Research On Detection Of Fastening State In Bolted Joints Using Vibrationacoustic Modulation Technology

Bolted joints are widely applied in various structures and equipment of different industries. The health status has a significant impact on safety and reliability of the working condition. Therefore, tightening degree detection of bolted joints is quite important. Traditional methods rely mainly on manual repair, which is very inefficiency. While vibration-acoustic modulation(VAM), a nonlinear acoustic detection technique, is sensitive to contact-type defects and it is easy to realize rapid online detection. So VAM technique is chosen to detect bolted joints in this paper and positioning of loose bolts is finally realized.At present, there are various theory models of VAM technique abroad. ‘Openclose theory’ is chosen to be the basic theory for further research. It is considered that low-frequency vibration makes contact interface open and close. In this case, high-frequency ultrasonic passing through the interface will be modulated. According to this theory model, loosening index(LI) is defined to estimate the health status of bolted joints and the calculation method is offered. Experimental results shows that LI can be used to reflect the connection condition very well.AS VAM technology is still in the research stage, there is no advanced equipment now. Therefore, hardware and software experimental platform is built to achieve VAM detection according to the research demand. The hardware platform realizes the excitation of high-frequency and low-frequency signals, signal transmission and signal reception. Lab VIEW is applied to accomplish software platform, which is mainly used to achieve signal processing and the calculation of LI. The detection experiment of two-beam structure with one bolt is finished using the VAM detection platform. The results show that there is obvious negative correlation between LI and the bolt torque. It is proved that the experiment and the platform is.Contrast tests are carried out to study the effects of frequency and amplitude of vibration signal and ultrasonic signal on VAM detection, as well as the effects of excitation and reception location. Results show that the frequency of vibration should be in agreement with the resonant frequencies of the structure and the modulation effect becomes greater with the increase of vibration amplitude. It is proved that the amplitude of ultrasonic has little effect on VAM. S weep-frequency signal is used in VAM detection, which can reduce error efficiently and improve detection efficiency. Experiments shows that vibration and ultrasonic should interact on contact interface before they are received. In this case, the VAM detection is more effective.Two-plate structure with four bolts is studied to verify the feasibility and reliability of this VAM detection platform. ANSYS simulation and experiments are combined to come up with the principle for the selection of the parameters. Two detection schemes are proposed to achieve the location of loosening bolts. ‘Multipoint-exciting method’ can evaluate the health status of the bolts around the excitation position. The whole bolted joints can be detected after several experiments. So there will be more workloads. Contrarily, the excitation position is fixed while the reception positions are placed around the bolts in ‘Multi- pointreceiving method’. The position of loosening bolt can be roughly detected by comparing the LIs which are received in different places.