This paper presents a frequency domain impedance-signature-based technique for health monitoring of an assembled truss structure. Unlike conventional modal analysis approaches, the technique uses piezoceramic (PZT) elements as integrated sensor-actuators for acquisition of signature pattern of the truss. The concept of the localization of sensing/actuation area for damage detection of an assembled structure is presented for the first time. Through a PZT patch bonded to a truss node and the measurement of its electric admittance, which is coupled with the mechanical impedance of the truss, the signature pattern of a truss is monitored. The admittance of a truss in question is compared with that of the original healthy truss. Statistic algorithm is then applied to extract a damage index of the truss based on the signature pattern difference. Experimental proof that over a selected band, the detection range of a bonded PZT sensor on a truss is highly constrained to its immediate neighborhood is presented. This characteristic allows accurate determination of the damage location in a complex real-world structure with a minimum mathematical modeling and numerical computation.

Described in this paper are the details of an automated real-time structure health monitoring system. The system is based on structural signature pattern recognition. It uses an array ofpiezoceramic (PZ1) patches bonded to the structure as integrated sensor-actuators, an electric impedance analyzer for structural frequency response function (FRF) acquisition and a PC for control and graphic display. An assembled 3-bay truss structure is employed as a test bed. Two issues, the localization of sensing area and the sensor temperature drift, which are Critical for the sneess of this technique are addressed and a novel approach of providing temperature compensation using probability correlation function is presentecL Due to the negligible weight and size of the solid-state sensor array and its ability to sense incipient-type damage, the system can eventually be implemented on many types of structures such as aircraft, spacecraft, large-span dome roof and steel bridges requiring multilocation and real-time health monitoring

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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A study of published literature and information from piezoelectric, electrostrictive, and magnetostrictive actuator vendors has been undertaken to establish the mechanical and electrical operating characteristics of the actuators, and to compare them using output energy density criteria. Output energy values of up to 0.666 J can be achieved with off-the-shelf actuators. Energy density per unit volume was found in the range 1.816-7.280 J/dm3. Energy density per unit mass was found in the range 0.233-0.900 J/kg. In one isolated case, higher energy densities of 11.9 J/dm3 and 1.09 J/kg were identified for a small co-fired PMN stack of 0.00481 J total energy. Energy transformation efficiency between input electric energy and output mechanical energy was found to be: 17-27% for adhesively-bonded PZT and PMN stacks, 5-20% for co-fired PZT and PMN stacks, and 67.1% for TERFENOL-D devices. The overall performance of induced-strain actuators based on output energy density criteria was found to vary widely from vendor to vendor, and even from one model to another within the same vendor catalogue list. These variations are attributed to progress being made currently in both the active material technology and in the detailed mechanical construction of induced-strain actuators based on these materials.