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.

The electro-mechanical (EM) impedance method is gradually emerging as a widely accepted technique for structural health monitoring and systems identification. The method utilizes smart piezoceramic (PZT) transducers intimately bonded to the surface of a structural substrate. Through the unique electro-mechanical properties of the PZT transducers, the presence of damage, as well as the dynamical properties of the host structure are captured and reflected in the electrical admittance response. In the present work, the effect of the bond layer on the electro-mechanical response of a smart system is being studied. Experiments with the EM impedance method were performed on laboratory-sized beams. Consequently, the effects of shear lag due to the finite thickness bond layer were successfully identified. This was followed by the theoretical analysis of shear lag effects. It was found that the induced strain behaviour of the structural specimen in question is inevitably modified by the presence of shear lag between the PZT transducer and the structural substrate. Subsequently, the EM admittance response of the beam specimens were simulated based on the results gathered from the theoretical analysis. Incidentally, it was found that the theoretical model clearly depicts the trends of the measured response.

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Modal analysis based damage detection techniques depend upon the accurate estimation of the changes in natural frequencies, damping ratio and mode shapes of a structure that occur with damage. But, even a large damage in a structure reflects only small shifts of the lower natural frequencies. At higher modes of vibration perceptible shifts can be observed even for occurrence of incipient damages. Yet, it is impractical to extract modal parameters at higher modes using conventional modal testing techniques. With the emerging smart piezoceramics (PZT) based damage detection technology, impedance response functions of the structure can be obtained in the higher frequencies ranges. The electromechanical admittance signature obtained in this method is a coupling of the impedances of the bonded PZT transducer and the host structure. In this paper, a unique method for damage detection that blends the modal analysis techniques with the emerging e/m impedance technique is presented. A finite element model of a cantilever beam is adopted to simulate damage and to demonstrate the possible damage identification by this method.