The bolt calibration and axial force measurement test conducted in the test chamber is based on the time difference of the propagation of ultrasonic longitudinal waves before and after the bolt is tightened. This time difference is typically just a few hundred nanoseconds (1 ns = 10â»â¹ seconds). During the measurement, the ultrasonic wave passes through the bolt twice, which makes the process sensitive to any irregularities. If the bolt’s end face is not properly treated or if the surface layer is uneven, the measurement error can become as significant as the actual measured value, making the results unreliable. Therefore, it's essential that the bolt’s end face is smooth and perpendicular to its axis to prevent refraction of the ultrasonic wave, which could otherwise distort the readings or even cause failure in the measurement.
Every time the probe is installed, it must apply consistent pressure to the bolt’s end face. Otherwise, variations in the thickness of the coupling layer may introduce errors. The group bolt test conducted during the site testing of the Wuhu Yangtze River Bridge aimed to study the relationship between the installation torque of the bolts and their axial force, as well as to measure the pre-tension loss before the steel beam was erected. The primary objective was to evaluate the reliability of the axial force tester. Based on this, the test focused solely on measuring the pre-tension of the bolts.
The calibration procedure for the test bolts follows a similar approach to the axial force measurement process. First, the end faces of the bolts are smoothed. Then, five bolts from the group are calibrated using a load cell, and the results are linearly regressed to determine the load factor KL. The average KL value from the five bolts is used for all bolts in the group. In this case, KL was found to be 21603.
The application and measurement steps are as follows: Before tightening, the ultrasonic longitudinal wave is measured using the ultrasonic axial force tester, and the initial pressure ring values are recorded with a static strain gauge. An electric wrench is then used to tighten each bolt sequentially. After completion, the static Pressure Gauge measures the pressure on the ring, representing the axial force. At the same time, the ultrasonic wave is measured again, and the sound signal is analyzed. Finally, the axial force is calculated using the formula P = Δt × (KL), where Δt is the time difference.
Test results showed that the ultrasonic axial force tester produced values that were about 6% different from the reference values obtained from the calibrated force rings, with an average relative error of only 28%. These results indicate that the method is both feasible and accurate for measuring bolt pre-tension.
Both the group plug test and earlier preliminary tests confirmed that using the ultrasonic axial force tester for pre-tension measurement is fully viable and provides satisfactory accuracy. However, during the test, it was observed that the torque coefficient of the bolt is influenced by ambient temperature and humidity. This is because changes in these conditions affect the friction coefficient of the phosphate coating on the bolt’s surface, leading to variations in the torque coefficient. Additionally, the inconsistent performance of the electric wrench used for tightening also contributed to the variability in the axial force. For example, the electronic control system of the wrench is sensitive to temperature and humidity, affecting its torque output.
In summary, if the test environment differs from the actual construction site, the torque coefficient of the field bolts may vary from that measured in the test chamber. It is also important to calibrate the electric wrench under the same environmental conditions as the construction site—temperature, humidity, and voltage. Otherwise, the torque applied at the site may differ from what was calibrated in the lab. To ensure precision, the axial force measured via the torque method must meet strict requirements, and all production, storage, transportation, calibration, and installation procedures must be strictly followed.
From the measurements taken, although the average axial force of the tested bolts fell within the normal range, a significant proportion of them were over-tightened, which cannot be ignored.
In conclusion, through the indoor tests, group plug tests, and on-site trials, the following conclusions were drawn: the hardware equipment used in this research is advanced and complete, the software is well-designed, and it meets the requirements for random testing of high-strength bolt axial forces.
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