Solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves

Abstract

The invention discloses a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves, and belongs to the technical field of nondestructive testing, the method comprises the following steps: S1, detection preparation; s2, selecting a fundamental frequency wave; s3, selecting a transducer; s4, signal excitation and receiving; s5, low-speed impact damage detection; and S6, data analysis and conclusion obtaining. By adopting the solid plate low-speed impact damage detection method based on the nonlinear ultrasonic guided waves, the design of the transducer is optimized, the flexibility of mode selection is improved, the frequency mixing area is accurately controlled, the specific mode guided waves are received bidirectionally, and the influence of other mode guided waves is reduced, so that the detection efficiency and accuracy are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of nondestructive testing, and in particular relates to a method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves. Background Art

[0002] Solid plate structures are widely used in the fields of shipbuilding, engineering applications, and aerospace. These structures are often subjected to low-speed impacts during use, such as bird strikes or accidental drops of maintenance tools during flight. Although this low-energy impact does not cause immediate obvious damage, it may cause small and hidden internal damage. Over time, these potential damages may gradually expand and eventually cause structural failure. Therefore, effective impact damage detection of solid plates is crucial to ensure the safety and reliability of the structure.

[0003] Traditional nondestructive testing methods include radiography, magnetic particle testing, and penetrant testing, but they are usually suitable for large-scale defect detection and have difficulty in detecting subtle early damage. In contrast, ultrasonic testing has become an ideal detection method due to its strong applicability, low cost, and high speed. In particular, Lamb waves, which can propagate over long distances in thin-walled structures without significant attenuation, are very suitable for detecting tiny defects in such structures.

[0004] Nonlinear ultrasonic testing uses the nonlinear effect of materials under stress to detect internal defects or changes. Compared with traditional linear ultrasound, nonlinear ultrasound can detect tiny defects more sensitively and provide higher detection accuracy. Although existing technologies have made some progress in the detection of low-speed impact damage to solid plates, they still face many challenges in terms of detection accuracy, experimental conditions, data analysis and application breadth. Therefore, a new solution is urgently needed. Summary of the invention

[0005] The purpose of the present invention is to provide a method for detecting low-speed impact damage of solid plates based on nonlinear ultrasonic guided waves. The method optimizes the transducer design, increases the flexibility of mode selection, accurately controls the mixing area, bidirectionally receives specific mode guided waves, and reduces the influence of other mode guided waves, thereby improving the detection efficiency and accuracy.

[0006] To achieve the above object, the present invention provides a method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves, comprising the following steps: S1. Test preparation; Aluminum alloy plates with damage defects were made, and a testing platform was built with the RAM-5000 SNAP nonlinear ultrasonic testing system as the main body; S2, select the fundamental frequency wave; Use software to draw the Lamb wave dispersion curve of the aluminum alloy plate, and obtain the phase velocity and group velocity of the fundamental wave according to the dispersion curve; calculate the location of the mixing area according to the selected fundamental wave; S3, select transducer; The transducer comprises an exciting transducer and a receiving transducer, wherein the exciting transducer adopts a wedge transducer and the receiving transducer adopts a piezoelectric chip comb transducer; S4, signal excitation and reception; Select aluminum alloy plates without damage defects for testing, first conduct a single excitation test, then conduct a joint excitation test, and record the excitation test data of aluminum alloy plates without damage defects; S5, low-speed impact damage detection; Select the aluminum alloy plate with damage defects in S1, set at least one sampling point on the aluminum alloy plate with damage defects, repeat the individual excitation test and the common excitation test in S4 for the sampling point, and record the excitation test data of the sampling point; S6. Data analysis and conclusions obtained; The excitation detection data of the aluminum alloy plate without damage defects in S4 is compared with the excitation detection data of the sampling point in S5. If it is found that the amplitude at a specific frequency increases significantly, it indicates that the aluminum alloy plate has low-speed impact damage at the sampling point.

[0007] Preferably, in S2, the fundamental frequency wave is controlled to produce a mixing effect at a specified position by adjusting the excitation time difference.

[0008] Preferably, in S2, the fundamental frequency wave is screened out by non-zero energy flow criterion and phase velocity matching.

[0009] Preferably, in S2, the frequencies of the two selected fundamental waves are 1.1 MHz S 0 mode and 2.78MHz A 1 model.

[0010] Preferably, in S3, a ruler is used to control the wedge transducer and the piezoelectric chip comb transducer to be in a straight line and maintain a fixed distance between them.

[0011] Preferably, in S3, the piezoelectric chip comb transducer includes a comb wedge, the fingers of the comb wedge have the same length, and the period length of the comb wedge is equal to the wavelength of the specific mode Lamb wave.

[0012] Preferably, in S3, according to 2.78MHz A 1 mode with 1.1MHz S 0 The difference frequency of the mode, the piezoelectric comb transducer is selected to be 1.7MHz A 0 mode as the receive mode.

[0013] Therefore, the present invention adopts the above-mentioned solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves. Compared with the prior art, the present invention has the following significant beneficial effects: (1) The present invention increases the range of mode selection by optimizing the transducer design, providing greater flexibility for selecting wave modes for specific application scenarios; (2) The present invention can accurately control the position of the mixing area, thereby achieving accurate positioning of local damage; (3) The piezoelectric chip comb transducer used in the present invention can effectively receive ultrasonic guided waves of a specific mode while reducing the influence of guided waves of other modes; (4) The present invention can not only cover a larger inspection area, but also identify multiple defects at the same time, thus improving the inspection efficiency and reducing the possibility of omissions; (5) The transducer design adopted by the present invention can simultaneously receive guided waves from two opposite directions while reducing the influence of guided waves in other modes; (6) The present invention adopts an improved transducer design to construct a relatively simple experimental platform, reducing the complexity and cost of the equipment.

[0014] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A front view of a damaged metal plate in a method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves according to the present invention; Figure 2 A back view of a damaged metal plate in a method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves according to the present invention; Figure 3 This is a Lamb wave phase velocity and group velocity dispersion curve of a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention. Figure 3 a in it represents the Lamb wave phase velocity dispersion curve; Figure 3 b in the figure represents the Lamb wave group velocity dispersion curve; Figure 4 A schematic diagram of a transducer for a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention; Figure 5 It is a schematic diagram of the structure of a piezoelectric chip comb transducer of a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention; Figure 6 It is a schematic diagram of the comb wedge structure of a piezoelectric chip comb transducer in a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention; Figure 7 It is a schematic diagram of the phase-colinear mixing of a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention; Figure 8 The present invention is a method for detecting low-speed impact damage of solid plates based on nonlinear ultrasonic guided waves, which is a single excitation 1.1 MHz S 0 The time domain waveform and frequency domain diagram of the Lamb wave of the mode, Figure 8 a in the figure represents the time domain waveform. Figure 8 The b in the figure represents the frequency domain diagram; Fig. 9 The present invention is a method for detecting low-speed impact damage of solid plates based on nonlinear ultrasonic guided waves, which is a single excitation 2.78MHz A 1 The time domain waveform and frequency domain diagram of the Lamb wave of the mode, Fig. 9 a in the figure represents the time domain waveform. Fig. 9 The b in the figure represents the frequency domain diagram; Fig.10 The figure is a time domain waveform diagram and a frequency domain diagram of Lamb waves of two modes excited together in a method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves of the present invention. Fig.10 a in the figure represents the time domain waveform. Fig.10 The b in the figure represents the frequency domain diagram; Fig.11 A schematic diagram of detecting low-velocity impact damaged aluminum alloy plates according to a method for detecting low-velocity impact damage of solid plates based on nonlinear ultrasonic guided waves of the present invention; Fig.12 It is a frequency domain diagram of the common excitation detection at the fourth point of a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention; Fig.13 This is a frequency domain diagram of the single excitation detection at the 4th point of a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention. Fig.13 The a in the formula represents 1.1MHz S 0 Frequency domain diagram of the mode Lamb wave, Fig.13 The b in the code indicates 2.78MHzA 1 Frequency domain diagram of the mode Lamb wave; Fig.14 It is a frequency domain diagram of the common excitation detection at the 5th point of a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention; Fig.15 This is a frequency domain diagram of the fifth point of single excitation detection in a solid plate low-speed impact damage detection method based on nonlinear ultrasonic guided waves of the present invention. Fig.15 The a in the formula represents 1.1MHz S 0 Frequency domain diagram of the mode Lamb wave, Fig.15The b in the code indicates 2.78MHzA 1 Frequency domain plot of the modal Lamb wave.

[0016] Reference numerals 1. Comb-shaped wedge. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used in the present invention should be the common meaning understood by people with general skills in the field to which the present invention belongs.

[0018] Embodiment 1 A method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves of the present invention comprises the following steps: S1. Test preparation; The present invention selects 6061 aluminum alloy plate as the detection object. This material is widely used in the fields of aerospace, automobile manufacturing, etc., and has good mechanical properties and processability.

[0019] In order to damage the 6061 aluminum alloy plate, a drop hammer impact system was built, which includes a punch (for impact), a high-speed camera (to record the impact process), a data acquisition device (to collect parameters such as impact force), and a control platform (to ensure the stability of the impact process). The drop hammer impact system generates impact energy by making the punch perform free fall motion at a specified height. Ignoring air resistance, the impact energy is equal to the potential energy of the punch height. The standard fixture ensures the stability of the impact process and eliminates secondary impact interference. The data uploaded to the data acquisition device by the PCB pressure sensor is used to calculate relevant parameters such as impact velocity and impact energy.

[0020] like Figure 1-Figure 2 As shown in the figure, 15J impact energy was used to impact the 6061 aluminum alloy plate. After the impact, there was no obvious crack on the surface, but there may be slight damage inside. Visual inspection was performed to confirm the front and back of the aluminum alloy plate to ensure that the impact damage met the test expectations.

[0021] Build a SNAP system, and use the RAM-5000 SNAP nonlinear ultrasonic test system as the main body to build a detection platform for the excitation and reception of Lamb wave mixing signals.

[0022] S2, select the fundamental frequency wave; Through the second-order perturbation method and mode expansion method, the nonlinear interaction of Lamb waves is theoretically analyzed, and it can be obtained that the displacement field of the mixing wave is a combination of a series of propagation modes with frequencies equal to the sum and difference of the frequencies of the two fundamental waves. The specific calculation method is as follows: ; in, z Direction is the direction of wave propagation, y The direction is perpendicular to the propagation direction and parallel to the plate thickness direction. t For time, and are the frequencies of the two fundamental waves, express m The wave structure of the order propagation mode, express m Amplitude of the first-order propagation mode.

[0023] Satisfies the following expression: ; in, is the composite energy flow of sum frequency and difference frequency modes, is the nonlinear surface driving force, is the nonlinear body driving force, and are the wave numbers of the fundamental frequency wave respectively.

[0024] when hour, It can be expressed as: ; The non-zero energy flow criterion is , the phase velocity matching condition is .

[0025] In order to ensure the existence of the mixing wave, the non-zero energy flow criterion and the phase velocity matching condition must be met at the same time. According to these two criteria, the appropriate excitation signal mode pair is selected. 0 Mode and A 1 The pattern is used as the excitation signal pattern pair.

[0026] like Figure 3 As shown in the figure, the Lamb wave dispersion curve of 6061 aluminum alloy plate is plotted using Dispersion Calculator software. The dispersion curve shows the variation of phase velocity and group velocity with frequency in different modes. The phase velocity and group velocity at a specific frequency can be read from the dispersion curve. The frequencies of the two selected fundamental waves are 1.1MHz (S 0 mode) and 2.78MHz (A 1mode). According to the dispersion curve, the corresponding group velocity values ​​are found at these two frequencies, 1.1MHz S 0 The theoretical group velocity of the mode is about 4990.4m / s, 2.78MHz A 1 The theoretical group velocity of the mode is about 3658.48 m / s. Based on the group velocities of the two fundamental frequency waves, their meeting position in the solid plate is calculated.

[0027] By controlling the excitation time difference, the two fundamental frequency waves can meet at this specified position and produce a mixing effect. The transducer excitation time difference is achieved through the SNAP system. The SNAP system adjusts the period of the Hanning window so that the two fundamental frequency waves can arrive at the specified position (such as 127mm) at the same time, thereby producing a mixing effect.

[0028] S3, select transducer; like Figure 4 As shown in the figure, the transducer includes an excitation transducer and a receiving transducer. The excitation transducer uses a wedge transducer to excite Lamb wave signals of different frequencies. Its advantage is that it can effectively convert electrical signals into mechanical vibrations, thereby generating the required Lamb waves. The 5-cycle Hanning window modulated sine signal (A) with a center frequency of 2.78MHz on the right is selected. 1 mode), the left side is excited by a 5-cycle Hanning window modulated sinusoidal signal with a center frequency of 1.1 MHz (S 0 mode). The two frequencies are chosen based on their propagation characteristics in the 6061 aluminum alloy plate and the mixing effects they produce.

[0029] like Figure 5-Figure 7 As shown, the receiving transducer uses a piezoelectric chip comb transducer to receive the mixed Lamb wave signal. The piezoelectric chip comb transducer consists of a comb wedge 1 and a piezoelectric chip bonded to the wedge. The comb wedge 1 consists of a series of parallel arranged fingers, and there is a certain interval between these fingers. By designing the finger width and the finger interval, the reception of a specific mode of Lamb wave is achieved. The piezoelectric chip can convert mechanical vibrations into electrical signals or vice versa. Due to the geometric symmetry of the piezoelectric chip comb transducer and the uniform spacing between the fingers, it can receive both forward propagating and reversely reflected guided waves. The structure of the piezoelectric chip comb transducer includes a piezoelectric chip, a bonding layer and a comb wedge 1. The geometric parameters of the comb wedge 1 include the finger length (aperture) W , refers to the number of bars n , cycle length L , finger width a and inter-finger spacing b , where the period length .

[0030] Comb wedge 1 each finger lengthW Due to the periodicity of the finger arrangement, the phase difference of the Lamb waves excited and received by adjacent finger electrodes It can be calculated by the following formula: ; in, is the angular frequency, For time, is the phase velocity.

[0031] Based on the output vector and method, when the phase difference between adjacent fingers is The total output reaches its maximum value when certain conditions are met. Specifically, when the phase difference When , the total output reaches the maximum value. Substituting the above phase difference formula, we get: ; in, is the wavelength, is the base frequency.

[0032] Therefore, the basic condition that the piezoelectric chip comb transducer needs to meet for receiving a specific mode of Lamb wave is that the period length L of the piezoelectric chip comb wedge 1 is consistent with the wavelength of the specific mode of Lamb wave. The design of this transducer allows it to receive Lamb waves of a specific mode and can select different modes as needed. By adjusting the period length (L) of the comb wedge 1 to be equal to the wavelength of the Lamb wave of a specific mode, effective excitation and reception of different modes can be achieved. The period length L of the piezoelectric chip comb transducer selected in this embodiment is 1.52mm.

[0033] The excitation mode is the symmetric mode S 0 (1.1MHz) and antisymmetric mode A 1 (2.78MHz). In order to receive the mixed signals of these modes, especially the difference frequency signal of the excitation frequency (1.68MHz), it is necessary to select a receiving mode. The piezoelectric chip comb transducer with a section length of 1.52mm is in the antisymmetric A 0 The frequency in the mode is 1.7MHz, which is very close to the difference frequency signal of the excitation frequency (1.68MHz), so the antisymmetric A is selected. 0 mode as the receive mode.

[0034] In order to ensure that the two Lamb waves propagate on the same path and meet at the predetermined position and mix, the relative position between the wedge transducer and the piezoelectric chip comb transducer needs to be precisely controlled. A 300mm ruler is used to control the wedge transducer and the piezoelectric chip comb transducer to always be in a line, and this length is used to determine the distance between the two wedge transducers, thereby ensuring the realization of two-way reception and maintaining high detection accuracy even in complex environments.

[0035] S4, signal excitation and reception; Choose a 395.5mm piece with a good surface and no damage inside or outside. 345mm 1mm 6061 aluminum alloy plate, single excitation test. First single excitation 1.1MHz S 0 The Lamb wave of the mode is then excited separately by the 2.78MHz A 1 The Lamb wave of the mode is recorded and the time domain and frequency domain graphs displayed by the oscilloscope are recorded.

[0036] like Figure 8-Figure 9 As shown, the 1.1 MHz S 0 The Lamb wave of the mode has obvious peaks only at the fundamental wave 1.1MHz and its harmonics (2.2MHz, 3.3MHz). 1 The Lamb wave of the mode has only an obvious peak at the fundamental wave of 2.73MHz, and no other additional frequency components appear.

[0037] Then conduct a joint excitation test, keep the transducer position unchanged, and excite the two frequencies of Lamb waves at the same time, and observe whether the difference frequency number appears when the difference frequency is 1.68MHz in the frequency domain diagram. Fig.10 As shown, when the aluminum alloy plate is undamaged, the frequency domain diagram has no obvious change at 1.68MHz, indicating that there is no mixing effect at this time.

[0038] S5, low-speed impact damage detection; First, arrange the sampling points, and arrange multiple sampling points from top to bottom at intervals of 10 mm on the aluminum alloy plate with damage defects. Then obtain the mixing signal, and repeat the individual and common excitation process in S4 for each sampling point, and ensure that the position of the transducer remains unchanged in each test, so that the low-speed impact damage is always within the Lamb wave mixing area.

[0039] like Fig.11As shown in the figure, a schematic diagram of the detection of an aluminum alloy plate damaged by low-speed impact is shown. There is a pit of about 10 mm in size on the aluminum alloy plate, which represents the damage caused by low-speed impact. In order to perform detailed Lamb wave mixing detection, 9 sampling points are set from top to bottom at intervals of 10 mm, numbered 1 to 9. Among them, the 4th and 5th points are located in the damaged area. Since the damage occurs between the 4th and 5th points, the mixing signals of these two points are mainly concerned.

[0040] S6. Data analysis and conclusions obtained; The data obtained from the SNAP system was imported into the origin software for processing, and the Lamb wave frequency domain waveform was plotted to compare the frequency domain characteristics with and without damage. The frequency domain diagrams of single excitation and simultaneous excitation were compared, especially focusing on the presence or absence of the difference frequency signal and its intensity change. If the amplitude at a specific frequency is found to increase significantly, it indicates that there is low-speed impact damage at that location. Based on the comprehensive test results, it is confirmed that the Lamb wave mixing effect can effectively detect low-speed impact damage in solid plates, and its sensitivity and accuracy are evaluated.

[0041] In order to verify whether the mixing signal does carry damage information, it is necessary to compare the data in the damaged case with the data in the undamaged case. The data at points 4 and 5 are compared with the frequency domain diagram of the Lamb wave mixing signal in the undamaged case.

[0042] like Figure 12-13 As shown in the figure, the frequency corresponding to the maximum amplitude of the mixing signal waveform at the fourth point marked is 1.66083MHz, and the amplitude is 1.229257. In contrast, at the same frequency, the single excitation 1.1MHz S 0 The amplitude of the modal is only 0.145837, while the single excitation 2.78MHz A 1 The amplitude in the modal state is even lower, only 0.015828. This indicates that in the damaged area (point 4), the two fundamental waves are mixed to produce a significantly enhanced difference frequency signal (1.66083MHz), which proves that the mixing defect signal is successfully received at this point.

[0043] like Figure 14-15 As shown in Figure 1, the data of point 5 are analyzed, and the tests of single excitation and common excitation are also performed. Although point 5 also shows the existence of mixed frequency defect signals, its amplitude is smaller than that of point 4. This difference may reflect the change of damage degree or location.

[0044] Therefore, the present invention adopts the above-mentioned method for detecting low-speed impact damage of solid plates based on nonlinear ultrasonic guided waves. This method optimizes the transducer design, increases the flexibility of mode selection, accurately controls the mixing area, bidirectionally receives specific mode guided waves, and reduces the influence of other mode guided waves, thereby improving detection efficiency and accuracy.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for detecting low-speed impact damage of solid plates based on nonlinear ultrasonic guided waves, characterized in that: The following steps are involved: S1. Test preparation; Aluminum alloy plates with damage defects were made, and a testing platform was built with the RAM-5000 SNAP nonlinear ultrasonic testing system as the main body; S2, select the fundamental frequency wave; Use software to draw the Lamb wave dispersion curve of the aluminum alloy plate, and obtain the phase velocity and group velocity of the fundamental wave according to the dispersion curve; calculate the location of the mixing area according to the selected fundamental wave; S3, select transducer; The transducer comprises an exciting transducer and a receiving transducer, wherein the exciting transducer adopts a wedge transducer and the receiving transducer adopts a piezoelectric chip comb transducer; S4, signal excitation and reception; Select aluminum alloy plates without damage defects for testing, first conduct a single excitation test, then conduct a joint excitation test, and record the excitation test data of aluminum alloy plates without damage defects; S5, low-speed impact damage detection; Select the aluminum alloy plate with damage defects in S1, set at least one sampling point on the aluminum alloy plate with damage defects, repeat the individual excitation test and the common excitation test in S4 for the sampling point, and record the excitation test data of the sampling point; S6. Data analysis and conclusions obtained; The excitation detection data of the aluminum alloy plate without damage defects in S4 is compared with the excitation detection data of the sampling point in S5. If it is found that the amplitude at a specific frequency increases significantly, it indicates that the aluminum alloy plate has low-speed impact damage at the sampling point.

2. The method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves according to claim 1, characterized in that: In S2, the fundamental frequency wave is controlled to produce a mixing effect at a specified position by adjusting the excitation time difference.

3. The method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves according to claim 1, characterized in that: In S2, the fundamental frequency wave is screened out by the non-zero energy flow criterion and phase velocity matching.

4. The method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves according to claim 3 is characterized in that: In S2, the frequencies of the two fundamental waves selected are 1.1 MHz S0 mode and 2.78 MHz A1 mode respectively.

5. The method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves according to claim 1, characterized in that: In S3, a ruler is used to control the wedge transducer and the piezoelectric crystal comb transducer to be in a straight line and to maintain a fixed separation distance.

6. The method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves according to claim 1, characterized in that: In S3, the piezoelectric chip comb transducer includes a comb wedge, the fingers of the comb wedge have the same length, and the period length of the comb wedge is equal to the wavelength of the specific mode Lamb wave.

7. The method for detecting low-speed impact damage of a solid plate based on nonlinear ultrasonic guided waves according to claim 4, characterized in that: In S3, according to the difference frequency between the 2.78 MHz A1 mode and the 1.1 MHz S0 mode, the piezoelectric chip comb transducer selects the 1.7 MHz A0 mode as the receiving mode.

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