| High strength bolts are widely used in bridges and other civil engineering structures due to their convenient installation,disassembly,good mechanical performance and high reliability.As the main force transfer component at the joint,high-strength bolts will produce preload loss under the action of earthquake,automobile and other dynamic loads,and even lead to the destruction of bolts or connectors,which brings huge security risks to the operation of the structure.Therefore,the connection condition monitoring of highstrength bolts has theoretical significance and engineering application value.In this paper,the acoustic emission technology is used to monitor the shear failure and bearing failure process of high-strength bolt connection under cyclic load.The mechanical behavior and acoustic emission mechanism of the structure in each stress stage of the two failure modes are studied,and the acoustic emission signal characteristics in different stress stages are extracted.Finally,the identification method of bolt friction failure and plate yield based on time-frequency energy distribution coefficient is established.The main contents of this paper are as follows:(1)This paper summarizes the research status of high-strength bolt connection condition monitoring,as well as the application and research of acoustic emission technology in structural health monitoring,and introduces the basic principle of acoustic emission and common signal processing methods.(2)The shear failure and compression failure of high-strength bolt specimens were simulated by finite element method,and the mechanical characteristics of the two failure modes were compared,which were used for the design and analysis of the experiment.Then,the cyclic loading experiments of high-strength bolt specimens were carried out to obtain the acoustic emission signals of four stress stages of shear failure mode friction,slip,screw elasticity and screw plasticity,and four stress stages of compression failure mode elasticity,yield,strengthening and necking.The main AE sources of shear failure are asperity wear and plastic deformation and fracture of screw;the main AE sources of pressure failure are asperity wear and plastic deformation and fracture of plate.(3)Through parameter analysis and spectrum analysis,it is found that the relationship between the duration and amplitude of AE signal and the peak frequency of AE signal in each stress stage of the two failure modes present different characteristics.Under the shear failure mode,the signal amplitude in the friction stage is generally 40-70 d B,and the peak frequency is mainly concentrated in 100-150 k Hz,while the signal amplitude in the later stage is generally 40-80 d B,and the peak frequency is 300-320 k Hz,which also has higher energy.Under the bearing failure mode,the signal amplitude of elastic stage is generally40-65 d B,and the peak frequency is mainly concentrated in 100-170 k Hz,while the signal amplitude of later stage is generally 40-80 d B,and the peak frequency is 460-480 k Hz,540-600 k Hz,and the energy proportion increases.(4)Based on the time-frequency analysis of acoustic emission signals in each loading stage,a state identification method of high-strength bolts based on time-frequency energy distribution coefficient is proposed.Firstly,the wavelet basis is selected by using the correlation coefficient and the number of decomposition layers is determined.Then,the radar image of the energy distribution coefficient of wavelet packet component is established.According to the statistical analysis results of the radar image center coordinate,the identification index of the stress state of high-strength bolt is established.When the center coordinate is below the straight line y=-3.091x-0.164,the specimen will be in friction failure,resulting in relative slip.When the center coordinate is below the straight line y=-2.343x-0.047,the specimen will reach the yield limit,so as to realize the monitoring of high-strength bolt connection state. |
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