The use of piezoelectric ceramic materials for structural actuation is a fairly well developed practice that has found use in a wide variety of applications. However, actuators with piezoceramic fibers and interdigitated electrodes have risen to the forefront of the intelligent structures community due to their increased actuation capability. However, their fiber-reinforced construction causes them to exhibit anisotropic piezomechanical properties, and the required larger driving voltages make the inherent piezoelectric nonlinearities more prevalent. In order to effectively utilize their increased performance, the more complicated behavior of these actuators must be sufficiently characterized. The current work is intended to provide a detailed nonlinear characterization of the mechanical and piezoelectric behavior of the Macro Fiber Composite actuator, which was developed at the NASA Langley Research Center. The mechanical behavior of this planar actuation device, which is both flexible and robust, is investigated by first developing a classical lamination model to predict its short-circuit linear-elastic properties, which are then verified experimentally. The sensitivity of this model to variations in constituent material properties is also studied. Phenomenological models are then used to represent the measured nonlinear short-circuit stress-strain response to various in-plane mechanical loads. Piezoelectric characterization begins with a nonlinear actuation model whose material parameters are determined experimentally for monotonically increasing electric fields. Next, the response of the actuator to a sinusoidal electric field input is measured under various constant mechanical loads and field amplitudes. From this procedure, the common linear piezoelectric strain coefficients are presented as a function of electric field amplitude and applied stress. In addition, a Preisach model is developed that uses the collected data sets to predict the hysteretic piezoelectric behavior of the MFC. Lastly, other related topics, such as manufacturing, cure kinetics modeling and linear thermoelasticity of the Macro Fiber Composite, are covered in the appendices. / Ph. D.

MEMS devices are finding application in diverse fields that include energy harvesting, microelectronics and sensors. In energy harvesting, MEMS scale devices are employed due to its efficiencies of scale. The miniaturization of energy harvesters permit them to be integrated as the power supply for sensors often in the same package and also extends their use to remote and extreme ambient applications. Unlike inductive harvesting, piezoelectric and magnetoelectric devices lend easily to MEMS scaling. The processing of such Piezo-MEMS devices often requires special fabrication, characterization and testing techniques. Our research work has focused on the development of the various technologies for a) the better characterization of the constituent materials that make up these devices, b) the conceptualization and structural design of unique MEMS energy harvesters and finally c) the development of the unit operations (many novel) for fabrication and the mechanical and electrical testing of these devices. In this research work, we have pioneered some new approaches to the characterization of thin films utilized in Piezo-MEMS devices: (1) Temperature-Time Transformation (TTT) diagrams are used to document texture evolution during thermal treatment of ceramics. Multinomial and multivariate regression techniques were utilized to create the predictor models for TTT data of Pb(Zr0.60Ti0.40 O3) sol-gel thin films. (2) We correlated the composition (measured using Energy Dispersive X-ray analysis (EDX) and Electron Probe Micro Analysis (EPMA)) of Pb(Zr0.52Ti0.48 O3) RF sputtered thin films to its optical dispersion properties measured using Variable Angle Spectroscopic Ellipsometry (VASE). Wemple-DiDomenico, Jackson-Amer, Tauc and Urbach optical dispersion factors and Lorentz Lorenz polarizability relationships were combined to realize a model for predicting the elemental content of any thin film system. (3) We developed in house capability for strain analysis of magnetostrictive thin films using laser Doppler Vibrometry (LDV). We determined a methodology to convert the displacements measurements of AC magnetic field induced vibrations of thin film samples into magnetostriction values. (4) Finally, we report the novel use of a thermo-optic technique, Time Domain Thermoreflectance (TDTR) in the study of Pb(Zr,Ti)O3 (PZT) thin film texturing. Time Domain Thermoreflectance (TDTR) has been proved to be capable of measuring thermal properties of atomic layers and interfaces. Therefore, we utilized TDTR to analyze and model the heat transport at the nano scale and correlate with different PZT crystalline orientations. To harvest energy at the low frequency (<100Hz) of ambient vibrations, MEMS energy harvesters require special structures. Extensive research has led us to the development of Circular Zigzag structure that permits inertial mass free attainment of such low frequencies. In addition to Si micromachining, we have fabricated such structures using a new Micro water jet micromachining of thin piezo sheets, unimorphs and bimorphs. For low frequency magnetic energy harvesting, we also fabricated the first magnetoelectric macro fiber composite. This device also employs a novel low temperature metallic bonding technique to fuse the magnetostrictive layer to the piezoelectric layers. A special low viscosity epoxy enabled the joining of the flexible circuit to the magnetoelectric fibers. Lastly, we developed a nondimensional tunable Piezo harvester, called PiezoCap, which decouples the energy harvesting component of the device from the resonant vibration component. We do so by using magnets loaded on piezo harvester strips, thereby making them piezomagnetoelastic and vary the spacing between 2 magnet+piezoelectric pairs to eliminate dimensionality and permit active tunability of the harvester's resonant frequency. / Ph. D.

Piezoelectrics offer high actuation authority and sensing over a wide range of frequencies. A Macro-Fiber Composite is a type of piezoelectric device that offers structural flexibility and high actuation authority. A challenge with piezoelectric actuators is that they require high voltage input; however the low power consumption allows for relatively lightweight electronic components. Another challenge, for piezoelectric actuated aerodynamic surfaces, is found in operating a relatively compliant, thin structure (desirable for piezoceramic actuators) in situations where there are relatively high external (aerodynamic) forces. Establishing an aeroelastic configuration that is stiff enough to prevent flutter and divergence, but compliant enough to allow the range of available motion is the central challenge in developing a piezocomposite airfoil. The research proposed here is to analyze and implement novel electronic circuits and structural concepts that address these two challenges. Here, a detailed theoretical and experimental analysis of the aerodynamic and electromechanical systems that are necessary for a practical implementation of a piezocomposite airfoil is presented. First, the electromechanical response of Macro-Fiber Composite based unimorph and bimorph structures is analyzed. A distributed parameter electromechanical model is presented for interdigitated piezocomposite unimorph actuators. Necessary structural features that result in large electrically induced deformations are identified theoretically and verified experimentally. A novel, lightweight electrical circuitry is proposed and implemented to enable the peak-to-peak actuation of Macro-Fiber Composite bimorph devices with asymmetric voltage range. Next, two novel concepts of supporting the piezoelectric material are proposed to form two types of variable-camber aerodynamic surfaces. The first concept, a simply-supported thin bimorph airfoil, can take advantage of aerodynamic loads to reduce control input moments and increase control effectiveness. The structural boundary conditions of the design are optimized by solving a coupled fluid-structure interaction problem by using a structural finite element method and a panel method based on the potential flow theory for fluids. The second concept is a variable-camber thick airfoil with two cascading bimorphs and a compliant box mechanism. Using the structural and aerodynamic theoretical analysis, both variable-camber airfoil concepts are fabricated and successfully implemented on an experimental ducted-fan vehicle. A custom, fully automated low-speed wind tunnel and a load balance is designed and fabricated for experimental validation. The airfoils are evaluated in the wind tunnel for their two-dimensional lift and drag coefficients at low Reynolds number flow. The effects of piezoelectric hysteresis are identified. In addition to the shape control application, low Reynolds number flow control is examined using the cascading bimorph variable-camber airfoil. Unimorph type actuators are proposed for flow control in two unique concepts. Several electromechanical excitation modes are identified that result in the delay of laminar separation bubble and improvement of lift. Periodic excitation to the flow near the leading edge of the airfoil is used as the flow control method. The effects of amplitude, frequency and spanwise distribution of excitation are determined experimentally using the wind tunnel setup. Finally, the effects of piezoelectric hysteresis nonlinearity are identified for Macro-Fiber Composite bimorphs. The hysteresis is modeled for open-loop response using a phenomenological classical Preisach model. The classical Preisach model is capable of predicting the hysteresis observed in 1) two cantilevered bimorph beams, 2) the simply-supported thin airfoil, and 3) the cascading bimorph thick airfoil. / Ph. D.

The purpose of this research is to characterize the response of an airfoil with an oscillatory morphing, Macro-fiber composite (MFC) trailing edge. Correlation of the airfoil lift with the oscillatory input is presented. Modal analysis of the test airfoil and apparatus is used to determine the frequency response function. The effects of static MFC inputs on the FRF are presented and compared to the unactuated airfoil. The transfer function is then used to determine the lift component due to cambering and extract the inertial components from oscillating airfoil. Finally, empirical wind tunnel data is modeled and used to simulate the deflection of airfoil surfaces during dynamic testing conditions. This research serves to combine modal analysis, empirical modeling, and aerodynamic testing of MFC driven, oscillating lift to formulate a model of a dynamic, loaded morphing airfoil. / Master of Science

The research presented in this thesis uses the macro-fiber composite (MFC) actuator that was recently developed at the NASA Langley Research Center for two major themes, sensing and actuation for vibration control, and power harvesting. The MFC is constructed using piezofibers embedded in an epoxy matrix and coated with Kapton skin. The construction process of the MFC affords it vast advantages over the traditionally used piezoceramic material. The MFC is extremely flexible, allowing it to be bonded to structures that have curved surface without fear of accidental breakage or additional surface treatment as is the case with monolithic piezoceramic materials. Additionally the MFC uses interdigitated electrodes that capitalize on the higher d33 piezoelectric coupling coefficient that allow it to produce higher forces and strain than typical monolithic piezoceramic materials. The research presented in this thesis investigates some potential applications for the MFC as well as topics in power harvesting. This first study performed was to determine if the MFC is capable of being used as a sensor for structural vibration. The MFC was incorporated into a self-sensing circuit and used to provide collocated control of an aluminum beam. It was found that the MFC makes a very accurate sensor and was able to provide the beam with over 80% vibration suppression at its second resonant frequency. Following this work, the MFC was used as both a sensor and actuator to apply multiple-input-multiple-output vibration control of an inflated satellite component. The control system used a positive position feedback (PPF) controller and two pairs of sensors and actuators in order to provide global vibration suppression of an inflated torus. The experiments found that the MFC and control system was very effective at attenuating the vibration of the first mode but ineffective at higher modes. It was found the positioning of the sensors and actuators on the structure contributed heavily to the controller's performance at higher modes. A discussion of the reasons for the controller's ineffectiveness is supply and a solution using self-sensing techniques for collocated vibration suppression was investigated. Subsequent to the research in vibration sensing and control, the ability to use piezoelectric materials to convert ambient vibration into usable electrical energy was tested and quantified. First, a model of a power harvesting beam is developed using variational methods and is validated on a composite structure containing four separate piezoelectric wafers. It is shown that the model can accurately predict the power generated from the vibration of a cantilever beam regardless of the load resistance or excitation frequency. The damping effects of power harvesting on a structure are also demonstrated and discussed using the model. Next, the ability of the piezoelectric material to recharge a battery and a quantification of the power generated are investigated. After determining that the rechargeable battery is compatible with the power generated through the piezoelectric effect, the MFC was compared with the traditional monolithic PZT for use as a power harvesting material. It was found that the MFC produces a very low current, making it less efficient than the PZT material and unable to charge batteries because of their need for relatively large current. Due to the MFC being incapable of charging batteries, only the PZT was used to charge batteries and the charge times for several nickel metal hydride batteries ranging from 40 to 1000mAh are supplied. / Master of Science

The application of smart materials to control the flight dynamics of a Micro Air Vehicle (MAV) has numerous benefits over traditional servomechanisms. Under study is wing morphing achieved through the use of piezoelectric Macro Fiber Composites (MFCs). These devices exhibit low power draw but excellent bandwidth characteristics. This thesis provides a background in the 2D analytical and computer modeling tools and methods needed to design and characterize an MFC-actuated airfoil. A composite airfoil is designed with embedded MFCs in a bimorph configuration. The deflection capabilities under actuation are predicted with the commercial finite element package NX Nastran. Placement of the piezoelectric actuator is studied for optimal effectiveness. A thermal analogy is used to represent piezoelectric strain. Lift and drag coefficients in low Reynolds number flow are explored with XFOIL. Predictions are made on static aeroelastic effects. The thin, cambered Generic Micro Aerial Vehicle (GenMAV) airfoil is fabricated with a bimorph actuator. Experimental data are taken with and without aerodynamic loading to validate the computer model. This is accomplished with in-house 2D wind tunnel testing. / Master of Science

O presente trabalho visa empregar uma metodologia numérica para determinar as propriedades macro mecânica de compósitos ativos (AFC - Active Fiber Composite ou MFC - Macro Fiber Composite), combinando o conceito de Volume Elementar Representativo (VER) com o Método dos Elementos Finitos (MEF). Inicialmente, apresenta-se a fundamentação teórica associada à abordagem numérica empregada. Posteriormente, os modelos numéricos desenvolvidos são aplicados na determinação dos coeficientes efetivos de materiais compósitos inteligentes transversalmente isotrópicos com fibras piezelétricas de seção com forma circular e quadrada, respectivamente. Finalmente, os resultados numéricos obtidos pela metodologia proposta são, então, comparados com resultados da literatura. Constata-se que os resultados obtidos são muito semelhantes aos resultados relatados pela literatura para arranjo quadrático e hexagonal com fibra de geometria circular, sendo que neste caso, compararam-se os resultados numéricos com analíticos obtidos através do Método de Homogeneização Assintótica. Em seguida, a metodologia é aplicada para determinação dos coeficientes efetivos para arranjo quadrático e hexagonal com fibra de geometria quadrada. Empregando diferentes frações volumétricas de fibras, os resultados via MEF foram comparados aos resultados analíticos obtidos através do Método dos Campos Uniformes (Uniform Field Method). Após a avaliação das limitações e potencialidades da metodologia, de forma direta, através de resultados analíticos, realizou-se a avaliação da mesma de forma indireta. Para tal, foram realizadas análises dinâmicas visando comparar as Funções de Resposta em Frequência (FRF) experimentais com as obtidas computacionalmente. Dessa forma, utilizou-se uma viga de alumínio estrutural engastada-livre, onde foram colados duas pastilhas piezelétricas, sendo uma para realizar a excitação da estrutura e, a outra para fazer a aquisição dos dados. Os modelos computacionais via MEF empregaram para o domínio das pastilhas, as propriedades efetivas determinadas através da metodologia desenvolvida. Os resultados obtidos demonstraram mais uma vez as potencialidades da metodologia proposta. Assim, conclui-se que a metodologia numérica não é somente uma boa alternativa para o cálculo de coeficientes efetivos de compósitos inteligentes, mas também uma ferramenta para o projeto de estruturas inteligentes monitoradas por materiais piezelétricos. / This work presents the development a numerical methodology to determine the mechanical properties of active macro composites (AFC - Active Fiber Composite, or MFC - Macro Fiber Composite), combining the concept of Representative Elementary Volume (REV) with the Finite Element Method (FEM). In the first instance, the theoretical framework associated with the numerical approach employed is presented. Later, numerical models based on unit cell are applied to predict the effective material coefficients of the transversely isotropic piezoelectric composite with circular cross section fibers. Finally, numerical results obtained by the proposed methodology are compared to other methods reported in the literature. It appears that the results are very similar to the literature results for square and hexagonal arrangement of fibers with circular geometry, in which case, it was compared numerical with analytical results calculated by Asymptotic Homogenization Method (AHM). After that, the methodology is applied to determine the effective coefficients for square and hexagonal array with square fiber geometry. Employing different fiber volume fractions, it follows that the results obtained by the proposed methodology were compared to analytical results calculated by the Uniform Field Method (UFM). After assessing the potential and limitations of the methodology, either directly, through analytical results, the evaluation took place in the indirect approach. Then, dynamic analyses were performed in order to compare the Frequency Response Functions (FRFs) determined by experimental tests with computational results. Thus, it was used a cantilever beam aluminum structure, which were bonded two piezoelectric patches, one to carry the excitement of the structure and the second to perform the data acquisition. The effective properties determined by the proposed methodology were applied for the dominium established by the piezoelectric patches. The results showed, again, the potential of the proposed methodology. Therefore, the numerical methodology is not only a good alternative for the calculation of effective coefficients of smart composite, but also a tool for the design of smart structures monitored by piezoelectric materials.

O presente trabalho visa empregar uma metodologia numérica para determinar as propriedades macro mecânica de compósitos ativos (AFC - Active Fiber Composite ou MFC - Macro Fiber Composite), combinando o conceito de Volume Elementar Representativo (VER) com o Método dos Elementos Finitos (MEF). Inicialmente, apresenta-se a fundamentação teórica associada à abordagem numérica empregada. Posteriormente, os modelos numéricos desenvolvidos são aplicados na determinação dos coeficientes efetivos de materiais compósitos inteligentes transversalmente isotrópicos com fibras piezelétricas de seção com forma circular e quadrada, respectivamente. Finalmente, os resultados numéricos obtidos pela metodologia proposta são, então, comparados com resultados da literatura. Constata-se que os resultados obtidos são muito semelhantes aos resultados relatados pela literatura para arranjo quadrático e hexagonal com fibra de geometria circular, sendo que neste caso, compararam-se os resultados numéricos com analíticos obtidos através do Método de Homogeneização Assintótica. Em seguida, a metodologia é aplicada para determinação dos coeficientes efetivos para arranjo quadrático e hexagonal com fibra de geometria quadrada. Empregando diferentes frações volumétricas de fibras, os resultados via MEF foram comparados aos resultados analíticos obtidos através do Método dos Campos Uniformes (Uniform Field Method). Após a avaliação das limitações e potencialidades da metodologia, de forma direta, através de resultados analíticos, realizou-se a avaliação da mesma de forma indireta. Para tal, foram realizadas análises dinâmicas visando comparar as Funções de Resposta em Frequência (FRF) experimentais com as obtidas computacionalmente. Dessa forma, utilizou-se uma viga de alumínio estrutural engastada-livre, onde foram colados duas pastilhas piezelétricas, sendo uma para realizar a excitação da estrutura e, a outra para fazer a aquisição dos dados. Os modelos computacionais via MEF empregaram para o domínio das pastilhas, as propriedades efetivas determinadas através da metodologia desenvolvida. Os resultados obtidos demonstraram mais uma vez as potencialidades da metodologia proposta. Assim, conclui-se que a metodologia numérica não é somente uma boa alternativa para o cálculo de coeficientes efetivos de compósitos inteligentes, mas também uma ferramenta para o projeto de estruturas inteligentes monitoradas por materiais piezelétricos. / This work presents the development a numerical methodology to determine the mechanical properties of active macro composites (AFC - Active Fiber Composite, or MFC - Macro Fiber Composite), combining the concept of Representative Elementary Volume (REV) with the Finite Element Method (FEM). In the first instance, the theoretical framework associated with the numerical approach employed is presented. Later, numerical models based on unit cell are applied to predict the effective material coefficients of the transversely isotropic piezoelectric composite with circular cross section fibers. Finally, numerical results obtained by the proposed methodology are compared to other methods reported in the literature. It appears that the results are very similar to the literature results for square and hexagonal arrangement of fibers with circular geometry, in which case, it was compared numerical with analytical results calculated by Asymptotic Homogenization Method (AHM). After that, the methodology is applied to determine the effective coefficients for square and hexagonal array with square fiber geometry. Employing different fiber volume fractions, it follows that the results obtained by the proposed methodology were compared to analytical results calculated by the Uniform Field Method (UFM). After assessing the potential and limitations of the methodology, either directly, through analytical results, the evaluation took place in the indirect approach. Then, dynamic analyses were performed in order to compare the Frequency Response Functions (FRFs) determined by experimental tests with computational results. Thus, it was used a cantilever beam aluminum structure, which were bonded two piezoelectric patches, one to carry the excitement of the structure and the second to perform the data acquisition. The effective properties determined by the proposed methodology were applied for the dominium established by the piezoelectric patches. The results showed, again, the potential of the proposed methodology. Therefore, the numerical methodology is not only a good alternative for the calculation of effective coefficients of smart composite, but also a tool for the design of smart structures monitored by piezoelectric materials.

The design and implementation of a morphing unmanned aircraft using smart materials is presented. Articulated lifting surfaces and articulated wing sections actuated by servos are difficult to instrument and fabricate in a repeatable fashion on thin, composite-wing micro-air-vehicles. Assembly is complex and time consuming. A type of piezoceramic composite actuator commonly known as Macro Fiber Composite (MFC) is used for wing morphing. The actuation capability of this actuator on fiberglass unimorph was modeled by the Rayleigh-Ritz method and quantified by experimentation. Wind tunnel tests were performed to compare conventional trailing edge control surface effectiveness to an MFC actuated wing section. The continuous surface of the MFC actuated composite airfoil produced lower drag and wider actuation bandwidth. The MFC actuators were implemented on a 0.76 m wingspan aircraft. The remotely piloted experimental vehicle was flown using two MFC patches in an elevator/aileron (elevon) configuration. Preliminary testing has proven the stability and control of the design. Flight tests were performed to quantify roll control using the actuators. Force and moment coefficients were measured in a low-speed, open section wind tunnel, and the database of aerodynamic derivatives were used to analyze control response. / Master of Science

Structural health monitoring (SHM) is the process of using measurements of a structure's response to known excitations and trying to determine if damage has occurred to the structure. This also fits the description of non-destructive evaluation (NDE). The main difference is that NDE takes place while the structure is out of service and SHM is intended to take place while the structure is in service. As such, SHM provides the opportunity to provide early warning against structural failure. This thesis intends to advance the state of the art in SHM by examining two approaches to SHM: vibration based and impedance based, and to associate these with the NDE method of stress intensity factors. By examining these methods the goal is to try and answer some of the important questions in SHM process. The first is to experimentally validate a crack model and to see how small of a crack can be detected by vibration methods. The second is to use the concept of stress intensity factor to perform an SHM type of measurement to determine the remaining life of a structure once the impedance method has determined that damage has occurred. The measurement system considered consists of using several different piezoceramic materials as self-sensing actuators and sensors. The structures are a simple beam and a more complex lug element used in aircraft applications. The approach suggested here is to use the impedance and vibration methods to detect crack initiation and then to use the proposed stress intensity method to measure the stress intensity factor of the structure under consideration. / Master of Science