Piezoelectric stacks are being sought to be used as actuators for precision positioning and deployment of mechanisms in future planetary missions. Beside the requirement for very high operation reliability, these actuators may be required to operate in space environments that are considered harsh compared to normal terrestrial conditions. These environmental conditions include low and high temperatures and vacuum or high pressure. Additionally, the stacks are subjected to high stress and in some applications need to operate for extended time periods. Many of these requirements are beyond the current industry design margins for nominal terrestrial applications. In order to investigate some of the properties to assess the durability of such actuators and their limitations we have developed a new type of test fixture that can be easily integrated in various test chambers for simulating environmental conditions, can provide access for multiple measurements while being exposed to adjustable stress levels. We have designed and built two versions of these test fixture and these fixtures were made to be adjustable for testing stacks with different dimensions and can be easily used in small or large numbers. The properties that were measured using these fixtures include impedance, capacitance, dielectric loss factor, leakage current, displacement, breakdown voltage, and lifetime performance. The fixtures characteristics and the test capabilities are presented in this paper.

Over the years, piezoelectric transformer (PT) was used in inverters to drive cold cathode fluorescent lamps (CCFL). As it stands now, CCFL still was considered to be the primary light source for the backlight module of LCD TVs. It is know that transformers play important role in backlight module performance. Since the LCD TV might be sold to different parts of the world and the inverters as well as the power boards do produce heat, the piezoelectric transformers are expected to perform well in vastly different environmental temperatures. Since temperature will influence the material properties, the resonant frequency of piezoelectric transformer gets affected easily by the environmental temperature. In this paper, we operated the Rosen-type piezoelectric transformers of various thicknesses in temperatures such as 25, 50, 70, and 90°C to examine the temperature dependency of the resonant frequency, the power efficiency, the step-up ratio, and the output current with CCFL as the loads. Simulation was also done to study the trend of resonant frequency variations in different temperatures. Furthermore, the effect of environmental temperature to the performance of various thicknesses of Rosen-type PT was examined. The relationship between the CCFL used and the operating temperature was investigated with an attempt to explore future applications.

The rate-dependence of piezoelectric materials resulting from the kinetics of domain switching is an important factor that needs to be included in realistic modeling attempts. This paper provides a systematic study of the rate-dependent hysteresis behavior of a commercially available PZT stack actuator. The stack actuator is coupled to a flexure system which provides mechanical loading via a spring. Experiments covering full as well as minor loops are conducted at different loading rates with polarization and strain recorded. In addition, the creep behavior at different constant levels of the electric field is observed. These experiments provide evidence of kinetics being characterized by strongly varying relaxation times that can be associated with different switching mechanisms.

Relaxor ferroelectric single crystals produce extraordinary strain levels in response to electric field, but the response is highly dependent on temperature and bias stress. This behavior must be well characterized for the successful development of actuation and sensor applications. This paper presents the results of experimental characterization of <001> oriented single crystal PMN-xPT (x = 0.27 and 0.29) is presented for combined electrical and mechanical loading at various temperatures. These data are contrasted with previously reported constitutive behavior of <001> single crystal PMN-xPT with different compositions. The effects of composition on the phase transformation behavior and linear material properties are compared and discussed.

Future NASA missions are increasingly seeking to use actuators for precision positioning to accuracies of the order of fractions of a nanometer. For this purpose, multilayer piezoelectric stacks are being considered as actuators for driving these precision mechanisms. In this study, sets of commercial PZT stacks were tested in various AC and DC conditions at both nominal and extreme temperatures and voltages. AC signal testing included impedance, capacitance and dielectric loss factor of each actuator as a function of the small-signal driving sinusoidal frequency, and the ambient temperature. DC signal testing includes leakage current and displacement as a function of the applied DC voltage. The applied DC voltage was increased to over eight times the manufacturers' specifications to investigate the correlation between leakage current and breakdown voltage. Resonance characterization as a function of temperature was done over a temperature range of -180°C to +200°C which generally exceeded the manufacturers' specifications. In order to study the lifetime performance of these stacks, five actuators from one manufacturer were driven by a 60volt, 2 kHz sine-wave for ten billion cycles. The tests were performed using a Lab-View controlled automated data acquisition system that monitored the waveform of the stack electrical current and voltage. The measurements included the displacement, impedance, capacitance and leakage current and the analysis of the experimental results will be presented.

The technique of birefringence imaging was exploited to observe the evolution of creeping strain fields in transparent PLZT 8/65/35 samples. PLZT samples with features that produce non-uniform fields were loaded with constant voltage boundary conditions. The resulting birefringence contours evolve with time and can be related to strain measurements. Three experimental arrangements are reported: partial surface electrodes producing intense fields near an electrode tip, a round insulating hole producing local concentration of electric field, and a thin, sharp crack producing crack tip fields. In each case, material was initially in the as-sintered (unpoled) state, and was loaded with nominal electric field strengths that were well below the coercive field. However, the birefringence imaging indicates significant remanent strain evolving over a time period of order 10³s. The resulting mean electric displacements are greatly enhanced relative to uniform field conditions at the same mean field strength. The measurements show only weak interaction between thin cracks and the applied electric field, suggesting that the thin cracks are effectively permeable. The results are of potential use in calibrating multi-axial and time dependent material models.

The effect of shear stress on polarization retention in ferroelectric thin films was assessed using a nonlinear finite element phase field model. It is shown that reverse switching can occur when tetragonal phase films are grown in the <111> orientation. The effect of a substrate and a top electrode are modeled by applying rigid constraints and shear loads to the finite element phase field model to predict the evolution of ferroelectric domain structures. The residual shear stress in the film is shown to increase when the film is rigidly clamped to a substrate. When shear stress is applied to the top surface of the thin film model, 90° domain walls move in the direction of shear loading. Model predictions are found to qualitatively correlate with piezoelectric response microscopy experiments given in the literature.

The paper presents continuous and discrete variational formulations for the treatment of the non-linear response of piezoceramics under electrical loading. The point of departure is a general internal variable formulation that determines the hysteretic response of the material as a generalized standard medium in terms of an energy storage and a rate-dependent dissipation function. Consistent with this type of standard dissipative continua, we develop an incremental variational formulation of the coupled electromechanical boundary value problem. We specify the variational formulation for a setting based on a smooth rate-dependent dissipation function which governs the hysteretic response. Such a formulation allows us to reproduce the dielectric and butterfly hysteresis responses characteristic of the ferroelectric materials together with their rate-dependency and to account for macroscopically non-uniform distribution of the polarization in the specimen. An important aspect is the numerical implementation of the coupled problem. The discretization of the two-field problem appears, as a consequence of the proposed incremental variational principle, in a symmetric format. The performance of the proposed methods is demonstrated by means of benchmark problems.

Some relaxor ferroelectric single crystals undergo a diffuse phase transformation while others undergo a step like transformation when driven by either stress or electric field. In this work the distributed transformation is modeled as a sequence of distributed transformations associated with compositional fluctuations typical or relaxor ferroelectrics. The distribution function is taken to be Gaussian. The results are in good agreement with observations of the response of <011> cut PMN-0.32PT single crystals.

Vehicle's components experience a variety of small to large strains during its operations. Monitoring a set of only large strains of the components is preferable for critical analysis of structural health of the vehicle. For data of all strain history including smaller amplitudes as a function of time require huge data storage system which in turn requires large power and communication equipment. Therefore, development of more energy efficient structure health monitoring (SHM) system is increasingly important for assessment of its long-term reliability. SHM has two components, one is to record history of large straining during vehicle's operation, and the other is to detect any larger damages hidden in a vehicle during or when it is at rest. This study is aimed at the first SHM issue. The concept of this new SHM is to measure the fatigue properties of piezoelectric sensors that are subjected to a set of loading. In this research, we used cantilever beam made of steel plate on which a piezoelectric thin plate is mounted. The cantilever beam is subjected to a set of known vibrations by electro-magnetic induction apparatus. Then we measured residual P-E curves of the piezo-sensor at a number of fatigue cycles. It is found that the residual polarization (Pr) of the P-E curves of fatigued piezo-sensor exhibit continuous reduction, which provides a useful data set to assess what level of loading that the cantilever beam has experienced.

Domain switching near a stationary crack tip in a single crystal of ferroelectric material is investigated. The phase-field approach applying the material polarization as the order parameter is used as the theoretical modeling framework, and the finite element method is used for the numerical solution technique. The electromechanical form of the J-integral is appropriately modified to account for the polarization gradient energy terms, and analyzed to illustrate the amount of shielding, or lack thereof, due to domain switching at the crack tip.

To improve the piezoelectricity of cellulose electro-active paper (EAPap), electrical field and magnetic field alignments were investigated. EAPap is made with cellulose by dissolving cotton pulp and regenerating cellulose with aligned cellulose fibers. EAPap made with cellulose has piezoelectric property due to its structural crystallinity. Noncentro-symmetric crystal structure of EAPap, which is mostly cellulose II, can exhibit piezoelectricity. However, EAPap has ordered crystal parts as well as disordered parts of cellulose. Thus, well alignment of cellulose chains in EAPap is important to improve its piezoelectricity. In this paper, uniaxial alignments of cellulose chains were investigated by applying electric field and magnetic field. As exposing different fields to EAPap samples, the changed characteristics were analyzed by X-Ray diffractometer (XRD) and Scanning electron microscopy (SEM). Finally, the piezoelectricity of EAPap samples was evaluated by comparing their piezoelectric charge constant [d₃₁]. As increasing applied electric field up to 40V/mm, d₃₁ value was gradually improved due to increased cellulose crystallinity as well as alignment of cellulose chains. Also the alignment of cellulose chains was improved with increasing the exposing time to magnetic field (5.3T) and well alignment was achieved by exposing EAPap sample on the magnetic field for 180min.

While the acidic polymer electrolyte membrane (PEM) Nafion has garnered considerable attention, the active response of basic PEMs offers another realm of potential applications. For instance, the basic PEM Selemion is currently being considered in the development of a CO₂ separation prototype device to be employed in coal power plant flue gas. The mechanical integrity of this material and subsequent effects in active response in this harsh environment will become important in prototype development. A multiscale modeling approach based on rotational isomeric state theory in combination with a Monte Carlo methodology may be employed to study mechanical integrity. The approach has the potential to be adapted to address property change of any PEM in the presence of foreign species (reinforcing or poisoning), as well as temperature and hydration variations. The conformational characteristics of the Selemion polymer chain and the cluster morphology in the polymer matrix are considered in the prediction of the stiffness of Selemion in specific states.

Ionomeric polymer transducers have received considerable attention in the past ten years due to their ability to generate large bending strain and moderate stress at low applied voltages. Ionic polymer transducers consist of an ionomer, usually Nafion, sandwiched between two electrically conductive electrodes. Recently, a novel fabrication technique denoted as the direct assembly process (DAP) enabled controlled electrode architecture in ionic polymer transducers. A DAP transducer usually consists of two high surface area electrodes made of uniform distributed particles sandwiching an ionomer membrane. Further enhancements to the DAP enabled sub-micron control of the electrode architecture. In this study a previously developed finite element model, capable of simulating ionic polymer transducers with high surface area electrodes is used to study the effect of electrode architecture on the actuation performance due to a unit volt step input. Four architectures are considered: Agglomerate, Gradient, Random, and Lines. The four architectures are simulated for low particle loading and high particle loading. The agglomerate presents the case of badly dispersed metal particles in the electrode. Simulation results demonstrate that particle aggregation reduces the actuation performance on an IPT. The Gradient simulates an IPT built using an Impregnation-Reduction method. The Gradient is compared to a randomly distributed electrode which represents an IPT built using the DAP method. Simulation results demonstrate that the DAP built IPT outperforms the one built using the impregnation-reduction method. Finally line architecture is simulated and results demonstrate that it outperforms random architecture especially at high particle loading.

Piezoelectricity is one of major actuating mechanisms of a cellulose-based Electro-Active paper (EAPap). Wet drawn stretching method was introduced in the fabrication process of cellulose film to increase piezoelectricity of EAPap. The characteristics of wet drawn cellulose were studied by scanning electron microscope (SEM), X-ray diffractogram (XRD) and pull test. The performance of EAPap was evaluated by measuring bending displacement and piezoelectric charge constant. The performance of EAPap was sensitive to the fabrication process and material orientation of cellulose film. Aligning cellulose fibers in the fabrication process was a critical parameter to improve mechanical and electromechanical properties of EAPap. The experimental results provided that wet drawn stretching is an effective fabrication method to improve mechanical stiffness and piezoelectricity of EAPap.

Recent results have demonstrated that lipid bilayers have the ability to "self-heal" after mechanical failure. In a previous study the maximum pressure that could be withstood by a bilayer lipid membrane (BLM) formed over a porous substrate was measured and reported. This paper expands on this subject by exploring the ability of a BLM to spontaneously "self-heal" or reform after having been pressurized until failure. A 1-Stearoyl-2-Oleoyl-sn-Glycero-3-Phosphocholine (SOPC) BLM is reconstituted over a silicon substrate that contains a single square aperture (25 x 25 μm) and is pressurized until failure. It is found that the BLM spontaneously reforms multiple times over the aperture after the initial failure. For each experiment the BLM is subjected to several pressurization cycles with a 70 mV potential applied across the BLM. The current is measured using an impedance analyzer and indicates the presence of a BLM formed over the aperture. It is found that electrical current conducted across the BLM increases from approximately 100 pA to 650 nA during each BLM failure and returns to 100 pA after BLM reformation. These results demonstrate that the bilayer is reforming because the electrical resistance across the aperture is increasing by several orders of magnitude.

In this paper we investigate the mechanical response from repeated high compressive strains on freestanding, long, vertically aligned multiwalled carbon nanotube membranes and show that the arrays of nanotubes under compression behave very similar to soft tissue and exhibit viscoelastic behavior. Under compressive cyclic loading, the mechanical response of nanotube blocks shows initial preconditioning and hysteresis characteristic of viscoeleastic materials. Furthermore, no fatigue failure is observed even at high strain amplitudes up to half million cycles. The outstanding fatigue life and extraordinary soft tissue-like mechanical behavior suggest that properly engineered carbon nanotube structures could mimic artificial muscles.

For Unmanned aerial vehicles, a morphing wing is desired to improve the maneuverability and reduce the total weight of structures. Our research group has developed a foldable composite structure for a morphing wing skin plate by using Carbon Fiber Reinforced Plastics (CFRP). The material system is called Partially Flexible Composites (PFC). In the present paper, PFC is introduced and a self-sensing system of the PFC is investigated. Since carbon fibers have electrical conductivity, damages of the PFC can be detected by monitoring electrical resistance changes of the PFC. This method is called Electrical Resistance Changes Method. An electrical resistance model of the PFC is built and a relationship of ratio of fiber fractures and electrical resistance changes is obtained. Then, to investigate the performance of the PFC, cyclic-bending tests are conducted. Damages of the PFC caused by cyclic-bending are detected by using ERCM. As a result, the PFC with more than 10mm-long flexible part has almost no damage; the stiffness of the structure remains unchanged. After that, a McKibben pneumatic artificial muscles actuator is made and it is founded that this can be applied to the PFC as an actuator. This actuator consists of a silicon rubber and a carbon fiber that are the same as the material of flexible part of the PFC. This enables us to make actuator-integrated composite structures. In the present study, the applicability of the McKibben pneumatic artificial muscles actuator is investigated.

We summarize the methodology that we have used to address integrating sensing network into composite materials for structural self diagnosis. First, we have examined the effect of stress concentration that arises due to the embedment of sensors and external devices on the strength and endurance of laminated glass fiber composites. To analyze the mechanical response of the composite material under study subjected to in-plane or impact loads, we have fabricated a series of samples, with and without embedded (dummy) sensors/micro-processors, using S2 glass fiber/epoxy, and have characterized their response by acoustic emission. Guided by the corresponding results, we can select sensors and other necessary components in such way as to minimize the impact of the embedded electronics on the material integrity and, at the same time, to implement acoustic sensing monitoring functionalities within the material. A 4-tree hierarchical network of PVDF sensors capable of acquiring signals typically related to resin micro cracking phenomena has been developed and partially integrated into a cross ply laminate. The achieved results and ongoing research will be discussed.

The acoustic performance of multifunctional aluminium foam panels has been studied in this paper. The closed cell foam structure was perforated using hole drilling techniques to produce an open cell structure for use in acoustic applications. Opening the cell structure allows permeability through the foam, thus improving acoustic absorption. The study analyzes the effects of hole size, perforation ratio and foam surface skin on acoustic performance. Mechanical testing was conducted to quantify the reduction in stiffness and strength of the perforated aluminum foam structure. Finally, optimal parameters for use in stiff automotive panel applications are identified.

Reconfigurable and morphing structures may provide significant improvement in overall platform performance through optimization over broad operating conditions. The realization of this concept requires structures, which can accommodate the large deformations necessary with modest weight and strength penalties. Other studies suggest morphing structures need new materials to realize the benefits that morphing may provide. To help meet this need, we have developed novel composite materials based on specially designed segmented reinforcement and shape memory polymer matrices that provide unique combinations of deformation and stiffness properties. To tailor and optimize the design and fabrication of these materials for particular structural applications, one must understand the envelope of morphing material properties as a function of microstructural architecture and constituent properties. Here we extend our previous simulations of these materials by using 3D models to predict stiffness and deformation properties in variable stiffness segmented composite materials. To understand the effect of various geometry tradeoffs and constituent properties on the elastic stiffness in both the high and low stiffness states, we have performed a trade study using a commercial FEA analysis package. The modulus tensor is constructed and deformation properties are computed from representative volume elements (RVE) in which all (6) basic loading conditions are applied. Our test matrix consisted of four composite RVE geometries modeled using combinations of 5 SMP and 3 reinforcement elastic moduli. Effective composite stiffness and deformation results confirm earlier evidence of the essential performance tradeoffs of reduced stiffness for increasing reversible strain accommodation with especially heavy dependencies on matrix modulus and microstructural architecture. Furthermore, our results show these laminar materials are generally orthotropic and indicate that previous calculations of matrix gap and interlaminar strains based on kinematic approximations are accurate to within 10-20% for many material systems. We compare these models with experimental results for a narrow geometry and material set to show the accuracy of the models as compared to physical materials. Our simulations indicate that improved shape memory polymer materials could enable a composite material that can accommodate ~30% strain with a cold state stiffness of ~30GPa. This would improve the current state of the art 5-10x and significantly reduce the weight and stiffness costs of using a morphing component.

In this paper we report a modeling technique and analysis of wave dispersion in a cellular composite laminate with spatially modulated microstructure, which can be modeled by parameterization and homogenization in an appropriate length scale. Higher order beam theory is applied and the system of wave equations are derived. Homogenization of these equations are carried out in the scale of wavelength and frequency of the individual wave modes. Smaller scale scattering below the order of cell size are filtered out in the present approach. The longitudinal dispersion relations for different values of a modulation parameter are analyzed which indicates the existence of stop and pass band patterns. Dispersion relations for flexural-shear case are also analyzed which indicates a tendency toward forming the stop and pass bands for increasing values of a shear stiffness modulation parameter. The effect the phase angle (θ) of the incident wave indicates the existence more number of alternative stop bands and pass bands for θ = 45°.

Over the past few decades, binary NiTi shape memory alloys have received attention due to their unique mechanical characteristics, leading to their potential use in low-temperature, solid-state actuator applications. However, prior to using these materials for such applications, the physical response of these systems to mechanical and thermal stimuli must be thoroughly understood and modeled to aid designers in developing SMA-enabled systems. Even though shape memory alloys have been around for almost five decades, very little effort has been made to standardize testing procedures. Although some standards for measuring the transformation temperatures of SMA's are available, no real standards exist for determining the various mechanical and thermomechanical properties that govern the usefulness of these unique materials. Consequently, this study involved testing a 55NiTi alloy using a variety of different test methodologies. All samples tested were taken from the same heat and batch to remove the influence of sample pedigree on the observed results. When the material was tested under constant-stress, thermal-cycle conditions, variations in the characteristic material responses were observed, depending on test methodology. The transformation strain and irreversible strain were impacted more than the transformation temperatures, which only showed an affect with regard to applied external stress. In some cases, test methodology altered the transformation strain by 0.005-0.01mm/mm, which translates into a difference in work output capability of approximately 2 J/cm³ (290 in•lbf/in³). These results indicate the need for the development of testing standards so that meaningful data can be generated and successfully incorporated into viable models and hardware. The use of consistent testing procedures is also important when comparing results from one research organization to another. To this end, differences in the observed responses will be presented, contrasted and rationalized, in hopes of eventually developing standardized testing procedures for shape memory alloys.

As the use of active structures continues to become more commercially viable, the need for accurate numerical modeling has gained importance. A current example of such a smart structure includes the variable geometry chevron. Future applications are also being designed, including a variable area jet engine nozzles and a torque tube actuators for rotor blades. This work concentrates on the FEA modeling of the Ni60Ti40 (wt %) SMA used in these applications and subsequent experimental validation. The constitutive model employed for the SMA material accounts for the full thermomechanical response and also accounts for such aspects as variable maximum transformation strain and smooth material hardening during transition. Model calibration is performed via uniaxial material testing. An overview of the model and material properties is presented followed by a discussion of the analysis results for the complex aerospace actuation applications. Comparisons to experimental validation of the overall system response are made.

Cyclic and time dependence of superelastic properties such as critical stress for stress-induced martensite (σ_SIM), irrecoverable strain levels, and stress hysteresis are crucial parameters to ensure stable operation in applications of shape memory alloys. In our studies on titanium-niobium shape memory alloys that have undergone various thermo-mechanical processes, declines in both σ_SIM and stress hysteresis with increasing cycle number were observed. More surprisingly, aging treatment at room temperature following cycling produced stress-strain behavior very similar to behavior of samples prior to cyclic deformation, and irrecoverable strain levels did not increase monotonically with increasing cycle number. Lowest irrecoverable strain levels and smallest evolution in the superelastic behavior were found in precipitated cold rolled or processed specimen by equal channel angular extrusion.

Within the last decade, the development of compact SMA actuators has led to the design of smart structures such as the Variable Geometry Chevron (VGC), designed by Boeing engineers. The chevrons are aerodynamic devices actuated by SMA beam actuators and placed along the trailing edge of a jet engine to provide noise reduction. The SMA actuators are clamped on an elastic substrate that provides a biasing force allowing repeated one-way shape memory effect under cyclic thermal actuation. In this work, a comprehensive characterization of thermally induced fatigue behavior of nickel-rich NiTi SMA actuators subject to different constant applied stresses is presented. The influence of various parameters is studied in order to assess the fatigue behavior of nickel-rich NiTi, namely: two heat treatments, two heat treatment environments, three fatigue test specimen thicknesses and four stress levels. The purpose of this thermomechanical fatigue study is to evaluate the shape recovery stability, the influence of large applied stresses, the amount of permanent deformation and the resulting failure mechanisms. Fatigue limits of ~ 5,000 to ~ 60,000 cycles were found for applied stress levels ranging from 250 MPa to 100 MPa.

Morphing or reconfigurable structures potentially allow for previously unattainable vehicle performance by permitting several optimized structures to be achieved using a single platform. The key to enabling this technology in applications such as aircraft wings, nozzles, and control surfaces, are new engineered materials which can achieve the necessary deformations but limit losses in parasitic actuation mass and structural efficiency (stiffness/weight). These materials should exhibit precise control of deformation properties and provide high stiffness when exercised through large deformations. In this work, we build upon previous efforts in segmented reinforcement variable stiffness composites employing shape memory polymers to create prototype hybrid composite materials that combine the benefits of cellular materials with those of discontinuous reinforcement composites. These composites help overcome two key challenges for shearing wing skins: the resistance to out of plane buckling from actuation induced shear deformation, and resistance to membrane deflections resulting from distributed aerodynamic pressure loading. We designed, fabricated, and tested composite materials intended for shear deformation and address out of plane deflections in variable area wing skins. Our designs are based on the kinematic engineering of reinforcement platelets such that desired microstructural kinematics is achieved through prescribed boundary conditions. We achieve this kinematic control by etching sheets of metallic reinforcement into regular patterns of platelets and connecting ligaments. This kinematic engineering allows optimization of materials properties for a known deformation pathway. We use mechanical analysis and full field photogrammetry to relate local scale kinematics and strains to global deformations for both axial tension loading and shear loading with a pinned-diamond type fixture. The Poisson ratio of the kinematically engineered composite is ~3x higher than prototypical orthotropic variable stiffness composites. This design allows us to create composite materials that have high stiffness in the cold state below SMP T_g (4-14GPa) and yet achieve large composite shear strains (5-20%) in the hot state (above SMP T_g).

This study presents the testing techniques used to thermomechanically characterize the material behavior of a shape memory polymer as well as the resulting data. An innovative visual-photographic apparatus, known as a Vision Image Correlation system was used to measure the strain. A series of tensile tests were performed on specimens in which strain levels of 10%, 25%, 50%, and 100% were applied to the material while above its glass transition temperature. After deforming the material to a specified applied strain, the material was constrained and cooled to below the glass transition temperature. Finally, the specimen was heated again to above the transition temperature, and the resulting shape recovery profile was measured. The dependence of the recoverable strain on the heating and cooling rate was investigated in this work. Results showed that strain recovery occurred in a nonlinear fashion with respect to temperature. Results also indicated that the ratio of recoverable strain to the applied strain was a constant value, and was independent of the level of applied strain.

Researchers at NASA Glenn Research Center have been investigating high temperature shape memory alloys as potential damping materials for turbomachinery rotor blades. Analysis shows that a thin layer of SMA with a loss factor of 0.04 or more would be effective at reducing the resonant response of a titanium alloy beam. Two NiTiHf shape memory alloy compositions were tested to determine their loss factors at frequencies from 0.1 to 100 Hz, at temperatures from room temperature to 300°C, and at alternating strain levels of 34-35x10^-6. Elevated damping was demonstrated between the M_s and M_f phase transformation temperatures and between the A_s and A_f temperatures. The highest damping occurred at the lowest frequencies, with a loss factor of 0.2-0.26 at 0.1 Hz. However, the peak damping decreased with increasing frequency, and showed significant temperature hysteresis in heating and cooling.

This paper discusses the 3-D numerical modeling of irrecoverable inelastic strain generation in shape memory alloys (SMAs), which is becoming increasingly important as more complicated engineering applications of SMAs are designed. Such behavior, although often rate-independent, can be rate-dependent at high temperatures. This work primarily addresses the modeling of rate-independent inelasticity in SMAs. A material behavior of particular interest occurs when plastic slip and martensitic transformation are occurring simultaneously and the influence of irrecoverable inelastic strain formation on phase transformation is considered. Motivated by experimental results obtained both from the laboratory and the literature, an SMA model which additionally captures the formation and evolution of plastic strains is proposed. The model is implemented into a 3-D finite element method framework and analysis results for two different boundary value problems are discussed. These problems include pre-working of an SMA beam actuator and micro-indentation of SMA thin films.

A series of experiments is presented examining the thermo-electro-mechanical response of commercially-available, conditioned, shape memory alloy (SMA) wires (Flexinol, from Dynalloy, Corp.) during cyclic thermomechanical loading. A specialized experimental setup enables temperature control via a thermoelectric/heatsink in thermal contact with the wire specimen during various modes of testing. It allows simultaneous measurement of elongation, load, strain and resistivity in a selected gage length. It also allows full-field optical and infrared imaging to be performed during testing. A moderately high transition temperature NiTi-based shape memory wire (90C Flexinol) is characterized first by differential scanning calorimetry and a series of isothermal experiments over a range of temperatures. Subsequent experiments examine the shakedown behavior over a range of dead loading temperature cycles. Results show a significant two-way shape memory effect, suggesting that both residual stresses and locked-in oriented Martensite are considerable in this commercial alloy. Repeatable behavior (little shakedown) is confirmed at relatively low stress levels, but significant evolution in the response (shakedown behavior) exists at higher stress levels during the first several temperature cycles.

Conventional structural cables (or wire ropes) are composed of steel wires helically wound into strands, which, in turn, are wound around a core. Cables made from shape memory alloy (SMA) wires are a new structural element with promising properties for a broad range of new applications. Among the many potential advantages of this form are increased bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal lag, redundancy, and signicant design flexibility. Currently there are no known studies of SMA cables in the literature, so exploratory thermo-mechanical experiments were performed on two commercially available cable designs as part of an ongoing research program to systematically characterize their thermomechanical behavior and demonstrate their potential utility as adaptive or resilient tension elements.

The nonlinear thermomechanical relationship in shape memory polymers (SMPs) has drawn considerable attention in many fields ranging from aeronautics to medicine largely due to their ability to withstand deformations several orders of magnitude larger than in shape memory alloys (SMAs). At high temperatures, SMPs share attributes with compliant elastomers and exhibit long-range reversibility. In contrast, at low temperatures they become very rigid and are susceptible to plastic, although recoverable, deformations.

The shape-memory alloy, for example NiTi (Nickel Titanium), is used for an actuator and frame by the shape- memory effect and the super-elasticity effect. And piezoelectric material, for example PZT (Lead Zirconate Titanate), is used for an actuator and sensor by the piezoelectric effect and the pyroelectric effect. However, each of the materials has been used for the sensor and the actuator independently because the principle of the function of each of materials is different. In this research, a new multi functional device combines these four functions is developed by coating the PZT thin film of 20μm to the surface of the NiTi wire by the chemical reaction by using the hydrothermal crystallization method. We succeeded in the detection of the displacement at a super-elastic deformation of this multi functional device from the piezoelectric effect of the PZT film on the surface. By combining each effects, the multifunctional device can be used for a self-sensing actuator, wherein the transformation by the shape-memory effect is detected by the piezoelectric effect or the pyroelectric effect, and the rough and precision actuator performs the rough movement by the shape memory effect and performs the precise movement by the piezoelectric effect.

Cellular materials are known to be useful in the application of designing light but stiff structures. This applies to various components used in various industries such as rotorcraft blades, car bodies or portable electronic devices. Structural application of the metal foam is typically confined to light weight sandwich panels, made up of thin solid face sheets and a metallic foam core. The resulting high-stiffness structure is lighter than that constructed only out of the solid metal material. The face sheets carry the applied in-plane and bending loads and the role of the foam core is separate the face sheets to carry some of the shear stresses, while remaining integral with the face sheet. Many challenges relating to the fabrication and testing of these metal foam panels continue to exist due to some mechanical properties falling short of their theoretical potential. Hence in this study, a detailed three dimensional foam structure is generated using series of 2D Computer Tomography (CT) scans, on Haynes 25 metal foam. Series of the 2D images are utilized to construct a high precision solid model including all the fine details within the metal foam as detected by the CT scanning technique. Subsequently, a finite element analysis is then performed on an as fabricated metal foam microstructures to evaluate the foam structural durability and behavior under tensile and compressive loading conditions. The analysis includes a progressive failure analysis (PFA) using GENOA code to further assess the damage initiation, propagation, and failure. The open cell metal foam material is a cobalt-nickel-chromium-tungsten alloy that combines excellent high-temperature strength with good resistance to oxidizing environments up to 1800 °F (980 °C) for prolonged exposures. The foam is formed by a powder metallurgy process with an approximate 100 pores per inch (PPI).

A commonly noted disadvantage of shape memory alloys is their frequency response which is limited by how fast the material can be cooled. This paper presents a feasibility study of using vertically aligned carbon nanotubes (CNT) as microscopic cooling fins to improve convective heat transfer. Using DC plasma enhanced chemical vapor deposition (PECVD), aligned CNT's were successfully grown directly on ½ of the surface of a 0.38 mm diameter SMA wire, achieving desirable thermal contact. Cooling speeds were measured with a thermal imaging camera, and the effective convective coefficient was extracted from the temperature profiles using a basic cooling model of the wire. From this model, the effective convective coefficient was estimated to have increased by 24% (from 50 W/m²K for untreated SMA wire to 62 W/m²K for the nanotube treated wire), indicating that the deposition of CNT's indeed increased performance. By extrapolating these results to full wire coverage, up to a 46% improvement in frequency response with zero weight or volumetric penalties is predicted. Further improvements in cooling performance are likely to occur with higher CNT densities and longer nanotube lengths, allowing further developments of this technology to benefit many future applications utilizing high-speed miniature/micro-scale SMA actuators.

This present paper is focused on the effect of conductive particulate and fibrous fillers on the characterized property of styrene-based shape memory polymer incorporating carbon black (CB) and short carbon fiber (SCF). It was shown that the particulate additives are dispersed homogeneously within matrix and served as interconnections between the fibers, while the fibrous additives may be considered as a rigid long aggregate of carbon, leading to easy formation of continuous conductive networks. The glass transition temperature of nanocomposites drops sharply as compared with that of pure SMP from the differential scanning calorimetry (DSC). For the composite containing 5 wt% CB and 2 wt% SCF, the storage modulus increases by 16.2% compared to that of the composite containing 5 wt% CB and 1 wt% SCF; the peak of tangent delta curve is an alternative definition of T_g, thus T_g defined in such a way is determined as 69.44°C from Dynamic Mechanical Analyzer (DMA) test which is higher than 25.78°C obtained from DSC test. The electrical conductivity of the composite achieves 3 S/cm by four-point Van De Pauw method, and the shape recovery can be activated with a constant voltage of 25 V through them.

A continuum thermodynamics formulation for micromagnetics coupled with mechanics is devised to model the evolution of magnetic domain and martensite twin structures in ferromagnetic shape memory alloys. The theory falls into the class of phase-field or diffuse-interface modeling approaches. In addition to the standard mechanical and magnetic balance laws, a two sets of micro-forces their associated balance laws are postulated, one set for the magnetization order parameter and one set for the martensite order parameter. The second law of thermodynamics is analyzed to identify the appropriate material constitutive relationships. The general formulation does not constrain the magnitude of the magnetization to be constant, allowing for the possibilities of spontaneous magnetization changes associated with strain and temperature. The equations governing the evolution of the magnetization are shown to reduce to the commonly accepted Landau-Lifshitz-Gilbert equations when the magnetization magnitude is constant. Numerical solutions to the governing equations are presented to investigate the fundamental interactions between the magnetic domain wall and the martensite twin boundary in ferromagnetic shape memory alloys. Calculations are performed to determine under what conditions the magnetic domain wall and the martensite twin boundary can be dissociated, resulting in a limit to the actuating strength of the material.

Due to magnetic field diffusion and structural dynamics, the relationship between magnetic field and strain in Ni-Mn-Ga changes significantly as the frequency of applied field is increased. In order to describe this behavior, which is critical for actuator applications, we present a strain model for Ni-Mn-Ga driven with dynamic magnetic fields. The magnitude and phase of the magnetic field inside the sample are modeled as a 1-D magnetic diffusion problem, from where an averaged or effective field is calculated. A continuum thermodynamics constitutive model is used to quantify the hysteretic response of the martensite volume fraction due to this effective magnetic field. The evolution of volume fractions with effective field is proposed to behave as a zero order system. To quantify the dynamic strain output, the actuator is represented as a lumped-parameter 1-DOF resonator with force input dictated by the twin-variant volume fraction. This results in a second order, linear ODE whose periodic force input is expressed as a summation of Fourier series terms. The total dynamic strain output is obtained by superposition of strain solutions due to each harmonic force input. The model accurately describes experimental measurements at frequencies of up to 250 Hz.

Galfenol alloys show promise as a new magnetically activated smart material based on their unique combination of relatively high magnetostrictive performance and good mechanical robustness. Investigations of aluminum additions to single crystal iron-gallium alloys have been done previously, and the magnetostrictive response seems to follow the rule of mixtures with decreasing saturation magnetostriction with increasing aluminum content. Aluminum is assumed to substitute for Ga directly in the alloy. Directionally solidified polycrystalline Galfenol alloys with aluminum additions were produced to determine the effects on the magnetic properties. Iron-gallium-aluminum alloys were investigated for two primary reasons: (1) Fe-Al alloys are well established and are typically manufactured using conventional thermo-mechanical processing techniques such as rolling; it is anticipated that aluminum additions will aid in the development of Galfenol alloy rolled sheets (2) Gallium prices continue to rise and a cost effective alternative needs to be investigated. Several Fe-Ga-Al alloy compositions were prepared using the Free Stand Zone Melting (FSZM) directional solidification technique. Alloy composition ranges investigated include: Fe_80.5Ga_xAl_19.5-x (4.9≤x≤13), Fe_81.6Ga_yAl_18.4-y (4.6≤y≤13.8), and Fe₈₅Ga_zAl_15-z (3.75≤z≤11.25). Alloys were studied using EDS (chemistry verification), EBSD (crystallite orientation), and magnetic characterization techniques to determine the effect of aluminum addition on the polycrystalline binary Fe-Ga system. Magnetic properties such as saturation magnetostriction (λ_sat), piezomagnetic constant (d₃₃), and relative magnetic permeability (μ_r) of directionally solidified Fe-Ga-Al polycrystalline alloys will be compared to binary Fe-Ga alloys including investigations into the crystal orientation effects on these properties. Results suggest that up to 50% aluminum can be substituted in the alloy while maintaining considerable saturation magnetostriction, > 200 ppm.

Iron-gallium alloys (known as Galfenol), are one of only a few metal alloys known to exhibit large auxetic or negative Poisson's ratio behavior. The mechanical properties, including the auxeticity, of Galfenol are strongly dependent on the composition. This research seeks to measure the elastic properties of Galfenol through a range of practical compositions in order to create a thorough database as well as present trends in the elastic properties. This is achieved through tensile testing of single-crystal Galfenol dog-bone-shaped specimens of varying compositions. For each composition, there is one specimen aligned along the [100] crystallographic axis and one aligned along the [110] axis. This project will enable future researchers to confidently know the elastic properties of the alloy, as well as enable them to select the alloy with optimum elastic properties for their applications.

We propose a miniature spherical motor using iron-gallium alloy (Galfenol). This motor consists of four rods of Galfenol with square cross-section, a wound coil, a permanent magnet, an iron yoke and a spherical rotor placed on the edge of the rods. The magnetomotive force of the magnet provides bias magnetostriction for the rods and an attractive force that maintains the rotor on the rods. When currents of 180 deg phase difference flow in pairs of opposing coils, a torque is exerted on the rotor is by pushing (expansion) and pulling (contraction) of the rods. Rotation about a single axis is realized by a sawtooth current, such that the rotor rotates with slow expansion and slips at the rapid contraction. The motor can be fabricated at small sizes and driven with a low voltage, suitable for application as a microactuator for rotating the camera and mirror in endoscopes.

A general framework is developed to model the nonlinear magnetization and strain response of cubic magnetostrictive materials to 3-D dynamic magnetic fields and 3-D stresses. Dynamic eddy current losses and inertial stresses are modeled by coupling Maxwell's equations to Newton's second law through a nonlinear constitutive model. The constitutive model is derived from continuum thermodynamics and incorporates rate-dependent thermal effects. The framework is implemented in 1-D to describe a Tonpilz transducer in both dynamic actuation and sensing modes. The model is shown to qualitatively describe the effect of increase in magnetic hysteresis with increasing frequency, the shearing of the magnetization loops with increasing stress, and the decrease in the magnetostriction with increasing load stiffness.

This work investigates the equivalence of thermodynamic potentials utilizing stress-induced anisotropy energy and potentials using elastic, magnetoelastic, and mechanical work energies. The former is often used to model changes in magnetization and strain due to magnetic field and stress in magnetostrictive materials. The enthalpy of a ferromagnetic body with cubic symmetry is written with magnetization and strain as the internal states and the equilibrium strains are calculated by minimizing the enthalpy. Evaluating the enthalpy using the equilibrium strains, functions of the magnetization orientation, results in an enthalpy expression devoid of strain. By inspecting this expression, the magnetoelastic, elastic, and mechanical work energies are identified to be equivalent to the stress-induced anisotropy plus magnetostriction-induced fourth order anisotropy. It is shown that as long as the value of fourth order crystalline anisotropy constant K₁ includes the value of magnetostriction-induced fourth order anisotropy constant ΔK₁, energy formulations involving magnetoelastic, elastic, and mechanical work energies are equivalent to those involving stress-induced anisotropy energy. Further, since the stress-induced anisotropy is only given for a uniaxial applied stress, an expression is developed for a general 3D stress.

The recent discovery of Galfenol as a "large" magnetostrictive material (as high as 400με) offers a particularly promising transducer material that combines largely desirable mechanical attributes with superior magnetic properties. The high permeability of this material makes it easy to magnetize, however it also causes a relatively low cutoff frequency in dynamic applications, above which eddy currents form and introduce significant power losses. To reduce the eddy current losses, magnetostrictive drivers used in dynamic applications are commonly laminated. A second transducer design consideration is that in materials which exhibit positive magnetostriction, it is common to impose an initial compressive "prestress" to the material that is sufficient to align the orientation of the magnetic moments within the material to a direction perpendicular to the stress axis. This is done to maximize the magnetostriction realized when a magnetic field applied along the stress axis rotates the moments parallel to the stress axis. An alternative to the application of a compressive prestress is to build-in a uniaxial magnetic anisotropy through stress annealing. Stress annealing is a high temperature process with simultaneous application of a compressive load and subsequent cooling under load in which the magnetic moment alignment developed at temperature is retained upon removal from the stress anneal fixture. The compressive load needed to build in a useful uniaxial magnetic anisotropy in Galfenol is greater than the buckling load for Galfenol laminae sized for use in high frequency dynamic applications. In this study, prior work on stress annealing of solid rods of single and polycrystalline samples of Galfenol is successfully extended to thin laminae of Galfenol by introducing fixtures needed to avoid buckling. The standard stress annealing device uses a hydraulic actuator to apply compressive stress to the sample. Two linear guides have been added to ensure a normal compression load path to reduce the potential for buckling of thin laminations. In addition, a mechanical holding fixture was used to maintain proper alignment of the thin laminations during stress annealing. Data are presented that demonstrate the magnetic uniaxial anisotropy developed by stress annealing of laminated Galfenol rods.

Energy density and coupling factor are widely used as figures of merit for comparing different active materials. These parameters are usually evaluated as material constants assuming a linear behavior of the material over all operating ranges. In this work it is shown that the operating conditions have an effect on the energy density and coupling factor which cannot be ignored. A single crystal rod of Fe₈₄Ga₁₆ was characterized as a magnetostrictive actuator and sensor under different quasi-static stress and magnetic field conditions. The material showed a saturation magnetostriction of 247 με and a maximum stress sensitivity of 45 T/GPa. A maximum energy density of 2.38 kJ/m³ and coupling factor higher than 0.6 were calculated from experimental results. The experimental behavior was modeled using an energy based non-linear approach which was further used to calculate the coupling factor and energy density as continuous functions of stress and magnetic field in the material. Guidelines on optimal operating conditions for magnetostrictive actuators and sensors using FeGa alloys have been suggested.

This paper is concerned with the development of a finite element model for fully-coupled magnetomechanical boundary value problems, which allows the implementation of nonlinear, anisotropic and hysteretic material models. The formulation is based on a vector-valued magnetic potential and a small strain setting. The derivation of the numerical algorithm is considered in detail. In order to test its implementation the case of piezomagnetism is considered, for which a thermodynamically-consistent formulation is presented. The results of the finite element analysis that have been obtained for two different boundary value problems, which involve the mechanical and magnetic loading of an elastic matrix material with a transversely isotropic piezomagnetic inclusion, are discussed.

We propose a three-DOF magnetostrictive micro actuator using Iron-Gallium alloy (Galfenol). The actuator consists of two parallel beam structure having a Galfenol core, located at either end of a Galfenol rod of 1 mm square cross-section and length 11 mm, with two orthogonal ditches cut down it of width 0.3 mm. Around the resulting prongs are wound, and the prongs are bonded to an iron end cap to close the magnetic circuit. When current is passed through a coil wound round one of the orthogonal parallel beams, the resulting magnetostriction enables the actuator to bend in two directions. In addition, longitudinal displacement with high frequency bandwidth can be generated by excitation of two or of all four coils. Maximum displacements were observed of 8 to 10 μm in bending and 2.2 μm in the longitudinal direction. This actuator is potentially applicable in machining (drilling), positioning, and in a micro-motor using wobbling or translational motion when powered by a small power supply.

A fully-coupled, nonlinear model is presented that characterizes the 3-D strain and magnetization response of magnetostrictive materials to magnetic fields and mechanical stresses. The model provides an efficient framework for characterization, design, and control of Galfenol (Fe_1-xGa_x) devices with 3-D functionality subjected to combined magnetic field and stress loading. A thermodynamic approach is taken to determine possible domain orientations considering the magnetocrystalline anisotropy, magnetomechanical, and Zeeman energies. The domain configuration is determined through minimization of the total Gibbs energy of a collection of domains. To incorporate material texture, the orientation of the applied field and stress with respect to the local crystal orientation is included as a statistically distributed parameter. Hysteresis due to irreversible domain wall motion is modeled by accounting for the energy loss due to domain wall pinning sites.

Consideration of demagnetization effect, the model used to calculate the magnetostriction of single particle under the applied field is firstly built up. Then treating the magnetostriction particulate as an eigenstrain, based on Eshelby equivalent inclusion and Mori-Tanaka method, the approach to calculate average magnetostriction of the composites under any applied field as well as saturation is studied. Results calculated by the approach indicate that saturation magnetostriction of magnetostrictive composites increases with increasing of particle aspect, particle volume fraction and decreasing of Young' modulus of matrix, and the influence of applied field on magnetostriction of the composites becomes more significant with larger particle volume fraction or particle aspect.

This paper motivates a one-dimensional thermo-magneto-mechanical free energy model for NiMnGa. Following a discussion of material behavior and modeling purpose, we present what might be referred to as a meso-scale model, incorporating micro-scale physics while striving for macro-scale simplicity. Development of the model begins with the construction of a free energy landscape for the material, with strain and magnetization as its order parameters. This landscape includes paraboloidal energy wells - isolated from each other by energy barriers - to represent stable states of the material. The energy well positions and barrier heights are allowed to vary as functions of stress, magnetic field, and temperature. The resulting equations are employed within the theory of thermally activated processes to find the phase-fraction evolution of a sample. Previous results demonstrating the potential of the modeling approach are included.

The electromechanical field concentrations due to surface and internal electrodes in multilayer piezoelectric film actuators are investigated through numerical and experimental characterization. A nonlinear finite element analysis is carried out to discuss the effects of electric field and poling on the displacement and internal electromechanical fields in fully and partially poled piezoelectric actuators, by introducing models for polarization switching. Displacement measurements are also performed for the actuators, and a comparison of the predictions with experimental data is conducted.

Piezoelectrics are the active material of choice in a wide range of electromechanical applications including SONAR, medical ultrasound and non-destructive evaluation. However, designers of high power piezoelectric systems have suggested that a discrepancy exists between mathematical modelling predictions and measured transducer performance. In most high power applications piezoelectric materials are operated under large compressive stresses. Manufacturers of piezoelectric materials publish a wide range of performance data, however, the majority of the data is acquired under no bias stress. In this paper, a new technique that facilitates the characterisation of piezoelectric materials over a wide range of operating stresses (0-140MPa) at their resonant frequency is described. It builds upon the IEEE techniques for piezoelectric characterisation and utilises measurement equipment found in the majority of piezoelectric development laboratories. The technique therefore offers a low cost extension to existing facilities for the accurate determination of piezoelectric properties under high stress loading. Results gained using the new technique confirm that substantial variation in electromechanical properties of piezoelectric materials occurs under stress loading. Using this derived data, a more informed evaluation of transducer materials and more accurate predictions of transducer performance can be made.

Cellulose based Electroactive paper (EAPap) has been reported as a smart material that can be used as sensors and actuator materials. It has merits in terms of lightweight, biodegradability, large displacement and low actuation voltage. Actuation principle of EAPap is a combination of piezoelectric and ion migration effect. However, the performance of actuator is sensitive to humidity levels, in other words it produces large bending displacement at high humidity levels. Thus, we made an attempt to develop an EAPap which produces large displacement at low humidity level by blending cellulose with small amount of poly (ethylene oxide)-poly (ethylene glycol) [PEO-PEG]. The interaction between cellulose and PEO-PEG is studied by means of SEM and FT-IR. The potential application of PEO-PEG/ cellulose blend film as an actuator working at low humidity level is demonstrated by testing the actuator performance in terms of bending displacement, power consumption with respect to actuation voltage, frequency and humidity level.

Ferrogels are soft polymer materials containing a filler of magnetic particles that allow the material to be activated by magnetic fields. These materials have shown capabilities for large strains, fast response, ease of synthesis and biocompatibility and have potential applications including artificial muscles, controlled drug release systems, and hyperthermia cancer treatment. In this work the actuator behavior for a selection of ferrogel compositions and synthesis methods are characterized including their free strain and loading behavior. Samples were synthesized using either chemical or physical methods for samples containing PVA of 4, 8, and 12 wt% and magnetic particles of 1, 5, and 10 wt%. This samples were then tested for free strain and strain under loads of up to 4 times their weight by exposing them of fields from between 0.2 and 0.25T. Results show that softer samples with the largest amount of iron achieve the largest strains. Thus, chemically crosslinked sample with 4 wt% PVA and 10 wt% iron achieved the largest strain of almost 40%. Soft samples however exhibit low loaded capabilities with a blocked load of 1.7g identified. The physically crosslinked samples which were stiffer achieved very good loading capabilities with only a 20% strain decrease when loaded up to 400% of their weight. This translated in to a energy density of 320 J/m³ making these materials very promising for actuator applications.

An interface electrode in a layered piezoelectric medium of finite thickness is investigated. Closed-form expressions for the electromechanical fields at the electrode tip are obtained in terms of the applied electric field intensity factor. Effect of layer thickness on the electrode tip fields is studied. The values of the electric field intensity factor are obtained exactly for infinite layer thickness and numerically for finite layer thickness. The stress, electric displacement, and electric field are graphically shown. It is found that the electromechanical intensity factors at the electrode tip can be reduced considerably by decreasing the thickness of the piezoelectric layer. From the solution of the single electrode problem, the solution for two identical and collinear interface electrodes in the piezoelectric medium are also obtained.

The ultimate aim of this study is the development of microwave absorbers containing both dielectric and magnetic lossy materials. Carbon nanofibers (CNFs) were used as dielectric lossy materials and NiFe particles were used as magnetic lossy materials. Total twelve specimens for the three types such as dielectric, magnetic and mixed radar absorbing materials (RAMs) were fabricated. Their complex permittivities and permeabilities in the range of 2~18 GHz were measured using the transmission line technique. The parametric studies in the X-band (8.2~12.4 GHz) for reflection loss characteristics of each specimen to design the single-layered RAMs were performed. The mixed RAMs generally showed the improved absorbing characteristics with thinner matching thickness. One of the mixed RAMs, S09 with the thickness of 2.00 mm had the 10 dB absorbing bandwidth of 4.0 GHz in the X-band. The experimental results for selected specimens were in very good agreements with simulation ones in terms of the overall reflection loss characteristics and 10 dB absorbing bandwidth.

As U.S. Army systems and vehicles become more dependent on electronic devices and subsystems, there is an increasing need for improving the mass- and volume-efficiency of energy storage components. The conventional approach for saving mass and volume is to increase component energy density. Alternatively, overall system weight can be reduced by replacing purely structural components, such as armor or frame members, with structures that also store energy. Specifically, we are developing capacitors that can also carry structural loads by intercalating glass fiber reinforced polymer dielectric layers with metallized polymer film electrodes. In previous work, we developed a metric, the multifunctional efficiency (MFE), for comparing various structural capacitor preparations and guiding multifunctional design. Modeling and characterization of fiber composite-based structural capacitors has shown that the MFE is sensitive to fiber shape, orientation, volume fraction, and dielectric constant. In this work, various dielectric materials are studied against this MFE metric and the effect of fiber properties and volume fraction on MFE is explored experimentally.

In aerospace structures, the increase of mechanical performance of materials such as Carbon Fiber Reinforced Polymers (CFRPs) is always a key goal. In parallel, there is a constant demand for multi-functional solutions that provide continuous, integrated damage monitoring in an efficient and cost affordable way. Structural Health Monitoring systems are crucial for a variety of aerospace applications where safety, operational cost and the maintenance have increased significantly. The Electrical Resistance Technique (ERT) as a promising damage monitoring technique uses the CFRP materials themselves as inherent damage sensors. Currently method's medium sensitivity does not allow the identification of early damage stages requested for a potential application. By using highly conductive carbon-nanotubes as filler material into the epoxy matrix of CFRP is expected to increase the sensitivity of the method, allowing for wider field of applications. In addition, it is expected that the introduction of CNTs into the polymer matrix of CFRP laminates will increase the overall mechanical and electrical performance of the composite. This double role of the CNTs is investigated in the present study. Quasi-static tensile, cyclic loading-unloading-reloading with increase load level at each loading cycle and tension-tension fatigue tests with parallel monitoring of the longitudinal resistance performed on CFRP laminates with various contents of CNTs in the epoxy matrix showed that matrix cracking and fiber breakage caused resistance to increase irreversibly. Although the individual damage mechanisms could not be easily distinguished the overall damage state can be reliably characterized. Moreover significant increase in the fracture resistance was shown, for both Mode I and Mode II tests in the case of CNT doped laminates, compared against the reference laminate where neat epoxy matrix was used. Finally, low velocity impact tests showed that the CNT doped laminates appear to have reduced damage area based on C-Scan evidences.

The focus of this research is to electrospin continuous yarns of piezoelectric nanofibers. Incorporating piezoelectric polymer fibers in traditional composites can add sensing and actuation capabilities, which creates a wide array of potential applications. To process nanofibers with piezoelectric properties, we are pursuing the electrospinning of poly (vinylidene fluoride) (PVDF) in DMAc. A method of electrospinning on water is used to form the continuous fibers, which are then tested using DSC, XRD, and microscopy. Through this technique, we see evidence that the non-polar α-phase of PVDF is converted to the polar β-phase, which is responsible for its piezoelectric behavior.