Electromechanical Impedance Modeling for Adhesively Bonded Piezo-Transducers

Abstract

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.

Modelling Shear Lag Phenomenon for Adhesively Bonded Piezo-Impedance Transducers

“…Presented in the following sections is a detailed review of the shear lag mechanism inherent in adhesively bonded PZT patches and its rigorous integration in 1D and 2D impedance models, as proposed by Bhalla & Soh (2004d). Further, a new simplified model model proposed by Bhalla et al (2009), which is especially suitable for solving the inverse problem (of extracting Z s ), considering the presence of bond layer, is also described.…”

“…The bond layer is assumed to be connected in between the PZT patch and the host structure such that it can transfer the force between the two through pure shear mechanism. Unlike the previous model of Bhalla & Soh (2004d), where shear strain varied along the patch, an average shear strain uniform along the length has been considered as a simplification. Let u p be the displacement at the tip of the PZT patch at any point of time.…”

“…In addition, since frequencies of the order of 30-400 kHz are involved in the EMI technique, quasi-static approximation (for calculating l eff ) is strictly not valid. The next section presents the model of Bhalla & Soh (2004d) to alleviate all the above shortcomings.…”

“…Replacing Z s, by Z s,eq in Eq. (7), the modified expression for admittance across the PZT patch can be obtained as in the case of the model of Bhalla & Soh (2004d).…”

Skin-Spar Failure Detection of a Composite Winglet Using FBG Sensors

“…The application of these methods is limited by the detection of only big damage, since a little damage cannot introduce considerable changes in the structure modal parameters. Conversely, the "local scale" methods allow for detecting a damage also when it is still of little dimensions, but they require a consistent number of sensors in order to guarantee adequate coverage of the whole system [8][9][10][11]. The number of sensors can drastically get off if the FBG sensors are used thanks to their capabilities to monitor more areas with a single optical fiber.…”