High-strength Bolt Shear Connection Monitoring Based On Acoustic Emission

In recent years,high-strength bolts have been widely used in civil engineering structures such as long-span bridges and super-tall buildings.As the main force transfer component of the joint connections,the high-strength bolt is easy to loose and lose the pretension force and even causes the screw to fall off,fracture and damage during its long-term service.It seriously affects the safety of the structure.Therefore,the on-line monitoring of high-strength bolt connection status has the important theoretical significance and practical engineering value.In view of the condition monitoring and failure mode identification requirements of high-strength bolt connections,this thesis implements the acoustic emission technology to monitor the shear failure of high-strength bolts.The mechanical behaviors and acoustic emission sources of the high-strength bolt are investigated during the four stages of shear failure process,respectively friction force transmission,relative sliding,screw elastic deformation and screw plastic deformation.Through the parameter analysis,spectrum analysis and time-frequency analysis,the characteristics of acoustic emission signals in different stages are extracted.An index is established to identify the friction failure of high-strength bolt shear connection.The main contents of this thesis are listed as follows:(1)The current research status of high-strength bolt state detection and monitoring,as well as the application and development of acoustic emission technology in bridge engineering are summarized.The basic principle of acoustic emission technology and common signal processing methods of acoustic emission signal are introduced.(2)The typical mechanical behavior and failure mode of shear connection of highstrength bolts are analyzed.The parameters of shear fracture test specimens of highstrength bolts by finite element simulation are determined.The micromechanical behavior of high-strength bolt in each shear stage and the main acoustic emission sources in each stage are studied.(3)The acoustic emission signals generated by different micromechanical behaviors in different stages are obtained by the screw shear failure experiments of high-strength bolt connections.Through the analysis of signal waveform characteristic parameters and spectrum analysis,it is found that there are different acoustic emission sources in each shear stage.But it is difficult to establish a quantitative index to accurately distinguish different types of acoustic emission sources and different stages of high-strength bolts.(4)The feature extraction of acoustic emission signals for high-strength bolts is carried out based on the wavelet transform.Firstly,the wavelet base parameters are selected by Shannon entropy optimization,and the acoustic emission signals at different stages of the shear connection are analyzed.Then,the characteristic frequency bands that can distinguish different acoustic emission sources are proposed,which are 100-280 k Hz and 280-700 k Hz respectively.Finally,according to the statistical analysis results of a large number of signals,the identification index of shear friction failure is established to realize the monitoring of high-strength bolt connection status.