Ultrasonic measurement method for pretightening force of bolt with variable clamping length

Abstract

The invention relates to an ultrasonic measurement method for the pre-tightening force of a bolt with a variable clamping length. The method comprises the following steps: performing an ultrasonic calibration experiment under a reference clamping length based on a calibration bolt, determining a reference pre-tightening force coefficient, and establishing a pre-tightening force coefficient theoretical model; inputting the actual clamping length of a to-be-measured bolt into the pre-tightening force coefficient theoretical model, and determining an actual pre-tightening force coefficient; performing ultrasonic pretightening force measurement on the to-be-measured bolt to obtain an ultrasonic signal transit time difference of the to-be-measured bolt; and calculating the pre-tightening force of the to-be-measured bolt based on the actual pre-tightening force coefficient and the ultrasonic signal transit time difference. By establishing a bolt pre-tightening force coefficient theoretical model, the pre-tightening force coefficient of the same specification of bolt at any clamping length can be accurately predicted only by performing a calibration experiment on the same specification of bolt at a single clamping length, on one hand, the pre-tightening force coefficient at different clamping lengths can be rapidly determined, and on the other hand, the pre-tightening force coefficient of the bolt at any clamping length can be accurately predicted; and on the other hand, high measurement precision can be ensured.

Description

Technical Field

[0001] This application relates to the field of ultrasonic nondestructive testing technology, and in particular to an ultrasonic method for measuring the preload of bolts with varying clamping length. Background Technology

[0002] In mechanical structures of major engineering fields such as aerospace and rail transportation, bolted connections are the core fastening method, and their reliability directly determines the overall integrity and service safety of the mechanical structure. However, bolts are prone to preload relaxation under long-term alternating loads, leading to functional degradation of the connection structure and, in severe cases, catastrophic cascading failures. Therefore, achieving real-time and accurate monitoring of bolt preload is a key technical requirement for ensuring the safe and stable operation of major engineering equipment.

[0003] Currently, there are various traditional methods in the field of bolt preload testing, such as torque control and resistance strain gauge methods. However, these methods generally have technical limitations: torque control is easily affected by fluctuations in the friction coefficient between the bolt and the connected parts, resulting in insufficient accuracy in preload control; resistance strain gauge methods require destructive installation of the bolts (such as attaching strain gauges), and each measurement takes a long time, resulting in low measurement efficiency. This makes it difficult to meet the needs of rapid monitoring of large quantities of bolts in engineering sites, thus limiting its engineering application scope.

[0004] In contrast, ultrasonic measurement technology based on the acoustoelastic effect has become the mainstream technology for preload monitoring in the industrial field due to its advantages of non-invasive measurement (no damage to the bolt structure) and millisecond-level dynamic response. The core principle of this technology is to accurately acquire the time delay change of ultrasonic waves during the axial propagation of the bolt and inversely obtain the current preload value of the bolt. The core limiting factor for its measurement accuracy lies in the accurate calibration of the preload coefficient—the accuracy of the preload coefficient directly determines the reliability of the mapping relationship between time delay and preload.

[0005] In practical engineering applications, due to the influence of machining tolerances of the clamped parts (such as thickness deviation), differences in structural form (such as flat parts and irregular parts), and the diversity of service conditions (such as vibration and temperature fluctuation), bolts of the same specification often need to be adapted to different clamping lengths (i.e. the actual force-bearing section length of the bolt) during the assembly process. Therefore, it is necessary to obtain the preload coefficient under the corresponding clamping length in order to ensure measurement accuracy.

[0006] However, the current method for calibrating the preload coefficient has significant drawbacks: for bolts of the same specification, a complete calibration experiment must be conducted separately for each change in clamping length, and according to industry standards, the number of calibration points for a single group of bolts must not be less than 5 (covering different preload levels). This "single clamping length corresponding to a single independent calibration" model results in a cumbersome and time-consuming calibration process, significantly reducing calibration efficiency. Furthermore, errors generated during multiple independent calibrations are easily accumulated and transmitted in subsequent field measurements, further reducing the accuracy of preload monitoring. Ultimately, this severely limits the potential for large-scale application of ultrasonic measurement technology based on the acoustoelastic effect in the engineering field.

[0007] Therefore, there is an urgent need in related technologies for a method that can achieve efficient and accurate bolt preload detection. Summary of the Invention

[0008] Therefore, it is necessary to provide an ultrasonic measurement method for bolt preload with varying clamping length that can achieve efficient and accurate detection of bolt preload, addressing the aforementioned technical problems.

[0009] In a first aspect, this application provides an ultrasonic measurement method for bolt preload force when clamping length varies. The method includes: Based on the ultrasonic calibration experiment of the calibration bolt under the reference clamping length, the reference preload coefficient was determined and a theoretical model of the preload coefficient was established. Input the actual clamping length of the bolt to be tested into the theoretical model of the preload coefficient to determine the actual preload coefficient. The ultrasonic preload force of the bolt under test is measured to obtain the ultrasonic signal transit time difference of the bolt under test. The preload of the bolt to be tested is calculated based on the actual preload coefficient and the ultrasonic signal transit time difference.

[0010] Optionally, in one embodiment of this application, the step of conducting an ultrasonic calibration experiment based on the calibration bolt at a reference clamping length to determine the reference preload coefficient and establish a theoretical model for the preload coefficient includes: A longitudinal wave ultrasonic transducer is placed at the center of the calibration bolt head, and a reference clamping length is set. Under no preload condition, ultrasonic preload is measured on the calibration bolt to obtain the reference ultrasonic signal transit time; Different preloads were applied to the calibration bolts, and ultrasonic preload measurements were performed on the calibration bolts under the preload condition to obtain the corresponding calibration ultrasonic signal transit time. Calculate the ultrasonic signal transit time difference under different preload conditions, and linearly fit the preload with the corresponding ultrasonic signal transit time difference to obtain the reference preload coefficient. The calibration coefficients of the theoretical model of preload coefficient are determined based on the theoretical expression of preload coefficient, reference preload coefficient, reference clamping length, and nominal bolt diameter.

[0011] Optionally, in one embodiment of this application, the expression for the theoretical model of the preload coefficient is:

[0012] in, This is the bolt preload coefficient, in kN / ns; These are calibration coefficients, in kN·mm / ns; Bolt clamping length, in mm; This is the nominal diameter of the bolt, in mm.

[0013] Optionally, in one embodiment of this application, the step of measuring the ultrasonic preload of the bolt under test to obtain the ultrasonic signal transit time difference of the bolt under test includes: A longitudinal wave ultrasonic transducer is placed at the center of the head of the bolt to be tested; When the bolt to be tested is not tightened, ultrasonic preload is measured to obtain the transit time of the reference ultrasonic signal; After the bolt to be tested is tightened, ultrasonic preload is measured to obtain the real-time ultrasonic signal transit time. The ultrasonic signal transit time difference of the bolt under test is calculated based on the reference ultrasonic signal transit time and the real-time ultrasonic signal transit time.

[0014] Optionally, in one embodiment of this application, the arrangement of the longitudinal wave ultrasonic transducer includes: The head of the calibration bolt or the bolt to be tested is ground, and a longitudinal wave ultrasonic transducer is attached to the ground bolt head. The surface roughness of the ground bolt head is <3.2µm.

[0015] Optionally, in one embodiment of this application, the longitudinal wave ultrasonic transducer has a center frequency of 10MHz and a diameter of 6mm.

[0016] Optionally, in one embodiment of this application, the actual clamping length is obtained by measuring the total thickness of the clamped part on-site using a laser rangefinder or digital caliper.

[0017] The above-mentioned ultrasonic measurement method for bolt preload with varying clamping length first involves conducting an ultrasonic calibration experiment on a calibrated bolt at a reference clamping length to determine a reference preload coefficient and establish a theoretical model for the preload coefficient. Then, the actual clamping length of the bolt to be tested is input into the theoretical model to determine the actual preload coefficient. Next, ultrasonic preload measurement is performed on the bolt to be tested to obtain the ultrasonic signal transit time difference. Finally, the preload of the bolt to be tested is calculated based on the actual preload coefficient and the ultrasonic signal transit time difference. In other words, by establishing a theoretical model for bolt preload coefficient, only one calibration experiment is needed for bolts of the same specification under a single clamping length to accurately predict the preload coefficient of bolts of the same specification at any clamping length, significantly improving the engineering efficiency of preload detection. Single calibration eliminates the error sources of the traditional method of multiple calibration processes under different clamping lengths, effectively suppressing the accumulation and propagation of errors, and greatly improving the accuracy and reliability of preload inversion. Addressing the core challenge in actual engineering where bolts of the same specification need to be adapted to multiple clamping lengths due to differences in the clamped parts, this model has universality and strong robustness, and can quickly and accurately adapt to working conditions with varying clamping lengths, breaking through the limitations of traditional calibration modes and greatly expanding the engineering application potential of ultrasonic technology. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating an ultrasonic measurement method for bolt preload with varying clamping length in one embodiment. Figure 2 This is a schematic diagram of bolt clamping in one embodiment; Figure 3 This is a schematic diagram of a linear fitting curve between the preload force and the corresponding ultrasonic signal transit time difference in one embodiment. Figure 4 This is a diagram showing the measurement error of the bolt preload in one embodiment. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0020] In one embodiment, such as Figure 1 As shown, an ultrasonic measurement method for bolt preload with varying clamping length is provided, comprising the following steps: S101: Based on the ultrasonic calibration experiment of the calibration bolt under the reference clamping length, the reference preload coefficient is determined and a theoretical model of the preload coefficient is established.

[0021] In this embodiment, a bolt of the same material and specification as the bolt to be tested is selected as the calibration bolt, and the reference clamping length is used. An ultrasonic calibration experiment was conducted to obtain the reference preload coefficient. Based on this, a theoretical model for the preload coefficient was established. Bolts of the same material and specification as the bolt under test refer to fully threaded hexagonal head bolts conforming to GB / T 5782 standard, with the same material, heat treatment process, nominal diameter, and thread specification, a strength grade of 8.8 or higher, and a nominal diameter range of M12-M30. In specific applications, four different lengths of 12.9 grade coarse-threaded bolts with a nominal diameter of 20 mm were used as examples to predict the preload coefficient and measure the preload under different clamping lengths. Detailed bolt parameters are shown in Table 1.

[0022] Table 1 Detailed parameters of the experimental bolts

[0023] In one embodiment of this application, the step of conducting an ultrasonic calibration experiment based on the calibration bolt at a reference clamping length to determine the reference preload coefficient and establish a theoretical model for the preload coefficient includes: S201: Arrange a longitudinal wave ultrasonic transducer at the center of the calibration bolt head and set the reference clamping length.

[0024] S202: Under no preload condition, ultrasonic preload is measured on the calibration bolt to obtain the reference ultrasonic signal transit time.

[0025] S203: Apply different preloads to the calibration bolts, and under the preload condition, perform ultrasonic preload measurement on the calibration bolts to obtain the corresponding calibration ultrasonic signal transit time.

[0026] S204: Calculate the ultrasonic signal transit time difference under different preload conditions, and linearly fit the preload with the corresponding ultrasonic signal transit time difference to obtain the reference preload coefficient.

[0027] S205: Calibration coefficients of the theoretical model for determining the preload coefficient based on the theoretical expression of the preload coefficient, the reference preload coefficient, the reference clamping length, and the nominal diameter of the bolt.

[0028] In one embodiment of this application, the head of the calibration bolt is first pre-treated, and a longitudinal wave ultrasonic transducer with a frequency of 10MHz and a diameter of 6mm is arranged at the center. The longitudinal wave ultrasonic transducer is attached to the bolt head with thread-locking adhesive, and the reference clamping length of the bolt is set by a tension clamp. , Figure 2The diagram shows the clamping of bolts, where 1 is tension clamp 1, 2 is longitudinal wave sensor, 3 is calibration bolt, 4 is clamping ring 1, 5 is limiting clamping device, 6 is clamping ring 2, and 7 is tension clamp 2. Table 2 shows the reference clamping length for bolts of various length specifications.

[0029] Table 2 shows the ultrasonic signal transit time difference for the calibration bolts.

[0030] Subsequently, under no preload condition, ultrasonic preload was measured on the calibration bolt to obtain the reference ultrasonic signal transit time. Subsequently, different preloads were applied to the calibration bolts using a tensile testing machine. Under preload conditions, ultrasonic preload measurements were performed on the calibration bolts to obtain the transit time of the calibration ultrasonic signal. , , ..., Then, a mutual algorithm was used to calculate the ultrasonic signal transit time difference under different preload conditions. As shown in Table 2, the reference preload coefficient is obtained by linearly fitting the preload force with the corresponding ultrasonic signal transit time difference. The linear fitting curve is as follows Figure 3 As shown. Finally, based on the theoretical expression of the preload coefficient and the reference preload coefficient... Reference clamping length and bolt nominal diameter Thus, the calibration coefficients are determined. The results are shown in Table 3.

[0031] Table 3 Bolt calibration results for calibrated bolts

[0032] In one embodiment of this application, the expression for the theoretical model of the preload coefficient is:

[0033] in, This is the bolt preload coefficient, in kN / ns; These are calibration coefficients, in kN·mm / ns; Bolt clamping length, in mm; This is the nominal diameter of the bolt, in mm.

[0034] In one embodiment of this application, based on Hooke's law and the principle of acoustoelasticity, the longitudinal wave transit time difference... With stress The relationship is:

[0035] in, The elastic modulus of the material; The acoustoelastic coefficient of the material; The equivalent stress length of the bolt, in mm; Wave velocity under zero stress, m / s; Based on bolt preload With axial force Relationship We can obtain:

[0036] in, The expression is:

[0037] in, The equivalent cross-sectional area of ​​the bolt is in mm. 2 .

[0038] When a bolt is subjected to axial force, the stress distribution along the central axis is uneven, and the stress-bearing length of the bolt includes not only the clamping length, but also the engagement area between the nut and the bolt and the bolt head, exhibiting a typical three-segment distribution characteristic, which can be divided into a stress-rising segment, a stress-uniform segment, and a stress-falling segment.

[0039] Among them, the magnitude of the stress along the central axis is related to the nominal diameter of the bolt. and clamping length The following functional relationship must be satisfied:

[0040] in, , which is the theoretical axial stress of the bolt, i.e., the stress magnitude in the uniform stress section; The equivalent stress length of a bolt can be expressed as:

[0041] Simplifying, we get:

[0042] Combining this with the expression for k, we get:

[0043] In the formula: , , These are all inherent properties of bolt materials, and are related to the bolt material and heat treatment method. This is related to the bolt specifications. Once the bolt material, heat treatment method, and specifications are determined, the original formula can be simplified to:

[0044] in, This is the bolt preload coefficient, in kN / ns; These are calibration coefficients, in kN·mm / ns; Bolt clamping length, in mm; This is the nominal diameter of the bolt, in mm.

[0045] S102: Input the actual clamping length of the bolt to be tested into the theoretical model of the preload coefficient to determine the actual preload coefficient.

[0046] In this embodiment of the application, based on the theoretical model of preload coefficient, the actual clamping length of the bolt to be tested is used. The corresponding preload coefficient is obtained. The theoretical models for preload of four different bolt length specifications (M20_L180, M20_L160, M20_L140, and M20_L120) are as follows: , , and Actual clamping length of four different bolt length specifications. As shown in Table 4, the corresponding preload coefficients are obtained based on the theoretical model of the preload coefficient. Furthermore, the preload coefficient calculated by the theoretical model was compared with the experimental calibration results. The results show that the theoretically calculated preload coefficient has good consistency with the experimental calibration results, and the relative error under all working conditions is less than 3.5%, which meets the engineering accuracy requirements.

[0047] Table 4 Prediction results of preload coefficient

[0048] In one embodiment of this application, the actual clamping length is obtained by measuring the total thickness of the clamped part on-site using a laser rangefinder or digital caliper.

[0049] S103: Perform ultrasonic preload measurement on the bolt to be tested to obtain the ultrasonic signal transit time difference of the bolt to be tested.

[0050] In this embodiment, a tensile testing machine is used to apply an axial load of 40-80kN (gradient 10kN) to simulate the preload under actual working conditions. Ultrasonic preload is measured on the bolt under test at each load stage to obtain the ultrasonic signal transit time difference of the bolt under test.

[0051] Specifically, in one embodiment of this application, the step of measuring the ultrasonic preload of the bolt under test to obtain the ultrasonic signal transit time difference of the bolt under test includes: S301: A longitudinal wave ultrasonic transducer is placed at the center of the head of the bolt to be tested.

[0052] S302: When the bolt to be tested is not tightened, perform ultrasonic preload measurement to obtain the transit time of the reference ultrasonic signal.

[0053] S303: After the bolt to be tested is tightened, ultrasonic preload is measured to obtain the real-time ultrasonic signal transit time.

[0054] S304: Calculate the ultrasonic signal transit time difference of the bolt under test based on the reference ultrasonic signal transit time and the real-time ultrasonic signal transit time.

[0055] In one embodiment of this application, the head of the bolt to be tested is first pre-treated, and a longitudinal wave ultrasonic transducer with a frequency of 10MHz and a diameter of 6mm is arranged on the bolt head using thread-locking adhesive. Then, under no preload condition, ultrasonic preload is measured to obtain the calibrated ultrasonic signal transit time. Subsequently, different preloads were applied to the calibration bolts using a tensile testing machine. Under preload conditions, ultrasonic preload measurements were performed to obtain the real-time ultrasonic signal transit time. Finally, the transit time difference of the ultrasonic signal in the bolt under test was calculated. .

[0056] In one embodiment of this application, the arrangement of the longitudinal wave ultrasonic transducer includes: The head of the calibration bolt or the bolt to be tested is ground, and a longitudinal wave ultrasonic transducer is attached to the ground bolt head. The surface roughness of the ground bolt head is <3.2µm.

[0057] In one embodiment of this application, the longitudinal wave ultrasonic transducer has a center frequency of 10MHz and a diameter of 6mm.

[0058] S104: Calculate the preload of the bolt to be tested based on the actual preload coefficient and the ultrasonic signal transit time difference.

[0059] In this embodiment of the application, the actual preload coefficient of the bolt to be tested is obtained. Time difference of ultrasonic signal transit with the bolt under test The preload of the bolt under test was calculated according to the bolt preload formula, and the results are shown in Table 5. The bolt preload formula is:

[0060] in, The bolt preload is expressed in kN. The bolt preload coefficient is expressed in kN / ns. Longitudinal wave transit time difference, ns.

[0061] Table 5. Bolt preload measurement results

[0062] Furthermore, calculate the measurement error of the bolt preload, such as... Figure 4 As shown, the results indicate that the relative error range of the bolt preload measured by the preload coefficient predicted by the theoretical model is 0.57%-3.5%, and the error amplitude decreases with the increase of axial load.

[0063] In the above-mentioned ultrasonic measurement method for bolt preload with varying clamping length, firstly, an ultrasonic calibration experiment is performed on the calibrated bolt at a reference clamping length to determine the reference preload coefficient and establish a theoretical model of the preload coefficient; then, the actual clamping length of the bolt to be tested is input into the theoretical model of the preload coefficient to determine the actual preload coefficient; next, ultrasonic preload measurement is performed on the bolt to be tested to obtain the ultrasonic signal transit time difference; finally, the preload of the bolt to be tested is calculated based on the actual preload coefficient and the ultrasonic signal transit time difference. In other words, by establishing a theoretical model for bolt preload coefficient, only one calibration experiment is needed for bolts of the same specification under a single clamping length to accurately predict the preload coefficient of bolts of the same specification at any clamping length, significantly improving the engineering efficiency of preload detection. Single calibration eliminates the error sources of the traditional method of multiple calibration processes under different clamping lengths, effectively suppressing the accumulation and propagation of errors, and greatly improving the accuracy and reliability of preload inversion. Addressing the core challenge in actual engineering where bolts of the same specification need to be adapted to multiple clamping lengths due to differences in the clamped parts, this model has universality and strong robustness, and can quickly and accurately adapt to working conditions with varying clamping lengths, breaking through the limitations of traditional calibration modes and greatly expanding the engineering application potential of ultrasonic technology.

[0064] It should be understood that although the steps in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the embodiments described above may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0065] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data used for analysis, data stored, data displayed, etc.) involved in this application are all information and data authorized by the user or fully authorized by all parties.

[0066] Those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium. When executed, the computer program can include the processes of the embodiments described above. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM). The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, etc., and are not limited to these.

[0067] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0068] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. An ultrasonic measurement method for bolt preload with varying clamping length, characterized in that, The method includes: Based on the ultrasonic calibration experiment of the calibration bolt under the reference clamping length, the reference preload coefficient was determined and a theoretical model of the preload coefficient was established. Input the actual clamping length of the bolt to be tested into the theoretical model of the preload coefficient to determine the actual preload coefficient. The ultrasonic preload force of the bolt under test is measured to obtain the ultrasonic signal transit time difference of the bolt under test. The preload of the bolt to be tested is calculated based on the actual preload coefficient and the ultrasonic signal transit time difference.

2. The ultrasonic measurement method for bolt preload with varying clamping length according to claim 1, characterized in that, The process of conducting ultrasonic calibration experiments on the calibration bolt at the reference clamping length to determine the reference preload coefficient and establish a theoretical model for the preload coefficient includes: A longitudinal wave ultrasonic transducer is placed at the center of the calibration bolt head, and a reference clamping length is set. Under no preload condition, ultrasonic preload is measured on the calibration bolt to obtain the reference ultrasonic signal transit time; Different preloads were applied to the calibration bolts, and ultrasonic preload measurements were performed on the calibration bolts under the preload condition to obtain the corresponding calibration ultrasonic signal transit time. Calculate the ultrasonic signal transit time difference under different preload conditions, and linearly fit the preload with the corresponding ultrasonic signal transit time difference to obtain the reference preload coefficient. The calibration coefficients of the theoretical model of preload coefficient are determined based on the theoretical expression of preload coefficient, reference preload coefficient, reference clamping length, and nominal bolt diameter.

3. The ultrasonic measurement method for bolt preload with varying clamping length according to claim 2, characterized in that, The expression for the theoretical model of the preload coefficient is as follows: in, This is the bolt preload coefficient, in kN / ns; These are calibration coefficients, in kN·mm / ns; Bolt clamping length, in mm; Nominal diameter of the bolt, in mm.

4. The ultrasonic measurement method for bolt preload with varying clamping length according to claim 1, characterized in that, The ultrasonic preload measurement of the bolt under test, and the ultrasonic signal transit time difference of the bolt under test, includes: A longitudinal wave ultrasonic transducer is placed at the center of the head of the bolt to be tested; When the bolt to be tested is not tightened, ultrasonic preload is measured to obtain the transit time of the reference ultrasonic signal; After the bolt to be tested is tightened, ultrasonic preload is measured to obtain the real-time ultrasonic signal transit time. The ultrasonic signal transit time difference of the bolt under test is calculated based on the reference ultrasonic signal transit time and the real-time ultrasonic signal transit time.

5. The ultrasonic measurement method for bolt preload with varying clamping length according to claim 2 or 4, characterized in that, The longitudinal wave ultrasonic transducer includes: The head of the calibration bolt or the bolt to be tested is ground, and a longitudinal wave ultrasonic transducer is attached to the ground bolt head. The surface roughness of the ground bolt head is <3.2µm.

6. The ultrasonic measurement method for bolt preload with varying clamping length according to claim 2 or 4, characterized in that, The longitudinal wave ultrasonic transducer has a center frequency of 10MHz and a diameter of 6mm.

7. The ultrasonic measurement method for bolt preload with varying clamping length according to claim 1, characterized in that, The actual clamping length is obtained by measuring the total thickness of the clamped part on-site using a laser rangefinder or digital caliper.

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