Experimental study on multi-bolt connection pre-tightening torque testing based on impedance method

Small satellites have much higher requirement of rapid response, but they contain a large number of bolted connections. Fast detection of bolt tightening torque after completing assembly is of great significance to improve the fast response of small satellites. In order to detect the bolt preload torque in the multi-bolt connection structures, the electromechanical impedance method is used to study how to detect the bolt pre-tightening torque in the two-bolt lap joint. The loosening of bolts leads to structural mechanical impedance changes. The electrical impedance of the macro-fiber composite (MFC) on the surface of structure changes due to the electromechanical coupling effect. According to the above principle, the MFC was pasted on the surface of the lap beam. When the torque on the bolt is reduced, thelmpedance curve of the MFC is measured by the impedance analyzer to search the sensitive frequency range. In the above sensitive range, the root mean square deviation (RMSD) of the real part of the impedance was calculated when the bolt is loose and in the healthy state. The experimental results show that the peak and its frequency of the real part of the impedance curve changes obviously with the loosening of the two bolts, and the RMSD gradually increases. A database containing the RMSD of the real number of the impedance and the known operating conditions is established by comparing the unknown conditions of the RMSD so as to determine the bolt loosening situation, and this method is also verified by experiments. Therefore, the electromechanical impedance method, which uses MFC as piezoelectric sensors, can successfully detect whether the two bolts of the lap joint are loose or not.