Bolt pretightening force ultrasonic detection method and system

Abstract

The invention discloses a bolt pre-tightening force ultrasonic detection method and system, and relates to the technical field of ultrasonic nondestructive testing, and the method comprises the steps: constructing a dual-echo bolt pre-tightening force detection model based on an acoustic elastic effect; the method comprises the following steps: acquiring an initial state of a bolt and an ultrasonic echo detection signal for applying a known load, and respectively intercepting a first echo wave band and a second echo wave band of the bottom surface of the bolt to obtain a pre-stored primary wave, a pre-stored secondary wave, a primary wave and a secondary wave; respectively obtaining a primary sound time difference and a secondary sound time difference according to the sound time differences of the primary wave, the pre-stored primary wave, the secondary wave and the pre-stored secondary wave, and drawing a calibration curve of the bolt pre-tightening force and the combined sound time difference; inputting ultrasonic detection echo data of the in-service bolt into the dual-echo bolt pre-tightening force detection model and the bolt pre-tightening force-combined sound time difference calibration curve to obtain the bolt pre-tightening force; according to the method, calibration errors caused by coupling layer error changes generated in multiple coupling measurement processes are reduced.

Description

Technical Field

[0001] This invention relates to the field of ultrasonic nondestructive testing technology, specifically to an ultrasonic testing method and system for bolt preload. Background Technology

[0002] Bolted connections are the most commonly used fasteners in industrial settings due to their detachability, high load-bearing capacity, low cost, and adaptability to harsh environments. They play an irreplaceable role in detachable parts and load transfer, and are widely used in connection structures such as spacecraft connecting plates, steel frame structure connections, rotor systems, pipeline connections, and wind turbine connections. However, excessive or insufficient bolt preload can lead to bolt loosening or even breakage. Therefore, measuring the preload of in-service bolts is of great significance, and ultrasonic testing technology is favored for its non-destructive and fast testing speed.

[0003] The acoustoelastic effect method has the advantages of simple principle and convenient implementation, and is the main method for determining the magnitude of bolt preload using ultrasonic testing. This method infers the current bolt preload by obtaining the calibration curve of a bolt based on the linear relationship between sound velocity and stress and the time of flight (TOF) difference between the preloaded and unpreloaded states.

[0004] Currently, many devices on the market use peak measurement or threshold measurement to extract TOF, which cannot overcome the weakness of the acoustic elastic effect and the limitations of equipment performance. They suffer from large data fluctuations, severe noise interference, and multiple coupling errors, making it difficult to achieve good detection quality in actual applications of judging the magnitude of bolt preload. Summary of the Invention

[0005] To address the shortcomings of existing technologies, such as the inability to overcome the weak acoustic elasticity effect and limitations in equipment performance, resulting in large data fluctuations, severe noise interference, and multiple coupling errors, this invention proposes an ultrasonic testing method and system for bolt preload. By using a combined acoustic time difference of the primary and secondary echoes to replace the primary acoustic time difference in plotting the bolt preload-acoustic time difference curve, the calibration error caused by the variation in coupling layer error during multiple coupling measurements is reduced, thereby solving the problems existing in the prior art.

[0006] An ultrasonic testing method for bolt preload includes the following steps: The ultrasonic echo detection signal of the bolt sample in its initial state is obtained, and the first echo band and the second echo band of the bottom surface of the bolt are extracted from the ultrasonic detection signal and recorded as the pre-stored first wave and the pre-stored second wave, respectively. After applying a known load to the bolt sample, ultrasonic echo detection signals were collected, and the first echo segment and the second echo segment on the bottom surface were extracted and recorded as the first wave and the second wave, respectively. Based on the acoustic time difference between the primary wave and the pre-stored primary wave, and the acoustic time difference between the secondary wave and the pre-stored secondary wave, the primary acoustic time difference and the secondary acoustic time difference are obtained respectively; with the applied load as the abscissa and the difference between the secondary acoustic time difference and the primary acoustic time difference as the ordinate, the calibration curve of bolt preload versus combined acoustic time difference is plotted. By combining the slope of the calibration curve with the double-echo bolt preload detection model based on the acoustoelastic effect, a new double-echo bolt preload detection model is obtained. The combined acoustic time difference of the ultrasonic echo detection signals of the in-service bolts is input into a new dual-echo bolt preload detection model to obtain the current preload of the bolts.

[0007] Furthermore, the ultrasonic echo detection signal is a time-domain echo signal received by a single ultrasonic probe at the head of the bolt under ultrasonic longitudinal wave pulse excitation; the time-domain echo signal includes a primary echo band and a secondary echo band at the bottom of the bolt.

[0008] Furthermore, the corresponding wavebands of the extracted ultrasonic echo detection signal are kept in the same position in their respective original signals.

[0009] Furthermore, the acoustic time difference between the primary wave and the pre-stored primary wave, and between the secondary wave and the pre-stored secondary wave, is calculated using a modal decomposition algorithm or an acoustic time extraction algorithm.

[0010] Furthermore, the ultrasonic echo detection signal of the bolt in its initial state is the ultrasonic echo detection signal collected from any bolt of the same batch and model under natural conditions.

[0011] Furthermore, the construction of the double-echo bolt preload detection model based on the acoustoelastic effect specifically includes the following steps: According to nonlinear acoustic theory, when an ultrasonic longitudinal wave propagates along the stress direction in a solid medium, changes in stress will cause corresponding changes in wave velocity. Therefore, the acoustoelastic effect can be expressed as: In the formula This represents the longitudinal wave velocity under applied stress. This represents the longitudinal wave velocity when no stress is applied. This represents stress in the same direction as the sound wave propagation. For the longitudinal wave, the acoustic elasticity factor is constant; The bolt length under zero load is defined as: ; In the formula Indicates the original length of the bolt as a whole; Indicates the original length of the non-clamped portion of the bolt; This indicates the original length of the bolt clamping portion; the clamping portion refers to the equivalent stress zone of the bolt under tension in a loaded state, and its length is consistent with the distance from the nut to the bolt. According to Hooke's Law, Elongation under load is represented as: ; In the formula This indicates the length of the bolt clamping portion under tension. Let the elastic modulus of the bolt material be represented, then the propagation time of the ultrasound is expressed as: ; ; ; ; in, When representing a single echo sound under zero load on a bolt, its sound path includes two coupling layers, two clamping regions, and two non-clamping regions; When representing the secondary echo sound under zero load condition of the bolt, its sound path includes two coupling layers, four clamping regions, and four non-clamping regions; This represents the composition of the sound path of a single echo sound under bolt loading conditions. They are consistent, but the thickness of their coupling layer and the length of their clamping region will change; This indicates the thickness of the coupling layer under zero load conditions. This indicates the thickness of the coupling layer under loaded conditions; similarly, When representing the secondary echo sound under bolt loading, its sound path includes two coupling layers, four clamping regions, and four non-clamping regions, but the thickness of the coupling layer and the length of the clamping region have changed; The combined acoustic time difference before and after bolt loading stress is then expressed as: ; in, This represents the combined echo time difference, which is the difference between the second echo time difference and the first echo time difference. Indicates the bolt stress calibration factor; The relationship between bolt preload and axial stress is expressed as follows: ; In the formula F Indicates the bolt preload. This represents the equivalent cross-sectional area of ​​the bolt. Using bolt preload force instead of axial stress, the relationship between bolt preload force difference and combined acoustic time difference is obtained: ; in, This indicates the bolt preload calibration factor.

[0012] This invention also includes a detection system for an ultrasonic testing method for bolt preload, comprising: The ultrasonic excitation module is used to output ultrasonic pulses to the bolt being tested according to user instructions; The echo acquisition module is used to receive the echo signal of the ultrasonic pulse rebounding at the bottom of the bolt being tested; The lower-level control module is used to receive instructions from the upper-level computer, send them to the excitation module, and upload the echo signal collected by the echo acquisition module to the upper-level computer. The host computer module is connected to the lower computer control module via a gigabit Ethernet PHY chip. It is used to receive the echo signal collected by the echo acquisition module and measure the bolt preload using the bolt preload ultrasonic detection method to assist the user in judging the bolt condition.

[0013] Furthermore, the ultrasonic excitation module includes a signal generation circuit, a power amplification circuit, and a piezoelectric ultrasonic transducer; the ultrasonic excitation module is used to generate an excitation waveform by sending an excitation command to the signal generation circuit, and the excitation waveform drives the piezoelectric ultrasonic probe to generate ultrasonic pulses via the power amplification circuit.

[0014] Furthermore, the echo acquisition module includes a low-voltage acquisition circuit, a signal amplification circuit, a high-speed ADC, and a piezoelectric ultrasonic transducer; the echo acquisition module is used to receive echo signals through a piezoelectric ultrasonic probe and convert them into electrical signals, and then convert the electrical signals into digital signals after passing through the signal amplification circuit.

[0015] Furthermore, the lower-level control module includes a ZYNQ development board, a direct memory access (DMA) controller, an interrupt system, and DDR memory. The lower-level control module is used to schedule the acquisition of data from the converted digital signal through the DMA controller of the ZYNQ development board and store it in the DDR memory space. The stored data is then transmitted to the upper-level computer via the gigabit network PHY interface using the UDP protocol.

[0016] This invention provides an ultrasonic testing method for bolt preload, which has the following advantages: This invention constructs a double-echo bolt preload detection model based on the acoustoelastic effect, records ultrasonic testing data of the bolt in its initial state, and plots its bolt preload-combined acoustic time difference calibration curve. The slope of the calibration curve is then combined with the double-echo bolt preload detection model based on the acoustoelastic effect to achieve bolt preload measurement from ultrasonic echo data of in-service bolts. This ultrasonic bolt preload detection method calculates the combined acoustic time difference of the primary wave and pre-stored primary wave, and the secondary wave and pre-stored secondary wave, to plot the bolt preload calibration curve. This reduces calibration errors caused by changes in coupling layer thickness during multiple coupled measurements and significantly enhances the detection system's noise suppression capability. It also provides a convenient and efficient ultrasonic testing system with advantages such as high accuracy, high reliability, and flexibility, making it of significant innovative and practical value in the field of ultrasonic nondestructive testing. Attached Figure Description

[0017] Figure 1 This is a flowchart of the ultrasonic testing method for bolt preload in an embodiment of the present invention; Figure 2 This is a schematic diagram of the ultrasonic testing method for bolt preload in an embodiment of the present invention; Figure 3 This is a flowchart of the method for drawing the combined acoustic time difference-preload calibration curve in an embodiment of the present invention; Figure 4 This is a schematic diagram of the overall architecture of the ultrasonic measurement system for bolt preload based on ZYNQ and a portable PC in an embodiment of the present invention. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0019] This invention proposes an ultrasonic testing method for bolt preload, such as... Figure 1 As shown, the specific steps include: S1. Construct a double-echo bolt preload detection model based on acoustoelastic effects. The double-echo bolt preload detection model includes:

[0020] According to nonlinear acoustic theory, the acoustoelastic effect can be described as follows: when an ultrasonic longitudinal wave propagates along the stress direction in a solid medium, the change in stress will cause a corresponding change in wave velocity, as shown in equation (1): (1) In the formula This represents the longitudinal wave velocity under applied stress. This represents the longitudinal wave velocity when no stress is applied. This represents stress in the same direction as the sound wave propagation. Denotes the second-order elastic constant. This represents the third-order elastic constant.

[0021] The above equation can be simplified to first order using the Taylor expansion method: (2) in, This represents the longitudinal wave velocity under applied stress. This represents the longitudinal wave velocity when no stress is applied. This represents stress in the same direction as the sound wave propagation. The acoustic elasticity factor constant for longitudinal waves is typically on the order of magnitude 1. Positive numbers.

[0022] like Figure 2 As shown, the bolt length under zero load is defined as: (3) In the formula This indicates the original length of the bolt as a whole. This indicates the original length of the non-clamped portion of the bolt. This represents the original length of the bolt clamping portion. The clamping portion refers to the equivalent stress zone of the bolt under tension under load; its length is approximately the same as the distance from the nut to the bolt. Specific influencing factors include the bolt clamping length, load type, and thread type. The original length of the clamping portion remains constant for a given bolt, according to Hooke's Law. Elongation under load is represented as:

[0023] (4) In the formula This indicates the length of the bolt clamping portion under tension. Let the elastic modulus of the bolt material be represented, then the propagation time of the ultrasound can be expressed as: (5) (6) (7) (8) in, When representing a single echo sound under zero load on a bolt, its sound path includes two coupling layers, two clamping regions, and two non-clamping regions; When representing the secondary echo sound under zero load condition of the bolt, its sound path includes two coupling layers, four clamping regions, and four non-clamping regions; This represents the composition of the sound path of a single echo sound under bolt loading conditions. While the results are consistent, the coupling layer thickness and clamping region length will vary. This is because ultrasonic measurements under normal conditions, both zero stress and loaded states, require two coupling operations with the ultrasonic probe, resulting in a change in the coupling layer thickness. This indicates the thickness of the coupling layer under zero load conditions. This indicates the thickness of the coupling layer under load; furthermore, due to the tension on the clamping portion, it will be affected by Hooke's Law, and its length will be determined by... become Similarly, When representing the secondary echo sound under bolt loading, its sound path includes two coupling layers, four clamping regions, and four non-clamping regions, but the thickness of the coupling layer and the length of the clamping region have changed.

[0024] In summary, the combined acoustic time difference before and after bolt loading stress can be expressed as: (9) Through experimental measurements and theoretical calculations, it can be known that... On the order of magnitude The positive number, while the axial tensile force on the bolt is only on the order of magnitude. Therefore, in equation (9) .in, This represents the combined echo time difference, which is the difference between the second echo time difference and the first echo time difference. This represents the bolt stress calibration factor, which is determined by the inherent properties of the bolt itself and the clamping length, and is a constant for installed bolts.

[0025] The relationship between bolt preload and axial stress can be expressed as: (10) In the formula F Indicates the bolt preload. This represents the equivalent cross-sectional area of ​​the bolt.

[0026] Using the bolt preload to replace the axial stress, we can obtain the expression: (11) in, Indicates the bolt preload calibration factor. Equation (11) is the relationship between the bolt preload difference and the combined acoustic time difference, and the two are linearly related. Compared with equation (9), equation (11) links the combined acoustic time difference with the preload, which is easier to measure in practical applications than stress.

[0027] The above-described double-echo bolt preload detection model based on acoustoelastic effect provided in this embodiment of the invention has no coupling layer error. The influence of this greatly optimizes the coupling error resistance of the acoustoelastic effect calibration method.

[0028] S2. Record the ultrasonic test data of a bolt of a certain specification under its initial state and plot its bolt preload-combined acoustic time-of-flight calibration curve. The ultrasonic test data under the initial state of the bolt refers to the ultrasonic echo detection signals collected from bolts of the same batch and model under natural conditions. Its time domain range should include the waveform characteristics of the first and second echoes from the bottom of the bolt. The method for plotting the bolt preload-combined acoustic time-of-flight calibration curve includes:

[0029] S2.1 Obtain ultrasonic test data of the bolt in its initial state.

[0030] S2.2. Extract the first and second echo segments from the bottom surface of the bolt and record them as the pre-stored first wave and the pre-stored second wave, respectively.

[0031] S2.3 Apply load to the bolt and acquire ultrasonic signals; The loading methods for the bolt in steps S2.1 and S2.3 include, but are not limited to, using loading equipment such as tensile testing machines and torque wrenches, and stress measurement equipment such as resistance strain gauges; The ultrasonic detection signals acquired in steps S2.1 and S2.3 are time-domain echo signals received by a single ultrasonic probe at the bolt head under ultrasonic longitudinal wave pulse excitation, and should include the primary and secondary echo bands at the bottom of the bolt.

[0032] S2.4. Extract the first and second echo bands from the bottom surface and label them as the first wave and the second wave, respectively. The corresponding bands extracted in steps S2.2 and S2.4 should maintain the same position in their respective original signals.

[0033] S2.5 Calculate the acoustic time difference between the primary wave and the pre-stored primary wave and record it as the primary acoustic time difference; S2.6 Calculate the acoustic time difference between the secondary wave and the pre-stored secondary wave, denoted as the secondary acoustic time difference; the acoustic time difference calculation methods in steps S2.5 and S2.6 include: mode decomposition algorithm and acoustic time extraction algorithm. The mode decomposition algorithm refers to the empirical wavelet transform (EWT) method, which includes time-frequency transformation, adaptive spectrum partitioning, configuration of wavelet filter banks, and modal quality assessment steps. Its principle is adaptive mode decomposition that combines the adaptivity of empirical mode decomposition and the theoretical completeness of wavelet transform; by decomposing the signal into multiple frequency bands, the longitudinal wave echo and noise, mode conversion wave, and guided wave signal in the ultrasonic detection signal are separated, thereby avoiding multi-mode waveform aliasing and greatly improving the anti-interference capability of the ultrasonic detection method.

[0034] The time-of-sound extraction algorithm refers to the cross-correlation algorithm, which includes signal denoising, cross-correlation value sequence calculation, and result evaluation steps. Its working principle is to find the optimal alignment position between the primary (secondary) wave signal and the pre-stored primary (secondary) wave signal by signal translation and cross-correlation function calculation. Compared with common peak comparison or threshold comparison methods, this method has higher time-of-sound resolution, which helps overcome the difficulty of insufficient equipment sampling frequency. Furthermore, as a global calculation method, it is less susceptible to interference from a few noise points with large amplitudes.

[0035] S2.7 Record data points with the applied load as the x-axis and the combined acoustic time difference as the y-axis.

[0036] S2.8 Determine whether the expected load value has been applied.

[0037] S2.9. Linear fitting of all data points yields the bolt preload-combined acoustic time difference calibration curve.

[0038] S3. Acquire ultrasonic measurement echo signals of in-service bolts.

[0039] S4. By combining the dual-echo bolt preload measurement model, the bolt preload-combined acoustic time difference calibration curve, and the ultrasonic detection echo data of currently in-service bolts, bolt preload measurement can be achieved.

[0040] Based on the same inventive concept, this invention also proposes an ultrasonic testing system for bolt preload, comprising: The ultrasonic excitation module includes a signal generation circuit, a power amplification circuit, and a piezoelectric ultrasonic transducer, which is used to output excitation pulses according to user instructions.

[0041] The echo acquisition module includes a low-voltage acquisition circuit, a signal amplification circuit, a high-speed ADC, and a piezoelectric ultrasonic transducer, used to receive echo signals from the object under test.

[0042] The lower-level control module, including the ZYNQ development board's ARM core, DMA controller, interrupt system, and DDR storage, is used to receive instructions from the upper-level computer, send them to the excitation module, and upload data from the acquisition module to the upper-level computer.

[0043] The host computer module includes peripherals such as a touch screen, keyboard, and mouse, as well as an interactive interface built on Qt software, which guides users to set control parameters and displays system configuration information in text and image form.

[0044] The host computer module should also include a data analysis module, which contains a double-echo bolt preload detection model based on the acoustoelastic effect, used for functions such as ultrasonic signal modal decomposition, high-precision acoustic time difference calculation, and bolt preload-acoustic time difference calibration curve fitting.

[0045] The lower-level control module and the upper-level computer provided in this embodiment of the invention are connected via a gigabit Ethernet PHY chip.

[0046] In this embodiment of the invention, the instructions from the host computer are stored in the configuration information of the ultrasonic excitation module through the lower computer control module. The DA module converts the digital signal into an analog signal and sends it to the signal amplifier before transmitting it to the ultrasonic transducer.

[0047] like Figure 4 As shown, the signal transmission path in this embodiment of the invention is as follows: First, the host computer is responsible for setting the parameters of the excitation signal, including the excitation center frequency and gain. Users can easily input the desired parameters through a human-machine interface, converting them into control commands, which are then transmitted to the lower-level control module via a gigabit network PHY chip. Upon receiving the control command, the lower-level control module uses an ARM chip to break it down and sends the excitation command to a signal generation circuit to generate an excitation waveform. This waveform is then amplified by a signal amplification circuit to drive a piezoelectric ultrasonic probe with a suitable excitation voltage to generate ultrasonic pulses. The ultrasonic pulses bounce off the bottom of the object being tested, and the echo signal is received by the piezoelectric ultrasonic probe and converted into an electrical signal. This analog signal is amplified and then transmitted to an analog-to-digital (AD) module for conversion into a digital signal. The data is then collected and stored in DDR memory via the ZYNQ (Zynq-7000 All Programmable SoC) system's Direct Memory Access (DMA) controller. Finally, the data is transmitted to the host computer via the gigabit network PHY interface using the UDP protocol. After receiving the data, the host computer uses the ultrasonic detection method for bolt preload described in the embodiments of the present invention to measure the bolt preload, and can draw its ultrasonic time domain, frequency domain and calibration or measurement images to assist the user in making a bolt status judgment. Finally, it can also save the ultrasonic data in combination with the auxiliary parameters input by the user in the human-machine interface, such as the approximate bolt length, bolt type, service time, etc.

[0048] This invention uses a combined acoustic time difference of primary and secondary echoes to replace the primary acoustic time difference when plotting the bolt preload-acoustic time difference curve. This reduces calibration errors caused by variations in coupling layer errors during multiple coupled measurements and overcomes the limitations of traditional methods that have stringent requirements for coupling quality. The introduction of wavelet empirical decomposition and cross-correlation combination algorithms significantly improves the stability of ultrasonic time measurement in the face of noise and guided wave interference of various modes. Furthermore, this invention proposes an ultrasonic testing system based on ZYNQ and a portable PC, enabling convenient human-computer interaction and high-quality excitation, reception, and analysis of ultrasonic testing signals. Therefore, this method and system offer significant advantages in terms of cost-effectiveness, ease of operation, reliability, and high accuracy, making them of significant innovative and practical value in the field of ultrasonic nondestructive testing.

[0049] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for ultrasonic testing of bolt preload, characterized in that, Includes the following steps: The ultrasonic echo detection signal of the bolt sample in its initial state is obtained, and the first echo band and the second echo band of the bottom surface of the bolt are extracted from the ultrasonic detection signal and recorded as the pre-stored first wave and the pre-stored second wave, respectively. After applying a known load to the bolt sample, ultrasonic echo detection signals were collected, and the first echo segment and the second echo segment on the bottom surface were extracted and recorded as the first wave and the second wave, respectively. Based on the acoustic time difference between the primary wave and the pre-stored primary wave, and the acoustic time difference between the secondary wave and the pre-stored secondary wave, the primary acoustic time difference and the secondary acoustic time difference are obtained respectively; with the applied load as the abscissa and the difference between the secondary acoustic time difference and the primary acoustic time difference as the ordinate, the calibration curve of bolt preload versus combined acoustic time difference is plotted. By combining the slope of the calibration curve with the double-echo bolt preload detection model based on the acoustoelastic effect, a new double-echo bolt preload detection model is obtained. The combined acoustic time difference of the ultrasonic echo detection signals of the in-service bolts is input into a new dual-echo bolt preload detection model to obtain the current preload of the bolts.

2. The ultrasonic testing method for bolt preload according to claim 1, characterized in that, The ultrasonic echo detection signal is a time-domain echo signal received by a single ultrasonic probe at the head of the bolt under ultrasonic longitudinal wave pulse excitation; the time-domain echo signal includes a primary echo band and a secondary echo band at the bottom of the bolt.

3. The ultrasonic testing method for bolt preload according to claim 1, characterized in that, The corresponding wavebands of the extracted ultrasonic echo detection signal are kept in the same position in their respective original signals.

4. The ultrasonic testing method for bolt preload according to claim 1, characterized in that, The acoustic time difference between the primary wave and the pre-stored primary wave, and between the secondary wave and the pre-stored secondary wave, is calculated using a modal decomposition algorithm or an acoustic time extraction algorithm.

5. The ultrasonic testing method for bolt preload according to claim 1, characterized in that, The ultrasonic echo detection signal of the bolt in its initial state is the ultrasonic echo detection signal collected from any bolt of the same batch and model under natural conditions.

6. The ultrasonic testing method for bolt preload according to claim 1, characterized in that, The construction of the double-echo bolt preload detection model based on the acoustoelastic effect specifically includes the following steps: According to nonlinear acoustic theory, when an ultrasonic longitudinal wave propagates along the stress direction in a solid medium, changes in stress will cause corresponding changes in wave velocity. Therefore, the acoustoelastic effect can be expressed as: In the formula This represents the longitudinal wave velocity under applied stress. This represents the longitudinal wave velocity when no stress is applied. This represents stress in the same direction as the sound wave propagation. For the longitudinal wave, the acoustic elasticity factor is constant; The bolt length under zero load is defined as: ； In the formula Indicates the original length of the bolt as a whole; Indicates the original length of the non-clamped portion of the bolt; This indicates the original length of the bolt clamping portion; the clamping portion refers to the equivalent stress zone of the bolt under tension in a loaded state, and its length is consistent with the distance from the nut to the bolt. According to Hooke's Law, Elongation under load is represented as: ； In the formula This indicates the length of the bolt clamping portion under tension. Let the elastic modulus of the bolt material be represented, then the propagation time of the ultrasound is expressed as: ； ； ； ； in, When representing a single echo sound under zero load on a bolt, its sound path includes two coupling layers, two clamping regions, and two non-clamping regions; When representing the secondary echo sound under zero load condition of the bolt, its sound path includes two coupling layers, four clamping regions, and four non-clamping regions; This represents the composition of the sound path of a single echo sound under bolt loading conditions. They are consistent, but the thickness of their coupling layer and the length of their clamping region will change; This indicates the thickness of the coupling layer under zero load conditions. This indicates the thickness of the coupling layer under loaded conditions; similarly, When representing the secondary echo sound under bolt loading, its sound path includes two coupling layers, four clamping regions, and four non-clamping regions, but the thickness of the coupling layer and the length of the clamping region have changed; The combined acoustic time difference before and after bolt loading stress is then expressed as: ； in, This represents the combined echo time difference, which is the difference between the second echo time difference and the first echo time difference. Indicates the bolt stress calibration factor; The relationship between bolt preload and axial stress is expressed as follows: ； In the formula F Indicates the bolt preload. This represents the equivalent cross-sectional area of ​​the bolt. Using bolt preload force instead of axial stress, the relationship between bolt preload force difference and combined acoustic time difference is obtained: ； in, This indicates the bolt preload calibration factor.

7. A detection system based on the ultrasonic testing method for bolt preload as described in claim 1, characterized in that, include: The ultrasonic excitation module is used to output ultrasonic pulses to the bolt being tested according to user instructions; The echo acquisition module is used to receive the echo signal of the ultrasonic pulse rebounding at the bottom of the bolt being tested; The lower-level control module is used to receive instructions from the upper-level computer, send them to the excitation module, and upload the echo signal collected by the echo acquisition module to the upper-level computer. The host computer module is connected to the lower computer control module via a gigabit Ethernet PHY chip. It is used to receive the echo signal collected by the echo acquisition module and measure the bolt preload using the bolt preload ultrasonic detection method to assist the user in judging the bolt condition.

8. The ultrasonic testing system for bolt preload according to claim 7, characterized in that, The ultrasonic excitation module includes a signal generation circuit, a power amplification circuit, and a piezoelectric ultrasonic transducer. The ultrasonic excitation module is used to generate an excitation waveform by sending an excitation command to the signal generation circuit, and the excitation waveform drives the piezoelectric ultrasonic probe to generate ultrasonic pulses via the power amplification circuit.

9. The ultrasonic testing system for bolt preload according to claim 7, characterized in that, The echo acquisition module includes a low-voltage acquisition circuit, a signal amplification circuit, a high-speed ADC, and a piezoelectric ultrasonic transducer. The echo acquisition module is used to receive echo signals through a piezoelectric ultrasonic probe and convert them into electrical signals, and then convert the electrical signals into digital signals after passing through the signal amplification circuit.

10. The ultrasonic testing system for bolt preload according to claim 7, characterized in that, The lower-level control module includes a ZYNQ development board, a direct memory access (DMA) controller, an interrupt system, and DDR memory. The lower-level control module is used to schedule the acquisition of data from the converted digital signal through the DMA controller of the ZYNQ development board and store it in the DDR memory space. The stored data is then transmitted to the upper-level computer via the gigabit network PHY interface using the UDP protocol.

Images