A nonlinear guided wave nondestructive testing method based on elastic wave metasurface

Abstract

The present invention discloses a nonlinear guided wave nondestructive testing method based on an elastic wave metasurface, comprising: arranging a metasurface on one side of a preset detection area on the surface of a test plate to be tested, and arranging a piezoelectric actuator on the other side of the detection area; generating Lamb waves through the piezoelectric actuator, collecting the displacement amplitude response signal on the test plate to be tested between two rows of supercells contained in the metasurface and using it as detection data after Fourier transformation; comparing the detection data with the baseline data obtained by Fourier transformation of the displacement amplitude response signal obtained from a healthy test plate after arranging the metasurface and piezoelectric actuator in the same way as the test plate to be tested, so as to determine whether the test plate to be tested has early damage. The present invention achieves a significant enhancement of the nonlinear signal of early material damage in a millimeter-scale structure through the design of a scatterer array at a subwavelength scale, while reducing the transmission of the center frequency guided wave signal, significantly improving the detection signal-to-noise ratio.

Description

Technical Field

[0001] The present invention relates to the field of nondestructive testing of materials, and in particular to a nonlinear guided wave nondestructive testing method based on elastic wave metasurface. Background Art

[0002] In recent years, active detection technology based on ultrasonic guided waves has garnered significant attention in the fields of nondestructive testing and structural health monitoring due to its long propagation distance, low attenuation, and ability to rapidly detect large areas. It has become one of the most active and promising detection technologies in this field. Ultrasonic guided waves, also known as Lamb waves, are plane waves that, as they propagate through a waveguide medium, undergo continuous reflection, refraction, and conversion between longitudinal and transverse waves at the medium's boundaries. Their vibrations vary with parameters such as plate thickness and frequency. Traditional linear guided wave detection techniques are highly sensitive to larger cracks but less sensitive to early fatigue damage and microcracks. In contrast, nonlinear guided wave detection techniques focus on high-frequency nonlinear signals, such as double and triple harmonics, generated by guided waves when encountering microdefects such as corrosion and cracks. These nonlinear effects are reliable and sensitive indicators of material damage. By arranging sensors to collect and process these signals, information such as the presence and location of defects can be determined.

[0003] However, the dispersion and multimodal nature of guided waves, as well as the modal conversion and interface scattering often associated with damage and boundary effects, lead to complex guided wave response signals. For metal materials used in aircraft, to improve detection accuracy and damage identification sensitivity, a large number of sensors are typically deployed to receive guided wave signals. This increases the complexity of the detection system, places higher demands on software and hardware processing, and increases the probability of hardware failures and false alarms. Furthermore, the high-frequency nonlinear signals generated by tiny defects are weak and often overwhelmed by the center frequency signal or noise, requiring complex signal processing and increasing the time cost of damage detection.

[0004] Currently, research on guided wave nondestructive testing is mainly focused on thin plate structures, where damage detection is achieved by analyzing different modal signals of Lamb waves, but complex signal processing is still required. Metamaterial research on elastic waves mainly focuses on the control of waveguide directionality. Relevant scholars have constructed chaotic cavity components for filtering and focusing, but they are far inferior to metasurfaces in terms of volume. The application of metasurfaces in nondestructive testing is mostly aimed at macroscopic damage, and there is relatively little research on the microscopic damage that causes early failure of materials. In nonlinear guided wave testing, how to use the signal amplification function of metasurfaces to quickly and effectively separate nonlinear high-order harmonics from low-frequency fundamental waves under the conditions of complex guided wave signals and weak effective signal-to-noise ratio is a challenge that needs to be solved urgently. Summary of the Invention

[0005] The purpose of the present invention is to provide a nonlinear guided wave nondestructive testing method based on elastic wave metasurface. Through the design of scatterer arrays at the subwavelength scale, a substantial enhancement of the nonlinear signal of early material damage can be achieved in millimeter-scale structures, while reducing the transmission of the center frequency guided wave signal, significantly improving the detection signal-to-noise ratio.

[0006] In order to achieve the above tasks, the present invention adopts the following technical solutions:

[0007] A nonlinear guided wave nondestructive testing method based on an elastic wave metasurface, comprising:

[0008] A metasurface is placed on one side of a predetermined detection area on the surface of a test plate to be tested, and a piezoelectric actuator is placed on the other side of the detection area. Lamb waves are generated by the piezoelectric actuator, and the displacement amplitude response signal on the test plate to be tested between two rows of supercells contained in the metasurface is collected and used as detection data after Fourier transformation.

[0009] The healthy test plate is set up with the same detection area, metasurface and piezoelectric actuator as the test plate to be tested. Lamb waves are generated and the displacement amplitude response signal on the healthy test plate between the supercells is collected. After Fourier transformation, it is used as the baseline data.

[0010] Based on the comparison of the test data and the baseline data, it is determined whether the test plate to be tested has early damage; wherein, the test plate to be tested is a test plate of the same type as the healthy test plate.

[0011] Furthermore, the metasurface comprises two rows of supercells, each row of supercells is composed of arranged unit cells, and each unit cell is a rectangular parallelepiped structure; the material of the unit cell is the same as that of the test plate to be tested.

[0012] Furthermore, the design method of the unit cell includes:

[0013] Determine the wavelength of Lamb wave in the material according to the material density, Poisson's ratio, Young's modulus and the size and thickness of the test board ; Calculate the transmittance of the unit cell and reflectivity :

[0014] ;

[0015] ;

[0016] First, a single unit cell is placed on the test board to be tested, with the left side of the unit cell facing the detection area; two straight lines s1 and s2 are set between the left side of the unit cell and the left edge of the test board to be tested, at a distance of l1 and l2 from the unit cell, where l2>l1; a straight line s3 is set between the right side of the unit cell and the right edge of the test board to be tested, at a distance of l3 from the unit cell; then a preset out-of-plane excitation is applied to the left edge of the test board to be tested, and the out-of-plane excitation is less than 10 -5 The small perturbation force of N; then the out-of-plane displacements at lines s1 and s2 are and , the out-of-plane displacement at line s3 is ; k is the wave number, , is the signal wavelength of the out-of-plane excitation, e is a natural constant, and i is an imaginary unit;

[0017] The transmittance and reflectivity of the unit cell are controlled by adjusting the size of the unit cell, and the size of the unit cell is finally determined.

[0018] Furthermore, the transmittance and reflectance of the unit cell can be controlled by adjusting the size of the unit cell, including:

[0019] Given a preset value of millimeter for the unit cell width b, the values of length a and height c are continuously calculated through simulation so that the transmittance As low as possible while ensuring transmittance and reflectivity The sum of the squares is 1; the size of the final output unit cell is optimized through simulation.

[0020] Furthermore, the designed unit cells are arranged along their front-to-back direction at preset intervals to form a column of supercells, and the overall length of the supercell is consistent with the side length of the detection area; then the formed column of supercells is copied to another column of supercells along the width direction to form a metasurface.

[0021] Furthermore, the signal acquisition equipment arranged on the test board to be tested includes: a piezoelectric actuator, a laser vibrometer, an oscilloscope, a robotic arm, a power amplifier and a data collector, wherein:

[0022] A piezoelectric actuator is arranged on one side of the detection area; an oscilloscope is used to modulate the excitation signal of the Lamb wave, and after amplification by a power amplifier, the excitation signal is applied to the piezoelectric actuator to generate a Lamb wave that propagates within the test board to be tested; the laser probe of the laser vibrometer is used to detect the displacement amplitude response signal of the Lamb wave at the laser irradiation point, where the laser irradiates the test board to be tested between two rows of supercells; the spatial position of the laser vibrometer is controlled by a robotic arm to adjust the laser irradiation position; the displacement amplitude response signal is collected by a data acquisition device and sent to the host computer for subsequent Fourier transform processing.

[0023] Furthermore, the piezoelectric actuator generates a Lamb wave after stimulating an excitation signal; wherein the excitation signal is in the following form:

[0024] ;

[0025] Where t is the time parameter, is the center frequency of the excitation signal, e is a natural constant; To control the excitation signal The length of The parts are all set to 0.

[0026] Furthermore, based on the comparison of the test data and the baseline data, it is determined whether the test board to be tested has early damage, including:

[0027] Compare the test data of the metal plate to be tested with the baseline data of the healthy metal plate. If the amplitude at the 2f frequency in the test data is five times greater than the amplitude at the 2f frequency in the baseline data, it is considered that there is early damage in the test area, otherwise there is no early damage. is the center frequency of the excitation signal.

[0028] A terminal device includes a processor, a memory, and a computer program stored in the memory; when the processor executes the computer program, the nonlinear guided wave nondestructive testing method based on the elastic wave metasurface is implemented.

[0029] A computer-readable storage medium stores a computer program; when the computer program is executed by a processor, the nonlinear guided wave nondestructive testing method based on an elastic wave metasurface is implemented.

[0030] Compared with the prior art, the present invention has the following technical features:

[0031] 1. Signal enhancement and noise reduction: By suppressing the center frequency waveguide in a confined state and amplifying nonlinear signals in an enhanced state, the signal-to-noise ratio is significantly improved, simplifying the back-end signal processing process.

[0032] 2. High-sensitivity detection: The millimeter-level structural design can sensitively capture the nonlinear response caused by early damage to the material.

[0033] 3. Wide range of applications: The device has a compact structure and can be attached to the surface of the sample to be tested, enabling rapid detection over a large area and providing new research ideas for damage location.

[0034] 4. Quantitative evaluation: By correlating the nonlinear signal intensity with the response of the multi-point compact metasurface array, the damage degree can be quantified and the spatial distribution analyzed. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A model diagram of a unit cell in one embodiment of the present invention;

[0036] Figure 2 Schematic diagram of the spacing between unit cells when two rows of super cells are arranged in one embodiment of the present invention;

[0037] Figure 3 This is a unit cell transmittance field diagram in one embodiment of the present invention;

[0038] Figure 4 A schematic diagram of the arrangement of a signal acquisition device in one embodiment of the present invention;

[0039] Figure 5 The transmittance results for an embodiment of the present invention when the unit cell width b = 3 mm are shown, where the left figure is for f = 50 kHz and the right figure is for f = 100 kHz.

[0040] Figure 6 : is a parameterized scan result of the distance n between two columns of supercells when two columns of supercells are arranged in one embodiment of the present invention;

[0041] Figure 7 Figure 1 is a normalized energy field diagram of enhanced state simulation in one embodiment of the present invention, where the left figure is for the case of f=50kHz and the right figure is for the case of f=100kHz;

[0042] Figure 8 This is a comparison diagram of the energy amplitude of a metal aluminum plate during simulation in one embodiment of the present invention, wherein the left diagram shows the energy amplitude of a healthy metal aluminum plate, and the right diagram shows the energy amplitude of a damaged metal aluminum plate;

[0043] Figure 9 This is the detection data obtained between supercells on a metal aluminum plate in one embodiment of the present invention, where the left figure shows the change of amplitude over time, and the right figure shows the change of Fourier transform results over time;

[0044] Figure 10 The experimental results of the enhanced and confined states of the metasurface in one embodiment of the present invention are shown. The left figure shows the experimental results of points taken on a line set on the metasurface, and the right figure shows the experimental results of points taken on a line set between supercells.

[0045] Figure 11 Extracting the out-of-plane displacement field of the point under the excitation signal in one embodiment of the present invention;

[0046] Figure 12 is the normalized result of the Fourier transform of the out-of-plane displacement field in one embodiment of the present invention;

[0047] Figure 131. A control experiment diagram of a healthy and damaged aluminum metal plate in one embodiment of the present invention, wherein the left diagram is a damaged aluminum metal plate, and the right diagram is a healthy aluminum metal plate;

[0048] Figure 14 The out-of-plane displacement amplitude signal in one embodiment of the present invention is subjected to Fourier transformation and normalization.

[0049] Figure 15 Comparative experimental diagram of a damaged aluminum plate with and without a metasurface in one embodiment of the present invention, where the left image is a damaged aluminum plate and the right image is a healthy aluminum plate;

[0050] Figure 16 This is the result of normalization processing after Fourier transform of the out-of-plane displacement amplitude signal in one embodiment of the present invention. DETAILED DESCRIPTION

[0051] When using nonlinear guided wave detection technology to detect tiny crack damage, the key is how to quickly and effectively separate the higher-order harmonic signals generated by the nonlinear effect from the fundamental signal. The emergence of metamaterials and metasurfaces has provided a new approach to solving this problem. As "sensors" that enhance guided wave signals, they can significantly improve the signal-to-noise ratio of nonlinear signals, demonstrating significant superiority over traditional nonlinear detection technologies. Elastic metasurfaces, as two-dimensional planar forms of elastic metamaterials, exhibit extraordinary physical properties unattainable by natural dielectric materials due to their unique microstructural design, such as negative Poisson's ratio, negative stiffness, zero shear modulus, and multistability. This has revolutionary significance for improving the performance of traditional materials.

[0052] The present invention provides a nonlinear guided wave nondestructive testing method based on elastic wave metasurface. By designing a metasurface on the surface of a test plate, two rows of supercells in the metasurface can be used on a millimeter-scale structure to significantly amplify Lamb wave signals affected by early micro- and nanoscale damage to the material.

[0053] The metasurface comprises two rows of supercells, each consisting of a single cell with a rectangular structure and made of the same material as the test plate. The metasurface is attached to one side of a predetermined detection area on the test plate, with a piezoelectric actuator placed on the other side. Lamb waves are generated by the electrical actuator using a predetermined excitation signal. The inventors' team discovered that when the test plate is healthy and undamaged, the displacement amplitude response signal collected during the Lamb wave propagation does not contain nonlinear second harmonics, or the second harmonics are very weak. However, when the test plate is damaged, the metasurface structure can effectively amplify the second harmonics. The displacement amplitude signal collected between the two rows of supercells is significantly amplified at a frequency of 2f (f is the center frequency of the excitation signal) compared to when the test plate is in a healthy state. The larger the peak value, the more severe the early damage to the test plate. Based on this principle, this scheme can use metasurfaces to perform nondestructive testing on the test plate to be inspected. The test plate can be made of metal, composite materials, etc.

[0054] The method of the present invention effectively simplifies the complexity of signal processing in nonlinear nondestructive testing, making nondestructive testing more efficient and rapid while reducing testing costs, providing a feasible solution for the wider application of nonlinear nondestructive testing. The specific implementation process of the present invention is described in detail below with reference to the accompanying drawings.

[0055] The present invention provides a nonlinear guided wave nondestructive testing method based on an elastic wave metasurface, comprising the following steps:

[0056] A metasurface is placed on one side of a predetermined detection area on the surface of a test plate to be tested, and a piezoelectric actuator is placed on the other side of the detection area. Lamb waves are generated by the piezoelectric actuator, and the displacement amplitude response signal on the test plate to be tested between two rows of supercells contained in the metasurface is collected and used as detection data after Fourier transformation.

[0057] The healthy test plate is set up with the same detection area, metasurface and piezoelectric actuator as the test plate to be tested. Lamb waves are generated and the displacement amplitude response signal on the healthy test plate between the supercells is collected. After Fourier transformation, it is used as the baseline data.

[0058] Based on the comparison of the test data and the baseline data, it is determined whether the test plate to be tested has early damage; wherein the test plate to be tested is the same type of test plate as the healthy test plate, and the same type means the same material and thickness; the detection area is a designated area on the test plate to be tested, such as Figure 4 As shown, the detection area in this example is a rectangular area, which can be adjusted according to actual needs.

[0059] 1. Metasurface.

[0060] See also Figure 1 and Figure 2The metasurface in the present invention is composed of two rows of supercells, each row of supercells is composed of unit cells, and each unit cell is a rectangular parallelepiped structure; the design process of the unit cell is as follows:

[0061] 1.1 First, determine the wavelength of Lamb wave in the material according to the material density, Poisson's ratio, Young's modulus and material size and thickness of the test board to be tested. The wavelength The determination can be obtained by COMSOL modeling and simulation.

[0062] 1.2 At known wavelengths Calculate the transmittance of the unit cell in the case of and reflectivity :

[0063] ;

[0064] ;

[0065] First, a single unit cell is placed on the test board to be tested, with the left side of the unit cell facing the detection area; two straight lines s1 and s2 are set between the left side of the unit cell and the left edge of the test board to be tested, at a distance of l1 and l2 from the unit cell, where l2>l1; a straight line s3 is set between the right side of the unit cell and the right edge of the test board to be tested, at a distance of l3 from the unit cell; then a preset out-of-plane excitation is applied to the left edge of the test board to be tested, and the out-of-plane excitation is less than 10 -5 The small perturbation force of N; then the out-of-plane displacements at lines s1 and s2 are and , the out-of-plane displacement at line s3 is ; k is the wave number, , is the signal wavelength of the out-of-plane excitation, e is a natural constant, and i is an imaginary unit.

[0066] 1.3 See Figure 3 , adjust the size of the supercell (length a, width b and height c) to control the transmittance and reflectance of the unit cell; see Figure 2 The specific adjustment method is: a preset value of millimeter level is given for the unit cell width b, such as 3mm; in actual application, it can be set according to the size of the test board, application scenario, etc.; the values of length a and height c are continuously calculated through simulation to make the transmittance As low as possible while ensuring transmittance and reflectivity The sum of the squares is 1; the size of the final output unit cell is optimized through simulation.

[0067] The design idea of this solution is to require the unit cell to have high reflectivity. Correspondingly, when the reflectivity is high, the transmittance is low. When the transmittance is low, the present invention seeks a smaller unit cell size to meet the compactness requirement of the design.

[0068] 1.4 The designed unit cells are arranged along their front-to-back direction at a preset interval H to form a row of supercells; the overall length of the supercell is consistent with the side length of the detection area, so that the unit cells can cover the edge of the detection area; in one embodiment of the present invention, the preset interval H is 6 mm; then the formed row of supercells is copied to another row of supercells along the width direction to form a metasurface; wherein, the spacing n between the two rows of supercells is related to the frequency and wavelength of the Lamb wave, and can be optimized by parametrically scanning the distance n through COMSOL simulation; that is, during the optimization process, the second harmonic amplification effect of the Lamb wave in the two rows of supercells is made as obvious as possible.

[0069] When only one row of supercells is provided, the row of supercells has the ability to isolate Lamb waves; however, when two rows of supercells are provided, the second harmonic can be effectively amplified between the supercells.

[0070] 2. Arrangement of signal acquisition equipment.

[0071] like Figure 4 As shown, the signal acquisition equipment in this solution includes a piezoelectric actuator, a laser vibrometer, an oscilloscope, a robotic arm, a power amplifier, and a data collector, wherein:

[0072] The piezoelectric actuator (piezoelectric sheet can be used) is placed on one side of the detection area; the oscilloscope is used to modulate the excitation signal of the Lamb wave The excitation signal is amplified by a power amplifier and then applied to the piezoelectric actuator to generate a Lamb wave that propagates within the test board. The laser probe of the laser vibrometer is used to detect the displacement amplitude response signal of the Lamb wave at the laser irradiation location. The laser is irradiated onto the test board between two rows of supercells. The spatial position of the laser vibrometer is controlled by a robotic arm to adjust the laser irradiation position. The displacement amplitude response signal is collected by a data acquisition device and sent to a host computer for subsequent Fourier transform and other processing.

[0073] In this solution, the piezoelectric actuator generates a Lamb wave after exciting the excitation signal modulated by the Hanning window and transmits it to one side of the metasurface; then the displacement amplitude response signal along the thickness direction can be collected on the test board to be tested; in one embodiment of the present invention, the designed excitation signal as follows:

[0074] ;

[0075] Where t is the time parameter, is the center frequency of the excitation signal, e is a natural constant; Used to control the length of the excitation signal, which means that it is greater than The parts are all set to 0.

[0076] 3. Damage detection.

[0077] In this scheme, it is first necessary to set up a detection area on the healthy test plate, then arrange the metasurface structure (bonded by epoxy glue) and signal acquisition equipment, and collect the displacement amplitude response signal between the supercells, which is used as the baseline data after Fourier transformation; in actual application, a test plate of the same type as the healthy test plate is selected, and after setting up the detection area at the same position, the metasurface and signal acquisition equipment are arranged. The displacement amplitude response signal obtained at this time is used as the detection data after Fourier transformation.

[0078] For damaged test panels, without the metasurface structure, the collected test data will generate second harmonics. However, this nonlinear second harmonic is very weak and buried in the noise signal, making it difficult to determine whether it exists, which in turn affects the determination of whether early damage exists.

[0079] In this scheme, after the metasurface is designed, the test data of the test plate to be tested and the baseline data of the healthy test plate are compared. If the amplitude at the 2f frequency in the test data is five times greater than the amplitude at the 2f frequency in the baseline data, it is considered that there is early damage in the inspection area of the test plate to be tested. Otherwise, there is no early damage; thus completing the non-destructive testing of the test plate to be tested.

[0080] The simulation experiment of the present invention is as follows:

[0081] In the embodiment verification of the present invention, aluminum is selected as the model material and the metal aluminum plate is used as the test plate to be tested. Its basic parameter density is 2700kg / m 3 , Young's modulus is 70 GPa, and Poisson's ratio is 0.33.

[0082] First, by arranging the metasurface and signal acquisition equipment on the metal aluminum plate, the baseline data of the metal aluminum plate in a healthy and non-destructive state is obtained; in this embodiment, the excitation signal Center frequency Set to 50kHz.

[0083] When the thickness of the metal aluminum plate is 1mm, the wavelength calculated in COMSOL is 11.488mm, and the transmittance of the unit cell is calculated at this wavelength. The transmittance calculation formula mentioned above can be entered into COMSOL for calculation. The transmittance is affected by the unit cell dimensions a, b, and c. By parametrically scanning the unit cell dimensions a and c, the same operation can be performed on dimensions a and b, and b and c. Figure 3The transmittance field diagram drawn from the COMSOL calculation results under the changes of unit cell size a and c when the unit cell width b is 3mm is shown in the figure. It is noted that as the unit cell size increases, the transmittance decreases. However, considering the compactness and small size of the unit cell, this solution hopes to achieve similar results with a smaller unit cell size. Figure 5 The transmittance results for a unit cell with a width of b = 3 mm are shown, using unit cell sizes with lower transmittance, i.e. a = 6 mm and c = 7 mm.

[0084] Finite element simulation software is used to verify the metal aluminum plate and the metasurface. During modeling, a low-reflection boundary is set around the established metal aluminum plate model to reduce reflection. The elastic wave excitation signal s(t) is applied to one side of the metal aluminum plate detection area. For a single-row supercell, a point source excitation method is used to ensure that the excitation is incident at all angles in the plane. Figure 5 The field diagram shows that the single-layer metasurface supercell has high reflection for 50kHz and 100kHz signals, which demonstrates the ultra-wideband isolation capability of the designed metasurface. Figure 6 As shown in the figure, when two rows of super cells are arranged, the energy between the two layers of super cells is collected. By parametrically scanning the distance n between the two rows of super cells, the energy variation with n at 50kHz and 100kHz can be obtained. When n=11.66mm, Figure 5 The energy of 100kHz is four orders of magnitude higher than that of 50kHz; therefore, when the distance between the two columns of supercells is , the enhanced state effect has a very obvious enhancement on the target frequency, while having no enhancement effect on the center frequency, thereby achieving the separation of the center frequency and the doubled frequency signal.

[0085] Figure 7 Shown by Figure 6 The results obtained are the normalized energy field diagrams of the double-layer metasurface for the confined state effect at a center frequency of 50 kHz and the enhanced state at its frequency multiplication of 100 kHz, respectively, when n = 11.66 mm. At f = 100 kHz, a significant energy enhancement is achieved in the double-layer metasurface supercell.

[0086] In COMSOL simulation, the nonlinearity caused by the early stage of material degradation is introduced to simulate the damage. The nonlinearity of the material is introduced by giving the Murnaghan third-order elastic modulus in the hyperelastic material. In the early stage of material degradation, the Murnaghan third-order elastic modulus will change dramatically. Therefore, the material damage can be simulated by changing the Murnaghan third-order elastic modulus parameter. At the left edge of the metal aluminum plate, a Hanning window with a center frequency of 50kHz is given. Wave packet signal. In the time domain simulation, the healthy metal aluminum plate has no obvious energy amplitude in the middle of the metasurface under a given excitation, while the damaged metal aluminum plate with the nonlinear condition added can see a significant energy amplitude, as shown in the following figure. Figure 8 shown; in Figure 9 The out-of-plane displacement between the double-layer supercells was collected and Fourier transform was performed to monitor the frequency information. It was noted that in the control simulation without the introduction of nonlinearity, the main peak of the collected wave packet signal after FFT was 50kHz. However, after the introduction of nonlinearity, the nonlinear signal 2f, that is, the frequency of 100kHz, was significantly enhanced after the metasurface sharpening and enhancement, with a numerical value of 43 times, which greatly improved the signal-to-noise ratio of the nonlinear signal.

[0087] Based on the size and arrangement parameters of the metasurface units determined by the above simulation, in one embodiment, the arrangement is as follows:

[0088] The dimensions of the metal plate (length * width * thickness) are 500 * 500 * 1 mm. 3 , made of aluminum, its elastic modulus GPa, Poisson's ratio is ,density This example uses 30 designed unit cells, each 6 mm long, 3 mm wide, and 7 mm high. The unit cells are attached to an aluminum plate using epoxy adhesive, with a unit cell spacing H of 6 mm and a distance of 11.66 mm between two rows of supercells. (Actual attachment errors may occur, resulting in a shift in the amplified frequency.)

[0089] In this embodiment, the excitation signal adopts the mixing signal s(t2):

[0090] ;

[0091] in , is the center frequency of the excitation signal.

[0092] In the detection area in front of the metasurface (such as Figure 4 A line is set at the mid-distance piezoelectric actuator position l) and between the supercells in the metasurface (such as the middle line between two rows of supercells). The displacement amplitude response signals of 180 points on the two lines are taken and the results are obtained after Fourier transformation. Figure 10 The first picture shows that f occupies the main part of the signal, and the signal of 2f is relatively weak. After being amplified by the metasurface, Figure 10 In the second picture, it is noted that the energy at f is significantly weakened, while the signal at 2f has an obvious peak, and its energy is significantly amplified, and the energy amplification amplitude reaches more than five times, which also verifies the enhanced state effect of the designed metasurface.

[0093] Furthermore, nonlinearity was introduced to verify the effectiveness of the metasurface in actual damage detection. A 10cm*10cm area of an aluminum plate was heated at 400°C for four hours using a constant-temperature heating station to ensure thermal aging of the material. This change in the material's microstructure can cause a nonlinear response. A drop hammer was then used to impact the aluminum plate to simulate impact damage, which also alters the material's original properties, thus revealing nonlinear effects.

[0094] The experimental steps are the same as above. Due to the error of metasurface pasting, the center frequency of the excitation signal is 45kHz. In this scheme, point A is taken between the two rows of twin cells on the metasurface, and point B is taken in the detection area between the metasurface and the piezoelectric actuator as the detection point to extract the out-of-plane displacement field of this point under the excitation signal. For details, see Figure 11 Compared with time domain signals, this solution focuses more on frequency domain information after Fourier transform. Figure 12 The normalized Fourier transform results of the out-of-plane displacement field at points A and B are shown. The line graph shows that before the metasurface, the second harmonic is very weak and buried within the center frequency signal. However, after passing through the metasurface, it produces a remarkable amplification effect, reaching approximately 25 times the original value. Although the nonlinear signal is very weak, a distinct peak is evident at the second harmonic. This significant peak can be passively manipulated on the metasurface, replacing complex signal processing, improving the signal-to-noise ratio and thus revealing material damage.

[0095] Furthermore, in order to verify that the amplification of the second harmonic is caused by the nonlinearity of the material itself, rather than the nonlinearity of the experimental instrument and the metasurface itself, this program conducted a control experiment on a healthy metal aluminum plate and a damaged metal aluminum plate. Figure 13 Using the control variable method, under the same excitation signal, the out-of-plane displacement amplitude signal at the same point in the middle of the metasurface is collected, and the signal is normalized after Fourier transformation. The results are shown as follows: Figure 14 As shown in the figure, it can be seen that in the presence of damage, a clear peak is observed at the second harmonic frequency of 90kHz. However, for a normal metal aluminum plate (undamaged), no second harmonic frequency is observed under center frequency excitation. This solution compares the signal of the damaged metal aluminum plate with that of the intact metal aluminum plate, and it can be seen that the metasurface amplifies the nonlinear second harmonic of the damaged plate by more than 20 times. The significant amplification effect at the double harmonic frequency also verifies the role of the metasurface in precisely selecting specific frequencies for amplification.

[0096] Similarly, this scheme requires a control experiment to eliminate the influence of the metasurface on the experiment. Therefore, this scheme conducted a comparative experiment on a damaged metal aluminum plate with and without the metasurface. The experimental arrangement is shown in Figure 15 .

[0097] When the experimental conditions and excitation signals remain the same, this scheme uses a laser vibrometer to collect the out-of-plane displacement signal at the same point and uses Fourier transform to study the frequency domain information; the normalized results are shown as follows Figure 16 As shown, even when the aluminum plate is damaged, the presence of the metasurface demonstrates significant amplification at the second harmonic of the center frequency. The results show a 50-fold amplification effect around 90kHz, with some frequency points experiencing a 70-fold signal enhancement, demonstrating the metasurface's remarkable ability to focus and amplify energy. However, without the metasurface structure, the displacement amplitude response signal measured by this solution at the same point, while containing a nonlinear second harmonic, is lost in the noise. Without complex post-processing, this solution is unable to distinguish the nonlinear second harmonic from the noise and the center frequency signal.

[0098] Through its structural design, the metasurface of this scheme can selectively amplify weak nonlinear second harmonics, weakening the center frequency detection signal while amplifying the second harmonic that reflects early damage to the material, significantly improving the signal-to-noise ratio while greatly reducing the complexity of subsequent signal processing.

[0099] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. A nonlinear guided wave nondestructive testing method based on elastic wave metasurface, characterized in that: include: Arrange a metasurface on one side of a predetermined detection area on the surface of the test plate to be inspected, and arrange a piezoelectric actuator on the other side of the detection area; Lamb waves are generated by a piezoelectric actuator, and the displacement amplitude response signal on the test plate to be tested between the two rows of supercells contained in the metasurface is collected and used as the test data after Fourier transformation; The healthy test plate is set up with the same detection area, metasurface and piezoelectric actuator as the test plate to be tested. Lamb waves are generated and the displacement amplitude response signal on the healthy test plate between the supercells is collected. After Fourier transformation, it is used as the baseline data. Based on the comparison of the test data and the baseline data, determine whether the test panel to be tested has early damage; wherein the test panel to be tested is the same type of test panel as the healthy test panel; The metasurface comprises two rows of supercells, each row of supercells is composed of arranged unit cells, and each unit cell is a rectangular parallelepiped structure; the material of the unit cell is the same as that of the test plate to be tested; The design method of the unit cell includes: Determine the wavelength of Lamb wave in the material according to the material density, Poisson's ratio, Young's modulus and the size and thickness of the test board ; Calculate the transmittance of the unit cell and reflectivity : ； ； First, a single unit cell is placed on the test board to be tested, with the left side of the unit cell facing the detection area; two straight lines s1 and s2 are set between the left side of the unit cell and the left edge of the test board to be tested, at a distance of l1 and l2 from the unit cell, where l2>l1; a straight line s3 is set between the right side of the unit cell and the right edge of the test board to be tested, at a distance of l3 from the unit cell; then a preset out-of-plane excitation is applied to the left edge of the test board to be tested, and the out-of-plane displacements at the straight lines s1 and s2 are respectively and , the out-of-plane displacement at line s3 is ; k is the wave number, , is the signal wavelength of the out-of-plane excitation, e is a natural constant, and i is an imaginary unit; The transmittance and reflectivity of the unit cell are controlled by adjusting the size of the unit cell, and the size of the unit cell is finally determined.

2. The nonlinear guided wave nondestructive testing method based on elastic wave metasurface according to claim 1 is characterized in that: By adjusting the size of the unit cell, the transmittance and reflectance of the unit cell can be controlled, including: Given a preset value of millimeter for the unit cell width b, the values of length a and height c are continuously calculated through simulation so that the transmittance Low and guaranteed transmittance and reflectivity The sum of the squares is 1; the size of the final output unit cell is optimized through simulation.

3. The nonlinear guided wave nondestructive testing method based on elastic wave metasurface according to claim 1, characterized in that: The designed unit cells are arranged along their front-to-back direction at preset intervals to form a row of supercells, whose overall length is consistent with the side length of the detection area; then the formed row of supercells is copied into another row of supercells along the width direction to form a metasurface.

4. The nonlinear guided wave nondestructive testing method based on elastic wave metasurface according to claim 1, characterized in that: The signal acquisition equipment arranged on the test board to be tested includes: piezoelectric actuator, laser vibrometer, oscilloscope, robotic arm, power amplifier and data acquisition device, among which: A piezoelectric actuator is arranged on one side of the detection area; an oscilloscope is used to modulate the excitation signal of the Lamb wave, and after amplification by a power amplifier, the excitation signal is applied to the piezoelectric actuator to generate a Lamb wave that propagates within the test board to be tested; the laser probe of the laser vibrometer is used to detect the displacement amplitude response signal of the Lamb wave at the laser irradiation point, where the laser irradiates the test board to be tested between two rows of supercells; the spatial position of the laser vibrometer is controlled by a robotic arm to adjust the laser irradiation position; the displacement amplitude response signal is collected by a data acquisition device and sent to the host computer for subsequent Fourier transform processing.

5. The nonlinear guided wave nondestructive testing method based on elastic wave metasurface according to claim 1, characterized in that: The piezoelectric actuator generates a Lamb wave after stimulating an excitation signal; wherein the excitation signal is in the following form: ； Where t is the time parameter, is the center frequency of the excitation signal, e is a natural constant; To control the excitation signal The length of The parts are all set to 0.

6. The nonlinear guided wave nondestructive testing method based on elastic wave metasurface according to claim 1, characterized in that: Based on the comparison of test data and baseline data, determine whether the test board has early damage, including: Compare the test data of the metal plate to be tested with the baseline data of the healthy metal plate. If the amplitude at the 2f frequency in the test data is five times greater than the amplitude at the 2f frequency in the baseline data, it is considered that there is early damage in the test area, otherwise there is no early damage. is the center frequency of the excitation signal.

7. A terminal device comprising a processor, a memory, and a computer program stored in the memory; characterized in that: When the processor executes the computer program, it implements the nonlinear guided wave nondestructive testing method based on the elastic wave metasurface according to any one of claims 1 to 6.

8. A computer-readable storage medium storing a computer program; wherein: When the computer program is executed by a processor, the nonlinear guided wave nondestructive testing method based on the elastic wave metasurface according to any one of claims 1 to 6 is implemented.

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