacturing processes.
Abstract: A numerical modeling framework for planar solid-oxide fuel cell (PSOFC) based vehicular auxiliary power unit (APU) is developed. The power-conditioning system (PCS) model comprises the comprehensive transient models of PSOFC, balance-of-plant and power-electronics subsystems (BOPS and PES, respectively) and application load (AL). It can be used for resolving the interactions among PSOFC, BOPS, PES and AL, control design and system optimization and studying fuel-cell durability. The PCS model has several key properties including: (i) it can simultaneously predict spatial as well as temporal dynamics; (ii) it has two levels of abstraction: comprehensive (for detailed dynamics) and reduced-order (for fast simulation); and (iii) the fast-simulation model can be implemented completely in Simulink/Matlab environment, thereby significantly reducing the cost as well as time and provides the avenue for real-time simulation and integration with vehicular power-train models employing the widely used ADVISOR. The computational overhead and accuracy of the fast-simulation and comprehensive models are compared. Significant savings in time compared to using the former were obtained, without compromising accuracy.
Abstract: This paper examines the effects of fusion zone size on failure modes, static strength and energy absorption of resistance spot welds (RSW) of advanced high strength steels (AHSS) under lap shear loading condition. DP800 and TRIP800 spot welds are considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. Static weld strength tests using lap shear samples were performed on the joint populations with various fusion zone sizes. The resulted peak load and energy absorption levels associated with each failure mode were studied for all the weld populations using statistical data analysis tools. The results in this study show that AHSS spot welds with conventionally required fusion zone size of 4 root t cannot produce nugget pullout mode for both the DP800 and TRIP800 welds under lap shear loading. Moreover, failure mode has strong influence on weld peak load and energy absorption for all the DP800 welds and the TRIP800 small welds: welds failed in pullout mode have statistically higher strength and energy absorption than those failed in interfacial fracture mode. For TRIP800 welds above the critical fusion zone level, the influence of weld failure modes on peak load and energy absorption diminishes. Scatter plots of peak load and energy absorption versus weld fusion zone size were then constructed, and the results indicate that fusion zone size is the most critical factor in weld quality in terms of peak load and energy absorption for both DP800 and TRIP800 spot welds. (C) 2007 Elsevier Ltd. All rights reserved.
Abstract: The interfacial adhesion strength between the oxide scale and the substrate is crucial to the reliability and durability of metallic interconnects in solid oxide fuel cell (SOFC) operating environments. It is necessary, therefore, to establish a methodology to quantify the interfacial adhesion strength between the oxide scale and the metallic interconnect substrate, and furthermore to design and optimize the interconnect material as well as the coating materials to meet the design life of an SOFC system. In this paper, we present an integrated experimental/analytical methodology for quantifying the interfacial adhesion strength between the oxide scale and a ferritic stainless steel interconnect. Stair-stepping indentation tests are used in conjunction with subsequent finite element analyses to predict the interfacial strength between the oxide scale and Crofer 22 APU substrate. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: The mechanical properties and response of two polypropylene (PP)-based composites have been determined for small strains and for a range of strain rates in the quasi-static domain. These two materials are talc-filled and unfilled high-impact PP. Uniaxial tensile tests were performed at different strain rates in order to characterize the mechanical response and the strain rate effect. The experimental results showed that both unfilled and talc-filled high-impact PP were sensitive to strain rate and exhibited nonlinear behavior even at relatively low strains. SEM analysis was conducted to obtain a better comprehension of deformation mechanisms involved during loading by observations of the microstructure evolution. For each of these two materials, two existing modeling approaches are proposed. The first one is a three-parameter nonlinear constitutive model based on the experimental results. The second is a micromechanically based approach for the elastic-viscoplastic behavior of the composite materials. The stress-strain curves predicted by these models are in fairly good agreement with our experimental results.
Abstract: A methodology for microstructure design of fuel cell materials is developed. Statistical continuum mechanics theory is used to link mechanical, magnetic, and transport properties to microstructures represented. Texture, composite volume fractions, grain boundary character distribution, particle to particle and the void distributions are taken into consideration by using pair correlation functions and higher order statistics. Effective properties for transport and mechanical properties were calculated. The use of scattering techniques provides 3-dimensional information on two-point statistics. The information for microstructure is used in a general framework for microstructure design to optimize the properties of fuel cell materials.
Abstract: The objective of this work was to provide a modeling tool for the design of reliable seals for SOFC stacks. The work consisted of experimental testing to determine thermal-mechanical properties of a glass-ceramic sealing material and numerical modeling of stack sealing systems. The material tests captured relevant temperature-dependent property data for Pacific Northwest National Laboratory's (PNNL) G18 sealant material as required by the analytical models. A viscoelastic continuum damage model for this glass-ceramic sealant was developed and implemented in the MSC MARC finite element code and used for a detailed analysis of a planar SOFC stack under thermal cycling conditions. Realistic thermal loads for the stack were obtained using PNNL's multiphysics solver SOFC-MP. The accumulated seal damage and component stresses were evaluated for multiple thermal loading cycles. The seals nearest the stack mount location were most susceptible to damage which began during the first operational cycle and accumulated during shutdown. Viscoelastic seal compliance was also found to beneficially reduce the stresses in the anode.
Abstract: Providing adequate and efficient cooling schemes for solid-oxide-fuel-cell (SOFC) stacks continues to be a challenge coincident with the development of larger, more powerful stacks. The endothermic steam-methane reformation reaction can provide cooling and improved system efficiency when performed directly on the electrochemically active anode. Rapid kinetics of the endothermic reaction typically causes a localized temperature depression on the anode near the fuel inlet. It is desirable to extend the endothermic effect over more of the cell area and mitigate the associated differences in temperature on the cell to alleviate subsequent thermal stresses. In this study, modeling tools validated for the prediction of fuel use, on-cell methane reforming, and the distribution of temperature within SOFC stacks are employed to provide direction for modifying the catalytic activity of anode materials to control the methane conversion rate. Improvements in thermal management that can be achieved through on-cell reforming is predicted and discussed. Two operating scenarios are considered, one in which the methane fuel is fully pre-reformed and another in which a substantial percentage of the methane is reformed on-cell. For the latter, a range of catalytic activity is considered, and the predicted thermal effects on the cell are presented. Simulations of the cell electrochemical and thermal performance with and without on-cell reforming, including structural analyses, show a substantial decrease in thermal stresses for an on-cell reforming case with slowed methane conversion rate.
Abstract: In this paper, a dynamic model of the solid-oxide fuel cell (SOFC) power unit is developed for the purpose of designing a controller to regulate fuel flow rate, fuel temperature, air flow rate, and air temperature to maintain the SOFC stack temperature, fuel utilization rate, and voltage within operati I on limits. A lumped model is used to consider the thermal dynamics and the electro-chemical dynamics inside an SOFC power unit. The fluid dynamics at the fuel and air inlets are considered by using the in-flow ramp-rates. A simulation result has been presented to illustrate the dynamic response of an SOFC stack when changing the electric load and fuel flow rates.
Abstract: This paper summarizes the dynamic joint strength evaluation procedures and the measured dynamic strength data for 13 joint populations of self-piercing rivets (SPR) and resistance spot welds (RSWs) joining similar and dissimilar metals. A state-of-the-art review of the current practice for conducting dynamic tensile/compressive strength tests in different strain rate regimes is first presented, and the generic issues associated with dynamic strength test are addressed. Then, the joint strength testing procedures and fixture designs used in the current study are described, and the typical load versus displacement curves under different loading configurations are presented. Uniqueness of the current data compared with data in the open literature is discussed. The majority of experimental results indicate that joint strength increases with increasing loading rate. However, the strength increase from 4.47 m/s (10 mph) to 8.94 m/s (20 mph) is not as significant as the strength increase from static to 4.47 m/s. It is also found that with increasing loading velocity, displacement to failure decreases for all the joint samples. Therefore, 'brittleness' of the joint sample increases with impact velocity. Detailed static and dynamic strength data and the associated energy absorption levels for all the samples in the 13 joint populations are also included. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: Statistical continuum approach is used to predict effective conductivity of anisotropic random porous heterogeneous media using two-point correlation functions. Probability functions play a critical role in describing the statistical distribution of different constituents in a heterogeneous media. In this study a 3-dimensional two-point correlation function is utilized to characterize the anisotropic porous media of a Cathode materials to incorporate all the details of the microstructure. These correlation functions are then linked to the effective properties using homogenization relations. An anisotropioc Green's function solution is used to solve the set of field equations. Examples in this study demonstrated how the model captured the anisotropy in effective conductivity of the random heterogeneous media. Predicted results showed the influence of microstructure on the effective conductivity tensor.
Abstract: This paper examines the effects of fusion zone size on failure modes, static strength, and energy absorption of resistance spot welds (RSW) of advanced high-strength steels (AHSS). DP800 and TRIP800 spot welds were considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. The critical fusion zone sizes to ensure nugget pullout failure mode are developed for both DP800 and TRIP800 using limit load based analytical model and microhardness measurements of the weld cross sections. Static weld strength tests using cross-tension samples were performed on the joint populations with controlled fusion zone sizes. The resulting peak load and energy absorption levels associated with each failure mode were studied for all the weld populations using statistical data analysis tools. The results in this study show that AHSS spot welds with fusion zone size of 4 root t-cannot produce nugget pullout mode for both the DP800 and TRIP800 materials examined. The critical fusion zone size for nugget pullout shall be derived for individual materials based on different base metal properties as well as different heat-affected zone (HAZ) and weld properties resulting from different welding parameters.
Abstract: This paper summarizes the fatigue test results of self-piercing rivet (SPR) joints between similar and dissimilar sheet metals. The influences of material grades, material thickness, piercing direction and the use of structural adhesive on the rivet samples' fatigue behaviors were investigated. Fatigue test results indicate that SPR joints have superior fatigue strength than resistance spot weld (RSW) joints for the same material combinations. The application of structural adhesive also significantly enhances the fatigue strength of the joint samples; this is particularly true for the lap-shear loading configuration. In addition, different piercing directions for SPR joints have a noticeable effect on the static and fatigue strength of the joints. The joint fatigue results presented in this paper offer design engineers the durability data for SPR joints with various material combinations under different loading conditions. Moreover, it provides manufacturing engineers with some insights on the effects of different manufacturing parameters on the strength and durability of these joints. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: This paper addresses the damage and fracture issues of glass and ceramic materials used in solid oxide fuel cells. Analyses of an internal crack and of an interface crack between dissimilar materials were conducted using a modified boundary layer modeling approach. In this approach, fracture is allowed to occur in a small process window situated at an initial crack tip. Elastic displacement crack-tip fields are prescribed as remote boundary conditions. Crack propagation was first modeled discretely. Next, a continuum damage mechanics (CDM) model for brittle materials was developed to capture damage and crack growth in the process window. In particular, the damage model was applied to a glass-ceramic material that had been developed in-house for sealing purposes. Discrete and continuum damage solutions were then compared. Finally, the CDM model was used to determine the crack propagation direction as a function of a mode mixity measure.
Abstract: This paper studies the nonlinear behavior of a glassceramic seal used in planar solid oxide fuel cells (SOFCs). To this end, a viscoelastic damage model has been developed that can capture the nonlinear material response due to both progressive damage in the glass-ceramic material and viscous flow of the residual glass in this material. The model has been implemented in the MSC MARC finite element code, and its validation has been carried out using the experimental relaxation test data obtained for this material at 700 degrees C, 750 degrees C, and 800 degrees C. Finally, it has been applied to the simulation of a SOFC stack under thermal cycling conditions. The areas of potential damage have been predicted.
Abstract: Statistical continuum approach is used to predict effective conductivity of anisotropic random porous heterogeneous media using two-point correlation functions. Probability functions play a critical role in describing the statistical distribution of different constituents in a heterogeneous media. In this study a three-dimensional two-point correlation function is utilized to characterize the anisotropic media without making any assumption on the microstructure. Examples in this study demonstrated how the model captured the anisotropy in effective conductivity of the random heterogeneous media. Predicted results showed the influence of microstructure on the effective conductivity tensor. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: In this research, a Simulink model of a standalone vehicular solid-oxide fuel cell (SOFC) auxiliary power unit (APU) is developed. The SOFC APU model consists of three major components: a controller model; a power electronics system model; and an SOFC plant model, including an SOFC stackmodule, two heat exchanger modules, and a combustor module. This paper discusses the development of the nonlinear dynamic models for the SOFC stacks, the heat exchangers and the combustors. When coupling with a controller model and a power electronic circuit model, the developed SOFC plant model is able to model the thermal dynamics and the electrochemical dynamics inside the SOFC APU components, as well as the transient responses to the electric loading changes. It has been shown that having such a model for the SOFC APU will help design engineers to adjust design parameters to optimize the performance. The modeling results of the SOFC APU heat-up stage and the output voltage response to a sudden load change are presented in this paper. The fuel flow regulation based on fuel utilization is also briefly discussed. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: A multiscale model based on statistical continuum mechanics is proposed to predict the mechanical and electrical properties of heterogeneous porous media. This model is applied within the framework of microstructure sensitive design (MSD) to guide the design of the microstructure in porous lanthanum strontium manganite (LSM) fuel cell electrode. To satisfy the property requirement and compatibility, porosity and its distribution can be adjusted under the guidance of MSD to achieve optimized microstructure.
Abstract: This paper summarises the authors' work on strength and failure mode estimation of self-piercing rivets (SPRs) for automotive applications. First, the static cross tension strength of an SPR joint is estimated using a lower bound limit load based strength estimator. Failure mode associated with the predicted failure strength can also be identified. It is shown that the cross tension strength of an SPR joint depends on the material and gage combinations, rivet design, die design and riveting direction. The analytical rivet strength estimator is then validated by experimental rivet strength measurements and failure mode observations from nine SPR joint populations with various material and gage combinations. Next, the estimator is used to optimise rivet strength. Two illustrative examples are presented in which rivet strength is improved by changing rivet length and riveting direction from the original manufacturing parameters.
Abstract: A constitutive model based on continuum damage mechanics is used to study the stone-impact resistance of automotive windshields. An axisymmetric finite element model is created to simulate the transient dynamic response and impact-induced damage tensors for laminated glass layers subject to stone-impact loading. The windshield glass consists of two glass outer layers laminated by a thin poly(vinyl butyral) (PVB) layer. The constitutive behaviour of the glass layers is simulated using the continuum damage mechanics model with linear damage evolution. The PVB layer is modelled with a linear viscoelastic solid. The model is used to predict and examine damage patterns on different glass surfaces for different windshield designs including variations in ply thickness and curvatures.
Abstract: A micro-macro mechanistic approach to damage in short-fiber composites is developed in this paper. At the microscale, a reference aligned fiber composite is considered for the analysis of the damage mechanisms such as matrix cracking and fiber-matrix debonding using the modified Mori-Tanaka model. The associated damage variables are defined, and the stiffness reduction law dependent on these variables is established. The stiffness of a random fiber composite containing random matrix microcracks and imperfect interfaces is then obtained from that of the reference composite, which is averaged over all possible orientations and weighted by an orientation distribution function. The macroscopic response is determined using a continuum damage mechanics approach and finite element analysis. Final failure resulting from saturation of matrix microcracks, fiber pull-out and breakage is modeled by a vanishing element technique. The model is validated using the experimental results found in literature as well as the results obtained for a random chopped fiber glass-vinyl ester system. Acoustic emission techniques were used to quantify the amount and type of damage during quasi-static testing.
Abstract: The stone-impact resistance of a monolithic glass ply is studied using a combined experimental and computational approach. Instrumented stone-impact tests are first carried out in a controlled environment. Explicit finite element analyses are then used to simulate the interactions of the indentor and the glass layer during the impact event, and a continuum damage mechanics (CDM) model is used to describe the constitutive behavior of glass. The experimentally measured strain histories for low-velocity impact serves as validation of the modeling procedures. Next, stair-stepping impact experiments are performed with two indenter sizes on two glass ply thicknesses, and the test results are used to calibrate the critical stress parameters used in the CDM constitutive model. The purpose of this study is to establish the modeling procedures and the CDM critical stress parameters under impact loading conditions. The modeling procedures and the CDM model will be used in our future studies to predict through-thickness damage evolution patterns for different laminated windshield designs in automotive applications.
Abstract: In this paper, we present two levels of electrochemical modeling for solid oxide fuel cells: cell continuum and microscale electrochemistry. The microscale electrochemistry model simulates the performance of porous electrode materials based on the microstructure of the material, the distribution of reaction surfaces, and the transport of oxygen ions through the material. The overall fuel cell current-voltage relations are obtained using the microscale electrochemistry modeling and form the basic input to the continuum level electrochemistry model. The continuum electrochemistry model calculates the current electrical density, cell voltage, and heat production in fuel cell stacks with H-2 or other fuels, taking into account as inputs local values of the gas partial pressures and temperatures. This approach is based on a parameterized current-voltage (I-V) relation and includes the heat generation from both joule heating and chemical reactions. It also accounts for species production and destruction via mass balance. The continuum electrochemistry model is then coupled with a flow-thermal-mechanical simulation framework for fuel cell stack design and optimizing operating conditions.
Abstract: In this paper, we report predicted results for texture evolution in FCC metals under uniaxial compression test. These results are computed using a newly developed nonlinear rigid viscoplastic crystal plasticity model based on an intermediate interaction law. This interaction law is formulated by the minimization of a normalized error function which combines the local fields' deviations, from the macroscopic ones, obtained by the classical upper bound (Taylor) and lower bound (Sachs) models. This interaction law leads to results lying between the upper and lower bound approaches by simply varying a scalar weight function phi (0 < phi < 1). A simple interaction law based on the linear mixture of the fields from the Taylor and Sachs models is also used. The results from these both the linear and nonlinear intermediate approaches are shown in terms of texture evolution under uniaxial compression. These results are discussed in comparison with the well known experimental textures in compressed FCC metals. Finally, we show that the linear intermediate approach yields fairly acceptable texture predictions under compression and that the fully non-linear approach predicts much better results.
Abstract: 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 l(max)) on the prediction accuracy of texture evolution is investigated.
Abstract: This paper summarizes work on finite element modeling of nugget growth for resistance spot welding of aluminum alloy to steel. It is a sequel to a previous paper on experimental studies of resistance spot welding of aluminum to steel using a transition material. Since aluminum alloys and steel cannot be readily fusion welded together due to their drastically different thermal physical properties, a cold-rolled clad material was introduced as a transition to aid the resistance welding process. Coupled electrical-thermal-mechanical finite element analyses were performed to simulate the nugget growth and heat generation patterns during the welding process. The predicted nugget growth results were compared to the experimental weld cross sections. Reasonable comparisons of nugget size were achieved. The finite element simulation procedures were also used in the electrode selection stage to help reduce weld expulsion and improve weld quality.
Abstract: This paper examines the effects of fusion zone size on failure modes, static strength, and energy absorption of aluminum spot welded samples using a combined experimental, statistical, and analytical approach. The main failure modes for aluminum spot welds are nugget pullout and interfacial fracture. First, static strength tests using coupon configurations of lap shear, cross tension, and coach peel were performed on the joint populations with a controlled fusion zone size. Thirty replicate static strength tests were performed for each coupon configuration. The resulting peak load and energy absorption levels associated with each failure mode were studied using statistical models. Next, an analytical model was developed to determine the failure mode of an aluminum resistance spot weld based on limit load analyses. It was found that fusion zone size, sheet thickness, and the level and location of weld porosity/defects are the main factors influencing the cross-tension failure mode of an aluminum spot weld. Two additional spot weld populations with different fusion zone sizes were then fabricated to validate the analytical failure mode model. Static cross-tension tests were again performed, and the experimental observations confirmed the analytically predicted failure modes for each population.
Abstract: This paper summarizes our work to date on resistance spot welding of aluminum alloy to steel, from process development to performance evaluation. Since aluminum alloys and steel cannot be readily fusion welded together due to their drastically different thermal physical properties, a cold-rolled clad material was introduced as a transition to aid the resistance welding process. The optimal welding parameters and electrode selections were established using experimental approaches. The welded samples' mechanical behaviors were then evaluated using static and dynamic weld strength tests as well as cyclic fatigue tests. The weld strength, failure mode, and fatigue life were then compared with self-piercing rivets of the same dissimilar metals combination. Statistical analyses were also performed to analyze the effects of different failure modes on samples' peak strength and energy absorption.
Abstract: A 3D simulation tool for modeling solid oxide fuel cells is described. The tool combines the versatility and efficiency of a commercial finite element analysis code, MARC((R)), with an in-house developed robust and flexible electrochemical (EC) module. Based upon characteristic parameters obtained experimentally and assigned by the user, the EC module calculates the current density distribution, heat generation, and fuel and oxidant species concentration, taking the temperature profile provided by MARC((R)) and operating conditions such as the fuel and oxidant flow rate and the total stack output voltage or current as the input. MARC((R)) performs flow and thermal analyses based on the initial and boundary thermal and flow conditions and the heat generation calculated by the EC module. The main coupling between MARC((R)) and EC is for MARC((R)) to supply the temperature field to EC and for EC to give the heat generation profile to MARC((R)). The loosely coupled, iterative scheme is advantageous in terms of memory requirement, numerical stability and computational efficiency. The coupling is iterated to self-consistency for a steady-state solution. Sample results for steady states as well as the startup process for stacks with different flow designs are presented to illustrate the modeling capability and numerical performance characteristic of the simulation tool. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: A numerical control algorithm is described that predicts the axial end-feed and internal pressure loads to give maximum formability of circular tubes during hydroforming. The controller tracks the stresses, strains and mechanical response of the incremental finite element solution to estimate the proper axial feed (end-feed) and internal pressure increments to apply in the next increment as the tube deforms. The algorithm uses the material stress-strain curve and the deformation theory of plasticity with Hill's criterion to relate the current stress and strain increments (from the finite element model) to the next applied load increments. A controlled increment in plastic strain is prescribed for the next solution increment, and the pressure and end-feed increments are calculated to give a constant ratio of incremental axial and hoop strains. Hydroforming simulations using this method were conducted to predict the load histories for controlled expansion of 6061-T4 aluminum tubes within a conical die shape and under free hydroforming conditions. The predicted loading paths were applied in hydroforming experiments to form the conical and free-formed tube shapes. The model predictions and experimental results are compared in this paper for deformed shape, strains and the extent of forming at rupture. (C) 2003 Published by Elsevier Ltd.
Abstract: A micro-macro mechanistic approach to damage in short-fibre composites is developed in this paper. At the micro-scale, the damage mechanisms such as matrix cracking and fibre/matrix debonding are analysed to define the associated damage variables. The stiffness reduction law dependent on these variables is then established using micromechanical models and average orientation distributions of fibres and microcracks. The macroscopic response is obtained by means of thermodynamics of continuous media, continuum damage mechanics and a finite element formulation.
Abstract: The response of soda-lime glass subjected to the stress field induced by the static indentation of a spherical indenter is studied using continuum damage mechanics (CDM). An anisotropic damage tensor with linear damage evolution law is chosen to model the cracking damage. An axisymmetric finite element model is generated to simulate the static indentation process. The damage pattern and zone size are predicted for both the loading cycle and the unloading cycle, and the comparison between the predictions and the experimental results reported in the open literature serves as a validation of the CDM model and the modeling procedure.
Abstract: A micro-macro mechanistic approach to matrix cracking in randomly oriented short-fiber composites is developed in this paper. At the micro-scale, the virgin and reduced elastic properties of the reference aligned fiber composite are determined using micromechanical models [Proc. Roy Soc. Lond. A241 (1957) 376; Acta Metall. 21 (1973) 571; Mech. Mater. 2 (1983) 123], and are then distributed over all possible orientations in order to compute the stiffness of the random fiber composite containing random matrix microcracks. After that the macroscopic response is obtained by means of a continuum damage mechanics formulation, which extends the thermodynamics based approach in [Comp. Sci. Technol. 46 (1993) 29] to randomly oriented short-fiber composites. Damage accumulations leading to initiation and propagation of a macroscopic crack are modeled using a vanishing element technique. The model is validated against the published experimental data and results [Comp. Sci. Technol 55 (1995) 171]. Finally, its practical application is illustrated through the damage analysis of a random glass/epoxy composite plate containing a central hole and under tensile loading. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: We have developed a method for measurement of internal stress in glass articles. The method uses Rayleigh-scattered light from a properly polarized laser beam propagating through glass at an oblique angle. This light is imaged with an electronic focal plane array camera. The method is similar to earlier published methods except for the inclusion of an externally controlled phase retarder. The phase retarder allows for the success of the method. The method is suitable for use on flat or curved glass and is applicable over a broad range of residual stresses. Experimental results are provided showing the in-plane stress throughout the thickness of a television glass sample.
Abstract: Through a variety of programs, Pacific Northwest National Laboratory is working with government agencies and private industries to help bring SOFC-based technology to the commercial marketplace. These activities include the development of advanced materials and fabrication techniques for SOFC stack components, the development and utilization of modeling tools for optimization of cell and stack designs, and the development and application of advanced characterization techniques to increase the understanding of fundamental electrochemical processes occurring in SOFCs.
Abstract: A micro-macro mechanistic approach to matrix cracking in randomly oriented short-fiber composites is used in this paper for the damage and failure analysis of a random glass/epoxy plate containing a central hole under tensile loading. At the micro-scale, the virgin and reduced elastic properties of the composite are computed using micromechanical models and are then averaged over all possible orientations and weighted by an orientation distribution function. Next, the macroscopic response is performed by means of a continuum damage mechanics formulation in which the damage evolution law is obtained using a damage threshold function and the concepts of thermodynamics of continuous media. Damage accumulations leading to initiation and propagation of a macroscopic crack are modeled using a vanishing element technique.
Abstract: The electrochemical performance of electrodes, such as those used in solid oxide fuel cells, depends to a large extent on the microstructure of the electrode material. A modeling approach, based on the lattice Boltzmann simulation method, has been developed to describe the detailed three-dimensional transport and reaction phenomena occurring in a representative sample of porous electrode material. The model geometry is obtained by obtaining statistical information from a two-dimensional micrograph and reconstructing an equivalent three-dimensional structure. The usefulness of this modeling approach is demonstrated by simulating a simplified two-dimensional electrode.
Abstract: In order to reduce the trial-and-error practices, which are time-consuming and expensive, numerical methods for predicting the material formability and tile hydroforming parameters are essential. This paper presents all inverse approach to tube hydroforming to efficiently predict the thickness, strain and pressure distributions for a given deformed configuration of aluminum AA6061-T4 tubes under free hydroforming conditions or hydroformed within a conical die. Tile analysis employed a membrane finite element formulation within the framework of the deformation theory and Hill's criterion to describe the plastic flow. Hydroforming experiments of aluminum tubes using a conical die and under free conditions were also conducted, and the experimental results were compared with the numerical predictions.
Abstract: The U. S. Department of Energy initiated the Solid State Energy Conversion Alliance (SECA) to facilitate the development of solid oxide fuel cell modules based on mass-customization concepts for use with commonly available fossil fuels at low cost. The U. S. Department of Energy's National Energy Technology Laboratory and Pacific Northwest National Laboratory coordinate SECA activities. Commercial developers, universities, and government agencies, and other national laboratories participate in the Alliance in a tightly coordinated structure to develop precommercial prototypes. The SECA Core Technology Program supports industrial development teams by providing problem-solving research to overcome common technical barriers identified by the industry teams. Core Technology activities include programs in fuel processing, manufacturing, controls and diagnostics, power electronics, modeling and simulation, and materials. The modeling and simulation program develops computational tools to support development and commercialization of SECA technology. This paper reviews the various development and validation activities and availability of modeling tools at DOE national laboratories as pan of the SECA Core Technology modeling and simulation program.
Abstract: A numerical process control method developed in [1] is used in this paper to determine the deformations observed in tube hydroforming experiments. This method was incorporated in the finite element simulations to predict the pressure/end-feed histories to achieve maximum tube deformations without wrinkling. Tile analysis used deformation theory and Hill's criterion to describe the plastic flow, and forming limit data to predict the onset of rupture. The computed loading paths were applied in forming experiments using extruded aluminum 6061-T4 tubes that were hydroformed under free conditions and within a conical die. Tile resulting strains, deformations and loads to failure were measured for comparison with the model predictions.
Abstract: A simulation tool for modeling planar solid oxide fuel cells is demonstrated. The tool combines the versatility of a commercial computational fluid dynamics simulation code with a validated electrochemistry calculation method. Its function is to predict the flow and distribution of anode and cathode gases, temperature and current distributions, and fuel utilization. A three-dimensional model geometry, including internal manifolds, was created to simulate a generic, cross-flow stack design. Similar three-dimensional geometries were created for simulation of co-flow, and counterflow stack designs. Cyclic boundary conditions were imposed at the top and bottom of the model domains, while the lateral walls were assumed adiabatic. The three cases show that, for a given average cell temperature, similar fuel utilizations can result irrespective of the flow configuration. Temperature distributions however, which largely determine thermal, stresses during operation, are dependent on the chosen design geometry/flow configuration. The co-flow case had the most uniform temperature distribution and the smallest thermal gradients, thus offers thermo-structural advantages over the other flow cases. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The gas transport in the porous electrode is treated by a phenomenological approach such that the gas concentration at the three-phase boundary (TPB) region is the additive superposition of that transported from the source, i.e. the gas channels. With plausible approximations and elemental algebra, analytical expressions are obtained to estimate the effects of ribs on the concentration polarization of planar fuel cell operations. It is shown that the model can closely reproduce the experimental concentration polarization curve for small and medium current density (up to about 2 A/cm(2)), providing a simple and effective method for engineering application. The concentration polarization caused by the presence of a rib is discussed and the concentration profiles with varying rib widths are illustrated. In connection with the electrical resistance, the determination of the optimal rib width for minimizing the overall polarization is also shown. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: This paper describes an inverse approach (IA) formulation for the analysis of tubes under free hydroforming conditions. The IA formulation is derived from that of Guo et al. established for flat sheet hydroforming analysis using constant strain triangular membrane elements. First, an incremental analysis of free hydroforming for a hot-dip galvanized (HG/Z140) DP600 tube is performed using the Marc finite element code. The deformed geometry obtained at the last converged increment is then used as the final configuration in the inverse analysis. This comparative study allows an assessment of the predictive capability of the inverse analysis. The results are compared with the experimental values determined by Asnafi and Skogsgardh. After that, a procedure based on a forming limit diagram (FLD) is described as a means to adjust the process parameters such as the axial feed and internal pressure. Finally, the adjustment process is illustrated through a reanalysis of the same tube using the inverse approach.
Abstract: A unified phenomenological model is developed to study the dislocation glide through weak obstacles during the first stage of plastic deformation in metals. This model takes into account both the dynamical responses of dislocations during the flight process and thermal activations while dislocations are bound by obstacle arrays. The average thermal activation rate is estimated using an analytical model based on the generalized Friedel relations. Then, the average flight velocity after an activation event is obtained numerically by discrete dislocation dynamics (DD). To simulate the dynamical dislocation behavior, the inertia term is implemented into the equation of dislocation motion within the DD code. The results from the DD simulations, coupled with the analytical model, determine the total dislocation velocity as a function of the stress and temperatures. By choosing parameters typical of the face centered cubic metals, the model reproduces both obstacle control and drag control motion in low and high velocity regimes, respectively. As expected by other string models, dislocation overshoots of obstacles caused by the dislocation inertia at the collisions are enhanced as temperature goes down. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: This paper presents a computational tool for the analysis of freely hydroformed tubes by means of an inverse approach. The formulation of the inverse method developed by Guo et al. [1] is adopted and extended to the tube hydroforming problems in which the initial geometry is a round tube submitted to hydraulic pressure and axial feed at the tube ends (end-feed). A simple criterion based on a forming limit diagram is used to predict the necking regions in the deformed workpiece. Although the developed computational tool is a stand-alone code, it has been linked to the Marc finite element code for meshing and visualization of results. The application of the inverse approach to tube hydroforming is illustrated through the analyses of the aluminum alloy AA6061-T4 seamless tubes under free hydroforming conditions. The results obtained are in good agreement with those issued from a direct incremental approach. However the computational time in the inverse procedure is much less than that in the incremental method.
Abstract: 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. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: The cold pilger metal forming technique is known to produce round titanium alloy tubing with mechanical properties that may be significantly anisotropic. These mechanical properties are of interest to both the manufacturers and consumers for defining initial manufacturing limitations and defining the final product design limitations. This study focuses on experimentally characterizing the yield locus development of Ti-3Al-2.5V seamless tubing during cold pilgering and a subsequent thermal stress relieving process. The materials are experimentally characterized using a biaxial testing apparatus, which subjects the specimen tubes to combined axial load and internal pressure. The Hill yield criterion is subsequently fit to the experimental results producing continuous yield loci. Each specimen is also experimentally characterized using X-ray diffraction to gain insight into the. material textures that accompany the macroscopic properties. All work is focused on one particular pilger pass at two different production rates. A second experimental variable is introduced to the study by using two significantly different input materials, as characterized by X-ray diffraction. This study also investigates the nature of the plastic deformation of the tubing developed during gold pilgering via finite element analysis and discusses the relationship between the finite element predictions and the mechanical anisotropy.
Abstract: The configuration of dislocation wall structures and the interactions between dislocations and dislocation walls play, a significant role in the understanding of deformation processes in metals. Samples of single-crystal aluminum deformed by tensile-straining (15%) were analyzed using TEM. In tensile-deformed (15%) single crystal aluminum, a cell structure is well developed and dislocations in the cell boundaries consist of either one set of Burgers vector or two sets of Burgers vector. The three-dimensional image of cell wall structure, misorientation angle across the cell boundaries and the Burgers vectors of dislocations in the cell boundaries are characterized.
Abstract: Modeling activities at PNNL support design and development of a modular SOFC systems. The SOFC stack modeling capability at PNNL has developed to a level at which planar stack designs can be compared and optimized for startup performance. Thermal-fluids and stress modeling is being performed to predict the transient temperature distribution and to determine the thermal stresses based on the temperature distribution. Current efforts also include the development of a model for calculating current density, cell voltage, and heat production in SOFC stacks with hydrogen or other fuels. The model includes the heat generation from both Joule heating and chemical reactions. It also accounts for species production and destruction via mass balance. The model is being linked to the finite element code MARC to allow for the evaluation of temperatures and stresses during steady state operations.
Abstract: 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 1x2 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. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: The superplastic deformation and cavitation damage characteristics of a modified aluminum ahoy are investigated at a temperature range from 500 to 550 degreesC. 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. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: In this work, we develop a framework coupling continuum elasto-viscoplasticity with three-dimensional discrete dislocation dynamics (micro3d). The main problem is to carry out rigorous analyses to simulate the deformation of single crystal metals (fcc and bcc) of finite domains. While the overall macroscopic response of the crystal is based on the continuum theory, the constitutive response is determined by discrete dislocation dynamics analyses using micro3d. Size effects are investigated by considering two boundary value problems: (1) uniaxial loading of a single crystal cube, and (2) bending of a single crystal micro-beam. It is shown that boundary conditions and the size of the computational cell have significant effect on the results due to image stresses from free-boundaries. The investigation shows that surface effects cannot be ignored regardless of the cell size, and may result in errors as much as 10%. Preliminary results pertaining to dislocation structures under bending conditions are also given. Published by Elsevier Science B.V.
Abstract: We have developed a method for measurement of temperature in glass products with the following features: (1) noncontacting, (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 degreesC to 550 degreesC (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. Published by Elsevier Science Ltd.
Abstract: 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 x 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. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: We developed a nondestructive and noncontact method for measuring stress at the midplane of tempered glass plates that uses Bragg scattering from a pair of thermal gratings. These gratings are formed by 1064-nm beams from a seeded Nd:YAG laser, and we measure the polarization state of light from a 532-nm beam that scatters from both thermal gratings. The change in polarization of the doubly scattered light with separation between the two gratings allows measurement of the in-plane stress. A model of the Bragg scattering efficiency, experimental investigations of the scattered beams, and stress measurements are reported. (C) 2001 Optical Society of America.
Abstract: The current work develops forming-limit diagrams (FLDs) for weld materials in aluminum tailor-welded 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 are welding of a 1- to 2-mm-thick 5000 series aluminum alloy sheet.
Abstract: 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. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: Measurements of surface residual stress at various locations were made for three front windshields from the Ford Contour automobile using a grazing angle surface polarimeter sensitive to the optical birefringence produced in stressed glass. The two in-plane principal stresses were determined from these measurements. Also, an uncertainly analysis was performed to evaluate the precision of these measurements. The principal stresses were shown to have a relatively high inherent uncertainty due to the nature of the measurement technique, On the windshield outside surface the stresses were apparently uniform and both principal stresses were equal in magnitude. For most of this surface the variation was within the uncertainty bounds. Only near the edges of the glass was the stress not uniform. On the inside surface the stresses were less uniform and the principal stresses were different in magnitude. On this surface the variation did exceed the uncertainty bounds. The variation of stress at a given location from one windshield to the next was within the uncertainty bounds.
Abstract: 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. (C) 2000 Elsevier Science S.A. All rights reserved.
Abstract: A closed-form analytical solution for the displacement, and strain-stress fields of a circular Volterra dislocation loop having a glide and prismatic components is obtained. Assuming linear elasticity and infinite isotropic material, the displacement field is found by integrating the Burgers displacement equation for a circular dislocation loop. The strain field is subsequently obtained and stresses follow from Hooke's law. The field equations are expressed in terms of complete elliptic integrals of the first, second, and/or third elliptic integrals. The general loop solution is, from the principle of superposition, the additive sum of the prismatic and glide solutions. (C) 2000 Elsevier Science Ltd. All rights reserved.
Abstract: 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 payer 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. (C) 2000 Elsevier Science S.A. All rights reserved.
Abstract: Recent studies have shown that for a variety of unirradiated and irradiated materials, a slope of similar to 2 is obtained for a correlation between yield in a shear punch test and yield in a uniaxial tensile test. Application of the von Mises yield criterion would predict a slope of root 3. A finite element model (FEM) of the shear punch test was developed to aid in understanding the experimentally obtained slope of similar to 2. FEM simulations of the sheer punch test were conducted using stress-strain data from uniaxial tensile tests on 316 stainless steel in four initial cold-work conditions. A correlation was developed between the FEM-evaluated effective sheer yield strength and the experimentally-evaluated uniaxial yield strength. The slope from this correlation was found to be nearly the same as for the slope from the correlation between the experimentally-evaluated effective shear yield strength and the experimentally-evaluated uniaxial yield strength. The finite element model showed that stresses other than pure shear exist in a specimen during a shear punch test, and these other stresses may explain why the slope of the experimental yield strength correlation is different than root 3.
Abstract: 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 are welding process.
Abstract: 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. (C) 2000 Elsevier Science S.A. All rights reserved.
Abstract: 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 relaxation of current flaw acceptance standards could negate the benefits of outstanding probability of detection capabilities.
Abstract: The objectives of the the work presented in this paper are to perform probabilistic calculations simulating 304 stainless steel piping under conditions of intergranular stress corrosion cracking (IGSCC);and to evaluate alternate inspection strategies and the associated reductions in failure probabilities. The stress corrosion cracking model that is part of the PRAISE code is applied, and the PRAISE predictions are compared with service experience. A simplified parametric approach has been adopted to characterize IGSCC by a single damage parameter, which depends on the service-related and residual stresses, environment, and degree of sensitization. A matrix of calculations that addresses a wide range of pipe sizes, materials, and Service conditions has been developed and executed. Sensitivity studies were performed to gain insight into the critical inputs to the model. probability of detection curves for IGSCC were established for the pipe wall thickness range of 2.54 cm (1.0 in.) and greater, and the effects of inservice inspection strategies were evaluated.
Abstract: 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.
Abstract: 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.
Abstract: Superplastic ductility, while known to be strongly related to the strain rate sensitivy m, is difficult to predict and many times can be difficult to understand. Cavitation is most often indicated as the cause of a lower than expected elongation; however, this consideration has not always explained the variation in ductility observed, such as a significant scatter in elongations measured for the same material and under the same test conditions. The range of factors which may be involved are suggested here to include initial gradients, or inhomogeneities, in such factors as cavitation, temperature, grain size as well as geometry (i.e. area, stress, or stress state). A model is summarized which constructs the tensile;test section of a number of elements, each allowing a different initial condition as well subsequent evolution. The model is applied to cavitation variations with supporting evidence from controlled experiment on the superplastic Pb-Sn alloy which had various patterns of pre-machined holes throughout the test section. Also, the potential effect of dynamic recrystallization is explored with the model, and which suggests that a substantial loss in ductility can result.
Abstract: The superplastic deformation characteristics of a modified aluminum alloy were investigated at temperature range from 500 to 550 degrees 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 1x10(-3) s(-1). When these pressure histories were used in tray forming experiments, good agreement was obtained between the forming and modeling results. (C) 1998 Elsevier Science Ltd. All rights reserved.
Abstract: Finite element models were constructed to simulate forming of a long rectangular tray. Control algorithms were developed to predict pressure history by maintaining a constant strain rate throughout forming. These forming cycles were applied in superplastic forming press, and partially and fully formed trays were produced to compare with finite element predictions. The partially formed tray touches the bottom of the die at time equal to that predicted by the model, indicating that the constitutive and friction models correctly simulate the forming process. Trays were sectioned and thinning profiles were measured in the width-wise, length-wise, and the diagonal. corner directions. Comparisons between the predicted thinning profiles and the measured ones for partially and completely formed trays are described. Based on these results, it appears that the coefficient of friction varies somewhat over time. An average coefficient of friction of 0.4 provides reasonably good agreement between the predicted and measured thinning profiles for completely formed trays.
Abstract: Uniaxial tension specimens were pulled to a prescribed strain at a constant baseline strain rate. These specimens were sectioned after testing to determine the extent of internal cavitation using digital image analysis. A void fraction evolution equation accounting for the nucleation and growth of voids during uniaxial deformation was derived. The experimental void fraction data was in good agreement with the proposed model. Cavitation in this alloy was found to be a function of strain rate, and more voids were seen at high strain rates. Pressure-time histories predicted using the finite element method were applied to a modified 5083 aluminum alloy in a superplastic press, and partial and fully formed trays were produced. These trays were sectioned and polished, and micrographs were taken to show the cavities in various sections of the tray. Experimental investigations focused on studying the effect of plane strain and general multiaxial deformations on the evolution of cavitation. The plane strain state of stress that occurs in the middle of the tray was more damaging in terms of the evolution of cavitation that the general multi-axial state. Through-thickness variation of cavity void fraction was observed in the entrance radius and the bottom radius regions of the formed tray. It was also observed that the application of hydrostatic pressure was beneficial in reducing cavitation levels.
Abstract: Results observed from a laboratory study concerning the degradation of an alluvial channel owing to the flow of clear water are reported. Two sizes of sand, of median diameter 0.47 mm and 0.79 mm and geometric standard deviation 4.65 and 3.54, respectively, are used as a bed material. This paper focuses on studying the variation of the surface layer size with time and distance, the variation of sediment discharge with time, the sediment size of the armoured layer, and the time required for stabilization of the channel bed. Some useful equations for predicting the sediment size of the armoured layer and the total time for the degradation are also given.
Abstract: 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 senstive to accidental or local damage of the exterior girders than the interior ones.
Abstract: 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.
