Structural health monitoring of piezoelectric structures by estimating energy conversion efficiency with capacitance measurements

In structures equipped with piezoelectric materials, the quantification of the efficiency of the energy conversion between mechanical and electrical domain (and vice versa) was already demonstrated to be a reliable feature for determining the presence of a structural damage. Usually, this efficiency is determined by means of vibration measurements to derive the value of the modal electro-mechanical coupling coefficient, which is the damage feature employed to calculate damage indexes. Nevertheless, in some cases, the estimation of this damage feature can be challenging because of different reasons, such as, e.g., non sufficient external excitation to the system, low values of the modal electro-mechanical coupling coefficient which makes its estimation uncertain and implies the need of shunting the piezoelectric elements with tailored electrical impedances. The present paper aims at showing that similar damage indexes can be estimated also by measuring capacitance trend as function of frequency for each piezoelectric element in the structure. This allows monitoring the structure without any need of vibration measurements and addition of external electric circuits used to shunt the piezoelectric elements, with significant simplification of the estimation procedure. Three different damage indexes are proposed, evidencing which of them are the most reliable for detecting the presence of the damage and getting information about its location. The method and the indexes are studied through numerical analyses and validated by means of an experimental campaign on a tailored set-up.