In recent years, the piezoelectric-ceramic (PZT) patches are increasingly been used as impedance transducers for non-destructive evaluation (NDE) of structures. In this application, the electrical admittance of a PZT patch surface bonded to the structure is utilized as a diagnostic signature of the structure. The operating frequency is typically maintained in the kHz range for optimum sensitivity in damage detection. The electro-mechanical interaction between the host structure and the bonded patch is key to the detection of damage in this NDE technique. Although the method is well established for a wide variety of structures and material types, very little research has focused on the fundamental structure-PZT interaction. This paper reviews the fundamental electro-mechanical coupling between the structure and the PZT patch and introduces a new concept of 'active' signatures, whereby it is possible to utilize the direct interactive component of the signature for NDE afte filtering the 'inert' component. Consequences of this concept, which include increased sensitivity to damage and reduced influence of temperature fluctuations on signatures are highlighted.
Detection of damages and progressive deterioration in structures is a critical issue. Visual inspections are tedious and unreliable. Incipient damages are often not discernible by low frequency dynamic response and other NDE techniques. Smart piezoelectric ceramic (PZT) transducers are emerging as an effective alternative in health monitoring of structures. The electro-mechanical impedance method employs the self-actuating and sensing characteristics of the PZT, without having to use actuators and sensors separately. When excited by an ac source, the PZT transducers bonded to the host structure activates the higher modes of vibration locally. Changes in the admittance response of the transducer serves as an indicator of damage around the transducer. In this paper, the effectiveness of PZT transducers for characterizing damages in concrete, in terms of the damage extent and location, is experimentally examined. The root mean square deviation (RMSD) index, adopted to quantify the changes in the admittance signatures, correlates with the damage extent. The damages on the surface that is not mounted by the PZT are also discernible. An array of transducers proves effective in detecting the damaged zone. The progressive incipient crack can be detected much before it actually becomes visible to the naked eye.
Modal analysis based damage detection techniques using only first few modes are not sensitive for damage identification. The sensitivity of the modal parameters to damage is greater at the higher modes of vibration. Yet, actuation of structures at high frequencies is very difficult with the conventional modal testing methods. In this paper, a new technique that uses smart piezoelectric (PZT) material to extract the modal frequencies for higher modes of vibration is presented. A PZT transducer possesses simultaneous actuating and sensing capabilities. The electromechanical (e/m) impedance method exploits this feature of the PZT transducer to measure its drive-point impedance characteristics when bonded to a structure. Damage location is identified using the natural frequency shifts obtained from the structural impedance signatures and the corresponding undamaged state modes shapes. This technique is superior to other methods, which rely only on statistical quantification of changes in the measured structural signatures. The damage locations were successfully identified by this method for a finite element simulated beam model. The natural frequencies obtained experimentally for longitudinal and bending modes were fairly consistent with the analytical predictions. However, the modeling of damage as merely a source of stiffness reduction proves insufficient to accurately estimate its location, experimentally.
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, surface-bonded 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 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 long-term 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.