A Modified Time Reversal Method for Guided Wave Based Bolt Loosening Monitoring in a Lap Joint

Loss of preload in pretensioned bolts is inevitable. Reliable monitoring of bolt preload is significant to ensure structural reliability and safety. This paper proposes a modified time reversal method for bolt loosening monitoring. In this method, both phase shift and signal amplitude of the focalized wave packet in the reconstructed signal are extracted as tightness indices. To this end, the conventional time reversal method is modified. Initially, for a nominal healthy structure, the conventional time reversal method is performed. The reemitted signal used in this step is recorded as a ‘standard’ reemitted signal. Then, the ‘standard’ reemitted signal is inputted back to the structure for detecting damage. If the structure is damaged, the focalized wave packet in the reconstructed response signal due to the ‘standard’ remitted signal can have both phase shift and peak amplitude difference compared to the focalized wave packet at the healthy state. The main difference between the modified time reversal method and the conventional method is that for all damage cases the proposed method uses the same remitted signal which is recorded for a nominal healthy structure. Numerical and experimental studies are performed to validate the effectiveness of the proposed method for quantitative monitoring of bolt preload in a lap-jointed specimen. The experimental results of the proposed method are compared with those the wave energy dissipation based and the conventional time reversal methods. It is found that the proposed method outperforms the other methods in terms of accuracy and the sensitive range for bolt loosening detection.