Study on Ultrasonic Testing Method for Residual Axial Force of Mounting to Yield Bolt

After the bolt is tightened to yield, the axial force of the bolt can be measured according to the ultrasonic yield tightening calibration curve. However, after the bolt has passed the test for a period of time and the heat engine test, the bolt will have a certain attenuation. At present, it is mainly evaluated by torque attenuation. The range of axial force attenuation.

Ultrasonic can now test the axial force of the bolt, and it can also detect and analyze the axial force after the tightened bolt axial force is attenuated. However, the axial force of the ultrasonically measured bolt after attenuation is found to be quite different from the actual axial force according to the conventional method. This paper studies the axial force after tightening and a new measurement method. If there is any good way to measure the bolt after yielding and tightening, please leave a message.

1 Introduction

In the automotive development process, the development of the fastening process occupies an important position, especially in the powertrain integration and chassis fastening connection, it is necessary to test whether the clamping force of the part meets the design requirements and needs stable and reliable detection technology. It is currently evaluated whether the reliability of a fastening joint has gradually changed from the original retest torque to the measurement of the residual axial force.

How to accurately measure the residual axial force of bolts is a topic of great concern to automotive fasteners. At present, the use of ultrasonic technology to measure the axial force of bolts has been widely used. Ultrasonic technology testing bolt axial force is an indirect test method. The basic principle is that according to the theory of acoustic elasticity, the bolt will stretch itself during the tightening process, and at the same time, an axial force will be generated, and the ultrasonic wave will be transmitted from one end of the bolt to the other. One end, then reflected back will form a time difference (Figure 1), the time difference is just proportional to the amount of elongation. In the elastic range, according to Hooke's law, it can be concluded that the axial force of the bolt is proportional to the time difference of the ultrasonic wave.

The test process is to first install and tighten the bolt according to the actual clamping length, and obtain the characteristic curve of the bolt axial force (measured by the axial force sensor) and the elongation (ultrasonic measurement) during the tightening process, and then use this relationship. Curve to measure the axial force of the bolt in the actual fastening subsystem. Because it is an indirect test, an accurate calibration curve is critical to the test results. At present, the axial force test is divided into two parts, one is the test of the initial axial force (that is, the dynamic axial force), and the other is the test of the residual axial force. At present, whether it is the R&D laboratory of the whole vehicle or the test room of the parts, the domestic Related companies are using imported equipment manufacturers to provide test methods to accurately test the initial axial force of bolts, but in the residual axial force test process, because of the lack of uniform and operational test standards at home or internationally, The confidentiality of foreign technology, there are still large deviations in the test results.

In this paper, the drawbacks of the traditional test method are analyzed for the full-threaded bolts. The method of accurately measuring the residual axial force of the yielded bolts is summarized. It provides some technical support for the development of the automotive fastening process and the solution of engineering problems.

2. Introduction of traditional ultrasonic methods

At present, in the fastening process of the automobile, in order to maximize the performance of the bolt, a torque tightening to corner (Torque & Angle to Yield, TAY) bolt tightening assembly strategy is often used. In the early stage of automotive development, the design engineer needed to measure the residual axial force of the TAY bolt after dynamic loading (such as bench test, road test, etc.) to evaluate whether the fastening point meets the design requirements. With the current level of ultrasonic testing and the instructions given by the manufacturer of the ultrasonic equipment, it is easy to implement for bolts with linear loads, but it is more difficult for TAY bolts.

Figure 1. Ultrasonic test schematic

The traditional ultrasonic test method is as follows: Figure 2 reflects the relationship between the elongation of the bolt and the axial force. It can be seen that when the bolt is tightened to a linear area for the first time, such as point A, the axial force attenuation of the bolt is performed down the linear section of the BA. The measurement only needs to use ultrasonic waves to measure the elongation of the bolt. The axial force is gone.

Figure 2. Schematic diagram of the axial force and elongation of the bolt

When the bolt is first tightened to yield, such as point D in Figure 2, the conventional method assumes that the bolt axial force is attenuated along the DE line parallel to the BA. For example, if the bolt has been attenuated to point E, the axial force f (kN) of the point F is obtained by linear fitting curve AF, and then the force of the EF section is subtracted from the force a of the measured point E ( kN). Among them, according to the parallelogram relationship, the force of the EF section is d(kN), which is measured directly by disassembling the bolt or by two different fitting curves (linear fitting and nonlinear fitting) to obtain the target torque. The dynamic axial force difference (ie DG segment), there is an equation relationship: DG = EF = d (kN).

3. The drawbacks of traditional ultrasonic methods

Using the friction coefficient test bench and the MC900 data acquisition instrument (see Figure 3 and Figure 4, respectively) to verify the traditional test method, we can quantitatively see the deviation range of the traditional method test.

Figure 3. Bolt friction coefficient test bench

Figure 4. MC900 Data Acquisition Instrument

Among them, the friction coefficient test bench is equipped with an electric torque gun and an axial force sensor, which can accurately apply torque and accurately measure the axial force of the bolt, that is, the clamping force, through the MC900 data acquisition instrument.

The verification process of the traditional method test is as follows: take out the new bolt from a batch of bolts that have been dynamically calibrated, and tighten to the target torque (assuming 100 Nm + 90 deg torque is applied to the M12 bolt). At this time, the bolt has yielded and the following can be passed. The method of artificially attenuating the clamping force is used to compare the obtained residual axial force value with the current sensor display value. That is, the manual wrench is used to control the axial force sensor on the friction coefficient test bench (ie, Load cell, level 1 accuracy, Load cell can display the current residual axial force value in real time), and unload the bolt to different levels (10% of the dynamic axial force value). ~40%), using the two methods of calculating and disassembling bolts, the residual axial force values ​​of the bolts in different relaxed states and the residual axial force values ​​displayed by the load cell in real time are obtained, and the results are recorded.

Table 1. Verification of residual axial force results from traditional test methods

As shown in Table 1. It can be seen that when the attenuation does not exceed 30%, the residual axial force error obtained by the calculation method has reached 9.6%, and the error of the disassembly method is also 3.7%. Such a large error simply cannot meet the test requirements.

4. Analysis of traditional ultrasonic methods

In the traditional test, the characteristic curve of static stretching of metal materials is referenced. It is considered that if the specimen is unloaded, the load is attenuated along a path parallel to the tightening curve. This is an ideal assumption based on the standard dumbbell pattern. The law of static stretching and unloading, but for the round rod material with the same diameter of the thread, it does not necessarily have such regularity. The curve obtained by tightening/unscrewing twice on the friction coefficient test bench is shown in Fig. 5. The abscissa is the elongation of the bolt, and the ordinate is the real-time axial force value measured by the axial force sensor.

Figure 5. Two times the bolt is tightened-unloaded (abscissa: elongation, ordinate: axial force)

It should be pointed out here that the data source of the slope calculation in the curve is based on: the tightening slope selects the data of the starting segment (about 0%~65% of the axial force), because the loading of the elastic segment in the traditional method is also the starting of the tightening. For the segment, the loosening slope is selected from the initial section of the loosening (about 50% to 100% of the axial force), because the attenuation of the axial force is generally within this range and will not be lower.

As can be seen from the figure, the slope of each tightening and loosening is different, the loosening slope is smaller than the tightening slope, and the slope decreases as the number of bolt tightening increases. This phenomenon was also verified in the bolt drawing/unloading test. The slope of the curve for the first unloading of static stretching is significantly smaller than the slope of the first loading, as shown in Fig. 6 and Fig. 7.

Figure 6. Bolt mounting diagram on the stretcher

Figure 7. Multiple tension/unload curves for bolts (abscissa: elongation, ordinate: tensile force)

Table 2. Comparison of several full-thread bolt tightening and loosening slopes

Table 2 quantitatively illustrates the difference between the first tightening slope and the loosening slope. The slope of the loosening is less than the slope of the tightening. Therefore, the idea of ​​using the path attenuation parallel to the tightening in the conventional method leads to a large measurement error. At the same time, the calibration process of the loosening is exactly the same as the attenuation behavior of the loaded bolts. That is to say, the process of attenuating the clamping force of the bolts is consistent with the effect of the process of manually loosening the bolts.

5. Research on the method of accurately measuring the residual axial force of bolts

Based on the analysis of the error source of the traditional test method, considering the typical characteristics of the calibration curve, for testing the residual axial force installed to the yield bolt, the calibration curve of the batch bolt loosening should be selected instead of the linear segment of the tightening curve.

Here, the new method is expressed as the loosening calibration method. The specific steps and precautions are as follows:

5.1 Bolt calibration

1) Tighten the bolt on the friction coefficient test bench, tighten it to the neck of the bolt, and record the bolt length change and the axial force value in real time as the tightening calibration curve.

2) Tighten the bolt according to the assembly torque on the friction coefficient test bench. After the ultrasonic wave is stabilized (to eliminate the temperature influence), loosen the bolt and record the bolt length change and the axial force value in real time as the loosening calibration curve.

3) Tighten and loosen the number of calibration bolts to not less than 5, and meet the absolute value of the calibration curve dispersion < ± 5%.

5.2 Residual axial force test

   1) For unyielded bolts, use the elastic section of the calibration curve to test the residual axial force, and directly measure and read with ultrasonic.

2) For the yielded bolt, you need to load the loosening calibration curve, first record the current axial force value, and then subtract the axial force value (for the dummy amount) after removing the bolt to obtain the static or residual axial force value of the bolt at that time.

6. Unscrewing calibration test results verification analysis

According to the method in Section 5, after tightening to the specified torque with the M12 ́ 1.75 bolt on the friction coefficient test bench (Fig. 2), the bolt has yielded, and then the wrench is used to start loosening, and the attenuation is 10%. A set of data, namely ultrasonic measurements and sensor readings, were tested in total for 3 samples, and the recorded data is shown in Table 3.

Table 3. Verification of test results for loosening calibration

It can be seen that the measurement error of the loosening calibration method has been greatly reduced, and the deviation within 30% of the bolt axial force attenuation can be reduced to ±2%, which far meets the test requirements. This fully demonstrates that the loosening calibration method solves the problem of large error in the long-term test. This method has formed an enterprise standard and is applied to the bolt rapid path verification development test of new models.

7. Conclusion

1) For testing the residual axial force of automotive high-strength fastening bolts, the traditional test method is based on the ideal assumption of bolt tightening, which has a large measurement error.

2) Using the friction coefficient test bench to simulate the tightening and attenuation of the bolts, the key steps of testing the residual axial force of the bolts are clarified, and the accuracy of the method of testing the residual axial force by using the loosening calibration curve is verified.

3) For the actual part test, it is necessary to study the material, force and temperature of the actual part to more accurately test the residual axial force of the fastening point bolt.

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