The test process divides the test into two groups. The first group is the vibration frequency constant, and the acoustic emission signals are collected under different accelerations: the vibration frequency is 55Hz, and the acceleration is 5m/s2, 15m/s2, 25m/s2 respectively. Acoustic emission signals are collected under tight torque.
The test conditions of the second group are the collection of acoustic emission signals at different frequencies: the acceleration is 15m/s2, and the acoustic frequencies are 35Hz, 55Hz, 75Hz, respectively, and the acoustic emission signals are collected under different pre-tightening torques. After the test device is installed according to the test conditions, the vibrometer is turned on. When the vibrating table vibrates into a stable state, the acoustic emission acquisition system is used to collect the rubbing signal generated by the bolt connecting member under the vibration environment. The pre-tightening torques in the two sets of experiments were 2Nm, 5Nm, 8Nm, 10Nm and 13Nm.
When the acceleration is 5m/s2 and 15m/s2, the fluctuation curve of the ringing count rate is increasing. The reason for the analysis may be that the connection state of the bolt and the connected part changes under the vibration environment, and the tightness of the bolt and the connected part gradually When the reduction is made, the looseness occurs, and with the increase of the looseness, the collected acoustic emission signal becomes more and more obvious, and thus the ringing count rate curve gradually increases with the sampling time. When the acceleration is taken as 25m/s2, the ringing count rate is increased relative to the first two accelerations, but the curve does not appear to gradually increase. By analyzing the test results of other working conditions, it can be seen that under different torques, the peak value of the ringing count rate and the value of the acceleration show a certain regularity, which increases with the increase of the acceleration.
It can be seen from the results of Test 1 that the larger the acceleration is, the larger the ringing count rate and the higher the energy, which means that the change of the vibration acceleration has an influence on the connection state of the bolted connection structure. The reason may be that the increase in acceleration causes an increase in the relative motion between the bolt head and the flange, the connecting flange, and the contact surface between the screw and the nut, resulting in a stronger acoustic emission signal.
The effect of different pre-tightening torque on the acoustic emission characteristics of the bolted joint structure In the vibration environment, when the acceleration is taken as 5m/s2, the energy curve comparison for different torque conditions can be seen, when the torque is increased from 2Nm to 10Nm. The corresponding energy curve is lowered, indicating that the larger the tightening torque applied to the bolt, the smaller the energy generated by the rubbing between the obtained bolt and the connected member, and the weaker the acoustic emission is. The reason for the analysis is that the greater the tightening torque applied to the bolt, the tighter the connection state of the bolt to its connected member, and the smaller the relative displacement between the bolt and its connected member, thereby reducing the activity of acoustic emission; The smaller the torque, the greater the relative displacement between the bolt and its connected member, which tends to cause loosening of the bolt, thereby causing a rubbing motion between the bolt and its connected structure, and generating a large amount of acoustic emission signals. When the torque continues to increase to 13Nm, the energy curve does not conform to the above rule, and the generated energy is rather large. The reason for this phenomenon is that when the torque of 13Nm is applied, the pre-tightening torque is too large, because the bolt is connected with the test piece. Very close, causing the bolt head and the flange to partially embed each other. Under the vibration condition, the edge of the bolt head and the flange form an edge rubbing effect. The energy generated by this situation is more than the bolt head plane and the flange. The signal is strong, so its energy is very high at a torque of 13Nm. It can be seen from the above law that when the pre-tightening torque is 10Nm, the generated energy is the smallest, indicating that when the pre-tightening torque is 10Nm, the connection between the bolt and its linked member is the closest. After processing and analyzing other groups of data, the same rules as above are obtained.
The effect of frequency on the acoustic emission characteristics of the bolted connection structure is reflected by the two characteristic parameters of the ringing count rate and the energy, respectively. The acceleration is taken as 15m/s2 and the pre-tightening torque is 5Nm. The ringing count rate and energy (~10) obtained after analyzing the data at 35 Hz, 55 Hz, and 75 Hz, respectively, are analyzed and compared, and it is seen that the peaks of the corresponding ringing count rates are respectively taken at frequencies of 35 Hz, 55 Hz, and 75 Hz. For 350N/s, 380N/s, 425N/s, the peak curve of the ringing count rate generally increases with increasing frequency, but the amplitude of the increase is small, and the relative acceleration affects the ringing rate. Say, the frequency does not change the ringing count rate. It can be seen from the energy (0) that the cumulative energy value resulting from the change of the connection state of the bolted structure in the vibration environment increases as the frequency increases.
From the above analysis results, it can be seen that when the vibration acceleration is kept constant and the applied pre-tightening torque value is constant, comparing the ringing count rate and energy of different frequencies, it can be seen that the change of the frequency has a change in the ringing count rate. A certain impact, but the impact is not large, the ringing count rate shows a slight increase with the increase of frequency; while the change of frequency is more obvious to the change of energy, the energy is increased with the increase of frequency. After analyzing and processing the data under other working conditions, the same law is obtained.
Conclusion The ringing count rate and energy of the bolted joints increase with the increase of the acceleration in the vertical vibration environment; the change of the frequency of the vibrating table also affects the ringing count rate and energy. The ringing count rate and energy are both frequency dependent. Increase and increase, the difference between the two is that the frequency has a great influence on the change of energy. Therefore, the two characteristic parameters of the ringing count rate and the energy can be used to identify the bolt connection state when the acceleration and the frequency change.
The energy generated by the friction between the bolt and its connecting member decreases with the increase of the pre-tightening torque, but when the pre-tightening torque increases to a certain value, the generated energy is the smallest. After exceeding the limit torque, due to the rubbing state The energy generated is rather high, and it is very important to find the characteristic value of this pre-tightening torque in the project to judge the elastic state of the bolt connection structure.
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