Mark E. Walter

Observations of serration characteristics and acoustic emission during serrated flow of an Al-Mg alloy

Abstract Study of serrated flow is important for the production of metal alloy parts with smooth surfaces and for fundamental understanding of microscale plastic deformation. In this experimental investigation of a rolled Al–Mg alloy, serrated flow was observed for the entire plastic flow region at three different strain rates. Strain and strain rate dependent serration amplitudes and widths were quantified. In addition, the acoustic emission (AE) signature from the plastic flow region was analyzed in detail and was related to the serrations in the macroscopic stress versus strain results. For all experiments, deformation bands on the surfaces of the specimens were found to be consistent with ‘Type B’ serrations. Video observations of the deformation bands were synchronized with the AE, and the deformation bands were found to coincide with more persistent AE bursts and anomalies in the serrations.

The mechanical response of three EB-PVD thermal barrier coating microstructures

The mechanical response of as-received thermal barrier coating specimens with three different microstructures was evaluated. Specimens consisted of rectangular bars of Inconel 617 substrates with NiCoCrAlY bond coats and EB-PVD zirconia top coats. The deposition conditions had been varied to produce columnar top coat microstructures with three different column sizes. Scanning electron microscopy, optical microscopy, and X-ray diffraction analyses were used to characterize the microstructures. Four-point bend experiments were performed with the top coat under either tension or compression. In situ microscopy identified the damage formation and propagation modes, and acoustic emission analyses also identified differences in the responses of each microstructure.

Response of fiber reinforced ceramic matrix composites through computational modeling of damage

Abstract Uni-axial tension experiments with a uni-directional ceramic matrix composite have indicated which damage mechanisms are active during the deformation history. A finite element model has been developed to investigate the onset and evolution of these experimentally observed damage mechanisms in a unit cell. This computational model is based on an axisymmetric unit cell which is discretized into cohesive interface elements and linearly elastic elements. As cohesive elements fail according to critical stress and energy release rate criteria, cracks form and propagate. To obtain converged solutions during crack propagation the full dynamic equations were solved implicitly using the Newmark method. The unit cell response was determined as a function of the fiber/matrix interface strength and toughness and as a function of the matrix toughness. Details of matrix crack propagation and the subsequent debond initiation were observed, and the implications for material toughening are discussed. The simplest unit cell response was averaged using the experimentally determined evolution of matrix crack density. The averaged response of the unit cell corresponds very well to the experimentally measured macroscopic composite response.

The thermomechanical stress-strain response of intumescent mat materials through experiments and strain decomposition

Intumescent mat materials in catalytic converters undergo chemical reactions that lead to material property changes and volume expansion during heating processes. Dead weight (load control) and displacement control compression experiments have been performed to explore static and transient stress-strain responses. The apparatus and methods for both experiments are described. The experimental results together with a strain decomposition procedure yield a master curve that can be employed for constitutive modelling.

Thermomechanical Properties of Cycled Ceramic/Glass Composite Seals for Solid Oxide Fuel Cells

The present research focuses on a novel ceramic/glass composite seal. These seals firstunderwent a curing cycle. The cycled seal was then characterized with a laser dilatome-ter to identify the glass transition, softening temperature, and thermal expansion proper-ties. High temperature ring-on-ring (RoR) experiments were performed to study the effectof glass transition and softening temperatures on mechanical response. X-ray diffraction(XRD) techniques in conjunction with post-test micrographs were used to understand theobserved mechanical response. In addition, Weibull statistical analysis performed oncycled seals showed that Weibull modulus had decreased and Weibull characteristicsstrength had increased with multiple thermal cycles. [DOI: 10.1115/1.4029876]Keywords: solid oxide fuel cell (SOFC), ceramic/glass seals, glass transition tempera-ture, biaxial flexural strength, Weibull parameters

Application of high-speed video extensometry for high-temperature tensile characterization of boron heat-treated steels

The increased desire to use advanced, high-strength steels for lightweight automotive structural components requires better understanding of thermo-mechanical behavior and appropriate experimental data for developing constitutive models. Thermo-mechanical studies are particularly important for understanding and optimizing hot-stamping processes which produce both complex and high-strength components. The experimental setup presented herein is capable of characterizing the thermo-mechanical behavior of such steels with strain rates up to approximately 1 s–1 and temperatures as high as 850 °C. The main parts of the apparatus are a high-speed camera, a load frame, and a box furnace. For the determination of strain, a simple image-processing program was developed. The strain was determined in three sections that span the entire gauge length of the specimen. Thus, the onset of localization could be more accurately determined. Stress versus strain data for various strain rates and temperatures are presented.

Mechanical Characterization and Modeling of Electrolyte Membranes in Electrolyte-Supported SOFCs

Planar Solid Oxide Fuel Cells (SOFCs) are made up of repeating sequences of thin layers of energy producing ceramics, seals, and current collectors. For electro-chemical reasons it is best to keep the ceramic layers as thin as possible, which also means that the cells are more susceptible to damage during production, assembly, and operation. The latest-generation electrolyte-supported SOFCs have a honeycomb-type support structure. The electrolyte membranes, which are much smaller in thickness than they are in area, require a two-scale approach for finite element modeling; the smaller scale focuses on analyzing a representative area of the cell, while the larger scale examines the cell as a whole. To provide the material data for the models, an array of experimental techniques are needed. The small scale model requires bulk elastic properties of the electrolyte material, which are measured over a range of temperatures using a sonic resonance technique. This model then outputs “effective” properties for the large scale, which must be experimentally validated using four-point bend tests on representative samples. Additionally, a series of compression tests are performed on cells for validate the performance of electrolytes in the context of a stack.

Using Specimen Geometry to Isolate Flexural and Torsional Vibration Modes During Sonic Resonance

The ASTM E1875 method of sonic resonance is a convenient means of obtaining the elastic material properties of materials, particularly at high temperatures. The technique is performed by exciting a rectangular bar specimen to vibrate over a range of frequencies while measuring the amplitude of the bar’s vibrations. At specific frequencies, a local maximum in the displacement amplitude denotes either flexural or torsional resonance. The elastic and shear moduli are then computed from the first flexural and torsional resonant frequencies, respectively, as well as from the mass and dimensions of the bar. However, for certain bar geometries, the sequence in which the different resonant frequencies occur depends on the Poisson’s ratio of the material. Because the amplitudes measured during the tests do not distinguish between flexure and torsion, and because Poisson’s ratio is generally not known a priori, it is desirable to select specimen geometries for which only one sequence of frequencies is possible. This paper presents an analytical approach to determine what Poisson’s ratio would need to be in order for the first torsional frequency to always be higher than the second flexural frequency. This critical value for Poisson’s ratio is shown to be a function solely of geometry and can therefore be selected at will through the choice of length, width, and height of the bar. When the critical Poisson’s ratio is chosen to be greater than 0.5 and therefore non-physical, the first torsional frequency of the bar will always be higher than the second flexural frequency, regardless of the material composing the bar. A comparable approach is used to identify geometries that ensure that the first torsional frequency is always less than the third flexural frequency. When one selects geometries for which the sequence of the different resonant modes is known, the actual Poisson’s ratio of the material is no longer needed to compute the elastic constants.

Mechanical Characterization for Identifying an Appropriate Thermal Cycle for Curing Ceramic/Glass Composite Seals

Solid oxide fuel cells (SOFCs) require seals that can function in severe environments at elevated temperatures. Sealing remains a significant issue for all types of planar SOFCs. In fact seals could be considered the most critical component for commercializing the entire SOFC technology. Not only do inadequate seals present a barrier to commercialization of SOFCs, but there are also significant and interesting fundamental issues in materials selection, characterization, and design. The present research focuses on a novel ceramic/glass composite seal that is produced by roller compaction. It was found that the high heating and cooling rates during binder burnout cycles resulted in seals that were significantly weaker in compression. Post-test analysis showed that the number of micro-voids and other surface anomalies increased significantly with faster heating and cooling rates. By studying the microstructures, surface topography, and mechanical responses of seals cured at different rates, the current research proposes an appropriate thermal cycle for curing the green seals. Seals, thus cured, include minimum micro-voids and surface anomalies and have better compressive mechanical response. These properties would help provide better sealing performance and increased longevity at SOFC operating conditions.

Mechanical Characterization and Modeling of Corrugated Metal Foams for SOFC Applications

Planar solid oxide fuel cells (SOFCs) are made up of repeating sequences of thin layers of cermet electrodes, ceramic electrolytes, seals, and current-collectors. For electro-chemical reasons it is best to keep the electrolyte layers as thin as possible. However, for electrolyte-supported cells, the thin electrolytes are more susceptible to damage during production, assembly, and operation. The latest-generation electrolyte-supported SOFCs feature metallic foam current-collectors which relay current between the energy-producing materials and the rest of the circuit. These foams are stamped into a corrugated shape which is intended to reduce the compressive loads which are transferred through the stack onto the brittle electrolyte, but the mechanical behavior of the foams remain to be fully understood. Characterization of the corrugated metal foams consists of comparison of load-vs.-displacement behavior between experimentally measured compression data and a single-component finite element model which isolates the foam from the rest of the stack. Mechanical properties of the foam are found using an iterative approach, in which the material properties used as inputs to the model are changed until the load-displacement data best agrees with experiments. The model explores the influence of elastic and plastic properties in combination with and without friction. Thus obtained, the properties can then be used in a stack model to determine which parameters can best reduce the demands on the electrolyte without sacrificing electrochemical performance.Copyright