The detection of damages by modal analysis and similar vibration techniques depends upon the knowledge and estimation of various modal parameters. In addition to the associated difficulties, such low-frequency dynamic response based techniques fail to detect incipient damages. Smart piezoelectric ceramic (PZT) transducers, which act both as actuators and sensors in a self-analyzing manner, are emerging to be effective in non-parametric health monitoring of structural systems. In this paper we present the results of an experimental study for the detection and characterization of damages using PZT transducers on aluminum specimens. The method of extracting the impedance characteristics of the PZT transducer, which is electromechanically coupled to the host structure, is adopted for damage detection. Three types of damage are simulated and assessed by the bonded PZT transducers for characterization. We present the effectiveness of PZT transducers in the detection and characterization of incipient damages without the need to know the modal parameters. The PZT transducers are found to have a significantly large sensing area for detecting even small incipient damages. The possibility of replicating the pristine state signatures of different transducers under similar conditions of bonding and geometrical location is also explored. For appropriate characterization of damages, a few statistical signature pattern recognition techniques are evaluated.

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The electro-mechanical impedance (EMI) technique, which utilizes 'smart' piezoceramic (PZT) patches as collocated actuator-sensors, has recently emerged as a powerful technique for diagnosing incipient damages in structures and machines. This technique utilizes the electro-mechanical admittance of a PZT patch surface bonded to the structure as the diagnostic signature of the structure. The operating frequency is typically maintained in the kHz range for optimum sensitivity in damage detection. However, there are many impediments to the practical application of the technique for NDE of real-life structures, such as aerospace systems, machine parts, and civil-infrastructures like buildings and bridges. The main challenge lies in achieving consistent behavior of the bonded PZT patch over sufficiently long periods, typically of the order of years, under 'harsh' environment. This necessitates protecting the PZT patch from environmental effects. This paper reports a dedicated investigation stretched over several months to ascertain the longterm consistency of the electro-mechanical admittance signatures of PZT patches. Possible protection of the patch by means of suitable covering layer as well as the effects of the layer on damage sensitivity of the patch are also investigated. It is found that a suitable cover is necessary to protect the PZT patch, especially against humidity and to ensure long life. It is also found that the patch exhibits a high sensitivity to damage even in the presence of the protection layer. The paper also includes a brief discussion on few recent applications of the EMI technique and possible use of multiplexing to optimize sensor interrogation time.

In the beginning, the electro-mechanical (EM) impedance method for structural health monitoring was recognized as a means of structural in-situ stress monitoring and measurement. Consequently, theoretical analysis based on the EM impedance method as a tool for in-situ stress identification in structural members was presented. A dynamic impedance model derived from the Euler-Bernoulli beam theory was developed to investigate the influence of in-situ stress on the dynamic and electro-mechanical response of a smart beam interrogated by a pair of symmetrically bounded, surfacebonded piezoceramic (PZT) transducers. Numerical simulation was performed for a laboratory sized smart beam subjected to a multitude of axial loads at the ends. It was found that natural frequency shifts takes place in the presence of in-situ stress. Furthermore, these shifts, which are linearly related to the magnitude of applied load, is directly reflected in the point-wise dynamic stiffness response. However, in terms of the electro-mechanical response, which can be measured directly, the shift of peaks of the EM admittance signature is not directly indicative of the natural frequency shifts. This arises as an inverse problem in engineering, which cannot be deciphered using direct approach. Back calculation of the in-situ stress using genetic algorithm (GA) was proposed.

The electromechanical (EM) impedance method is emerging as an effective tool for structural damage detection. Damage is detected by changes in the EM impedance signatures of the smart piezoelectric transducer bonded on the structure. The damage quantification has so far been restricted to using non-parametric statistical indices to measure changes in the signatures. Such measures, although simplistic, fail to correlate the changes in the signatures to physical parameters of the structures. Thus, although effective in detecting the presence of damage, the method fails to give further information about the location and severity of the damage. In this paper, the EM impedance method integrated with a finite element (FE) model is presented as a means for characterizing damage growth. Damage growth is characterized by quantifying the changes in the natural frequency shifts of the structure extracted from the EM admittance signatures. A new damage characterization index is derived, which is experimentally validated to be capable of distinguishing a localized increase in severity from damage propagation through the structure.