Bolt axial force measurement method based on absolute acoustic time ratio of ultrasonic transverse and longitudinal waves

Abstract

The invention relates to a method for measuring the axial force of a bolt based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves, comprising the following steps: 1. measuring the delay time of an ultrasonic stress detection system; 2. selecting bolts with the same material and heat treatment method as those of in-service bolts for laboratory calibration to obtain the absolute acoustic time ratio of transverse and longitudinal waves and a double-wave stress coefficient under zero stress; 3. obtaining the external dimensions of the in-service bolts, and using an ultrasonic stress detection system to obtain the absolute acoustic time of transverse waves and the absolute acoustic time of longitudinal waves of the in-service bolts after deducting the system delay; 4. performing temperature correction on the measured absolute acoustic time to obtain the corrected absolute acoustic time ratio of transverse and longitudinal waves; 5. correcting the double-wave stress coefficient of the in-service bolt obtained in step 2; 6. correcting the absolute acoustic time ratio of transverse and longitudinal waves of the in-service bolt under zero stress obtained in step 2; 7. obtaining the axial force of the in-service bolt according to the corrected absolute acoustic time ratio of transverse and longitudinal waves and the double-wave stress coefficient under zero stress in steps 5 and 6.

Description

Technical Field

[0001] The invention relates to the technical field of ultrasonic nondestructive testing, in particular to a method for measuring the axial force of a bolt based on absolute acoustic time ratio of ultrasonic transverse and longitudinal waves. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Bolts are commonly used fasteners. In important connection parts in the fields of wind turbines, turbines, bridges, aviation, etc., the axial force of the bolts needs to be strictly controlled. Inappropriate axial force will cause the nut to loosen or the bolt to break, causing the bolt connection to fail prematurely.

[0004] The bolt axial force test methods usually include torque wrench method, resistance strain gauge method and optical mechanics method, etc. These methods have certain limitations. The ultrasonic measurement technology of bolt axial force based on acoustic elastic effect is a low-cost, fast and non-destructive method for bolt axial force measurement. There are two main types of such ultrasonic methods:

[0005] The first is the single-wave method, which uses only longitudinal waves. This method has high measurement accuracy, but it requires measuring the length of the bolt and the acoustic time under zero stress before installation. It is suitable for measuring the preload force of newly installed bolts, but cannot be used to measure the preload force of in-service bolts.

[0006] The second is the dual-wave method, which uses longitudinal and shear waves at the same time, usually using coordination equations to achieve measurement. The dual-wave measurement technology has the following limitations:

[0007] It is necessary to accurately calculate the transverse and longitudinal wave speeds of the bolt under zero stress, which requires very high accuracy in measuring the length of the bolt.

[0008] The zero deviation of the ultrasonic probe is not considered when calculating the transverse and longitudinal wave acoustic time, or the secondary wave is used for zero deviation measurement without considering the reflection loss;

[0009] Bolts of the same specification and material are required for laboratory calibration. Calibration cannot be achieved if the equipment has no spare bolts, which limits the detection application of in-service bolts. Summary of the invention

[0010] In order to solve the technical problems existing in the above-mentioned background technology, the present invention provides a method for measuring the axial force of a bolt based on the absolute sound-time ratio of ultrasonic transverse and longitudinal waves. Based on an ultrasonic stress detector and a piezoelectric probe integrating transverse and longitudinal waves, the delay time of the ultrasonic stress detection system is firstly measured, and then the bolts with the same material and heat treatment method as the measured bolts are calibrated to measure the dual-wave stress coefficient, the absolute sound-time of transverse and longitudinal waves of the in-service bolts is measured, and the measured sound-time is corrected for temperature and stress coefficient, and finally the axial force of the bolt is calculated according to the corrected sound-time ratio of transverse and longitudinal waves and the dual-wave stress coefficient.

[0011] In order to achieve the above object, the present invention adopts the following technical solution:

[0012] The first aspect of the present invention provides a method for measuring the axial force of a bolt based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves, comprising the following steps:

[0013] Step 1: Measure the delay time of the ultrasonic stress detection system;

[0014] Step 2: Select bolts with the same material and heat treatment method as the in-service bolts for laboratory calibration to obtain the absolute acoustic time ratio of transverse and longitudinal waves and the double-wave stress coefficient under zero stress;

[0015] Step 3: Obtain the external dimensions of the in-service bolts, and obtain the absolute sound time of the shear wave and the absolute sound time of the longitudinal wave of the in-service bolts after deducting the system delay based on the ultrasonic stress detection system;

[0016] Step 4: Perform temperature correction on the measured absolute acoustic time to obtain the corrected absolute acoustic time of transverse waves, the absolute acoustic time of longitudinal waves, and the corrected absolute acoustic time ratio of transverse and longitudinal waves;

[0017] Step 5: Correct the double-wave stress coefficient of the in-service bolt obtained in step 2;

[0018] Step 6: Correct the absolute acoustic time ratio of the transverse and longitudinal waves of the in-service bolt under zero stress obtained in step 2;

[0019] Step 7: Based on the results of steps 5 and 6, the axial force of the in-service bolt is obtained.

[0020] Step 1, specifically:

[0021] Workpieces with the same material and heat treatment method as the in-service bolts are selected to make comparison specimens of different lengths. The length range of the comparison specimens covers the range of the in-service bolts being tested. The first echo sound time of the shear wave and longitudinal wave of the specimens of each length are measured respectively, and the system delay time of the shear wave and the system delay time of the longitudinal wave are obtained after linear fitting; the first echo sound time is t=t0+2S / V, t0 is the system delay time, S is the length of the specimen, and V0 is the speed of sound in the specimen.

[0022] In step 1, if the detection instrument or ultrasonic probe is replaced, the system delay time measurement is performed again.

[0023] Step 2, specifically:

[0024] Select a bolt with the same material and heat treatment method as the in-service bolt and fix it on the calibration platform. Obtain the length L and clamping length L1 of the bolt, and record the temperature C and the shear wave acoustic time T under zero stress at this time. S0 and longitudinal wave sound time T L0 , pressurize the calibration platform and record the shear wave acoustic time T of the bolt under different stresses S1 and longitudinal wave sound time T L1 , deduct the system delay time of transverse wave and longitudinal wave respectively, and obtain the absolute sound time of transverse wave and longitudinal wave in the bolt;

[0025] According to the measured absolute acoustic time of the shear wave and the longitudinal wave, the absolute acoustic time ratio of the shear wave and the longitudinal wave under different stresses is obtained;

[0026] All stress values ​​σ and the corresponding absolute acoustic time ratios of transverse and longitudinal waves After linear fitting, the absolute acoustic time ratio of transverse and longitudinal waves under zero stress is obtained: and the value of the double-wave stress coefficient A;

[0027] After repeating the above steps for a set number of times, the average value is taken as the final value of the transverse and longitudinal wave acoustic time ratio and the double-wave stress coefficient under zero stress.

[0028] The calibration platform in step 2 includes a hydraulic sleeve, an integrated ultrasonic longitudinal and shear wave probe and a temperature sensor. The bolt to be tested is fixed on the hydraulic sleeve by a nut. The hydraulic sleeve is respectively connected to the dynamometer and the hydraulic device. The integrated ultrasonic longitudinal and shear wave probe and the temperature sensor are arranged at both ends of the bolt to be tested. The integrated ultrasonic longitudinal and shear wave probe has a magnetic suction device. The integrated ultrasonic longitudinal and shear wave probe is connected to the bolt ultrasonic stress detector through a cable.

[0029] Step 3 is as follows:

[0030] Obtain the length L' and clamping length L1' of the in-service bolt to be tested, and use the ultrasonic stress detection system to obtain the shear wave acoustic time T of the in-service bolt S1 and longitudinal wave sound time T L1 , and the absolute acoustic time of the shear wave of the in-service bolt after deducting the system delay is obtained respectively. and the absolute acoustic time of longitudinal waves

[0031] Step 4 is as follows:

[0032] The absolute acoustic time obtained in step 3 is corrected for temperature according to the following formula to obtain the corrected absolute acoustic time of shear wave: and the absolute acoustic time of longitudinal waves according to The corrected absolute acoustic time ratio of transverse and longitudinal waves is obtained

[0033]

[0034]

[0035] Where L' is the total length of the in-service bolt, Vs0 is the sound velocity of the shear wave under zero stress obtained in step 1, C2 is the temperature of the in-service bolt during inspection, C1 is the temperature during laboratory calibration, Cs is the temperature coefficient of the shear wave sound velocity, V L0 is the sound velocity of the longitudinal wave under zero stress obtained in step 1, C L is the temperature coefficient of longitudinal wave speed of sound;

[0036] Step 5 is as follows:

[0037] The double wave stress coefficient of the in-service bolts is corrected as follows:

[0038]

[0039] Where A1 is the corrected double-wave stress coefficient, L is the total length of the calibrated bolt, L0 is the stress-bearing length of the calibrated bolt, L' is the total length of the in-service bolt, and L'0 is the stress-bearing length of the in-service bolt.

[0040] Step 6 is as follows:

[0041] The absolute acoustic time ratio of transverse and longitudinal waves of in-service bolts under zero stress is corrected by the following formula:

[0042]

[0043] in, is the absolute acoustic-to-temporal ratio of transverse and longitudinal waves under zero stress after correction, L' is the total length of the in-service bolt, C1 is the temperature during laboratory calibration, C2 is the temperature during in-service bolt testing, C L is the temperature coefficient of the longitudinal wave speed of sound, Cs is the temperature coefficient of the transverse wave speed of sound, is the absolute acoustic time ratio of transverse and longitudinal waves under zero stress, is the absolute acoustic time of the shear wave under zero stress, is the absolute acoustic time of longitudinal waves under zero stress.

[0044] Step 7 is as follows:

[0045] Using the corrected absolute acoustic time ratio of transverse and longitudinal waves under zero stress The axial force σ of the in-service wind turbine bolts is calculated using the double-wave stress coefficient A1, as shown in the following formula:

[0046]

[0047] in, is the corrected absolute acoustic time ratio of transverse and longitudinal waves.

[0048] Compared with the prior art, one or more of the above technical solutions have the following beneficial effects:

[0049] 1. By measuring the delay time of the ultrasonic stress detection system and selecting bolts with the same material and heat treatment method as the in-service bolts for laboratory calibration, on the one hand, the delay time of the ultrasonic stress detection system is measured, and on the other hand, the laboratory calibration and on-site detection both use the absolute acoustic time ratio of the transverse and longitudinal waves after deducting the system delay time, which is more in line with the theory of acoustic elasticity and has higher measurement accuracy.

[0050] 2. During laboratory calibration, bolts with the same material and heat treatment method as the in-service bolts can be selected. The same specifications are not required. When the equipment lacks spare bolts of the same specifications, it will not affect the axial stress detection of the in-service bolts. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0052] Figure 1 A schematic diagram of linear fitting of primary echo acoustic time data and sample length provided by one or more embodiments of the present invention;

[0053] Figure 2 A schematic diagram of a laboratory system calibration of bolts provided for one or more embodiments of the present invention;

[0054] Figure 3 A schematic diagram of dimensioning of calibrated bolts and in-service bolts provided for one or more embodiments of the present invention;

[0055] Figure 4 A linear fitting diagram of stress-transverse and longitudinal wave acoustic time ratio provided for one or more embodiments of the present invention;

[0056] In the figure, 1-bolt, 2-hydraulic sleeve, 3-nut, 4-dynamometer, 5-hydraulic device, 6-magnetic device, 7-ultrasonic longitudinal and transverse wave integrated probe, 8-temperature sensor, 9-cable, 10-bolt ultrasonic stress detector. DETAILED DESCRIPTION

[0057] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0058] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0059] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0060] As described in the background technology, in the method of using ultrasound to detect the axial force of bolts, the single-wave method is usually used before the bolts are installed and cannot be applied in scenarios where the bolts are in use, that is, it is not suitable for in-service bolts; the dual-wave method has certain limitations. It requires accurate calculation of the transverse and longitudinal wave sound velocities of the bolts under the zero stress state, and has high requirements for the measurement accuracy of the bolt length. It also needs to consider the zero bias of the ultrasonic probe when calculating the transverse and longitudinal wave sound velocities, or use secondary waves to calculate the reflection loss when measuring the zero bias. It also needs to use bolts of the same specification and material for laboratory calibration. When the equipment does not have spare bolts of the same specification and material, calibration cannot be achieved.

[0061] Therefore, the following embodiment provides a method for measuring the axial force of a bolt based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves. Based on an ultrasonic stress detector and a piezoelectric probe integrating transverse and longitudinal waves, the delay time of the ultrasonic stress detection system is first measured, and then the bolts with the same material and heat treatment method as the measured bolts are calibrated to measure the dual-wave stress coefficient. The absolute acoustic time of transverse and longitudinal waves of the in-service bolts is measured, and the measured acoustic time is corrected for temperature and stress coefficient. Finally, the axial force of the bolt is calculated based on the corrected acoustic time ratio of transverse and longitudinal waves and the dual-wave stress coefficient.

[0062] In the following embodiments, bolts installed on wind towers in a wind farm are taken as in-service bolts as examples, wherein no spare bolts are provided for the in-service wind towers.

[0063] Embodiment 1:

[0064] like Figure 1-4 As shown, the bolt axial force measurement method based on the absolute acoustic-time ratio of ultrasonic transverse and longitudinal waves includes the following steps:

[0065] Step 1: Measure the delay time of the ultrasonic stress detection system.

[0066] First, select workpieces with the same material as the bolts to be tested and make comparison samples of different lengths. The length range should cover the range of wind power bolts to be tested, such as samples with lengths of 100mm, 150mm, 200mm, 250mm, 300mm, 350mm, and 400mm. Apply a thin and uniform layer of shear wave special coupling agent to the end of the sample, then adsorb the ultrasonic longitudinal and shear wave integrated probe with a magnetic device to the end of the bolt, connect the bolt ultrasonic stress detector, and then connect the piezoelectric longitudinal and shear wave integrated probe to the ultrasonic stress detector, and measure and record the first echo sound time of the shear wave and longitudinal wave corresponding to each length of the sample.

[0067] The acoustic time t of the first echo of the sample received by the ultrasonic stress detector is composed of two parts: the propagation time t1 of the ultrasonic wave in the workpiece and the delay time t0 of the detection system, that is, the formula t = t0 + t1. The delay time t0 of the detection system is mainly composed of the delay of the electronic system of the instrument and the ultrasonic delay time of the probe protective layer.

[0068] Theoretically, for the same detection instrument and probe, the corresponding shear wave and longitudinal wave detection system delay time is a fixed value. According to the ultrasonic formula t = t0 + 2S / V0 (where S represents the length of the sample, V0 represents the speed of sound in the sample), it can be seen that the acoustic time t of the sample's first echo is linearly related to the sample L.

[0069] Linear fitting is performed on the first echo acoustic time data of the shear wave and longitudinal wave of the tested sample, respectively, as shown in Figure 1 As shown, the system delay time t of the shear wave measured this time can be obtained. s0 is 0.7015μs, and the system delay time of the longitudinal wave is t L0 It is 0.854μs.

[0070] If the detection instrument or ultrasonic probe is replaced, the system delay time must be measured again.

[0071] Step 2: Select bolts with the same material as the bolts being tested for laboratory calibration, and calculate the absolute acoustic time ratio of transverse and longitudinal waves and the double-wave stress coefficient under zero stress respectively.

[0072] like Figure 2 As shown, first install the bolt 1 in the hydraulic sleeve 2 in the calibration platform and tighten it with a nut 3. The sleeve is equipped with a dynamometer 4 and connected to a hydraulic device 5. Apply a thin and uniform layer of shear wave special coupling agent to the end of the bolt, then adsorb the ultrasonic longitudinal and shear wave integrated probe 7 with a magnetic device 6 and the temperature sensor 8 to the end of the bolt, and connect the bolt ultrasonic stress detector 10 through a cable 9. Before pressurizing, use a micrometer to measure the length L and the clamping length L1 of the calibration bolt, as shown in the figure. Figure 3 As shown in the figure, read the temperature C and the shear wave acoustic time T under zero stress.S0 and longitudinal wave sound time T L0 , and then use Figure 2 The hydraulic device 5 of the calibration platform is pressurized, and the shear wave sound time T of the detector under different stresses is recorded. S1 and longitudinal wave sound time T L1 , and then deduct the system delay time of the transverse wave and the longitudinal wave respectively, and the absolute sound time of the transverse wave and the longitudinal wave in the bolt can be obtained. That is:

[0073]

[0074]

[0075]

[0076]

[0077] in, The absolute acoustic time of a shear wave under zero stress is: is the absolute acoustic time of the longitudinal wave under zero stress, is the absolute acoustic time of shear waves under different stresses, Absolute acoustic time of longitudinal waves under different stresses.

[0078] Will and Substitute into formula (5) and calculate the absolute acoustic time ratio of transverse waves and longitudinal waves under different stresses:

[0079]

[0080] According to the theory of acoustic elasticity, it can be concluded that the absolute acoustic time ratio of transverse waves and longitudinal waves is linearly related to the magnitude of the axial stress, that is:

[0081]

[0082]

[0083] in, is the absolute acoustic time ratio of the shear wave and the longitudinal wave under zero stress, A is the double-wave stress coefficient, σ is the axial stress on the bolt, S is the cross-sectional area of ​​the bolt, and F is the axial tension on the bolt.

[0084] All stress values ​​σ and the corresponding absolute acoustic time ratios of transverse and longitudinal waves Perform a linear fit and get Curve graphs, such as Figure 4 As shown in the figure above, the absolute acoustic time ratio of transverse and longitudinal waves under zero stress can be obtained. The value of double wave stress coefficient A is 1.81983587 and -0.00001632.

[0085] Repeat step 2 to calibrate the sample 5 times, and take the 5 measurements. The average value of and A is taken as the final value of the transverse and longitudinal wave acoustic time ratio and the double-wave stress coefficient under zero stress.

[0086] Step 3: If Figure 3 , use a micrometer to measure the length L' and clamping length L1' of the in-service wind turbine bolt to be tested, then perform the test according to the method in step 2 and record the shear wave sound time T of the in-service bolt to be tested. S1 and longitudinal wave sound time T L1 Then, according to Formula 3 and Formula 4, the absolute acoustic time of the shear wave of the bolt under test after deducting the system delay is calculated. and the absolute acoustic time of longitudinal waves

[0087] Step 4: Perform temperature correction on the measured acoustic time according to formula 8 and formula 9 respectively to obtain the corrected absolute acoustic time of shear wave: and the absolute acoustic time of longitudinal waves Then, the corrected absolute acoustic time ratio of transverse and longitudinal waves is calculated according to formula 10.

[0088]

[0089]

[0090]

[0091] Where L' is the length of the in-service wind turbine bolt, Vs0 is the sound velocity of the shear wave under zero stress measured in step 1, C2 is the temperature of the in-service bolt during inspection, C1 is the temperature during laboratory calibration, Cs is the temperature coefficient of the shear wave sound velocity, V L0 is the sound velocity of the longitudinal wave under zero stress measured in step 1, C L is the temperature coefficient of longitudinal wave speed.

[0092] Step 5: Correct the double-wave stress coefficient of the in-service bolts according to formula 11.

[0093]

[0094] Where A1 is the corrected double-wave stress coefficient, L is the total length of the calibration bolt, and L0 is the stress-bearing length of the calibration bolt (e.g. Figure 3 Take L1+D, that is, the total length of the bolt + diameter), L' is the total length of the in-service bolt, and L'0 is the force-bearing length of the in-service bolt (such as Figure 3 Take L1'+D').

[0095] Step 6: Correct the absolute acoustic-to-time ratio of transverse and longitudinal waves of the in-service bolts under zero stress according to formula 12.

[0096]

[0097] in, is the absolute acoustic time ratio of transverse and longitudinal waves under corrected zero stress.

[0098] Step 7: Use the corrected absolute acoustic time ratio of transverse and longitudinal waves under zero stress and the double-wave stress coefficient to calculate the axial force σ of the in-service wind turbine bolts according to formula 13.

[0099]

[0100] The above embodiment uses a linear fitting method to measure the delay time of the ultrasonic stress detection system. Both laboratory calibration and on-site testing use the absolute acoustic time ratio of transverse and longitudinal waves after deducting the system delay time, which is more in line with the theory of acoustic elasticity and has higher measurement accuracy. The axial stress detection of the in-service bolts can be achieved by performing laboratory calibration on bolt specimens of the same material but different specifications as the in-service bolts to be tested, which solves the problem of having no spare bolts from the same batch for laboratory calibration of in-service wind towers in wind farms. The method can be applied to the axial stress detection of bolts on in-service wind towers in wind farms.

[0101] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for measuring the axial force of a bolt based on the absolute acoustic-to-temporal ratio of ultrasonic transverse and longitudinal waves, characterized in that: The following steps are involved: Step 1: Measure the delay time of the ultrasonic stress detection system; Step 2: Select bolts with the same material and heat treatment method as the in-service bolts for laboratory calibration to obtain the absolute acoustic time ratio of transverse and longitudinal waves and the double-wave stress coefficient under zero stress; Step 3: Obtain the external dimensions of the in-service bolts, and obtain the absolute sound time of the shear wave and the absolute sound time of the longitudinal wave of the in-service bolts after deducting the system delay based on the ultrasonic stress detection system; Step 4: Perform temperature correction on the measured absolute acoustic time to obtain the corrected absolute acoustic time of transverse waves, the absolute acoustic time of longitudinal waves, and the corrected absolute acoustic time ratio of transverse and longitudinal waves; Step 5: Correct the double-wave stress coefficient of the in-service bolt obtained in step 2; Step 6: Correct the absolute acoustic time ratio of the transverse and longitudinal waves of the in-service bolt under zero stress obtained in step 2; Step 7: Based on the results of steps 5 and 6, the axial force of the in-service bolt is obtained.

2. The bolt axial force measurement method based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 1 is characterized in that: The step 1 is specifically as follows: Workpieces with the same material and heat treatment method as the in-service bolts are selected to make comparison specimens of different lengths. The length range of the comparison specimens covers the range of the in-service bolts being tested. The first echo sound time of the shear wave and longitudinal wave of the specimens of each length are measured respectively, and the system delay time of the shear wave and the system delay time of the longitudinal wave are obtained after linear fitting; the first echo sound time is t=t0+2S / V, t0 is the system delay time, S is the length of the specimen, and V0 is the speed of sound in the specimen.

3. The bolt axial force measurement method based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 2 is characterized in that: In step 1, if the detection instrument or ultrasonic probe is replaced, the system delay time measurement is performed again.

4. The bolt axial force measurement method based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 1 is characterized in that: The step 2 is specifically: Select bolts with the same material and heat treatment method as the in-service bolts and fix them using the calibration platform, and obtain the length of the bolts L and clamping length L 1. Record the temperature at this time C and the shear wave acoustic time under zero stress T S0 and longitudinal sound T L0 , pressurize the calibration platform and record the shear wave sound time of the bolt under different stresses T S1 and longitudinal sound T L1 , deduct the system delay time of transverse wave and longitudinal wave respectively, and obtain the absolute sound time of transverse wave and longitudinal wave in the bolt; According to the measured absolute acoustic time of the shear wave and the longitudinal wave, the absolute acoustic time ratio of the shear wave and the longitudinal wave under different stresses is obtained; All stress values and the corresponding absolute acoustic time ratio of transverse and longitudinal waves After linear fitting, the absolute acoustic time ratio of transverse and longitudinal waves under zero stress is obtained: and double wave stress coefficient A The value of After repeating the above steps for a set number of times, the average value is taken as the final value of the transverse and longitudinal wave acoustic time ratio and the double-wave stress coefficient under zero stress.

5. The method for measuring the bolt axial force based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 4, characterized in that: The calibration platform includes a hydraulic sleeve, an integrated ultrasonic longitudinal and shear wave probe and a temperature sensor. The bolt to be tested is fixed to the hydraulic sleeve by a nut. The hydraulic sleeve is respectively connected to a dynamometer and a hydraulic device. The integrated ultrasonic longitudinal and shear wave probe and the temperature sensor are arranged at both ends of the bolt to be tested. The integrated ultrasonic longitudinal and shear wave probe has a magnetic suction device. The integrated ultrasonic longitudinal and shear wave probe is connected to the bolt ultrasonic stress detector through a cable.

6. The method for measuring the bolt axial force based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 1, characterized in that: The step 3 is specifically as follows: Get the length of the in-service bolt to be tested L＇ and clamping length L 1 ＇ , using the ultrasonic stress detection system to obtain the shear wave sound of the in-service bolt T S1 and longitudinal sound T L1 , and the absolute acoustic time of the shear wave of the in-service bolt after deducting the system delay is obtained respectively. and the absolute acoustic time of longitudinal waves .

7. The method for measuring the bolt axial force based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 1, characterized in that: The step 4 is specifically as follows: The absolute acoustic time obtained in step 3 is corrected for temperature according to the following formula to obtain the corrected absolute acoustic time of shear wave: and the absolute acoustic time of longitudinal waves ,according to The corrected absolute acoustic time ratio of transverse and longitudinal waves is obtained : ； ； in, is the total length of the bolts in service, V s0 is the sound velocity of shear wave under zero stress obtained in step 1, C 2 is the temperature of the in-service bolt during inspection, C 1 is the temperature during laboratory calibration, C s is the temperature coefficient of the shear wave velocity, V L0 is the sound velocity of the longitudinal wave under zero stress obtained in step 1, C L is the temperature coefficient of longitudinal wave speed.

8. The method for measuring the bolt axial force based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 1, characterized in that: The step 5 is specifically as follows: The double wave stress coefficient of the in-service bolts is corrected as follows: ； in A 1 is the corrected double-wave stress coefficient, L To calibrate the total length of the bolt, L 0 is the stress-bearing length of the calibrated bolt, is the total length of the bolt in service, is the stress-bearing length of the bolt in service.

9. The method for measuring the bolt axial force based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 1, characterized in that: The step 6 is specifically as follows: The absolute acoustic time ratio of transverse and longitudinal waves of in-service bolts under zero stress is corrected by the following formula: ； in, is the absolute acoustic time ratio of transverse and longitudinal waves under corrected zero stress, is the total length of the bolt in service, C 1 is the temperature during laboratory calibration, C 2 is the temperature of the in-service bolt during inspection, C L is the temperature coefficient of longitudinal wave speed of sound, Cs is the temperature coefficient of the shear wave velocity, is the absolute acoustic time ratio of transverse and longitudinal waves under zero stress, is the absolute acoustic time of the shear wave under zero stress, is the absolute acoustic time of longitudinal waves under zero stress.

10. The method for measuring the bolt axial force based on the absolute acoustic time ratio of ultrasonic transverse and longitudinal waves according to claim 1, characterized in that: The step 7 is specifically as follows: Using the corrected absolute acoustic time ratio of transverse and longitudinal waves under zero stress and double wave stress coefficient A 1 Calculate the axial force of the in-service wind turbine bolts , as shown below: ； in, is the corrected absolute acoustic time ratio of transverse and longitudinal waves.

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