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The transient thermal response of a 15-cell, 48 V, lithium-ion battery pack for an unmanned ground vehicle (UGV) was simulated using ANSYS fluent. Heat generation rates and specific heat capacity of a single cell were experimentally measured and used as input to the thermal model. A heat generation load was applied to each battery, and natural convection film boundary conditions were applied to the exterior of the enclosure. The buoyancy-driven natural convection inside the enclosure was modeled along with the radiation heat transfer between internal components. The maximum temperature of the batteries reached 65.6 °C after 630 s of usage at a simulated peak power draw of 3600 W or roughly 85 A. This exceeds the manufacturer's maximum recommended operating temperature of 60 °C. We present a redesign of the pack that incorporates a passive thermal management system consisting of a composite expanded graphite (EG) matrix infiltrated with a phase-changing paraffin wax. The redesigned battery pack was similarly modeled, showing a decrease in the maximum temperature to 50.3 °C after 630 s at the same power draw. The proposed passive thermal management system kept the batteries within their recommended operating temperature range.

We present an analysis of the fatigue failure of an 18 tooth star–ratchet gear (SRG). The subject gear was implemented in the freewheel assembly of a mountain bicycle. After 6 years of service, the gear failed unexpectedly during a typical off-road ride. The unique geometry of SRGs precluded a simple comparison to existing gear lifetimes. Scanning Electron Microscopy (SEM) analysis of the failed gears showed crack initiation at the root of the gear teeth, followed by fatigue crack propagation and eventual chip-out. A biomechanical analysis of pedaling forces, coupled with explicit power data obtained from instrumented rides over the same trails, in conjunction with a Finite Element Analysis (FEA) of the gear, were used to determine stress amplitudes for fatigue calculations. Energy dispersive spectroscopy (EDS) determined the alloy composition of the gear and thus set the strength and fatigue properties of the gear. Basquin’s law, Goodman’s mean stress correction, and Miner’s rule were used to estimate the lifetime, in bike rides, of the gear. Our analysis led to an estimate of 2288 rides, while failure was reported after roughly 312 rides. Given the uncertainties in fatigue life estimation and service use, we find this estimate acceptable.

The United States Department of Homeland Security (DHS) charge is to, “Build a safer, more secure, and more resilient America by preventing, deterring, neutralizing, or mitigating the effects of deliberate efforts by terrorists to destroy, incapacitate, or exploit elements of our Nation’s CIKR …” using an all-hazards approach. The effective implementation of this strategy hinges on understanding catastrophes and their potential effect on the functioning of infrastructure. Unfortunately, there has been no unifying theory of catastrophe to guide decision-making, preparedness, or response. In this paper, the authors present a framework based on network science and normal accident theory that can be used to guide policy decisions for homeland security. They show that exceedance probability encompasses operational definitions of risk and resilience and provides a unifying policy framework for homeland security investments. Such an approach allows one to classify hazards as ‘high’ or ‘low’ risk, according to the resiliency exponent, and guide investments toward prevention or response. This framework is applied to cyber exploits and electric power grid systems to illustrate its generality.

Characterizing the mechanical properties of materials and biological systems at the nanoscale requires accurate measurement of forces on the order of μN and less. Due to the scale of the measurements and size of the instrumentation, calibration of nanoscale devices presents a new challenge in metrology. In order to ensure accuracy of results, traceable calibrations must be performed on nanoscale instrumentation. Our group recently developed a novel MEMS-based high resolution load cell with force resolution on the order of μN. This paper reports on a simple method for traceably calibrating our device using dead weights that could be generalized to other MEMS-based load cells. In this article, fabrication of a MEMS load cell is detailed and we compare our calibrated force–displacement curves to a non-linear theoretical prediction, revealing errors as great as 29%.

This paper identifies dynamic excitation parameters that promote decohesion of stiction-failed microcantilevers. The dynamic response of "s-shaped" adhered beams subjected to harmonic loading is described using modal analysis; this model is then used to predict the onset of debonding in the context of a critical interface energy. These theoretical results are used to rationalize preliminary experiments, which illustrate that dynamic excitation may be used to affect partial or complete repair of stiction-failed microcantilevers. The theoretical results provide fundamental insight regarding regimes where resonant effects trigger debonding and can serve as a potential mechanism for stiction repair. The models illustrate that driving a structure at resonance is usually beneficial with regards to debonding. However, this is not universally true; there is no benefit to driving a device at frequencies with unfavorable mode-shapes. Thus, these results provide a reasonable physical and mathematical explanation for the preliminary experimental results, while providing a roadmap for identifying parameters in future tests

We have developed a new scientific tool for the mechanical characterization of axisymmetric freestanding nano-thickness films. The tool consists of a calibrated MEMS-based load cell with a spherical loading tip, and a positioning system capable of sub-nm accuracy. The loading tip is positioned over the center of a free-standing film using a two-axis positioning stage. A Michelson Interferometer is positioned below the film to record the displacement field of the membrane, in situ, as the load cell deflects the membrane from above. As the MEMS load cell is lowered into the axisymmetric freestanding thin film, concentric fringes emanate from the center of the thin film sample. This tool allows us to record the applied load and membrane displacement simultaneously. To date, load cells with stiffnesses as low as 2 nN/μm have been fabricated and calibrated. We have validated the operation of the apparatus using gold films 1080 μm in diameter and 170 nm thick.

In this paper, we present experimental measurements of the strain energy release rate for stiction-failed polysilicon microcantilevers using a newly developed single cantilever beam peel test. Our experiments show that dry-contacting microcantilevers adhere exclusively as tip-stuck, "arc-shaped" stiction failures, while adhesion under "wet" conditions generate exclusively "s-shaped" stiction failures. Microcantilevers were "peeled" from the substrate under displacement control using a piezoelectric actuator attached to one end of an array of microcantilever beams. The crack length was monitored using interferometric imaging, and related to the applied displacement using established equations from linear elastic fracture mechanics. The pull-off forces associated with "arc-shaped" stiction failures were an average value of 89.7 nN, for 1000 mum long beams, and an average value of 123 nN for 1500 mum long beams. Adhesion energies for s-shaped failures were measured as 13.7 mJ/m2 for IPA released beams and 15.4 mJ/m2 for deionized water released beams. These values are in good agreement with previous measurements. The proposed experimental method enables application of a simple fracture mechanics model using a standard specimen geometry. These experiments, using both wet and dry adhesion failure conditions, show that the quality of the adhesive failure depends upon the magnitude of the forces pulling the microcantilever into contact with the underlying substrate

Nanoscale surface features at the interface of bonded silicon wafers are critical to achieving high yield wafer bonding processes. Interfacial features such as surface patterning, trapped particles and gasses, and wafer surface nanotopography can generate residual stresses that may affect subsequent device lifetimes, processing and performance. This paper presents a numerical model that captures a more complete description of bonding physics and mechanics than extensions of closed form thin-plate solutions. The model is verified by experimentally measuring residual stresses arising from patterned nanoscale surface features using an infrared grey- field polariscope. The proposed numerical model captures the functional form of the observed residual stress fields arising from interfacial defects and is in good agreement with experiments. These simulations indicate that high bond energies may produce debonds around small defects that are not possible to detect using infrared transmission inspection techniques, yet the associated residual stresses are increasingly large in magnitude.

Infrared imaging was used to measure the thickness of a Krylon® flat black spray paint coating applied to the surface of steel test coupons. Thermal signals were generated in the test samples by applying a cyclic tensile load at various frequencies, which generated a corresponding thermo-elastic temperature change. Since the substrate is a better thermo-elastic generator than the paint, the heat generated in the metal conducts through the paint during each cycle of loading and is easily distinguished from the paint signal. Test samples were prepared with a stepped progression of controlled paint thickness, and infrared images taken using a Delta Therm 1000 IR imaging system. The infrared signal intensity as well as the phase lag relative to the applied loading were directly related to the paint thickness. The experimental results are described well using existing heat conduction models. Though the proposed method was proven using thermoelastic heating, any heat source below the paint can be used to provide the necessary reference signal. Furthermore, the method can also be used for any coating type and is not limited to paint coatings.

Fiber pushout tests were used to quantify the effects of fiber coating thickness on the mechanical properties of two model composite systems: a monazite-coated (LaPO 4 -coated) alumina (A1 2 O 3 ) fiber in an A1 2 O 3 matrix and a LaPO 4 -coated yttrium aluminum garnet (YAG) fiber in an A1 2 O 3 matrix. Interface properties were quantified using the Liang and Hutchinson (LH) pushout model and mechanistically rationalized by considering the change in residual thermal stresses with changes in the coating thickness. Measures of the pure Mode II interfacial fracture energy, the coefficient of friction, and a radial clamping pressure are extracted by fitting the LH equations to the experimental results. Using the approach that has been developed herein, a methodology is available for measuring the interfacial properties, predicting the effect of coating thickness, and selecting the coating thickness to alter the interfacial properties.

This study explores the effect of interfacial properties on damage evolution and damage mechanisms in model glass fiber reinforced epoxy matrix composites. The composite properties were varied by changing the interfacial bond between the fiber and the matrix. Doubleedge-notched specimens were tested in tension and evaluated using thermoelastic stress analysis (TSA) to observe damage initiation and evolution, and to identify the operative damage mechanism. Changing the interface properties was found to change the operative damage mechanism. This study conclusively demonstrates that the damage mechanism in composites can be controlled by judicious changes in the interface properties. POLYM. COMPOS., 26:241–246, 2005. © 2005 Society of Plastics Engineers

Thermoelastic stress analysis (TSA) has been used to monitor damage evolution in several composite systems. The method is used to measure full-field hydrostatic stress maps across the entire visible surface of a sample, to quantify the stress redistribution that is caused by damage and to image the existing damage state in composites. Stress maps and damage images are constructed by measuring the thermoelastic and dissipational thermal signatures during cyclic loading. To explore the general utility of the method, test samples of several ceramic-matrix and cement-matrix composites have been fabricated and tested according to a prescribed damage schedule. The model materials have been chosen to illustrate the effect of each of three damage mechanisms: a single crack that is bridged by fibers, multiple matrix cracking, and shear bands. It is shown that the TSA method can be used to quantify the effect of damage and identify the operative damage mechanism. Each mechanism is identified by a characteristic thermal signature, and each is shown to be effective at redistributing stress and diffusing stress concentrations. The proposed experimental method presents a new way to measure the current damage state of a composite material.

We describe indentation testing of freestanding elastomeric films with thickness ranging from 0.1 to 3 mm. The relationship between load–deflection measurements and mechanical properties is discussed in the context of constitutive behavior, test variables (such as span length, film thickness and indenter size) and analytical indentation models. Regimes are identified wherein elastic properties can be obtained from either linear or non-linear load–deflection response, using point-load plate models and finite-contact membrane models, respectively. Testing of cast specimens of a poly(dimethyl siloxane)-based elastomer illustrates that elastic moduli determined via indentation is consistent with uni-axial tests. The models for point loads on plates include the effects of both indenter penetration and plate deflection; both are important for soft materials, such as elastomers and biological materials. The results demonstrate measurement approaches to characterize soft materials and ultra-thin freestanding films with load–deflection stiffness on the order of 0.01 N/m. The macroscale experiments and theoretical framework outlined here can be extended to films with sub-micron thickness and moduli greater than ∼1 GPa, or samples with thickness on the order of 100 μm and moduli less than ∼1 MPa.

We present theoretical and experimental results to describe the mechanics of indentation of a clamped circular membrane with a frictionless spherical indenter. Analytical expressions and numerical simulations are presented for the relationships between contact radius, finite indentation strains (and stresses), pre-stretch, loads and deflection. These closed-form solutions are contrasted with point-load models that neglect the contact size (i.e. classical Schwerin-type solutions), and lead to important differences in the indentation strain and load–deflection response. The accuracy of these closed form expressions is illustrated by comparisons with detailed numerical results and experiments on thin elastomer films. We show that the closed-form solutions can be used to extract mechanical properties from indentation testing of freestanding films, with important implications for developing new tests on nanoscale films and/or compliant materials such as polymers and biological substances.

This paper presents a new impression method for fatigue testing polymer membranes that circumvents many of the problems associated with traditional uniaxial testing of thin, compliant films. The proposed test geometry subjects the polymer membrane to a biaxial stress state by displacing the center of a clamped circular specimen with a spherical indenter. The initiation of fatigue cracks in the membrane is monitored using optical and/or infrared sensing. Finite element and non-linear membrane theory models, which take into account large-displacement kinematics, non-linear material behavior and friction between the indenter and membrane, are used to establish connections between the applied indenter displacement, or load, and the stress in the film. A key result is a closed-form relationship between applied load and membrane stress, which can be used for a wide range of friction and material response. The broadly applicable approach is demonstrated using two types of laminated polymer membranes. Polym. Compos. 25:442–450, 2004. © 2004 Society of Plastics Engineers.

Presents a thermomechanical model that describes adhesion reduction in MEMS structures using laser heating. A fracture mechanics model is developed where the interface between the stiction-failed microcantilever and the substrate is treated as a crack, and the energy release rate is calculated using elastic theory. In order to include the effect of a temperature difference between the microcantilever and the substrate, an associated thermal strain energy is included in the fracture model. If the free length is longer than the critical buckling length, the beam buckles decreasing the strain energy of the system. For surface-micromachined polycrystalline silicon cantilevers with an initial crack length of 400 /spl mu/m, the model predicts that a temperature difference of 100 K repairs microcantilevers as long as 1300 /spl mu/m. The peeling of adhered beams from the substrate after laser irradiation is experimentally shown with measured crack lengths within 15% of predicted values indicating that the proposed model establishes the mechanism of adhesion reduction by laser irradiation.

A new experimental method is presented for quantifying machining damage in polymer matrix composites. The method consists of capturing infrared images of machined samples and using thermoelastic stress analysis to quantify damage from the machining event. A modified stress concentration factor is presented as an easily measured and useful damage parameter. Circular holes were drilled into the center of plate specimens fabricated from a commercially available glass fiber reinforced composite. A standard drill bit, brad point drill bit, and abrasive water jet machining were the three machine tools investigated. Infrared images were used to quantify the machining damage by assigning a thermoelastic stress analysis based stress concentration factor (mSCF) to each machined hole. The mSCF was then used to rank the damage inherent to each machining method. Optical and electron microscopy were utilized to identify the types of damage associated with the three machining methods. Finally, each sample was fatigued to failure to substantiate the IR results. The ranking of damage based upon the mSCF was in good agreement with the fatigue lifetime rankings: higher mSCF is associated with shorter fatigue lifetimes.

Disc brakes are exposed to large thermal stresses during routine braking and extraordinary thermal stresses during hard braking. High-g decelerations typical of passenger vehicles are known to generate temperatures as high as 900°C in a fraction of a second. These large temperature excursions have two possible outcomes: thermal shock that generates surface cracks; and/or large amounts of plastic deformation in the brake rotor. In the absence of thermal shock, a relatively small number of high-g braking cycles are found to generate macroscopic cracks running through the rotor thickness and along the radius of the disc brake. The analysis herein shows that rotor failure is a consequence of low cycle thermo-mechanical fatigue. An analysis of the vehicle dynamics was used to find a heat flux equation related to braking forces. The heat flux equation was then used in a finite element analysis to determine the temperature profile in the brake. Once the brake temperature was obtained, a simplified shrink fit analysis was used to estimate the stresses that arise during hard braking. This approach shows that plastic deformation occurs due to the large thermal strains associated with high-g braking. The calculated strain amplitude was then used in a Coffin–Manson law to predict the number of high-g braking cycles to failure. Good agreement was obtained between reported braking cycles to failure and the proposed theoretical approach

This paper presents a new experimental method that utilizes the thermoelastic effect to detect sub-surface cracks in a laminated polymer membrane. A highly accurate infrared camera is used to measure the thermoelastic and dissipational heat signatures associated with bi-axial fatigue loading of membranes. Changes in these thermal signatures arise whenever cracks form in any layer of the laminate, including fully embedded layers, thereby providing a novel method for experimentally measuring the initiation and growth of damage in sub-surface layers. The proposed method is illustrated using a model 3-layer system of EVOH sandwiched between two polyurethane layers. Bi-axial fatigue loading was used to initiate cracking in the central EVOH layer without damaging the outer polyurethane layers. Cracking in the central layer resulted in a distinct thermal signature that was plainly visible using the proposed method.

A new experimental method is presented for quantifying impact damage and estimating the remaining fatigue lifetime of impact damaged polymer matrix composite materials. The procedure is demonstrated using composites of glass fiber reinforced polyurethane produced by injection molding and structural reaction injection molding. Thermoelastic stress analysis (TSA) was used to quantify the stress concentration associated with impact-damage in test samples of each composite. Following impact and TSA imaging, the samples were fatigued to failure over a range of stress amplitudes. The TSA-derived stress concentration factors were used to determine a modified stress amplitude that collapsed the impact-fatigue data onto a master stress-life curve. This approach provides a quantitative measure of impact damage and a practical methodology for estimating the residual fatigue lifetime of impact; damaged composites.

CMCs offer measurable advantages in specific strength and stiffness, but have yet to overcome their relatively low fracture toughness. However, new understanding about composite damage mechanisms and the potential design benefits of stress redistribution are, again, igniting the potential use of CFCCs in structural applications. Engineers simply need to develop the necessary design framework, including an understanding of damage and a new approach to estimating residual life in order to confidently develop applications for CFCCs. This note shows that oxidation resistant high temperature, damage-tolerant CFCCs are currently available.

The following paper is an engineering failure analysis of an adjustment pin used in a leg press exercise machine. The pin is used to allow adjustment of the machine for people of different heights. It was modeled as a cantilever beam subjected to varying forces depending on the weight stack setting. The fracture occurred at a point of localized stress concentration. The analysis shows an elastic stress concentration of 1.67 which would cause plastic deformation and a fatigue stress concentration of 1.38 that led to the eventual fatigue failure. Based on actual user data, a block loading analysis was used to calculate a pin lifetime of approximately one year. The pin actually failed after 1 years of use. Design changes are recommended to reduce the size of the stress concentration and subsequently increase the life of the adjustment pin.

Summary Thermoelastic stress analysis was used to document the effect of composite damage on the stress distribution in three ceramic matrix composites. Composite damage was found to significantly alter the thermoelastic response of each material, with the greatest effect noted in SiC/CAS. Thermoelastic imaging of these materials affords a more complete picture of how the various damage mechanisms affect the stress distribution. In particular, a stress concentration factor computed from thermoelastic images, serves as an indicator of stress redistribution. The stress concentration factors were computed by comparing notch root to far field temperatures, and monitored after the introduction of various amounts of damage. In each material, the stress concentration factor diminished as the damaging load approached the ultimate stress. Reduction in the stress concentration is associated with local changes in modulus, mechanistically arising from combinations of fiber, matrix and interface fracture. Stress redistribution occurs as a consequence of modulus changes, leading to lower notch sensitivity in each of the tested composites.

Strain development in several notched ceramic-matrix composites (CMCs) has been monitored over a broad range of load. This was achieved by using phase-shifting moire interferometry, which provides a map of the surface strains. A sequence of fringe patterns was used to chart the evolution of strain redistribution as a function of load. The ensuing strains were related to the micromechanical damage mechanisms. Stress concentrations were estimated from the strains by using stress/strain curves. Implications for the notch sensitivity of CMCs are discussed.

Interfacial mechanical properties of SiC monofilament-reinforced β′-SiAlON composites were characterized by a single fiber push-out technique. Interfacial parameters were studied as a function of embedded filament lengths, including comparisons of linear, nonlinear shear-lag, and progressive debonding analysis models. The interfacial debonding peak load ( P p ) and maximum frictional sliding load ( P max ) were both measured from the apparent load-displacement curves. Linear and shear-lag analyses were fitted to the data as a function of embedded filament lengths, respectively. In comparison, the progressive debonding analysis was conducted by fitting the effective load-displacement curves obtained by subtraction of machine compliance from the apparent load-displacement curves. The nonlinear shear-lag model gave better regression fits to the data than did the linear model, while the progressive debonding model provided much more interfacial information than did the shear-lag model. In addition to the coefficient of friction (μ) and radial residual stress ( σ N ), axial residual load ( P r ), critical load for interfacial crack initiation or propagation ( P d ), interfacial fracture toughness ( G i ), as well as the interfacial roughness amplitude ( A ) and its contribution to the interfacial normal stress ( σ r ) were extracted from the progressive debonding model, using a three-parameter, non-linear least squares fitting method on the effective load-displacement curves.

Fiber-reinforced ceramic-matrix composites (CMCs) depend upon inelastic mechanisms to diffuse stress concentrations associated with holes, notches, and cracks. These mechanisms consist of fiber debonding and pullout, multiple matrix cracking, and shear band formation. In order to understand these effects, experiments have been conducted on several double-edge-notched CMCs that exhibit different stress redistribution mechanisms. Stresses have been measured and mechanisms identified by using a combination of methods including X-ray imaging, edge replication, and thermoelastic analysis. Multiple matrix cracking was found to be the most effective stress redistribution mechanism.

Tension experiments performed on a 0/90 laminated silicon carbide/calcium aluminosilicate composite at room temperature establish that this material is notch insensitive. Multiple matrix cracking is determined to be the stress redistribution mechanism. This mechanism is found to provide a particularly efficient means for creating local inelastic strains, which eliminate stress concentrations.

Performance and reliability of silicon based microelectromechanical systems (MEMS) and photovoltaic (PV) devices are often strongly affected by defects and residual stresses. As a result, both industry and academia need new tools that can rapidly locate and quantify defects. The grey-field photoelastic technique has been shown to improve detection and residual stress quantification of defects in wafer bonded MEMS structures. In the PV industry, the tool has proven capable of detecting high bulk stress in wafers, damage due to wafer cutting, and stresses associated with trapped cracks. In this paper, we describe the development of an infrared photoelastic tool that captures full-field residual stress images at camera framing rates. This solid state tool, which adapts technology developed for visible light residual stress inspection, reduces the data collection time for residual stress maps from several minutes to a fraction of a second. We demonstrate the tool on canonical samples including a beam in bending and disk in compression to verify stress results and identify the lower limits of detection. We then demonstrate the tool on several industrial applications.

Push-out experiments on sapphire fibers in both glass and γ-TiAl matrices show a sinusoidal modulation in the load–displacement trace that is identical in wavelength to the surface roughness of the sapphire fibers. A model, presented in a previous paper, is used to describe the push-out behavior, and is shown to agree with experiment. The results quantitatively validate the key role of fiber surface roughness during fiber sliding.

The presence of asperites at the fiber-matrix interface is noted experimentally as a load rise during the initial stages of push-out and as a reseating load drop during push-back. These effects are modelled by considering the elastic deformation of asperites at the interface. During fiber sliding, the decorrelation of initially matching fiber and matrix geometries results in an asperity pressure at the interface. Fractal models of interface roughness are incorporated into an equation of fiber sliding that is found to accurately reproduce experimental observations. Additionally, an asperity wear mechanism must be introduced to explain the effects of fatigue, the variation of fiber reseating with sliding distance, and the rapid decay in sliding stress during pristine fiber push-out.

An improved fiber push-through test has been designed and used to obtain new information about interfaces in composites consisting of matrices of a Ti alloy and borosilicate glass, both reinforced with SiC fibers. Interpretation of these results is accomplished through an analysis of coupled debonding and push-through, followed by push-back. The sliding stress is found to vary with push-out distance and to be substantially reduced in the vicinity of a fatigue crack in the Ti matrix composite. These effects are attributed to asperity wear, matrix plasticity and fragmentation of the fiber coating around the debonded interface. Reseating effects on push-back have been demonstrated, but have been found to diminish as the relative fiber-matrix displacement increases. Fiber roughness has been identified as an important aspect of interface sliding.

We present ultra low-cycle fatigue experiments of axisymmetric nano-crystalline gold films of nano-thickness deformed by a spherical indenter using a recently developed freestanding membrane test. Freestanding membranes of gold were centrally deflected using a spherical indenter attached to a MEMS load cell. Fabrication of the films for these experiments yielded films 100 nm thick and 500 μm in diameter. We observed that the plastically deformed thin films recover completely in time at room temperature. One particular film was loaded 4 consecutive times, with the fourth loading leading to its fracture after full recovery from the three prior loadings. Observation of this phenomenon directed us to term this behavior “Ultra Low cycle fatigue”.

We present experimental measurements of the mechanical properties of nanoscale-thick materials using a recently developed free-standing membrane test. Freestanding membranes of gold and a nano-porous silicate were centrally deflected using a spherical indenter attached to a microfabricated fixed-fixed beam device. Experiments of this type allow us to extract mechanical properties of the thin films without the complicating effects of the substrate, enabling accurate study of length scale effects. These experiments differ from previous work by testing freestanding films and by including the finite contact size of the indenter tip.Copyright © 2006 by ASME

Self-monitoring of blood glucose has become an important and critical tool for effective management of diabetes. A closed-loop, real-time sensing and dispensing system, however, has been elusive. We describe here, a novel MEMS-based sensor intended to measure, in realtime, the concentration of glucose in a solution. The device utilizes a bi-material membrane with a functional hydrogel layer that swells reversibly, and in proportion to, the presence of glucose. The hydrogel is composed of 2-hydroxyethyl methacrylate (HEMA) functionalized with 3-acrylamidophenylboronic acid (AAPBA) groups as the glucose-sensitive moiety. Phenylboronic acid (PBA) derivatives interact with glucose by forming a charged complex, causing the hydrogel to swell by solvent intake [1].Copyright © 2006 by ASME

We present results from a new procedure developed to quantify the pull-off force and strain energy release rates associated with stiction-failure in microdevices. The method is analogous to a standard, macro-scale peel test, but carried out using micro-scale devices. Adhesion is initiated by lowering an array of microcantilevers that protrude from a substrate into contact with a separate substrate. Displacement is controlled by a piezo-actuator with sub-nm resolution while alignment is controlled using linear and tilt stages. An interferometric microscope is used to align the array and the substrate and to record deflection profiles and adhesion lengths during peel-off. This geometry is accurately modeled using linear elastic fracture mechanics, creating a robust, reliable, standard method for measuring adhesion energies in stiction-failed microdevices.Copyright © 2006 by ASME

We present a new experimental method for measuring the mechanical properties of nanoscale-thick materials using the free-standing membrane test. Freestanding circular membranes are centrally deflected using a spherical indenter attached to a microfabricated fixed-fixed beam device. Experiments of this type allow us to extract mechanical properties of the thin films without the complicating effects of the substrate, enabling accurate study of length scale effects. These experiments differ from previous work by testing freestanding films and by including the finite contact size of the indenter tip.

A novel glucose sensor based on the deflection of a thin membrane is proposed in the article. The sensor consists of a glucose-sensitive hydrogel layer which exhibits swelling when immersed in a glucose-containing solution. A detailed overview of the fabrication procedure and preliminary experimental results for real-time glucose monitoring using our prototype devices will be presented.Copyright © 2005 by ASME

Surface micromachined structures with high aspect ratios are often utilized as sensor platforms in microelectromechanical systems (MEMS) devices. These structures generally fail by stiction or adhesion to the underlying substrate during operation, or related initial processing. Such failures represent a major disadvantage in mass production of MEMS devices with highly compliant structures. Fortunately, most stiction failures can be prevented or repaired in a number of ways. Passive approaches implemented during fabrication or release include: (1) utilizing special low adhesion coatings and (2) processing with low surface energy rinse agents. These methods, however, increase both the processing time and cost and are not entirely effective. Active approaches, such as illuminating stiction-failed microstructures with pulsed laser irradiation, have proven to be very effective for stiction repair [1–5]. A more recent and promising method, introduced by Gupta et al. [6], utilized laser-induced stress waves to repair stiction-failed microstructures. This approach represents a logical extension of the laser spallation technique for debonding thin films from substrates [7–9]. The method transmits stress waves into MEMS structures by laser-irradiating the back side of the substrate opposite the stiction-failed structures. This paper presents an experimental study that compares the stress wave repair method with the thermomechanical repair method on identical arrays of stiction-failed cantilevers.Copyright © 2004 by ASME

We present a model for reducing adhesion in MEMS structures using laser heating and compare the model to experimental results. Using a fracture mechanics model, the interface between the stiction-failed microcantilever and the substrate is treated as a crack, and the energy release rate is calculated using elastic theory. In order to include the effect of laser irradiation of the microcantilevers, an associated thermal strain energy is included in the fracture model. As the beam peels, the free length reaches a critical value where the beam buckles, decreasing the energy of the system. The results of the model predict a temperature difference of 100 K is able to repair microcantilevers as long as 600 μm. Experiments are performed that demonstrate the peeling of stiction-failed beams from the substrate after laser irradiation as predicted by the thermomechanical model.

The following paper is an engineering failure analysis of an adjustment pin used in a leg press exercise machine. The pin is used to allow adjustment of the machine for people of different heights. It was modeled as a cantilever beam subjected to varying forces depending on the weight stack setting. The fracture occurred at a point of localized stress concentration. The analysis shows an elastic stress concentration of 1.67 which would cause plastic deformation and a fatigue stress concentration of 1.38 that led to the eventual fatigue failure. Based on actual user data, a block loading analysis was used to calculate a pin lifetime of approximately one year. The pin actually failed after 1 1 2 years of use. Design changes are recommended to reduce the size of the stress concentration and subsequently increase the life of the adjustment pin. © 1998 Elsevier Science Ltd. All rights reserved.

An experimental technique for the measurement of interfacial sliding properties in fiber-reinforced materials is presented. The technique involves pushing a bundle of fibers simultaneously through a thin section of the composite. Using this technique, sample averages are immediately available, eliminating the need for repeated single-fiber tests. The technique is particularly useful for composites that contain small-diameter fibers. Salient features of the technique are demonstrated through tests on a CAS/SiC composite.

An experimental method is presented for quantifying impact damage in composite materials using specimens of glass fiber reinforced polyurethane and epoxy. Thermoelastic stress analysis (TSA) was used to quantify the stress concentration associated with impact-damage. Following impact and TSA imaging, the samples were fatigued to failure over a range of stress amplitudes. The stress concentration factors acquired from TSA were used to determine a modified stress for each sample that collapsed the impact, then fatigue data onto a master curve on stress-life axes. This approach provides a quantitative measure of impact damage and a rational estimate of the expected residual fatigue lifetime of impact damaged composites.

Damage tolerance, which depends upon inelastic mechanisms, is crucial to the general utility of ceramic matrix composites (CMCs). Recent experiments have confirmed several damage mechanisms and revealed that stress redistribution does occur in CMCs. The current composite paradigm relies on fiber coatings to achieve this damage tolerance. More recently, an Al2O3/ Al2O3 CMC was developed which attains damage tolerance through the use of a porous matrix. To evaluate this all-oxide material, tensile experiments were conducted on double edge-notched test specimens. An infrared imaging technique, known as thermoelastic stress analysis, was used to classify the damage mechanism and measure the extent of stress redistribution in double edge-notched specimens. Results indicate that the subject alumina/alumina composite exhibits a measurable degree of ductility by developing shear band damage zones.

A new generation of relatively ‘ductile’ CMCs is being developed that expands the general utility of structural ceramic composites. These new materials rely on inelastic mechanisms such as interface failure, matrix cracking, fiber failure and fiber pullout to redistribute stress away from locations of stress concentration (Evans et al., 1995). The combined effect of these mechanisms can be summarized in three macroscopic damage classifications (Mackin et al., 1995): Class I damage is the development of a single matrix crack bridged by fibers; Class II damage involves the development of multiple cracks in the matrix; and Class III damage involves the development of a shear damage zone. The operative damage mechanism depends upon the composite constituent properties, while the extent of stress redistribution depends upon the damage mechanism.

Thermoelastic stress analysis was used to document the effect of composite damage on the stress distribution in three brittle matrix composites. Thermoelastic imaging of these materials affords a qualitative picture of how the various damage mechanisms affect stress redistribution in the composite. In particular, a stress concentration factor computed form TSA images serves as an indicator of stress redistribution. The stress concentration factors were computed by comparing notch root to far field temperatures, and monitored after the introduction of various amounts of damage. In each material, the stress concentration factor diminished as the damaging load approached the ultimate stress. Reduction in the stress concentration is associated with local changes in modulus, mechanistically arising from combinations of fiber, matrix and interface fracture. Stress redistribution occurs as a consequence of modulus changes, leading to lower notch sensitivity in each of the tested composites. In addition to quantifying stress redistribution, the operative damage mechanism can be imaged by capturing the second harmonic of the applied stress frequency. TSA provides a valuable, non-contacting tool for measuring stress redistribution as well as visualizing the mechanism responsible for the resulting stress redistribution.

The mechanical properties of fiber reinforced composites depends strongly upon the properties of the fiber/matrix interface. Enhanced fracture resistance and strain to failure are synonymous with debonding and sliding of the reinforcement phase. Thus, the two key properties of the composite are the interfacial toughness and the post-debond sliding stress. After debonding a variety of interfacial sliding phenomena are noted, including: stick-slip, steady-state, and roughness dominated sliding. The interfacial properties, including the coefficient of friction, the radial clamping pressure, asperity amplitude, the elastic properties of the constituents, and the compliance of the test machine, each play a role in the operative sliding phenomenon. Experiments have been conducted to explore each of these phenomena. In addition, models have been developed that rationalize all of the observed behavior.