Browsing by Author "Lalande, Frederic"
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- Analysis of the sensing region of a PZT actuator-sensorEsteban, 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.
- Ballistic Impact Resistance of Graphite Epoxy Composites With Shape Memory Alloy and Extended Chain Polyethylene Spectra™ Hybrid ComponentsEllis, 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.
- Effects of temperature on the electrical impedance of piezoelectric elementsKrishnamurthy, 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.
- Implementing Impedance - Based Health MonitoringRaju, Vinod (Virginia Tech, 1997-11-14)This work is an experimental study of applying an impedance-based health monitoring technique to complex structures. The work is presented in three parts. In the first part we consider effects of the following three factors on damage detection abilities: actuator excitation level, test wire length and ambient conditions (temperature, structural loading and vibration). It was concluded that increasing the applied voltage improves the signal to noise ratio and damage detection abilities. Test wire lengths under 30m do not affect damage detection abilities. The technique is able to distinguish and detect damage even with variations in temperature, structural loading and vibration. In the second part we apply our health monitoring technique to a complex truss structure and a massive steel steam header. We discovered that with multiplexing (acquiring a single signal from distributed actuators) the actuators on the truss structure we could detect damage but with less location information. Damage detection on the steel pipe ended in inconclusive results. The use of this technique on massive structures needs further investigation. Finally, we conducted a detailed experimental study of monitoring the integrity of composite-reinforced masonry structures. We developed a software package which enables even a casual user to determine if significant damage has occurred in these structures. The technique was successfully applied to detect damage (particularly due to delamination) in these composite-concrete structures. Most significantly, the technique was also able to detect damage well in advance of actual failure. This work relies mainly on frequency response plots and damage metric charts to present the data and to arrive at any conclusions. While frequency response plots give a qualitative approach to the analysis, damage metric charts attempt to quantify the data.
- Modeling of induced strain actuation of shell structuresChaudhry, Zaffir; Lalande, Frederic; Rogers, Craig A. (Acoustical Society of America, 1995-05-01)Based on the thin-shell Donnell theory, a model to represent the action of discrete induced strain actuator patches symmetrically bonded to the surface of a circular cylindrical shell has been developed. The model provides estimates of the bending curvatures due to the out-of-phase actuation and the in-plane strains due to the in-phase actuation of the bonded actuator patches. The magnitudes of the induced curvature and the in-plane strain are found to be identical to those of plates; however, due to the strain-displacement relations in cylindrical coordinates, the in-plane and out-of-plane displacements are coupled. Expressions for the equivalent forces and moments that represent the action of the actuator patches have been developed. Due to the curvature of the shell, the representation of the in-phase actuation with an equivalent in-plane line force applied along the edge of the actuator results in the application of erroneous rigid-body transverse forces. To avoid these rigid body forces, a method to represent the in-phase actuation with a system of self-equilibrating forces is proposed. The action of the actuator is then represented by an equivalent in-plane force and a transverse distributed pressure applied in the region of the actuator patch. Finite element verification of the proposed model is presented. The displacements due to the actual actuator actuation are compared with the proposed model, and very good agreement is found.
- Modeling of the induced strain actuation of shell structuresLalande, Frederic (Virginia Tech, 1995-04-05)When discrete piezoelectric actuator patches bonded on structures are used for active shape, vibration, and acoustic control, the desired deformation field in the structure is obtained through the application of localized line forces and moments generated by expanding or contracting bonded piezoelectric actuators. An impedance-based model to predict the dynamic response of cylindrical shells subjected to excitation from surface-bonded induced strain actuators is presented. The essence of the impedance approach is to include the actuator/structure impedance ratio in the calculations of the actuator forces applied to the structure, which will retain the dynamic characteristics of the actuators. The appropriate representation of the loading due to in-phase and out-of-phase actuation is discussed. Due to the curvature of the shell, the representation of the in-phase actuation with an equivalent in-plane line force applied along the edge of the actuator results in the application of erroneous rigid-body transverse force. To avoid these rigid body forces, the action of the actuator needs to be represented by am equivalent in-plane force and a transverse distributed pressure applied in the region of the actuator patch to maintain the structure self-equilibrium. A full derivation of the impedance model is included, taking great care in the structural and actuator impedance definition. It is found that the actuator's output dynamic force in the axial and tangential direction are not equal. Various case studies of a cylindrical thin shell are performed to illustrate the capabilities of the developed impedance model. The in-phase and out-of-phase actuation authority of induced strain actuators bonded to the surface of a shell is compared. It is shown that out-of-phase actuation has better authority in exciting the lower order bending modes, while in-phase actuation has better authority in exciting the higher order circumferential modes. Dynamic finite clement analysis has been performed using piezoelectric elements available in ANSYS 5.0. The good correlation between the finite clement results and the impedance model confirms the analytical solution. Experimental data of a circular ring actuated in-phase and out-of-phase by a piazoelectric material (PZT) were also compared to the derived impedance model.
- Qualitative health monitoring and incipient damage inspection/evaluationAyres, John W. (Virginia Tech, 1996-05-01)Real-time structural integrity monitoring is a concept that is becoming a reality in the engineering community. It will soon be possible for a structure to warn the user when its own structural integrity has been altered. A qualitative impedance-based health monitoring technique, which can be implemented for real-time damage evaluation of complex structures, is investigated. The basic principle of the technique is to monitor the structure's mechanical impedance which will be changed with the presence of damage. The mechanical impedance variations are monitored by measuring the electrical impedance of a bonded piezoelectric actuator/sensor (PZT). This mechanical-electrical impedance relation is due to the electro-mechanical coupling property of piezoelectric materials. This health monitoring technique can be easily adapted to existing structures, since only a small non-intrusive PZT patch is needed. This impedance-based method operates at high frequencies (generally above 100kHz), which enables it to detect incipient type damage in a localized region. The localized sensing region offers the advantage of not being affected by nonnal operating conditions or by changing boundary conditions. In this thesis, a complete theoretical background on the impedance-based technique is derived. Then, the technique is applied successfully to a variety of case studies; such as composite patch repair, aircraft structures, precision parts, and civil infrastructure. By simplifying the impedance measurement interpretation through a simple scalar damage metric, the real-time implementation of the impedance-based technique has been proven feasible.
- SMA-Induced Deformations In general Unsymmetric LaminatesDano, Marie-Laure (Virginia Tech, 1997-04-22)General unsymmetric laminates exhibit large natural curvatures at room temperature. Additionally, inherent to most unsymmetric laminates is the presence of two stable configurations. Multiple configurations and stability issues arise because of the geometric nonlinearities associated with the large curvatures. The laminate can be changed from one stable configuration to the other by a simple snap-through action. This situation offers the opportunity to use shape memory alloys (SMA) attached to the laminate to generate the snap-through forces and change the shape of the laminate on command. Presented is a model which can predict SMA-induced deformations in general unsymmetric laminates and, particularly, the occurrence of the snap through. First, a methodology is developed to predict the deformations of flat general unsymmetric epoxy-matrix composite laminates as they are cooled from their elevated cure temperature. Approximations to the strain fields are used in the expression for the total potential energy, and the Rayleigh-Ritz approach is used to study equilibrium. To further study the laminate deformations, finite-element analyses are performed. Experimental results are presented which confirm the predictions of the developed theory and the finite-element analyses regarding the existence of multiple solutions and the magnitude of the deformations. Results are compared with those of several other investigators. Next, the deformation behavior of general unsymmetric laminates subjected to applied forces is studied. The principle of virtual work is used to derive the equilibrium equations relating the laminate deformations to the applied forces. By solving the equilibrium equations as a function of the force level, relations between the laminate deformations and the applied force are derived, and the force level at which the laminate changes shape is determined. Finally, an existing SMA constitutive model is implemented into the developed theory to predict the deformations of simple structures to SMA-induced forces. Experiments on a narrow aluminium plate with an externally attached SMA actuator are conducted. The experimental results show good agreement with the predictions from the developed theory. Next, the deformation behavior of general unsymmetric laminates subjected to SMA actuators is predicted using the developed theory. Experiments using SMA actuators to generate the snap through of nsymmetric laminates are conducted. Good correlation with the developed theory is obtained.