A closed crack imaging device and method based on nonlinear Lamb waves

Abstract

The present invention relates to a closed crack imaging device and method based on nonlinear Lamb waves. A measurement system is constructed using a signal generator, a high-voltage amplifier, an excitation probe, a receiving probe, a voltage amplifier, and an oscilloscope. A linear sensor array is established to record the reflection of the Lamb wave on the structure to be measured. The received signal is decomposed using a "db8" wavelet to obtain a harmonic component of the excitation signal at the second harmonic frequency, i.e., the second harmonic, as the nonlinear Lamb wave reflected by the closed crack. The Hilbert transform value of the nonlinear Lamb wave is associated with the pixel value of a given point on the structure to generate a damage image, in which the position of the closed crack is displayed at a position with a higher pixel value. The present invention can promptly detect closed cracks in the structure to be measured, and is of great significance for detecting structural fatigue and corrosion and providing early warning of structural failure and damage.

Description

Technical Field

[0001] The present invention relates to a closed crack imaging device and method based on nonlinear Lamb waves, belonging to the technical field of nondestructive testing. Background Art

[0002] Lamb waves hold great promise for application in structural health monitoring and nondestructive testing. Damage can be detected, located, and quantified based on Lamb wave characteristics, such as propagation velocity, dispersion, frequency shift, attenuation, modal conversion, transmission, and reflection. However, linear Lamb waves are not effective for detecting closed cracks, such as fatigue, corrosion, and thermal cracks.

[0003] Since closed cracks are generally related to early damage of structures, the discovery of closed cracks is of particular importance for early warning of structural failure and damage. There is an urgent need to propose a new imaging method for closed cracks in structures. Summary of the Invention

[0004] The present invention provides a closed crack imaging device and method for nonlinear Lamb waves, which can timely discover closed cracks in a structure to be tested, and is of great significance for early warning of structural failure and damage.

[0005] The technical solution adopted by the present invention to solve its technical problem is:

[0006] A closed crack imaging device for nonlinear Lamb waves, comprising a signal generator, a high-voltage amplifier, an excitation probe, a receiving probe, a voltage amplifier, and an oscilloscope;

[0007] The receiving probes and the excitation probes include a plurality of receiving probes and the excitation probes, and the plurality of receiving probes and the excitation probes are arranged on the structure to be measured;

[0008] The output end of the signal generator is connected to the input end of the high-voltage amplifier, the output end of the high-voltage amplifier is connected to the excitation probe, and the receiving probe is connected to the input end of the voltage amplifier. The excitation probe and the receiving probe are connected to the structure to be tested through a coupling agent to respectively transmit and receive Lamb waves in the structure to be tested. The output end of the signal generator, the monitoring end of the high-voltage amplifier, and the output end of the voltage amplifier are all connected to the oscilloscope;

[0009] As a further preferred embodiment of the present invention, the receiving probes include six, the receiving probes and the excitation probes are arranged in a linear array, and three receiving probes are evenly arranged on both sides of the excitation probe;

[0010] As a further preferred embodiment of the present invention, the distance between adjacent receiving probes and the distance between a receiving probe and an adjacent excitation probe are both 2 cm;

[0011] A closed crack imaging method based on nonlinear Lamb waves specifically comprises the following steps:

[0012] Step S1: Equipment preparation, including signal generator, high-voltage amplifier, excitation probe, receiving probe, voltage amplifier, and oscilloscope;

[0013] Step S2: based on the material properties and thickness of the structure to be tested, a dispersion curve of the Lamb wave is drawn to obtain wave velocity information; the dispersion curve shows the change of the group velocity of the Lamb wave in the structure to be tested as a function of the frequency-thickness product;

[0014] Step S3: Build a test platform and connect all the equipment in step S1. The incident wave amplitude generated by the signal generator is set to 4 Vpp and the center frequency is set to 200 kHz.

[0015] Step S4: Apply coupling agent to the excitation probe and the receiving probe respectively to remove the air between the excitation probe and the receiving probe and the structure to be measured, so as to ensure linear and distortion-free conversion between the electrical signal and the Lamb wave signal;

[0016] Step S5: six receiving probes and one excitation probe are placed on the structure to be measured, and the receiving probes and the excitation probes are arranged in a linear array, with the distance between adjacent positions in the linear array being 2 cm;

[0017] Step S6: Obtain the signal wave of the receiving probe through steps S3, S4, and S5, and use discrete wavelet transform to extract the nonlinear wave, that is, separate the second harmonic generated by the slapping and friction of the crack interface in the structure to be tested;

[0018] Step S7: Establishing Cartesian coordinates on the surface of the structure to be measured, calculating the time difference between the emission and reception of the Lamb wave based on the positions of the imaging point, the excitation probe, and the receiving probe within the Cartesian coordinates, and selecting the amplitude of the nonlinear component in the received signal as the pixel value of the imaging point based on the time difference; wherein, the linear Lamb wave propagates from the excitation probe to the imaging point at a speed v1, and the nonlinear Lamb wave propagates from the imaging point to the receiving probe at a speed v2;

[0019] Step S8: Six pairs of transmit-receive Lamb waves are formed between the six receiving probes and the excitation probes. For each imaging point, each pair of transmit-receive Lamb waves generates a pixel value. The six pairs of pixel values are added together to obtain the final pixel value of the imaging point, and the damage image is reconstructed. At the same time, the damage image is normalized, and the maximum pixel value is retained.

[0020] As a further preferred embodiment of the present invention, in step S2, when the structure to be measured is a uniform isotropic plate, the characteristic equation followed by Lamb wave propagation is:

[0021]

[0022] in, h is half the thickness of the structure to be measured, k is the wave number, ω is the circular frequency, c L is the velocity of the longitudinal wave, c T is the velocity of the shear wave, +1 and -1 in formula (1) represent the symmetric and antisymmetric modes respectively;

[0023] As a further preferred embodiment of the present invention, in step S3, the signal generator generates an incident wave, the high-voltage amplifier amplifies the energy of the incident wave and transmits it to the structure through the excitation probe, the receiving probe receives the Lamb wave transmitted through the structure, and the voltage amplifier amplifies the energy of the received wave;

[0024] The oscilloscope displays the vibration wave generated by the signal generator, the vibration wave amplified by the high-voltage amplifier, and the vibration wave amplified by the voltage amplifier;

[0025] The incident wave generated by the signal generator consists of a 20-cycle Hanning window sine wave, and the voltage waveform transmitted from the excitation probe to the structure follows the rule:

[0026]

[0027] Among them, f c is the center frequency, t is the time series;

[0028] As a further preferred embodiment of the present invention, in step S6, the mother wavelet is selected as db8, the signal wave received by the receiving probe is decomposed into a high-frequency component and a low-frequency component, and the low-frequency component is decomposed again. The above process is repeated six times until the frequency of the high-frequency component obtained by decomposition is twice the center frequency of the incident wave. At this time, the high-frequency component is a nonlinear Lamb wave;

[0029] As a further preferred embodiment of the present invention, in step S7, in Cartesian coordinates, the coordinates of the imaging point are (x, y), and the coordinates of the excitation probe are (x T ,y T ), the receiving probe coordinates are (x R ,y R ), assuming the distance from the imaging point to the excitation probe is d1, and the distance from the imaging point to the receiving probe is d2, the time calculation formula for Lamb wave vibration propagation is:

[0030]

[0031] The excitation probe is located at the origin (0, 0), and the six receiving probes are marked as #1, #2, #3, #4, #5, and #6, and their positions in Cartesian coordinates are (-6, 0), (-4, 0), (-2, 0), (2, 0), (4, 0), and (6, 0) cm respectively.

[0032] Calculate the Lamb wave vibration propagation time Δt obtained by each receiving probe. The amplitude of the nonlinear wave at Δt represents the reflection intensity of the imaging point.

[0033] As a further preferred embodiment of the present invention, in step S7, when the imaging point coincides with the crack, a strong reflection is generated, and a peak is generated in the nonlinear Lamb wave; when the imaging point is not on the crack, no obvious reflection occurs, and the amplitude of the nonlinear Lamb wave is close to 0; therefore, the amplitude of the nonlinear Lamb wave can be used as the pixel value to characterize the reflection intensity of the imaging point;

[0034] The instantaneous amplitude of the nonlinear Lamb wave is not selected as the pixel value of the closed crack image. The calculation method of the instantaneous amplitude a(t) is:

[0035]

[0036] Where x(t) is the nonlinear Lamb wave signal, P is the Cauchy principal value, t is the time series, τ is the integral variable of H(t), and it disappears after integration;

[0037] As a further preferred embodiment of the present invention, in step S8, formula (4) is matched with formula (3), and six pixel values of six pairs of transmitted and received Lamb waves are calculated respectively, and the six pixel values are added to determine the final pixel value of the imaging point;

[0038] The damaged image was normalized and pixel values greater than 0.9 were retained.

[0039] Through the above technical solution, compared with the existing technology, the present invention has the following beneficial effects:

[0040] 1. The closed crack imaging device provided by the present invention can excite and collect nonlinear Lamb wave signals through a contact or non-contact transducer;

[0041] 2. The closed crack imaging method provided by the present invention uses the wavelet method to separate the second harmonics generated by the crack interface slapping and friction;

[0042] 3. The closed crack imaging method provided by the present invention realizes the imaging of closed cracks through second harmonics, and the pixels of each point on the damage map represent the degree of damage.

[0043] 4. The imaging device and method of the present invention are applicable to both above-water and underwater structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention will be further described below with reference to the accompanying drawings and examples.

[0045] Figure 1 The present invention provides a test platform for a closed crack imaging device for nonlinear Lamb waves.

[0046] Figure 2 Schematic diagram of the imaging point position in Cartesian coordinates provided by the present invention;

[0047] Figure 3 It is a test platform for glass plate closed crack detection in a preferred embodiment provided by the present invention;

[0048] Figure 4 Schematic diagram of Lamb wave group velocity dispersion curve of glass plate in a preferred embodiment provided by the present invention;

[0049] Figure 5a is a waveform diagram of the incident wave in a preferred embodiment provided by the present invention, Figure 5b This is the time domain waveform of the signal received by the #1 receiving probe;

[0050] Figure 6a is a frequency domain diagram of the incident wave in the preferred embodiment provided by the present invention, Figure 6b This is the frequency domain waveform of the signal received by the #1 receiving probe;

[0051] Figure 7a This is a time domain waveform diagram of the nonlinear wave extracted from the #1 receiving probe in the preferred embodiment provided by the present invention. Figure 7b This is the frequency domain waveform of the nonlinear wave extracted from the #1 receiving probe;

[0052] Figure 8 is a time-pixel value diagram of the nonlinear wave extracted from the #1 receiving probe in the preferred embodiment provided by the present invention;

[0053] Figure 9 This is a schematic diagram of closed crack imaging formed when conducting a test using the closed crack imaging device provided by the present invention. DETAILED DESCRIPTION

[0054] The present invention will now be described in further detail with reference to the accompanying drawings. In the description of this application, it should be understood that the terms "left side", "right side", "upper", "lower", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are intended only to facilitate the description of the present invention and simplify the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. "First", "second", etc. do not indicate the importance of the components and therefore should not be understood as limiting the present invention. The specific dimensions used in this embodiment are only for illustrative purposes only and do not limit the scope of protection of the present invention.

[0055] As described in the background technology, the existing Lamb wave cannot effectively detect closed cracks when detecting, locating and quantifying damage. Therefore, after many experiments, the applicant of this application discovered a closed crack imaging device and method based on nonlinear Lamb waves. The main principle is to obtain nonlinear Lamb wave signals through a series of means, and separate the second harmonics generated by the impact and friction of the crack interface through the wavelet method, so as to realize the imaging of closed cracks. The pixels of each point on the obtained damage map represent the degree of damage. This method can timely detect closed cracks in the structure to be tested and timely warn of the failure of the structure to be tested.

[0056] This application first gives a detailed description of the closed crack imaging device to be used. Through such a setting, the extraction of nonlinear Lamb waves can be achieved, such as Figure 1 As shown, the apparatus mainly includes the following components: a signal generator (Tektronix AFG31000), a high-voltage amplifier (Aigtek ATA4051), an excitation probe (Fuji AE1045SW), a receiving probe (Fuji AE1045SW), a voltage amplifier (PXPA3 amplifier), and an oscilloscope (Tektronix MSO64); wherein, the receiving probes include several, and the several receiving probes and the excitation probes are arranged on the structure to be tested; the output end of the relevant signal generator is connected to the input end of the high-voltage amplifier, the output end of the high-voltage amplifier is connected to the excitation probe, and the receiving probe is connected to the input end of the voltage amplifier; the excitation probe and the receiving probe are connected to the structure through a coupling agent, and transmit and receive Lamb waves in the structure to be tested, respectively; the output end of the signal generator, the monitoring end of the high-voltage amplifier, and the output end of the voltage amplifier are all connected to the oscilloscope.

[0057] In the preferred embodiment, the present application sets the number of receiving probes to six, and also places restrictions on their arrangement, such as Figure 1 As shown, six receiving probes and the excitation probe are arranged in a linear array, and three receiving probes are evenly arranged on both sides of the excitation probe. The distance between each position point in the linear array is 2 cm, and six different signals can be recorded simultaneously.

[0058] Next, the present application conducts an imaging method test based on the above-mentioned closed crack imaging device, which specifically includes the following steps:

[0059] Step S1: Equipment preparation: prepare a signal generator, a high-voltage amplifier, an excitation probe, a receiving probe, a voltage amplifier, and an oscilloscope.

[0060] Step S2: Based on the material properties and thickness of the structure to be tested (generally a plate-like structure), a dispersion curve of the linear Lamb wave is drawn to obtain wave velocity information; the dispersion curve shows the change of the group velocity of the Lamb wave in the structure to be tested as a function of the frequency-thickness product;

[0061] When the structure to be measured is a uniform isotropic plate, the characteristic equation followed by Lamb wave propagation is:

[0062]

[0063] in, h is half the thickness of the structure to be measured, k is the wave number, ω is the circular frequency, c L is the velocity of the longitudinal wave, c T is the velocity of the shear wave, and +1 and -1 in formula (1) represent the symmetric and antisymmetric modes, respectively.

[0064] Step S3: Build a test platform and assemble and connect all the equipment in step S1. The incident wave amplitude generated by the signal generator is set to 4 Vpp and the center frequency is set to 200 kHz.

[0065] Here are the specific steps for building the test platform for easy understanding: (1) Connect the signal generator to the input of the high-voltage amplifier. The signal generator is used to generate the incident wave (composed of a 20-cycle Hanning window sine wave, which follows the rule shown in formula (2)).

[0066]

[0067] Among them, f c is the center frequency, t is the time series;

[0068] (2) The output end of the high-voltage amplifier is connected to the excitation probe. The high-voltage amplifier is used to amplify the energy of the incident wave, and the excitation probe is used to transmit the incident wave to the structure.

[0069] (3) The receiving probe is connected to the input end of the voltage amplifier. The receiving probe is used to receive the vibration wave after being transmitted through the structure, and the voltage amplifier is used to amplify the energy of the received wave.

[0070] (4) The output end of the signal generator, the monitoring end of the high-voltage amplifier and the output end of the voltage amplifier are all connected to the oscilloscope. The oscilloscope is used to display the vibration wave generated by the signal generator, the vibration wave amplified by the high-voltage amplifier and the received vibration wave amplified by the voltage amplifier.

[0071] Step S4: Coupling agent is applied to the excitation probe and the receiving probe respectively to remove air between the excitation probe, the receiving probe and the structure to be measured, so as to ensure linear and distortion-free conversion between the electrical signal and the Lamb wave signal.

[0072] Step S5: Six receiving probes and one excitation probe are placed on the structure to be measured, and the receiving probes and the excitation probes are arranged in a linear array. The distance between adjacent positions in the linear array is 2 cm. It should be emphasized again here that the number, spacing and position of the receiving probes can be adjusted according to actual conditions, and there is no requirement to be six or linear arrangement.

[0073] Step S6: Since the incident wave of the test system is linear, the nonlinear Lamb wave can be regarded as the reflection of the closed crack. The signal wave of the receiving probe is obtained through steps S3, S4 and S5, and the nonlinear wave is extracted using discrete wavelet transform, that is, the second harmonic generated by the slapping and friction of the crack interface in the test structure is separated.

[0074] Specifically, the mother wavelet db8 is selected, and the signal wave received by the receiving probe is decomposed into high-frequency and low-frequency components. The low-frequency component is decomposed again, and the above process is repeated six times until the frequency of the decomposed high-frequency component is twice the center frequency of the incident wave. At this time, the high-frequency component is a nonlinear Lamb wave.

[0075] Step S7: Establishing Cartesian coordinates on the surface of the structure to be measured, calculating the time difference between the emission and reception of the Lamb wave based on the positions of the imaging point, the excitation probe, and the receiving probe within the Cartesian coordinates, and selecting the amplitude of the nonlinear component in the received signal as the pixel value of the imaging point based on the time difference; wherein, the linear Lamb wave propagates from the excitation probe to the imaging point at a speed v1, and the nonlinear Lamb wave propagates from the imaging point to the receiving probe at a speed v2;

[0076] Specifically, such as Figure 2 As shown, in Cartesian coordinates, the coordinates of the imaging point are (x, y), and the coordinates of the excitation probe are (x T ,y T ), the receiving probe coordinates are (x R ,y R ), assuming the distance from the imaging point to the excitation probe is d1, and the distance from the imaging point to the receiving probe is d2, the time calculation formula for Lamb wave vibration propagation is:

[0077]

[0078] The excitation probe is located at the origin (0, 0), and the six receiving probes are marked as #1, #2, #3, #4, #5, and #6, respectively. Their positions in Cartesian coordinates are (-6, 0), (-4, 0), (-2, 0), (2, 0), (4, 0), and (6, 0) cm, respectively. The linear Lamb wave propagates from the excitation probe to the imaging point at a speed of v1, and the nonlinear Lamb wave propagates from the imaging point to the receiving probes #1, #2, #3, #4, #5, and #6 at a speed of v2.

[0079] The Lamb wave vibration propagation time Δt acquired by each receiving probe is calculated. The amplitude of the nonlinear wave at Δt represents the reflection intensity at the imaging point. This is because when the imaging point coincides with a crack, a strong reflection occurs, creating a peak in the nonlinear Lamb wave. When the imaging point is not on the crack, no significant reflection occurs, and the amplitude of the nonlinear Lamb wave approaches 0. Therefore, the amplitude of the nonlinear Lamb wave can be used as the pixel value to represent the reflection intensity of the imaging point.

[0080] The instantaneous amplitude of the nonlinear Lamb wave is selected as the pixel value of the closed crack image. The calculation method of the instantaneous amplitude a(t) is:

[0081]

[0082] Where x(t) is the nonlinear Lamb wave signal, P is the Cauchy principal value, t is the time series, and τ is the integral variable of H(t), which disappears after integration.

[0083] Step S8: Six pairs of transmit-receive Lamb waves are formed between the six receiving probes and the excitation probes, and each pair of transmit-receive Lamb waves generates a pixel value. The six pairs of pixel values are added together to obtain the final pixel value of the imaging point, and the damage image is reconstructed. At the same time, the damage image is normalized, and the larger pixel values are retained (in the preferred embodiment test, pixel values greater than 0.9 are retained);

[0084] It should be noted that since the coordinates of the excitation and receiving probes are known, formula (4) is matched with formula (3) to assign a pixel value to the imaging point by referencing the corresponding pixel value at Δt. Each transmitted-received vibration wave generates a pixel value. By adding the pixel values generated by several pairs of transmitted-received vibration waves (in the preferred embodiment, since six receiving probes are included, six pairs of pixel values are added), the final pixel value of the imaging point can be determined. Based on this principle, the damage image is reconstructed.

[0085] Example:

[0086] This application uses a glass plate with dimensions of 400×400×3 mm as an example, and conducts experiments on its surface to verify the accuracy of the proposed method. The material properties of the plate are shown in Table 1.

[0087] Table 1 Material properties of glass sheets

[0088] Young's modulus Poisson's ratio density 55GPa 0.2 <![CDATA[2400kg / m 3 ]]>

[0089] The closed crack is 1mm wide, 2cm long and 1mm deep. Figure 3 As shown in Figure 2, under Lamb wave incidence, the contact crack will reflect the nonlinear Lamb wave.

[0090] According to step S2, the dispersion curve of the Lamb wave group velocity and the frequency-thickness product of the glass plate is plotted, and the Figure 4 Information about wave speed is shown.

[0091] According to step S3, step S4 and step S5, the signal waves of receiving probes #1, #2, #3, #4, #5 and #6 are obtained. Figure 5a The waveform of the incident wave is shown. Figure 5b The waveform of the receiving wave of the receiving probe #1 is shown. At this time, both linear Lamb waves and nonlinear Lamb waves exist. It can be seen from the figure that the nonlinear wave appears at about 60μs. The spectrum is calculated using fast Fourier transform, and the matching is obtained as follows Figure 6a The frequency domain diagram of the incident wave and Figure 6b The frequency domain diagram of the received wave of the #1 receiving probe. It can be seen from the figure that in addition to the fundamental frequency (200kHz), the second harmonic appears at 400kHz.

[0092] According to step S6, the "db8" discrete wavelet decomposition is performed, and the decomposition level is 20. Figure 7a The time domain waveform of the 12th level component is shown. It can be seen from the figure that the peak appears at about 60μs. Figure 7b The frequency domain waveform of the 12th component is shown. It can be seen from the figure that its center frequency is at 400kHz, which is consistent with the second harmonic frequency. Therefore, the 12th component is a nonlinear wave.

[0093] According to step S7, the excitation probe is located at the origin (0,0), and the receiving probes #1, #2, #3, #4, #5, and #6 are located at (-6,0)(-4,0)(-2,0)(2,0)(4,0)(6,0) cm respectively; the linear Lamb wave propagates from the excitation probe to the imaging point at a speed of v1, and the nonlinear Lamb wave propagates from the imaging point to the receiving probes #1, #2, #3, #4, #5, and #6 at a speed of v2. Since the center frequency of the incident wave is 200kHz, v1 corresponds to a group velocity of 200kHz, and v2 corresponds to a group velocity of 400kHz; matching Figure 4 , v1=4901m / s, v2=4700m / s.

[0094] Figure 8 Draw conclusions for Figure 7a The time-correlated pixels of the signal generate a series of pixel signals for different transmission-reception waves by repeating the above process.

[0095] According to step S8, since the coordinates of the transmitting probe and the receiving probe are known, the propagation time Δt of the vibration wave to a given imaging point can be calculated according to formula (4). By referring to the pixel value corresponding to Δt, a pixel value is assigned to the imaging point; one transmitting-receiving pair generates one pixel value, and the six pixel values generated by six pairs of transmitting-receiving Lamb wave pairs are added together to determine the final pixel value of the imaging point. Based on this principle, the damage image is reconstructed, and then the image is normalized, retaining pixel values greater than 0.9. The final image is as follows: Figure 9 As shown, high pixel values coincide with closed cracks, and the positions of the crack, transmitting probe, and receiving probe are represented by white triangles and white circles, respectively, imaging the cracks in the glass plate.

[0096] In summary, the present application stimulates and collects nonlinear Lamb wave signals through contact or non-contact transducers; separates the second harmonics generated by the impact and friction of the crack interface through the wavelet method; and realizes the imaging of closed cracks through the second harmonics. The pixels of each point on the damage map represent the degree of damage.

[0097] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as such herein, will not be interpreted in an idealized or overly formal sense.

[0098] The meaning of "and / or" in this application means that both situations where each exists alone or both exist at the same time are included.

[0099] The term “connection” as used in this application may mean a direct connection between components or an indirect connection between components via other components.

[0100] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A closed crack imaging method based on nonlinear Lamb waves, wherein the closed crack imaging device used includes a signal generator, a high-voltage amplifier, an excitation probe, a receiving probe, a voltage amplifier, and an oscilloscope; The receiving probes and the excitation probes include a plurality of receiving probes and the excitation probes, and the plurality of receiving probes and the excitation probes are arranged on the structure to be measured; The output end of the signal generator is connected to the input end of the high-voltage amplifier, the output end of the high-voltage amplifier is connected to the excitation probe, and the receiving probe is connected to the input end of the voltage amplifier. The excitation probe and the receiving probe are connected to the structure to be tested through a coupling agent, respectively transmitting and receiving Lamb waves in the structure to be tested. The output end of the signal generator, the monitoring end of the high-voltage amplifier, and the output end of the voltage amplifier are all connected to the oscilloscope. The characteristics are as follows: The specific steps include: Step S1: Equipment preparation, including signal generator, high-voltage amplifier, excitation probe, receiving probe, voltage amplifier, and oscilloscope; Step S2: based on the material properties and thickness of the structure to be tested, a dispersion curve of the Lamb wave is drawn to obtain wave velocity information; the dispersion curve shows the change of the group velocity of the Lamb wave in the structure to be tested as a function of the frequency-thickness product; Step S3: Build a test platform and assemble and connect all the equipment in step S1. The incident wave amplitude generated by the signal generator is set to 4 Vpp and the center frequency is set to 200 kHz. Step S4: Apply coupling agent to the excitation probe and the receiving probe respectively to remove the air between the excitation probe and the receiving probe and the structure to be measured, so as to ensure linear and distortion-free conversion between the electrical signal and the Lamb wave signal; Step S5: six receiving probes and one excitation probe are placed on the structure to be measured, and the receiving probes and the excitation probes are arranged in a linear array, with the distance between adjacent positions in the linear array being 2 cm; Step S6: Obtain the signal wave of the receiving probe through steps S3, S4, and S5, and use discrete wavelet transform to extract the nonlinear wave, that is, separate the second harmonic generated by the slapping and friction of the crack interface in the structure to be tested; Step S7: Establishing Cartesian coordinates on the surface of the structure to be measured, calculating the time difference between the emission and reception of the Lamb wave based on the positions of the imaging point, the excitation probe, and the receiving probe within the Cartesian coordinates, and selecting the amplitude of the nonlinear component in the received signal as the pixel value of the imaging point based on the time difference; wherein, the linear Lamb wave propagates from the excitation probe to the imaging point at a speed v1, and the nonlinear Lamb wave propagates from the imaging point to the receiving probe at a speed v2; Step S8: Six pairs of transmit-receive Lamb waves are formed between the six receiving probes and the excitation probes. For one imaging point, each pair of transmit-receive Lamb waves generates a pixel value. The sum of the six pairs of pixel values is the final pixel value of the imaging point, and the damage image is reconstructed. At the same time, the damage image is normalized and the maximum pixel value is retained.

2. The imaging method of the closed crack imaging device based on nonlinear Lamb waves according to claim 1, characterized in that: In step S2, when the structure to be measured is a uniform isotropic plate, the characteristic equation followed by Lamb wave propagation is: in, h is half the thickness of the structure to be measured, k is the wave number, ω is the circular frequency, c L is the velocity of the longitudinal wave, c T is the velocity of the shear wave, and +1 and -1 in formula (1) represent the symmetric and antisymmetric modes, respectively.

3. The imaging method of the closed crack imaging device based on nonlinear Lamb waves according to claim 2, characterized in that: In step S3, the signal generator generates an incident wave, the high-voltage amplifier amplifies the incident wave energy and transmits it to the structure through the excitation probe, the receiving probe receives the Lamb wave transmitted through the structure, and the voltage amplifier amplifies the energy of the received wave; The oscilloscope displays the vibration wave generated by the signal generator, the vibration wave amplified by the high-voltage amplifier, and the vibration wave amplified by the voltage amplifier; The incident wave generated by the signal generator consists of a 20-cycle Hanning window sine wave, and the voltage waveform transmitted from the excitation probe to the structure to be measured follows the rule: Among them, f c is the center frequency, and t is the time series.

4. The closed crack imaging method based on nonlinear Lamb waves according to claim 3, characterized in that: In step S6, the mother wavelet db8 is selected to decompose the signal wave received by the receiving probe into high-frequency and low-frequency components, and the low-frequency component is decomposed again. The above process is repeated six times until the frequency of the high-frequency component obtained by decomposition is twice the center frequency of the incident wave. At this time, the high-frequency component is a nonlinear Lamb wave.

5. The closed crack imaging method based on nonlinear Lamb waves according to claim 4, characterized in that: In step S7, in Cartesian coordinates, the imaging point coordinates are (x, y), and the excitation probe coordinates are (x T ,y T ), the receiving probe coordinates are (x R ,y R ), assuming the distance from the imaging point to the excitation probe is d1, and the distance from the imaging point to the receiving probe is d2, the Lamb wave vibration propagation time calculation formula is The excitation probe is located at the origin (0, 0), and the six receiving probes are marked as #1, #2, #3, #4, #5, and #6. Their positions in Cartesian coordinates are (-6, 0), (-4, 0), (-2, 0), (2, 0), (4, 0), and (6, 0), respectively. The coordinate axis unit is cm. The Lamb wave vibration propagation time Δt obtained by each receiving probe is calculated, and the amplitude of the nonlinear wave at Δt represents the reflection intensity of the imaging point.

6. The closed crack imaging method based on nonlinear Lamb waves according to claim 5, characterized in that: In step S7, when the imaging point coincides with the crack, a strong reflection occurs, and a peak is generated in the nonlinear Lamb wave. When the imaging point is not on the crack, no obvious reflection occurs, and the amplitude of the nonlinear Lamb wave is close to 0. Therefore, the amplitude of the nonlinear Lamb wave can be used as the pixel value to represent the reflection intensity of the imaging point. The instantaneous amplitude of the nonlinear Lamb wave is selected as the pixel value of the closed crack image. The calculation method of the instantaneous amplitude a(t) is: Where x(t) is the nonlinear Lamb wave signal, P is the Cauchy principal value, t is the time series, and τ is the integral variable of H(t), which disappears after integration.

7. The closed crack imaging method based on nonlinear Lamb waves according to claim 6, characterized in that: In step S8, formula (4) is matched with formula (3) to calculate six pixel values of six pairs of transmitted and received Lamb waves respectively, and the six pixel values are added to determine the final pixel value of the imaging point; The damaged image was normalized and pixel values greater than 0.9 were retained.

Images