A bolt axial stress and thread area micro-crack detection system and detection method

Abstract

The application discloses a bolt axial stress and thread area micro-crack detection system and a detection method. The detection system comprises an array ultrasonic detection unit and a signal processing unit. The array ultrasonic detection unit is arranged on the end face of a bolt to be detected and comprises a central electrode arranged at the geometric center of the end face and used for emitting and receiving ultrasonic signals propagating along the bolt axis, a plurality of edge electrodes arranged in a ring array along the edge of the end face, and each edge electrode is arranged at a position corresponding to one thread area of the bolt and used for emitting and receiving ultrasonic signals covering the corresponding thread area, and a signal processing unit electrically connected with the central electrode and all the edge electrodes and used for receiving and processing the electrical signals transmitted by each electrode, calculating the axial stress of the bolt and determining whether the micro-crack exists in the corresponding thread area. The application has the advantages of accurate crack positioning and depth measurement.

Description

Technical Field

[0001] This invention mainly relates to the field of ultrasonic nondestructive testing and structural health monitoring technology, specifically to a bolt axial stress and threaded microcrack detection system and method. Background Technology

[0002] Under long-term alternating loads, fatigue cracks are prone to initiation at stress concentration points such as the thread root and the bolt transition zone on the turbine top cover bolts. Traditional ultrasonic testing often uses single-crystal probes or linear arrays, which have the following drawbacks: (1) It can only shoot directly along the axis, which can easily form a blind spot for the near end face area such as the root of the thread; (2) Bolts need to be removed or the probe needs to be moved manually, making in-situ online monitoring impossible; (3) The sound field coverage is incomplete, making it difficult to simultaneously take into account the crack detection sensitivity of the near-end thread area and the far-end screw / bottom.

[0003] Existing technologies also have their limitations. For example, the bolt failure early warning method for axial-flow hydro-generator units proposed in patent application CN115950957A relies on the synergistic effect of a multivariate logistic regression model and a convolutional neural network model. It requires the collection of a large number of bolt failure samples for model training, which is complex and costly. Moreover, this method does not solve the problem of blind spots in crack detection in the edge and thread areas of large / giant bolts. Its crack identification mainly relies on image analysis of ultrasonic waveform images, without utilizing the relative change law of ultrasonic propagation time. The identification speed is slow, the anti-interference ability is weak, and it is not suitable for on-site online monitoring. The remote status diagnosis and traceability system for wind turbine cluster bolts in patent application CN118934485A focuses on remote data traceability and fault alarm of wind turbine bolts. Its crack identification depends on whether the acoustic time exceeds the preset threshold range of "no tension - complete tightening". It cannot achieve the location and depth measurement of crack defects, let alone the early detection of latent defects in areas such as threads and tooth roots. It has not formed a targeted detection scheme for cracks in the thread area of ​​large / giant bolts using array electrodes. Summary of the Invention

[0004] To address the technical problems existing in the prior art, this invention provides a bolt axial stress and thread zone microcrack detection system and method for achieving precise crack location and depth measurement.

[0005] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows: A bolt axial stress and threaded microcrack detection system includes an array ultrasonic testing unit and a signal processing unit. The array ultrasonic testing unit is arranged on the end face of the bolt to be inspected, and includes: A central electrode, located at the geometric center of the end face, is used to transmit and receive ultrasonic signals propagating along the bolt axis; Multiple edge electrodes are arranged in a ring array along the edge of the end face. The position of each edge electrode corresponds one-to-one with a threaded area of ​​the bolt, and is used to transmit and receive ultrasonic signals covering the corresponding threaded area. The signal processing unit is electrically connected to the center electrode and all edge electrodes, respectively, and is used to receive and process the electrical signals transmitted from each electrode, and to perform the following operations: The axial stress of the bolt is calculated based on the ultrasonic propagation time difference between the center electrode in the zero-stress state and the service state. Based on the echo signal received by any edge electrode in service, it is determined that there is a microcrack in the corresponding threaded area.

[0006] Preferably, the detection system further includes: The data storage unit is used to store baseline data during the system calibration phase, as well as real-time and historical data during the online testing process. The early warning unit, connected to the signal processing unit, is used to issue audible and visual alarms and / or remote communication alarms when the axial stress value exceeds the preset safety range or when a microcrack is detected.

[0007] Preferably, the array ultrasonic detection unit adopts a 2D ring piezoelectric array structure, and the working frequency of the center electrode and the edge electrodes is 15-50MHz; the center electrode and all edge electrodes are made of PZT-1-3 composite piezoelectric material, and the size and working frequency are consistent; each electrode is electrically connected to the external ultrasonic excitation module and ultrasonic receiving module through flexible circuits, and is fixed on a high-strength insulating ceramic base.

[0008] Preferably, the array ultrasonic detection unit operates in a time-division multiplexing manner according to the following transmission mode: Proximal thread crack detection mode: Triggers synchronous in-phase excitation of all edge electrodes, while the center electrode remains silent and receives echo signals across the entire frequency band, forming a 0° parallel sound beam along the bolt axis, used to detect near-end thread cracks within a range of 0-100mm from the electrode mounting end face. Remote screw / bottom crack detection mode: The timing difference excitation strategy of "opposite edge electrode - center electrode - local edge electrode" is adopted to detect the direction corresponding to each edge electrode in sequence. By controlling the excitation time delay, a directional focused sound beam with a preset deflection angle β to the bolt axis is formed to detect cracks in the remote area from 100mm from the electrode mounting end face to the bottom of the bolt. Acoustic stress measurement mode: Only the center electrode is triggered for individual excitation, while all edge electrodes are silent and synchronously turned on to receive signals, used to measure the total acoustic time of the bottom wave propagating along the bolt axis.

[0009] This invention also discloses a detection method based on the bolt axial stress and thread zone microcrack detection system described above, comprising the following steps: S1, An array of ultrasonic testing units is set on the upper end face of the bolt to be inspected; the included angle between the centers of any two adjacent edge electrodes is equal, and the center electrode coincides with the geometric center of the upper end face; S2, crack detection is performed on the near-end thread area of ​​the bolt to be inspected using the near-end crack detection emission mode, and the near-end crack detection result is obtained; S3, using the far-end crack detection emission mode, performs crack detection on the far-end thread area of ​​the bolt under inspection and obtains the far-end crack detection result; S4, through the acoustic time stress measurement mode, quantitatively measure the axial stress of the bolt under inspection and obtain the axial stress measurement result; S5, based on the results of near-end crack detection, far-end crack detection, and axial stress measurement, comprehensively judges whether the preset safety threshold is exceeded; if not exceeded, outputs a detection report; if exceeded, outputs a detection report and warning information.

[0010] Preferably, step S2 specifically includes: S2.1, adopts the near-end crack detection emission mode to output synchronous trigger signal; S2.2, according to the synchronization trigger signal, control all edge electrodes to be synchronously and in phase excited, while the center electrode remains silent and the full-band receiving mode is turned on; S2.3 controls all edge electrodes to emit preset ultrasonic longitudinal wave pulse signals of equal amplitude, phase and frequency at the same time, forming a parallel sound beam along the bolt axis. S2.4 receives echo signals in real time through all edge electrodes and the center electrode; S2.5, Crack analysis is performed based on the echo signal to obtain the near-end crack detection results.

[0011] Preferably, step S3 specifically includes: S3.1, adopt the far-end crack detection emission mode to set the timing difference excitation strategy of "opposite edge electrode - center electrode - local edge electrode"; S3.2, Set the first excitation timing of the far circumferential region corresponding to the first edge electrode according to the strategy; S3.3, according to the first excitation timing sequence, the preset power ultrasonic pulse signals are emitted sequentially, and interference superposition occurs in the propagation path of the distal thread region to form a first directional focused sound beam with a preset deflection angle β to the bolt axis; S3.4, Receive the first directional echo signal of the first directional focused sound beam through the center electrode; S3.5, execute steps S3.2 and S3.3 sequentially according to the circumferential rotation rule to obtain all directional echo signals in the full circumference; S3.6, Crack analysis is performed based on each directional echo signal to obtain the remote crack detection results.

[0012] Preferably, the first excitation timing sequence includes: first wave excitation of the first edge electrode, excitation of the center electrode after a fixed delay Δt1, and excitation of the target edge electrode opposite to the first edge electrode after a fixed delay Δt2; the fixed delays Δt1 and Δt2 are both in the range of 10-50ns, and the preset deflection angle β is in the range of 5°-15°.

[0013] Preferably, step S4 specifically includes: The central electrode is individually excited to emit an ultrasonic pulse signal for stress measurement via the acoustic time-stress measurement mode, while all edge electrodes remain silent and simultaneously turn on the receiving mode. The bottom echo signal is received synchronously by the center electrode and the edge electrode, and the total acoustic time t of the bottom echo is collected. σ ; Based on the pre-calibrated zero-stress state bottom wave acoustic time t0, the acoustic time difference Δt = t σ -t0; Substituting the acoustic time difference Δt into the preset quantitative formula for axial stress σ=k1×Δt+b1, the current axial stress value is calculated; k1 and b1 are preset parameters.

[0014] Preferably, a system calibration process is included before step S1: Axial stress calibration: Apply a gradient axial load to a standard bolt of the same specification as the bolt to be tested, and record the acoustic time t of the center electrode under service conditions. σ In conjunction with the zero-stress acoustic time t0, a quantitative relationship formula is established between axial stress σ and acoustic time difference Δt: σ = k1 × Δt + b1; k1 and b1 are preset parameters. Crack depth calibration: Microcracks of different depths are prefabricated in the threaded area of ​​a standard bolt, and the acoustic time difference values ​​of the corresponding edge electrodes are recorded to establish a crack depth-acoustic time difference correlation model.

[0015] Compared with the prior art, the advantages of the present invention are as follows: Full thread coverage is achieved through two time-division multiplexing modes of a 2D ring piezoelectric array: In the near-end thread region, all edge electrodes are synchronously and in phase excited to form a 0° parallel sound beam coinciding with the bolt axis, with a beam spread angle ≤5°, eliminating detection blind spots; in the far-end region, a time-difference excitation strategy of "opposite edge electrode - center electrode - local edge electrode" is employed, forming a directional focused sound beam at a preset angle of 5°~15° to the bolt axis through interference superposition. The two modes work in tandem to cover the entire thread region from end face to bottom in one go, fundamentally solving the problem of missed detection in stress concentration areas such as the thread root and tooth base using traditional methods. Furthermore, the operating frequency of the center electrode and all edge electrodes is 15~50MHz, far exceeding the 5MHz of conventional ultrasonic testing probes. This high frequency brings higher resolution—combined with coherent superposition and directional focusing technology, it can identify microcracks as small as 1mm, ensuring high sensitivity in microcrack monitoring.

[0016] Edge electrodes are evenly distributed in a ring array along the edge of the end face (typically 6, 8, or 12 electrodes), with each electrode's position corresponding to a threaded area of ​​the bolt. When an edge electrode detects an abnormal echo signal time difference, the circumferential location of the crack can be uniquely determined by the electrode's number. Simultaneously, the axial depth of the crack can be quantitatively estimated using the echo time difference value of the edge electrode, providing maintenance personnel with a precise maintenance decision-making basis down to the specific threaded area, avoiding blind troubleshooting.

[0017] The center electrode and edge electrodes share the same array in a time-division multiplexing manner, without interfering with each other: In the acoustic time-stress measurement mode, only the center electrode is excited to emit ultrasonic pulses, while all edge electrodes remain silent and synchronously turn on to receive. The total acoustic time t of the bottom wave is measured. σ And compared with the acoustic time t0 under zero stress state, the acoustic time difference Δt = t is obtained. σ Substituting -t0 into the pre-calibrated quantitative formula for axial stress σ=k1×Δt+b1, the current axial stress value can be calculated. Crack detection is primarily performed by the edge electrode in both near-end and far-end modes. Both functions can achieve simultaneous perception of bolt structural health and load status without additional sensors, significantly reducing system complexity and deployment costs.

[0018] In summary, the ultrasonic emission control method based on a multi-chip ring array probe of the present invention can realize the integration and alarm of online bolt detection, full-domain crack detection, crack detection and axial stress detection, and is especially suitable for long-term online monitoring and early warning of large and giant bolts in heavy equipment fields such as wind power, hydropower, and nuclear power. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the array ultrasound detection unit in an embodiment of the present invention.

[0020] Figure 2 This is a flowchart of the bolt axial stress and threaded area microcrack detection method according to an embodiment of the present invention.

[0021] Figure 3 This is a structural diagram of the bolt axial stress and threaded microcrack detection system according to an embodiment of the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0023] like Figure 2 As shown, the bolt axial stress and threaded microcrack detection system provided in this embodiment of the invention includes an array ultrasonic detection unit and a signal processing unit; An array ultrasonic testing unit, arranged on the end face of the large / giant bolt to be inspected, includes: A central electrode, located at the geometric center of the end face, is used to transmit and receive ultrasonic signals propagating along the bolt axis to measure the axial stress of the bolt. Multiple edge electrodes are arranged in a ring array along the edge of the end face. The position of each edge electrode corresponds one-to-one with a threaded area of ​​the bolt. They are used to transmit and receive ultrasonic signals covering the corresponding threaded area to identify microcracks in that area. The signal processing unit is electrically connected to the center electrode and all edge electrodes, and is used to receive and process the electrical signals transmitted from each electrode, and perform the following operations: The axial stress of the bolt is calculated based on the ultrasonic propagation time difference between the center electrode in the zero-stress state and the service state. Based on the characteristic that the acoustic time of the echo signal received by any edge electrode in service is less than that in zero stress state, it is determined that there is a microcrack in the corresponding threaded area.

[0024] In addition, the signal processing unit is also used to circumferentially locate the crack based on the number of the edge electrode where the microcrack is determined to exist; and to determine the real-time service status of the edge electrode based on the acoustic time t. σ,i Its zero-stress state acoustic time t 0i The axial depth of the crack is calculated by combining the acoustic time difference value with the pre-stored crack depth-acoustic time difference correlation model.

[0025] In addition, the detection system also includes: The data storage unit is used to store baseline data during the system calibration phase, as well as real-time and historical data during the online testing process. The early warning unit, connected to the signal processing unit, issues an audible and visual alarm and / or a remote communication alarm when the axial stress value calculated by the signal processing unit exceeds the preset safety range, or when a microcrack is detected. The preset safety range is ±15% of the bolt design stress; the early warning unit's threshold for detecting microcracks is a crack depth ≥2mm.

[0026] Specifically, the array ultrasonic testing unit adopts a high-temperature and corrosion-resistant sealed structure to adapt to the service environments of hydropower, wind power, heavy machinery, nuclear power, or bridge engineering. The array ultrasonic testing unit uses a 2D ring piezoelectric array structure, with the central and edge electrodes operating at frequencies of 15-50MHz. The central electrode and all edge electrodes are made of PZT-1-3 composite piezoelectric material. The piezoelectric wafer size and operating frequency of the central electrode and all edge electrodes are consistent. The central electrode and all edge electrodes are electrically connected to the external ultrasonic excitation module and ultrasonic receiving module via flexible circuitry and are fixed to a high-strength insulating ceramic base to avoid signal crosstalk between electrodes.

[0027] Specifically, the system calibration process includes: Axial stress calibration: Apply a gradient axial load to a standard bolt of the same specification as the bolt to be tested, and record the acoustic time t of the center electrode under service conditions. σ And combining the zero-stress acoustic time t0, the relationship between axial stress σ and acoustic time difference Δt=t is established. σ The quantitative relationship between -t0 is expressed by the formula σ = k1 * Δt + b1.

[0028] The online testing process includes: Axial stress measurement: The central electrode is driven to emit ultrasonic waves, and the echo signal is received to obtain the real-time acoustic time t of the service status. σ Combine the zero-stress acoustic time t0 to calculate the real-time acoustic time difference Δt, substitute it into the quantitative relationship formula, and calculate the real-time axial stress value; Microcrack identification: Drive all edge electrodes to emit ultrasonic waves and receive echo signals, and compare the real-time service state acoustic time tσ,i of any edge electrode with its zero-stress state acoustic time t 0i Compare, if t σ,i <t 0i If so, it is determined that there is a microcrack in the threaded area corresponding to the edge electrode.

[0029] During the online testing process, the array ultrasonic testing unit operates in a time-division manner according to the following transmission modes: Proximal thread crack detection mode: Triggers synchronous in-phase excitation of all edge electrodes, while the center electrode remains silent and receives echo signals across the entire frequency band, forming a 0° parallel sound beam along the bolt axis, used to detect near-end thread cracks within a range of 0-100mm from the electrode mounting end face. Remote screw / bottom crack detection mode: The timing difference excitation strategy of "opposite edge electrode - center electrode - local edge electrode" is adopted to detect the direction corresponding to each edge electrode in sequence. By controlling the excitation time delay, a directional focused sound beam with a preset deflection angle β to the bolt axis is formed to detect cracks in the remote area from 100mm from the electrode mounting end face to the bottom of the bolt. Acoustic stress measurement mode: Only the center electrode is triggered for individual excitation, while all edge electrodes remain silent and simultaneously activate for reception. This mode is used to measure the total acoustic time t of the bottom wave propagating along the bolt axis. σ .

[0030] The near-end thread region is the range of [0, 100 mm] from the upper end face, including the stress concentration area at the thread root and the root of the thread, while the far-end thread region is the thread region from 100 mm to the bottom of the bolt on the side of the upper end face.

[0031] In the remote screw / bottom crack detection mode, the specific excitation timing for detecting any direction, such as the direction of electrode 1, is as follows: The first step is to individually excite the first edge electrode (No. 1) corresponding to that direction. The second step is to individually excite the center electrode (0) after a first time delay Δt1. The third step is to delay the second time Δt2 and then individually excite the second edge electrode (No. 4) which is 180° opposite to the first edge electrode. The range of the first time Δt1 and the second time Δt2 is 10-50 ns, and is calibrated according to the bolt diameter to achieve a directional focused sound beam with a preset deflection angle β, which is in the range of 5°-15°.

[0032] Specifically, the fabrication process of each electrode is as follows: Substrate pretreatment: The end face of the bolt to be inspected is cleaned and ion etched to make its surface clean and activated; First mask loading: A first mask is loaded on the substrate end face, and the first mask covers all non-deposited areas of the electrodes on the bolt end face; Piezoelectric functional layer deposition: A ZnO piezoelectric functional layer was deposited at the center of the substrate end face using magnetron sputtering. The sputtering parameters were as follows: sputtering temperature 100~200℃, target-substrate distance 40~100mm, a mixed gas of argon and oxygen in a volume ratio of 3:1~1:3 was introduced to a pressure of 1.0~3.5Pa in the cavity, sputtering power 200~900W, sputtering time 3~15h, and substrate bias voltage 0~150V. Protective layer deposition steps: An AlCrNbSiTiO or SiO2 protective layer is deposited on the surface of the already deposited ZnO piezoelectric functional layer using magnetron sputtering. The deposition parameters are: temperature 150℃, cavity gas pressure 1.5~2.0Pa, and deposition time 2~5h. Electrode layer forming: Remove the first mask and replace it with an annular hole mask adapted to a single electrode. Deposit an AgCr electrode layer on the surface of the piezoelectric functional layer using magnetron sputtering. Annealing treatment: After removing the mask, the integrally formed concentric ring array electrodes are annealed to obtain stable piezoelectric and conductivity properties.

[0033] like Figure 2 As shown, the bolt axial stress and threaded microcrack detection method provided in this embodiment of the invention includes: S1. A 2D ring piezoelectric array is set on the upper end face of the large bolt to be tested. The 2D ring piezoelectric array consists of a central electrode and 2N edge electrodes evenly distributed along the circumference. The included angle between the centers of any two adjacent edge electrodes is equal, and the central electrode coincides with the geometric center of the upper end face. In this embodiment, a schematic diagram of the 2D ring piezoelectric array is shown below. Figure 1 As shown, the ultrasonic electrode consists of a central electrode and 2N edge electrodes evenly distributed along the circumference. Figure 1 The specific number of edge electrodes is 6, but it can also be 8 or 10, etc. The center electrode coincides with the geometric center of the upper end face, and the included angle between the centers of any two adjacent edge electrodes is 60°. The center electrode and all edge electrodes are made of PZT-1-3 composite piezoelectric material; this material has both a high electromechanical coupling coefficient and low acoustic impedance, which can realize efficient transmission of high-frequency ultrasonic pulses and high-sensitivity reception of echo signals. The piezoelectric crystal size and operating frequency of the center electrode and all edge electrodes are consistent. The center electrode and all edge electrodes are electrically connected to the external ultrasonic excitation module and ultrasonic receiving module through flexible circuits and are fixed on a high-strength insulating ceramic base to avoid signal crosstalk between electrodes.

[0034] Preferably, the center electrode and all edge electrodes are 20~30MHz longitudinal wave probes.

[0035] S2, crack detection is performed on the near-end thread region of the large bolt to be tested using the near-end crack detection emission mode, and the near-end crack detection result is obtained; where the diameter of the large / giant bolt is ≥50mm and the length is ≥200mm. In this embodiment, the proximal thread region is the range of [0, 100 mm) from the upper end face, including the stress concentration area at the thread root and the root of the thread, and the distal thread region is the thread region from 100 mm to the bottom of the bolt on the side of the upper end face. S2.1, adopts the near-end crack detection emission mode to output synchronous trigger signal; S2.2, control all edge electrodes to be synchronously and in phase excited according to the synchronization trigger signal, keep the center electrode in a silent state, and turn on the full-band reception mode; S2.3 controls all edge electrodes to emit preset ultrasonic longitudinal wave pulse signals of equal amplitude, phase and frequency at the same time, and the ultrasonic beam energy of the preset ultrasonic longitudinal wave pulse signal is concentrated in the near-end thread region. because Figure 1 The six edge electrodes are evenly distributed in a regular hexagonal ring. The emitted pre-set ultrasonic longitudinal wave pulse signal is coherently superimposed inside the bolt's metal medium, forming a parallel sound beam that coincides with the bolt's axial direction at 0° along the positive direction of the bolt axis. The sound beam covers the entire circumferential thread area near the bolt end, with no detection dead angles, and the sound beam diffusion angle is less than or equal to 5°. S2.4 receives the echo signal of the preset ultrasonic longitudinal wave pulse signal in real time through all edge electrodes and center electrodes. The echo signal is a bolt axial parallel sound beam, and the sound beam covers the entire circumferential thread area near the bolt end, with no detection dead angle. S2.5, Crack analysis is performed based on the echo signal to obtain the near-end crack detection results.

[0036] Time-domain feature analysis of the echo signal is performed to extract characteristic parameters such as peak position, acoustic time, and amplitude. The reflected waves of bolt structures, bottom waves, and crack reflected waves are distinguished. Combined with the direction of sound beam propagation, the circumferential location, axial depth quantification, and severity assessment of fatigue cracks are achieved. The sound beam diffusion angle is less than or equal to 5°, which can effectively identify microcracks at the near end of 1 mm. The signal-to-noise ratio of the echo signal is ≥20dB, ensuring the sensitivity of microcrack monitoring.

[0037] S3, using the far-end crack detection emission mode, performs crack detection on the far-end thread area of ​​the large bolt to be inspected, and obtains the far-end crack detection result; In this embodiment, after the near-end thread crack detection is completed, the far-end crack detection is performed. The steps are as follows: S3.1, adopt the far-end crack detection emission mode to set the timing difference excitation strategy of the opposite edge electrode-center electrode-this side edge electrode; S3.2, set the first excitation timing of the far circumferential region corresponding to the first edge electrode according to the timing difference excitation strategy of the opposite edge electrode-center electrode-this side edge electrode; The first incentive sequence includes the first wave of incentives, the second incentive, and the third incentive; S3.3, according to the first excitation timing sequence, the preset power ultrasonic pulse signals are emitted sequentially, and interference superposition occurs in the propagation path of the distal thread region to form a first directional focused sound beam with a preset deflection angle β to the bolt axis; The first edge electrode is controlled according to the first wave of excitation in the first excitation timing sequence (e.g., Figure 1 The first preset power ultrasonic pulse signal is emitted from edge electrode 1. After a fixed delay Δt1, the second preset power ultrasonic pulse signal is emitted according to the second excitation control center electrode; After a fixed delay Δt2, the target edge electrode opposite the first edge electrode is controlled according to the third excitation. Figure 1 The fourth edge electrode in the middle emits a third preset power ultrasonic pulse signal; The first preset power ultrasonic pulse signal, the second preset power ultrasonic pulse signal, and the third preset power ultrasonic pulse signal interfere and superimpose in the propagation path of the distal thread region to form a first directional focused sound beam at a preset angle β to the bolt axis. The first directional focused sound beam points to the circumferential region corresponding to the No. 4 edge electrode in the distal thread region. Preferably, the range of β is 5°~15°, which can adapt to the distal detection requirements of bolts of different lengths and can effectively identify microcracks and structural defects pointing in the direction of the No. 4 edge electrode. S3.4, Receive the first directional echo signal of the first directional focused sound beam through the center electrode; S3.5, execute steps S3.2 and S3.3 sequentially according to the circumferential rotation rule to obtain all directional echo signals in the full circumference; Following the circumferential rotation rule, the timing excitation is performed sequentially in the order of "edge electrode 2 → center electrode → edge electrode 5", with the delay parameter consistent with the direction of edge electrode 4; the timing excitation is performed sequentially in the order of "edge electrode 3 → center electrode → edge electrode 6", with the delay parameter consistent with the direction of edge electrode 4; in the same way, the distal cracks in the directions of all edge electrodes are completed sequentially; thus completing the full circumferential distal region scan coverage; S3.6, Crack analysis is performed based on each directional echo signal to obtain the remote crack detection results.

[0038] Time-domain feature analysis is performed on the directional echo signal to extract characteristic parameters such as peak position, acoustic time, and amplitude. The reflected waves of bolt structures, bottom waves, and crack reflected waves are distinguished. Combined with the direction of sound beam propagation, the circumferential location, axial depth quantification, and severity assessment of fatigue cracks are realized, and the detection results of far-end cracks are finally obtained.

[0039] S4, through the acoustic time-stress measurement mode, quantitatively measure the axial stress of the large bolt to be tested, and obtain the axial stress measurement result; The stress measurement ultrasonic pulse signal is emitted by the center electrode under controlled acoustic time-stress measurement mode, while all edge electrodes remain silent and simultaneously activated in high-resolution receiving mode. The stress measurement ultrasonic pulse signal emitted by the center electrode propagates along the positive direction of the bolt axis to the lower end face of the bolt, reflects off the end face, and returns along the original path, forming a bottom echo signal. The total acoustic time t of the bottom echo signal from the emission to the reception of the stress measurement ultrasonic pulse signal is accurately acquired by the center electrode and edge electrodes simultaneously (edge ​​electrodes assist in verifying the uniformity of sound beam propagation).σ ; Based on the pre-calibrated zero-stress state bottom wave acoustic time t0, the acoustic time difference Δt = t σ -t0; Substituting the acoustic time difference Δt into the preset quantitative formula for axial stress σ=k1×Δt+b1, the current axial stress value of the large bolt to be tested is calculated and used as the axial stress measurement result. k1 and b1 are preset parameters.

[0040] S5, based on the results of near-end crack detection, far-end crack detection, and axial stress measurement, comprehensively judges whether the preset safety threshold is exceeded. If it is not exceeded, outputs a detection report; if it is exceeded, outputs a detection report and warning information.

[0041] like Figure 3 As shown, this embodiment of the invention also provides a bolt axial stress and threaded zone microcrack detection system, comprising: The ultrasonic electrode setting module is used to set a 2D ring piezoelectric array on the upper end face of the large bolt to be tested. The 2D ring piezoelectric array consists of a central electrode and 2N edge electrodes evenly distributed along the circumference. The included angle between the centers of any two adjacent edge electrodes is equal, and the central electrode coincides with the geometric center of the upper end face. The near-end crack detection module is used to detect cracks in the near-end thread region of the large bolt to be inspected through the near-end crack detection emission mode, and obtain the near-end crack detection results. The remote crack detection module is used to detect cracks in the remote thread area of ​​the large bolt to be inspected through the remote crack detection emission mode, and obtain the remote crack detection results. The axial stress measurement module is used to quantitatively measure the axial stress of the large bolt under test through the acoustic time stress measurement mode, and obtain the axial stress measurement result. The detection and early warning module is used to comprehensively determine whether the preset safety threshold is exceeded based on the detection results of near-end cracks, far-end cracks, and axial stress. If it is not exceeded, a detection report is output; if it is exceeded, a detection report and early warning information are output.

[0042] This invention employs a 2D annular piezoelectric array on the upper surface of a large bolt to be inspected. Near-end and far-end crack detection emission modes control electrodes for near-end and far-end crack detection, while an acoustic time-stress measurement mode controls electrodes for axial stress measurement, enabling online inspection of large bolts. During crack detection, the near-end crack detection emission mode detects cracks in the near-end thread region, and the far-end crack detection emission mode detects cracks in the far-end thread region, achieving complete coverage of the large bolt's thread. This allows for precise crack location and depth measurement, solving the problem of ultrasonic field coverage for large bolts. Incomplete coverage creates blind spots in crack detection, making it difficult to simultaneously detect cracks in both the near-end and far-end thread regions. This invention comprehensively judges whether a preset safety threshold is exceeded based on the near-end crack detection results, far-end crack detection results, and axial stress measurement results. If the threshold is exceeded, a detection report and warning information are output, achieving integrated crack detection and axial stress detection with warning. In summary, the ultrasonic emission control method based on a multi-chip ring array probe of this invention can realize online detection of large bolts, full-domain crack detection, and integrated crack detection and axial stress detection with warning.

[0043] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0044] Example 1 For M80 large / giant bolts with a length of 810mm, made of 45 steel, and a rated working stress of 210MPa, the following testing steps are performed: (1) Electrode layout: One center electrode with a diameter of 8 mm, a longitudinal wave straight probe, and a frequency of 20 MHz is set at the center of one end face of the bolt. Eight edge electrodes are evenly distributed along the edge of the end face with an electrode spacing of 45°, a diameter of 5 mm, a longitudinal wave straight probe, and a frequency of 20 MHz. The position of each edge electrode corresponds to one of the eight threaded areas of the bolt. (2) System calibration: Axial stress calibration: Fix a standard M80 bolt to a universal testing machine and apply gradient axial loads of 100kN, 200kN, 300kN, 400kN, and 500kN, holding each load level for 5 minutes. Record the service time (t) of the center electrode. σ Simultaneously, the acoustic time t0 = 274576 ns under zero stress was recorded, and the acoustic time difference Δt corresponding to each load was calculated. The results after data correction are shown in Table 1 below: Table 1 Corrected data for acoustic time difference under different axial loads

[0045] Based on the cross-sectional area of ​​the M80 bolt, the actual stress corresponding to each axial load is calculated; based on the corrected acoustic time difference and the corresponding stress data, the least squares method is used for linear fitting, and the fitting process is as follows: Let the fitted line be σ = k1 × Δt + b1, in: σ is the axial stress (MPa). Δt is the corrected acoustic time difference (ns). Calculations show that: The final axial stress fitting formula is: σ = 0.377 × Δt + 0.9, with a goodness of fit R. 2 =0.999.

[0046] (3) Simulated microcrack measurement: On standard M80 stainless steel bolts, standard microcracks with depths of 1mm, 2mm, 3mm, 4mm, and 5mm were pre-fabricated at different distances from the bolt end face using wire cutting. The bolts were then fixed on a universal testing machine, and a constant axial stress of 210MPa was applied. The edge electrode echo signals and the acoustic time t for the presence of cracks of different depths were recorded. ci Calculate the acoustic time difference Δt i The results are shown in Table 2 below: Table 2. Acoustic time and bottom wave signal data corresponding to cracks at different depths.

[0047] (4) Online detection: Axial stress measurement: An ultrasonic electrode is attached to the non-stressed end face of the M80 stainless steel bolt to be inspected. An ultrasonic sensor drives the central electrode to emit ultrasonic signals, and the real-time acoustic time difference under service conditions is detected as Δt = 368 ns. Substituting this into the previously calibrated axial stress fitting formula σ = 0.377 × Δt + 0.9, the axial stress is calculated as follows: 0.377×368+0.9≈139.6MPa The rated working stress of the bolt is 210MPa, and the preset safety range is ±15% of the rated working stress. The calculated stress of 139.6MPa is lower than the safety range, and the system triggers a stress anomaly warning, which requires maintenance personnel to check and handle it in a timely manner. Threaded area microcrack monitoring: Synchronously drive 8 edge electrodes to emit ultrasonic signals. The 5th edge electrode detects the real-time service sound time, with a measured t5=16975ns. Its zero-stress sound time t0=274576ns. The service sound time t5 is less than the zero-stress sound time t0. Combined with the one-to-one correspondence between the edge electrodes and the threaded area, such as the 5th edge electrode corresponding to the 5th threaded area of ​​the bolt, and based on the correlation between the sound time difference and the crack location, the system automatically alarms about 50mm away from the non-stressed end face of the bolt and pushes a warning message to the maintenance personnel, indicating that there is a 3mm microcrack in the 5th threaded area. (5) Result verification: Metallographic analysis was used to inspect the fifth threaded area of ​​the bolt, confirming the presence of a 3mm microcrack in that area. The axial stress of the bolt was measured using a universal testing machine, and the measured value was 136.0MPa, with an error of 2.7% compared to the calculated value. Both measurements meet the accuracy requirements for testing large / giant bolts.

[0048] Example 2 For the giant connecting bolts of the main tower of the wind farm, the bolt specifications are M60, the length is 700mm, the material is 42CrMo, and the rated working stress is 300MPa. Online monitoring was carried out for 60 consecutive days, and the axial stress and crack status were recorded in real time. The specific process is as follows: (a) Electrode layout: One central electrode and 12 edge electrodes are symmetrically arranged on the non-stressed end of the bolt. The distance between the edge electrodes is 30°. The 12 edge electrodes correspond one-to-one with the 12 threaded areas of the bolt. All electrodes are connected to ultrasonic sensors and integrated into a complete monitoring system.

[0049] (II) System Calibration: Acoustic time is related to bolt length: With a bolt length L = 700 mm and under zero stress, the ultrasonic wave is emitted from the central electrode, reflected off the other end of the bolt, and then received. The acoustic time under zero stress is... t0 = 2L / v = 1.4 / 5900 = 237288ns Axial stress calibration: Following the gradient load calibration method in Example 1, M60 and 42CrMo standard bolts were fixed on a universal testing machine, and gradient axial loads of 100kN, 200kN, 300kN, 400kN, and 500kN were applied. The actual stress corresponding to each load was first calculated, and then the service life t of the center electrode under each load level was recorded. σ Calculate the acoustic time difference Δt=t σ -t0, the least squares method was used to perform linear fitting on the stress-acoustic time difference data; the final fitted formula for axial stress was obtained: σ = 0.042Δt - 2.8 Goodness of fit R 2=0.99. (III) Online monitoring: The system operated continuously for 60 days, acquiring real-time acoustic time signals from the center and edge electrodes to calculate axial stress and crack depth. Specific monitoring results are as follows: Stress monitoring: Within 60 days, the axial stress of the bolts remained stable between 280-320MPa, which did not exceed the preset safety range (±15% of the design stress, i.e., 255-345MPa, where the calculation error of the safety range is corrected to match the rated working stress of 300MPa), indicating that the bolt connection was in good condition and there were no abnormal stress fluctuations.

[0050] Crack monitoring: For the first 51 days, the acoustic time difference values ​​of all edge electrodes were normal, indicating that there were no microcracks in the threaded area. On the 52nd day, the system detected that the real-time service acoustic time t3 of the No. 3 edge electrode was 20400ns, which was less than its zero stress acoustic time t0 = 237288ns, indicating that there was a crack in the thread near the end of the bolt 6cm. The system immediately issued an alarm and pushed a warning message to the maintenance personnel, indicating that there was a microcrack in the third threaded area.

[0051] (iv) On-site verification: After receiving the warning information, the maintenance personnel promptly shut down the machine for inspection. Through magnetic particle testing, they confirmed that there was a 4mm microcrack in the third threaded area and replaced the bolt in time. This prevented safety accidents such as loose connection of the wind farm main tower and bolt breakage, and verified the practicality and reliability of the invention.

[0052] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A bolt axial stress and threaded zone microcrack detection system, characterized in that, Includes an array ultrasound detection unit and a signal processing unit: The array ultrasonic testing unit is arranged on the end face of the bolt to be inspected, and includes: A central electrode, located at the geometric center of the end face, is used to transmit and receive ultrasonic signals propagating along the bolt axis; Multiple edge electrodes are arranged in a ring array along the edge of the end face. The position of each edge electrode corresponds one-to-one with a threaded area of ​​the bolt, and is used to transmit and receive ultrasonic signals covering the corresponding threaded area. The signal processing unit is electrically connected to the center electrode and all edge electrodes, respectively, and is used to receive and process the electrical signals transmitted from each electrode, and to perform the following operations: The axial stress of the bolt is calculated based on the ultrasonic propagation time difference between the center electrode in the zero-stress state and the service state. Based on the echo signal received by any edge electrode in service, it is determined that there is a microcrack in the corresponding threaded area.

2. The bolt axial stress and threaded microcrack detection system according to claim 1, characterized in that, The detection system also includes: The data storage unit is used to store baseline data during the system calibration phase, as well as real-time and historical data during the online testing process. The early warning unit, connected to the signal processing unit, is used to issue audible and visual alarms and / or remote communication alarms when the axial stress value exceeds the preset safety range or when a microcrack is detected.

3. The bolt axial stress and threaded zone microcrack detection system according to claim 1, characterized in that, The array ultrasonic detection unit adopts a 2D ring piezoelectric array structure, with the central electrode and edge electrodes operating at a frequency of 15-50MHz. The central electrode and all edge electrodes are made of PZT-1-3 composite piezoelectric material, and their size and operating frequency are consistent. Each electrode is electrically connected to the external ultrasonic excitation module and ultrasonic receiving module through flexible circuits and is fixed on a high-strength insulating ceramic base.

4. The bolt axial stress and threaded zone microcrack detection system according to claim 1, 2, or 3, characterized in that, The array ultrasonic detection unit operates in a time-division manner according to the following transmission modes: Proximal thread crack detection mode: Triggers synchronous in-phase excitation of all edge electrodes, while the center electrode remains silent and receives echo signals across the entire frequency band, forming a 0° parallel sound beam along the bolt axis, used to detect near-end thread cracks within a range of 0-100mm from the electrode mounting end face. Distant screw / bottom crack detection mode: The timing difference excitation strategy of "opposite edge electrode - center electrode - local edge electrode" is adopted. The direction corresponding to each edge electrode is detected in sequence. By controlling the excitation time delay, a directional focused sound beam with a preset deflection angle β to the bolt axis is formed to detect cracks in the distal area from 100mm from the electrode mounting end face to the bottom of the bolt. Acoustic stress measurement mode: Only the center electrode is triggered for individual excitation, while all edge electrodes are silent and synchronously turned on to receive signals, used to measure the total acoustic time of the bottom wave propagating along the bolt axis.

5. A detection method based on the bolt axial stress and threaded zone microcrack detection system according to any one of claims 1-4, characterized in that, Includes the following steps: S1, An array of ultrasonic testing units is set on the upper end face of the bolt to be inspected; the included angle between the centers of any two adjacent edge electrodes is equal, and the center electrode coincides with the geometric center of the upper end face; S2, crack detection is performed on the near-end thread area of ​​the bolt to be inspected using the near-end crack detection emission mode, and the near-end crack detection result is obtained; S3, using the far-end crack detection emission mode, performs crack detection on the far-end thread area of ​​the bolt under inspection and obtains the far-end crack detection result; S4, through the acoustic time stress measurement mode, quantitatively measure the axial stress of the bolt under inspection and obtain the axial stress measurement result; S5. Based on the results of the near-end crack detection, the far-end crack detection, and the axial stress measurement, determine whether the preset safety threshold has been exceeded. If the error does not exceed the limit, output a test report; If the error exceeds the limit, output a detection report and warning information.

6. The detection method according to claim 5, characterized in that, Step S2 specifically includes: S2.1, adopts the near-end crack detection emission mode to output synchronous trigger signal; S2.2, according to the synchronization trigger signal, control all edge electrodes to be synchronously and in phase excited, while the center electrode remains silent and the full-band receiving mode is turned on; S2.3 controls all edge electrodes to emit preset ultrasonic longitudinal wave pulse signals of equal amplitude, phase and frequency at the same time, forming a parallel sound beam along the bolt axis. S2.4 receives echo signals in real time through all edge electrodes and the center electrode; S2.5, Crack analysis is performed based on the echo signal to obtain the near-end crack detection results.

7. The detection method according to claim 5 or 6, characterized in that, Step S3 specifically includes: S3.1, adopt the far-end crack detection emission mode to set the timing difference excitation strategy of "opposite edge electrode - center electrode - local edge electrode"; S3.2, Set the first excitation timing of the far circumferential region corresponding to the first edge electrode according to the strategy; S3.3, according to the first excitation timing sequence, the preset power ultrasonic pulse signals are emitted sequentially, and interference superposition occurs in the propagation path of the distal thread region to form a first directional focused sound beam with a preset deflection angle β to the bolt axis; S3.4, Receive the first directional echo signal of the first directional focused sound beam through the center electrode; S3.5, execute steps S3.2 and S3.3 sequentially according to the circumferential rotation rule to obtain all directional echo signals in the full circumference; S3.6, Crack analysis is performed based on each directional echo signal to obtain the remote crack detection results.

8. The detection method according to claim 7, characterized in that, The first excitation timing sequence includes: the first wave excites the edge electrode opposite the first edge electrode, after a fixed delay Δt1, the center electrode is excited, and after a fixed delay Δt2, the first edge electrode is excited; the fixed delays Δt1 and Δt2 are both in the range of 10-50ns, and the preset deflection angle β is in the range of 5°-15°.

9. The detection method according to claim 5 or 6, characterized in that, Step S4 specifically includes: The central electrode is individually excited to emit an ultrasonic pulse signal for stress measurement via the acoustic time-stress measurement mode, while all edge electrodes remain silent and simultaneously turn on the receiving mode. The bottom echo signal is received synchronously by the center electrode and the edge electrode, and the total acoustic time t of the bottom echo is collected. σ ; Based on the pre-calibrated zero-stress state bottom wave acoustic time t0, the acoustic time difference Δt = t σ -t0; Substituting the acoustic time difference Δt into the preset quantitative formula for axial stress σ=k1×Δt+b1, the current axial stress value is calculated; k1 and b1 are preset parameters.

10. The detection method according to claim 5 or 6, characterized in that, The system calibration process is included before step S1: Axial stress calibration: Apply a gradient axial load to a standard bolt of the same specification as the bolt to be tested, and record the service time t of the center electrode. σ In conjunction with the zero-stress acoustic time t0, a quantitative relationship formula is established between axial stress σ and acoustic time difference Δt: σ = k1 × Δt + b1; k1 and b1 are preset parameters. Crack depth calibration: Microcracks of different depths are prefabricated in the threaded area of ​​a standard bolt, and the acoustic time difference values ​​of the corresponding edge electrodes are recorded to establish a crack depth-acoustic time difference correlation model.

Images