Fracture Monitoring Of High Strength Bolts Based On Acoustic Emission Technology

Steel structure has the advantages of high strength,good seismic performance,easy construction and environmental protection,so it has become the preferred structural form in large-scale civil engineering infrastructure construction.High strength bolt is an important connection mode and key force transmission component of steel structure joints.Due to stress corrosion and other reasons,high strength bolts in long-span bridges are often found to be broken and missing.The fracture of high-strength bolts will cause great security risks,and the sudden fracture of a large number of bolts will lead to the instability and even collapse of the steel structure system.Therefore,timely detection and replacement of high-strength bolts is of great significance to the safe operation of steel structures.Although researchers have reached a certain consensus on the fracture mechanism of high-strength bolts,there is no effective measure to early warn the fracture of high-strength bolts.In practical engineering,it is basically by manual inspection to find the broken bolts and then replace them.As a dynamic nondestructive testing technology,acoustic emission technology is often used to monitor the acoustic emission phenomenon of material damage.However,there is no in-depth study on the AE characteristics of high-strength bolts before fracture,especially in the stress corrosion environment.Firstly,the monotonic tensile test of high strength bolt with notch is designed to reveal the acoustic emission signal characteristics of high strength bolt before fracture;Then the stress corrosion fracture test of high-strength bolt is carried out,and the sound emission technology is used to monitor the health state of high-strength bolt in the test.The main research contents are as follows(1)Through monotonic loading tensile test,the acoustic emission signals of the whole process of bolt fracture were obtained.Based on the principle of acoustic emission,the signal characteristics of high-strength bolt fracture are analyzed,and the fracture process of highstrength bolt is divided into three damage stages.Through the analysis of cumulative impact experience diagram and parameter distribution diagram of acoustic emission signal,the damage stage of high strength bolt can be identified qualitatively.Then,combined with waveform analysis,the energy of each frequency band of the signal is extracted by wavelet packet decomposition.On this basis,the damage mechanism of high-strength bolt is introduced,and the damage index of high-strength bolt is put forward to quantitatively identify the fracture damage stage of high-strength bolt,so as to provide early warning for the fracture of highstrength bolt.(2)Through the stress corrosion test,the acoustic emission signals of the whole process of stress corrosion fracture were obtained.Based on the characteristics of AE parameters in corrosion damage,the AE signals are clustered by K-means.According to the mechanism of metal stress corrosion,the corresponding relationship between AE signal types and AE sources is established.According to the specific signal,the corrosion damage stage of high strength bolt is divided;Based on the frequency of signal occurrence,the dynamic threshold of dimensionless index acquisition is proposed;Through real-time processing of acoustic emission data,the damage stage of corroded high-strength bolt can be identified,which can provide early warning for stress corrosion fracture of high-strength bolt.