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Über dieses Buch

This the fifth volume of six from the Annual Conference of the Society for Experimental Mechanics, 2010, brings together 25 chapters on Emerging Energy Systems. It presents early findings from experimental and computational investigations including Material State Changes in Heterogeneous Materials for Energy Systems, Characterization of Carbon Nanotube Foam for Improved Gas Storage Capability, Thermoresponsive Microcapsules for Autonomic Lithium-ion Battery Shutdown, Service Life Prediction of Seal in PEM Fuel Cells, and Assessing Durability of Elastomeric Seals for Fuel Cell Applications.



Experimental Mechanics for Prognosis of Material State Changes in Heterogeneous Materials for Energy Systems

The concept of prognosis is typically discussed in terms of mechanical characteristics such as structural integrity, durability, damage tolerance, fracture toughness, etc. These familiar concepts are usually addressed by considering balance equations, crack growth relationships, and constitutive equations with constant material properties, and constant or cyclically applied load conditions. Loading histories are represented by changing stress (or strain) states, only. But for many situations, especially associated with high performance engineering structures, the local state of the material may also change during service, so that the properties used in those equations are functions of time and history of applied conditions. But for many energy systems, a broader definition is required. For example, in fuel cells, properties such as conductivity and electrochemical character are altered by material degradation, so that “property fields” replace the global constants in multiphysics balance equations, and time and history enter into the governing equations. The present paper will examine a small set of such problems which involve novel experimental methods of following the accumulation of distributed damage and changes in material state. Specifically, the paper discusses this problem in the context of material state changes measured by impedance variations as a method of following and interpreting those changes in terms of functional performance. The application of these concepts is extended from mechanical structures to energy systems, wherein the material state changes result in variations in electrochemical properties that directly control functional performance of devices such as fuel cells.
Kenneth L. Reifsnider, Prasun K. Majumdar, Paul D. Fazzino, Fazle Rabbi

Characterisation of Carbon Nanotube Foam for Improved Gas Storage Capability

Advanced fuel cells require efficient hydrogen storage tanks. This study presents preliminary results on a novel compound based on an alumina substrate coated with carbon nanotube foam (CNF) that is expected to improve substantially the hydrogen storage capability. A catalytic chemical vapour deposition (CCVD) technique was applied for obtaining the desired structure. It involved the organometallic compound ferrocene (a simultaneous source of iron and carbon), H2 as reducing gas, and Ar as dragging gas. The CNF-alumina system formed was characterised by means of scanning and transmission electron microscopy (SEM, TEM, resp.). Applying the BET method with N2 as carrier gas, it was found that the novel compound exhibits a high specific surface area, due to the porous morphology, and a high thermal stability. These aspects are very promising for the application intended. The sponge-like structure of the CNF may store hydrogen (or other gases) due to physical adsorption in much larger quantities as compared to conventional storage tanks.
Armando Peña, Aimé Guerrero, Julio Puerta, Joaquín L. Brito, Thomas K. Heckel

Thermoresponsive Microcapsules for Autonomic Lithium-ion Battery Shutdown

Lithium-ion batteries are commonly used in consumer electronics applications such as cellular phones and computers. However, there are safety concerns, such as external heating, over-charging, high current charging, or physical damage, which prevent their full market acceptance. Autonomic shutdown of lithium-ion batteries, through functionalization of battery electrodes with thermoresponsive microcapsules, is proposed as a fail-safe mechanism. The proposed concept relies on monomer-filled microcapsules that can be triggered to rupture within a desired temperature range and deliver an electrically isolating core to the electrode surface to shut down the battery cell. Preparation of thermoresponsive microcapsules that can be triggered to rupture using a low-boiling point solvent and deliver a thermally polymerizable core is described. Additionally, we demonstrate that the rupture temperature can be controlled by appropriate selection of microencapsulated trigger solvents. Initial work on the coating of battery materials with thermoresponsive spheres is also described.
M. Baginska, B. J. Blaiszik, S. A. Odom, A. E. Esser-Kahn, M. M. Caruso, J. S. Moore, N. R. Sottos, S. R. White

Service Life Prediction of Seal in PEM Fuel Cells

Proton exchange membrane fuel cell (PEMFC) is a promising power source for automobiles in the near future. During operation, there are gases and liquids inside the fuel cell. Sealing around the peripherals of the cell is therefore required to prevent the gases/liquids inside the cell from leaking. Polymers are usually used as the sealing or gasket materials. They in general possess the property of viscoelasticity. The stress relaxation behavior of liquid silicon rubber, a type of polymer, is studied in this article. Applying the time-temperature superposition, master curve is generated for prediction of service life of this material used as seals in PEMFC.
Tong Cui, C-W. Lin, C. H. Chien, Y. J. Chao, J. Van Zee

Experimental Validation of A Constitutive Model for Ionomer Membrane in Polymer Electrolyte Membrane Fuel Cell (PEMFC)

The authors have implemented a nonlinear constitutive modeling for ionomer membranes with application in polymer electrolyte membrane fuel cells. The constitutive model features multiplicative decomposition of viscoelastic and plastic deformation gradient tensors, micro-mechanism inspired viscous flow rule, nonlinear viscoelastic Bergström-Boyce model, and hydration and temperature dependent elastic modulus of ionomer membrane. In this work, the authors attempted to experimentally validate the constitutive model. Experimental results obtained from uniaxial tension tests of perfluorosulfonic acid membrane (e.g Nafion® from DuPont.) under wellcontrolled environments were used to fit the model parameters, which are subsequently used in a finite element (FEM) code to predict stress and deformation of ionomers in complex loading cases. The proposed model after validation showed fairly good predictive capabilities for the large deformation behavior of Nafion membrane subjected to the creep and relaxation at different strain rates in a wide range of relative humidity.
Wonseok Yoon, Xinyu Huang, Roham Solasi

Evidence of piezonuclear reactions: From geological and tectonic transformations to neutron detection and measurements

Neutron emission measurements, by means of helium-3 and neutron bubble detectors, were performed on solid specimens during three different kinds of mechanical tests: compression tests under displacement control, under cyclic loading, and by ultrasonic vibration. The material used for the tests was Green Luserna granite. Since the analyzed material contains iron, our conjecture was that piezonuclear fission reactions involving fission of iron into aluminum, and of iron into magnesium and silicon, should have occurred during compression damage and failure. It is also interesting to emphasize that the present natural abundances of aluminum (~8%), and silicon (28%) and scarcity of iron (~4%) in the continental Earth’s crust should be possibly due to the piezonuclear fission reactions considered above.
A. Carpinteri, G. Lacidogna, A. Manuello, O. Borla

Energy dispersive X-ray spectroscopy analysis on rock samples subjected to piezonuclear tests

In the present paper, Energy Dispersive X-ray Spectroscopy (EDS) was performed on different samples of external or fracture surfaces coming from specimens used in piezonuclear tests [1,2]. For each sample, different measurements of the same crystalline phases (phengite and biotite) were performed in order to get averaged information of the chemical composition and to detect possible piezonuclear transmutations from iron to lighter elements. The results of EDS analyses show that, in the fracture surface samples, a considerable reduction in the iron content (~25%) seems to be counterbalanced by an increase in Al, Si, and Mg concentrations.
A. Carpinteri, A. Chiodoni, A. Manuello, R. Sandrone

Acoustic and electromagnetic emissions in rocks under compression

The present paper focuses on Acoustic Emission (AE) and Electromagnetic Emission (EME) detected during laboratory compression tests. We investigated the mechanical behaviour of granite rock (Luserna stone) specimens with different size and shape. The recorded AE and EME signals were related to the time history of the load applied to the specimens. The results show the frequency range in which the EME due to fracture phenomena take place. In addition, the experimental evidence demonstrates how AE can be considered as a fracture precursor, since it precedes EME events, which accompany stress drops and related discontinuous fracture advancements.
G. Lacidogna, A. Manuello, A. Carpinteri, G. Niccolini, A. Agosto, G. Durin

Gear with Asymmetric Teeth for use in Wind Turbines

In the US, wind energy is one of the electrical energy sources that are growing fastest. The growth has been linear at a rate of about 20% to 30% per year over the last decade. For the next two decades, the wind industry has set its goal to more than 20%. However, there still remains concern on the reliability of wind turbines. This concern is often directed to the fact that gearbox failure has been a major problem in the wind industry. Compared to the other wind turbine components, gearbox failures result in the second highest down time per failure. Currently, there are several initiatives underway to improve gearbox reliability in wind turbines. Additionally, due to the increasing performance requirements, there has also been need of new gear designs. Recently, theoretical analyses have shown that asymmetric gears may offer a potential to reduce the costs associated with the gear failures, while at the same time maintaining the fatigue life. Also, for wind turbine gearboxes, the gears experience only uni-directional loading. In these instances, the geometry of the drive side does not have to be symmetric to the coast side. This allows for the designing of gears with asymmetric teeth. The objective of this research was to design and construct a testbed for testing the performance of asymmetric gears. The tests to be performed on the testbed include gear dynamics, tip relief modification, high-contact-ratio, and wear.
S. Ekwaro-Osire, T.-H Jang, A. Stroud, I. Durukan, F. M. Alemayehu, A. Swift, J. Chapman

Material Brittleness and the Energetics of Acoustic Emission

This paper will review energy aspects of the acoustic emission (AE) phenomenon and its relationship to material properties especially brittleness. The spectral energy density of the AE wave at low frequencies is related to the moment tensor, but this is only a fraction of the total energy converted in the deformation or damage process. The “conversion efficiency” from static elastic energy to dynamic AE energy is governed by the source speed, and this in turn is related to the brittleness of the material. Meanwhile, the spectral bandwidth of the AE near the source is governed by the duration of the source event. The resulting relationships between brittleness and acoustic emissivity will be discussed. Examples will be drawn from metals, fiber reinforced composites and geological materials. A further factor that has a strong influence on a material’s damage tolerance is its heterogeneity. This also has a strong influence on its acoustic emissivity, specifically on the amplitude distribution. In a recent development in the practical application of AE to industrial plant monitoring, these factors and others are integrated in a model of the Probability of Detection (POD) for fatigue cracks growing in a mixed mode comprising both ductile and brittle deformation mechanisms.
Adrian A. Pollock

b-value of plain concrete beams based on AE Quanta

In seismology, Gutenberg-Richter relationship \(\log_{10} \ N = a - bM\) is an empirical relationship between the magnitude of earthquake and its recurrence frequency. The constant ‘b’, is called the b-value and is the log linear slope of frequency-magnitude distribution. An analogy is drawn between earthquake and failure process in concrete. During the failure process of concrete, Acoustic emission (AE) energy is released in the form of energy waves having certain peak amplitudes. While estimating the b-value during fracture in concrete, peak amplitudes of the AE signals are used. Right from the onset of micro-cracks till failure, the AE events are recorded with their peak amplitudes and AE energies. Interestingly, the AE energy release has been observed to be in “packets” or bursts. These energy packets have been called by the authors as AE quanta. In the estimation of b-value, peak amplitudes of events of groups having a definite number are used. Instead of using amplitudes of arbitrary group of events, quanta are utilized as groups for obtaining the b-value,. Unlike in seismology, wherein the b-value could be nearly unity, it is found that the b-value from quanta is much less than that obtained from the amplitudes.
S. Muralidhara, Hamid Eskandari, B. K. Raghu Prasad, R. K Singh

AE monitoring of the Syracuse Athena Temple: Scale invariance in the timing of ruptures

We present a comparative statistical analysis between the time series of the acoustic emission (AE) events detected from the ancient Greek Athena temple in Syracuse (Eastern Sicily, UNESCO World Heritage List since 2005), and the time series of small and intermediate earthquakes occurred in this part of Sicily during the AE monitoring period. The waiting-time distributions for both time series and different magnitude thresholds are described by a unique scaling function indicating self-similarity over a wide range of magnitude scales. The similarity between the waiting times in AEs and earthquakes suggests a correlation between the microfracturing process accompanying ageing and deterioration of the monument and the regional seismicity. Our results reveal that the structure of the Athena temple is particularly sensitive to the normal seismic activity, suggesting structural AE monitoring as a useful tool for seismic hazard assessment.
G. Niccolini, G. Durin, G. Lacidogna, A. Manuello, A. Carpinteri

Analysis of energy released by elastic emission in brittle materials under compression

Experimental results are presented for fracture tests carried out on concrete and rock specimens under compression, and an analysis is performed for low-frequency acoustic emissions (elastic emissions, or ELE) due to crack growth. ELEs are vibrations of the specimen surface with relevant amplitudes and low frequencies (between 1 kHz and 20 kHz), appearing at the very last stages of the test and then revealing imminent failure. A spectral analysis of the ELEs is performed by measuring with a calibrated transducer the local acceleration of the specimen surface in the application point of the transducer. Quantitative estimation of ELE released energy is given in terms of kinetic energy using a simple kinematic model.
A. Schiavi, G. Niccolini, P. Tarizzo, G. Lacidogna, A. Manuello, A. Carpinteri

Numerical simulation of AE activity in quasi-brittle materials under compression

We present some numerical simulations of AE due to damage propagation in quasi-brittle materials under compression. To this purpose, the AE cumulative number, the time frequency analysis and the statistical properties of AE time series will be numerically simulated adopting the so-called “particle method strategy”. The method provides the velocity of particles in a set simulating the behavior of a granular system and, therefore, is suitable to model the compressive wave propagation and acoustic emission (corresponding to cracking) in a solid body. The numerical simulations correctly describe the compression test in terms of mean stress-strain and crack pattern. The size effects on the peak compressive strength and on the AE count are correctly reproduced. In addition, the amplitude distribution (b-value) and temporal evolution of AE events due to cracking, crucial for the evaluation of damage and remaining lifetime, was simulated according to the experimental evidences.
Stefano Invernizzi, Alberto Carpinteri, Giuseppe Lacidogna, Amedeo Manuello

Mechanical characterization and AE of translucent self-compacting concrete plates in bending

An experimental and numerical study on the mechanical behaviour of an innovative composite material based on the combination of a self-compacting concrete (SCC) matrix with transparent glass inclusions is proposed. The experimental tests have been monitored by an acoustic emission (AE) device. The results are interpreted by a FEM model accounting for the fracture of the two different materials and the interface between them. The AE monitoring is used for the definition of the crack pattern, and to determine the fracture energy dissipation domain.
A. Manuello, A. Carpinteri, S. Invernizzi, G. Lacidogna, S. Pagliolico, A. Torta

Reconfiguration of Large Leaf under Wind Load

In this study, the reconfiguration of Heliconia rostrata leaf was investigated by using fluid structure interaction (FSI) analysis at different wind speeds. In order to perform FSI analysis, the time dependent mechanical properties of petiole and lamina were modeled by using linear viscoelastic constitutive laws and the parameters were fitted from 3 point bending tests and tensile tests experimental data. Besides, in fluid domain, air is considered as incompressible fluid in this study. The drag force and banding moment of Heliconia rostrata leaf at different wind speeds (4 and 6 m/sec) under headwind condition were examined. The FSI analyses show that, compared with drag forces of rigid leaf under headwind, the drag forces of Heliconia rostrata leaf reduce 35% and 63% under 4 and 6 m/sec headwind conditions respectively. The findings of this study can help us design better flexible structures such as wind turbine blades.
N.-S. Liou, S.-S. Tsai, H.-H. Yen

Manufacturing Challenges for the Modern Wind Turbine Rotor

As blades have gotten larger, challenges must be met to assure delivery of the power producing element of the turbine. The industry does not allow for years of testing and development; the investment is enormous. Not all of the assumptions about how blades could be scaled have proven out
Gary Kanaby

Structural Monitoring of Wind Turbine Blades Using Fiber Optic Bragg Grating Strain Sensors

Over the last few years, fiber optic sensors (FOS) have seen increased acceptance and widespread use in civil engineering, aerospace, marine, oil & gas, composites and smart structure applications. More and more, different research groups and blade manufacturers worldwide have started adopting fiber sensors and fiber Bragg gratings (FBGs) in particular, as practical sensing technology for wind blades. FOS are an attractive technology and reliable sensing solution due to the fact that are completely immune to electromagnetic interference, lightning and electric noise, unlike more conventional electronic sensors that are prone to failure given the harsh and exposed environmental conditions under which wind turbines normally operate. Typically, FBG sensor arrays–either surface-mounted or embedded–have been used to monitor the mechanical behavior of composite rotor blades during the design and qualification stages, as well as in service, to help monitor, on–line, the blades’ condition under rotating, stationary and different wind load conditions. In this paper, will present test field results on the mechanical measurements from an experimental composite blade developed under Sandia Lab’s S–Blade experimental wind turbine program, instrumented with FBG temperature and strain sensors. A discussion of the methodology, on-line monitoring electronic system, and results obtained will be presented.
Alan Turner, Tom W. Graver

Degradation of Polymer Electrolyte Membranes

Premature failure of polymer electrolyte membranes used in proton exchange membrane fuel cells result in short life and degradation of performance of the fuel cell stack. Changes in the humidity and temperature cause swelling and shrinking of the membrane which result is stresses in the membrane. Stress relaxation and changes in conductivity will occur in the membrane. A novel experimental facility which allows the control of humidity, temperature, load or strain and the simultaneous measurement of the proton impedance has been developed and used to measure the stress relaxation and associated changes in conductivity of Nafion membranes. It was found that at constant strain, both stress relaxation and a drop in the conductivity of the membrane occurs.
Alan Jones, Jaya Malladi

The Nonlinear Viscoelastic Properties of PFSA Membranes in Water-immersed and Humid Air Conditions

Proton exchange membranes (PEM) in an automotive fuel cell stack can experience significant temperature and hydration changes as the stack responds to the demanding automotive duty cycle. Since mechanical stresses resulting from the hygrothermal cycles are believed to contribute to the loss of mechanical durability that are sometimes experienced in operating PEM fuel cells, it is important to characterize the mechanical behavior of PEMs over a wide range of hygrothermal conditions. In this study, the linear and nonlinear viscoelastic properties of PEMs equilibrated with both humidified air and liquid water are characterized using a custom-built multistation stress relaxation fixture. Specifically, relaxation data of a commercially available, perfluorosulfonic acid PEM was collected over a temperature range of 30-90°C and strain levels from less than 1% to over 20% or more. A comparison of immersed data to dry conditions and a range of humidity levels is presented in this paper. Significant nonlinearity is observed in the membrane, but becomes less pronounced at longer times. Cyclic tests with various strain levels were carried out on the membranes at 70o C in immersed conditions. The nonlinearity exhibited by the PEM under the larger strain levels was represented quite accurately with a Schapery unaxial hereditary single integral model. For this initial effort, material nonlinear parameters were chosen to simulate the stress output from larger strain levels. Complex loading profiles at various rates were used to validate the model and good agreement was achieved between experimental results and numerical predictions.
Lei Yan, Timothy A. Gray, Kshitish A. Patankar, Scott W. Case, Michael W. Ellis, Robert B. Moore, David A. Dillard, Yeh-Hung Lai, Yongqiang Li, Craig S. Gittleman

Assessing Durability of Elastomeric Seals For Fuel Cell Applications

Proton exchange membrane fuel cells typically consist of stacks of membrane electrode assemblies sandwiched between bipolar plates, effectively combining the individual cells in series to achieve the desired voltage levels. Elastomeric gaskets are commonly used between each cell to insure that the reactant gases are isolated; any failure of a fuel cell gasket can cause the reactants to mix and can lead to failure of the fuel cell. An investigation of the durability and lifetime of these fuel cell seals was performed by using accelerated characterization methods. A hydrocarbon sealant was tested in five different environments to simulate fuel cell conditions. Material properties such as secant modulus at 100% strain, tensile strength and strain at failure were determined using dogbone samples aged at several different imposed strains and aging times in environments of interest. Tearing energy was evaluated using trouser test samples tested under different rates and temperatures after various environmental aging conditions. Viscoelastic properties of these seals were analyzed using momentary and relaxation compressive stress tests. A viscoelastic and mechanical property characterization of these elastomeric seals under accelerated aging conditions could help understand their behavior and predict their durability in the presence of mechanical and environmental loading.
Justin E. Klein, Gilles M. Divoux, Hitendra K. Singh, Scott W. Case, David A. Dillard, John G. Dillard, Wonho Kim, Robert B. Moore, Jason B. Parsons

Compression of Seals in PEM Fuel Cells

Seals or gaskets are used in PEM fuel cells (PEMFC) or stacks to prevent leaking of the liquid and gas inside the cell. The fuel cells or the stacks are normally assembled with nuts and bolts or a combination of nuts, bolts, and springs. As the seal is typically made of polymers, the level of the compressive stress applied to the seal during long term operation of the fuel cell relaxes. In addition, the amount of compression applied to the seal may vary due to temperature changes during the fuel cell operation which arises from thermal expansion and contraction of all components in the cell. To understand the sealing force existed in a fuel cell during operation, all these factors must be fully understood. In this study, the compression of the seal in a PEMFC was investigated experimentally. Specifically the amount of compression was measured in-situ, i.e. immediately after the assembly and during the normal operation of the PEMFC. The objective of this study is to gain an understanding of the variation of compressive strain applied to the seal as the temperature of the PEMFC changes and cycles. This information is useful in estimating the sealing force in the cell and consequently the life of the seal.
Chi-Hui Chien, Chih-Wei Lin, Yuh-Jin Chao, Cui Tong, John Van Zee

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.
Ryan Berke, Angel Suresh, Mark E. Walter

Metal Foils in Clean, Renewable Energy Applications

Metal strip and sheet thinner than 125 microns (0.005 inch) is, by definition, foil. Foils significantly thinner than this value have found application in two different alternative energy applications. Foils in the 15 to 5 micron range (0.0006 to 0.0002 inch) are finding use in fuel cell balance of plant applications. One hurdle to the implementation of fuel cell technology is the lack of widespread hydrogen supply infrastructure. Reforming of hydrocarbon fuels such as natural gas, propane or diesel fuel on site is one way to enable the use of fuel cells in many applications. Reforming of hydrocarbon fuels results in a reformate gas stream containing species other than hydrogen, such as nitrogen and carbon monoxide. Pure hydrogen gas can be extracted from the reformate gas stream using a permselective metal foil membrane. Hydrogen flux increases and cost decreases with membrane thickness. In another renewable energy application, stainless steel foils 25 to 50 microns (0.001 to 0.002 inches) thick are being used as substrates for advanced thin film photovoltaic cells. The use of foil substrates instead of glass makes the solar cells lighter, more rugged and processable in a continuous fashion, lowering manufacturing costs.
Mark Robinson

Experimental and Probabilistic Analysis of Asymmetric Gear Tooth

Photoelasticity has been used previously in experimental studies on gears. With the advantages of today’s high capability of computing and software, photoelastic analysis can be combined with probabilistic analysis. The objective of this study was to perform an experimental study and probabilistic analysis of an asymmetric gear tooth. The uncertainty considered involves the material properties and geometry. In this study, different asymmetric gear tooth profiles were manufactured from PSM-5 photoelastic material. This study shows that the combination of photoelastic experiments and probabilistic analyses may be an effective tool to study the advantages of asymmetric gear over the symmetric gears.
S. Ekwaro-Osire, I. Durukan, F. M. Alemayehu
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