Bolt looseness monitoring based on damping measurement by using a quantitative electro-mechanical impedance method

In-situbolt looseness monitoring is very important for the stability and safety of engineering structures. The electro-mechanical impedance (EMI) method, which had been intensively studied in structure health monitoring, has offered a feasible solution. However, in traditional EMI method, the lack of modal pre-selection leads to dense resonant peaks which brings difficulty to signal analysis. The commonly used statistical damage indices typically do not have physics meaning and may fail to distinguish the defects from the environmental changes. In this work, a quantitative EMI (Q-EMI) method was proposed to measure the damping of structures for bolt looseness monitoring. The in-plane excitation method based on distributed thickness-shear-mode piezoelectric transducers was used for selective excitation of vibration modes. The modal-related sensitivity variation was theoretically analyzed and then validated by experiments. Three different specimens were tested and the results showed that during bolt loosening, the modal damping increased by 100%–300%, while the resonance frequency of the structure only decreased by several percent. Moreover, it was found that the loosened bolt may be located using the variation of modal-related sensitivity. Due to its good performance and easy mounting, the proposed Q-EMI method is very promising for bolt looseness monitoring and is expected to be widely used in near future.