Nonlinear guided wave nondestructive testing method based on elastic wave metasurface

Abstract

The invention discloses a nonlinear guided wave nondestructive testing method based on an elastic wave metasurface, which comprises the following steps: arranging a metasurface on one side of a preset detection area on the surface of a test panel to be detected, and arranging a piezoelectric exciter on the other side of the detection area; the method comprises the following steps: generating Lamb waves through a piezoelectric exciter, collecting a displacement amplitude response signal on a to-be-detected test plate between two columns of supercells contained in the metasurface, and taking the displacement amplitude response signal as detection data after Fourier transform; and the detection data is compared with baseline data obtained by Fourier transform of an obtained displacement amplitude response signal after the metasurface and the piezoelectric exciter are arranged on the health test panel by adopting the same method as that of the test panel to be detected, so that whether the test panel to be detected has early damage or not is determined. Through the design of the scatterer array under the sub-wavelength scale, material early-stage damage nonlinear signals are greatly enhanced in a millimeter-scale structure, meanwhile, transmission of center frequency guided wave signals is reduced, and the detection signal-to-noise ratio is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the field of non-destructive testing of materials, and particularly to a non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface. Background Art

[0002] In recent years, due to characteristics such as long propagation distance, low attenuation, and the ability to achieve rapid detection of large areas, the active detection technology based on ultrasonic guided waves has attracted much attention in the fields of non-destructive testing and structural health monitoring, and has become one of the most active and promising detection technologies in this field. Ultrasonic guided waves, namely Lamb waves, are a kind of plane waves. When propagating in a waveguide medium, they continuously undergo reflection, refraction, and the conversion between longitudinal and transverse waves with the medium boundary, and the vibration changes with parameters such as plate thickness and frequency. Traditional linear guided wave detection technology is highly sensitive to large crack defects, but less sensitive to early fatigue damage, micro-cracks, etc. In contrast, nonlinear guided wave detection technology pays more attention to high-frequency nonlinear signals such as second harmonics and third harmonics generated when guided waves encounter micro-defects such as corrosion and cracks. These nonlinear effects are reliable and sensitive indicators of material damage. By arranging sensors to collect and process signals, information such as the presence and location of defects can be judged.

[0003] However, due to the dispersion and multimodal characteristics of guided waves, as well as the mode conversion and interface scattering that often accompany the interaction with damage and boundaries, the guided wave response signals are complex. For the metal materials used in aircraft, to improve the detection accuracy and damage recognition sensitivity, it is usually necessary to arrange a large number of sensors to receive guided wave signals, which increases the complexity of the detection system, poses higher requirements for software and hardware processing, and at the same time increases the probability of hardware failures and false alarms. In addition, the high-frequency nonlinear signals generated by micro-defects are weak and are often submerged by the central frequency signals or noise, and complex signal processing is required, increasing the time cost of damage detection.

[0004] Currently, the research on guided wave non-destructive testing mainly focuses on thin plate structures, and damage detection is achieved by analyzing different modal signals of Lamb waves, but still requires a complex signal processing process. The research on elastic waves by metamaterials mainly focuses on the directional control of guided waves. 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 non-destructive testing mostly targets macroscopic damage, and relatively less research has been done on the micro-damage of early material failure. In nonlinear guided wave detection, under the conditions of complex guided wave signals and weak signal-to-noise ratio of effective signals, how to use the signal amplification function of metasurfaces to quickly and effectively separate nonlinear higher harmonics and low-frequency fundamental waves is an urgent challenge currently faced. Summary of the Invention

[0005] The object of the present invention is to provide a non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface. Through the design of a scatterer array at the sub-wavelength scale, a significant enhancement of the nonlinear signal of early damage to the material is achieved in a millimeter-scale structure, while reducing the transmission of the guided wave signal at the central frequency, and significantly improving the detection signal-to-noise ratio.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface, comprising:

[0008] Arranging a metasurface on one side of a preset detection area on the surface of a test plate to be detected, 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 signals on the test plate to be detected between two columns of supercells included in the metasurface, and using them as detection data after Fourier transform;

[0009] After setting the detection area, arranging the metasurface and the piezoelectric actuator for the healthy test plate in the same manner as the test plate to be detected, generating Lamb waves and collecting the displacement amplitude response signals on the healthy test plate between the supercells, and using them as baseline data after Fourier transform;

[0010] Based on the comparison between the detection data and the baseline data, determine whether there is early damage to the test plate to be detected; wherein, the test plate to be detected is of the same type as the healthy test plate.

[0011] Further, the metasurface includes two columns of supercells, each column of supercells is composed of a single-cell arrangement, and each single cell is a cuboid structure; the material of the single cell is the same as the material of the test plate to be detected.

[0012] Further, the design method of the single cell includes:

[0013] Determine the wavelength of Lamb waves in the material according to the material density, Poisson's ratio, Young's modulus and the size and thickness of the material of the test plate to be detected ; calculate the transmittance and reflectance :

[0014] ;

[0015] ;

[0016] First, arrange a single unit cell on the test plate to be detected, with the left side of the unit cell facing the detection area. Set two straight lines s1 and s2 at positions with distances l1 and l2 from the left side of the unit cell to the left edge of the test plate to be detected, where l2 > l1. Set a straight line s3 at a position with a distance l3 from the right side of the unit cell to the right edge of the test plate to be detected. Then, apply a preset out-of-plane excitation at the left edge of the test plate to be detected, and this out-of-plane excitation is a small perturbation force less than 10 -5 N. Then, the out-of-plane displacements at the straight lines s1 and s2 are respectively and , and the out-of-plane displacement at the straight line s3 is ; k is the wave number, , is the signal wavelength of the out-of-plane excitation, e is the natural constant, and i is the imaginary unit;

[0017] Regulate the transmittance and reflectance of the unit cell by adjusting the size of the unit cell, and finally determine the size of the unit cell.

[0018] Furthermore, regulating the transmittance and reflectance of the unit cell by adjusting the size of the unit cell includes:

[0019] Given a preset millimeter-level value for the width b of the unit cell, continuously calculate the values of the length a and height c through simulation, such that the transmittance is as low as possible and ensure that the sum of the squares of the transmittance and the reflectance is 1. Optimize through simulation and finally output the size of the unit cell.

[0020] Furthermore, arrange the designed unit cells in a row along their front-back direction at a preset interval to form a column of supercells, and the overall length of the supercells is the same as the side length of the detection area. Then, copy another column of supercells along the width direction of the formed column of supercells to form a metasurface.

[0021] Furthermore, the signal acquisition devices arranged on the test plate to be detected include: a piezoelectric actuator, a laser vibrometer, an oscilloscope, a robotic arm, a power amplifier, and a data collector, where:

[0022] The piezoelectric actuator is arranged on one side of the detection area. The oscilloscope is used to modulate the excitation signal of the Lamb wave, and after amplifying the excitation signal by the power amplifier, apply it to the piezoelectric actuator to generate the Lamb wave propagating in the test plate to be detected. The laser probe of the laser vibrometer is used to detect the displacement amplitude response signal of the Lamb wave at the laser irradiation position, where the laser irradiates on the test plate to be detected between the two columns of supercells. Control the spatial position of the laser vibrometer through the robotic arm to adjust the laser irradiation position. The displacement amplitude response signal is collected by the data collector and sent to the upper computer for subsequent Fourier transform processing.

[0023] Furthermore, after the piezoelectric actuator excites an excitation signal, Lamb waves are generated; the form of the excitation signal is as follows:

[0024] ;

[0025] where t is the time parameter, is the center frequency of the excitation signal, and e is the natural constant; is used to control the length of the excitation signal and its meaning is that the part greater than is set to 0.

[0026] Furthermore, based on the comparison of the detection data and the baseline data, it is determined whether there is early damage to the test plate to be detected, including:

[0027] Compare the detection data of the metal plate to be detected with the baseline data of the healthy metal plate. If the amplitude at the 2f frequency in the detection 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 detection area; otherwise, there is no early damage; where 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 non-destructive testing method based on nonlinear guided waves of an 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 non-destructive testing method based on nonlinear guided waves of an elastic wave metasurface is implemented.

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

[0031] 1. Signal enhancement and noise reduction: By suppressing the center frequency guided wave in the confined state and amplifying the nonlinear signal in the enhanced state, the signal-to-noise ratio is significantly improved, and the backend signal processing flow is simplified.

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

[0033] 3. Wide-range application: The structure is small and can be attached to the surface of the sample to be detected, enabling wide-range and rapid detection and providing a new research idea for damage location.

[0034] 4. Quantitative evaluation: By correlating the nonlinear signal intensity with the response of the multi-point compact metasurface array, the quantification of the damage degree and the analysis of the spatial distribution are realized. Description of the Drawings

[0035] Figure 1 Model diagram of a single cell in an embodiment of the present invention;

[0036] Figure 2 Schematic diagram of the spacing between single cells when two columns of supercells are arranged in an embodiment of the present invention;

[0037] Figure 3 Transmittance field diagram of a single cell in an embodiment of the present invention;

[0038] Figure 4 Schematic diagram of the arrangement of signal acquisition devices in an embodiment of the present invention;

[0039] Figure 5 Transmittance result diagram when the width b of a single cell in an embodiment of the present invention is 3 mm. The left figure shows the case of f = 50 kHz, and the right figure shows the case of f = 100 kHz;

[0040] Figure 6 Parametric scan result of the distance n between two columns of supercells when two columns of supercells are arranged in an embodiment of the present invention;

[0041] Figure 7 Normalized energy field diagram of enhanced state simulation in an embodiment of the present invention. The left figure shows the case of f = 50 kHz, and the right figure shows the case of f = 100 kHz;

[0042] Figure 8 Energy amplitude comparison diagram of a metal aluminum plate during simulation in an embodiment of the present invention. The left figure shows the energy amplitude of a healthy metal aluminum plate, and the right figure shows the energy amplitude of a damaged metal aluminum plate;

[0043] Figure 9 Detection data obtained between supercells on a metal aluminum plate in an embodiment of the present invention. The left figure shows the change of amplitude with time, and the right figure shows the change of Fourier transform result with time;

[0044] Figure 10 Experimental results of enhanced state and confined state of metasurface in an embodiment of the present invention. The left figure shows the experimental results of taking points on a line set on the metasurface, and the right figure shows the experimental results of taking points on a line set between supercells;

[0045] Figure 11 Out-of-plane displacement field of this point under the extracted excitation signal in an embodiment of the present invention;

[0046] Figure 12 Normalized result after Fourier transform of the out-of-plane displacement field in an embodiment of the present invention;

[0047] Figure 13This is a control experiment diagram of healthy and damaged metal aluminum plates in an embodiment of the present invention. The left figure shows the damaged metal aluminum plate, and the right figure shows the healthy metal aluminum plate;

[0048] Figure 14 This is the result after normalization processing of the out-of-plane displacement amplitude signal in an embodiment of the present invention after Fourier transform;

[0049] Figure 15 This is a comparison experiment diagram of damaged aluminum plates with and without a metasurface in an embodiment of the present invention. The left figure shows the damaged metal aluminum plate, and the right figure shows the healthy metal aluminum plate;

[0050] Figure 16 This is the result after normalization processing of the out-of-plane displacement amplitude signal in an embodiment of the present invention after Fourier transform. Detailed implementation manners

[0051] When using the nonlinear guided wave detection technology to detect microcrack damage, the key lies in how to quickly and effectively separate the high-order harmonic signals generated by the nonlinear effect from the fundamental wave signals. The emergence of metamaterials and metasurfaces provides a new idea for solving this problem. As the "sensor" for enhancing guided wave signals, they can significantly improve the signal-to-noise ratio of nonlinear signals and show great superiority compared with traditional nonlinear detection technologies. As the two-dimensional planar form of elastic metamaterials, elastic metasurfaces exhibit many extraordinary physical properties that cannot be achieved by natural medium materials due to their unique microstructural design, such as negative Poisson's ratio, negative stiffness, zero shear modulus, multi-stability, etc., which are of revolutionary significance for improving the performance of traditional materials.

[0052] The present invention provides a nonlinear guided wave non-destructive testing method based on an elastic wave metasurface. By designing a metasurface on the surface of the test plate, two supercells in the metasurface can be used to greatly amplify the Lamb wave signal affected by early micro-nano scale damage of the material on a millimeter-scale structure.

[0053] Among them, the metasurface includes two columns of meta-cells, each column of meta-cells is composed of unit cells, the unit cell is a cuboid structure, and the material is the same as that of the test plate; the metasurface is attached to one side of the preset detection area on the test plate, and a piezoelectric actuator is arranged on the other side; a Lamb wave is generated by the preset excitation signal through the piezoelectric actuator; through the research of the inventor team, it is found that when the test plate is in a healthy state without damage, there is no non-linear second harmonic in the displacement amplitude response signal collected during the propagation of the Lamb wave, or the second harmonic is very weak; when there is damage in the test plate, the metasurface structure can effectively amplify the second harmonic at this time, and the displacement amplitude signal collected between the two columns of meta-cells has a significant amplification at the frequency of 2f (f is the center frequency of the excitation signal) compared with the test plate in the healthy state; and the greater the peak value, the more serious the early damage of the test plate. Based on this principle, this solution can use the metasurface to achieve non-destructive testing of the test plate to be detected; the material of the test plate can be metal, composite material, etc.

[0054] The method of the present invention effectively simplifies the complexity of signal processing in non-linear non-destructive testing, makes non-destructive testing more efficient and fast, and at the same time reduces the testing cost, providing a feasible solution for the wider application of non-linear non-destructive testing. The following will specifically describe the implementation process of the present invention with reference to the accompanying drawings.

[0055] A non-linear guided wave non-destructive testing method based on an elastic wave metasurface provided by the present invention includes the following steps:

[0056] Arrange a metasurface on one side of the preset detection area on the surface of the test plate to be detected, and arrange a piezoelectric actuator on the other side of the detection area; generate a Lamb wave through the piezoelectric actuator, collect the displacement amplitude response signal on the test plate to be detected between the two columns of meta-cells included in the metasurface, and use it as detection data after Fourier transform;

[0057] After setting the detection area, arranging the metasurface and the piezoelectric actuator on the healthy test plate in the same way as the test plate to be detected, generate a Lamb wave and collect the displacement amplitude response signal on the healthy test plate between the meta-cells, and use it as baseline data after Fourier transform;

[0058] Based on the comparison of the detection data and the baseline data, determine whether there is early damage to the test plate to be detected; among them, the test plate to be detected 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 specified area on the test plate to be detected, such as Figure 4 As shown, in this example, the detection area is a rectangular area, which can be adjusted according to actual needs.

[0059] 1. Metasurface.

[0060] See Figure 1 and Figure 2, the metasurface in the present invention consists of two columns of supercells, each column of supercells is composed of a single-cell arrangement, and each single cell is a cuboid structure; the design process of the single cell is as follows:

[0061] 1.1 First, determine the wavelength of Lamb waves in the material according to the material density, Poisson's ratio, Young's modulus of the test plate to be detected, as well as the size and thickness of the material, etc. ; The determination of this wavelength can be obtained by COMSOL modeling and simulation.

[0062] 1.2 In the case of a known wavelength , calculate the transmittance and reflectance of the single cell:

[0063] ;

[0064] ;

[0065] Among them, first arrange a single single cell on the test plate to be detected, with the left side of the single cell facing the detection area; set two straight lines s1 and s2 at positions with distances l1 and l2 from the left edge of the test plate to the left side of the single cell, where l2 > l1; set a straight line s3 at a position with a distance l3 from the right edge of the test plate to the right side of the single cell; then apply a preset out-of-plane excitation at the left edge of the test plate to be detected, and this out-of-plane excitation is a small perturbation force less than 10 -5 N; then the out-of-plane displacements at the straight lines s1 and s2 are respectively and , and the out-of-plane displacement at the straight line s3 is ; k is the wave number, , is the signal wavelength of the out-of-plane excitation, e is the natural constant, and i is the imaginary unit.

[0066] 1.3 Refer to Figure 3 , adjust the size (length a, width b, and height c) of the supercell to control the transmittance and reflectance of the single cell; refer to Figure 2 , the specific adjustment method is: give a preset value in millimeters for the width b of the single cell, such as 3 mm; in actual application, it can be set according to the size of the test plate, application scenario, etc.; continuously calculate the values of the length a and height c through simulation, so that the transmittance is as low as possible and ensure that the sum of the squares of the transmittance and reflectance is 1; optimize the final output of the size of the single cell through simulation.

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

[0068] 1.4 Arrange the designed unit cells in a column along their front-back direction at a preset interval H to form a column of supercells; the overall length of the supercell is the same as 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 copy another column of supercells along the width direction of the formed column of supercells to form a metasurface; among them, the distance n between the two columns of supercells is related to the frequency and wavelength of the Lamb wave, and can be optimized by parametric scanning of this distance n through COMSOL simulation; that is, during the optimization process, the second harmonic amplification effect of the Lamb wave in the two columns of supercells is made as obvious as possible.

[0069] When only one column of supercells is set, this column of supercells has the ability to isolate the Lamb wave; while when two columns of supercells are set, the second harmonic can be effectively amplified between the supercells.

[0070] 2. Arrangement of the signal acquisition device.

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

[0072] The piezoelectric actuator (a piezoelectric wafer can be used) is arranged on one side of the detection area; the oscilloscope is used to modulate the excitation signal of the Lamb wave , and after amplifying this excitation signal by the power amplifier, apply it to the piezoelectric actuator to generate a Lamb wave propagating in the test plate to be detected; the laser probe of the laser vibrometer is used to detect the displacement amplitude response signal of the Lamb wave at the laser irradiation position; among them, the laser irradiates the test plate to be detected between the two columns of supercells; the spatial position of the laser vibrometer is controlled by the robotic arm to adjust the laser irradiation position; the displacement amplitude response signal is collected by the data collector and sent to the upper computer for subsequent Fourier transform and other processing.

[0073] In this solution, after the piezoelectric actuator excites the excitation signal modulated by the Hanning window, a Lamb wave is generated and transmitted to the metasurface side; then a displacement amplitude response signal along the thickness direction can be collected on the test plate to be detected; in one embodiment of the present invention, the designed excitation signal is as follows:

[0074] ;

[0075] where t is the time parameter, is the center frequency of the excitation signal, and e is the natural constant; used to control the length of the excitation signal, which means that the part greater than is set to 0.

[0076] 3. Damage detection.

[0077] In this solution, first, a detection area needs to be set on a healthy test plate, and then a metasurface structure (bonded by epoxy glue) and signal acquisition equipment are arranged, and the displacement amplitude response signal between supercells is collected. After Fourier transform, it is used as baseline data. In actual application, a test plate to be detected of the same type as the healthy test plate is selected, and after setting the detection area at the same position, the metasurface and signal acquisition equipment are arranged. At this time, the displacement amplitude response signal obtained is used as detection data after Fourier transform.

[0078] For the test plate to be detected with damage, in the case where the metasurface structure is not arranged, although the detection data collected will generate second harmonics, this non-linear second harmonic is very weak and submerged in the noise signal, making it difficult to determine whether it exists, thus affecting the determination of whether there is early damage.

[0079] In this solution, after the design of the metasurface, the detection data of the test plate to be detected and the baseline data of the healthy test plate are compared. If the amplitude at the 2f frequency in the detection 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 detection area of the test plate to be detected; otherwise, there is no early damage, thus completing the non-destructive testing of the test plate to be detected.

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

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

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

[0083] When the thickness of the metal aluminum plate is 1 mm, the wavelength is calculated to be 11.488 mm in COMSOL, and the transmittance of the unit cell is calculated at this wavelength; the transmittance calculation formula mentioned above can be input into COMSOL for calculation. The transmittance is affected by the unit cell sizes a, b, and c. By parametric scanning of the unit cell sizes a and c, similarly, the same operations can also be performed on the sizes a and b and b and c; in Figure 3The transmittance field diagram plotted from the COMSOL-calculated transmittance results with the change of unit cell sizes a and c when the unit cell width b is 3 mm is shown. It is noted that the larger the unit cell size, the lower its transmittance. However, considering the compactness and small size of the unit cell, this solution hopes to achieve a similar effect with a smaller unit cell size. As Figure 5 The transmittance result diagram of the unit cell when the width b = 3 mm is shown. The unit cell size with a lower transmittance is selected, that is, a = 6 mm and c = 7 mm.

[0084] The finite element simulation software is used to verify the finite element simulation of the metal aluminum plate and the metasurface; when 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 detection area of the metal aluminum plate. For a single row of supercells, the point source excitation method can be used to ensure that the excitation is incident at various angles in the plane. From Figure 5 the field diagram, it can be obtained that the single-layer metasurface supercells have a high reflection for signals of 50 kHz and 100 kHz, which demonstrates the ultra-wideband isolation ability of the designed metasurface; as Figure 6 shown, when arranging two rows of supercells, the energy between the two layers of supercells is collected. By parametrically scanning the parameter n of the distance between the two rows of supercells, the change of energy with n at 50 kHz and 100 kHz can be obtained; when n = 11.66 mm, at Figure 5 100 kHz of has four orders of magnitude higher energy than 50 kHz; therefore, when the distance between the two rows of supercells is, the enhanced state effect has a very obvious enhancement for the target frequency and no enhancement effect on the center frequency, and thus the separation of the center frequency and the harmonic signal can be realized.

[0085] Figure 7 shows the result obtained from Figure 6 When n = 11.66 mm, the normalized energy field diagrams of the confined state effect of the double-layer arranged metasurface for the center frequency 50k and its harmonic 100k are simulated. At f = 100 kHz, a great energy enhancement is achieved in the double-layer metasurface supercells.

[0086] Nonlinearity caused by early material degradation is introduced in the COMSOL simulation for damage simulation. The nonlinearity of the material is introduced by giving the Murnaghan third-order elastic modulus in the hyperelastic material. At the initial stage of material degradation, the Murnaghan third-order elastic modulus will change violently. Therefore, the change of the Murnaghan third-order elastic modulus parameter can be used to simulate material damage; at the left edge of the metal aluminum plate, a Hann window modulated with a center frequency of 50 kHz is given. Wave packet signal. In the time-domain simulation, for a healthy metal aluminum plate, no obvious energy amplitude is seen in the middle of the metasurface under a given excitation, while for a damaged metal aluminum plate with added nonlinear conditions, an obvious energy amplitude can be seen. The specific display is as shown in Figure 8 ; in Figure 9 It shows that out-of-plane displacement is collected between double-layer supercells and Fourier transform is performed to monitor frequency information. Note that in the control simulation without introducing nonlinearity, the main peak of the collected wave packet signal after FFT is 50 kHz. However, after introducing nonlinearity, the nonlinear signal 2f at a frequency of 100 kHz has a significant enhancement after being sharpened and enhanced by the metasurface, with a numerical value of 43 times, greatly improving the signal-to-noise ratio of the nonlinear signal.

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

[0088] The size (length * width * thickness) of the metal plate is specifically 500 * 500 * 1 mm 3 , the material is aluminum, and its elastic modulus GPa, the Poisson's ratio is , and the density kg / m³. A total of 30 designed unit cells are used in this example, with a length of 6 mm, a width of 3 mm, and a height of 7 mm; the unit cells are pasted on the aluminum plate with epoxy glue, the unit cell spacing H is 6 mm, and the distance between two columns of supercells is 11.66 mm (there may be errors in actual pasting, which will cause the amplified frequency to shift).

[0089] In this embodiment, the excitation signal uses a mixed-frequency signal s(t2):

[0090] ;

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

[0092] In the detection area in front of the metasurface (such as at a position l away from the piezoelectric exciter in Figure 4 ), and between the supercells inside the metasurface (such as the middle line of two columns of supercells), a line is set respectively, and the displacement amplitude response signals of 180 points are taken on the two lines and the results after Fourier transform. In Figure 10 The first figure shows that f occupies the main part of the signal, and the signal of 2f is relatively weak. After being amplified by the metasurface, in Figure 10 The second figure shows that the energy at f is significantly weakened, while the signal at 2f has an obvious peak, 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 is introduced to verify the effectiveness of the metasurface in actual damage detection. A constant-temperature heating stage is used to heat a 10 cm * 10 cm area of a metal aluminum plate at a constant temperature of 400 °C for 4 hours to ensure that the material undergoes thermal aging. The change in the microstructure of this material will cause the material to exhibit a nonlinear response. At the same time, a drop hammer is used to impact the metal aluminum plate to simulate impact damage, and this damage will also change the original characteristics of the material and thus exhibit a nonlinear effect.

[0094] The experimental procedure is the same as above. Due to the error in pasting the metasurface, the center frequency of the excitation signal is 45 kHz. In this scheme, point A is taken between two columns of twins on the metasurface, and point B in the detection area between the metasurface and the piezoelectric exciter is used as the detection point to extract the out-of-plane displacement field at this point under the excitation signal. For details, see Figure 11 . Compared with the time-domain signal, this scheme pays more attention to the frequency-domain information after Fourier transform. Figure 12 Shows the normalized results of the Fourier transform of the out-of-plane displacement fields at points A and B. It can be seen from the line graph that before the metasurface, the second harmonic is very weak and submerged in the center frequency signal. After passing through the metasurface, a strange amplification effect occurs, and the amplification factor reaches about 25 times the original. Although the nonlinear signal is very weak, there is an obvious peak at the second harmonic. This significant peak can replace complex signal processing with passive regulation of the metasurface, improve the signal-to-noise ratio, and thus reflect the damage situation of the material.

[0095] Furthermore, 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 instruments and the metasurface itself, this scheme conducts a control experiment on healthy and damaged metal aluminum plates as Figure 13 shown. Using the method of controlling variables, under the same excitation signal, the out-of-plane displacement amplitude signals at the same point in the middle of the metasurface are collected, and after Fourier transform, normalization processing is carried out. The results are shown as Figure 14 shown. It can be seen that in the case of damage, there is an obvious peak at the second harmonic frequency of 90 kHz. For a normal metal aluminum plate (without damage treatment), no second harmonic can be seen under the center frequency excitation. By comparing the signals of the damaged metal aluminum plate and the undamaged metal aluminum plate in this scheme, it can be seen that the metasurface amplifies the nonlinear second harmonic of the damaged one by more than twenty times; the obvious amplification effect at the second harmonic frequency also verifies the role of the metasurface in accurately selecting the amplification of specific frequencies.

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

[0097] Under the condition that the experimental conditions and excitation signals remain the same, this scheme collects the out-of-plane displacement signals at the same point by a laser vibrometer and studies the frequency-domain information through Fourier transform; the normalization results are shown as Figure 16 shown. It can be obtained that when there are damages on the metal aluminum plate and there is a metasurface, there is an obvious amplification at the second harmonic frequency of the central frequency. The illustrated result has an amplification effect of fifty times near 90 kHz, and the signal enhancement at individual frequency points is about seventy times. This demonstrates the strange function of the metasurface in energy focusing and amplification. However, in the case of no metasurface structure, for the displacement amplitude response signal measured at the same point by this scheme, although it contains non-linear second harmonics, they are submerged in the noise signal. Without complex signal processing in the later stage, this scheme cannot distinguish the non-linear second harmonics from the noise and the signal at the central frequency.

[0098] The metasurface of this scheme can selectively amplify the weak non-linear second harmonics through structural design, while weakening the detection signal at the central frequency and amplifying the second harmonic frequency that reflects the early damage of the material, significantly improving the signal-to-noise ratio and greatly reducing the complexity of later signal processing.

[0099] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A non-destructive testing method for nonlinear guided waves based on elastic wave metasurfaces, characterized in that, include: A metasurface is arranged on one side of a detection area preset on the surface of the test plate to be detected, and a piezoelectric actuator is arranged 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; After setting the detection area, arranging the metasurface and piezoelectric actuator in the same way as the test plate to be tested, Lamb waves are generated and the displacement amplitude response signals on the healthy test plate between the supercells are collected and used as baseline data after Fourier transformation. Based on the comparison of the detection 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.

2. The non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface according to claim 1, wherein The supersurface 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.

3. The non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface according to claim 2, wherein The design method of the unit cell comprises: Determine the wavelength of Lamb waves in the material according to the material density, Poisson's ratio, Young's modulus of the test plate to be detected, and the size and thickness of the material ; Calculate the transmittance of the unit cell and reflectivity : ； ； Among them, first arrange a single unit cell on the test plate to be detected, with the left side of the unit cell facing the detection area; set two straight lines s1 and s2 at positions with distances l1 and l2 from the unit cell between the left side of the unit cell and the left edge of the test plate to be detected, where l2 > l1; set a straight line s3 at a position with a distance l3 from the unit cell between the right side of the unit cell and the right edge of the test plate to be detected; then apply a preset out-of-plane excitation at the left edge of the test plate to be detected, and this out-of-plane excitation is a small perturbation force less than 10 -5 N; then the out-of-plane displacements at the straight lines s1 and s2 are respectively and , and the out-of-plane displacement at the straight line s3 is ; k is the wave number, , is the signal wavelength of the out-of-plane excitation, e is the natural constant, and i is the 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.

4. The non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface according to claim 3, characterized in that, The transmittance and reflectivity of the unit cell can be controlled by adjusting the size of the unit cell, including: Given a preset value of one millimeter for the width b of the unit cell, the values of the length a and the height c are continuously calculated through simulation such that the transmittance is as low as possible and it is ensured that the sum of the squares of the transmittance and the reflectivity is 1; the size of the unit cell is finally output through simulation optimization.

5. The non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface according to claim 2, wherein The designed unit cells are arranged along the front-to-back direction at preset intervals to form a row of supercells, and the overall length of the supercell is consistent with the side length of the detection area; then the formed row of supercells is copied to another row of supercells along the width direction to form a hypersurface.

6. The non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface according to claim 1, wherein 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: The piezoelectric actuator is arranged on one side of the detection area; the oscilloscope is used to modulate the excitation signal of the Lamb wave, and the excitation signal is amplified by the power amplifier and applied to the piezoelectric actuator to generate the Lamb wave propagating in the test board to be detected; 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, wherein the laser is irradiated on the test board to be detected between two columns of super cells; the spatial position of the laser vibrometer is controlled by the mechanical arm to adjust the laser irradiation position; the displacement amplitude response signal is collected by the data acquisition device and sent to the host computer for subsequent Fourier transform processing.

7. The non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface according to claim 1, characterized in that The piezoelectric actuator generates a Lamb wave after exciting 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, and e is the natural constant; used to control the excitation signal length, whose meaning is that the part greater than is set to 0.

8. The non-destructive testing method for nonlinear guided waves based on an elastic wave metasurface according to claim 1, wherein Based on the comparison of the test data and the baseline data, determine whether the test board to be tested has early damage, including: Compare the detection data of the metal plate to be detected with the baseline data of the healthy metal plate. If the amplitude at the 2f frequency in the detection 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 detection area; otherwise, there is no early damage. Among them is the center frequency of the excitation signal.

9. 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 supersurface according to any one of claims 1 to 8.

10. A computer-readable storage medium storing a computer program therein; characterized in that, When the computer program is executed by a processor, the nonlinear guided wave nondestructive testing method based on the elastic wave supersurface according to any one of claims 1 to 8 is implemented.

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