VTechWorks Repository :: Browsing by Author "Rogers, Craig A."

Browsing by Author "Rogers, Craig A."

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Active control of acoustic radiation due to discontinuities on thin beams
Frampton, Kenneth D. (Virginia Tech, 1991-04-15)
Two experiments were conducted to study the active control of acoustic radiation due to discontinuities on thin beams. One experiment investigated the radiation from a clamped end condition and the other investigated the radiation from a blocking mass. The beams were excited by subsonic flexural traveling waves which "scattered" (or produced reflected and transmitted traveling and near-field waves) when they encountered the discontinuity. This "scattering" produced supersonic wave number components in the beam vibrational response which were responsible for the acoustic radiation. The main purpose of these experiments was to control the acoustic radiation from discontinuities on beams by actively changing the characteristics of the "scattered" waves with control actuators. In each experiment the system was disturbed by a harmonic, subsonic input from a point force shaker. Control actuator( s) (in the form of shakers and piezoelectric actuators) were attached to the beam near the discontinuity. Error microphone(s) were positioned in the acoustic field which supplied an error signal to the digital controller. The digital controller employed was the filtered-x version of the adaptive LMS algorithm programmed on a dedicated signal processing board in a personal computer. An array of accelerometers was attached to the beam which were used to decompose the complex amplitudes of an assumed displacement equation. By applying a spatial Fourier transform to the displacement equation the wavenumber components present in the beam displacement were calculated. This aided in the investigation of the mechanism by which control of the acoustic field was affected. Results from these experiments showed that large attenuations at the error microphones were possible (as much as 50dB) along with global attenuation of the acoustic field. The mechanism by which the control of the acoustic far-field was achieved was demonstrated as a decrease in the supersonic wavenumber components in the beam vibrational response.
Active control of sound transmission through plates in a reverberant environment
Zhou, Ning (Virginia Tech, 1992)
Active control of sound transmission through an elastic plate placed between two reverberation chambers is studied experimentally. Active acoustic control is performed using piezoelectric sensors and actuators bonded to the plate. The control technique uses an adaptive control algorithm. Results are presented for harmonic excitation provided by a speaker in the source chamber at two resonant frequencies of the plate. Influence of different types of error sensors, varied actuator locations, and varied speaker locations are studied. Compared to microphone sensors in the receiving chamber, piezoelectric sensors are shown to be effective in reducing sound transmission through the plate. Average reduction of sound pressure level (SPL) on the order of 20 dB or 13 dB are achieved when the plate vibrates at mode (3,1) or (3,3). Microphone sensor locations are shown to influence the controlled sound field, those located where the direct sound field is dominant result in larger SPL reductions. SPL reductions are caused by two mechanisms: modal reduction and modal restructuring, and the dominance of either is shown to depend on actuator locations. When the sound field is non-diffuse, speaker locations influence the SPL and the SPL reduction by changing the plate's structural response. Also included in this work, previously developed one-dimensional (I-D) modal sensor theory for beams is used to develop modal sensors for a clamped plate. Two I-D modal sensors are applied to a fully clamped plate and each shown to observe a particular subset of plate vibration modes. Previous work developed the theory for two-dimensional (2-D) modal sensors for simply-supported plates. A necessary and sufficient condition for the spatial functions of 2-D modal sensors are developed for plates with arbitrary boundary conditions.
Active control of sound transmission/radiation from elastic plates using multiple piezoelectric actuators
Wang, Bor-Tsuen (Virginia Tech, 1991)
This thesis presents a theoretical analysis of active control of sound radiation from elastic plates with the use of piezoelectric transducers as actuators. A strain-energy model (SEM), based upon the conservation of strain energy, for a laminate beam with attached or embedded finite-length spatially distributed induced strain actuators was first developed to determine the induced strain distribution. The equivalent axial force and bending moment induced by the embedded or surface bonded actuators were also calculated. The one-dimensional SEM was then extended to a two-dimensional model by employing the classical laminate plate theory and utilizing Heaviside functions to integrate the actuator influence on the substructure. The mechanics model can determine the structural coupling effect and predict the structural response as a result of piezoelectric actuation. A baffled simply-supported rectangular plate subjected to harmonic disturbances was considered as the plant. Piezoceramic materials bonded to the surfaces of the plate or point force shakers were applied as control actuators. Both microphones in the radiated far-field and accelerometers located on the plate were considered as error sensors. In addition, distributed sensors for pressure and structural motion were modelled. The cost function was formulated as the modulus squared of the error signal. Linear quadratic optimal control theory was then applied to minimize the cost function to obtain the optimal input voltages to the actuators. Both near-field and far-field pressure and intensity responses as well as plate displacement distributions were presented to show the effectiveness and mechanisms of control for various configurations of the actuators and sensors. Plate wavenumber analysis was also shown to provide a further insight into control technique. The results show that piezoelectric actuators perform very well as control sources, and that pressure sensors have many advantages over acceleration sensors while distributed sensors are superior to discrete sensors. The optimal placement of multiple fixed size piezoelectric actuators in sound radiation control is also presented. A solution strategy is proposed to calculate the applied voltages to piezoelectric actuators with the use of linear quadratic optimal control theory. The location of piezoelectric actuator is then determined by minimizing an objective function, which is defined as the sum of the mean square sound pressure measured by a number of error microphones. The optimal location of piezoelectric actuators for sound radiation control is found so as to minimize the objective function and shown to be dependent on the excitation frequency. In particular, the optimal placement of multiple piezoelectric actuators for on-resonance and off-resonance excitation is presented. Results show that the optimally placed piezoelectric actuators perform far better in sound radiation control than arbitrarily selected. This work leads to a design methodology for adaptive or intelligent material systems with highly integrated actuators and sensors. The optimization procedure also leads to a reduction in the number of control transducers.
Active damage control using artificial intelligence: initial studies into identification and mitigation
Kiel, David H. (Virginia Tech, 1993-06-05)
This thesis presents an initial investigation into Active Damage Control (AD C) using Artificial Intelligence (AI). AI can alleviate the sometimes complicated task of modelling the system and also provides an adaptable solution process. The two research areas of ADC, damage identification and damage control, are studied in separate investigations. An AI technique called "rule induction" is used for the damage identification study. Velocity data from three plates (one without damage, one with damage at the center, and one with damage at the edge) are acquired using a laser data acquisition system. A set of rules is then induced from these data which accurately identifies which plates have damage and where the damage is located. With regard to the damage control, a real-time, machine-learning technique called "BOXES" is used to locally control the vibration of various systems by identifying their vibrational patterns. Using this technique, it is shown that the computer successfully learns an effective control law for various simulations using its trials and failures as the only learning information. It is also seen that the learning algorithm is somewhat less effective when experimentally applying this method to a pin-pin, aluminum beam. A discussion of possible improvements are presented in the future work section.
Active dynamic response tuning of adaptive composites utilizing embedded nitinol actuators
Barker, Daniel Keith (Virginia Tech, 1989-07-04)
Adaptive composites utilizing embedded nitinol fibers have the unique ability to change their material properties, induce large internal distributed forces in a structure, and can modify the stress and strain distribution within a structure in a controlled manner. In this study, nitinol fibers are embedded in graphite-epoxy and are used as distributed actuators to actively tune the dynamic response of clamped-clamped beams. The natural frequencies of clamped-clamped nitinol composite beams are shown, experimentally. to increase linearly as a function of temperature. Beams with nitinol volume fractions of 5% 10%, and 15% can increase their first natural frequency by factors of 1.7, 2.5, and 3.0 respectively. Classical lamination theory is used to formulate a mathematical model of the dynamic response which includes the adaptive properties of the embedded nitinol fibers as a function of temperature, as well as the thermal aspects of the matrix material. Experimental characterization of nitinol for use as constrained thermosets is performed and the results are used in the mathematical model. The mathematical model is used to calculate the natural frequencies of clamped-clamped nitinol composite beams and the results are compared to experimental results. It is clear that adaptive composites represent a new concept in active control of structural responses and may act as a catalyst for future developments in both material and structures technology. Demonstrating, experimentally and computationally, the ability to alter the dynamic response using unique adaptive qualities will hopefully inspire new material/structural interaction paradigms.
Active vibration and structural acoustic control of shape memory alloy hybrid composites: Experimental results
Rogers, Craig A. (Acoustical Society of America, 1990-12-01)
Shape memory alloy hybrid composites have been shown both by analytical simulations and experiments to be effective adaptive materials for active vibration and structural acoustics control [Rogers and Robertshaw, Engineering Science Preprints 25, ESP25.88027, Society of Engineering Sciences (1988) and ASME Paper 88-WA/DE-9 ( 1988); Rogers et al., in Proceedings of the 30th Structures, Structural Dynamics, and Materials Conference, AIAA Paper 89-1389 (1989)]. Structural acoustics is the study of how elastic structures radiate or receive sound, and in its most fundamental form involves the simultaneous solution of the differential equations describing the structure and fluid media with appropriate boundary conditions between the two, i.e., a "fully" coupled analysis. This paper will review the state-ofthe-art of active control utilizing shape memory alloy hybrid composites and present experimental results showing active dynamic tuning by a method called active strain energy tuning (ASET), active control of sound radiation from a clamped-baffled beam, and transient vibration control of a cantilevered beam.
Analysis and Compensation of Imperfection Effects in Piezoelectric Vibratory Gyroscopes
Loveday, Philip Wayne (Virginia Tech, 1999-01-29)
Vibratory gyroscopes are inertial sensors, used to measure rotation rates in a number of applications. The performance of these sensors is limited by imperfections that occur during manufacture of the resonators. The effects of resonator imperfections, in piezoelectric vibratory gyroscopes, were studied. Hamilton's principle and the Rayleigh-Ritz method provided an effective approach for modeling the coupled electromechanical dynamics of piezoelectric resonators. This method produced accurate results when applied to an imperfect piezoelectric vibrating cylinder gyroscope. The effects of elastic boundary conditions, on the dynamics of rotating thin-walled cylinders, were analyzed by an exact solution of the FlÃ¼gge shell theory equations of motion. A range of stiffnesses in which the cylinder dynamics was sensitive to boundary stiffness variations was established. The support structure, of a cylinder used in a vibratory gyroscope, should be designed to have stiffness outside of this range. Variations in the piezoelectric material properties were investigated. A figure-of- merit was proposed which could be used to select an existing piezoceramic material or to optimize a new composition for use in vibratory gyroscopes. The effects of displacement and velocity feedback on the resonator dynamics were analyzed. It was shown that displacement feedback could be used to eliminate the natural frequency errors, that occur during manufacture, of a typical piezoelectric vibrating cylinder gyroscope. The problem of designing the control system to reduce the effects of resonator imperfections was investigated. Averaged equations of motion, for a general resonator, were presented. These equations provided useful insight into the dynamics of the imperfect resonator and were used to motivate the control system functions. Two control schemes were investigated numerically and experimentally. It was shown that it is possible to completely suppress the first-order effects of resonator mass/stiffness imperfections. Damping imperfections, are not compensated by the control system and are believed to be the major source of residual error. Experiments performed on a piezoelectric vibrating cylinder gyroscope showed an order of magnitude improvement, in the zero-rate offset variation over a temperature range of 60°C, when the control systems were implemented.
Analysis of the sensing region of a PZT actuator-sensor
Esteban, Jaime (Virginia Tech, 1996-07-15)
A high frequency impedance-based qualitative non-destructive evaluation (NDE) technique has been successfully applied for structural health monitoring at the Center for Intelligent Material Systems and Structures (CIMSS) [1-3]. This new technique uses piezoceramic (PZT) patches as actuator-sensors to provide a low-power driven constant voltage dynamic excitation, and to record the modulated current flow through the structure. Therefore, it relies on tracking the electrical point impedance to identify incipient level damage. The high frequency excitation provided by the PZT, ensures the detection of minor changes in the monitored structure. It also limits the sensing area to a region close to the PZT source, therefore only changes in the near field of the PZT are detected, enhancing the ability of this technique to localize incipient damage. The phenomena of the PZT's sensing region localization has been the driving motivation for this research. More fundamental analytical research should be performed before full application of this technique is possible. Thereby, a wave propagation continuum mechanics based approach has been applied to model the high frequency vibrations of one dimensional structures. Energy dissipation mechanisms, such as bolted connections and internal friction, are considered to have a major role in the attenuation of the PZT's induced wave, therefore these mechanisms has been extensively studied. To analyzed bolted connections, linear and nonlinear joint models have been used to describe the wave interaction with such nonconservative discontinuities. Also, with the use of an impedance based model, the electromechanical coupling of the PZT and the host structure is added into the formulation. The wave interaction and energy dissipated at the bolted discontinuity has been assessed with energy flux computations of the incident, transmitted, and reflected waves. The effect of loosening the bolted joint has been also analyzed by reducing the spring stiffness and increasing the damping in the dash pots for the linear joint model, and reducing the Coulomb stiffness and shearing force at the interface for the nonlinear case. A scheme based on the correspondence principle has been applied to calculate the specific damping capacity of a system, at any given frequency, as a quantification of the energy dissipated through the system. The material damping was added into the formulation assuming the modulus to have a complex representation, and therefore the corresponding loss factors were found with active measurement of the material properties of the specimen via a wave propagation method, that monitories the wave's speed at two locations. Once the bases of the analytical model have been set up and corroborated with experiments, a parametric study has been developed to account for the various factors that can affect the sensing range of the PZT’s induced wave, and therefore to have a “rule of thumb on how to go about” when bonding PZTs to structures to monitor them. Apart from the energy dissipation mechanisms, other parameters responsible for the reflection of the incoming wave, and its consequent attenuation, has also been reconstructed. With the extensive analysis of these parameters, an impedance damage metric, based on the undamaged and damaged impedance, has been developed for various factors that can be the source of incipient damage. An attenuation metric has also been introduced to identify the degree of transmission of the propagating wave at certain discontinuities. The analysis of the case scenarios reproduced in this parametric study will aid in the knowledge about the number of PZTs needed to be placed in the monitored structure, the most critical locations, and when a monitored member in a system need to be replaced.
An axisymmetric linear/high-order finite element for filament wound composite structures
Rogers, Craig A. (Virginia Polytechnic Institute and State University, 1987)
The development of an axisymmetric linear by high-order finite element to model filament-wound structures is presented. The primary objective of this work was to develop a ’design code' to analyze filament wound spherical pressure vessels. In order to develop a design-oriented analysis capability which can produce accurate results rather quickly with reduced input-data requirements, the total number of system equations must be reduced. To accomplish this task, a linear by high-order element was formulated which uses a single high-order displacement field finite element to model the total thickness of an axisymmetric composite structure. The displacement order for the in-plane direction remains linear, while the transverse order is user selectable. Numerical integration for stiffnesses is evaluated with respect to varying material properties and lamirna thicknesses in each individual element. Results from a computational economy study are presented showing potential time savings of 40 percent when compared to the conventional modeling scheme of using bi-linear elements. Actual test cases indicate that computation time savings may be as great as 55 percent when using linear by fourth-order elements and 45 percent when using linear by sixth-order elements. The accuracy of the element was evaluated by comparing the finite element results to elasticity solutions for isotropic, orthotropic, and filament-wound cylindrical pressure vessels. Most of the finite element results indicated a ±3 percent maximum error of the stresses compared to the elasticity results. The new linear by high order element stress results were nominally within ±2 percent of stresses calculated with conventional bilinear elements. Comparisons of finite element results for an actual filament-wound spherical pressure vessel slowed that linear by third- or fourth-order elements may be adequate for preliminary design purposes while the higher-order elements generally correlated better with the conventional bi-linear elements. Also presented is an outline of the design code and sample results for spherically wound pressure vessels.
Ballistic Impact Resistance of Graphite Epoxy Composites With Shape Memory Alloy and Extended Chain Polyethylene Spectra™ Hybrid Components
Ellis, Roger L. (Virginia Tech, 1996-12-09)
Graphite epoxy composites lack effective mechanisms for absorbing local impact energy often resulting in penetration and a structural strength reduction. The effect of adding small amounts of two types of high strain hybrid components on the impact resistance of graphite epoxy composites subjected to projectiles traveling at ballistic velocities (greater than 900 ft/sec) has been studied. The hybrid components tested include superelastic shape memory alloy (SMA), a material having an unusually high stra in to failure (15 - 20%), and a high performance extended chain polyethylene (ECPE) known as Spectra™, a polymer fiber traditionally used in soft and hard body armor applications. 1.2% volume fraction superelastic SMA fiber layer was embedded on the specimens front, middle, and backface to determine the best location for a hybrid component in the graphite composite. From visual observation and energy absorption values, it was concluded that the backface is the most suitable location for a high strain hybrid component. Unlike the front and middle locations, the hybrid component is not restricted from straining by surrounding graphite material. However, no significant increases in energy absorption were found when two perpendicular SMA layers and an SMA-aramid weave configuration were tested on the backface. In all cases, the embedded SMA fibers were pulled through the graphite without straining to their full potential. It is believed that this is due to high strain rate effects coupled with a strain mismatch between the tough SMA and the brittle epoxy resin. However, a significant increase in energy absorption was found by adding ECPE layers to the backface of the composite . With only a 12% increase in total composite mass, a 99% increase in energy absorption was observed.
The constitutive modeling of shape memory alloys
Liang, Chen (Virginia Tech, 1990-08-15)
This dissertation presents a one-dimensional thermomechanical constitutive model for shape memory alloys based on basic concepts of thermodynamics and phase transformation kinetics. Compared with other developed constitutive relations, this thermomechanical constitutive relation not only reflects the physical essence of shape memory alloys, i.e., the martensitic phase transformation involved, but also provides an easy-to-use design tool for engineers. It can predict and describe the behavior of SMA quantitatively. A multi-dimensional constitutive relation for shape memory alloys is further developed based on the one-dimensional model. It can be used to study the mechanical behavior including shape memory effect of complex SMA structures that have never been analytically studied, and provide quantitative analysis for many diverse applications of shape memory alloys. A general design method for shape memory alloy actuators has also been developed based on the developed constitutive relation and transient thermal considerations. The design methodology provides a quantitative approach to determine the design parameters of shape memory alloy force actuators, including both bias spring SMA force actuators and differential SMA force actuators.
Coupled electro-mechanical system modeling and experimental investigation of piezoelectric actuator-driven adaptive structures
Zhou, Su-Wei (Virginia Tech, 1994)
Of primary importance to the design and application of adaptive structures is a modeling method to allow for performance prediction and parametric optimization of the integrated system. The statics-based modeling approaches have been applied to model piezoelectric (PZT) actuator-driven adaptive structures. The dynamic interaction between the actuators and their host structures has been ignored, and the system energy conversion can’t be predicted. As a matter of fact, PZT actuator-driven smart structures are complex electromechanical coupling systems in which electrical energy is converted into mechanical energy and vice-versa. The actuator outputs and the system energy conversion are dominated by the complex electro-mechanical impedance of the system. The entire actuator/substrate system can thus be essentially represented by a coupled impedance-based system model. This research presents such an impedance-based electro-dynamics analytical method and the experimental investigation for integrated PZT/substrate systems. When compared with the conventional static models, the system modeling method has revealed the physical essence and the interconnections among the intelligent elements and supporting structures. The frequency-dependent behaviors of the actuator and the dynamic response of the integrated system are accurately predicted. The theoretical model was developed for generic PZT actuator-driven active structures. The actuation force was evaluated as a result of the dynamic interaction between the actuator and the host structure. The model was then extended to include the electrical parameters of the PZT actuator such that the power flow and consumption of the integrated system can be predicted. The system dissipative power was then treated as the equivalent generation source to evaluate a temperature rise and thermal damage of the actuator. To examine the utility and generality of the system modeling method, the developed model was applied to typical two-dimensional structures such as thin plates and thin shells, and to one-dimensional structures such as the circular rings and beams. The design-related mechanical and thermal stress characteristics of the actuators were also specifically investigated. In addition to the theoretical work, experiments were conducted. The PZT actuator-driven simply-supported plate was built and tested. The velocity response of the integrated plate and the dynamic strain of the PZT actuators were measured. The coupled electromechanical admittance of the real system was also directly measured using an impedance analyzer. The predicted solutions agree with the experimental results in all of the tested cases, verifying the theoretical model.
Design, fabrication, and calibration of an instrumented drop weight impact tester
Dempsey, Craig Thomas (Virginia Tech, 1994-02-05)
In this thesis, the complete design, fabrication, and calibration of an instrumented drop weight impact tester is described. Included in this description are all the sketches and drawings that are needed to duplicate this project, if so desired. This impact tester was built for around $23,000 less than it would have cost to buy and modify a commercial tester for the intended research application. This tester, as designed, was intended to be used in the field of impact location detection using artificial neural networks. Even though this impact tester was built for a specific research application, the design concepts that are presented can easily be adapted to a variety of testing needs. This impact tester was built using an non-working milling machine for a base. This provides a rigid, stable base along with a moveable X-Y table. The tester itself has the capability for drop weights ranging from 3.518 Ib up to 15.408 lb, and impact energy levels ranging from 0.6 ft-lb up to 45.6 ft-lb. Also, it is capable of impacting multiple locations of large plates with variable boundary condition sizes up to 12" x 24". Furthermore, it uses a computer program written using a data acquisition software package to provide output plots for the impact event, including the force and energy applied to the specimen versus time and the force versus displacement. Finally, initial experimental results obtained from this tester agree very well with those obtained from a commercially available tester, allowing it to be used in future tests involving intelligent material systems.
Dynamic analysis of piezoelectric actuator-driven circular rings using an impedance approach
Rossi, Anna; Liang, Chen; Rogers, Craig A. (Acoustical Society of America, 1994-09-01)
This paper presents a dynamic model for the response of a circular ring excited by piezoelectric transducer (PZT) actuators bonded on the ring surface. The dynamic response is determined based on the dynamic interaction between the PZT actuators and the structure using an impedance approach. Compared with the conventional static approach, in which a statically determined ''equivalent force'' of the actuator is used as the forcing function in the dynamic analysis, the impedance approach cannot only capture the physics of the actuator/structure interaction, but also accurately predict the structural dynamic response. Experiments have also been conducted to verify the theoretical model. The predicted dynamic response using the impedance approach agrees very well with the experimental results. Comparison of the conventional static approach and the impedance model has also been presented.
Dynamic transduction characterization of magnetostrictive actuators
Ackerman, Anthony E. (Virginia Tech, 1993-12-06)
The objective of this thesis is to develop an analysis approach for formulation of transduction or input/output representations for magnetostrictive actuators. This transduction model is developed through application of an electro-magneto-mechanical impedance modeling approach which combines both the mechanical dynamics and coupled behavior of the actuator device. Lumped and continuous mechanical impedance elements model the actuator dynamics and the constitutive relationships for Terfenol-D characterize the electro-magneto-mechanical interaction. Experimental analysis of a Terfenol-D actuator serves to verify the developed models and provides an indication of actuator non-linearity. The developed transduction model allows for various device behavior analysis including dissipative power consumption, force and stroke output, and efficiency as a transducer. An actuator design strategy based upon the dynamics of the actuator and a driven external system is presented and allows for analysis of various actuator behaviors in terms of device parameters. The Terfenol-D actuator as a collocated actuator/sensor is also made possible with the transduction model.
Effects of fiber type on the tribological behavior of polyamide composites
Weick, Brian L. (Virginia Tech, 1993)
An experimental and analytical study of the tribological behavior of polymer composites is presented. Glass, aramid, and carbon fiber-filled polyamide (Nylon 6,6) composites serve as models for understanding friction and wear processes encountered when polymer composites are used in tribological applications. Experimental results not only include measurements of friction and wear, but surface temperatures produced by frictional processes during oscillating contact experiments. Since an optically flat, transparent sapphire disk is used as the oscillating countersurface, surface temperatures can be measured directly at the interface using an infrared microscope. Experimental results show that the presence of fibers in the polyamide matrix lowers wear, friction, and surface temperature when compared with the unfilled polymer. Rationale for this improved tribological behavior is presented and discussed. Fiber-type is shown to have a direct influence on the tribological behavior of the polymer composite, and the chemical behavior at and near the interface is shown to be significant by examining worn and transferred material through surface analytical techniques. In particular, evidence is presented for the tribochemical degradation of intramolecular bonds in the polyamide macromolecule. Measurements of surface temperatures are compared with theoretical predictions using models for the real area(s) of contact, and results from “scanning” experiments are also presented in which the infrared microscope is used to measure surface temperatures at possible real areas of contact within the apparent contact region. Instantaneous measurements of surface temperature and friction over a single cycle of motion are also presented which allows for the performance of a frequency domain analysis. This technique not only shows the frequency content of the friction and surface temperature signals, but it also shows correlations between these two parameters. The role of intermolecular attractions in frictional processes is addressed, and evidence for relatively strong intermolecular attractions between the polyamide surface and sapphire disk is discussed.
Effects of temperature on the electrical impedance of piezoelectric elements
Krishnamurthy, Karthik Chandran (Virginia Tech, 1996-02-05)
A structural health monitoring technique, developed at the Center for Intelligent Material Systems and Structures, employs piezoelectric (PZT) materials for tracking the structural impedance to qualitatively identify damage. The mechanical impedance of a structure is a function of the structure's mass, stiffness, damping, and structural boundary conditions. Changes in any of the above-mentioned properties lead to a change in the mechanical impedance of the structure and a change in the impedance pattern of the structure. The mechanical impedance of a structure can be measured by coupling the electrical and mechanical impedances via PZT patches. Therefore any change in the mechanical impedance leads to a change in the electrical impedance of the PZT bonded to the structure of interest. However, change of the electrical impedance can also occur due to changes in temperature. Piezoelectric materials have been known to have temperature dependency regarding their basic properties, such as the dielectric constant and the piezoelectric coefficient. In this thesis, this temperature dependency will be investigated. The motivation of this work is linked to the impedance-based nondestructive evaluation (NDE) technique which employs PZT sensors for tracking changes in the structural impedance, by measuring the electrical impedance, to qualitatively identify damage. However, for this NDE technique to be successful in all types of environments, it must be insensitive to temperature variations. As mentioned earlier, piezoelectric materials have strong temperature dependency and a temperature compensation procedure is necessary. Therefore, two software correction techniques were developed to eliminate the effects of temperature in the electrical impedance measurements of PZT sensors. (NDE) technique which employs PZT sensors for tracking changes in the structural impedance, by measuring the electrical impedance, to qualitatively identify damage. However, for this NDE technique to be successful in all types of environments, it must be insensitive to temperature variations. As mentioned earlier, piezoelectric materials have strong temperature dependency and a temperature compensation procedure is necessary. Therefore, two software correction techniques were developed to eliminate the effects of temperature in the electrical impedance measurements of PZT sensors. The second correction technique is based on the sensor output. Through experimental investigation, it was found that temperature will have the effect of shifting the electrical impedance magnitude of the piezoelectric sensor, while leaving the impedance phase unaffected. To characterize the temperature effects in PZT materials, a temperature coefficient which is independent of frequency has defined. Finally, based on the defined temperature coefficient, a simple temperature compensation technique has been implemented successfully, eliminating the effects of temperature on PZT sensors while not eliminating the effects of temperature on the structure.
Electro-dynamic analysis of stack actuators and active members integrated within truss structures
Flint, Eric Michael (Virginia Tech, 1994-08-15)
In this thesis, a method of predicting the steady state, dynamic, electromechanical behavior of stack actuators (both electrostrictive and piezoelectric) integrated within complex structures is developed and experimentally verified. This research was motivated by a need to accurately predict transmission force, velocity output, and power consumption for a wide range of applications both terrestrial and space based. The relevant transduction equation / parameters are derived from basic principles. These results are experimentally verified with a PZT stack active member. The derivations are then extended to incorporate the effects of integrating the actuator within a host structure. Specifically, the equations needed to predict actuator output force, resulting velocity and drawn current are derived. To implement and test these results in a structure, the equivalent host structure impedance must be determined. This is done experimentally for a complex truss structure representative of a small satellite. These results are then used to prepare theoretical predictions which compare well with experimentally measured output force. Finally, the derivations are extended to the electrical behavior of active members integrated within truss structures. It is now possible to predict the electrical load imposed by the active member on the power supply system including the effects of coupling with the host structure dynamic boundary conditions. Two implications of this are considered. First, the required power demands directly influence the design and sizing of amplifiers, applied voltage levels and power systems. Second, the dissipative power from actuation losses contributes directly towards raising the internal temperature of an operating stack actuator.
Enhanced induced strain actuator performance through discrete attachment to structural elements
Chaudry, Zaffir Ahmed (Virginia Tech, 1992-07-02)
In intelligent structures, structural deformation is generally controlled by either embedding or surface bonding the induced strain actuator to the structure. With bonded or embedded actuators used for inducing flexure, the developed in-plane force contributes indirectly through a locally-generated moment. Control authority in this configuration is thus limited by actuator offset distance. The focus of this research was to investigate a new concept in which the actuator, as opposed to being bonded, is attached to the structure at discrete points. This configuration is fundamentally different from the bonded/embedded configuration in that the actuator and the structure between the two discrete points can deform independently; and the in-plane force of the actuator, which contributes only indirectly in the case of bonded actuator, can directly influence out-of-plane displacements of the structure. Additionally, the actuator offset distance can be optimized with respect to actuator force/strain saturation for increased authority. Two implementations of this concept as applied to beam structures were investigated. In the first, the actuator (e.g., shape memory alloy actuator wire) does not possess any flexural stiffness; and therefore, remains straight between the two attachment points. In the second implementation, the actuator (PZT's and electrostrictive) possesses flexural stiffness, and bends with the structure. The formulation and experimental results for both implementations are presented. Enhanced authority is demonstrated by comparing the static response of the discretely attached actuator beam systems with their bonded counterpart systems.
An experimental investigation of the behavior of Nitinol
Dye, Tracy Earl (Virginia Tech, 1990-08-05)
Shape memory alloys (SMA) have the unique ability to recover large strains and generate large recovery stresses via a repeatable martensitic transformation. Stress-strain and shape memory effect characteristics are needed in order to develop SMA force actuator design methods. Moreover, constitutive models able to quantitatively predict these characteristics and thus be useful as engineering design tools are also needed. An experimental apparatus designed to characterize the mechanical behavior of SMA was built and utilized. The apparatus is used specifically to gather stress-strain and shape memory effect characteristics from nitinol wire whereby mechanical properties associated with the material are determined. Phenomena such as the R-phase and stress induced martensite serration are investigated. A one-dimensional constitutive model is presented that quantitatively predicts stress-strain and shape memory effect behavior and was developed with the intention of being an engineering design tool for SMA force actuators. Experimental stress-strain and shape memory effect results are compared against that predicted by the model with the intention of verifying the model. The model displays the ability to predict stress-strain behavior that is in good quantitative agreement with experiment. The model also displays the ability to predict hysteric shape memory effect behavior for free, controlled, and restrained recovery cases of selected prestrains that is in good quantitative agreement with experiment. The model is unable to predict shape memory effect behavior such as the R-phase. Demonstrating the ability to experimentally investigate a constitutive model will hopefully inspire further combined experimental and theoretical SMA research.

Browsing by Author "Rogers, Craig A."

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