The aim of this paper is to show that Lamb waves may be effectively generated using piezoelectric transducers embedded inside a composite plate, for nondestructive evaluation and health monitoring applications. A cylindrical transducer embedded in a composite host plate is considered. The electrical impedance of the transducer alone in vacuum and then of the embedded transducer, which allows the identification of the resonance modes, have been obtained by the finite element method (FEM). Moreover, the displacement fields in the plate, which allow the identification of the types of Lamb waves, have been computed at the resonance frequencies. Comparison between the FEM results and the Lamb wave dispersion curves of the host material are in good agreement. Experimental results (electrical impedance, frequency response, and phase velocities) concerning a composite plate specimen containing the same piezoelectric transducer inside it are shown. A good agreement is generally obtained between numerical and exper...

Piezoelectric transducer embedded in a composite plate: Application to Lamb wave generation

Smart structures based on carbon fibre reinforced polymers with embedded piezoceramic patches demand and offer permanent and integral monitoring of structural and functional integrity. Electrical impedance spectroscopy is one method that can be implemented in a structural health monitoring system. Experimental investigations with strip-shaped specimens show the close connection between mechanical properties and impedance. An electrical equivalent circuit diagram and electromechanical finite-element modelling explain the general shape of the impedance spectrogram and the peaks due to the excited eigenmodes of the structure. Damage, caused by low-speed impact, is experimentally detected by changes in the impedance resonance peaks. The finite-element model confirms this theoretically. The influence of disturbing factors on the impedance such as ageing effects, mechanical loading and different electrical contacting is also considered.

https://iopscience.iop.org/article/10.1088/0964-1726/10/4/328/pdf

Damage detection in smart CFRP composites using impedance spectroscopy

Smart materials based on carbon-fibre-reinforced plastic (CFRP) with embedded PZT sensors and actuators are expected to be a favourite composite for vibration damping and noise reduction. Due to the wide variety of physical properties of the components, various damage mechanisms may reduce or even remove the sensing and actuating capabilities of the piezoceramic material. Comprehensive non-destructive characterization and integral health monitoring help to optimize the structure and its manufacturing and are essential prerequisites to ensure performance and availability of smart components during their lifetime. The first part of the paper presents high-resolution non-destructive imaging methods including microfocus x-rays, ultrasonics and eddy currents. These methods are used to characterize damage resulting from non-optimal manufacturing and external load. The second part is dedicated to newly developed imaging techniques using the active piezoceramics as transmitters of acoustic, electromagnetic and thermal fields. The third part focuses on health monitoring by impedance spectroscopy using the same piezoceramics as for vibration damping. Electromechanical finite-element modelling and experimental investigations of strip-shaped specimens have shown the close connection between mechanical properties and electrical impedance.

https://iopscience.iop.org/article/10.1088/0964-1726/12/6/019/pdf

Non-destructive characterization of smart CFRP structures

When a piezoelectric element is inserted into a medium, it is now well-established that the viscoelastic properties of this medium can easily be deduced from the measurement of the electrical impedance of this element and by using an appropriate optimization algorithm. This paper essentially deals with the influence of bonding defects at the interface between the piezoelectric sensor and the tested medium. First, a model is proposed to calculate the modifications of the electrical impedance due to defects of this type. The validity of this approach is demonstrated by comparing the theoretical predictions with the results obtained experimentally for different samples with well-defined bonding defects. Then, with this model, it is easy to determine the electrical impedance of an embedded piezoelectric element, whatever the value of the relative area on which the bonding defect occurs. In order to simulate what will happen in practice when we ignore that these defects are present, such simulated electrical impedances are processed with the usual optimization algorithm, assuming perfect bonding. Obviously, the viscoelastic properties of the tested medium are subject to error, as the size of the non-bonded area increases. It is shown that a very efficient tool based on the Kramers - Kronig relationships can be used to check the correctness of the extracted parameters.

I. Perrissin-Fabert

Papers

Simulated and experimental study of the electric impedance of a piezoelectric element in a viscoelastic medium