A metal plate thickness measurement method based on a hybrid system zero group velocity Lamb wave technology

Abstract

The application discloses a metal plate thickness measurement method based on a hybrid system zero group velocity Lamb wave technology, and belongs to the field of nondestructive testing and evaluation. The method comprises the following steps: a hybrid detection system is built, and an out-of-plane displacement vibration signal of a metal plate is obtained based on the hybrid detection system; the out-of-plane displacement vibration signal is converted into a frequency domain signal through fast Fourier transform, and a resonance frequency corresponding to a resonance peak of a zero group velocity Lamb wave mode in the frequency domain signal is extracted; a frequency-thickness product is calculated based on material parameters of the metal plate; and a thickness value of the metal plate is obtained through inversion based on the frequency-thickness product, the resonance frequency and a pre-established frequency-thickness product-thickness function relationship.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of nondestructive testing and evaluation, and particularly relates to a metal plate thickness measurement method based on a hybrid system zero group velocity Lamb wave technology. BACKGROUND

[0002] Conventional thickness measurement mostly adopts a time-of-flight method of a body wave pulse echo, and thickness calculation is realized by calculating the time delay of ultrasonic reflection in the material. This method has the advantage of real-time, but has significant technical limitations: first, in order to realize the measurement of thin plates, high-precision time domain analysis equipment needs to be provided, which requires the clock synchronization accuracy and signal-to-noise ratio of the detection system; second, the roughness and curvature radius of the material surface will distort the sound wave propagation path, and the exponential attenuation characteristics of high-frequency signals in the medium will cause the resolution of deep measurement to decrease; third, the detection of plate-shaped materials is mainly concentrated on thin plate samples, and the thickness of these thin plates is generally below 5mm, so it is more difficult to measure the thickness using traditional body waves. The superposition of wave packets on multiple interfaces in the collected signals requires certain requirements for signal processing, which seriously restricts its application performance in precision thin plate detection.

[0003] In recent years, Lamb wave-based nondestructive testing technology has attracted attention due to its sensitivity to thin plate structures. The frequency dispersion characteristics and multi-mode characteristics of Lamb waves enable thickness characterization through the correlation of the frequency-thickness product (the product of frequency and thickness) and phase velocity, group velocity. However, the traditional Lamb wave technology has the following problems: (1) complex mode selection: it needs to rely on dispersion curves and complex calculations to determine wave modes, which are easily affected by material parameter fluctuations (such as temperature-induced changes in sound velocity) in actual applications, resulting in unstable excitation modes; (2) signal attenuation and noise interference: signal aliasing is serious when multiple modes coexist, and energy attenuation is significant during long-distance propagation, reducing the signal-to-noise ratio; (3) difficulty in detecting layers and interfaces: the sensitivity of traditional Lamb detection methods to layered defects is insufficient, and there is a lack of effective means to decouple signal aliasing.

[0004] Therefore, the present application proposes a metal plate thickness measurement method based on a hybrid system zero group velocity Lamb wave technology to solve the problems existing in the prior art. SUMMARY

[0005] To solve the above technical problems, the present application proposes a metal plate thickness measurement method based on a hybrid system zero group velocity Lamb wave technology to solve the problems existing in the prior art.

[0006] To achieve the above purpose, the present application provides a metal plate thickness measurement method based on a hybrid system zero group velocity Lamb wave technology, which comprises:

[0007] building a hybrid detection system, and obtaining the out-of-plane displacement vibration signal of the metal plate based on the hybrid detection system;

[0008] converting the out-of-plane displacement vibration signal into a frequency domain signal through fast Fourier transform, and extracting a resonance frequency corresponding to a resonance peak of a zero group velocity Lamb wave mode in the frequency domain signal;

[0009] calculating a product of frequency and thickness based on material parameters of the metal plate;

[0010] obtaining a thickness value of the metal plate through inversion based on the product of frequency and thickness, the resonance frequency, and a pre-established product of frequency and thickness-thickness function relationship.

[0011] Optionally, the hybrid detection system comprises an electromagnetic acoustic sensor, a laser interferometer, a high-power pulse excitation device, and a digital oscilloscope.

[0012] The process of building the hybrid detection system and obtaining the out-of-plane displacement vibration signal based on the hybrid detection system comprises: disposing the electromagnetic acoustic sensor on the upper surface of the metal plate, disposing the receiving sensor on the lower surface of the metal plate, exciting the Lamb wave on the upper surface of the metal plate through the high-power pulse excitation device by the electromagnetic acoustic sensor, and receiving the out-of-plane displacement vibration signal on the lower surface of the metal plate through the receiving sensor by the laser interferometer in a non-contact manner.

[0013] Optionally, the electromagnetic acoustic sensor comprises a permanent magnet, a spiral coil, and the measured metal plate.

[0014] The process of obtaining the non-contact excited Lamb wave based on the electromagnetic acoustic sensor comprises:

[0015] generating a modulation signal through the high-power pulse excitation device;

[0016] injecting an excitation current into the spiral coil based on the modulation signal to generate an electromagnetic induction phenomenon;

[0017] The electromagnetic induction phenomenon excites the non-contact excited Lamb wave in the measured technical plate.

[0018] Optionally, the process of calculating the product of frequency and thickness based on the material parameters of the metal plate comprises:

[0019] obtaining the Young's modulus, Poisson's ratio, and density of the metal plate, and calculating the transverse wave velocity and longitudinal wave velocity based on the theory of elasticity;

[0020] Based on the transverse wave velocity and longitudinal wave velocity, the product of frequency and thickness corresponding to the zero group velocity point of the symmetric mode is solved in combination with the Lamb wave dispersion equation.

[0021] Optionally, the expressions of the transverse wave velocity and longitudinal wave velocity are:

[0022]

[0023] wherein v T is the transverse wave velocity, v L is the longitudinal wave velocity, E is the Young's modulus, v is the Poisson's ratio, and p is the density.

[0024] Optionally, the Lamb wave dispersion equation is:

[0025]

[0026] wherein d=2h, ω=2πf, k is the wave number, c p is the phase velocity, d is the thickness in the frequency-thickness product, f is the Lamb wave frequency, ω is the angular frequency, +1 represents the symmetric mode, -1 represents the anti-symmetric mode, h is the thickness of the plate to be measured, c L is the longitudinal wave velocity, and c T is the transverse wave velocity.

[0027] Optionally, the expression of the frequency-thickness product-thickness function relationship is:

[0028]

[0029] wherein fd is the frequency-thickness product, is the resonance frequency, and h is the thickness of the metal plate to be measured.

[0030] Compared with the prior art, the present application has the following advantages and technical effects:

[0031] The present application realizes non-contact high-precision measurement of the thickness of a metal plate through a hybrid detection system composed of an electromagnetic acoustic sensor and a laser interferometer. The method uses the local resonance characteristics of zero group velocity Lamb waves, extracts the frequency domain resonance peak signal, and effectively overcomes the limitations of traditional detection methods in thin plate measurement. The non-contact working mode makes it suitable for complex working conditions such as high temperature and rough surface, and avoids the interference caused by coupling agent. The system combines the high efficiency of electromagnetic excitation and the high sensitivity of laser detection, has strong anti-interference ability and small signal attenuation. The technical scheme has compact structure and simple operation, provides a reliable thickness measurement method for industrial non-destructive testing, and is particularly suitable for online detection and quality control application scenarios. BRIEF DESCRIPTION OF DRAWINGS

[0032] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application and are incorporated in and constitute a part of this application. The embodiments of the present application illustrated in the drawings, and their description, are presented to explain the present application and are not intended to limit the present application. In the drawings:

[0033] Figure 1A schematic diagram of an electromagnetic acoustic sensor and a laser-ultrasonic hybrid detection system according to an embodiment of the present application;

[0034] Figure 2 A schematic diagram of a metal plate thickness measurement experiment based on a zero group velocity Lamb wave hybrid system according to an embodiment of the present application;

[0035] Figure 3 Time and frequency domain waveform diagrams of a zero group velocity Lamb wave signal according to an embodiment of the present application, wherein (a) is a time domain diagram and (b) is a frequency diagram;

[0036] Figure 4 Time domain waveform diagrams of zero group velocity Lamb wave signals of metal plates of different thicknesses according to an embodiment of the present application, wherein (a) is a time domain waveform diagram of a zero group velocity Lamb wave signal of a 1mm aluminum plate, (b) is a time domain waveform diagram of a zero group velocity Lamb wave signal of a 2mm aluminum plate, (c) is a time domain waveform diagram of a zero group velocity Lamb wave signal of a 3mm aluminum plate, and (d) is a time domain waveform diagram of a zero group velocity Lamb wave signal of a 4mm aluminum plate;

[0037] Figure 5 Frequency domain waveform diagrams of zero group velocity Lamb wave signals of metal plates of different thicknesses according to an embodiment of the present application, wherein (a) is a frequency domain waveform diagram of a zero group velocity Lamb wave signal of a 1mm aluminum plate, (b) is a frequency domain waveform diagram of a zero group velocity Lamb wave signal of a 2mm aluminum plate, (c) is a frequency domain waveform diagram of a zero group velocity Lamb wave signal of a 3mm aluminum plate, and (d) is a frequency domain waveform diagram of a zero group velocity Lamb wave signal of a 4mm aluminum plate;

[0038] Figure 6 A dispersion curve diagram of a 2mm thick aluminum plate according to an embodiment of the present application;

[0039] Figure 7 A flowchart of a metal plate thickness measurement method based on a zero group velocity Lamb wave technology according to an embodiment of the present application. DETAILED DESCRIPTION

[0040] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments.

[0041] It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described herein can be executed in an order different from that shown herein.

[0042] Zero group velocity Lamb wave technology provides a new way to solve the above problems. The zero group velocity mode forms local resonance at a certain frequency-thickness product, and its energy is not propagated along the plate but localized, forming a sharp resonance peak, which is extremely sensitive to thickness changes. The corresponding relationship between resonance frequency and thickness can be used to achieve high-precision measurement, especially for plate thickness measurement. However, existing zero group velocity Lamb wave technology relies on single excitation method (such as piezoelectric probe or laser ultrasound), and more or less has the problems of unstable coupling performance, low excitation efficiency, complex equipment, high cost, and poor environmental interference resistance. For example, laser ultrasound requires a precise optical system and is sensitive to surface roughness, while the performance of piezoelectric ultrasound is greatly affected by coupling.

[0043] In view of the above-mentioned limitations in metal plate thickness measurement, the present application proposes a hybrid electromagnetic acoustic sensor and laser ultrasound method using zero group velocity Lamb wave phenomenon, which is used for non-contact thickness measurement of aluminum plate with thickness gradient at room temperature. The present application takes advantage of the advantages of optical and electromagnetic excitation mechanisms to build a hybrid electromagnetic acoustic sensor-laser ultrasound detection system, obtain the frequency shift of the resonance peak of the zero group velocity Lamb wave signal in the frequency domain, and calculate the frequency-thickness product combined with the sample material parameters. Innovative integration realizes accurate thickness characterization.

[0044] The present application proposes a metal plate thickness measurement method based on hybrid system zero group velocity Lamb wave technology, aiming to solve the shortcomings of traditional full-laser zero group velocity Lamb wave measurement method. The proposed method combines the advantages of electromagnetic acoustic sensor and laser ultrasound, and has wider adaptability to harsh environments and engineering practicability.

[0045] The present application builds a hybrid detection system of electromagnetic acoustic sensor and laser ultrasound. The present application obtains the zero group velocity mode related parameters by combining the high-resolution reception of non-contact excitation of electromagnetic acoustic sensor and laser ultrasound detection technology, and the good local detection characteristics and thickness sensitivity of zero group velocity Lamb wave, and measures the thickness of the material plate combined with the material parameters.

[0046] As shown in Figure 7 The present application provides a metal plate thickness measurement method based on hybrid system zero group velocity Lamb wave technology, including the following steps: building a hybrid detection system, obtaining the out-of-plane displacement vibration signal of the metal plate based on the hybrid detection system; converting the out-of-plane displacement vibration signal into a frequency domain signal by fast Fourier transform, and extracting the resonance frequency corresponding to the resonance peak of the zero group velocity Lamb wave mode in the frequency domain signal; calculating the frequency-thickness product based on the material parameters of the metal plate; based on the frequency-thickness product, the resonance frequency and the pre-established frequency-thickness product-thickness function relationship, the thickness value of the metal plate is obtained by inversion.

[0047] Step one: build the mixed detection system of electromagnetic acoustic sensor and laser ultrasound.

[0048] The mixed detection system is shown in Figure 1 . The system includes: a metal plate sample, an electromagnetic acoustic sensor, a laser interferometer, wherein the laser interferometer contains a receiving sensor, a high-power pulse excitation device, and a digital oscilloscope.

[0049] The experiment adopts a one-sending and one-receiving mode. The electromagnetic acoustic sensor is placed on one side of the metal plate, and the high-power pulse excitation device is connected to the electromagnetic acoustic sensor for exciting ultrasonic signals. The receiving sensor of the laser interferometer is placed on the other side of the metal plate in a non-contact manner for receiving ultrasonic signals.

[0050] Further, the process of building the mixed detection system and obtaining the out-of-plane displacement vibration signal based on the mixed detection system includes: placing the electromagnetic acoustic sensor on the upper surface of the metal plate, placing the receiving sensor on the lower surface of the metal plate, exciting Lamb waves on the upper surface of the metal plate through the high-power pulse excitation device, and receiving the out-of-plane displacement vibration signal on the lower surface of the metal plate through the receiving sensor in a non-contact manner.

[0051] Step two: metal plate thickness measurement experiment.

[0052] As shown in Figure 2 , the high-power pulse excitation device is connected to the electromagnetic acoustic sensor on one side of the metal plate to excite ultrasonic signals. The ultrasonic signals propagate inside the plate, where the transverse waves and longitudinal waves are constantly reflected and coupled to form Lamb waves. The receiving sensor of the laser interferometer on the other side of the metal plate emits detection laser, which hits the same position on the other side of the plate excited by the electromagnetic acoustic sensor. The received ultrasonic signal is the zero group velocity Lamb wave.

[0053] Step three: metal plate thickness measurement based on zero group velocity Lamb wave.

[0054] The collected zero group velocity Lamb wave signal is obtained, and the corresponding frequency domain waveform is obtained through FFT processing. The time domain and frequency domain waveforms of the collected signal are shown in Figure 3 . In the frequency domain waveform, a sharp and obvious resonance peak appears. The resonance frequency corresponding to the resonance peak value is extracted, combined with the frequency-thickness product calculated according to the material parameters, and according to the function relationship between the zero group velocity Lamb wave resonance frequency, the metal plate material parameters and the metal plate thickness value, the metal plate thickness is calculated to realize the thickness measurement.

[0055] Figure 4 The time domain waveform diagram of the zero group velocity Lamb wave signal obtained by using the mixed system of electromagnetic acoustic sensor and laser ultrasound to detect the aluminum plate with a thickness of 1-4mm (step 1mm) is shown.Figure 5 The frequency domain waveform diagram of the zero group velocity Lamb wave signal obtained by using the electromagnetic acoustic sensor and laser ultrasonic hybrid system to detect the aluminum plate with a thickness of 1-4 mm (step 1 mm) is shown. The Poisson's ratio of the aluminum plate used is 0.33, the Young's modulus is 72 GPa, and the density is 2680 kg / m 3 .

[0056] The frequency-thickness product is calculated based on the material parameters of the aluminum plate, and the thickness value of the plate is obtained by inverse calculation based on the functional correspondence relationship between the resonance frequency of the zero group velocity Lamb wave and the material parameters and thickness of the plate, so as to realize the thickness detection. The measurement results and errors of the zero group velocity Lamb wave of the metal plate hybrid system are shown in Table 1.

[0057] Table 1

[0058]

[0059] The metal plate thickness measurement method based on the hybrid system zero group velocity Lamb wave technology provided by the present application innovatively combines the electromagnetic acoustic transducer and laser ultrasonic technology, and constructs a hybrid nondestructive testing system based on the propagation mechanism of the zero group velocity Lamb wave. The technology breaks through the limitations of traditional laser ultrasonic detection in structural complexity, cost control and anti-interference performance, realizes high-precision thickness measurement of metal plates with different thicknesses, and exhibits unique thickness-related structural defect diagnosis capability.

[0060] As a specific embodiment of the present embodiment, an electromagnetic acoustic sensor and laser ultrasonic hybrid detection system is built. The system uses an electromagnetic acoustic sensor excitation device to excite a specific mode of Lamb wave in the measured metal plate, and a laser interferometer is configured at the receiving end to collect the off-plane displacement vibration signal of the monitoring point on the opposite side of the excitation source, i.e. the zero group velocity Lamb wave signal. For the collected zero group velocity Lamb wave signal, the characteristic resonance peak is extracted by using fast Fourier transform. By establishing a mapping model of the frequency domain characteristics-material parameters-thickness value, and combining the frequency-thickness product calculated based on the material parameters, the thickness of the metal plate is detected.

[0061] The traditional experimental system based on Lamb wave technology with zero group velocity mainly adopts a full laser experimental system with laser excitation and laser receiving. The present application adopts a hybrid experimental system with electromagnetic acoustic sensor excitation and laser receiving, adopts a one-excitation-one-receiving mode, i.e. the electromagnetic acoustic sensor is placed on one side of the surface of the plate sample, the laser receiving sensor is placed on the other side of the plate in a non-contact mode, and the detection laser is shot on the same position. A permanent magnet provides a stable magnetic field. According to the electromagnetic induction principle, the coil with alternating current is in the stable magnetic field provided by the permanent magnet, and the Lorentz force or magnetostrictive force is generated in the metal plate sample, thereby causing the periodic elastic deformation and vibration in the sample, and thereby exciting the ultrasonic wave. The generated ultrasonic wave propagates in the thin plate structure (finite boundary), wherein the transverse wave and the longitudinal wave are reflected, superimposed and coupled multiple times, and thereby the Lamb wave is formed and propagated along the plate direction to the far field.

[0062] As a specific embodiment of the present embodiment, the core components of the electromagnetic acoustic sensor excitation device are composed of a permanent magnet, a spiral coil and a measured metal plate. A modulation signal is generated by a high-power pulse excitation device, the excitation current is injected into the spiral coil, the electromagnetic induction phenomenon is generated, and thereby the Lamb wave of a specific mode is excited in the measured metal plate.

[0063] The electromagnetic acoustic sensor comprises a permanent magnet, a spiral coil and a measured metal plate. The process of obtaining the non-contact excitation Lamb wave based on the electromagnetic acoustic sensor comprises: generating a modulation signal by a high-power pulse excitation device; injecting the excitation current into the spiral coil based on the modulation signal, generating the electromagnetic induction phenomenon; and exciting the non-contact Lamb wave in the measured metal plate by the electromagnetic induction phenomenon.

[0064] As a specific embodiment of the present embodiment, the receiving sensor of the laser interferometer in the laser interferometer receiving device emits the detection laser to the surface of the plate sample, collects the out-of-plane displacement, causes the change of the detection laser, converts the optical signal into the electrical signal, and collects the out-of-plane displacement vibration signal at the same position on the other side of the plate opposite to the excitation device.

[0065] The working principle of the laser interferometer mainly involves a demodulator, a beam splitter, a continuous laser and a laser receiving sensor. The continuous laser emits the detection laser through the laser receiving sensor, and the detection laser is reflected after being shot on the metal plate sample. The reflected light and the reference light beam (from the same laser source, obtained through the beam splitter) are combined and interfered. The ultrasonic wave causes the out-of-plane displacement of the sample surface, changes the optical path length of the reflected light, changes the phase difference between the reflected light and the reference light, and thereby changes the interference light intensity. The laser receiving sensor receives the interfered optical signal and converts it into an electrical signal, and thereby the displacement signal of the ultrasonic wave is demodulated by the demodulator and transmitted to the signal display interface.

[0066] The S1 (symmetric mode 1st order)-ZGV (zero group velocity) mode (i.e. S1-ZGV mode) characteristic information contained in the out-of-plane displacement vibration signal is obtained, and the resonance peak frequency in the frequency domain of the mode is the resonance frequency, and the value of the resonance peak frequency is the frequency value corresponding to the peak value of the resonance peak. In this embodiment, the ZGV Lamb wave signal is subjected to Fast Fourier Transform (FFT) processing to obtain a frequency spectrum, and the resonance frequency value is obtained by picking up the frequency spectrum. The resonance frequency and the material elastic parameters (including density, Young's modulus, Poisson's ratio) and the thickness of the sample have a strict functional relationship, which provides key data support for the thickness measurement of the metal plate.

[0067] The process of calculating the product of frequency and thickness based on the material parameters of the metal plate includes: obtaining the Young's modulus, Poisson's ratio and density of the metal plate, calculating the transverse wave velocity and longitudinal wave velocity based on the theory of elasticity; based on the transverse wave velocity and the longitudinal wave velocity, the product of frequency and thickness corresponding to the zero group velocity point of the symmetric mode is solved by combining the Lamb wave dispersion equation.

[0068] The functional relationship can be calculated by the following formula:

[0069]

[0070] wherein, v T is the transverse wave velocity, v L is the longitudinal wave velocity, E is the Young's modulus, v is the Poisson's ratio, and p is the density.

[0071] The dispersion equation of the Lamb wave can be expressed as:

[0072]

[0073] wherein, d=2h,ω=2πf,k is the wave number, c p is the phase velocity, d is the thickness in the product of frequency and thickness, f is the Lamb wave frequency, and w is the angular frequency. +1 represents the symmetric mode, -1 represents the anti-symmetric mode, h is the thickness of the plate to be measured, c L is the longitudinal wave velocity, and c T is the transverse wave velocity.

[0074] According to the above formula, the dispersion relationship of the Lamb wave can be calculated, as shown in the following formula: Figure 6 As an example, the k-fd dispersion curve of a certain 2mm thick aluminum plate is plotted:

[0075] wherein, fd is the product of frequency and thickness, i.e. the product of frequency f and thickness d. The solid line represents the symmetric mode (S), and the dashed line represents the anti-symmetric mode (A). Figure 6 The S1 branch inflection point in the formula is the ZGV point corresponding to the S1 mode, and the corresponding ordinate is the product of frequency and thickness of the S1-ZGV.

[0076] The out-of-plane displacement vibration signal is collected, fast Fourier transform is carried out, and a frequency spectrum diagram is obtained. According to the relationship among the frequency-thickness product, the resonance frequency and the thickness, the thickness h of the to-be-measured plate can be calculated.

[0077]

[0078] As a specific embodiment of the present embodiment, the construction process of the mapping model of the frequency domain feature-material parameter-thickness includes: first, the Lamb wave dispersion relationship is calculated according to the material parameters, the dispersion curve is drawn, and the frequency-thickness product is obtained. Then, the electromagnetic acoustic sensor excitation device generates ultrasonic waves in the plate, the laser interferometer receiving device collects the out-of-plane displacement vibration signal, and the signal is converted to the frequency domain through fast Fourier transform. The resonance frequency is identified from the frequency domain diagram, and the thickness of the metal plate is calculated by combining the function relationship among the frequency-thickness product, the resonance frequency and the thickness.

[0079] The electromagnetic acoustic sensor used to excite the signal is a non-contact sensor, which reduces the influence of coupling on the signal quality, has the advantages of high temperature resistance and simple structure, and further reduces the detection cost.

[0080] The present application adopts a hybrid system of electromagnetic acoustic sensors and laser ultrasonic technology, combines the advantages of electromagnetic acoustic sensors and laser ultrasonic technology, and enhances the robustness of work in complex operating environments.

[0081] The detection method based on zero group velocity Lamb wave has high thickness sensitivity, realizes high-precision measurement of the thickness of the metal plate, reduces signal attenuation, has high energy, and further improves the detection performance in local areas, and has the detection advantage in narrow space.

[0082] The zero group velocity Lamb wave signal collected by the present application has obvious characteristics, which is convenient for feature extraction. The metal plate thickness calculation method is optimized, and the detection accuracy is improved.

[0083] The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto, any skilled person in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for measuring the thickness of metal plates based on hybrid system zero-group velocity Lamb wave technology, characterized in that, Includes the following steps: A hybrid detection system is constructed, and the out-of-plane displacement vibration signal of the metal plate is obtained based on the hybrid detection system; The out-of-plane displacement vibration signal is converted into a frequency domain signal by fast Fourier transform, and the resonance frequency corresponding to the resonance peak of the zero group velocity Lamb wave mode in the frequency domain signal is extracted. Calculation of frequency-thickness product based on material parameters of metal sheet; Based on the frequency-thickness product, resonant frequency, and the pre-established frequency-thickness product-thickness function relationship, the thickness value of the metal sheet is obtained by inversion.

2. The method for measuring the thickness of metal plates based on hybrid system zero-group velocity Lamb wave technology according to claim 1, characterized in that, The hybrid detection system includes: an electromagnetic acoustic sensor, a laser interferometer, a high-power pulse excitation device, and a digital oscilloscope; The process of building a hybrid detection system and obtaining out-of-plane displacement vibration signals based on the hybrid detection system includes: setting an electromagnetic acoustic sensor on the upper surface of the metal plate and a receiving sensor on the lower surface of the metal plate; the electromagnetic acoustic sensor non-contactly exciting Lamb waves on the upper surface of the metal plate through a high-power pulse excitation device; and the laser interferometer receiving the out-of-plane displacement vibration signals in a non-contact manner on the lower surface of the metal plate through the receiving sensor.

3. The method for measuring the thickness of metal plates based on hybrid system zero-group velocity Lamb wave technology according to claim 2, characterized in that, The electromagnetic acoustic sensor includes: a permanent magnet, a spiral coil, and a metal plate to be tested; The process of obtaining non-contact excited Lamb waves based on the electromagnetic acoustic sensor includes: A modulation signal is generated by a high-power pulse excitation device; An excitation current is injected into the spiral coil based on the modulation signal, generating an electromagnetic induction phenomenon. The electromagnetic induction phenomenon excites the non-contact Lamb wave in the tested technical board.

4. The method for measuring the thickness of metal plates based on hybrid system zero-group velocity Lamb wave technology according to claim 1, characterized in that, The process of calculating the frequency-thickness product based on the material parameters of metal sheets includes: Obtain the Young's modulus, Poisson's ratio, and density of the metal sheet, and calculate the transverse wave velocity and longitudinal wave velocity based on the theory of elasticity. Based on the transverse and longitudinal wave velocities, the frequency-thickness product corresponding to the zero group velocity point of the symmetric mode is solved by combining the Lamb wave dispersion equation.

5. The method for measuring the thickness of metal plates based on hybrid system zero-group velocity Lamb wave technology according to claim 4, characterized in that, The expressions for the transverse wave velocity and the longitudinal wave velocity are as follows: In the formula, v T For transverse wave velocity, v L Let E be the longitudinal wave velocity, E be Young's modulus, ν be Poisson's ratio, and ρ be density.

6. The method for measuring the thickness of metal plates based on hybrid system zero-group velocity Lamb wave technology according to claim 5, characterized in that, The Lamb wave dispersion equation is: In the formula, d = 2h, ω = 2πf, k is the wave number, c p d is the phase velocity, f is the thickness in the frequency-thickness product, ω is the Lamb wave frequency, +1 represents the symmetric mode, -1 represents the antisymmetric mode, h is the thickness of the plate being measured, and c is the phase velocity. L For the longitudinal wave velocity, c T This refers to the transverse wave velocity.

7. The method for measuring the thickness of metal plates based on hybrid system zero-group velocity Lamb wave technology according to claim 6, characterized in that, The expression for the frequency-thickness product-thickness function relationship is as follows: In the formula, fd is the frequency-thickness product. is the resonant frequency, and h is the thickness of the metal plate to be measured.

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