Axial force attenuation characteristic testing method of bolt connection structure

Abstract

The invention relates to an axial force attenuation characteristic test method of a bolt connection structure. The method comprises six steps to obtain a real residual axial force Fi'and an axial force error percentage eta i after a corresponding temperature cycle test. According to the invention, initial and secondary calibration of the to-be-tested bolt is carried out in S1 and S5, and F-t curve tests are respectively calibrated before and after temperature circulation of the to-be-tested bolt, so that plastic deformation and real axial force change of the to-be-tested bolt are separated. In addition, through temperature circulation environment simulation, multiple rounds of heating-constant temperature-cooling circulation are set, and therefore the connection behavior in the actual service environment is truly simulated. The invention also creatively provides a false axial force correction method, and the false increment axial force introduced by plastic deformation is deducted through secondary calibration, so that the accuracy and repeatability of axial force measurement are improved. According to the invention, matching analysis can be carried out on magnesium alloy and to-be-tested bolt materials, and aluminum alloy and carbon steel to-be-tested bolts are selected to be assembled with a magnesium alloy shell, so that the axial force retention performance under different material combinations is compared.

Description

Technical Field

[0001] This invention relates to the field of mechanical connection and material mechanics testing technology, and in particular to a method for testing the axial force attenuation characteristics of bolted connection structures. Background Technology

[0002] With the development of lightweight structures, magnesium alloys, due to their low density and high specific strength, are widely used in aerospace, automotive electronics, and precision structural components. However, magnesium alloys have poor thermal stability and creep properties, and are prone to plastic deformation and creep relaxation at high temperatures, leading to a gradual decrease in the clamping force at connection points in fasteners, including bolts and connecting parts connected to them. In fastener design, carbon steel or aluminum alloy bolts are often used in conjunction with magnesium alloy housings. Due to the significant differences in the coefficients of linear expansion among carbon steel, aluminum alloy, and magnesium alloy (e.g., carbon steel approximately 11 × 10⁻⁶), the use of these materials is problematic. / K~13×1 / K, aluminum alloy approximately 23×1 / K, magnesium alloy approximately 26×1 Under temperature cycling or long-term high-temperature service conditions, joints may exhibit axial force fluctuations, localized relaxation, and plastic elongation. Current research primarily focuses on measuring static clamping force at room temperature, lacking a systematic experimental method for understanding the attenuation of axial force at joints under temperature cycling conditions. The reasons for these problems are: 1. Failure to consider the superposition of multiple deformation factors caused by temperature cycling: Traditional experiments only test the initial axial force and the final residual axial force, failing to distinguish between the contributions of magnesium alloy creep and bolt plastic tension. 2. Insufficient bolt calibration: Most experiments do not recalibrate bolts after temperature cycling, leading to an inability to identify spurious axial force errors caused by bolt plastic deformation after temperature cycling. 3. Lack of full-process residual axial force tracking: Most existing experiments only measure axial force data at two points before and after high temperature, ignoring the dynamic changes during the intermediate process. 4. The fundamental reason lies in the lack of an experimental correction mechanism coupling the thermal expansion difference of multiple materials with the creep effect, and the lack of comparative data on secondary calibration of bolts after temperature cycling.

[0003] Therefore, it is essential to provide a test method for the axial force attenuation characteristics of bolted connection structures to address the shortcomings of existing technologies. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for testing the axial force attenuation characteristics of bolted connections. This method can accurately reflect the mechanical behavior of bolted connections under temperature cycling conditions, accurately assess the actual axial force attenuation of the bolts, and is applicable to experiments with different material combinations. It also offers high repeatability and facilitates the establishment of predictive models for bolted connections under temperature cycling.

[0005] The above-mentioned objectives of the present invention are achieved through the following technical measures: A method for testing the axial force attenuation characteristics of bolted connection structures is provided, comprising the following steps: S1. Measure the flight time change t and axial force F of the bolt to be tested to obtain the initial calibration Ft curve; S2. Assemble the bolt to be tested with the assembly clamp and the assembly counter to obtain a bolted connection structure, then proceed to S3. S3. Perform n temperature cycle tests on the bolted connection structure sequentially; after each temperature cycle, cool the bolted connection structure to room temperature and measure the uncorrected residual axial force F of the bolt under test. i and the corresponding flight time t i And there exists 1≤i≤n and n and i are both integers. After all temperature cycle tests are completed, proceed to S4; S4. Disassemble the bolted connection structure, and then test the no-load flight time t of the bolt under test in an unloaded state after treatment in S3. n+1 Enter S5; S5. Measure the flight time change t' and residual axial force F' of the bolt under test after processing by S4 to obtain the secondary calibration F'-t' curve of the bolt under test; S6. Flight time t based on the temperature cycle test corresponding to S3. i and uncorrected residual axial force F i The idle flight time t obtained by S4 n+1 And the secondary calibration F'-t' curve obtained from S5 yields the actual residual axial force F after the corresponding temperature cycling test. i and percentage of axial force error η i .

[0006] Preferably, the above-mentioned actual residual axial force F i The method to obtain ' is as follows: A1. Calculate the absolute value Δt of the difference between the flight time and the empty flight time for the corresponding temperature cycle test using equation (1). i : △t i =|t i -t n+1 |……Equation (1); A2. Based on the initial calibration Ft curve obtained from S1 and the absolute value Δt of the difference between the flight time and the empty flight time during the corresponding temperature cycle test. i Find the actual residual axial force F after the corresponding temperature cycling test. i Specifically, in the quadratic calibration F'-t' curve obtained in S5, Δt is used as... i The value on the ordinate corresponding to the horizontal axis is the actual residual axial force F after the corresponding temperature cycling test.i '.

[0007] Another preferred embodiment is the aforementioned actual residual axial force F. i The method to obtain ' is as follows: B1. Integrate the initial calibration Ft curve obtained in S1 and the secondary calibration F'-t' curve obtained in S5 into the same coordinate system; B2. Translate the quadratic calibration curve F'-t' in the same coordinate system along the x-axis, and the translation distance is equal to the empty flight time t. n+1 This yields the double calibration curve; B3. Based on the dual calibration curve obtained in B2 and the corresponding flight time t of the temperature cycling test. i Find the actual residual axial force F after the corresponding temperature cycling test. i Specifically, in the dual-calibration curve, the horizontal axis represents the flight time t. i The ordinate value corresponding to the F'-t' curve after time translation and secondary calibration is the true residual axial force F after the corresponding temperature cycle test. i '.

[0008] Preferably, the above-mentioned axial force error percentage η i We obtain the following from equation (2): ... Equation (2).

[0009] Preferably, S1 includes the following steps: S1.1 Assemble the bolt to be tested with the calibration clamp, calibration opponent, front fixture and rear fixture, and then clamp the whole assembly on the torque machine, ensuring that the clamping length, engagement length and other parameters are consistent with the actual use conditions during clamping; then assemble the ultrasonic length measuring instrument and axial force sensor with the bolt to be tested respectively, wherein the piezoelectric ceramic plate of the ultrasonic length measuring instrument is attached to the center of the axis of the head of the bolt to be tested, and the calibration opponent is a calibration toothed sleeve or nut; S1.2 Tighten the bolt to the point exceeding the maximum axial force of the bolt under test using the over-yield tightening method to obtain the axial force response characteristics of the bolt under test in the full working strain range. The torque and angle parameters are collected in real time by a torque sensor and an angle encoder to determine whether the maximum axial force of the bolt under test has been exceeded. S1.3. Measure the flight time change t of the bolt under test using the ultrasonic length measuring instrument, directly measure the axial force F using the axial force sensor, and obtain the initial calibration Ft curve. Read the maximum axial force Fmax in the initial calibration Ft curve and compare it with the specified installation axial force range. If the maximum axial force Fmax is less than the specified installation axial force range, replace the bolt under test or lubricate the calibration pair before re-calibrating and returning to S1.1; if the maximum axial force Fmax is within the specified axial force range, proceed to S2.

[0010] Preferably, S2 above includes the following steps: S2.1 Connect the bolt to be tested to the assembly clamp and the assembly handpiece. During the tightening process of the bolt to be tested, the torque and rotation angle parameters are collected in real time by the torque sensor and the angle encoder to obtain the axial force-torque-angle curve of the assembly process. S2.2. Compare the maximum torque Tmax in the axial force-torque-angle curve with the specified installation torque range of the bolt to be tested. If the maximum torque Tmax is less than the specified installation torque range, replace the bolt to be tested and return to S1 or adjust the assembly process parameters and return to S2.1. If the maximum torque Tmax is within the installation torque range, define the bolt to be tested, the assembly clamp and the assembly counter as a bolted connection structure and proceed to S3.

[0011] Preferably, S3 above includes the following steps: S3.1. Under room temperature conditions, after the bolted connection structure is left to stand for 0.5h to 2h, the flight time t0 and the uncorrected residual axial force F0 of the bolt to be tested are directly measured by the ultrasonic length measuring instrument. The uncorrected residual axial force F0 is obtained by the ultrasonic length measuring instrument according to the initial calibration Ft curve obtained in S1, and then proceeds to S3.2. S3.2. The bolted connection structure is subjected to n temperature cycle tests in sequence; and after each temperature cycle, the bolted connection structure is cooled to room temperature, and the uncorrected residual axial force F is directly measured using the ultrasonic length measuring instrument. i and the corresponding flight time t i And there exists 1≤i≤n, where n and i are both integers, the uncorrected residual axial force F i The initial calibration Ft curve obtained from S1 is obtained using an ultrasonic length measuring instrument. After all temperature cycling tests are completed, proceed to S4. Preferably, S4 specifically involves: disassembling the bolted connection structure and recording the disassembly torque; after disassembly, using the ultrasonic length measuring instrument to test the no-load flight time t of the bolt under test treated in S3 under no-load conditions. n+1 Enter S5.

[0012] Preferably, S5 specifically involves: assembling the bolt to be tested, processed by S4, with the calibration clamp and calibration counterpart; then assembling the ultrasonic length measuring instrument and the axial force sensor with the bolt to be tested respectively; then, the ultrasonic length measuring instrument measures the time-of-flight change t' of the bolt to be tested; and the axial force sensor measures the residual axial force F' of the bolt to be tested, thereby obtaining the secondary calibration F'-t' curve of the bolt to be tested.

[0013] During the process of measuring the flight time change by the ultrasonic length measuring instrument, the real-time temperature is measured simultaneously, and the ultrasonic length measuring instrument performs compensation calculations based on the temperature difference between the real-time temperature and the temperature at the calibration time.

[0014] The present invention discloses a method for testing the axial force attenuation characteristics of a bolted connection structure, comprising the following steps: S1, measuring the flight time change t and axial force F of the bolt to be tested to obtain an initial calibration Ft curve; S2, assembling the bolt to be tested with an assembly clamp and an assembly counter to obtain a bolted connection structure, proceeding to S3; S3, subjecting the bolted connection structure to n temperature cycle tests sequentially; after each temperature cycle, cooling the bolted connection structure to room temperature, and measuring the uncorrected residual axial force F of the bolt to be tested. i and the corresponding flight time t i And there exists 1≤i≤n where n and i are both integers. After all temperature cycling tests are completed, proceed to S4; S4: Disassemble the bolted connection structure, and then test the no-load flight time t of the bolt under test in the no-load state after treatment in S3. n+1 Proceed to S5; S5, measure the flight time change t' and residual axial force F' of the bolt under test after processing in S4, and obtain the secondary calibration F'-t' curve of the bolt under test; S6, based on the flight time t of the temperature cycle test corresponding to S3... i and uncorrected residual axial force F i The idle flight time t obtained by S4 n+1 And the secondary calibration F'-t' curve obtained from S5 yields the actual residual axial force F after the corresponding temperature cycling test. i and percentage of axial force error η iThis invention performs initial and secondary calibration of the bolt under test in steps S1 and S5, and calibrates the Ft curve of the bolt before and after temperature cycling, thereby separating the plastic deformation and the actual axial force change of the bolt. This invention also simulates the connection behavior under actual service conditions by setting multiple rounds of heating-isothermal-cooling cycles through temperature cycling environment simulation. Furthermore, this invention creatively proposes a spurious axial force correction method, which subtracts the spurious incremental axial force introduced by plastic deformation through secondary calibration, thereby improving the accuracy and repeatability of axial force measurement. This invention can perform material matching analysis between magnesium alloy and the bolt under test, selecting aluminum alloy and carbon steel bolts to be assembled with magnesium alloy shells, thereby comparing the axial force retention performance under different material combinations. The beneficial effects of this invention are: 1. It accurately reflects the mechanical behavior of the bolt connection under temperature cycling conditions; 2. It distinguishes between elastic and plastic contributions, enabling accurate assessment of the actual axial force attenuation of the bolt under test; 3. It is applicable to different material combinations, such as carbon steel-magnesium and aluminum-magnesium, facilitating engineering design optimization; 4. It has strong experimental repeatability, making it easy to establish a predictive model for the bolt connection under temperature cycling; 5. It has good scalability, and this method can also be applied to the life assessment and calibration of other lightweight metal connection systems. Attached Figure Description

[0015] The invention will be further described with reference to the accompanying drawings, but the contents of the drawings do not constitute any limitation on the invention.

[0016] Figure 1 This is a schematic diagram of the first assembly method of the bolt to be tested, the calibration clamp, the calibration opponent, the front fixture, and the rear fixture of the present invention.

[0017] Figure 2 This is a schematic diagram of the second assembly method of the screw to be tested, the calibration clamp, the calibration opponent, the front fixture, and the rear fixture of the present invention.

[0018] Figure 3 This is a schematic diagram of the assembly of the bolt to be tested with the actual clamping part and the hand part according to the present invention.

[0019] Figure 4 This is the initial calibration Ft curve for Example 2.

[0020] Figure 5 The torque-axial force-angle curve is for the formal assembly of component No. 1 in Example 2.

[0021] Figure 6 The torque-axial force-angle curve is for the formal assembly of component No. 2 in Example 2.

[0022] Figure 7 The F'-t' curve is the secondary calibration curve of the bolt to be tested after all temperature cycles of part 1 in Example 2.

[0023] Figure 8 The F'-t' curve is the secondary calibration curve of the bolt to be tested after all temperature cycles of part No. 2 in Example 2.

[0024] Figure 9 The F'-t' curve of the bolt under test after all temperature cycles of part 1 in the first method of Example 2 is the secondary calibration curve.

[0025] Figure 10 The F'-t' curve of the bolt under test after all temperature cycles of part No. 2 in the first method of Example 2 is a secondary calibration curve.

[0026] Figure 11 This is the equivalent correction curve of the axial force attenuation of the bolt under test after all temperature cycles of part 1 in the second method of Example 2.

[0027] Figure 12 The equivalent correction curve for the axial force attenuation of the bolt under test after all temperature cycles of part No. 2 in Example 2, the second method. Detailed Implementation

[0028] The technical solution of the present invention will be further described in conjunction with the following embodiments.

[0029] Example 1

[0030] A method for testing the axial force attenuation characteristics of a bolted connection structure comprises the following steps: S1. Measure the flight time change t and axial force F of the bolt to be tested to obtain the initial calibration Ft curve; S2. Assemble the bolt to be tested with the assembly clamp and the assembly counter to obtain a bolted connection structure, then proceed to S3. S3. Perform n temperature cycle tests on the bolted connection structure sequentially; after each temperature cycle, cool the bolted connection structure to room temperature and measure the uncorrected residual axial force F of the bolt under test. i and the corresponding flight time t i And there exists 1≤i≤n and n and i are both integers. After all temperature cycle tests are completed, proceed to S4; S4. Disassemble the bolted connection structure, and then test the no-load flight time t of the bolt under test in an unloaded state after treatment in S3. n+1 Enter S5; S5. Measure the flight time change t' and residual axial force F' of the bolt under test after processing by S4 to obtain the secondary calibration F'-t' curve of the bolt under test; S6. Flight time t based on the temperature cycle test corresponding to S3. i and uncorrected residual axial force F i The idle flight time t obtained by S4 n+1And the secondary calibration F'-t' curve obtained from S5 yields the actual residual axial force F after the corresponding temperature cycling test. i and percentage of axial force error η i .

[0031] The true residual axial force F of the present invention i There are two methods to obtain ', specifically: The first type is the actual residual axial force F. i The method to obtain ' is as follows: A1. Calculate the absolute value Δt of the difference between the flight time and the empty flight time for the corresponding temperature cycle test using equation (1). i : △t i =|t i -t n+1 |……Equation (1); A2. Based on the initial calibration Ft curve obtained from S1 and the absolute value Δt of the difference between the flight time and the empty flight time during the corresponding temperature cycle test. i Find the actual residual axial force F after the corresponding temperature cycling test. i Specifically, in the quadratic calibration F'-t' curve obtained in S5, Δt is used as... i The value on the ordinate corresponding to the horizontal axis is the actual residual axial force F after the corresponding temperature cycling test. i '.

[0032] The second type is the actual residual axial force F. i The method to obtain ' is as follows: B1. Integrate the initial calibration Ft curve obtained in S1 and the secondary calibration F'-t' curve obtained in S5 into the same coordinate system; B2. Translate the quadratic calibration curve F'-t' in the same coordinate system along the x-axis, with the translation distance being equal to the no-load flight time t. n+1 This yields the double calibration curve; B3. Based on the dual calibration curve obtained in B2 and the corresponding flight time t of the temperature cycling test. i Find the actual residual axial force F after the corresponding temperature cycling test. i Specifically, in the dual-calibration curve, the horizontal axis represents the flight time t. i The ordinate value corresponding to the F'-t' curve after time translation and secondary calibration is the true residual axial force F after the corresponding temperature cycle test. i '.

[0033] It should be noted that the two types of actual residual axial forces F in this invention i The actual residual axial force F obtained by the method of ' is i 'Approximately the same.'

[0034] It should also be noted that in the initial calibration, the test object was an unused bolt to be tested, and its axial force was measured using an ultrasonic length measuring instrument to obtain the result F. n The test object for the secondary calibration is the bolt under test after temperature cycling. Although its physical state changes slightly due to the temperature history (which can be equivalently regarded as a new bolt under test with a slightly different length), the test principle and method are completely consistent with the initial calibration. Therefore, both calibrations in this invention are performed under a clear reference state, using the same standardized measurement procedure to obtain data, and the resulting corrected true residual axial force F i It has the same reliability and authenticity.

[0035] Among them, the percentage of axial force error η i We obtain the following from equation (2): ... Equation (2).

[0036] It should be noted that the percentage of axial force error... η i This invention can be used to quantitatively assess the degree of axial force attenuation and plasticity effects caused by temperature cycling. The present invention aims to obtain the true residual axial force F of a bolt after temperature cycling. i Because ultrasound can only detect changes in total length and cannot distinguish between elastic and plastic deformation, the uncorrected residual axial force F obtained without secondary calibration will be insufficient. i The value is artificially inflated, which may lead people to mistakenly believe that the bolt axial force meets the usage requirements when in fact the bolt axial force is even smaller, resulting in unreliable axial force at the connection point.

[0037] S1 of the present invention includes the following steps: S1.1 Assemble the bolt to be tested with the calibration clamp, calibration opponent, front fixture and rear fixture, and then clamp the whole assembly on the torque machine, ensuring that the clamping length, engagement length and other parameters are consistent with the actual use conditions during clamping; then assemble the ultrasonic length measuring instrument and axial force sensor with the bolt to be tested respectively, wherein the piezoelectric ceramic plate of the ultrasonic length measuring instrument is attached to the center of the axis of the head of the bolt to be tested, and the calibration opponent is a calibration toothed sleeve or nut; S1.2 Tighten the bolt to the point exceeding the maximum axial force of the bolt under test using the over-yield tightening method to obtain the axial force response characteristics of the bolt under test in the full working strain range. The torque and angle parameters are collected in real time by a torque sensor and an angle encoder to determine whether the maximum axial force of the bolt under test has been exceeded. S1.3. Measure the flight time change t of the bolt under test using the ultrasonic length measuring instrument, directly measure the axial force F using the axial force sensor, and obtain the initial calibration Ft curve. Read the maximum axial force Fmax in the initial calibration Ft curve and compare it with the specified installation axial force range. If the maximum axial force Fmax is less than the specified installation axial force range, replace the bolt under test or lubricate the calibration pair, re-calibrate, and return to S1.1. If the maximum axial force Fmax is within the specified axial force range, proceed to S2.

[0038] It should be noted that ultrasonic length measuring instruments are based on the acoustoelastic effect, where the propagation speed of ultrasonic waves in stressed materials changes with the stress state. By measuring the change in the propagation time (time of flight) of ultrasonic waves in a bolt, the axial stress of the bolt is calculated, and thus the axial force is obtained. The advantages of ultrasonic axial force testing are non-destructive testing, in-service testing capability, high accuracy, and dynamic detection capability. The time of flight measured by an ultrasonic length measuring instrument is caused by two factors: the length change caused by elastic deformation—caused by the actual axial force, which is reversible; and the length change caused by plastic deformation—caused by material yielding, creep, high temperature, etc., which is irreversible. If elastic deformation and plastic deformation are not separated, the ultrasonic length measuring instrument can only detect the change in total length, interpreting all length changes as changes in axial force. However, in reality, during plastic deformation, the axial force may remain unchanged or even decrease. It should also be noted that the lubrication calibration in S1.3 aims to reduce the friction coefficient of the thread teeth of the handpiece.

[0039] S2 includes the following steps: S2.1 Connect the bolt to be tested to the assembly clamp and the assembly handpiece. During the tightening process of the bolt to be tested, the torque and rotation angle parameters are collected in real time by the torque sensor and the angle encoder to obtain the axial force-torque-angle curve of the assembly process. S2.2. Compare the maximum torque Tmax in the axial force-torque-angle curve with the specified installation torque range of the bolt to be tested. If the maximum torque Tmax is less than the specified installation torque range, replace the bolt to be tested or adjust the assembly process parameters and return to S2.1. If the maximum torque Tmax is within the installation torque range, define the bolt to be tested, the assembly clamp and the assembly counter as a bolted connection structure and proceed to S3.

[0040] S3 includes the following steps: S3.1 Under room temperature conditions, after the bolted connection structure is left to stand for 0.5h to 2h, the flight time t0 and the uncorrected residual axial force F0 of the bolt to be tested are directly measured by an ultrasonic length measuring instrument. The uncorrected residual axial force F0 is obtained by the ultrasonic length measuring instrument according to the initial calibration Ft curve obtained in S1, and then proceeds to S3.2. S3.2. Conduct n temperature cycle tests on the bolted connection structure sequentially; and after each temperature cycle, cool the bolted connection structure to room temperature, and then directly measure the uncorrected residual axial force F using an ultrasonic length measuring instrument. i and the corresponding flight time t i Furthermore, there exists a condition where 1 ≤ i ≤ n, and n and i are both integers, and the uncorrected residual axial force F i The initial calibration Ft curve obtained from S1 is obtained using an ultrasonic length measuring instrument. After all temperature cycling tests are completed, the process proceeds to S4.

[0041] Specifically, S4 involves: disassembling the bolted connection structure and recording the disassembly torque; after disassembly, using the ultrasonic length measuring instrument to test the no-load flight time t of the bolt treated in S3 under no-load conditions. n+1 Enter S5.

[0042] S5 specifically involves assembling the bolt to be tested, processed by S4, with the calibration clamp and calibration counterpart. Then, the ultrasonic length measuring instrument and the axial force sensor are respectively assembled with the bolt to be tested. The ultrasonic length measuring instrument measures the time-of-flight change t' of the bolt to be tested, and the axial force sensor measures the residual axial force F' of the bolt to be tested, thus obtaining the secondary calibration F'-t' curve of the bolt to be tested.

[0043] It should be noted that the axial force attenuation characteristic test method for bolted connection structures of the present invention can be used simultaneously with multiple bolts from the same batch to obtain an average value. The initial calibration Ft curve is common to bolts from the same batch. If the axial force fails to meet the requirements in S3, a new bolt can be replaced and the test repeated. However, the bolts used in S4 and S5 tests must be bolts that have undergone all temperature cycle tests sequentially.

[0044] During the process of measuring the flight time change t using the ultrasonic length measuring instrument, the real-time temperature is measured, and the ultrasonic length measuring instrument performs compensation calculations based on the temperature difference between the real-time temperature and the temperature at calibration.

[0045] This invention performs initial and secondary calibration of the bolt under test in steps S1 and S5, and conducts Ft curve tests on the bolt before and after temperature cycling to separate the changes in plastic deformation and actual axial force. Furthermore, this invention monitors residual axial force throughout the entire process, such as after assembly, after 0.5-2 hours of static rest, and at each stage of temperature cycling, thereby obtaining the dynamic attenuation law of axial force at the bolt connection point. This invention also simulates temperature cycling environments (high temperature / high and low temperature) by setting multiple rounds of heating-isothermal-cooling cycles to realistically simulate connection behavior under actual service conditions. This invention also creatively proposes a spurious axial force correction method, subtracting the spurious incremental axial force introduced by plastic deformation through secondary calibration, thereby providing accuracy and repeatability in axial force measurement. This invention can perform material matching analysis between magnesium alloy and the bolt under test, selecting aluminum alloy and carbon steel bolts to be assembled with magnesium alloy shells, thereby comparing the axial force retention performance under different material combinations.

[0046] It should be noted that the piezoelectric ceramic plate of the ultrasonic length measuring instrument is attached to the center of the axis of the bolt head to be tested; the bolt to be tested is a carbon steel bolt or an aluminum alloy bolt; the calibration device is a nut or a calibration threaded sleeve. The reference standard for the ultrasonic length measuring instrument of this invention is QC / T1153 Ultrasonic piezoelectric ceramic plate method for axial force testing of automotive fastening bolts. The assembly process of the bolt to be tested in this invention must meet the process requirements, specifically according to the actual process requirements. These process requirements include, but are not limited to, target axial force, torque, and rotation angle parameters, which need to be preset according to the bolt specifications and material properties.

[0047] In S1 or S5, the assembly method for the bolt to be tested involves clamping the bolt, calibration clamp, and calibration device (with piezoelectric ceramic sheet) in a torque machine or other integrated testing equipment with torque-angle-axial force measurement functions. During clamping, ensure that parameters such as clamping length and engagement length are consistent with the actual usage conditions. The torque machine's accessories include a front fixture, a rear fixture, and an axial force sensor. The front fixture is used to fix the calibration clamp; the rear fixture is used to fix the calibration device; and the axial force sensor is used to monitor the axial force of the bolt to be tested. The calibration gear set of this invention is placed in the rear fixture and then installed as a whole into the torque machine. The calibration gear set can be clamped in the following two forms: The first type involves calibrating the braces without inserting them into the anterior fixture, such as... Figure 1 At this point, the thickness of the calibration clamp should be thinner than the actual clamping thickness, i.e., calibration clamp thickness = actual clamping thickness - front fixture thickness - (2-3mm). Furthermore, a 2-3mm gap should be maintained between the calibration clamp and the front fixture to avoid direct contact that could lead to destructive torque testing. Figure 1 The arrow in the bolt to be tested indicates the ultrasonic wave propagation path.

[0048] The second method involves inserting the calibration brace into the anterior fixture, such as... Figure 2At this point, a 2-3mm gap should be maintained between the calibration clamp and the calibration holder to prevent direct contact that could damage the torque test. The thickness of the calibration holder should be slightly thinner than the actual clamping thickness by 2-3mm, i.e., calibration holder thickness = actual clamping thickness - (2-3mm). This ensures clamping accuracy and calibration reliability. Figure 2 The arrow in the bolt to be tested indicates the ultrasonic wave propagation path.

[0049] It should be noted that due to the different fit between the anterior fixture hole and the calibration aligner, the only difference between the two clamping methods is the thickness of the calibration clamp. The two clamping methods do not affect the results of the initial calibration Ft curve or the secondary calibration F'-t' curve.

[0050] After the bolt to be tested is clamped, confirm that the support surface of the bolt to be tested fits well with the clamping parts. Then tighten the bolt to be tested to a suitable torque or angle value. Generally, over-yield tightening is preferred, that is, tightening to a point exceeding the maximum axial force of the bolt to be tested, in order to obtain the axial force response characteristics of the bolt to be tested throughout the entire working strain range.

[0051] After clamping the bolt to be tested with the calibration fixture, calibration opponent, front jig, and rear jig in S1 or S5, confirm that the support surface of the bolt to be tested fits well with the fixture. Then tighten the bolt to be tested to a suitable torque or angle value. Generally, over-yield tightening is preferred, that is, tightening to a point exceeding the maximum tightening axial force of the bolt to be tested, in order to obtain the axial force response characteristics of the bolt to be tested throughout the entire working strain range.

[0052] In S2, the pre-applied bolt to be tested is assembled with the actual clamping part and the hand part according to the assembly process parameters, such as... Figure 3 A torque sensor was used to record the torque and angle during the assembly process. Simultaneously, an ultrasonic length gauge combined with a temperature sensor was used to measure the axial force, flight time, bolt length, and temperature of the assembled bolted connection structure. Figure 3 The arrow in the bolt to be tested indicates the ultrasonic wave propagation path.

[0053] The temperature cycling test in S3 is a high temperature / high and low temperature cycling test. The temperature cycling test is carried out in a constant temperature test chamber. The cycling conditions of the high temperature / high and low temperature cycling test are set, including the cycling temperature range, heating and cooling rate, cycling time and number of cycles.

[0054] This method for testing the axial force attenuation characteristics of bolted connections involves initial and secondary calibration of the bolt under test in S1 and S5. Ft curve tests are performed on the bolt before and after temperature cycling to separate the plastic deformation and actual axial force changes. Furthermore, the invention monitors residual axial force throughout the entire process, measuring axial force after assembly, after 0.5-2 hours of static rest, and at each stage of temperature cycling to obtain the dynamic attenuation law of the axial force at the bolt connection point. The invention also simulates temperature cycling environments (high temperature / high and low temperatures) by setting multiple rounds of heating-isothermal-cooling cycles to realistically simulate connection behavior under actual service conditions. The invention also creatively proposes a spurious axial force correction method, subtracting the spurious incremental axial force introduced by plastic deformation through secondary calibration, thereby improving the accuracy and repeatability of axial force measurements. This invention can perform material matching analysis between magnesium alloy and the bolt under test, selecting aluminum alloy and carbon steel bolts to be assembled with magnesium alloy shells to compare the axial force retention performance under different material combinations. The beneficial effects of this invention are: 1. It accurately reflects the mechanical behavior of bolted connections under temperature cycling conditions; 2. It distinguishes between elastic and plastic contributions, enabling precise assessment of the actual axial force attenuation of bolts; 3. It is applicable to different material combinations, such as carbon steel-magnesium and aluminum-magnesium, facilitating engineering design optimization; 4. It has strong experimental repeatability, making it easy to establish predictive models for bolted connections under temperature cycling; 5. It has good scalability, and this method can also be applied to the life assessment and calibration of other lightweight metal connection systems. Applying this invention, the actual axial force changes of bolts at each stage of temperature cycling can be accurately detected under non-destructive conditions, effectively eliminating errors caused by bolt plastic deformation, sound velocity changes, and temperature drift. Compared with traditional methods, this invention has high testing accuracy and good repeatability, and is suitable for fields such as high-temperature structural connectors, lightweight alloy fasteners, and high and low temperature fatigue connection reliability verification.

[0055] Example 2

[0056] Application of a test method for axial force attenuation characteristics of a bolted connection structure in Embodiment 1.

[0057] I. Initial calibration of the bolt to be tested, i.e., S1 in Example 1: (1) Sample preparation: Bolt to be tested: M8X26 aluminum bolt; Clamping and gripping parts material: Mg; Clamping length: 10mm; The condition of the bolts, clamps, and counters to be tested is guaranteed to be the same as that of the actual assembled clamps and counters.

[0058] (2) Calibration of the bolt to be tested

[0059] After the support surface of the bolt to be tested is in contact with the clamping device, the bolt is tightened to a suitable torque or angle value, using an over-yield tightening method, i.e., tightening to a point exceeding the maximum tightening axial force of the bolt to obtain the axial force response characteristics of the bolt across its entire working strain range. Simultaneously, an ultrasonic length gauge and a temperature sensor are used to measure the time-of-flight change t and temperature T of the bolt in real time. An automatic temperature compensation function is set in the ultrasonic length gauge to automatically correct the effect of temperature changes on the sound velocity in the measured axial force value. The synchronous data of the axial force F, time-of-flight t, and temperature T of the bolt are recorded. In this embodiment, four bolt samples were tested, as shown in Table 1-1.

[0060] Table 1-1. M8 Aluminum Bolt - Initial Calibration Axial Force F and Flight Time t Data

[0061] (3) Data processing and calibration curve establishment

[0062] The F-t data curves of the four tested bolts were fitted to establish a quantitative relationship curve between the axial force F and flight time t of the tested bolts, i.e., the initial Ft calibration curve, as shown below. Figure 4 .

[0063] (4) Judgment of the rationality of the axial force range of the initial calibration curve

[0064] The maximum axial force Fmax calibrated by the initial calibration curve (after fitting) should be judged against the installation axial force range in Table 1-2 below. If Fmax is within the installation axial force range of the M8 (corresponding specification) aluminum bolt to be tested, the formal assembly test in step 2 can be carried out; if Fmax is not within the installation axial force range of the M8 aluminum bolt to be tested, lubricate the calibration pair to reduce the friction coefficient of the thread teeth of the calibration pair, and then re-perform the initial calibration.

[0065] Table 1-2 Installation Axial Force Range for Common Aluminum Bolt Specifications

[0066] II. Formal assembly and testing, i.e., S2 of Example 1

[0067] (1) Assembly quantity: 10 PCS of bolts to be tested

[0068] (2) Assemble the patched bolts to be tested with the actual clamping parts and the hand parts according to the assembly process parameters (9Nm+45°). Use a torque sensor to record the torque and angle of each bolt to be tested during the assembly process, as shown in Table 2-1. At the same time, use an ultrasonic length measuring instrument with a temperature sensor to measure the axial force of each bolt to be tested after assembly, as shown in Table 2-2.

[0069] Table 2-1, M8 Aluminum Bolts - Formal Assembly Torque, Axial Force, and Angle Data

[0070] Table 2-2, M8 Aluminum Bolts - Formal Assembly Torque and Axial Force Data

[0071] (3) Torque range determination

[0072] The maximum torque Tmax of the bolt to be tested during formal assembly should be determined by referring to the installation torque range in Table 2-3 below. If Tmax is within the installation torque range of the M8 aluminum bolt, then the in-service residual axial force measurement process and temperature cycling process in step 3 can be carried out. If Tmax is not within the installation torque range of the M8 aluminum bolt, then the assembly process parameters should be reassembled or adjusted according to the original process parameters.

[0073] Table 2-3 Installation Torque Range for Common Aluminum Bolt Specifications

[0074] It should be noted that: 1. During the tightening process of the bolts to be tested, the torque and angle, two dynamic control parameters, are directly collected in real time through a torque sensor and an angle encoder to ensure that the assembly process conforms to the process specifications. This is a direct means of achieving the target axial force; 2. In this embodiment, each of the 10 bolts to be tested has its own corresponding torque-axial force-angle assembly data. Only the data for bolts No. 1 and No. 2 are listed here. Figure 5 and Figure 6 The assembly data of the other samples are not listed one by one; 3. Assembly torque value = maximum torque value in the torque-axial force-angle curve, assembly axial force = maximum axial force value in the torque-axial force-angle curve.

[0075] III. Measurement process of residual axial force in service and temperature cycling process, i.e., S3 of Example 1

[0076] (1) Short-term relaxation axial force test (after 1 hour of assembly)

[0077] After assembly, the bolt connection points to be tested are left to stand at room temperature for 1 hour. Then, the residual axial force F0 and flight time t0 of each bolt are measured using an ultrasonic length measuring instrument. The system automatically completes temperature compensation.

[0078] (2) Temperature cycling test

[0079] Place the entire bolted connection structure in a constant temperature test chamber and set the temperature cycling conditions. Temperature cycling conditions: cycling temperature 90℃, heating and cooling rate 1K / min (depending on the actual equipment capacity), cycling time 100h per cycle, and 3 cycles.

[0080] (3) Residual axial force test at each stage of temperature cycling

[0081] After each cycle, the components were cooled to room temperature and stabilized for a period of time. Then, an ultrasonic length gauge with a temperature sensor was used to measure and record the uncorrected residual axial force F of each bolt under test. i The corresponding flight time t is shown in Table 3 below. The instrument can record the real-time temperature and automatically perform compensation calculations based on the temperature difference between the real-time temperature and the temperature at calibration time to correct the axial force error caused by the change in ultrasonic propagation speed due to temperature.

[0082] Table 3. Measurement of residual axial force F of the bolt under test under various temperature cycles (uncorrected) i Flight time t data

[0083] IV. Disassemble the bolt to be tested and test the flight time of the bolt under no-load conditions, i.e., S4 in Example 1.

[0084] (1) After all temperature cycles are completed, remove all bolts to be tested and record the removal torque; (2) Then, under no-load conditions, the no-load flight time t4 of each bolt to be tested was measured and recorded using an ultrasonic length measuring instrument, as shown in Table 4.

[0085] Table 4. Disassembly torque and no-load flight time data of the bolt under test after completing all temperature cycles.

[0086] V. Secondary calibration of the bolt to be tested, i.e., S5 in Example 1

[0087] (1) Perform secondary calibration on each bolt to be tested that has been removed after all temperature cycles in step four. The operation method is the same as the initial calibration as step one. The initial calibration axial force F and flight time t data are shown in Table 5. The secondary calibration F'-t' curve of the bolt to be tested is obtained through the data in Table 5. (2) In this embodiment, each of the 10 bolts to be tested has its own corresponding secondary calibration F'-t' curve. Here, only the secondary calibration data of parts 1 and 2 are listed, such as Figure 7 and Figure 8 The secondary calibration data of the other samples are not listed one by one.

[0088] Table 5. Secondary calibration data for M8 aluminum bolts

[0089] VI. Actual residual axial force F i 'Calculation, i.e., S6 of Example 1'

[0090] 6.1 The first type of real residual axial force Fi How to obtain '

[0091] (1) The flight time t of each stage of the temperature cycle of the bolt under test. n The difference between the flight time and the no-load flight time tn+1, and the absolute value of the difference, is Δt = |t n -t n+1 Substituting △t into the secondary calibration curve, the corresponding axial force F' is the true residual axial force of the bolt to be tested after deducting the "false axial force" introduced by plastic deformation at each stage.

[0092] (2) All 10 bolts to be tested in this case have their own corresponding secondary calibration data. Only the secondary calibration correction data of bolts No. 1 and No. 2 are listed here, as shown in Table 6. Figure 9 and Figure 10 The secondary calibration correction data for the other samples are not listed one by one.

[0093] Table 6.1. Actual residual axial force F of the bolt under test after deducting the effect of plastic deformation during various temperature cycles. i 'data

[0094] Note: The data in each column of the table below must match the corresponding serial number of the bolt specimen to be tested.

[0095] 1. Calculation of flight time t: Flight time t1 is the third step to complete the temperature cycle. The flight time t1 of test 1, such as t1=100.6ns for component 1.

[0096] Flight time t2 is the third step to complete the temperature cycle. 2. The flight time t2 of the test, such as t2=98.1ns for item 1.

[0097] Flight time t3 is the third step to complete the temperature cycle. 3. The flight time t3 of the test, such as t3=95.8ns for component 1.

[0098] 2. Calculation of flight time difference Δt: The flight time difference Δt1 = |flight time t1 - no-load flight time t4|, as in item 1.

[0099] Flight time difference Δt1 = |flight time t1 - no-load flight time t4| = |100.6 - 12.6| = 88.0 ns.

[0100] The flight time difference Δt2 = |flight time t2 - no-load flight time t4|, as in item 1.

[0101] The flight time difference Δt2 = |flight time t2 - no-load flight time t4| = |98.1 - 12.6| = 85.5 ns.

[0102] The flight time difference Δt3 = |flight time t3 - no-load flight time t4|, as in item 1.

[0103] The flight time difference Δt3 = |flight time t3 - no-load flight time t4| = |95.8 - 12.6| = 83.2 ns.

[0104] 3. Corrected true residual axial force F i Calculation of ': 1) Substitute the flight time difference Δt1 as the abscissa into the second-calibrated F'-t' curve, and read the corresponding ordinate value, which is the temperature cycle. 1. Corrected actual residual axial force F1'.

[0105] If the flight time difference Δt1 = 88.0 ns for component 1 is used as the abscissa and substituted into the F'-t' curve of the secondary calibration in step 5, the corresponding ordinate value F1' = 8.1 kN is read. Figure 9 Finally, the percentage of axial force error is obtained according to equation (1): η 1 =(F1-F1') / F1×100%=17.73%.

[0106] 2) Substitute the flight time difference Δt2 as the abscissa into the second-calibrated F'-t' curve, and read the corresponding ordinate value, which is the temperature cycle. 2. Corrected actual residual axial force F2'.

[0107] If the flight time difference Δt2 = 85.5 ns for component 1 is used as the horizontal axis and substituted into the F'-t' curve of the secondary calibration in step 5, the corresponding vertical axis value F2' = 7.9 kN is read. Figure 9 Finally, the percentage of axial force error is obtained according to equation (1): η 2 =(F2-F2') / F2×100%=17.19%.

[0108] 3) Substitute the flight time difference Δt3 as the abscissa into the second-calibrated F'-t' curve, and read the corresponding ordinate value, which is the temperature cycle. 3. Corrected actual residual axial force F3'.

[0109] If the flight time difference Δt3 = 83.2 ns for component 1 is used as the horizontal axis and substituted into the F'-t' curve of the secondary calibration in step 5, the corresponding vertical axis value F3' = 7.7 kN is read. Figure 9 Finally, the percentage of axial force error is obtained according to equation (1): η3 =(F3-F3′) / F3×100%=12.42%.

[0110] 6.2 The second type of real residual axial force F i How to obtain '

[0111] (1) In the same coordinate system (the horizontal axis is the flight time t, and the vertical axis is the axial force F of the bolt to be measured), the entire secondary calibration curve is shifted along the positive direction of the horizontal axis by the unloaded flight time t. n+1 The translation correction curve is obtained. At this point, the flight time t of each stage of the temperature cycle is used as the reference. n x-axis: Then, the ordinate value of its intersection with the initial calibration curve represents the uncorrected residual axial force F (including plastic deformation). i ; The ordinate of the intersection point with the translated quadratic calibration curve represents the true residual axial force F after deducting the effect of plastic deformation. i '.

[0112] The difference between the two is ΔF = F i -F i ' is the illusory axial force introduced by plastic deformation.

[0113] (2) In this embodiment, each of the 10 bolts to be tested has its own corresponding secondary calibration data. Here, only the secondary calibration correction data of parts 1 and 2 are listed, as shown in Table 6.2-1 and Table 6.2-2. Figure 11 and Figure 12 The secondary calibration correction data for the other samples are not listed one by one.

[0114] Table 6.2-1. Actual residual axial force F of the bolt under test after deducting the influence of plastic deformation during various temperature cycles. i 'data

[0115] Table 6.2-2 Initial Calibration and Secondary Calibration Correction Data

[0116] Note: The data in each column of the table must match the corresponding serial number of the bolt specimen to be tested.

[0117] 1. Flight time after secondary calibration correction = Secondary calibration flight time t' + Idle flight time t4; 2. Calculation of flight time t: 1) Flight time t1 is the time when the temperature cycle is completed in step 3. 1. The flight time t1 of the test, such as component 1: t1 = 100.6 ns; 2) Flight time t2 is the time when the temperature cycle is completed in step 3. 2. The flight time t2 of the test, such as for component 1: t2 = 98.1 ns; 3) Flight time t3 is the time when the temperature cycle is completed in step 3. 3. The flight time t3 of the test, such as for item 1: t3 = 95.8 ns.

[0118] 3. Calculation of the corrected true residual axial force F': 1) Substitute the flight time t1 as the abscissa into the quadratic calibration correction curve in the axial force attenuation equivalent correction curve (e.g.) Figure 11 The temperature cycle is represented by the black dashed line in the middle. 1. Corrected actual residual axial force F1'; For example, for component number 1: the flight time t1 = 100.6 ns is used as the horizontal axis and substituted into the second calibration correction curve of the axial force attenuation equivalent correction curve. The corresponding vertical axis value F1' = 8.1 kN is then read. Figure 11 ; 2) Substitute the flight time t2 as the x-axis into the quadratic calibration correction curve in the axial force attenuation equivalent correction curve, and read the corresponding y-axis value, which is the temperature cycle. 2. Corrected actual residual axial force F2'; For example, for component No. 1: the flight time t2 = 98.1 ns is used as the abscissa and substituted into the second calibration correction curve of the axial force attenuation equivalent correction curve (e.g. Figure 11 (The black dashed line in the middle) Read the corresponding ordinate value F2' = 7.9kN, as shown. Figure 11 ; 3) Substitute the flight time t3 as the x-axis into the quadratic calibration correction curve in the axial force attenuation equivalent correction curve, and read the corresponding y-axis value, which is the temperature cycle value. 3. Corrected actual residual axial force F3'; For example, for component No. 1: the flight time t3 = 95.8 ns is used as the abscissa and substituted into the second calibration correction curve of the axial force attenuation equivalent correction curve (e.g. Figure 11 (The black dashed line in the middle) Read the corresponding ordinate value F3' = 7.7kN, as shown. Figure 11 ; If F is needed i The calculation process for ΔF then continues from step 3 in note 3: the flight time t... n Substitute the x-axis into the initial calibration curve (e.g.) Figure 11 The blue curve in the middle represents the uncorrected residual axial force F after temperature cycling. The corresponding ordinate value is the value of the curve. i The uncorrected residual axial force F can be calculated. i With the actual residual axial force F i The difference ΔF between 'n =F i -F i ', that is, the illusory axial force introduced by plastic deformation.

[0119] For example, for component number 1: the flight time t1 = 100.6 ns is used as the horizontal axis and substituted into the initial calibration curve. The corresponding vertical axis value is then read to obtain the temperature cycle value. 1. The uncorrected spurious axial force F1 = 9.846 kN. Therefore, the spurious axial force ΔF1 introduced by plastic deformation can be calculated as ΔF1 = F1 - F1' = 9.846 - 8.1 = 1.746 kN. Finally, the percentage of axial force error is obtained according to equation (1): η 1 =(F1-F1') / F1×100%=17.73%.

[0120] For example, for component number 1: the flight time t2 = 98.1 ns is used as the horizontal axis and substituted into the initial calibration curve. The corresponding vertical axis value is then read to obtain the temperature cycle value. 2. The uncorrected spurious axial force F2 = 9.540 kN, from which the spurious axial force ΔF2 introduced by plastic deformation can be calculated as F2 - F2' = 9.540 - 7.9 = 1.64 kN. Finally, the percentage of axial force error is obtained according to equation (1): η 2 =(F2-F2') / F2×100%=17.19%.

[0121] For example, for component number 1: the flight time t3 = 95.8 ns is used as the horizontal axis and substituted into the initial calibration curve. The corresponding vertical axis value is then read to determine the temperature cycle. 3. The uncorrected spurious axial force F3 = 9.474 kN, from which the spurious axial force ΔF3 introduced by plastic deformation can be calculated as: ΔF3 = F3 - F3' = 9.474 - 7.7 = 1.774 kN. Finally, the percentage of axial force error is obtained according to equation (1): η 3 =(F3-F3′) / F3×100%=12.42%.

[0122] Taking component number 1 as an example, the first type of real residual axial force F i The method used to obtain F' yielded F1' of 8.1 kN, F2' of 7.9 kN, and F3' of 7.7 kN; the second method yielded the actual residual axial force F i The method used to obtain F' yields F1' as 8.1 kN, F2' as 7.9 kN, and F3' as 7.7 kN. This demonstrates that the actual residual axial force F in both cases obtained using the present invention is... i The actual residual axial force F obtained by the method of ' is i 'same.

[0123] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for testing the axial force attenuation characteristics of a bolted connection structure, characterized in that, The process is as follows: S1. Measure the flight time change t and axial force F of the bolt to be tested to obtain the initial calibration Ft curve; S2. Assemble the bolt to be tested with the assembly clamp and the assembly counter to obtain a bolted connection structure, then proceed to S3. S3. Perform n temperature cycle tests on the bolted connection structure sequentially; after each temperature cycle, cool the bolted connection structure to room temperature and measure the uncorrected residual axial force F of the bolt under test. i and the corresponding flight time t i And there exists 1≤i≤n and n and i are both integers. After all temperature cycle tests are completed, proceed to S4; S4. Disassemble the bolted connection structure, and then test the no-load flight time t of the bolt under test in an unloaded state after treatment in S3. n+1 Enter S5; S5. Measure the flight time change t' and residual axial force F' of the bolt under test after processing by S4 to obtain the secondary calibration F'-t' curve of the bolt under test; S6. Flight time t based on the temperature cycle test corresponding to S3. i and uncorrected residual axial force F i The idle flight time t obtained by S4 n+1 And the secondary calibration F'-t' curve obtained from S5 yields the actual residual axial force F after the corresponding temperature cycling test. i and percentage of axial force error η i .

2. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 1, characterized in that, The actual residual axial force F i The method to obtain ' is as follows: A1. Calculate the absolute value Δt of the difference between the flight time and the empty flight time for the corresponding temperature cycle test using equation (1). i : △t i =|t i -t n+1 |……Equation (1); A2. Based on the initial calibration Ft curve obtained from S1 and the absolute value Δt of the difference between the flight time and the empty flight time during the corresponding temperature cycle test. i Find the actual residual axial force F after the corresponding temperature cycling test. i Specifically, in the quadratic calibration F'-t' curve obtained in S5, Δt is used as... i The value on the ordinate corresponding to the horizontal axis is the actual residual axial force F after the corresponding temperature cycling test. i '.

3. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 1, characterized in that, The actual residual axial force F i The method to obtain ' is as follows: B1. Integrate the initial calibration Ft curve obtained in S1 and the secondary calibration F'-t' curve obtained in S5 into the same coordinate system; B2. Translate the quadratic calibration curve F'-t' in the same coordinate system along the x-axis, and the translation distance is equal to the empty flight time t. n+1 This yields the double calibration curve; B3. Based on the dual calibration curve obtained in B2 and the corresponding flight time t of the temperature cycling test. i Find the actual residual axial force F after the corresponding temperature cycling test. i Specifically, in the dual-calibration curve, the horizontal axis represents the flight time t. i The ordinate value corresponding to the F'-t' curve after time translation and secondary calibration is the true residual axial force F after the corresponding temperature cycle test. i '.

4. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 2 or 3, characterized in that: The percentage of axial force error η i We obtain the following from equation (2): ... Equation (2).

5. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 4, characterized in that, S1 includes the following steps: S1.1 Assemble the bolt to be tested with the calibration clamp, calibration opponent, front fixture and rear fixture, and then clamp the whole assembly on the torque machine, ensuring that the clamping length, engagement length and other parameters are consistent with the actual use conditions during clamping; then assemble the ultrasonic length measuring instrument and axial force sensor with the bolt to be tested respectively, wherein the piezoelectric ceramic plate of the ultrasonic length measuring instrument is attached to the center of the axis of the head of the bolt to be tested, and the calibration opponent is a calibration toothed sleeve or nut; S1.2 Tighten the bolt to the point exceeding the maximum axial force of the bolt under test using the over-yield tightening method to obtain the axial force response characteristics of the bolt under test in the full working strain range. The torque and angle parameters are collected in real time by a torque sensor and an angle encoder to determine whether the maximum axial force of the bolt under test has been exceeded. S1.

3. Measure the flight time change t of the bolt under test using the ultrasonic length measuring instrument, directly measure the axial force F using the axial force sensor, and obtain the initial calibration Ft curve. Read the maximum axial force Fmax in the initial calibration Ft curve and compare it with the specified installation axial force range. If the maximum axial force Fmax is less than the specified installation axial force range, replace the bolt under test or lubricate the calibration pair, re-calibrate, and return to S1.

1. If the maximum axial force Fmax is within the specified axial force range, proceed to S2.

6. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 5, characterized in that, S2 includes the following steps: S2.1 Connect the bolt to be tested to the assembly clamp and the assembly handpiece. During the tightening process of the bolt to be tested, the torque and rotation angle parameters are collected in real time by the torque sensor and the angle encoder to obtain the axial force-torque-angle curve of the assembly process. S2.

2. Compare the maximum torque Tmax in the axial force-torque-angle curve with the specified installation torque range of the bolt to be tested. If the maximum torque Tmax is less than the specified installation torque range, replace the bolt to be tested or adjust the assembly process parameters and return to S2.

1. If the maximum torque Tmax is within the installation torque range, define the bolt to be tested, the assembly clamp and the assembly counter as a bolted connection structure and proceed to S3.

7. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 6, characterized in that, S3 includes the following steps: S3.

1. Under room temperature conditions, after the bolted connection structure is left to stand for 0.5h to 2h, the flight time t0 and the uncorrected residual axial force F0 of the bolt to be tested are directly measured by the ultrasonic length measuring instrument. The uncorrected residual axial force F0 is obtained by the ultrasonic length measuring instrument according to the initial calibration Ft curve obtained in S1, and then proceeds to S3.

2. S3.

2. The bolted connection structure is subjected to n temperature cycle tests in sequence; and after each temperature cycle, the bolted connection structure is cooled to room temperature, and the uncorrected residual axial force F is directly measured using the ultrasonic length measuring instrument. i and the corresponding flight time t i And there exists 1≤i≤n, where n and i are both integers, the uncorrected residual axial force F i The initial calibration Ft curve obtained from S1 is obtained using an ultrasonic length measuring instrument. After all temperature cycling tests are completed, the process proceeds to S4.

8. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 7, characterized in that, S4 specifically involves: disassembling the bolted connection structure and recording the disassembly torque; after disassembly, using the ultrasonic length measuring instrument to test the no-load flight time t of the bolt treated in S3 under no-load conditions. n+1 Enter S5.

9. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 8, characterized in that, S5 specifically involves assembling the bolt to be tested, processed by S4, with the calibration clamp and calibration counterpart, then assembling the ultrasonic length measuring instrument and the axial force sensor with the bolt to be tested respectively. The ultrasonic length measuring instrument then measures the time-of-flight change t' of the bolt to be tested, and the axial force sensor measures the residual axial force F' of the bolt to be tested, thus obtaining the secondary calibration F'-t' curve of the bolt to be tested.

10. The method for testing the axial force attenuation characteristics of a bolted connection structure according to claim 9, characterized in that: During the process of measuring the flight time change by the ultrasonic length measuring instrument, the real-time temperature is measured simultaneously, and the ultrasonic length measuring instrument performs compensation calculations based on the temperature difference between the real-time temperature and the temperature at the calibration time.

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