Frederic Lalande

Qualitative impedance-based health monitoring of civil infrastructures

This paper presents a qualitative health monitoring technique to be used in real-time damage evaluation of civil infrastructures such as bridge joints. The basic principle of the technique is to monitor the structural mechanical impedance which will be changed by the presence of structural damage. The mechanical impedance variations are monitored by measuring the electrical impedance of a bonded piezoelectric actuator/sensor patch. This mechanical-electrical impedance relation is due to the electromechanical coupling property of piezoelectric materials. This health monitoring technique can be easily adapted to existing structures, since only a small PZT patch is needed, giving the structure the ability to constantly monitor its own structural integrity. This impedance-based method operates at high frequencies (above 50 kHz), which enables it to detect incipient-type damage and is not confused by normal operating conditions, vibrations, changes in the structure or changes in the host external body. This health monitoring technique has been applied successfully to a variety of light structures. However, the usefulness of the technique for massive structures needs to be verified experimentally. For this purpose, a 500 lb quarter-scale deck truss bridge joint was built and used in this experimental investigation. The localized sensing area is still observed, but the impedance variations due to incipient damage are slightly different. Nevertheless, by converting the impedance measurements into a scalar damage index, the real-time implementation of the impedance-based technique has been proven feasible.

MONITORING THE INTEGRITY OF COMPOSITE PATCH STRUCTURAL REPAIR VIA PIEZOELECTRIC ACTUATORS/SENSORS

Critical to the success of composite repair of metallic aircraft structures is the integrity of the bond between the base aluminum panel and the reinforcing high-strength composite patch. Monitoring of the repair is equally important to insure the composite patch integrity throughout the service life of the structure. Described in this paper are the test methods and results of a vibration signature-based technique used to qualitatively identify a de-bond on several different composite repair coupons. The high-frequency domain vibration signature from the test coupon is obtained using a single patch of piezoelectric material (Lead Zirconate Titanate or PZT), functioning both as an actuator and sensor. The vibration signature is obtained as a variation in electrical impedance of the piezoelectric patch, while driven by a fixed alternating electric field over a frequency range. The current drawn by the actuator is modulated due to the structures inherent dynamic characteristics. The modulated electrical impedance which is analogous to the frequency response function, but much more easily obtainable, is an indication of vital dynamic structural behavior and is used to identify damage. Damage is simulated by either growing an existing pre-crack under the composite patch through cyclic loading or by creating a de-bond close to the edge of the repair patch. High frequency excitation, which is greatly facilitated by the electrically driven low-power compact PZT patch, is critical to the success of this technique because it assures a clearly visible change in the impedancelvibration signature even for very minor damage/changes. The technique has met with great success in the first stage of this development effort. Even a very minor de-bond or crack growth, has been clearly detected.

High-frequency impedance analysis for NDE of complex precision parts

A high-frequency, impedance-based structural health monitoring technique developed at the Center for Intelligent Material Systems and Structures (CIMSS) has been extended to the NDE of complex precision parts. Like many other non-destructive evaluation (NDE) techniques, this method relies on the detection of change in the dynamic properties of the structure when damage occurs. This NDE technique relies on the measurement of the electrical impedance at high frequencies (100 kHz - 275 kHz) with bonded non-intrusive piezoelectric actuator/sensors. Since the electrical and mechanical impedances of the bonded actuator/sensor are directly coupled, the proposed NDE method is able to detect incipient damage in the structure. Gears were chosen as complex precision parts for the experimental procedure because of their tight tolerances, high quality, and broad use. The goal is to show that incipient damage in the gear teeth, which are an extension of the cylindrical base structure, can be monitored. The most common types of damage in gears, i.e., abrasive wear and bending fatigue, were successfully detected. The impedance measurements before and after damage were converted into a scalar damage metric, which was used to detect the presence of damage when a threshold value was exceeded. Also, quality inspection was successfully demonstrated using the impedance-based technique.

Ballistic Impact Resistance of SMA and Spectra Hybrid Graphite Composites

The effect of adding small amounts 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) and a high performance extended chain polyethylene (ECPE) known as Spectram. 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 and ECPE/SMA layers to the backface of the composite.

Modeling of induced strain actuation of shell structures

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 ...

Impedance-Based Modeling of Actuators Bonded to Shell Structures

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 match the actuator impedance with the structural impedance at the ends of the actuators, which will retain the dynamic characteristics of the actuators. A detailed derivation of the actuator and structural impedance is included. It is found that the actuators 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. Out-of-phase actuation is shown to be the most efficient in exciting the lower order bending modes of shell structures. The paper is concluded with a finite element analysis verification of the derived impedance model.

Effects of temperature on the electrical impedance of piezoelectric sensors

Piezoelectric materials have been known to have temperature dependency regarding their basic properties, such as the dielectric constant and the piezoelectric coefficient. In this paper, this temperature dependency is investigated. The motivation of this work is linked to the impedance-based nondestructive evaluation (NDE) technique which employs piezoceramic (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. The approach used in this paper is empirical due to the complexity of the thermoelectromechanical constitutive modeling. 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. For a PZT PSI-5A, the variation was found to be linear in the 80 degree F to 160 degree F temperature range. To characterize the temperature effects in piezoelectric materials, a temperature coefficient which is independent of frequency has been 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.

Modeling considerations for in-phase actuation of actuators bonded to shell structures

A closed-form model to represent the in-phase actuation of induced strain actuators bonded to the surface of a circular shell is developed. Because of the inherent shell curvature, the equivalent discrete tangential forces generally used to represent the in-phase actuation of the actuators (such as in pin-force models) are not colinear and result in the application of rigid body forces on the shell. This nonequilibrium state violates the principle of self-equilibrium of fully integrated structures, such as piezoelectrically actuated shells. The solution to this nonequilibrium problem is to apply a uniform transverse pressure over the actuator region to maintain equilibrium. Using this adequate equivalent loading scheme for in-phase actuation, a response model for a circular ring is derived based on shell governing equations. To verify the in-phase actuation response model, finite element analysis is performed. A perfect match between the in-phase actuation response model and the finite elements results, when the actuator mass and stiffness are neglected, validates the derived analytical model. If the self-equilibrium is not maintained (point-force model), the predicted deformed shape is completely different from the actual shell response to in-phase actuation. Thus, by simply applying a uniform transverse pressure along with the discrete tangential forces to maintain the self-equilibrium of the shell, the shell response can be modeled accurately.

Passive and Active Tagging of Glass-Fiber Polymeric Composites for In-Process and In-Field Non-Destructive Evaluation

Conventional non-destructive evaluation (NDE) methods are not very effective in monitoring the material conditions of advanced composite and adhesive joints. A technology that has been proposed to enhance the inspectability of advanced composites is the particle tagging technique. Two theoretical models were recently proposed to characterize the dynamic behavior of ferromagnetic and magnetostrictive tagging particles. These theoretical models concerning the development of an active tagging technique with embedded ferromagnetic and magnetostrictive particles and magnetic excitation are now experimentally verified. The experimental results of the active particle tagging shows a variation in the dynamic response of the specimens when defects and/or damage are present. The sensory signature from a tagged polymer is extracted as a result of the interaction between the embedded particles and their host matrix. A study of various types of composites and tagging particles for passive and active tagging was performed. Experimental validation of concepts for tagging of structural materials for on-site inspection prior to installation have also been explored. The on-site particle tagging inspection has been verified on laboratory specimens obtained from industry and was shown to be very efficient.

Special considerations in the modeling of induced strain actuator patches bonded to shell structures

Based on the thin-shell Donnel theory, a model to represent the action of discrete induced strain actuator patches bonded to the surface of a circular cylindrical shell has been developed. The model provides estimates of the curvature 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 cylindrical strain- displacement relations, 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.