Electro-Mechanical Impedance Assessment of a Bolted Circular Plate Element of a Space Structure

Electro-mechanical impedance diagnostic is one of key structural health monitoring approaches in aerospace structures. Considerable number of studies have demonstrated its efficiency in monitoring bolted joints. This investigation focuses on effect of a bolted boundary on the electro-mechanical impedance response of the space structure. Many space vehicles incorporate cylindrical payloads featuring multiple plates connected with threaded rods. Position of nuts on the threaded rods determine layered structure of the payload. Because of the cylindrical configuration of the payload, internal layers are formed by circular plates bolted to the connection rods. The number of connection rods determines the number of bolted boundary conditions around plate’s circumference. In this case, the boundary of the plate is essentially a mix of bolted and free segments and is not associated with a classical boundary condition. It is suggested that this case may be represented by an elastic boundary conditions with boundary stiffnesses depending on torque applied to each bolted joint. Vibrations of a circular plate with indicated complex boundary conditions were studied in this contribution theoretically and experimentally. As a result of numerical studies, a range of stiffnesses was suggested to model the bolted boundary. An analytical expression for the electro-mechanical impedance of a circular plate was presented and was utilized in the calculation of the response of a circular plate with the complex boundary. Structural damage was modeled as deviation of the stiffness associated with the bolted joint. Experimental studies were carried out to validate results of theoretical investigations. Electro-mechanical impedance signatures of the circular plate with an attached piezoelectric active sensor were collected for different sets of boundary conditions representing theoretical scenarios. Effect of the compromised bolted joint on the electro-mechanical impedance response of the whole circular plate was explored and the analysis of changes due to different conditions of the bolted boundary was provided.