Abstract
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.
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