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.

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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 structure's 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.t~e s e a r c h Scientist, Member AIAA, ASME. f~r a d u a t e

This paper presents a qualitative health monitoring technique to be used in real-time damage evaluation of massive complex structures such as bridge joints. The basic principle of the technique is to monitor the structural mechanical impedance which will be changed with the presence of damage. The mechanical impedance variations are monitored by measuring the electrical impedance with a piezoelectric actuator/sensor. 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 (generally above 100kHz), 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 only been applied successfully to a variety of light structures. However, the usefulness of the NDE technique for massive structures is uncertain and needs to be investigated. For this purpose, a 500-LB, '/4-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 metric, the real-time implementation of the impedance-based technique has been proven feasible.

No abstract