High-Strength Bolt Pre-Tightening Force Monitoring Based On Time Reversal Method

As one of the main connecting modes in steel structures,the high-strength bolts are widely used in long-span bridges,high-rise buildings and other engineering fields.They are easy to install and tight for connection.The high-strength bolts are able to withstand large loads with detachable and replaceable advantages.However,due to the operational environments and time-varying load during the long-term service,the high-strength bolts suffer from the pre-tightening force failure and even the loosening and shedding,which seriously threaten the safe operation and service life of the structures.Therefore,it is of great practical significance to monitor the preload tension status of high-strength bolts.Due to the advantages of strong penetration ability,long propagation distance,high sensitivity and fast detection speed of ultrasonic,the ultrasonic-based techniques can be used to detect or monitor the health status of high-strength bolt connections.To make use of the adaptive focusing ability of the time reversal method that helps to improve the signal-to-noise ratio of ultrasonic signals propagating in complex structures,this thesis proposes a time reversal-based method for high-strength bolt pre-tightening force monitoring.The main contents of this thesis are listed as follows:(1)The state of the arts review on high-strength bolt pre-tightening monitoring,piezoelectric ceramic ultrasonic wave,time reversal method and its application in the field of structural health monitoring are carried out.The basic principle of ultrasonic wave and piezoelectric sensor are summarized focused on the energy transfer path and process of ultrasonic wave.(2)A time reversal-based method for high-strength bolt pre-tightening force monitoring is proposed.The piezoelectric ceramic is used as exciter and sensor.The reversal signal is obtained by the time reversal processing and emit again as the excitation signal.After propagation along the same path,the focusing signal can be obtained.It is demonstrated that the signal-to-noise ratio of ultrasonic signals can be effectively improved by using time reversal method.The pre-tightening force monitoring of high-strength bolts can be herein achieved by establishing the relationship between the peak value of focusing signal and the pre-tightening force.(3)Numerical simulation of the focusing property of time reversal method is carried out using the spectral element method.Taking the simple rod structure as an example,the feasibility of using ultrasonic time reversal for structure damage identification is verified.It provides the basis for the theoretical and experimental study of pre-tightening force monitoring of high-strength bolts.(4)The proposed pre-tightening force monitoring method for the high-strength bolt connection is verified through the experiments in the laboratory.The piezoelectric ceramic sensors are installed at different locations on the high-strength bolt connection specimen to collect the ultrasonic signals that are processed by the time reversal method.The experimental results show that the time reversal focusing signals has obvious focusing peak value relative to the response signal.The peak value of the focusing signal decreases with the decrease of the pre-tightening force of the high-strength bolt,which is more sensitive to the change of the pre-tightening force.It is further demonstrated that the relationship between the normalized focusing peak value and the pre-tightening force of high-strength bolt is not sensitive to the propagation distance of ultrasonic signals.The proposed high-strength bolt pre-tightening force monitoring method is proved to be a high potential for practical applications.