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LiNi0.5Mn1.5O4 is a promising high-voltage cathode for lithium-ion battery fast charging applications. Aware of its electrochemical stability issues, the material's surface is modified with small amounts of zirconia (ZrO2) ranging from 0.5 to 2 wt% using a scalable ball milling process. The advantage of the coating has been demonstrated in electrochemical measurements performed at room temperature and 55 ?C, and in cells discharged under high-rate conditions up to 80C. Of significance, the material coated with 1.0 wt% ZrO2 has been cycled at the 40C rate for over a thousand cycles and retains 86% of its initial capacity. The material with 2.0 wt% ZrO2 modification preserves 76% of its initial capacity when cycled at the 40C rate and 55 ?C. The coated materials have shown excellent cycling stability when subjected to 6C (10-min) fast charging and C/3 discharging for 300 cycles. Compared to the uncoated material, the interfacial resistance of the zirconia modified LiNi0.5Mn1.5O4 has been found to be much lower and does not significantly increase with increasing the coating amount. However, the electrochemical performances are still partly limited by both interfacial resistance at the beginning of charge and electrolyte diffusivity, particularly under higher rate cycling conditions. Overall, the strategy of ZrO2 surface modification applied to LiNi0.5Mn1.5O4 unveils the potential that the material could play in extreme fast charged electric vehicles. This research is sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory , managed by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 for the U.S. Department of Energy. Scopus

The viscoplastic intermediate ϕ-model was applied in this work to predict the deformation behavior and texture evolution in a magnesium alloy, an HCP material. We simulated the deformation behavior with different intergranular interaction strengths and compared the predicted results with available experimental results. In this approach, elasticity is neglected and the plastic deformation mechanisms are assumed as a combination of crystallographic slip and twinning systems. Tests are performed for rolling (plane strain compression) of random textured Mg polycrystal as well as for tensile and compressive tests on rolled Mg sheets. Simulated texture evolutions agree well with experimental data. Activities of twinning and slip, predicted by the intermediate ϕ-model, reveal the strong anisotropic behavior during tension and compression of rolled sheets.

Deformation asymmetry of magnesium alloys is an important factor on machine design in the automobile industry. Represented by the ratio of compressive yield stress (CYS) against tensile yield stress (TYS), deformation asymmetry is strongly related to texture and grain size. A polycrystalline viscoplasticity model, modified intermediate ϕ-model, is used to predict the deformation behavior of magnesium alloys with different grain sizes. Validated with experimental results, integrated computational materials engineering is applied to find out the route in achieving desired asymmetry via thermomechanical processing. For example, CYS/TYS in rolled texture is smaller than 1 under different loading directions. In other textures, such as extruded texture, CYS/TYS is large along the normal direction. Starting from rolled texture, asymmetry will increase to close to 1 along the rolling direction after being compressed to a strain of 0.2. Our modified ϕ-model also shows that grain refinement increases CYS/TYS. Along with texture control, grain refinement also can optimize the yield asymmetry. After the grain size decreases to a critical value, CYS/TYS reaches to 1 because CYS increases much faster than TYS. By tailoring the microstructure using texture control and grain refinement, it is achievable to optimize yield asymmetry in wrought magnesium alloys.

Applications of membrane technology in water desalination and wastewater treatment have increased significantly in the past few decades due to its many advantages over other water treatment technologies. Water treatment membranes provide high flux and contaminant rejection ability and require good mechanical strength and durability. Thus, assessing the mechanical properties of water treatment membranes is critical not only to their design, but also for studying their failure mechanisms, including the surface damage, mechanical and chemical ageing, delamination and loss of dimensional stability of the membranes. The various experimental techniques to assess the mechanical properties of wastewater treatment and desalination membranes are reviewed. Uniaxial tensile test, bending test, dynamic mechanical analysis, nanoindentation and bursting tests are the most widely used mechanical characterization methods for water treatment membranes. Mechanical degradations induced by fouling, chemical cleaning as well as membrane delamination are then discussed. Moreover, in order to study the membranes mechanical responses under similar loading conditions, the stress-state of the membranes are analyzed and advanced mechanical testing approaches are proposed. Some perspectives are highlighted to study the structure-properties relationship for wastewater treatment and water desalination membranes.

The use of self-healing glass as hermetic seals is a recent advancement in sealing technology development for the planar solid oxide fuel cells (SOFCs). Because of its capability to restore mechanical properties at elevated temperatures, the self-healing glass seal is expected to provide high reliability in maintaining the long-term structural integrity and functionality of SOFCs. To accommodate the design and evaluate the effectiveness of these engineered seals under various thermomechanical operating conditions, a computational modeling framework must be developed to accurately capture and predict the healing behavior of the glass material. In the present work, a mechanistic-based, two-stage model was developed to study the stress and temperature-dependent crack healing of the self-healing glass materials. The model initially was first calibrated by experimental measurements combined with kinetic Monte Carlo (kMC) simulation results and then implemented into finite element analysis (FEA). The effects of various factors, e.g., stress, temperature, and crack morphology, on the healing behavior of the glass were investigated and discussed.

Mechanical properties of solid oxide fuel cell glass-ceramic seal material, G18, are studied at high temperatures. Samples of G18 are aged for either 4 h or 100 h, resulting in samples with different crystallinity. Reduced modulus, hardness, and time-dependent behavior are measured by nanoindentation. The nanoindentation is performed at room temperature, 550, 650, and 750 °C, using loading rates of 5 mN s−1 and 25 mN s−1. Results show a decrease in reduced modulus with increasing temperature, with significant decrease above the glass transition temperature. Hardness generally decreases with increasing temperature, with a slight increase before Tg for the 4 h-aged sample. Dwell tests show that creep increases with increasing temperature, but decrease with further aging.

High energy particle irradiation of structural polycrystalline materials usually produces irradiation hardening and embrittlement. The development of predictive capability for the influence of irradiation on mechanical behavior is very important in materials design for next-generation reactors. A multiscale approach was implemented in this work to predict irradiation hardening of iron based structural materials. In the microscale, dislocation dynamics models were used to predict the critical resolved shear stress from the evolution of local dislocation and defects. In the macroscale, a viscoplastic self-consistent model was applied to predict the irradiation hardening in samples with changes in texture. The effects of defect density and texture were investigated. Simulated evolution of yield strength with irradiation agrees well with the experimental data of irradiation strengthening of stainless steel 304L, 316L and T91. This multiscale modeling can provide a guidance tool in performance evaluation of structural materials for next-generation nuclear reactors.

A self-healing glass seal has the potential to restore its mechanical properties upon being reheated to the solid oxide fuel cell (SOFC) stack operating temperature. Such a self-healing feature is desirable for achieving high seal reliability during thermal cycling. Self-healing glass is also characterized by its low mechanical stiffness and high creep rate at SOFC operating temperatures. Therefore, the geometric stability and structural integrity of the glass seal system are critical to its successful application in SOFCs. This paper describes studies of the geometric stability and structural integrity of the self-healing glass seal system and the influence of various interfacial conditions during the operating and cooling-down processes using finite element analyses. For this purpose, the test cell used in the leakage tests for compliant glass seals, conducted at Pacific Northwest National Laboratory (PNNL), was taken as the initial modeling geometry. The effect of the ceramic stopper on the geometric stability of the self-healing glass sealants was studied first. Two interfacial conditions of the ceramic stopper and glass seals, i.e., bonded (strong) or unbonded (weak), were considered. Then the influences of interfacial strengths at various interfaces, i.e., stopper/glass, stopper/PEN, as well as stopper/IC plate, on the geometric stability and reliability of glass during the operating and cooling processes were examined.

TWinning Induced Plasticity (TWIP) steel is a typical representative of the 2nd generation advanced high strength steels (AHSS) which exhibits a combination of high strength and excellent ductility due to the deformation twinning mechanisms. This paper discusses the principal features of deformation twinning in faced-centered cubic austenitic steels and shows how a physically based macroscopic model can be derived from microscopic-level considerations. In fact, a dislocation-based phenomenological model, with internal state variables including dislocation density and micro-twins volume fraction describing the microstructure evolution during deformation process, is proposed to model the deformation behavior of TWIP steels. The originality of this work lies in the incorporation of a physically based model on twin nucleation and volume fraction evolution in a conventional dislocation-based approach. Microstructural level experimental observations with scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques together with the macroscopic quasi-static tensile test, for the TWIP steel Fe–17.5 wt.% Mn–1.4 wt.% Al–0.56 wt.% C, are used to validate and verify the modeling assumptions. The model could be regarded as a semi-phenomenological approach with sufficient links between microstructure and the overall mechanical properties, and therefore offers good predictive capabilities. Its simplicity also allows a modular implementation in finite element-based metal forming simulations.

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 4 h 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.

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 °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.

High operating temperatures of solid oxide fuel cells (SOFCs) require that the sealant must function at a high temperature between 600°C and 900°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.

The creep behavior of glass or glass–ceramic sealant materials used in solid oxide fuel cells (SOFCs) becomes relevant under SOFC operating temperatures. In this paper, the creep of glass–ceramic sealants was experimentally examined, and a standard linear solid model was applied to capture the creep behavior of glass–ceramic sealant materials developed for planar SOFCs at high temperatures. The parameters of this model were determined based on the creep test results. Furthermore, the creep model was incorporated into finite-element software programs SOFC-MP and Mentat-FC developed at Pacific Northwest National Laboratory for multiphysics simulation of SOFCs. The effect of creep of glass–ceramic sealant materials on the long-term performance of SOFC stacks was investigated by studying the stability of the flow channels and the stress redistribution in the glass seal and on the various interfaces of the glass seal with other layers. Finite-element analyses were performed to quantify the stresses in various parts. The stresses in glass seals were released because of creep behavior during operations.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

Interconnects in solid oxide fuel cells (SOFCs) provide the electrical connection between the individual cells, as well as separate the anode air from the cathode fuel for each cell. The recently developed anode-supported planar SOFCs design allows the cell working temperature to be 800oC and lower, which led to the proposed use of ferritic stainless steel as interconnects and current collectors in the SOFC systems. Many experimental studies have shown that metal based interconnects will go through surface oxidation under the typical operating temperature and environment of an SOFC system. Since the oxide scale is formed at the outer surface of interconnect, its microstructure (morphology, texture), properties (electrical, thermal and mechanical properties), and structural integrity/stability will have a significant impact on the performance of interconnect. In this paper, we study the oxide growth kinetics and stresses generated during scale growth the thermal cycling for Cr-Fe based alloy, e.g. Crofer 22 APU. The goal is to predict the interconnect life under typical operating conditions and thermal cycles. First, a mathematical model is developed based on the growth of scale and coupling relation between diffusion and stress field. The following phenomena are considered: 1) The predominant cationic diffusion and cation distribution withinmore » the oxide scale in transient state and steady state; 2) Chemical reaction within the scale as well as the growth kinetics of the oxide scale; 3) Stresses distribution during the oxidation, especially for mid- and long-term treatment; 4) Coupling relationship or interaction between oxidation induced stresses and mass diffusion within the scale. Next, thermal stresses generated due to temperature changes in thermal cycling are predicted using finite element analyses for different substrate and scale thickness. In general, very high compressive in-plane stresses are predicted for the oxide layer. Also, high shear stress is also predicted on the oxide/substrate interface. The predicted shear stress on the interface is used to compare with the experimentally determined bond strength to predict delamination. Finally, scale spallation and IC life prediction are predicted by comparing the compressive stress in the scale with the critical buckling load for the delaminated scale.« less

Numerical simulations were performed to determine the effect that varying the percent on-cell steam-methane reformation would have on the thermal, electrical, and mechanical performance of generic, planar solid oxide fuel cell stacks. The study was performed using three-dimensional model geometries for cross-, co-, and counter-flow configuration stacks of 10x10- and 20x20-cm cell sizes. The analysis predicted the stress and temperature difference would be minimized for the 10x10-cm counter- and cross-flow stacks when 40 to 50% of the reformation reaction occurred on the anode. Gross electrical power density was virtually unaffected by the reforming. The co-flow stack benefited most from the on-cell reforming and had the lowest anode stresses of the 20x20-cm stacks. The analyses also suggest that airflows associated with 15% air utilization may be required for cooling the larger (20x20-cm) stacks.

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.

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.

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.

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.

Interfacial analysis utilizing interfacial indentation methodology (where micro/nano indentation is performed at the oxide scale/substrate interface to create and propagate cracks at the interface) was completed for the 14,000 and 20,000 hours, 800oC surface modified specimens. The localized oxide thickness and crack lengths were measured after applying loads ranging from 0.2 to 1.8 N. Post-processing of the results to date indicate that the critical load for surface blasted (SB) surface modified specimens is greater than the equivalent surface grind (SG) surface modified specimens which is indicative of a ~ 25% increase in interfacial strength in the SB specimens in comparisons to the SG specimens. However, more data is needed to further quantify the interfacial strength and to reduce any uncertainty due to the approach applied.

High-level radioactive waste (HLW) vitrification is a manufacturing process designed to dispose of nuclear energy fission products over long-term timescales. We studied and modeled the thermomechanical phenomena occurring during the processing of the glass blocks, e.g. during their solidification and their cooling down. The thermomechanical modeling takes place in 3D FEM simulations. The relaxations of the borosilicate glass are to be taken into account through scripted algorithms. They allow us to describe accurately the evolution of the glass properties over its phase transition (the glass transition temperatures are non-uniform in the HLW package). A damage behavior within the frame of Continuum Damage Mechanics is also used to predict the glass cracking surface area.

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 cold pilgering via finite element analysis and discusses the relationship between the finite element predictions and the mechanical anisotropy.

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.

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

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

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

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

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

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

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

In this paper, results of finite element crash simulation are presented for a TRIP steel side rail with and without considering the phase transformation during forming operations. A homogeneous phase transformation model is adapted to model the mechanical behavior of the austenite-to-martensite phase. The forming process of TRIP steels is simulated with the implementation of the material model. The distribution and volume fraction of the martensite in TRIP steels may be greatly influenced by various factors during forming process and subsequently contribute to the behavior of the formed TRIP steels during the crushing process. The results indicate that, with the forming induced phase transformation, higher energy absorption of the side rail can be achieved. The phase transformation enhances the strength of the side rail

As its name implies, self-healing glass seal has the potential of restoring its mechanical properties upon being reheated to stack operating temperature, even when it has experienced some cooling induced damage/crack at room temperature. Such a self-healing feature is desirable for achieving high seal reliability during thermal cycling. On the other hand, self-healing glass is also characterized by its low mechanical stiffness and high creep rate at the typical operating temperature of SOFCs. Therefore, from a design’s perspective, it is important to know the long term geometric stability and thermal mechanical behaviors of the self-healing glass under the stack operating conditions. These predictive capabilities will guide the design and optimization of a reliable sealing system that potentially utilizes self-healing glass as well as other ceramic seal components in achieving the ultimate goal of SOFC. In this report, we focused on predicting the effects of various generic seal design parameters on the stresses in the seal. For this purpose, we take the test cell used in the leakage test for compliant glass seals conducted in PNNL as our initial modeling geometry. The effect of the ceramic stopper on the geometry stability of the self-healing glass sealants is studied first. Then wemore » explored the effect of various interfaces such as stopper and glass, stopper and PEN, as well stopper and IC plate, on the geometry stability and reliability of glass during the operating and cooling processes.« less

High operating temperature of solid oxide fuel cells require that sealant must function at high temperature between 600o and 900oC and in the oxidizing and reducing environments of fuel and air. It should be noted that creep deformation becomes relevant for a material when the operating temperature is near or exceeds half of its melting temperature (in degrees of Kelvin). The operating temperatures for most of the solid oxide fuel cells (SOFC) under development in the SECA program are around 800oC, which exceeds the glass transition temperature Tg for most glass ceramic materials. 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 self-consistent 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.

Thermal stresses and warpage of the PEN are unavoidable due to the temperature changes from the stress-free sintering temperature to room temperature and mismatch of the coefficients 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.

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.

Creep deformation becomes relevant for a material when the operating temperature is near or exceeds half of its melting temperature (in degrees of Kelvin). The operating temperatures for most of the solid oxide fuel cells (SOFC) under development in the SECA program are around 1073oK. 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. Since the melting temperature of most stainless steel is around 1800oK, 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 SOFC 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 thatmore » creep behavior of the ferritic 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.« less

Planar Solid Oxide Fuel Cells (SOFC) stacks are multi-material layered systems with different thermo-mechanical properties. Due to their severe thermal loading, these layers have to meet high demands to preserve their mechanical integrity without any cracks initiation and propagation. In this paper, cracks propagation in a typical unit cell of the stack which consists of positive electrode–electrolyte–negative electrode (PEN) is modeled. Based on the mechanical properties of each layer and their interfaces, an energy criterion as a function of crack length is used for the prediction of possible crack extensions in the PEN. An analysis of the competition between crack deflections in the interfaces and crack penetration in layers is presented.

Microstructure level inhomogeneity between the harder martensite phase and the softer ferrite phase renders the dual phase (DP) steels more complicated failure mechanisms and associated failure modes compared to the conventionally used low alloy homogenous steels. This paper examines the failure mode DP780 steel and DP 980 steel under different loading conditions using finite element analyses on the microstructure levels. Micro-mechanics analyses based on the actual microstructures of DP steel are performed. The two-dimensional microstructure of DP steel was recorded by scanning electron microscopy (SEM). Mechanical properties of the ferrite and martensite phases exhibit the dependency of the different chemistry and thermal mechanical processes. The plastic work hardening properties of the ferrite phase was determined by the synchrotron-based high-energy X-ray diffraction technique. The work hardening properties of the martensite phase were calibrated and determined based on the uniaxial tensile test results. Under different loading conditions, different failure modes are predicted in the form of plastic strain localization. The local failure mode was 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 was strictly constrained,more » splitting failure perpendicular to the loading direction is predicted. On the other hand, under plane strain loading condition, commonly observed necking phenomenon is predicted which leads to the final failure of the material. Numerical predictions were well validated by the experimental.« less