The bolt calibration and axial force measurement test in the test chamber is proportional to the propagation time difference of the ultrasonic longitudinal wave before and after the bolt is applied, and the time difference is only a few hundred nanoseconds (1 nanosecond is equal to 10-9 seconds). During the measurement, the ultrasonic longitudinal wave passes through the twisted layer twice. Therefore, if the treatment of the bolt end face and the uniformity of the kneading layer are not paid attention to, the measurement error may reach the same order of magnitude as the measured value, and the measurement cannot be performed effectively. For this reason, the end face of the bolt is required to have a certain degree of smoothness, and the end face is perpendicular to the axis of the bolt to ensure that the ultrasonic longitudinal wave does not refract on the side of the bolt, which may affect the measurement accuracy or cause measurement failure.
Each time the probe is installed, the pressure of the probe on the end face of the bolt should be kept equal, otherwise the error will be caused by the thickness of the composite layer. The test of the bolt test group of the site test of the Wuhu Yangtze River Bridge is a test for the relationship between the construction torque of the bolt and the axial force of the bolt and the measurement of the pre-tension loss value of the bolt before the steel beam is erected. The main purpose of the test was to investigate the reliability of the axial force tester for measuring bolts. Based on the above purposes, this test only measures the bolt pretension. The calibration test bolt calibration test procedure is similar to the bolt axial force measurement test procedure. That is, the group bolt test bolts are first subjected to end surface smoothing treatment, and then 5 of the bolts used in the group plug test are calibrated. The calibration is based on the load of the axial force meter, and the calibration result is linearly regression to obtain the slope, ie the load factor KL. The average of the load factors calibrated by the five bolts is used as the load factor of each bolt of the group bolt test. KL=21603. The steps of applying and measuring are as follows: 1 Before the screwing, the ultrasonic longitudinal axis is measured by the ultrasonic bolt axial force tester, and the initial value of each pressure ring is measured by a static strain gauge. Use an electric wrench to screw one by one. After the final screwing is completed, the static pressure gauge is used to measure the pressure that the pressure ring loves, that is, the axial force. At the same time, the ultrasonic longitudinal wave is measured by the ultrasonic bolt axial force tester, and the sound of the ultrasonic longitudinal wave in the bolt is dry. Finally, the formula P △ "(Sand KL) ), the axial force of the bolt is obtained. Test results The purpose of this group bolt test is to test the measurement accuracy of the ultrasonic bolt axial force tester. Therefore, the force value of the force ring calibrated by the test machine is used as the reference and the ultrasound. The measured values ​​of the bolt axial force tester are compared. The results of the group plug test, such as the measured value of the ultrasonic bolt axial force tester, are 6% compared with the standard value of the measuring ring, and the average relative error is only 28%.
The group plug test and the preliminary test in the early stage fully show that it is completely feasible to measure the pre-tension of the bolt by the ultrasonic bolt axial force tester, and the measurement accuracy is satisfactory. In the test, it was found that the torque coefficient of the bolt is related to the ambient temperature and humidity. The reason is that when the ambient temperature and humidity change, the friction coefficient of the phosphating layer on the surface of the bolt will also change, resulting in a change in the torque coefficient of the bolt. In addition, the unstable performance of the part of the electric wrench used for the screwing is also the cause of the dispersion of the axial force. For example, the torque controller of the electric wrench is an electronic control device, so its control characteristics are related to temperature and humidity.
In summary, if the test environment of the torque coefficient of the bolt is different from the construction site, the torque coefficient of the field bolt and the torque coefficient of the bolt in the test chamber may also differ. Another point to note is that when calibrating the electric wrench, it must be carried out according to the temperature, humidity and voltage of the construction site. Otherwise, the output torque of the rice on the construction site will be different from the calibration of the laboratory. Torque. In summary, it is necessary to ensure that the axial force of the torque method meets certain precision requirements, and the welding process and procedures must be strictly observed in the production, storage, transportation, calibration, and screwing inspection of the bolts. From the results of several measurements, although the average axial force of the bolts under test is in the normal range, the proportion of over-bolted bolts is large and cannot be ignored. Conclusion Through the above indoor test, group plug test and construction test, the following conclusions are drawn: The hardware equipment of this research is advanced and complete; the corresponding software is reasonable, and it meets the requirements of random testing of high-strength bolt axial force.
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