Bolt Looseness Monitoring Based On The Peak Frequency Shift Of Piezoelectric Material On The Nut

Bolts,as the main connection and force transmission parts,are widely used in engineering practice.Its safety performance is directly related to the safety of the entire organization.Loose connection of bolts may cause failure of the entire structure and equipment,and may even cause catastrophic accidents.The degree of tightness of bolts and fatigue damage are often difficult to directly judge.To avoid the catastrophic consequences caused thereby,bolts are subjected to real-time Online health monitoring is especially important.For bolt connection device such as an anchor that the bolt is buried in the structure,it is convenient to extract information from the piezoelectric patch which is bonded on the outside surface of the nut.Therefore,in this paper,the bolt connection status is monitored based on the frequency change of the impedance of the piezoelectric patch outside the nut.The main contents are as follows:(1)The theory of electrical admittance for electromechanical coupling structures is theoretically analyzed and the point-by-point dynamic stiffness theory is introduced.Since the dynamic stiffness of the structure will change the resonant frequency of the piezoelectric patch,the change of the axial force of the bolt will affect the change of the dynamic stiffness of the structure,so that the axial force of the bolt and the admittance peak frequency of the coupling structure are established.The relationship provides theoretical support for monitoring the preload of bolts.(2)The finite element simulation of the stress state of the nut reveals the relationship between the stress state of the nut side and the pre-tightening force of the bolt.Combining with the dynamic stiffness theory,a method of monitoring the state of the bolt connection is proposed by pasting the piezoelectric patch on the side of the nut and extracting the peak frequency change of the impedance of the piezoelectric material.(3)The bolt connection status monitoring experimental device was constructed and the corresponding experimental fixture was made.Different experimental schemes were designed to study the influence of different sizes of the piezoelectric material stuck on the side of the nut and different attachment positions to the admittance peak frequency.The relationship between bolt preload and admittance peak frequency was established,and the relationship between the bolt preload and the admittance peak frequency was established.