Electromechanical impedance method for the health monitoring of bonded joints: Numerical modelling and experimental validation

The electromechanical impedance (EMI) method is one of the many nondestructive evaluation approaches proposed for the health monitoring of aerospace, civil, and mechanical structures. The method consists of attaching or embedding one or more wafer-type piezoelectric transducers (PZTs) to the system of interest, the host structure, and measuring certain electrical characteristics of the transducers. As these characteristics are also related to the impedance of the host structure, they can be used to infer the mechanical properties of the monitored structure. In the study presented in this paper, we utilize the EMI to monitor the quality of adhesively bonded joints. A finite element formulation was implemented to predict the electromechanical response of PZT attached to simple adhesively-bonded joints. The model was coded in Matlab and its results were validated by comparing them with the results associated with a few case studies implemented using commercial software. The finite element code was used to model the EMI measurement of a simple adhesively bonded joint subjected to adhesive layers of different qualities. Finally, the EMI was validated experimentally. A low cost circuit was designed and assembled to measure the electromechanical characteristics of a PZT glued to the structure of interest. We monitored the curing process of the adhesive forming the joint. This simulates at large the inverse of the degradation that may occur in bonded joints. The experimental results show that the EMI technique is able to capture variations of the stiffness of the adhesive layer