EMI Method Used in Structural Health Monitoring: A Literature Review

Structural health monitoring (SHM) allows damage to the structure or its components to be identified at an early stage by self-diagnosis with the aid of sensors. The usage of SHM enables safe engineering and reduces the maintenance costs of the industry. The sensors used in SHM are called piezoelectric wafer active sensors (PWAS) as they can simultaneously interrogate. PWAS transducers are widely used in mechanical engineering and civil engineering, especially aviation and space engineering. One of the most widely used methods in structural health monitoring is the Electromechanical Impedance (EMI) method. In this method, structural health is monitored by lead zirconate titanate (PZT) ceramic sensors embedded or bonded with the aid of high strength epoxy adhesive. By using the impedance analyzer, the characteristics of the real parts of impedance and admittance are obtained. The impedance analyzer implements an alternating voltage signal to the bonded / embedded PZT converter over the predefined frequency range. The magnitude and phase of the steady-state current are recorded directly as conductance and resistance characteristics without any conversion. These characteristics are used as a reference in assessing structural health. Whenever it is desired to assess the health of the structure, these characteristics are remeasured and compared with the reference characteristics. In this way, it can easily be determined whether there is a change in the structure. Ideally, the change in characteristics should only be observed due to the deterioration of the health situation. However, it is possible that the PZT characteristics may change due to other reasons such as aging, mechanical constraints, electrical constraints, thermal constraints and temperature fluctuations. In this study, a literature search was conducted for the electromechanical impedance method used in structural health monitoring. Advantages and disadvantages of the method have been examined.