An electromagnetic ultrasonic transverse wave detection device and method suitable for underwater concrete structure cracks

Abstract

The application discloses an electromagnetic ultrasonic transverse wave detection device and method suitable for underwater concrete structure cracks, and belongs to the technical field of nondestructive testing of hydraulic engineering; the method is characterized by the following steps: a sealing module is pressed tightly on the surface of a dam body by an underwater robot to form a sealed cavity; after the closed crack is actively opened by pumping and depressurizing, the crack plane profile is obtained by using electromagnetic ultrasonic transverse waves; the transverse wave is emitted at a repetition frequency of 100 Hz to 1000 Hz during the process of restoring and increasing the water pressure; the transverse wave sound time data under different pressures are recorded as second detection data; the first detection data, the reference sound time data and the second detection data are fused; the crack depth direction opening degree distribution is inversed based on the relationship between the transverse wave speed and the pressure of the acoustic elasticity effect; and the three-dimensional geometric evaluation result is generated; the application improves the crack detection conditions by actively regulating and controlling the local water pressure, and realizes the accurate three-dimensional nondestructive testing of underwater concrete structure cracks by using the high sensitivity of the transverse wave to the closed crack.

Description

Technical Field

[0001] This invention relates to the field of non-destructive testing technology for water conservancy projects, specifically to an electromagnetic ultrasonic shear wave testing device and method for detecting cracks in underwater concrete structures. Background Technology

[0002] As an important hydraulic engineering structure, concrete dams are subjected to a long-term service environment of high water level, high hydrostatic pressure, and complex temperature and stress fields. Cracks of different sizes and orientations are prone to form inside and on the surface of the dam body. Under high water level operation conditions, the cracks tend to close under the action of hydrostatic pressure, the difference in acoustic impedance at the crack interface decreases, and the ultrasonic reflection and scattering effects weaken. This makes it difficult for conventional ultrasonic testing methods to effectively detect such hidden cracks, posing a potential threat to the overall safety assessment and operational risk warning of the dam body.

[0003] In existing technologies, ultrasonic non-destructive testing of concrete cracks often employs piezoelectric ultrasonic transducers with fixed incident angles. Crack identification relies on a single longitudinal wave echo amplitude or detection parameters set based on human experience. Other methods divide the dam area under test into sections, incorporating the detection of water level depth and longitudinal wave echo characteristics to determine the degree of crack closure, identifying acoustically transparent risk sections, and then adjusting the incident deflection angle of the ultrasonic probe in conjunction with the crack's stress direction. However, this method still relies on longitudinal wave detection. When the crack interface is tightly closed under high water pressure, the longitudinal wave reflection energy decreases sharply, posing a risk of missed detections. Furthermore, this method passively adapts to the crack closure state, improving detection effectiveness through algorithmic compensation, without actively altering the crack's acoustic response conditions.

[0004] Electromagnetic ultrasonic testing technology is widely used in the non-destructive testing of metallic materials due to its advantages such as not requiring coupling agents and non-contact testing. Some technologies disclose systems and methods for non-contact testing of underwater pipes, risers, and other metal structures using electromagnetic ultrasonic transducers. However, the working principle of electromagnetic ultrasonic testing is to induce eddy currents on the surface of the tested material and use the Lorentz force to excite ultrasonic waves. This technology relies on the conductivity of the tested material. Concrete, as a non-ferromagnetic and weakly conductive composite material, is difficult for traditional electromagnetic ultrasonic transducers to excite sufficiently strong ultrasonic signals on its surface, especially the excitation efficiency of shear wave signals is extremely low, making it difficult to meet engineering requirements for the signal-to-noise ratio.

[0005] To address the coupling issue in ultrasonic testing of rough concrete surfaces, one technique employs a traction support platform on the workpiece to be inspected. The platform has through-holes for mounting, within which transverse wave ultrasonic sensor elements are installed and can move freely axially, ensuring good coupling between each sensor element and the workpiece. This device uses a rigid traction support platform, suitable for plate testing in laboratory or dry environments. However, it struggles to maintain stable sealing and coupling conditions in high-pressure underwater environments and does not address the acoustic transparency issue caused by elastic compression of closed cracks under high water pressure.

[0006] In the field of underwater structure inspection, one technical solution employs an underwater robot equipped with a depth camera. This robot utilizes an improved YOLOv8 image recognition algorithm to identify crack direction, and combines this with depth data from the depth camera to construct three-dimensional features of the crack. An autonomous navigation system then plans the optimal path to the crack location for grouting repair. However, this method relies on optical imaging technology, which limits its ability to detect cracks in turbid water or low visibility environments. Furthermore, it can only detect surface-opening cracks and cannot effectively detect surface-closed or shallowly extending cracks. The detection results are also greatly affected by ambient lighting conditions.

[0007] In summary, existing technologies for detecting closed cracks in high-pressure underwater concrete have the following main shortcomings: Longitudinal wave detection methods are greatly affected by crack closure. Under high water pressure, the crack interface is tightly adhered due to compressive stress, the acoustic impedance difference decreases, and the reflected energy of longitudinal waves weakens sharply. Conventional longitudinal wave detection methods are prone to missed detections. Existing technologies only passively adapt to the crack closure state through angle adjustment and algorithm compensation, and fail to fundamentally improve the acoustic response conditions of cracks.

[0008] The application of electromagnetic ultrasonic technology in concrete is limited. Existing electromagnetic ultrasonic testing methods are mainly for metallic materials, relying on the conductivity of the tested object to generate Lorentz force to excite ultrasonic waves. Concrete is a weakly conductive material, and traditional electromagnetic ultrasonic transducers are unable to excite detectable shear wave signals with sufficient intensity and penetration depth. The shear wave excitation efficiency and signal-to-noise ratio are both difficult to meet practical requirements.

[0009] Underwater optical inspection methods have poor adaptability. Crack detection methods based on underwater robots and visual images rely on water transparency and lighting conditions. In environments with large water flow disturbances and high sediment content leading to turbidity, image acquisition quality deteriorates, recognition accuracy decreases significantly, and these methods cannot detect underwater cracks with closed surfaces.

[0010] There is a lack of means to proactively improve detection conditions. Existing technologies have failed to design methods to proactively change the stress state of cracks in the high-pressure underwater environment, and can only passively adapt to the closure state of cracks. Detection conditions are greatly limited by environmental factors, making it difficult to achieve stable and reliable detection of closed cracks in concrete under various operating conditions.

[0011] Therefore, there is an urgent need for a device and method that can actively improve crack detection conditions in underwater high-pressure environments and achieve reliable detection by utilizing the high sensitivity of transverse waves to closed cracks. Summary of the Invention

[0012] To overcome the problems of the prior art, this invention discloses an electromagnetic ultrasonic shear wave detection device and method for detecting cracks in underwater concrete structures.

[0013] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: The first aspect provides an electromagnetic ultrasonic shear wave detection method suitable for underwater concrete structure cracks, comprising the following steps: S1, deploy an underwater robot equipped with an electromagnetic ultrasonic transverse wave detection module and a pressure control module, press the sealing module set at the front end of the underwater robot tightly against the surface of the concrete dam to be tested, forming a sealed cavity isolated from the surrounding water. S2, activate the pressure control module to pump water and reduce the pressure in the sealed cavity, so that the water pressure inside the cavity is lower than the preset difference of the external static water pressure, maintain the low pressure state for a predetermined time, and cause the closed cracks on the concrete surface to open actively due to elastic rebound. S3, under low pressure, controls the electromagnetic ultrasonic shear wave detection module to emit a shear wave pulse sequence to the detection area, receives the echo signal reflected by the crack interface, and obtains the first detection data reflecting the plane profile of the crack. S4, turn off the pumping, restore the water pressure inside the cavity to be equal to the external hydrostatic pressure, and record the reference acoustic time data in this state; S5, start pressurization to make the water pressure inside the cavity higher than the external hydrostatic pressure, and emit transverse waves at a repetition frequency of 100Hz to 1000Hz during the pressurization process and receive transverse wave signals that pass through the crack area, and record the transverse wave acoustic time data under different water pressure stages as the second detection data. S6. The first detection data, the reference acoustic time data and the second detection data are fused and processed. An inversion algorithm is used to calculate the crack opening distribution in the depth direction based on the relationship between the transverse wave velocity and pressure of the acoustoelastic effect, and generate the three-dimensional geometric evaluation result of the crack.

[0014] Preferably, the front edge of the sealing module is provided with an elastic sealing ring, and a pressure monitoring module for monitoring the water pressure inside the cavity is integrated inside the sealing module or on the cavity wall. The pressure regulation module includes a vacuum pump and a high-pressure water pump, with the vacuum pump pumping water to reduce pressure and the high-pressure water pump pressurizing.

[0015] Preferably, the preset difference in step S2 is 0.02MPa to 0.08MPa, and the preset time is 5 seconds to 15 seconds; The pressure is increased to a value of 0.05MPa to 0.15MPa higher than the external static water pressure, and a step-by-step pressure increase method is adopted.

[0016] Preferably, the electromagnetic ultrasonic transverse wave detection module is a non-contact electromagnetic ultrasonic transducer, which includes a permanent magnet or electromagnet that generates a bias magnetic field and an excitation coil for exciting eddy currents. The center frequency of the transverse wave pulse sequence is 50kHz to 500kHz.

[0017] Preferably, obtaining the first detection data reflecting the plane contour of the crack in step S3 specifically includes: performing time window interception, envelope detection and amplitude threshold determination on the received echo signal, extracting spatial location points where the echo amplitude is greater than a preset threshold, and connecting them to form the two-dimensional boundary contour of the crack.

[0018] Preferably, the recording of transverse wave acoustic time data under different water pressure stages in step S5 includes: performing cross-correlation time delay estimation on the transverse wave received signal under each pressure step, calculating the difference between the transit time of the transverse wave in the cracked region and the reference region without cracks, and forming an acoustic time difference versus pressure curve.

[0019] Preferably, the inversion algorithm in step S6 is at least one of least squares fitting or finite element inversion; The inversion of the transverse wave velocity and pressure relationship based on the acoustoelastic effect includes: establishing an acoustoelastic relationship model between the change in transverse wave velocity and the normal stiffness of the crack interface; using the acoustic time difference and pressure relationship curve in the second detection data as input; and inverting the crack opening depth by depth.

[0020] Preferably, the underwater robot also includes a positioning and navigation system, which uses a combination of a visual camera, an inertial measurement unit and a multibeam sonar to guide the sealing module to move point by point to cover the entire dam body area to be tested, generating a three-dimensional distribution map of the cracks.

[0021] Preferably, after generating the three-dimensional geometric evaluation result of the crack in step S6, the method further includes: comparing the three-dimensional geometric evaluation result of the crack with a preset safety threshold; if the crack depth or maximum opening exceeds the threshold, outputting a repair prompt signal to the repair execution module of the underwater robot.

[0022] On the other hand, an electromagnetic ultrasonic shear wave detection device suitable for underwater concrete structure cracks is provided for implementing the above method, comprising: Underwater robots; The sealing module, installed at the front end of the underwater robot, forms a sealed cavity that is isolated from the surrounding water when it is pressed tightly against the surface of the concrete dam to be tested. The pressure regulation module is connected to the interior of the sealing module's cavity, and can pump water to reduce pressure or inject water to increase pressure in the sealing cavity. The electromagnetic ultrasonic transverse wave detection module is located inside the cavity of the sealed module, with its emitting surface facing the surface of the concrete dam to be tested. It emits a transverse wave pulse sequence and receives the echo signal. The pressure monitoring module is located inside the cavity of the sealing module to detect the water pressure inside the sealing cavity in real time. The control module is electrically connected to the pressure regulation module, the electromagnetic ultrasonic transverse wave detection module, the pressure monitoring module, and the underwater robot's drive system, and is used to control the execution of steps S1 to S6.

[0023] The beneficial effects of this invention are as follows: Compared with the prior art, the technical solution provided by the present invention has the following significant advantages: By actively regulating the water pressure inside the sealed cavity, the closed crack can be actively opened under reduced pressure, which improves the acoustic response conditions of the crack and solves the technical problems of weak longitudinal wave reflection of closed cracks under high water pressure and easy to miss by conventional detection. Employing electromagnetic ultrasonic shear wave detection technology, which requires no coupling agent and is suitable for underwater operating environments, it achieves highly sensitive detection of closed cracks in high-attenuation concrete media by optimizing the shear wave repetition frequency and center frequency. A dual-modal detection method combining pressure reduction detection and pressure increase acoustic time measurement was adopted. The pressure reduction mode obtained the crack plane profile, and the pressure increase mode obtained the transverse wave acoustic time data under different stress states. The two types of data were fused to invert the crack opening distribution in the depth direction based on the acoustoelastic effect, thus realizing the three-dimensional geometric evaluation of the crack. By integrating the detection device into an underwater robot platform, the sealing module is guided by a positioning and navigation system to move point by point to cover the entire dam body area to be tested, realizing large-scale, automated, and high-precision underwater crack detection. The detection results can provide a basis for concrete structure safety assessment and repair decisions. Attached Figure Description

[0024] Figure 1 This is a flowchart of an electromagnetic ultrasonic shear wave detection method for underwater concrete structure cracks provided in Embodiment 1 of the present invention. Figure 2 This is a flowchart illustrating the operation of an electromagnetic ultrasonic shear wave detection device for underwater concrete structure cracks, as provided in Embodiment 1 of the present invention. Detailed Implementation

[0025] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, exemplary embodiments will be described in detail below, examples of which are illustrated in the accompanying drawings. In the following description relating to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of methods and systems consistent with some aspects of this application as detailed in the appended claims.

[0026] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The singular forms “a” and “the” as used in this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0027] The following detailed description of the specific implementation methods, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided in detail.

[0028] Example 1 Please refer to Figure 1 This embodiment provides an electromagnetic ultrasonic shear wave detection method for cracks in underwater concrete structures, including the following steps: S1, Deploy an underwater robot equipped with an electromagnetic ultrasonic transverse wave detection module and a pressure control module. The underwater robot has autonomous or remote-controlled movement capabilities, and a sealing module is fixedly installed at its front end. The underwater robot pushes the sealing module to the surface of the concrete dam to be tested, and uses the thrust generated by the thruster to press the sealing module tightly against the dam surface; The front edge of the sealing module is equipped with an elastic sealing ring. When compressed, the sealing module forms a sealed cavity that is completely isolated from the surrounding water body between itself and the dam surface. This sealed cavity provides a closed environment for subsequent pressure regulation and ultrasonic testing.

[0029] S2, start the pressure control module. The pressure control module includes a vacuum pump and a high-pressure water pump. The vacuum pump is connected to the inside of the sealed cavity through a sealed pipeline. The vacuum pump is started to pump water and reduce the pressure in the sealed cavity, so that the water pressure in the sealed cavity is reduced to a value lower than the preset difference of the external static water pressure. The preset difference is set based on the elastic modulus of concrete and the crack closure stress, and can be set to 0.02 MPa to 0.08 MPa to maintain this low pressure state for a predetermined period of time, such as 5 to 15 seconds. Under this pressure difference, the cracks on the concrete surface that were originally tightly closed under high water pressure actively open due to the elastic rebound effect, and micron-sized gaps are generated at the crack interface, restoring the ability to reflect ultrasonic waves.

[0030] S3, while maintaining a low-pressure state, controls the electromagnetic ultrasonic transverse wave detection module to emit a transverse wave pulse sequence toward the detection area; The electromagnetic ultrasonic transverse wave detection module is a non-contact electromagnetic ultrasonic transducer. It contains a permanent magnet or electromagnet to generate a bias magnetic field and is equipped with an excitation coil to generate eddy currents on the surface of the concrete being tested. The center frequency of the transverse wave pulse sequence is preferably between 50 kHz and 500 kHz. The emitted transverse wave propagates in the concrete medium and is reflected when it encounters the opened crack interface. The reflected echo is received by the same module. The received echo signal is subjected to time window truncation, envelope detection and amplitude threshold determination. Spatial location points with echo amplitude greater than the preset threshold are extracted. Connecting these location points forms the two-dimensional boundary contour of the crack, which is used as the first detection data.

[0031] S4, turn off the vacuum pump to stop pumping water, and allow the water pressure in the sealed cavity to gradually return to the same level as the external static water pressure; In this balanced state, the electromagnetic ultrasonic transverse wave detection module is controlled to re-emit the transverse wave pulse sequence and the acoustic time data of the received transverse wave echo signal is recorded as the reference acoustic time data. The reference acoustic time data reflects the standard propagation time of transverse waves in concrete under no pressure difference conditions, and is used for subsequent comparative analysis.

[0032] S5, start the high-pressure water pump of the pressure regulation module to inject water into the sealed cavity and pressurize it, so that the water pressure in the cavity is higher than the external static water pressure setting value, which is 0.05 MPa to 0.15 MPa, and gradually increase the pressure in a step-by-step manner; During the pressurization process, the electromagnetic ultrasonic transverse wave detection module is controlled to continuously transmit transverse wave pulse sequences at a repetition frequency of 100 Hz to 1000 Hz, and simultaneously receive transverse wave signals propagating through the crack region. The repetition frequency range of 100 Hz to 1000 Hz was set based on the following considerations: When the repetition frequency is below 100 Hz, the time interval between two adjacent transmissions exceeds 10 milliseconds. During the rapid pressure change in the step-up process, the number of acoustic time data points collected per unit time is too small to accurately capture the dynamic process of shear wave acoustic time changing with pressure. This results in insufficient fitting accuracy of the subsequent acoustic time difference versus pressure relationship curve, affecting the accuracy of crack opening inversion. When the repetition frequency is higher than 1000 Hz, the time interval between two adjacent transmissions is less than 1 millisecond. The propagation distance of the transverse wave in concrete is usually tens of centimeters, and the single propagation time is about 0.1 to 0.3 milliseconds. If the repetition frequency is too high, the transmitted pulse and the echo signal will overlap on the time axis, resulting in electrical crosstalk and acoustic aliasing, which will significantly reduce the signal-to-noise ratio of the echo signal and make it impossible to effectively extract transverse wave acoustic time data. Therefore, controlling the repetition frequency within the range of 100 Hz to 1000 Hz can ensure sufficient time resolution to track acoustic time changes during the boost process, while also avoiding signal aliasing and ensuring the reliability of the detection data and the accuracy of the inversion. Cross-correlation time delay estimation is performed on the transverse wave signals received under each pressure step, and the difference between the transit time of the transverse wave in the cracked region and the reference region without cracks is calculated. This forms the relationship curve between acoustic time difference and pressure, and these data are recorded as the second detection data.

[0033] S6, the first detection data obtained in step S3, the reference acoustic time data obtained in step S4, and the second detection data obtained in step S5 are fused together. An inversion algorithm is used, which is at least one of least squares fitting or finite element inversion, and the inversion calculation is performed based on the relationship between transverse wave velocity and pressure in the acoustoelastic effect. An acoustoelastic model is established to show the relationship between the change in transverse wave velocity and the normal stiffness of the crack interface. Based on acoustoelastic theory, this model states that when a transverse wave propagates perpendicular to the crack surface, its wave velocity changes with the normal stress at the crack interface. The relationship between wave velocity and pressure is described by the following equation:

[0034] Where ΔV represents the change in shear wave velocity, V0 represents the shear wave velocity corresponding to the reference acoustic time data, S is the acoustoelastic coefficient, and σ is the change in normal stress applied to the crack interface. The method for determining the acoustic elastic coefficient S is as follows: The acoustoelastic coefficient S reflects the sensitivity of transverse wave velocity to normal stress. Its value depends on the elastic modulus E, Poisson's ratio ν, and the roughness characteristics of the crack interface of the concrete material. For underwater concrete structures, the typical range of S is 1.0 × 10⁻⁶. -7 Pa -1 Up to 5.0×10 -7 Pa -1 ; In practical engineering applications, the S-value can be obtained through one of the following two methods: Method 1: Artificial cracks are pre-cast on concrete test blocks poured in the same batch and cured under the same conditions as the dam body to be tested. These test blocks are placed in a simulated water pressure environment, and a known pressure change Δσ is applied. The corresponding change in transverse wave velocity ΔV is measured, and the S value is obtained through formula fitting.

[0035] The calibration test should be conducted at least 5 different pressure steps, and the results with a fitting correlation coefficient greater than 0.95 should be used. Method 2: If experimental calibration is not possible, select an empirical value based on the concrete strength grade. For C25 to C35 concrete, S is taken as 2.0 × 10⁻⁶. -7 Pa -1 ; For C40 to C50 concrete, S is taken as 1.5 × 10⁻⁶. -7 Pa -1For high-strength concrete with a strength of C50 or higher, S is taken as 1.0 × 10⁻⁶. - 7 Pa -1 ; In this embodiment, the concrete strength grade of the dam body to be tested is C30, and S is taken as 2.0 × 10⁻⁶ using method two. -7 Pa -1 ; The change in transverse wave velocity under different pressures can be calculated using the acoustic time difference versus pressure curve obtained in step S5. Using this change as input, the above acoustoelastic relationship model is substituted to inversely calculate the crack opening distribution at each depth. The crack plane profile data is then fused with the opening distribution in the depth direction to finally generate a three-dimensional geometric evaluation result of the crack, including the crack length, width, depth, and the curve of the opening as a function of depth.

[0036] Specifically, this invention actively regulates the water pressure within the sealed cavity, causing closed cracks to open under reduced pressure and obtaining a planar profile using transverse wave reflection. Then, by combining the transverse wave acoustic time variation during the pressurization process with the acoustoelastic effect, the crack opening degree is inverted, achieving high-precision three-dimensional non-destructive testing of cracks in underwater concrete structures. This method effectively solves the technical problem of difficult detection of closed cracks under high water pressure and has advantages such as high detection accuracy, strong applicability, and good repeatability.

[0037] Furthermore, for the sealing module deployed in step S1, an elastic sealing ring is provided at the front edge. The elastic sealing ring is made of a hydrolysis-resistant and aging-resistant rubber material. When the sealing module is pressed against the surface of the concrete dam by the propulsion force of the underwater robot, the elastic sealing ring undergoes elastic deformation, filling the micro-uneven gap between the sealing module and the dam surface, forming a sealed cavity that is completely isolated from the surrounding water between the sealing module and the dam surface. A pressure monitoring module is integrated inside the sealing module or on the cavity wall to monitor water pressure changes inside the sealing cavity in real time; The pressure monitoring module can use a diffused silicon pressure sensor or a ceramic capacitive pressure sensor. Its sensitive element is in direct contact with the water in the sealed cavity, and can convert the water pressure signal into an electrical signal and transmit it to the control module. By using the real-time water pressure data fed back by the pressure monitoring module, the control module can accurately determine the difference between the current water pressure inside the sealed cavity and the external static water pressure, providing a closed-loop control basis for the pumping and depressurization process in step S2 and the pressurization process in step S5. The pressure control module involved in steps S1 and S2 includes a vacuum pump and a high-pressure water pump. The vacuum pump is connected to the inside of the cavity of the sealing module through a sealed pipeline, and the high-pressure water pump is also connected to the inside of the cavity of the sealing module through an independent sealed pipeline. Vacuum pumps are used to pump water and reduce pressure. When they are working, they extract water from the sealed cavity, reducing the water pressure inside the cavity to below the external hydrostatic pressure. High-pressure water pumps are used for pressurization. When they are working, they pump external water into a sealed cavity, raising the water pressure inside the cavity to a level higher than the external static water pressure. The start-up, shutdown, and operating intensity of the vacuum pump and high-pressure water pump are all automatically adjusted by the control module based on the water pressure signal fed back by the pressure monitoring module; Through the coordinated operation of the sealing module, pressure monitoring module and pressure regulation module, the present invention can accurately control the water pressure in the sealed cavity in an underwater environment, providing stable and controllable detection conditions for the active opening of cracks and the acquisition of transverse wave acoustic time data. Specifically, this specific embodiment is closely integrated with steps S1 to S6. The sealing module and its elastic sealing ring ensure the formation of the sealing cavity, the pressure monitoring module provides real-time feedback for pressure control in each step, and the vacuum pump and high-pressure water pump realize the pressure reduction operation in step S2 and the pressure increase operation in step S5, which together constitute the basic hardware support of the present invention.

[0038] Furthermore, for the pumping and depressurization operation in step S2, the preset difference is set to 0.02 MPa to 0.08 MPa. The selection of this preset difference is based on the elastic modulus of concrete and the mechanical response characteristics of closed cracks. The elastic modulus of concrete is typically between 20 GPa and 40 GPa. When a crack is closed under high water pressure, the concrete on both sides of the crack undergoes elastic compression due to compressive stress. When the water pressure inside the sealed cavity drops below the external hydrostatic pressure, the pressure difference acts on the crack area, causing the concrete on both sides of the crack to rebound elastically. According to theoretical calculations, a pressure difference of 0.02 MPa is sufficient to produce a rebound displacement at the micrometer level at the crack interface, and this displacement is within the sensitive range of shear wave detection. When the pressure difference exceeds 0.08 MPa, the gain of increasing the pressure difference on the crack opening tends to saturate, and an excessively large pressure difference will increase the risk of the sealing module falling off the dam surface. Therefore, limiting the preset difference to the range of 0.02 MPa to 0.08 MPa can both enable the closed crack to actively open to a detectable level and ensure the reliability of the seal and the safety of the detection.

[0039] In step S2, the predetermined time for maintaining the low-pressure state is set to 5 to 15 seconds. The selection of this predetermined time takes into account the response time of the crack's elastic rebound and the data acquisition time required for shear wave detection. The elastic rebound of a crack under pressure difference is a transient process, which usually takes 1 to 2 seconds to complete. However, in order to ensure that the crack fully opens and reaches a stable state, a certain amount of stress equilibrium time is required. Meanwhile, in step S3, a transverse wave pulse sequence needs to be transmitted and an echo signal needs to be received under low pressure. The time required for a single detection is about 2 to 3 seconds. Setting the lower limit of the predetermined time to 5 seconds can ensure that the crack opens stably and completes at least one complete shear wave scan. Setting the maximum time limit to 15 seconds avoids unnecessary impacts on the sealing performance of the sealing module and the local concrete structure caused by maintaining a low-pressure state for a long time. For the pressurization operation in step S5, the pressure value that makes the water pressure in the sealed cavity higher than the external hydrostatic pressure is set to 0.05 MPa to 0.15 MPa; The selection of this pressure range is based on the sensitivity of transverse wave velocity to normal stress in the acoustoelastic effect and the mechanical properties of the concrete crack interface. When the applied pressure is below 0.05 MPa, the change in normal stress at the crack interface is too small, and the resulting change in transverse wave velocity is lower than the resolution of the detection system, making it difficult to accurately extract the change characteristics from the acoustic time data. When the applied pressure is higher than 0.15 MPa, the excessive pressure may cause excessive elastic compression at the crack interface, or even cause local damage to the crack tip. At the same time, the excessive water pressure inside the cavity will also pose a challenge to the sealing performance of the sealing module. Therefore, a pressure range of 0.05 MPa to 0.15 MPa can avoid adverse effects on cracks and sealing structures while ensuring that the change in shear wave velocity is measurable.

[0040] Step S5 employs a stepped pressurization method, which divides the pressurization process from external hydrostatic pressure to the target pressure value into multiple pressure steps. The pressure is maintained stable for a period at each pressure step before further pressurization. The specific implementation method of the stepped pressurization is as follows: The control module first starts the high-pressure water pump to inject water into the sealed cavity. When the water pressure signal fed back by the pressure monitoring module reaches the first preset pressure step value, the control module shuts off the high-pressure water pump and maintains the pressure value stable for 2 to 5 seconds. During this stable period, the electromagnetic ultrasonic transverse wave detection module is controlled to emit transverse waves at a repetition frequency of 100 Hz to 1000 Hz and receive echo signals, recording the transverse wave acoustic time data under this pressure step. After completing the data acquisition for this step, the control module restarts the high-pressure water pump to increase the pressure to the next higher pressure step value, repeating the above stabilization and data acquisition process until the target pressure value is reached. The advantage of the step-by-step pressurization method is: Compared with continuous pressure increase, step pressure increase can obtain a stable stress state at each pressure point, eliminate the interference of pressure fluctuations on the transverse wave acoustic time measurement, make the acoustic time difference versus pressure relationship curve smoother and more accurate, and improve the accuracy of subsequent inversion calculation of crack opening. By rationally selecting the above numerical range and designing the stepped pressure increase method, the present invention realizes the active and controllable opening of the crack in step S2, and realizes the precise acquisition of the precise application of the stress state at the crack interface and the acoustic time data in step S5, laying a reliable data foundation for the high-precision inversion calculation in step S6.

[0041] Furthermore, the electromagnetic ultrasonic transverse wave detection module used in steps S1 to S6 is a non-contact electromagnetic ultrasonic transducer. This non-contact electromagnetic ultrasonic transducer does not directly contact the surface of the concrete dam being tested, but maintains a small gap. It excites ultrasonic waves on the concrete surface and receives echo signals through electromagnetic coupling. It does not require the use of a coupling agent, thus avoiding the problem of the coupling agent being difficult to apply and maintain in an underwater environment. The non-contact electromagnetic ultrasonic transducer includes a permanent magnet or electromagnet that generates a bias magnetic field and an excitation coil for exciting eddy currents. The permanent magnet is made of neodymium iron boron material with high remanence, which can generate a stable static bias magnetic field in the space around the transducer; Alternatively, an electromagnet can be used to generate a bias magnetic field, and the magnetic field strength can be adjusted by controlling the excitation current of the electromagnet coil to adapt to different concrete material properties or different detection depth requirements. The excitation coil is a planar spiral coil or a racetrack-shaped coil, arranged between the permanent magnet or electromagnet and the surface of the concrete dam. When a high-frequency pulse current is applied to the excitation coil, an alternating magnetic field is generated around the coil, which induces eddy currents on the concrete surface. The induced eddy current is subjected to Lorentz force under the action of static bias magnetic field. This Lorentz force causes the particles on the surface of concrete to vibrate at high frequency and radiate transverse waves into the concrete. When the transducer receives the echo signal, the reflected transverse wave at the crack interface causes the particles on the concrete surface to vibrate. This vibration cuts the magnetic field lines in the static bias magnetic field, causing an induced electromotive force to be generated at both ends of the excitation coil. This induced electromotive force is the received echo signal. The center frequency of the transverse wave pulse sequence is 50 kHz to 500 kHz. The determination of this center frequency range is based on the acoustic attenuation characteristics of concrete materials and the detection resolution requirements. To address the problems of weak electrical conductivity in concrete and low excitation efficiency of traditional electromagnetic ultrasonic shear waves, this invention can be solved through the following design: Based on the order-of-magnitude estimation of the Lorentz force density, the wet electrical conductivity of concrete is typically 10. -3 ~10-2 Under conditions of an excitation frequency of 200 kHz, a peak excitation current of 200 A, and 10-20 coil turns, the induced eddy current density on the concrete surface is approximately 10 S / m. 2 ~10 3 A / m 2 ; Combined with the static magnetic field of 0.5~1.2T provided by the permanent magnet, the Lorentz force density can reach 10. 2 ~10 3 N / m 3 Although the force density is lower than that of metallic materials, it is sufficient to generate nanometer- to micrometer-level particle vibration displacement at the elastic modulus of concrete, which is about 30 GPa. This displacement is within the detectable range, and the sensitivity of piezoelectric sensors or electromagnetic ultrasonic receivers can reach the nm level. Therefore, by optimizing the excitation current and magnetic field strength, usable transverse wave signals can be excited in concrete.

[0042] The pressure reduction in the sealed cavity indirectly improves the excitation conditions. In step S2, the pressure reduction in this invention causes the closed crack to actively open. After the gap at the crack interface increases, the acoustic impedance difference increases significantly, and the transverse wave reflection coefficient increases accordingly. This means that even if the absolute amplitude of the transverse wave excited in the concrete is limited, its crack echo signal-to-noise ratio can still meet the detection requirements. Compared with direct detection on closed cracks under high water pressure, the detection conditions of this invention have been actively optimized, reducing the requirements for transverse wave excitation efficiency.

[0043] The engineering adjustable range of transducer parameters: Based on the concrete strength grade and aggregate particle size, those skilled in the art can select the center frequency in the range of 50kHz to 500kHz, and adjust the number of turns, wire diameter, pulse voltage (100V to 1000V), and permanent magnet remanence (0.5T to 1.5T) of the excitation coil accordingly to obtain a recognizable echo signal on concrete in a specific engineering project. The above parameter adjustment is a routine skill in the design of electromagnetic ultrasonic transducers and does not require creative labor. In summary, the electromagnetic ultrasonic shear wave detection module described in this invention can effectively excite shear waves in weakly conductive concrete, thus meeting the requirements for crack detection.

[0044] When the center frequency is below 50 kHz, the transverse wave wavelength is relatively long, about 60 to 80 mm. For concrete microcracks with widths in the millimeter or sub-millimeter range, the reflection and scattering effects of long-wavelength transverse waves at the crack interface are weak, and the detection resolution is insufficient to accurately identify the crack boundary and internal morphology. When the center frequency is higher than 500 kHz, the wavelength of the transverse wave is short, about 4 to 6 mm. Although the resolution of fine cracks is improved, the particle size of concrete aggregate is usually 5 to 30 mm. When the short wavelength transverse wave encounters aggregate particles during propagation, it will be strongly scattered and attenuated, resulting in a sharp decrease in the penetration depth of the transverse wave, making it impossible to detect deep cracks. By controlling the center frequency within the range of 50 kHz to 500 kHz, it is possible to ensure that the transverse wave has sufficient penetration depth in the concrete medium, typically reaching 200 mm to 500 mm, and to obtain sufficient resolution for millimeter-level cracks, thus meeting the engineering requirements for crack detection in underwater concrete structures. In step S3, the electromagnetic ultrasonic shear wave detection module emits a shear wave pulse sequence within the aforementioned center frequency range under low pressure, receives the echo signal reflected from the crack interface, and obtains the first detection data reflecting the plane profile of the crack. In step S5, during the pressurization process, the module continuously transmits a sequence of transverse wave pulses with the same center frequency at a repetition frequency of 100 Hz to 1000 Hz, and receives transverse wave signals that pass through the crack region, and records transverse wave acoustic time data at different water pressure stages as the second detection data. The non-contact nature of this module avoids the coupling state between the probe and the dam surface being affected by changes in water pressure within the sealed cavity, ensuring the stability and consistency of signal transmission and reception throughout the entire detection process; Specifically, through the structural design of the non-contact electromagnetic ultrasonic transducer and the reasonable selection of the center frequency, this invention achieves reliable transverse wave detection of concrete cracks in an underwater high-pressure environment. This overcomes the shortcomings of traditional piezoelectric ultrasound which requires a coupling agent, and also solves the contradiction between transverse wave propagation and resolution in high-attenuation concrete media.

[0045] Furthermore, in step S3, after the electromagnetic ultrasonic shear wave detection module transmits a shear wave pulse sequence to the detection area under low pressure, it receives the echo signal reflected by the crack interface. In order to accurately extract information reflecting the plane contour of the crack from the received original echo signal, a series of processing is required on the echo signal, including time window truncation, envelope detection and amplitude threshold determination. Time window interception refers to the calculation of the time window for the arrival of crack echo signals based on the propagation speed of transverse waves in concrete medium and the distance between the sealing module and the surface of the dam to be tested. The propagation speed of transverse waves in concrete is known to be 2000 m / s to 3000 m / s. The distance between the emitting surface of the sealing module and the dam surface is a fixed value, usually in the range of 5 mm to 20 mm. Based on the above parameters, the time range required for the transverse wave to propagate from the emitting surface to the crack interface and return to the emitting surface is calculated, and a time window is set with this time range as the center. In the received continuous echo signal, only the signal segment within the time window is extracted for processing. Direct waves, multiple reflection waves and environmental noise outside the time window are removed to obtain the effective signal that mainly contains the reflection information of the crack interface. Envelope detection refers to the process of extracting the envelope of the effective signal after time window truncation. The original echo signal is a high-frequency oscillating waveform, and its amplitude changes rapidly with time and spatial location, making it difficult to use directly for amplitude comparison. Envelope detection converts a high-frequency oscillating signal into a smooth amplitude variation curve by calculating the instantaneous amplitude envelope of the signal. The specific implementation method is as follows: Perform a Hilbert transform on the signal after the time window is truncated to obtain the analytical form of the signal, and then calculate the magnitude of the analytical signal to obtain the envelope of the signal. Each point on the envelope curve represents the instantaneous amplitude of the echo signal at that moment, eliminating the interference of high-frequency oscillations on amplitude determination; Amplitude threshold determination refers to setting a preset threshold for the amplitude curve obtained after envelope detection; The preset threshold is determined based on the background noise level in the crack-free area, and is usually taken as 3 to 5 times the root mean square value of the background noise. Points on the envelope curve with amplitudes greater than a preset threshold are marked as effective reflection points. These effective reflection points correspond to the interface where the transverse wave encounters a sudden change in acoustic impedance on the propagation path, i.e., the location of the crack. After extracting all spatial locations where the echo amplitude is greater than the preset threshold, these locations are connected according to their geometric coordinates within the detection area to form the two-dimensional boundary profile of the crack. Since cracks are usually distributed in continuous linear or planar patterns, and the spatial distance between adjacent effective reflection points is less than the set connection threshold, discrete points can be connected into a closed or open continuous curve through a connection algorithm. This curve is the two-dimensional boundary projection of the crack on the dam surface and its shallow interior. Specifically, through the above-mentioned time window interception, envelope detection and amplitude threshold determination processing, step S3 can accurately extract crack reflection information from complex echo signals and form clear and distinguishable crack plane contour data, providing a basis for the three-dimensional geometric evaluation in the subsequent step S6.

[0046] Furthermore, in step S5, the pressure control module adopts a step-by-step pressure increase method to gradually increase the water pressure in the sealed cavity from the external static water pressure to a target pressure value that is 0.05 MPa to 0.15 MPa higher than the external static water pressure; At each pressure step, the control module stabilizes the pressure and triggers the electromagnetic ultrasonic transverse wave detection module to emit a transverse wave pulse sequence at a repetitive frequency of 100 Hz to 1000 Hz, while simultaneously receiving the transverse wave signal propagating through the crack region. In order to quantitatively analyze the acoustic response of the crack interface under different pressure conditions, it is necessary to extract acoustic time data from the received shear wave signal, which is specifically achieved through cross-correlation time delay estimation. The basic principle of cross-correlation delay estimation is as follows: For the same detection location, the transit time of two transverse wave signals received under different pressure steps will be slightly shifted in the crack region due to the change in the stress state at the crack interface. Using one signal as the reference signal and the other signal as the signal to be measured, calculate the cross-correlation function between the two; The cross-correlation function is defined as the similarity between the reference signal and the signal under test under different time delays. The cross-correlation function reaches its maximum value when the time delay is exactly equal to the actual transit time difference between the two signals. By searching for the peak position of the cross-correlation function, the time delay difference between the two signals can be accurately estimated. Specifically, in the implementation process of this invention, it is first necessary to obtain the transverse wave received signal of the crack-free reference area. The crack-free reference area refers to the intact concrete area within the coverage of the sealing module, which has been pre-scanned and confirmed to be free of cracks. In this reference area, following the same pressure step procedure in step S5, the transverse wave received signal under each pressure step is recorded as a reference signal. For the crack region, the transverse wave received signal under each pressure step is also recorded as the detection signal; For each pressure step, the detection signal from the cracked region is cross-correlated with the reference signal from the crack-free reference region under the same pressure step to estimate the time delay. The reference signal is x(t), and the detection signal is y(t). The cross-correlation function between the two is R. xy (τ) is expressed by the following formula:

[0047] Where t is a continuous time variable, in seconds, representing the time axis of the signal; τ is a time delay variable, representing the amount by which the detected signal y(t) is shifted to the left along the time axis, i.e., τ is negative or to the right, i.e., τ is positive translation. x(t) is the reference signal, i.e. the shear wave signal received in the crack-free reference region, which is a function of time t; y(t+τ) is the waveform of the detection signal after a time delay of τ; The integration interval is the effective duration of the signal. In actual calculations, the signal length T is taken, which is 1.5 times the duration of the transverse wave pulse, approximately 1 to 3 milliseconds. When τ equals the actual transit time difference Δt between the detected signal and the reference signal, the cross-correlation function R xy (τ) reaches its maximum value. By searching, R can be maximized. xy The maximum τ value can be used to obtain the difference Δt between the transit time of the transverse wave in the cracked region and the reference region without cracks. Cross-correlation delay estimation has higher accuracy and noise resistance compared to directly reading the time difference of waveform peak points; Because transverse waves are affected by aggregate scattering and environmental noise when propagating in concrete, the position of the peak point of the direct waveform is prone to fluctuation. However, the cross-correlation function utilizes the complete waveform information of the signal and smooths out the noise effect through integration, which can achieve the time delay estimation accuracy of sub-sampling interval, typically reaching 0.1 microseconds to 0.5 microseconds. After calculating the transit time difference under each pressure step, the data points are plotted on the coordinate system with the pressure value on the x-axis and the transit time difference value on the y-axis. The data points are then connected in sequence to form the curve of the relationship between acoustic transit time and pressure. This curve reflects the trend of the time required for a transverse wave to pass through the cracked area gradually increasing or decreasing relative to the intact area as the water pressure inside the sealed cavity increases. For closed cracks, the crack interface is in a relaxed state at low pressure, and the transverse wave transit time is relatively long. As the pressure increases, the crack interface is gradually compressed, the equivalent stiffness on the transverse wave propagation path increases, and the transit time gradually shortens. The slope and shape of the curve are directly related to the normal stiffness of the crack interface, and can be used to invert the crack opening distribution in step S6. Specifically, through the above cross-correlation time delay estimation and difference calculation, step S5 can obtain an accurate acoustic time difference versus pressure relationship curve, providing quantitative input data for subsequent crack opening inversion based on acoustoelastic effects.

[0048] Furthermore, in step S6, after fusing the first detection data, the reference acoustic time data and the second detection data, the crack opening distribution in the depth direction is calculated using an inversion algorithm. The inversion algorithm is at least one of least squares fitting or finite element inversion. The appropriate algorithm can be selected according to the actual detection accuracy requirements and computing resources. Alternatively, the two algorithms can be combined to verify the reliability of the inversion results. The specific process of inverting the relationship between transverse wave velocity and pressure based on the acoustoelastic effect is as follows: First, an acoustoelastic relationship model is established between the change in transverse wave velocity and the normal stiffness of the crack interface. Acoustoelastic theory shows that when a transverse wave propagates perpendicular to the crack interface, there is a quantitative relationship between the change in transverse wave velocity and the change in normal stress on the crack interface. This relationship is described by the normal stiffness of the crack interface as an intermediate variable. Crack interface normal stiffness K N Defined as the ratio of the change in normal stress at the crack interface, dσ, to the change in crack opening, dδ, i.e.:

[0049] The relationship between the effective wave velocity V of a transverse wave in a fractured medium and the transverse wave velocity V0 in a intact medium can be expressed by the following formula:

[0050] Where V represents the shear wave velocity of the fractured medium, V0 represents the shear wave velocity of the intact, uncracked medium, and K N ω represents the normal stiffness of the crack interface, Z represents the transverse wave angular frequency, and Z represents the acoustic impedance of the medium. When the applied pressure change Δσ is small, the change in transverse wave velocity ΔV and the pressure change Δσ can be approximated as a linear relationship, and the proportionality coefficient of this linear relationship can be obtained through experimental calibration. On precast concrete test blocks with cracks of different opening degrees, a known pressure change is applied, and the corresponding change in transverse wave velocity is measured to establish the correspondence between ΔV and Δσ. For actual testing, this linear relationship can be used to estimate ΔV from the measured Δσ, or to estimate the effective stress on the crack interface from the measured ΔV. The acoustic time difference versus pressure curve obtained in step S5 records the transverse wave transit time difference under different pressures. Given that the shear wave propagation distance L is the distance from the transmitting surface of the sealed module to the crack interface and back, the relationship between the shear wave transit time t and the shear wave velocity V is:

[0051] Therefore, the change in transverse wave velocity ΔV can be calculated from the acoustic time difference Δt. The specific conversion formula is as follows:

[0052] Where Δt represents the acoustic time difference, L represents the transverse wave propagation distance, V represents the transverse wave velocity in the cracked medium, and V0 represents the transverse wave velocity in the intact medium without cracks. Substituting the measured Δt into the above formula, and combining it with the known L and V0, V can be calculated to obtain the change in transverse wave velocity:

[0053] The normal stiffness K at the crack interface can be calculated from the change in transverse wave velocity ΔV using the acoustoelastic relationship model.N Substitute ΔV and V0 into the following equation:

[0054] Normal stiffness K N There is a one-to-one correspondence between the normal stiffness and the crack opening δ. For closed cracks, the normal stiffness decreases as the opening increases. This relationship can be given by experimental calibration or theoretical models. The experimental calibration method is as follows: Multiple sets of concrete crack specimens with different opening degrees were prepared, and their normal stiffness was measured to establish K. N The mapping relationship with δ can be represented by a table or fitted empirical formula. The theoretical model can be simplified using the following relationship:

[0055] Where δ represents crack opening, δ0 represents initial opening, and K0 is the reference stiffness coefficient; Due to the heterogeneity of concrete materials and the roughness of crack interfaces, to ensure inversion accuracy, this invention provides the following standard calibration procedure for determining the acoustoelastic coefficient S applicable to the concrete to be tested: For calibration test block preparation, samples are taken from concrete of the same batch and under the same curing conditions as the dam body to be tested, or core samples are drilled from non-critical parts of the dam body. Artificial cracks are pre-fabricated in the center of the test block. The method of pre-embedding thin sheet or three-point bending can be used. The crack opening is controlled to a typical value, such as 0.2 mm, by using a shim. This value is within the expected opening range to be detected within the pressure range of step S5. For pressure-time measurement, the calibration test block is placed in a simulated hydrostatic chamber. Using the same sealing module and electromagnetic ultrasonic shear wave detection module as in steps S1 to S5, the same pressure gradient is applied from external hydrostatic pressure to +0.15 MPa. The shear wave transit time at each pressure is recorded, and the acoustic time difference Δt relative to the crack-free reference test block is calculated. Based on the relationship between wave velocity and pressure in the acoustoelastic relationship model, combined with the conversion of the Δt formula, the S value is obtained through linear regression fitting. The fitting correlation coefficient should be greater than 0.95. Reasonableness of linear approximation: The pressure applied in step S5 of this invention varies from 0.05MPa to 0.15MPa, which is extremely small relative to the compressive strength of concrete (20MPa to 40MPa), and the change in normal stress at the crack interface is within the linear elastic range. Therefore, the linear acoustoelastic model using a constant S is an engineering-acceptable approximation. For cracks with significantly different openings, the relationship curve between S and opening can be obtained through multiple sets of calibration test blocks with different openings. However, in actual engineering, the variation of S is usually less than 10% in the pressure range of 0.05MPa to 0.15MPa. The opening inversion error introduced by taking a constant S can be controlled within ±0.02mm, which meets the engineering requirements. If calibration test blocks cannot be prepared, the reference acoustic time data measured in the crack-free area in step S4 can be used, with the concrete strength grade referenced from the empirical range of 1.0 × 10⁻⁶. -7 Pa -1 ~5.0×10 -7 Pa -1 Initial values ​​are selected, and the self-consistency is verified using the acoustic time difference data under different pressure steps in step S5: If the trend of the crack opening change with pressure is reasonable, that is, the crack opening decreases monotonically when the pressure increases, then the selected S value is usable. Otherwise, adjust the S value within the empirical range until it is self-consistent. This self-calibration method does not change the model form where S is a constant. Through the above calibration or self-calibration, the acoustoelastic model of the present invention can be reliably applied to the inversion of the opening of closed cracks in underwater concrete.

[0056] The relationship curve between acoustic time difference and pressure in the second detection data is used as input. The input data includes the pressure value and its acoustic time difference value corresponding to each pressure step. When using least squares fitting as the inversion algorithm, we assume a distribution model of crack opening along the depth direction, which can be in the form of a piecewise linear function or an exponential function. The theoretical acoustic time difference versus pressure relationship curve is obtained through forward modeling, and the sum of squares of the residuals between the theoretical curve and the measured curve is calculated. By using the least squares optimization algorithm, the parameters of the crack opening distribution model are continuously adjusted to minimize the sum of squared residuals. The crack opening distribution at this point is the inversion result. When using finite element inversion as the inversion algorithm, a local finite element model of the concrete containing cracks is established. The model discretizes the region to be detected into a finite number of elements, sets the crack interface as a contact element, and uses its normal stiffness as a parameter to be inverted. The acoustic time difference versus pressure curve obtained in step S5 is used as the target response. Through the inverse analysis module of the finite element software, iterative methods such as gradient optimization or genetic algorithm are used to continuously update the normal stiffness parameters of the crack contact element, so that the error between the acoustic time difference and pressure response calculated by the finite element and the measured curve gradually decreases. When the error is less than the preset convergence threshold, the iteration stops, and the distribution of crack opening along the depth direction is calculated from the final normal stiffness parameter distribution.

[0057] The distribution of crack opening along the depth direction obtained by the above inversion process is essentially an average effect or one-dimensional equivalent distribution along the sound wave propagation path. In order to more accurately obtain the difference in crack opening at different depth positions, such as 0~20mm and 20mm~40mm from the surface, this invention further provides the following two inversion methods that can achieve depth resolution. Those skilled in the art can choose to use them according to the actual detection conditions: Multi-frequency shear wave joint inversion: During the boosting process in step S5, the electromagnetic ultrasonic shear wave detection module is controlled to sequentially emit at least two shear wave pulse sequences with different center frequencies, such as f1=80kHz, f2=200kHz, and f3=400kHz. Transverse waves of different frequencies have different penetration depths and spatial resolutions in concrete. Lower frequencies, such as 80kHz, have longer wavelengths and greater penetration depths, up to 500mm, but are less sensitive to the opening of shallow cracks. Higher frequencies, such as 400kHz, have shorter wavelengths and shallower penetration depths (approximately 150mm), but are more sensitive to changes in shallow layer opening. The acoustic time difference-pressure relationship curves at each frequency are denoted as ΔT. f (P); The crack is discretized into N thin layers along the depth direction, where N≥3, and the thickness of each layer is d=10mm~20mm. Let the average opening of the i-th layer be δ. i For a transverse wave with frequency f, the overall acoustic time difference can be expressed as a linear superposition of the contributions from each layer:

[0058] Where, α f,i α is the sensitivity coefficient, representing the degree of influence of a unit change in the opening degree of the i-th layer on the transverse wave time difference at frequency f. f,i It can be obtained in advance through finite element simulation or calibration experiments, and its value is related to the sound velocity of concrete, attenuation coefficient, layer depth and frequency; For measurement data at least three times across multiple frequencies, an overdetermined system of equations is constructed, and the Tikhonov regularized least squares method is used to solve for the opening δ of each layer. i The regularization parameter is determined by the L-curve method, which is well known in the field and can stably obtain the layered distribution of opening in the depth direction.

[0059] Multi-angle incident tomography uses a multi-degree-of-freedom robotic arm equipped with an underwater robot to change the incident angle θ of the electromagnetic ultrasonic transverse wave detection module relative to the crack interface. For example, θ is 0°, 15°, 30°, and 45°. Different incident angles correspond to different sound wave propagation paths, and the sampling weights for different depth regions of the crack are different. Record the acoustic time difference-pressure relationship curve at each angle, establish the projection matrix by combining the ray tracing algorithm, and use the algebraic reconstruction technique ART or the joint iterative reconstruction technique SIRT to inversely retrieve the crack opening distribution in the depth direction. The aforementioned multi-frequency or multi-angle detection data can be continuously collected after a sealed cavity is formed, without the need to move the underwater robot; During the inversion calculation, the first detection data, namely the crack plane profile, is used as a priori constraint to limit the plane position of the crack. Only the crack opening distribution in the depth direction is inverted in layers, which significantly reduces ill-conditionedness and improves inversion stability. Verification was conducted through finite element numerical simulation and concrete test blocks containing precast non-uniform opening cracks. The test blocks were divided into three layers in the depth direction: 0~20mm with an opening of 0.3mm, 20~40mm with an opening of 0.15mm, and 40~60mm with an opening of 0.05mm. Using the above-mentioned multi-frequency joint inversion method, the error between the inverted value of the opening crack in each layer and the true value did not exceed ±0.03mm, and the depth resolution reached 15mm, which meets the requirements for crack assessment in water conservancy projects.

[0060] Specifically, the crack plane contour data obtained in step S3 is fused with the depth direction opening distribution obtained in step S6. That is, the corresponding depth opening data is assigned to each location point of the plane contour to construct a three-dimensional geometric model of the crack. This model can intuitively show the distribution pattern of the crack on the dam surface and the change of the crack opening in the depth direction, providing a basis for the safety assessment and repair decision of the concrete structure.

[0061] Furthermore, the underwater robot deployed in step S1 also includes a positioning and navigation system, which uses a combination of a visual camera, an inertial measurement unit and a multibeam sonar to guide the sealing module to move point by point to cover the entire dam body area to be tested, and finally generate a three-dimensional distribution map of the cracks. The vision camera is installed at the front end of the underwater robot or near the sealing module, and its optical axis is aligned with the pressing direction of the sealing module. With the assistance of an underwater searchlight, a visual camera collects image data of the dam surface in real time. Image processing algorithms are used to identify existing markers or natural texture features on the dam surface, enabling the underwater robot to achieve precise positioning within a local area. When the sealing module is pressed tightly against the dam surface, the vision camera records the image information of the current detection position and stores the spatial coordinates of that position in association with the image features. The inertial measurement unit is integrated into the sealed cabin of the underwater robot and includes a three-axis gyroscope and a three-axis accelerometer. The inertial measurement unit measures the angular velocity and linear acceleration of the underwater robot in real time. The attitude angle is obtained by integrating the angular velocity, and the displacement change is obtained by integrating the linear acceleration twice. The inertial measurement unit is used to provide continuous dead reckoning and positioning information in underwater environments when the visual camera cannot obtain clear images due to water turbidity, ensuring that the underwater robot can move stably along the preset detection path; Multibeam sonar is installed at the front or bottom of the underwater robot to emit multiple sound pulses in front of or to the side of the underwater robot, and to receive sound waves reflected back from the dam surface and the surrounding environment. Multibeam sonar can detect the relative distance between underwater robots and the dam surface in real time, as well as the macroscopic topographic undulations and obstacle distribution in the detection area, providing a wide range of environmental perception information for underwater robots to avoid obstacles and plan their paths. During the testing process, the workflow of the positioning and navigation system is as follows: Operators set the boundary coordinates and detection path of the dam area to be tested at the water surface control terminal. The detection path usually adopts a serpentine scanning method or a grid scanning method to ensure that the coverage area of ​​the sealing module is not missed. The control module converts the detection path into the target motion trajectory of the underwater robot, and integrates real-time data from the vision camera, inertial measurement unit and multibeam sonar, and makes the optimal estimate of the current pose of the underwater robot through Kalman filtering or particle filtering algorithm. When the underwater robot moves along the planned path, the control module adjusts the speed and direction of the thrusters in real time based on the pose information fed back by the positioning and navigation system, so that the sealing module is precisely aligned with the next detection point; After completing the detection operations of steps S1 to S6 at each detection point, the positioning and navigation system records the spatial coordinates of that point; After the sealing module completes the test at this point, the control module drives the underwater robot to move to the next adjacent point. The sealing module is then pressed against the dam surface again for testing. This process is repeated until all preset test points are covered. After all the data from the detection points have been collected, the control module will stitch and fuse the three-dimensional geometric evaluation results generated for each point according to their corresponding spatial coordinates. Linear interpolation or spline interpolation algorithms are used to smoothly transition the crack data between adjacent detection points, ultimately generating a three-dimensional distribution map of cracks covering the entire dam body area to be tested; This three-dimensional distribution map can intuitively display the planar distribution pattern, depth extension range, and changes in the opening degree of each crack with spatial location; Specifically, by combining a visual camera, an inertial measurement unit, and a multibeam sonar, the positioning and navigation system of this invention achieves high-precision autonomous positioning and path guidance in complex underwater environments, ensuring that the sealing module can move point by point to cover the entire dam body area to be tested, and providing a reliable position reference for generating a complete three-dimensional distribution map of cracks.

[0062] Furthermore, after generating the three-dimensional geometric evaluation results of the crack in step S6, this embodiment performs a safety assessment on the detection results and outputs corresponding repair guidelines; The crack three-dimensional geometric evaluation results generated in step S6 are compared with the preset safety threshold. The crack three-dimensional geometric evaluation results include the planar contour data of the crack on the dam surface and in the shallow interior, as well as the opening distribution data along the depth direction. Two key evaluation indicators were extracted from the assessment results: crack depth and maximum crack opening. Crack depth refers to the maximum vertical distance a crack extends from the surface of the dam into the interior of the concrete, while maximum crack opening refers to the maximum width of the interface gap at any point along the crack's extension path. The preset safety threshold is determined in advance based on the structural design grade of the concrete dam, the operating head height, and relevant water conservancy engineering specifications. The safety threshold specifically includes the depth threshold and the opening threshold. The value range of the depth threshold is determined according to the dam structure design drawings and the thickness of the protective layer, and is usually set to one-half to two-thirds of the thickness of the protective layer. The opening threshold is determined according to the concrete structure crack control code. For underwater concrete structures, it is usually set to 0.2 mm to 0.5 mm. The aforementioned thresholds are pre-stored in the non-volatile memory of the control module and can be updated and adjusted according to different dam types and detection tasks; The control module reads the depth and maximum opening value of each crack from the three-dimensional geometric evaluation results of the crack, compares the depth value with the depth threshold, and compares the maximum opening value with the opening threshold. The comparison operation uses conditional judgment logic. That is, if the crack depth is greater than the depth threshold or the maximum crack opening is greater than the opening threshold, the crack is determined to have exceeded the safety limit and needs to be repaired. When the crack depth or maximum opening is determined to exceed the corresponding safety threshold, the control module generates a repair prompt signal; The repair prompt signal includes a trigger identifier and the spatial coordinates of the crack. The repair prompt signal is transmitted to the repair execution module of the underwater robot via wired or wireless communication. The repair execution module is a functional unit installed on an underwater robot. Its specific implementation includes, but is not limited to, grouting nozzles, repair agent storage tanks, and corresponding fluid drive devices. After receiving the repair prompt signal, the repair execution module moves to the location of the crack autonomously or remotely according to the crack location coordinates carried in the signal, and starts the grouting or filling operation of the repair execution module to repair the crack that exceeds the safety threshold. Specifically, through the above comparison and prompting steps, the present invention achieves automatic linkage between the detection results and the repair action while completing the non-destructive testing of cracks. This avoids the tedious process of manual interpretation and manual positioning and repair, and improves the overall efficiency of underwater concrete structure crack detection and maintenance. This step enables the entire detection method to have an integrated function of detection evaluation and repair guidance, enhancing the practicality of the technical solution and its engineering application value.

[0063] Please refer to Figure 2 This embodiment also provides an electromagnetic ultrasonic shear wave detection device for implementing the method of the present invention, the structure of which is as follows: The underwater robot serves as the carrier and mobile platform for the entire device. It has autonomous or remote-controlled movement capabilities and is equipped with a power battery, thrusters, a sealed cabin, and a communication module inside or outside. The underwater robot's drive system includes multiple horizontal and vertical thrusters, enabling six degrees of freedom of movement and ensuring that the device accurately reaches the surface of the dam to be tested in complex underwater environments. The sealing module is installed at the front end of the underwater robot. The sealing module consists of a rigid cavity and an elastic sealing ring. The rigid cavity is made of corrosion-resistant metal material or high-strength engineering plastic, and the elastic sealing ring is embedded at the edge of its front opening. As the underwater robot moves forward, the elastic sealing ring of the sealing module is pressed tightly against the surface of the concrete dam to be tested. The elastic sealing ring undergoes elastic deformation, filling the microscopic uneven gap between the sealing module and the dam surface, forming a sealed cavity inside the sealing module that is completely isolated from the surrounding water. The pressure regulation module is connected to the interior of the sealing module's cavity via a sealed pipeline; The pressure control module includes a vacuum pump and a high-pressure water pump, both of which are installed inside the underwater robot or outside the sealed module. The inlet of the vacuum pump is connected to the sealed cavity through a pipeline, and its outlet is connected to the external water body. When the vacuum pump is started, the water in the sealed cavity is extracted, so that the water pressure inside the sealed cavity is reduced to a level lower than the external static water pressure. The inlet of the high-pressure water pump is connected to an external water body, and its outlet is connected to a sealed cavity through a pipeline. When the high-pressure water pump starts, it pumps the external water body into the sealed cavity, causing the water pressure inside the sealed cavity to rise to a level higher than the external static water pressure. The start-up, shutdown, and operating intensity of the vacuum pump and high-pressure water pump are automatically adjusted by the control module based on the signal fed back by the pressure monitoring module; The electromagnetic ultrasonic transverse wave detection module is located inside the cavity of the sealed module; The electromagnetic ultrasonic transverse wave detection module is a non-contact electromagnetic ultrasonic transducer, which contains a permanent magnet or electromagnet and an excitation coil. The module is encapsulated in a waterproof housing, with its emitting surface facing the surface of the concrete dam to be tested and maintaining a predetermined gap from the dam surface. Triggered by the control module, the electromagnetic ultrasonic transverse wave detection module transmits a sequence of transverse wave pulses to the detection area and receives echo signals reflected from the crack interface or transmitted through the crack area. The center frequency of the transverse wave pulse sequence is 50 kHz to 500 kHz, and the repetition frequency of the transverse wave emitted in step S5 is 100 Hz to 1000 Hz. The pressure monitoring module is located inside the cavity of the sealing module; The pressure monitoring module uses a diffused silicon pressure sensor or a ceramic capacitive pressure sensor, whose sensitive element is in direct contact with the water in the sealed cavity. The pressure monitoring module detects the water pressure inside the sealed cavity in real time and converts the water pressure signal into an electrical signal, which is then transmitted to the control module. The control module determines the difference between the current water pressure inside the sealed cavity and the external static water pressure based on the real-time water pressure data fed back by the pressure monitoring module, providing a closed-loop control basis for the pumping and depressurization operation in step S2 and the pressurization operation in step S5. The control module is electrically connected to the pressure regulation module, the electromagnetic ultrasonic shear wave detection module, the pressure monitoring module, and the drive system of the underwater robot. The control module includes a central processing unit, memory, and input / output interfaces. The central processing unit runs a pre-programmed control program. The control module receives the water pressure signal fed back by the pressure monitoring module, controls the pressure regulation module to pump water to reduce the pressure so that the water pressure in the sealed cavity is lower than the preset difference of the external static water pressure and maintains it for a predetermined time. While maintaining this pressure reduction state, the electromagnetic ultrasonic transverse wave detection module is triggered to emit the first transverse wave pulse sequence and receive the first echo signal. The control module then controls the pressure regulation module to inject water and pressurize it, so that the water pressure in the sealed cavity is higher than the external static water pressure set value. During the pressurization process, the electromagnetic ultrasonic transverse wave detection module is triggered to emit a second transverse wave pulse sequence at a repetition frequency of 100 Hz to 1000 Hz and receive the second echo signal, while recording the transverse wave acoustic time data corresponding to different water pressure stages. The control module also generates crack plane contour data based on the first echo signal, calculates the crack opening distribution in the depth direction based on the transverse wave acoustic time data and the relationship between transverse wave velocity and pressure of the acoustoelastic effect, and integrates the generated three-dimensional geometric evaluation results of the crack. The control module simultaneously drives the underwater robot's propulsion system, guiding the sealing module to move point by point to cover the entire dam area to be tested; Through the coordinated operation of the above modules, this device can automatically execute all the detection processes described in steps S1 to S6, and realize high-precision three-dimensional non-destructive testing of cracks in underwater concrete structures.

[0064] Although alternative embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make further changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0065] The above specific embodiments further illustrate the purpose, technical solution and beneficial effects of this application. It should be understood that the above are only specific embodiments of this application and are not intended to limit the scope of protection of this application. Any modifications, equivalent substitutions, improvements, etc., made on the basis of the technical solution of this application should be included within the scope of protection of this invention.

Claims

1. An electromagnetic ultrasonic shear wave detection method suitable for cracks in underwater concrete structures, characterized in that, Includes the following steps: S1, Deploy an underwater robot equipped with an electromagnetic ultrasonic transverse wave detection module and a pressure control module, press the sealing module set at the front end of the underwater robot tightly against the surface of the concrete dam to be tested, forming a sealed cavity isolated from the surrounding water. S2, activate the pressure control module to pump water and reduce the pressure in the sealed cavity, so that the water pressure inside the cavity is lower than the preset difference of the external static water pressure, maintain the low pressure state for a predetermined time, and cause the closed cracks on the concrete surface to open actively due to elastic rebound. S3, under the low pressure state, the electromagnetic ultrasonic transverse wave detection module is controlled to emit a transverse wave pulse sequence to the detection area, receive the echo signal reflected by the crack interface, and obtain the first detection data reflecting the plane profile of the crack. S4, turn off the pumping, restore the water pressure inside the cavity to be equal to the external hydrostatic pressure, and record the reference acoustic time data in this state; S5, start pressurization to make the water pressure inside the cavity higher than the external hydrostatic pressure, and emit transverse waves at a repetition frequency of 100Hz to 1000Hz during the pressurization process and receive transverse wave signals that pass through the crack area, and record the transverse wave acoustic time data under different water pressure stages as the second detection data. S6, the first detection data, the reference acoustic time data and the second detection data are fused and processed. An inversion algorithm is used to calculate the crack opening distribution in the depth direction based on the relationship between the transverse wave velocity and pressure of the acoustoelastic effect, and a three-dimensional geometric evaluation result of the crack is generated.

2. The method of claim 1, wherein, The sealing module has an elastic sealing ring at its front edge. The sealing module has a pressure monitoring module integrated inside or on its cavity wall to monitor the water pressure inside the cavity. The pressure control module includes a vacuum pump and a high-pressure water pump. The vacuum pump pumps water to reduce pressure, and the high-pressure water pump pressurizes the water.

3. The method of claim 1, wherein, The preset difference in step S2 is 0.02MPa to 0.08MPa, and the preset time is 5 seconds to 15 seconds; The pressurization process raises the internal water pressure to 0.05 MPa to 0.15 MPa above the external hydrostatic pressure, and employs a stepped pressurization method.

4. The method of claim 1, wherein, The electromagnetic ultrasonic transverse wave detection module is a non-contact electromagnetic ultrasonic transducer, which includes a permanent magnet or electromagnet that generates a bias magnetic field and an excitation coil for exciting eddy currents. The center frequency of the transverse wave pulse sequence is 50kHz to 500kHz.

5. The method according to claim 1, characterized in that, The first detection data reflecting the plane contour of the crack in step S3 specifically includes: performing time window interception, envelope detection and amplitude threshold determination on the received echo signal, extracting spatial location points where the echo amplitude is greater than a preset threshold, and connecting them to form the two-dimensional boundary contour of the crack.

6. The method according to claim 1, characterized in that, The step S5, which records the transverse wave acoustic time data under different water pressure stages, includes: performing cross-correlation time delay estimation on the transverse wave received signal under each pressure step, calculating the difference between the transverse wave transit time in the cracked region and the reference region without cracks, and forming an acoustic time difference versus pressure curve.

7. The method according to claim 1, characterized in that, The inversion algorithm in step S6 is at least one of least squares fitting or finite element inversion; The inversion of the transverse wave velocity and pressure relationship based on the acoustoelastic effect includes: establishing an acoustoelastic relationship model between the change in transverse wave velocity and the normal stiffness of the crack interface, using the acoustic time difference and pressure relationship curve in the second detection data as input, and inverting the crack opening depth by depth.

8. The method according to claim 1, characterized in that, The underwater robot also includes a positioning and navigation system, which uses a combination of a visual camera, an inertial measurement unit, and a multibeam sonar to guide the sealing module to move point by point to cover the entire dam area to be tested, generating a three-dimensional distribution map of the cracks.

9. The method according to claim 1, characterized in that, After generating the three-dimensional geometric evaluation result of the crack in step S6, the method further includes: comparing the three-dimensional geometric evaluation result of the crack with a preset safety threshold; if the crack depth or maximum opening exceeds the threshold, outputting a repair prompt signal to the repair execution module of the underwater robot.

10. An electromagnetic ultrasonic shear wave detection device for underwater concrete structure cracks, used to implement the method of any one of claims 1-9, characterized in that, include: Underwater robots; The sealing module, installed at the front end of the underwater robot, forms a sealed cavity that is isolated from the surrounding water when it is pressed tightly against the surface of the concrete dam to be tested. The pressure regulation module is connected to the interior of the sealing module's cavity, and can pump water to reduce pressure or inject water to increase pressure in the sealing cavity; An electromagnetic ultrasonic transverse wave detection module is installed inside the cavity of the sealed module, with its emitting surface facing the surface of the concrete dam to be tested. It emits a transverse wave pulse sequence and receives the echo signal. A pressure monitoring module is installed inside the cavity of the sealing module to detect the water pressure inside the sealing cavity in real time. The control module is electrically connected to the pressure regulation module, the electromagnetic ultrasonic transverse wave detection module, the pressure monitoring module, and the drive system of the underwater robot, respectively, and is used to control the execution of steps S1 to S6.

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