A defect detection method and device based on multi-modal lamb waves

Abstract

The application relates to the fields of sensors and nondestructive testing, in particular to a defect detection method and device based on multi-modal Lamb waves. In order to solve the problem that the effective modal signal corresponding to defects is difficult to accurately identify due to the multi-modal characteristics of Lamb waves, a new defect detection method and device based on multi-modal Lamb waves are provided, which comprises the following steps: 1) simultaneously exciting two Lamb waves with different frequencies in a sample; 2) receiving the detection echo of the sample and forming a time domain signal diagram; 3) using a wavelet transform high-efficiency time-frequency analysis method to generate a time-frequency diagram; 4) selecting two frequency values, respectively constructing frequency straight lines parallel to the time axis, and taking the two frequency straight lines as the reference to perform a vertical direction slicing operation on the generated time-frequency diagram, and finally obtaining a slice diagram; 5) analyzing the slice diagram to obtain the position of the defects in the sample. The method can accurately identify and position the defects.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of sensors and non-destructive testing, in particular to a defect detection method and device based on multi-modal Lamb waves. BACKGROUND

[0002] Non-destructive testing technology accurately identifies internal defects of materials and structures through non-destructive techniques, providing core support for high-end manufacturing quality control, safe operation of industrial equipment, and infrastructure life assessment. Electromagnetic ultrasonic guided waves, as a new technology in the field of non-destructive testing, use the interaction of electromagnetic induction and ultrasonic effect to excite ultrasonic guided waves that propagate along the surface or internal structure of the material to detect possible defects in the material or structure. It has the advantages of high detection efficiency, ability to achieve large-scale rapid scanning through single-point excitation, easy operation, no need for coupling agent, and suitability for high-temperature environments.

[0003] Lamb waves, as a type of ultrasonic guided wave, are sensitive to defects such as delamination, cracks, and corrosion, and are widely used in non-destructive testing of various materials or structures, such as wing skin detection in the aerospace field, thin plate body structure detection in the automotive and rail transportation industries, and thin plate material detection in the electronics industry. However, the multi-modal characteristics of Lamb waves have been a key problem restricting their application and development. The multi-modal characteristics of Lamb waves mainly manifest in the simultaneous excitation of multiple modes, such as symmetric modes S0, S1 and anti-symmetric modes A0, A1, as shown in Figure 1 Due to the differences in speed and dispersion characteristics of different modes, as well as the non-uniformity of energy distribution, modal aliasing is severe, making it difficult to accurately identify defect information corresponding to effective modal signals, as shown in Figure 2 The traditional detection method described in Figure 2 excites a single frequency of Lamb waves, which further affects the reliability of defect positioning, quantification, and type judgment.

[0004] Existing technologies suppress non-target modes from the wave source end by optimizing the structural parameters of electromagnetic ultrasonic transducers (such as coil layout, permanent magnet configuration) or use signal processing algorithms to separate the existing multi-modal signals. However, optimizing the transducer parameters can only suppress a limited number of modes. For example, when Lamb waves simultaneously exist in A0 and S0 modes at low frequencies, suppressing one of them for non-destructive testing is not effective when multiple modes (more than three) exist simultaneously. Moreover, using algorithms to process multi-modal signals relies on accurate dispersion models and has high computational complexity, making it difficult to meet the needs of on-site rapid detection. SUMMARY

[0005] To solve the problem of accurate identification of effective modal signals corresponding to defects due to the multi-modal characteristics of Lamb waves, a new defect detection method and device based on multi-modal Lamb waves are provided.

[0006] The application is implemented by adopting the following technical scheme:

[0007] A defect detection method based on multi-modal Lamb waves comprises the following steps:

[0008] 1) simultaneously exciting two different frequency Lamb waves in a sample;

[0009] 2) receiving the detection echo of the sample and forming a time domain signal diagram;

[0010] 3) using a wavelet transform high-efficiency time-frequency analysis method to map the signal from the time domain to the time-frequency domain, and generating a time-frequency diagram showing the change of the signal frequency with time;

[0011] 4) selecting two frequency values respectively consistent with the two excitation frequencies in step 1), respectively constructing frequency straight lines parallel to the time axis, and taking the two frequency straight lines as the reference to perform a vertical direction (the vertical direction is parallel to the amplitude direction) slicing operation on the generated time-frequency diagram, and finally obtaining a slice diagram of the change of the modal signal amplitude with time;

[0012] 5) analyzing the slice diagram according to the feature that the A0 modal speed detected by different excitation frequencies is similar, so as to obtain the position of the defects of the sample.

[0013] Further, the two different frequency Lamb waves excited in step 1) are composed of a high-frequency Lamb wave and a low-frequency Lamb wave, and the high-frequency Lamb wave facilitates the detection of smaller defects.

[0014] Further, the frequency of the high-frequency Lamb wave is 1.13 MHz, and the frequency of the low-frequency Lamb wave is 510 KHz.

[0015] A defect detection device based on multi-modal Lamb waves comprises a double-frequency excitation sensor and a double-frequency receiving sensor, a double-channel ultrasonic flaw detector, and an upper computer.

[0016] The double-frequency excitation sensor is used for exciting two different frequency Lamb waves, and the double-frequency receiving sensor is used for receiving the detection echo, the double-frequency excitation sensor is connected with two excitation ports of the double-channel ultrasonic flaw detector, the double-frequency receiving sensor is connected with a receiving port of the double-channel ultrasonic flaw detector, and a communication port of the double-channel ultrasonic flaw detector is connected with the upper computer.

[0017] The upper computer is used for sending command signals to the double-channel ultrasonic flaw detector, receiving and storing digital signals sent by the double-channel flaw detector, and converting the digital signals into time domain signal graphs. Then, the wavelet transform high-efficiency time-frequency analysis means is used to map the signals from the time domain to the time-frequency domain, to generate a time-frequency graph that can clearly show the characteristics of the signal frequency changing with time. Finally, two excitation frequency values are selected to construct two frequency straight lines parallel to the time axis, and the generated time-frequency graph is sliced in the vertical direction (the vertical direction is parallel to the amplitude direction) with the two frequency straight lines as the reference, and finally the slice graph of the modal signal amplitude changing with time is obtained.

[0018] In use, the aluminum plate is used as a sample, the double-frequency excitation sensor and the double-frequency receiving sensor are placed in a linear type on the aluminum plate along the length direction of the aluminum plate, the double-channel ultrasonic flaw detector inputs two excitation electrical signals with different frequencies to the double-frequency excitation sensor, the double-frequency excitation sensor excites two Lamb waves with different frequencies in the aluminum plate, then the double-frequency receiving sensor receives the detection echo, and after amplification and filtering processing by the double-channel ultrasonic flaw detector, the signals are discretized into digital signals, and then the digital signals are transmitted to the upper computer. The upper computer converts the digital signals into time domain signal graphs and generates slice graphs reflecting the details of the marker modal signal changing with time according to the time domain signal graphs. The signal characteristics are extracted by analyzing the slice graphs, and the signal characteristics include the marker modal direct wave flight time and the defect echo flight time. Finally, the defect information in the straight line range of the two sensors on the aluminum plate is obtained through signal characteristic analysis (signal characteristic analysis belongs to the common knowledge of those skilled in the art, that is, the defect position is obtained by formula d=v×t q , where v=H / t z , v is the propagation speed of the marker modal, H is the distance between the double-frequency excitation sensor and the double-frequency receiving sensor, t z is the marker modal direct wave flight time, t q is the defect echo flight time, and d is the distance between the double-frequency excitation sensor and the defect). When the defect detection corresponding to the straight line position is completed, the double-frequency excitation sensor and the double-frequency receiving sensor are moved to another straight line position along the width direction of the aluminum plate, and the defect corresponding to the straight line position is detected, until the detection of all defects on the aluminum plate is completed.

[0019] Further, the double-frequency excitation sensor includes a double-pitch stacked collaborative coil and an excitation permanent magnet pressed thereon, and the double-pitch stacked collaborative coil includes two excitation folded coils with different pitch intervals (it is known to those skilled in the art that the folded coil described in the present application is a rectangular wave folded coil), and the two excitation folded coils are arranged in a stacked manner.

[0020] Further, the two excitation meander coils are stacked up and down through a PCB process, improving the space coupling efficiency.

[0021] Further, the dual-frequency excitation sensor comprises a first upper shell, a first lower shell, the first upper shell is a rectangular cover with an opening downward, and the inner side of the cover top is vertically fixed with a first pressing strip for pressing the excitation permanent magnet between the first upper shell and the first lower shell, the first lower shell is a rectangular box structure with an opening upward, the first upper shell and the first lower shell are provided with first connecting holes at four corners, and the first upper shell and the first lower shell are fixedly connected through the first fixing bolt screwed in the first fixing nut after passing through the first connecting holes of the first upper shell and the first lower shell, facilitating the integration and standardization of the sensor structure as a whole.

[0022] Further, the meander interval of the excitation meander coil in the upper layer is 2.5mm, and the meander interval of the excitation meander coil in the lower layer is 1.25mm.

[0023] Further, the dual-frequency receiving sensor comprises a receiving meander coil and an array magnet pressed on the receiving meander coil, the array magnet is composed of a plurality of receiving permanent magnets arranged side by side along the meander direction of the receiving meander coil, and the meander interval of the receiving meander coil is the least common multiple of the A0 mode wavelength under two excitation frequencies. The design of the meander interval of the receiving meander coil is to enable the receiving of guided wave signals of two frequencies at the same time.

[0024] Further, the dual-frequency receiving sensor comprises a second upper shell, a second lower shell, the second upper shell is a rectangular cover with an opening downward, and the inner side of the cover top is vertically fixed with a second pressing strip for pressing the array magnet between the second upper shell and the second lower shell, the second lower shell is a rectangular box structure with an opening upward, the second upper shell and the second lower shell are provided with second connecting holes at four corners, and the second upper shell and the second lower shell are fixedly connected through the second fixing bolt screwed in the second fixing nut after passing through the second connecting holes of the second upper shell and the second lower shell, facilitating the integration and standardization of the sensor structure as a whole.

[0025] The beneficial effects of the present application are as follows: the detection method and device of the present application ingeniously utilize the A0 mode speed proximity characteristic for defect detection, overcome the difficulty of accurately identifying the effective mode signal corresponding to the multi-mode characteristic of the Lamb wave and positioning the defect, and enable accurate positioning of the defect; at the same time, the detection method can still realize accurate identification and positioning of the defect even in the case of more than three modes of excitation Lamb wave, has less computing resource occupation, and can adapt to on-site rapid detection. BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings, which are incorporated herein and constitute part of the specification, illustrate embodiments consistent with the present application and, together with the description, further serve to explain the principles of the application.

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, those skilled in the art can further obtain other drawings from these drawings without any creative effort.

[0028] Figure 1 A schematic diagram of a frequency dispersion curve of a Lamb wave in a 3mm aluminum plate;

[0029] Figure 2 A time-domain signal diagram obtained by using a conventional detection method;

[0030] Figure 3 A flowchart of the detection method according to the present application;

[0031] Figure 4 A schematic diagram of the structure of a dual-frequency excitation sensor according to the present application;

[0032] Figure 5 A schematic diagram of the structure of a dual-frequency receiving sensor according to the present application;

[0033] Figure 6 A schematic diagram of the arrangement of the two sensors according to the present application when in use;

[0034] Figure 7 A time-domain signal diagram detected by the detection method according to the present application;

[0035] Figure 8 A time-frequency diagram obtained by transforming the detection signal according to the present application:

[0036] Figure 9 A slice diagram obtained by slicing the time-frequency diagram according to the present application.

[0037] In the figure: 1-aluminum plate, 2-dual-frequency excitation sensor, 3-dual-frequency receiving sensor, 4-defect, 5-first upper shell, 6-first pressing strip, 7-exciting permanent magnet, 8-dual-interval laminated cooperative coil, 9-first lower shell, 10-second upper shell, 11-second pressing strip, 12-array magnet, 13-receiving folded coil, 14-second lower shell, 15-marked modal direct wave, 16-marked modal defect echo. DETAILED DESCRIPTION

[0038] In order to enable a more clear understanding of the above-mentioned objects, features and advantages of the present application, the following will further describe the schemes of the present application. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0039] In the description, it should be noted that the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance. It should be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting" should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be the communication inside two elements. For those skilled in the art, the specific meanings of the above terms can be understood according to the specific circumstances.

[0040] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein; obviously, the examples in the description are only some of the embodiments of the present application, not all the embodiments.

[0041] The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0042] A defect detection method based on multi-modal Lamb waves, as shown in Figure 3 includes the following steps:

[0043] 1) simultaneously exciting two different frequency Lamb waves in the sample;

[0044] 2) receiving the detection echo of the sample and forming a time domain signal diagram;

[0045] 3) using wavelet transform high-efficiency time-frequency analysis means, mapping the signal from time domain to time-frequency domain, and generating a time-frequency diagram showing the change of signal frequency with time;

[0046] 4) selecting two frequency values respectively consistent with the two excitation frequencies in step 1), respectively constructing frequency straight lines parallel to the time axis, and taking the two frequency straight lines as the reference, performing vertical direction (the vertical direction is parallel to the direction of the amplitude of the time-frequency diagram) slicing operation on the generated time-frequency diagram, and finally obtaining a slice diagram of the change of modal signal amplitude with time;

[0047] 5) analyzing the slice diagram according to the characteristics of similar A0 modal speed detected by different excitation frequencies, so as to obtain the position of the defect 4 of the sample.

[0048] In specific implementation, the two different frequency Lamb waves excited in step 1) are composed of a high-frequency Lamb wave and a low-frequency Lamb wave, and the high-frequency Lamb wave facilitates detection of the small defect 4.

[0049] In specific implementation, the high-frequency Lamb wave has a frequency of 1.13 MHz, and the low-frequency Lamb wave has a frequency of 510 KHz.

[0050] A defect detection device based on multi-modal Lamb waves, comprising a dual-frequency excitation sensor 2 and a dual-frequency receiving sensor 3, a dual-channel ultrasonic flaw detector, and an upper computer;

[0051] The dual-frequency excitation sensor 2 is used to excite two different frequency Lamb waves, and the dual-frequency receiving sensor 3 is used to receive detection echoes. The dual-frequency excitation sensor 2 is connected to two excitation ports of the dual-channel ultrasonic flaw detector, the dual-frequency receiving sensor 3 is connected to a receiving port of the dual-channel ultrasonic flaw detector, and a communication port of the dual-channel ultrasonic flaw detector is connected to the upper computer.

[0052] The upper computer is used to send a command signal to the dual-channel ultrasonic flaw detector, receive and store a digital signal sent by the dual-channel flaw detector, and convert the digital signal into a time-domain signal graph. Then, a wavelet transform high-efficiency time-frequency analysis method is used to map the signal from the time domain to the time-frequency domain to generate a time-frequency graph that can clearly display the characteristics of the signal frequency change over time. Finally, two excitation frequency values are selected to construct frequency straight lines parallel to the time axis, and the generated time-frequency graph is operated in the vertical direction (the vertical direction is parallel to the amplitude direction) with the two frequency straight lines as the reference, and finally the slice graph of the modal signal amplitude change over time is obtained.

[0053] In specific implementation, as shown in Figure 4 The dual-frequency excitation sensor 2 comprises a dual-spacing stacked cooperative coil 8 and an excitation permanent magnet 7 pressed thereon. The dual-spacing stacked cooperative coil 8 comprises two excitation meander coils with different meander spacings (as known to those skilled in the art: the meander coil described in the present application is a meander coil arranged in a rectangular wave), and the two excitation meander coils are arranged in a stacked manner. The dual-spacing stacked cooperative coil 8 is used to input two different frequency excitation electric signals.

[0054] In specific implementation, the two excitation meander coils are stacked in a stacked manner through a PCB process, which improves the spatial coupling efficiency.

[0055] In specific implementation, the dual-frequency excitation sensor 2 comprises a first upper shell 5 and a first lower shell 9. The first upper shell 5 is a rectangular cover with an opening downward, and the inner side of the cover top is vertically fixed with a first pressing strip 6 for pressing the excitation permanent magnet 7 between the first upper shell 5 and the first lower shell 9. The first lower shell 9 is a rectangular box structure with an opening upward. The first upper shell 5 and the first lower shell 9 are both provided with first connecting holes at the corners. The first upper shell 5 and the first lower shell 9 are fixedly connected by the first fixing bolt and the first fixing nut.

[0056] In specific implementation, the fold interval of the excitation fold coil in the upper layer is 2.5 mm, and the fold interval of the excitation fold coil in the lower layer is 1.25 mm.

[0057] In specific implementation, as shown in the figure, Figure 5 The dual-frequency receiving sensor 3 comprises a receiving fold coil 13 and an array magnet 12 pressed on the receiving fold coil 13. The array magnet 12 is composed of a plurality of receiving permanent magnets arranged side by side along the fold direction of the receiving fold coil 13. The fold interval of the receiving fold coil 13 is the least common multiple of the A0 mode wavelength of the two excitation frequencies. The design of the fold interval of the receiving fold coil 13 is to enable the simultaneous reception of guided wave signals of the two frequencies.

[0058] In specific implementation, the dual-frequency receiving sensor 3 comprises a second upper shell 10 and a second lower shell 14. The second upper shell 10 is a rectangular cover with an opening downward, and the inner side of the cover top is vertically fixed with a second pressing strip 11 for pressing the array magnet 12 between the second upper shell 10 and the second lower shell 14. The second lower shell 14 is a rectangular box structure with an opening upward. The second upper shell 10 and the second lower shell 14 are both provided with second connecting holes at the corners. The second upper shell 10 and the second lower shell 14 are fixedly connected by the second fixing bolt and the second fixing nut.

[0059] In use, the dual-frequency excitation sensor 2 and the dual-frequency receiving sensor 3 are placed in line along the length direction of the aluminum plate 1 on the aluminum plate 1 as shown in the figure, Figure 6 The dual-channel ultrasonic flaw detector inputs excitation electrical signals of two different frequencies (1.13 MHz and 510 KHz, respectively) to the dual-frequency excitation sensor 2. The dual-frequency excitation sensor 2 excites Lamb waves of two different frequencies in the aluminum plate 1. Then, the dual-frequency receiving sensor 3 receives the detection echo. After amplification and filtering processing by the dual-channel ultrasonic flaw detector, the digital signal is discretized. The digital signal is transmitted to the upper computer. The upper computer converts the digital signal into a time domain signal graph as shown in the figure, Figure 7The time-frequency diagram is converted from the time-domain signal diagram as shown in Figure 8 Finally, the slice diagram reflecting the details of the change of the marker modal signal over time is generated as shown in Figure 9 The signal features are extracted by analyzing the slice diagram, including the marker modal direct wave 15 flight time and the marker modal defect echo 16 flight time, and finally the defect 4 information in the range of the straight line where the two sensors are located on the aluminum plate 1 is obtained through signal feature analysis (signal feature analysis belongs to the common knowledge of those skilled in the art, that is, the defect 4 position is obtained by formula d=v×t q , where v=H / t z , v is the propagation speed of the marker modal, H is the distance between the dual-frequency excitation sensor 2 and the dual-frequency receiving sensor 3, t z is the marker modal direct wave 15 flight time, t q is the marker modal defect echo 16 flight time, and d is the distance between the dual-frequency excitation sensor 2 and the defect 4), when the detection of the defect 4 corresponding to the straight line position is completed, the dual-frequency excitation sensor 2 and the dual-frequency receiving sensor 3 are moved to another straight line position along the width direction of the aluminum plate 1, and the detection of the defect 4 corresponding to the straight line position is performed, until the detection of all defects 4 on the aluminum plate 1 is completed.

[0060] Compared with the various modal wave packets in Figure 2 , the defect 4 information cannot be obtained because the defect echo wave packet cannot be identified, while the defect 4 information can be accurately detected and analyzed by Figure 9 .

[0061] The above is only a specific embodiment of the present application, which enables those skilled in the art to understand or implement the present application. Although detailed description is made with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solution deviate from the scope of the technical solutions of the embodiments, and they should be covered in the protection scope of the claims.

Claims

1. A method for defect detection based on multi-modal Lamb waves, characterized in that, It comprises the following steps: 1) simultaneously exciting two different frequency Lamb waves in the sample; 2) receiving the detection echo of the sample and forming a time domain signal diagram; 3) using wavelet transform high-efficiency time-frequency analysis means to map the signal from time domain to time-frequency domain, generating a time-frequency diagram showing the change of signal frequency with time; 4) selecting two frequency values respectively consistent with the two excitation frequencies in step 1), respectively constructing frequency straight lines parallel to the time axis, and taking the two frequency straight lines as the reference to perform vertical slicing operation on the generated time-frequency diagram, finally obtaining a slice diagram of the modal signal amplitude changing with time; 5) analyzing the slice diagram according to the characteristics of the A0 modal speed detected by different excitation frequencies, thereby obtaining the position of the sample defect (4).

2. The method of claim 1, wherein, The two different frequency Lamb waves excited in step 1) are composed of a high-frequency Lamb wave and a low-frequency Lamb wave.

3. The method of claim 2, wherein, The frequency of the high-frequency Lamb wave is 1.13 MHz, and the frequency of the low-frequency Lamb wave is 510 KHz.

4. A multi-modal Lamb wave based defect detection apparatus, characterized by, It comprises a dual-frequency excitation sensor (2) and a dual-frequency receiving sensor (3), a dual-channel ultrasonic flaw detector, and an upper computer. The dual-frequency excitation sensor (2) is used to excite two different frequency Lamb waves, and the dual-frequency receiving sensor (3) is used to receive detection echoes. The dual-frequency excitation sensor (2) is connected with two excitation ports of the dual-channel ultrasonic flaw detector, the dual-frequency receiving sensor (3) is connected with a receiving port of the dual-channel ultrasonic flaw detector, and a communication port of the dual-channel ultrasonic flaw detector is connected with the upper computer. The upper computer is used to send command signals to the dual-channel ultrasonic flaw detector, receive and store digital signals sent by the dual-channel flaw detector, and convert the digital signals into a time domain signal diagram. Then, using wavelet transform high-efficiency time-frequency analysis means, the signal is mapped from time domain to time-frequency domain to generate a time-frequency diagram that clearly shows the characteristics of the change of signal frequency with time. Finally, two excitation frequency values are selected to construct frequency straight lines parallel to the time axis, and the generated time-frequency diagram is vertically sliced as the reference to obtain a slice diagram of the modal signal amplitude changing with time.

5. The multi-modal Lamb wave based defect detection apparatus of claim 4, wherein, The dual-frequency excitation sensor (2) comprises a dual-spacing stacked collaborative coil (8) and an excitation permanent magnet (7) pressed thereon. The dual-spacing stacked collaborative coil (8) comprises two excitation backfolded coils with different fold spacings, which are arranged in a stacked manner. The dual-frequency receiving sensor (3) comprises a receiving backfolded coil (13) and an array magnet (12) pressed on the receiving backfolded coil (13). The array magnet (12) is composed of multiple receiving permanent magnets arranged side by side along the backfolding direction of the receiving backfolded coil (13). The fold spacing of the receiving backfolded coil (13) is the least common multiple of the A0 modal wavelengths at the two excitation frequencies. The two excitation backfolded coils are stacked in a stacked manner by PCB technology.

6. The multi-modal Lamb wave based defect detection apparatus of claim 5, wherein, The double-frequency excitation sensor (2) comprises a first upper shell (5) and a first lower shell (9). The first upper shell (5) is a rectangular cover with an opening downward, and the inner side of the cover top is vertically fixed with a first pressing strip (6) for pressing the excitation permanent magnet (7) between the first upper shell (5) and the first lower shell (9). The first lower shell (9) is a rectangular box structure with an opening upward. The first upper shell (5) and the first lower shell (9) are provided with first connecting holes at four corners. The first upper shell (5) and the first lower shell (9) are fixedly connected by first locking bolts and first locking nuts.

7. The multi-modal Lamb wave based defect detection apparatus of claim 6, wherein, The fold interval of the excitation fold coil on the upper layer is 2.5 mm, and the fold interval of the excitation fold coil on the lower layer is 1.25 mm.

8. The multi-modal Lamb wave based defect detection apparatus of claim 7, wherein, The double-frequency receiving sensor (3) comprises a second upper shell (10) and a second lower shell (14). The second upper shell (10) is a rectangular cover with an opening downward, and the inner side of the cover top is vertically fixed with a second pressing strip (11) for pressing the array magnet (12) between the second upper shell (10) and the second lower shell (14). The second lower shell (14) is a rectangular box structure with an opening upward. The second upper shell (10) and the second lower shell (14) are provided with second connecting holes at four corners. The second upper shell (10) and the second lower shell (14) are fixedly connected by second locking bolts and second locking nuts.

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