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A high-frequency-electrical-impedance-signature-based technique for structural integrity monitoring is presented. The technique which has been under investigation at the Center for Intelligent Material Systems and Structures at Virginia Tech for the last 18 months is unique and different from conventional non-destructive damage identification and structural integrity monitoring methods. It relies on tracking the high-frequency (typically > 50 kHz) point impedance of the structure to identify damage. At such high frequencies, the technique is comparable in sensitivity to sophisticated traditional NDE techniques, such as ultrasonics, and is capable of qualitatively detecting incipient-type damage by looking at changes in structural impedance. As yet, it can be implemented in a remote sensing scenario with small non- intrusive piezoelectric (PZT) materials. The structure's high-frequency electrical impedance signature, which is functionally equivalent to its mechanical impedance signature, is obtained through a bonded PZT functioning both as actuator and sensor. A statistic algorithm based on the difference in the electrical impedance of a healthy and a damaged structure, is then applied to extract an index of the health of the structure. High-frequency excitation, which is greatly facilitated by the electrically driven low-power compact PZT patch, assures a clearly visible change in the impedance/vibration signature even for very minor damage/changes. It also limits the actuation/sensing area to a small region, `local-area', close to the PZT patch. Because of the limited actuation/sensing area, the impedance signature is affected only by changes in the structural properties close to the sensor-actuator and is insensitive to changes in far-field boundary conditions, mass loading, etc., which may be part of the normal usage of the structure. As a case study, an application to a real complex aircraft structure is presented. Experimental proof that very minor alterations in the structure are easily identified and the fact that the detection range of the bonded PZT actuator/sensor is constrained to its immediate neighborhood is presented.
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