Suresh Bhalla

Electromechanical Impedance Modeling for Adhesively Bonded Piezo-Transducers

The electromechanical impedance (EMI) technique for structural health monitoring (SHM) and nondestructive evaluation (NDE) employs piezoelectric-ceramic (PZT) patches, which are surface bonded to the monitored structures using adhesives. The adhesive forms a finitely thick, permanent interfacial layer between the host structure and the patch. Hence, the force transmission between the structure and the patch occurs through the bond layer, via shear mechanism, invariably causing shear lag. However, the impedance models developed so far ignore the associated shear lag and idealize the force transfer to occur at the ends of the patch. This paper analyses the mechanism of force transfer through the bond layer and presents a step-by-step derivation to integrate the shear lag effect into impedance formulations, both one-dimensional and two-dimensional. Further, using the integrated model, the influence of various parameters (associated with the bond layer) on the electromechanical admittance response is studied by means of a parametric study. It is found that the bond layer can significantly modify the measured electromechanical admittance if not carefully controlled during the installation of the PZT patch.

Effects of adhesive on the electromechanical response of a piezoceramic-transducer-coupled smart system

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 behavior 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.

Damage detection in concrete structures with smart piezoceramic transducers

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.

Incipient damage localization with smart piezoelectric transducers using high-frequency actuation

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.

An impedance-based piezoelectric-structure interaction model for smart structure applications

Publisher Summary This chapter presents the theoretical development and the experimental verification of a new simplified approach to model the multiphysics dynamic interaction between the PZT patches and their host structures in the smart systems. The mechanical coupling between the patch and the structure is modeled in two dimensions to accurately consider the planar vibrations. The derived equations are simple enough to be directly utilized for extracting the mechanical impedance of an unknown structure from the experimental signatures of a PZT patch surface bonded to it. This is an improvement over the existing models whose complexity prohibits direct application in similar practical scenarios. The presented formulations are experimentally verified by means of test on a smart system comprising an aluminum block instrumented with a PZT patch.