Metal plate thickness measuring method based on hybrid system zero group velocity Lamb wave technology

Abstract

The invention discloses a sheet metal 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 that a hybrid detection system is built, and an out-of-plane displacement vibration signal of a sheet metal is obtained based on the hybrid detection system; converting the out-of-plane displacement vibration signal into a frequency domain signal through fast Fourier transform, and extracting a resonant frequency corresponding to a formant of a zero group velocity Lamb wave mode in the frequency domain signal; calculating a frequency-thickness product based on material parameters of the metal plate; and based on the frequency thickness product, the resonant frequency and a pre-established frequency thickness product-thickness function relationship, performing inversion to obtain a thickness value of the metal plate.

Description

Technical Field

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

[0002] Conventional thickness measurement often uses the transit time method of body wave pulse echoes, which calculates thickness by calculating the time delay of ultrasonic wave reflection within the material. Although this method has the advantage of real-time performance, it has significant technical limitations: First, to achieve thin plate measurement, high-precision time domain analysis equipment is required, which places high demands on the detection system's clock synchronization accuracy and signal-to-noise ratio. Second, the surface roughness and curvature radius of the material distort the sound wave propagation path. In addition, the exponential decay characteristics of high-frequency signals in the medium lead to a decrease in deep measurement resolution. Third, the detection of plate-like materials is mainly focused on thin plate samples, which are generally less than 5mm thick. Using traditional body waves for thickness measurement is relatively ineffective. The collected signal is prone to the superposition of wave packets on multiple interfaces, which places certain requirements on signal processing, seriously restricting its application effectiveness in precision thin plate detection.

[0003] In recent years, nondestructive testing technology based on Lamb waves has attracted attention due to its sensitivity to thin plate structures. The dispersion characteristics and multi-mode characteristics of Lamb waves enable thickness characterization by correlating the frequency-thickness product (the product of frequency and thickness) with phase velocity and group velocity. However, traditional Lamb wave technology has the following problems: (1) Complex mode selection: It relies on dispersion curves and complex calculations to determine the wave mode. In practical applications, it is easily affected by fluctuations in material parameters (such as changes in sound velocity caused by temperature), resulting in unstable excitation modes; (2) Signal attenuation and noise interference: When multiple modes coexist, signal aliasing is serious, and energy attenuation is significant during long-distance propagation, reducing the signal-to-noise ratio; (3) Difficulty in delamination and interface detection: Traditional Lamb detection methods are not sensitive enough to delamination defects, and lack effective means to decouple signal aliasing.

[0004] Therefore, the present invention proposes a metal sheet thickness measurement method based on a hybrid system zero group velocity Lamb wave technology to solve the problems existing in the above-mentioned prior art. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention proposes a metal sheet thickness measurement method based on hybrid system zero group velocity Lamb wave technology to solve the problems existing in the above-mentioned prior art.

[0006] To achieve the above object, the present invention provides a metal sheet thickness measurement method based on a hybrid system zero group velocity Lamb wave technology, comprising:

[0007] Building a hybrid detection system, and obtaining an off-plane displacement vibration signal of the metal sheet based on the hybrid detection system;

[0008] Converting the off-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;

[0009] Calculate the frequency-thickness product based on the material parameters of the metal sheet;

[0010] Based on the frequency-thickness product, the resonant frequency and the pre-established frequency-thickness product-thickness function relationship, the thickness value of the metal plate is obtained by inversion.

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

[0012] Among them, the process of building a hybrid detection system and obtaining an off-plane displacement vibration signal based on the hybrid detection system includes: setting an electromagnetic acoustic sensor on the upper surface of the metal plate, setting a receiving sensor on the lower surface of the metal plate, the electromagnetic acoustic sensor non-contactly excites Lamb waves on the upper surface of the metal plate through a high-power pulse excitation device, and the laser interferometer receives the off-plane displacement vibration signal in a non-contact manner on the lower surface of the metal plate through the receiving sensor.

[0013] Optionally, the electromagnetic acoustic sensor comprises: a permanent magnet, a spiral coil and a metal plate to be measured;

[0014] The process of obtaining non-contact excitation Lamb waves based on the electromagnetic acoustic sensor includes:

[0015] generating a modulation signal by a 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 excitation Lamb wave in the technical plate under test.

[0018] Optionally, the process of calculating the frequency-thickness product based on the material parameters of the metal sheet includes:

[0019] Obtain the Young's modulus, Poisson's ratio, and density of the metal sheet, and calculate the shear wave velocity and longitudinal wave velocity based on the theory of elastic mechanics;

[0020] Based on the shear wave velocity and longitudinal wave velocity, the frequency-thickness product 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 for the shear wave velocity and the longitudinal wave velocity are:

[0022]

[0023] Where, v T is the shear wave velocity, v L is the longitudinal wave velocity, E is Young's modulus, ν is Poisson's ratio, and ρ is the density.

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

[0025]

[0026] Where, 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 antisymmetric mode, h is the thickness of the plate to be tested, c L is the longitudinal wave velocity, c T is the shear wave velocity.

[0027] Optionally, the frequency-thickness product-thickness function relationship is expressed as:

[0028]

[0029] Where 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 invention has the following advantages and technical effects:

[0031] The present invention achieves non-contact, high-precision measurement of the thickness of metal plates through a hybrid detection system consisting of an electromagnetic acoustic sensor and a laser interferometer. This method utilizes the local resonance characteristics of zero-group-velocity Lamb waves and effectively overcomes the limitations of traditional detection methods in thin plate measurement by extracting frequency-domain resonance peak signals. The non-contact working mode makes it suitable for complex working conditions such as high temperature and rough surfaces, while avoiding interference caused by coupling agents. The system combines the high efficiency of electromagnetic excitation with the high sensitivity of laser detection, and has the characteristics of strong anti-interference ability and low signal attenuation. This technical solution has a compact structure and is easy to operate. It provides a reliable thickness measurement method for industrial non-destructive testing and is particularly suitable for application scenarios such as online detection and quality control. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of this application. The exemplary embodiments and descriptions of this application are intended to explain this application and do not constitute an improper limitation on this application. In the accompanying drawings:

[0033] Figure 1Schematic diagram of an electromagnetic acoustic sensor and laser ultrasonic hybrid detection system according to an embodiment of the present invention;

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

[0035] Figure 3 1 is an example diagram of the time domain and frequency domain waveforms of a zero group velocity Lamb wave signal according to an embodiment of the present invention, wherein (a) is a time domain diagram and (b) is a frequency diagram;

[0036] Figure 4 Figures 1 and 2 are time-domain waveforms of zero-group-velocity Lamb wave signals of metal plates of different thicknesses according to an embodiment of the present invention, wherein (a) is a time-domain waveform of a zero-group-velocity Lamb wave signal obtained for a 1 mm aluminum plate, (b) is a time-domain waveform of a zero-group-velocity Lamb wave signal obtained for a 2 mm aluminum plate, (c) is a time-domain waveform of a zero-group-velocity Lamb wave signal obtained for a 3 mm aluminum plate, and (d) is a time-domain waveform of a zero-group-velocity Lamb wave signal obtained for a 4 mm aluminum plate.

[0037] Figure 5 Figures 2 and 3 are frequency domain waveforms of zero group velocity Lamb wave signals of metal plates of different thicknesses according to an embodiment of the present invention, wherein (a) is a frequency domain waveform of a zero group velocity Lamb wave signal obtained for a 1 mm aluminum plate, (b) is a frequency domain waveform of a zero group velocity Lamb wave signal obtained for a 2 mm aluminum plate, (c) is a frequency domain waveform of a zero group velocity Lamb wave signal obtained for a 3 mm aluminum plate, and (d) is a frequency domain waveform of a zero group velocity Lamb wave signal obtained for a 4 mm aluminum plate.

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

[0039] Figure 7 This is a flow chart of a metal sheet thickness measurement method based on a hybrid system zero group velocity Lamb wave technology according to an embodiment of the present invention. DETAILED DESCRIPTION

[0040] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

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

[0042] Zero group velocity Lamb wave technology provides a new approach to solving the above problems. The zero group velocity mode forms a local resonance at a specific frequency-thickness product. Its energy does not propagate along the plate but is localized, forming a sharp resonance peak. It is extremely sensitive to thickness changes and can achieve high-precision measurement through the correspondence between the resonance frequency and thickness. It is especially suitable for measuring the thickness of plates. However, existing zero group velocity Lamb wave technologies mostly rely on a single excitation method (such as piezoelectric probes or laser ultrasound), and more or less have problems such as unstable coupling performance, low excitation efficiency, complex equipment, high cost, and poor resistance to environmental interference. 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] To address the aforementioned limitations in metal sheet thickness measurement, this paper proposes a hybrid electromagnetic acoustic sensor and laser ultrasonic method that leverages the zero-group-velocity Lamb wave phenomenon for non-contact thickness measurement of thickness-gradient aluminum sheets at room temperature. Leveraging the advantages of both optical and electromagnetic excitation mechanisms, this paper constructs an electromagnetic acoustic sensor-laser ultrasonic hybrid detection system. This system measures the frequency shift of the zero-group-velocity Lamb wave signal resonant peak in the frequency domain of the metal sheet. This system, combined with the frequency-thickness product calculated from the sample material parameters, achieves precise thickness characterization through innovative integration.

[0044] This paper proposes a metal sheet thickness measurement method based on a hybrid system using zero-group-velocity Lamb waves. This method aims to address the shortcomings of traditional all-laser zero-group-velocity Lamb wave measurement methods. By combining the advantages of electromagnetic acoustic sensors and laser ultrasound, this method offers greater adaptability to harsh environments and greater engineering practicality.

[0045] This paper builds a hybrid detection system using electromagnetic acoustic sensors and laser ultrasonics. By integrating the non-contact excitation of electromagnetic acoustic sensors with the high-resolution reception of laser ultrasonic detection technology, and combining the excellent local detection and thickness sensitivity of zero-group-velocity Lamb waves, this system acquires zero-group-velocity modal parameters and, in combination with material parameters, measures the thickness of sheet materials.

[0046] like Figure 7 As shown, this embodiment provides a metal plate thickness measurement method based on a hybrid system zero group velocity Lamb wave technology, comprising the following steps: building a hybrid detection system, and obtaining an off-plane displacement vibration signal of the metal plate based on the hybrid detection system; converting the off-plane displacement vibration signal into a frequency domain signal through fast Fourier transform, and extracting the resonant frequency corresponding to the resonant 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; and inverting the frequency-thickness product, the resonant frequency, and a pre-established frequency-thickness product-thickness function relationship to obtain the thickness value of the metal plate.

[0047] Step 1: Build a detection system that combines electromagnetic acoustic sensors and laser ultrasonic sensors.

[0048] Hybrid detection systems such as Figure 1 The system includes: a metal plate sample, an electromagnetic acoustic sensor, and a laser interferometer, wherein the laser interferometer includes a receiving sensor, a high-power pulse excitation device, and a digital oscilloscope.

[0049] The experiment employed a single-excitation, single-receiver model. An electromagnetic acoustic sensor was placed on one side of the metal plate, and a high-power pulse excitation device was connected to the electromagnetic acoustic sensor to stimulate the ultrasonic signal. A laser interferometer's receiving sensor was placed on the other side of the metal plate in a contactless manner to receive the ultrasonic signal.

[0050] Furthermore, a hybrid detection system is built, and the process of obtaining an off-plane displacement vibration signal based on the hybrid detection system includes: setting an electromagnetic acoustic sensor on the upper surface of the metal plate, and setting a receiving sensor on the lower surface of the metal plate. The electromagnetic acoustic sensor excites Lamb waves on the upper surface of the metal plate in a non-contact manner through a high-power pulse excitation device, and the laser interferometer receives the off-plane displacement vibration signal in a non-contact manner on the lower surface of the metal plate through the receiving sensor.

[0051] Step 2: Metal sheet thickness measurement experiment.

[0052] like Figure 2 As shown, a high-power pulse excitation device is connected to an electromagnetic acoustic sensor on one side of a metal plate, stimulating an ultrasonic signal. The ultrasonic signal propagates within the plate, where transverse and longitudinal waves continuously reflect and couple to form Lamb waves. A laser interferometer's receiving sensor on the other side of the metal plate emits a detection laser, striking the same location on the opposite side of the plate stimulated by the electromagnetic acoustic sensor. The received ultrasonic signal is a zero-group-velocity Lamb wave.

[0053] Step 3: Metal sheet 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 as follows: Figure 3 As shown in the figure, a sharp and obvious resonance peak appears in the frequency domain waveform. The resonance frequency corresponding to the resonance peak is extracted and combined with the frequency-thickness product calculated based on the material parameters. Based on the functional relationship between the zero-group-velocity Lamb wave resonance frequency, the metal sheet material parameters, and the metal sheet thickness, the thickness of the metal sheet can be calculated and measured.

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

[0056] The frequency-thickness product is calculated based on the aluminum plate material parameters. Using the functional relationship between the zero-group-velocity Lamb wave resonant frequency and the plate material parameters and thickness, the plate thickness is obtained through inversion calculation, enabling thickness detection. The zero-group-velocity Lamb wave measurement results and errors for the metal plate hybrid system are shown in Table 1.

[0057] Table 1

[0058]

[0059] This paper proposes a hybrid system for measuring the thickness of metal sheets using zero-group-velocity Lamb waves. This innovatively integrates electromagnetic acoustic transducers with laser ultrasonic technology to create a hybrid nondestructive testing system based on the zero-group-velocity Lamb wave propagation mechanism. This technology overcomes the limitations of traditional laser ultrasonic testing in terms of structural complexity, cost control, and anti-interference performance, enabling high-precision thickness measurement of metals of varying thicknesses and demonstrating unique diagnostic capabilities for thickness-related structural defects.

[0060] As a specific implementation of this embodiment, a hybrid detection system combining an electromagnetic acoustic sensor and laser ultrasonic wave detection is constructed. The system utilizes an electromagnetic acoustic sensor excitation device to excite a specific mode of Lamb wave in the metal plate being tested. A laser interferometer is configured at the receiving end to collect, in real time, the off-plane displacement vibration signal (i.e., the zero-group-velocity Lamb wave signal) from the monitoring point opposite the excitation source. A fast Fourier transform is used to extract the characteristic resonance peaks of the collected zero-group-velocity Lamb wave signal. By establishing a mapping model between frequency domain characteristics, material parameters, and thickness values, combined with the frequency-thickness product calculated from the material parameters, the thickness of the metal plate can be detected.

[0061] Traditional experimental systems based on zero-group-velocity Lamb wave technology detection mainly use full-laser experimental systems with laser excitation and laser reception. The present invention adopts a hybrid experimental system with electromagnetic acoustic sensor excitation and laser reception, using a one-excitation-one-reception method, that is, the electromagnetic acoustic sensor is placed on one side of the plate sample surface, and the laser receiving sensor is placed on the other side of the plate in a non-contact manner, so that the detection laser hits the same position. The permanent magnet provides a stable magnetic field. According to the principle of electromagnetic induction, the coil with alternating current is in the stable magnetic field provided by the permanent magnet, generating Lorentz force or magnetostrictive force in the metal plate sample, causing periodic elastic deformation and vibration in the sample, thereby exciting ultrasonic waves. The generated ultrasonic waves propagate in the thin plate structure (finite boundary), wherein the transverse wave and the longitudinal wave are repeatedly reflected, superimposed, and coupled to form Lamb waves that propagate along the plate direction to the far field.

[0062] As a specific implementation of this embodiment, the core components of the electromagnetic acoustic sensor excitation device consist of a permanent magnet, a spiral coil, and the metal plate being measured. A high-power pulse excitation device generates a modulated signal, which injects an excitation current into the spiral coil, generating electromagnetic induction and exciting a specific mode of Lamb wave in the metal plate being measured.

[0063] The electromagnetic acoustic sensor includes: a permanent magnet, a spiral coil and a metal plate to be measured; wherein, the process of obtaining non-contact excitation Lamb waves based on the electromagnetic acoustic sensor includes: generating a modulation signal through a high-power pulse excitation device; injecting an excitation current into the spiral coil based on the modulation signal to generate an electromagnetic induction phenomenon; and the electromagnetic induction phenomenon excites non-contact Lamb waves in the metal plate to be measured.

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

[0065] Among them, the working principle of the laser interferometer mainly involves several parts such as the demodulator, spectrometer, continuous laser and laser receiving sensor. The continuous laser emits a detection laser through the laser receiving sensor, which is reflected after hitting the metal sheet sample. This beam of reflected light merges with the reference beam (from the same laser source, obtained through the spectrometer) to produce optical interference. The ultrasonic wave causes the surface of the sample to produce off-plane displacement, which will change the optical path length of the reflected light, causing the phase difference between it and the reference light to change, thereby causing the intensity of the interference light to change. The laser receiving sensor receives the light signal after interference and converts it into an electrical signal, thereby demodulating the ultrasonic displacement signal through the demodulator and transmitting it to the signal display interface.

[0066] The off-plane displacement vibration signal contains the characteristic information of the S1 (symmetric mode 1)-ZGV (zero group velocity) mode (i.e., the S1-ZGV mode). The resonance peak frequency in the frequency domain of this mode is the resonance frequency, and its value is the frequency value corresponding to the peak value of the resonance peak. In this embodiment, the ZGV Lamb wave signal is processed by Fast Fourier Transform (FFT) to obtain a spectrum, and the resonance frequency value is picked up on the spectrum. This resonance frequency has a strict functional relationship with the elastic parameters of the material (including density, Young's modulus, Poisson's ratio) and the thickness of the sample, providing key data support for the thickness measurement of metal plates.

[0067] The process of calculating the frequency-thickness product based on the material parameters of the metal sheet includes: obtaining the Young's modulus, Poisson's ratio and density of the metal sheet, calculating the shear wave velocity and longitudinal wave velocity based on the theory of elastic mechanics; based on the shear wave velocity and longitudinal wave velocity, combining the Lamb wave dispersion equation to solve the frequency-thickness product corresponding to the zero group velocity point of the symmetric mode.

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

[0069]

[0070] Among them, v T is the shear wave velocity, v L is the longitudinal wave velocity, E is Young's modulus, ν is Poisson's ratio, and ρ is the density.

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

[0072]

[0073] in, 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, and ω is the angular frequency. +1 represents the symmetric mode, -1 represents the antisymmetric mode, h is the thickness of the plate to be tested, and c L is the longitudinal wave velocity, c T is the shear wave velocity.

[0074] According to the above formula, Lamb's dispersion relation can be calculated, such as Figure 6 As shown in the figure, taking a 2mm thick aluminum plate as an example, the k-fd dispersion curve is drawn:

[0075] Where fd is the frequency-thickness product, which is the product of frequency f and thickness d. The solid line represents the symmetric mode (S), and the dashed line represents the antisymmetric mode (A). Figure 6 The inflection point of the S1 branch is the ZGV point corresponding to the S1 mode, and its corresponding ordinate is the frequency-thickness product of S1-ZGV.

[0076] Collect the off-plane displacement vibration signal, perform fast Fourier transform, and obtain the spectrum. Identify the resonant frequency of the S1 mode from the frequency domain diagram. The thickness h of the plate to be measured can be calculated based on the relationship between frequency-thickness product-resonance frequency-thickness. The relationship between frequency-thickness product-resonance frequency-thickness is as follows:

[0077]

[0078] As a specific implementation of this embodiment, the process of constructing a mapping model between frequency domain characteristics, material parameters, and thickness includes the following steps: first, calculating the Lamb wave dispersion relationship based on the material parameters, plotting the dispersion curve, and obtaining the frequency-thickness product. Next, an electromagnetic acoustic sensor excitation device generates ultrasonic waves in the sheet material, and a laser interferometer receiving device collects the off-plane displacement vibration signal. This signal is converted to the frequency domain through a fast Fourier transform. The resonant frequency is identified from the frequency domain plot, and the thickness of the metal sheet is calculated by combining the functional relationship between the frequency-thickness product, the resonant frequency, and the thickness.

[0079] The electromagnetic acoustic sensor used for exciting signals of the present invention is a non-contact sensor, which reduces the influence of coupling on signal quality, has the advantages of high temperature resistance and simple structure, and further reduces detection costs.

[0080] The present invention 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 working in complex operating environments.

[0081] This invention, based on a zero-group-velocity Lamb wave detection method, has high thickness sensitivity, enabling high-precision thickness measurement of metal sheets. Reduced signal attenuation and high energy improve localized detection performance, making it particularly suitable for detection in confined spaces.

[0082] The zero-group-velocity Lamb wave signal collected by the present invention has significant characteristics, which facilitates feature extraction, optimizes the metal plate thickness calculation method, and improves detection accuracy.

[0083] The above are merely preferred embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A metal sheet thickness measurement method based on hybrid system zero group velocity Lamb wave technology, characterized in that: The following steps are involved: Building a hybrid detection system, and obtaining an off-plane displacement vibration signal of the metal sheet based on the hybrid detection system; Converting the off-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; Calculate the frequency-thickness product based on the material parameters of the metal sheet; Based on the frequency-thickness product, the resonant frequency and the pre-established frequency-thickness product-thickness function relationship, the thickness value of the metal plate is obtained by inversion.

2. The metal sheet thickness measurement method based on the hybrid system zero group velocity Lamb wave technology according to claim 1 is 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; Among them, the process of building a hybrid detection system and obtaining an off-plane displacement vibration signal based on the hybrid detection system includes: setting an electromagnetic acoustic sensor on the upper surface of the metal plate, setting a receiving sensor on the lower surface of the metal plate, the electromagnetic acoustic sensor non-contactly excites Lamb waves on the upper surface of the metal plate through a high-power pulse excitation device, and the laser interferometer receives the off-plane displacement vibration signal in a non-contact manner on the lower surface of the metal plate through the receiving sensor.

3. The metal sheet thickness measurement method based on the hybrid system zero group velocity Lamb wave technology according to claim 2 is characterized in that: The electromagnetic acoustic sensor comprises: a permanent magnet, a spiral coil and a metal plate to be measured; The process of obtaining non-contact excitation Lamb waves based on the electromagnetic acoustic sensor includes: generating a modulation signal by a high-power pulse excitation device; injecting an excitation current into the spiral coil based on the modulation signal to generate an electromagnetic induction phenomenon; The electromagnetic induction phenomenon excites the non-contact excitation Lamb wave in the technical plate under test.

4. The metal sheet thickness measurement method based on the hybrid system zero group velocity Lamb wave technology according to claim 1 is characterized in that: The process of calculating the frequency-thickness product based on the material parameters of the metal sheet includes: Obtain the Young's modulus, Poisson's ratio, and density of the metal sheet, and calculate the shear wave velocity and longitudinal wave velocity based on the theory of elastic mechanics; Based on the shear wave velocity and longitudinal wave velocity, the frequency-thickness product corresponding to the zero group velocity point of the symmetric mode is solved in combination with the Lamb wave dispersion equation.

5. The metal sheet thickness measurement method based on the hybrid system zero group velocity Lamb wave technology according to claim 4 is characterized in that: The expressions of the shear wave velocity and the longitudinal wave velocity are: Where, v T is the shear wave velocity, v L is the longitudinal wave velocity, E is Young's modulus, ν is Poisson's ratio, and ρ is the density.

6. The metal sheet thickness measurement method based on the hybrid system zero group velocity Lamb wave technology according to claim 5 is characterized in that: The Lamb wave dispersion equation is: Where, 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 antisymmetric mode, h is the thickness of the plate to be tested, c L is the longitudinal wave velocity, c T is the shear wave velocity.

7. The metal sheet thickness measurement method based on the hybrid system zero group velocity Lamb wave technology according to claim 6 is characterized in that: The expression of the frequency-thickness product-thickness function relationship is: Where fd is the frequency-thickness product, is the resonance frequency, and h is the thickness of the metal plate to be measured.

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