Method and device for measuring thickness of each layer of double-layer thin plate based on ultrasonic Lamb wave zero group velocity resonance

Abstract

The present invention discloses a method and device for measuring the thickness of each layer of a double-layer thin plate based on ultrasonic Lamb wave zero group velocity resonance. The method utilizes a special position in the Lamb wave S1 mode where the wave number is not zero but the frequency is the smallest. Energy does not propagate at this position, and a sharp resonance peak is formed in the frequency domain, which is called ZGVLamb wave. This mode is very sensitive to the thickness of the plate material, and has a sharp resonance peak at a certain frequency-thickness product. The relationship between the resonance peak frequency and the thickness of the layer is used to characterize the thickness of each layer of the double-layer thin plate. When the ZGV frequency is measured, the corresponding thickness ratio is found, and the thickness of each layer can be calculated. The present invention overcomes the disadvantage of the traditional ultrasonic pulse echo method that when the plate thickness is less than the wavelength level, the echo signals overlap each other and cannot be measured.

Description

Technical Field

[0001] The invention relates to the field of ultrasound, and in particular to a method and a device for measuring the thickness of each layer of a double-layer thin plate based on ultrasonic Lamb wave zero group velocity resonance. Background Art

[0002] Double-layer thin plate materials are widely used in the construction industry, aerospace, medicine, pressure vessel manufacturing, coating technology, etc. Thickness is an important parameter of plate materials, so it is of great significance to characterize the thickness. Ultrasonic measurement methods have the advantages of low requirements for the measured structure, high spatial resolution, and little influence from the test environment, and are widely used to measure the thickness of objects. At present, the commonly used ultrasonic measurement methods include pulse echo method and the like. The pulse echo method measures the propagation time of the ultrasonic pulse in the sample, and then calculates the sample thickness according to the propagation speed. For example, reference 1 (Zheng Wei Lei, Man Ke Niu and Xiang Hua. Multilayer Ultrasonic Echo Structured Sparse Model and its Application [J]. Advanced Materials Research, 2013, 2438 (718-720): 1043-1048.). When the thin layer thickness is less than the wavelength level, the reflected echoes at the upper and lower surfaces and interfaces of the measured sample are overlapped, resulting in the inability to measure the thickness of the thin layer.

[0003] Therefore, a method is needed to solve the above problem by which the thickness of each layer can be well measured when the thickness of the double-layer thin plate is at the wavelength level or even smaller than the wavelength. Summary of the invention

[0004] The purpose of the present invention is to provide a method and device for measuring the thickness of each layer of a double-layer thin plate based on ultrasonic Lamb wave zero group velocity resonance, utilizing the existence of a special position in the Lamb wave S1 mode with a non-zero wave number but the smallest frequency, where energy does not propagate, forming a sharp resonance peak in the frequency domain, and utilizing the relationship between the resonance peak frequency value and the thickness ratio of the double-layer plate to characterize the thickness of each layer of the double-layer thin plate.

[0005] The technical solution to achieve the purpose of the present invention is as follows: In the first aspect, the present invention provides a method for measuring the thickness of each layer of a double-layer thin plate based on ultrasonic Lamb wave zero group velocity resonance. The zero group velocity frequency of the Lamb wave of the double-layer thin plate is related to the thickness ratio of the two plates. The relationship between the wave number k and frequency f of the Lamb wave of the double-layer thin plate and the thickness ratio of the thin plates is theoretically derived. The Lamb wave dispersion curve under different thickness ratios is drawn. The frequency of the point on the dispersion curve with a slope of zero but a non-zero wave number is called the ZGV frequency, wherein the thickness of each layer is described by the thickness ratio of the two plates and the total thickness. The zero group velocity frequency is experimentally measured, the corresponding thickness ratio is determined, and the thickness of each layer is calculated.

[0006] In one embodiment, an excitation and detection experiment of the Lamb wave ZGV resonance mode is carried out by laser excitation and laser Doppler vibrometer detection to obtain the ZGV frequency of the sample to be tested, and then the corresponding thickness ratio is found by comparing the theoretical ZGV frequency with thickness ratio curve, thereby obtaining the thickness of each layer.

[0007] In one embodiment, the density of the two plates is assumed to be ρ i , the longitudinal wave velocity is c Li , the shear wave velocity is c Ti , thickness ratio is r, i represents the layer, i=1, 2, total thickness is d, so we can get the matrix of double-layer plate Lamb wave frequency f and thickness ratio r that satisfies the following determinant value zero:

[0008]

[0009] Where: p i satisfy q i satisfy k is the wave number, ω is the angular frequency, ω=2πf, f is the frequency, is the Lamé constant, is the transverse wave vector.

[0010] In one embodiment, when the material density, shear wave velocity and total thickness are known, the Lamb wave dispersion curves under different thickness ratios r are obtained through the matrix determinant with a value of zero, and the ZGV frequency corresponds to the point on the curve with a slope of zero but a wave number k≠0, and a curve of the ZGV frequency changing with the thickness ratio r is drawn.

[0011] In one embodiment, the frequency value of the spectrum resonance peak is compared with the curve of the ZGV frequency changing with the thickness ratio, and the corresponding thickness ratio r is found, and the thickness h1 of the first layer of the double-layer thin plate can be obtained as follows: The thickness of the second layer h2 is: Where d is the total thickness.

[0012] In a second aspect, the present invention provides a device for measuring the thickness of each layer of a double-layer thin plate based on ultrasonic Lamb wave zero group velocity resonance, comprising an Nd:YAG laser, an attenuation plate, a convex lens, a laser Doppler vibrometer, an oscilloscope and a computer; the Nd:YAG laser emits a pulsed laser, which is incident on the convex lens through the attenuation plate, and the energy of the incident laser is controlled by adjusting the attenuation plate. The light beam is focused by the convex lens into a circular spot and irradiated on the front surface of the sample; the laser Doppler vibrometer emits a laser, which is self-focused by the system into a tiny light spot and vertically incident on the rear surface of the sample, and the detection light and the excitation light are aligned; the ultrasonic signal is converted into an electrical signal and displayed on the oscilloscope, and the time domain signal is Fourier transformed to obtain a spectrum, and the spectrum resonance peak frequency value is compared with the curve of the ZGV frequency changing with the thickness ratio to find the corresponding thickness ratio r, and the thickness h1 of the first layer of the double-layer thin plate is calculated as follows: The thickness of the second layer h2 is: Where d is the total thickness.

[0013] In one embodiment, the wavelength of the Nd:YAG laser is 1060 nm and the pulse width is 7 ns.

[0014] In a third aspect, the present invention provides a computer device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the method described in the first aspect when executing the program.

[0015] In a fourth aspect, the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method described in the first aspect.

[0016] In a fifth aspect, the present invention provides a computer program product, comprising a computer program, which implements the steps of the method described in the first aspect when executed by a processor.

[0017] Compared with the prior art, the beneficial features of the present invention are: Lamb wave is a plate wave. When the thickness of the double-layer thin plate is of the order of the wavelength or even smaller than the wavelength, the thickness of each layer can be determined by detecting the ZGV frequency of the Lamb wave of the double-layer thin plate. Compared with the traditional pulse echo method, there is no echo aliasing at the upper and lower surfaces and the interface, and the thickness of the layer cannot be measured.

[0018] The present invention is further described in detail below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the geometric description of the aluminum / tin double-layer thin plate.

[0020] Figure 2 It is the curve of the ZGV frequency of Lamb wave S1 mode of aluminum / tin double-layer thin plate changing with the thickness ratio r.

[0021] Figure 3 The figure is a schematic diagram of the experimental setup for implementing this method. DETAILED DESCRIPTION

[0022] The present invention proposes a Lamb wave zero group velocity method for measuring the thickness of each layer of a double-layer thin plate. The frequency of the Lamb wave zero group velocity (Zero-Group-Velocity, ZGV) of the double-layer thin plate is related to the thickness ratio of the two plates. The relationship between the wave number k and frequency f of the Lamb wave of the double-layer thin plate and the thickness ratio of the thin plates is theoretically derived. Lamb wave dispersion curves under different thickness ratios are drawn. The frequency of the point on the dispersion curve with a slope of zero but a non-zero wave number is called the ZGV frequency, wherein the thickness of each layer is described by the thickness ratio of the two plates and the total thickness.

[0023] Assume that the density of the two plates is ρ i , the longitudinal wave velocity is c Li , the shear wave velocity is c Ti , thickness ratio is r, i represents the layer, i=1, 2, total thickness is d, so we can get the matrix of double-layer plate Lamb wave frequency f and thickness ratio r that satisfies the following determinant value zero:

[0024]

[0025] Where: p i satisfy q i satisfy k is the wave number, ω is the angular frequency, ω=2πf, f is the frequency, is the Lamé constant, is the transverse wave vector.

[0026] The Lamb wave dispersion curve of a double-layer thin plate of known material at different thickness ratios is plotted using the above formula to determine the ZGV frequency, and a curve showing the ZGV frequency changing with the thickness ratio is plotted. An excitation detection experiment of the Lamb wave ZGV resonance mode is performed to obtain the ZGV frequency of the sample, and then the corresponding thickness ratio is found by comparing the theoretical ZGV frequency curve showing the change with the thickness ratio of the double-layer thin plate, thereby obtaining the thickness of each layer.

[0027] The specific implementation of this method is as follows:

[0028] In the first step, when the material density, shear wave velocity and total thickness of the double-layer thin plate are known, the Lamb wave dispersion curves under different thickness ratios r are obtained. The frequency of the point on the dispersion curve with a slope of zero but a non-zero wave number is the ZGV frequency.

[0029] The second step is to draw a curve of the ZGV frequency changing with the thickness ratio r of the two plates;

[0030] The third step is to conduct an excitation and detection experiment of the Lamb wave ZGV resonance mode by laser excitation and laser Doppler vibrometer detection to obtain the ZGV frequency of the sample;

[0031] The Nd:YAG laser with a wavelength of 1060nm and a pulse width of 7ns emits pulsed laser light, which is incident on the convex lens through the attenuation plate. The energy of the incident laser light can be controlled by adjusting the attenuation plate. The light beam is focused into a circular spot by the convex lens and irradiated on the front surface of the sample. The laser Doppler vibrometer emits laser light, which is self-focused into a tiny light spot on the rear surface of the sample by the system, and the detection light and the excitation light are aligned. The ultrasonic signal is converted into an electrical signal and displayed on the oscilloscope. The time domain signal is Fourier transformed to obtain the spectrum, and the resonance peak frequency value in the spectrum is read.

[0032] Step 4: Use the ZGV frequency measured in step 3 to find the corresponding thickness ratio r in the curve of step 2, and then the thickness of each layer of the double-layer plate can be calculated. The thickness ratio of the two plates is r, and the total thickness is d, then the thickness of the first layer h1 is: The thickness of the second layer h2 is:

[0033] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0034] Example

[0035] The present invention provides a Lamb wave zero group velocity method for measuring the thickness of each layer of a double-layer thin plate. Taking an aluminum / tin double-layer thin plate as an example, the density and shear wave velocity of the aluminum plate and the tin plate are known, and the thickness of each of the two plates is to be measured. Figure 1 Schematic diagram of the geometry of the double-layer plate.

[0036] The first step is to calculate the Lamb wave dispersion curve of the aluminum / tin double-layer thin plate under different thickness ratios r through the equation satisfied by the Lamb wave wave number k and frequency f, and find the S1 mode ZGV frequency on the dispersion curve. The ZGV frequency corresponds to the frequency of the point on the dispersion curve where the slope is zero but the wave number is not zero, where r is the ratio of the aluminum layer thickness to the tin layer thickness, and its range is [1 / 9,9];

[0037] The second step is to draw a curve of the ZGV frequency of Lamb wave S1 mode of aluminum / tin double-layer thin plate as the thickness ratio r of the two plates changes. Figure 2 ,It can be found from the figure that the ZGV frequency of Lamb wave S1 mode of double-layer thin plates composed of these two materials increases monotonically with the thickness ratio r of the two plates, and the two are in a one-to-one correspondence;

[0038] The third step is to conduct an excitation and detection experiment of the Lamb wave ZGV resonance mode by using laser excitation and laser Doppler vibrometer detection. Figure 3The experimental setup diagram is shown in Figure 2. The S1 modal ZGV frequency of the aluminum / copper double-layer sheet sample is obtained by detection, where the thickness of the aluminum layer and the tin layer in the double-layer sheet is unknown, and the total thickness can be read by a vernier caliper;

[0039] The Nd:YAG laser with a wavelength of 1060nm and a pulse width of 7ns emits pulsed laser light, which is incident on the convex lens through the attenuation plate. The energy of the incident laser light can be controlled by adjusting the attenuation plate. The light beam is focused into a circular spot by the convex lens and irradiated on the front surface of the sample. The laser Doppler vibrometer emits laser light, which is self-focused into a tiny light spot on the rear surface of the sample by the system, and the detection light and the excitation light are aligned. The ultrasonic signal is converted into an electrical signal and displayed on the oscilloscope. The time domain signal is Fourier transformed to obtain the spectrum, and the resonance peak frequency value in the spectrum is read.

[0040] Step 4: Since the Lamb wave S1 mode ZGV frequency of the aluminum / tin double-layer thin plate corresponds to the thickness ratio r of the two plates, the S1 mode ZGV frequency measured in step 3 can be used to find the only thickness ratio in the curve of step 2. The total thickness of the double-layer board is The thickness of each layer can be calculated. Aluminum layer thickness for: Tin layer thickness for:

Claims

1. A method for measuring the thickness of each layer of a double-layer plate based on ultrasonic Lamb wave zero group velocity resonance, characterized in that: The zero group velocity frequency of Lamb waves in double-layer thin plates is related to the thickness ratio of the two plates. The relationship between the wave number k and frequency f of Lamb waves in double-layer thin plates and the thickness ratio of the thin plates is derived. The dispersion curves of Lamb waves under different thickness ratios are plotted. The frequency of the point on the dispersion curve with a slope of zero but a non-zero wave number is called the ZGV frequency, where the thickness of each layer is described by the thickness ratio of the two plates and the total thickness. The zero group velocity frequency is measured experimentally, the corresponding thickness ratio is determined, and the thickness of each layer is calculated. Among them, the excitation and detection experiment of Lamb wave ZGV resonance mode is carried out by laser excitation and laser Doppler vibrometer detection to obtain the ZGV frequency of the sample to be tested, and then the corresponding thickness ratio is found by comparing the theoretical ZGV frequency with thickness ratio curve, so as to obtain the thickness of each layer; The method for measuring the thickness of each layer of a double-layer plate based on ultrasonic Lamb wave zero group velocity resonance is as follows: Assuming the density of the two plates is ρ i , the longitudinal wave velocity is c Li , the shear wave velocity is c Ti , thickness ratio is r, i represents the layer, i=1, 2, total thickness is d, so we can get the matrix of double-layer plate Lamb wave frequency f and thickness ratio r that satisfies the following determinant value zero: Where: p i satisfy q i satisfy k is the wave number, ω is the angular frequency, ω=2πf, f is the frequency, is the Lamé constant, is the transverse wave vector; When the material density, shear wave velocity and total thickness are known, the Lamb wave dispersion curves under different thickness ratios r are obtained through the matrix determinant with a value of zero. The ZGV frequency corresponds to the point on the curve with a slope of zero, but the wave number k≠0, and the curve of the ZGV frequency changing with the thickness ratio r is drawn; By comparing the frequency value of the spectrum resonance peak with the curve of the ZGV frequency changing with the thickness ratio, and finding the corresponding thickness ratio r, the thickness h1 of the first layer of the double-layer plate can be obtained as: The thickness of the second layer h2 is: Where d is the total thickness.

2. A device for measuring the thickness of each layer of a double-layer thin plate based on ultrasonic Lamb wave zero group velocity resonance, characterized in that: The invention comprises an Nd:YAG laser, an attenuation plate, a convex lens, a laser Doppler vibrometer, an oscilloscope and a computer; the Nd:YAG laser emits pulsed laser, which is incident on the convex lens through the attenuation plate, and the energy of the incident laser is controlled by adjusting the attenuation plate, and the light beam is focused by the convex lens into a circular light spot and irradiated on the front surface of the sample; the laser Doppler vibrometer emits laser, which is self-focused by the system into a tiny light spot and vertically incident on the rear surface of the sample, and the detection light and the excitation light are aligned; the ultrasonic signal is converted into an electrical signal and displayed on the oscilloscope, and the time domain signal is Fourier transformed to obtain a spectrum, and the spectrum resonance peak frequency value is compared with the curve of the ZGV frequency changing with the thickness ratio, and the corresponding thickness ratio r is found, and the thickness h1 of the first layer of the double-layer thin plate is calculated as follows: The thickness of the second layer h2 is: Where d is the total thickness.

3. The device for measuring the thickness of each layer of a double-layer thin plate based on ultrasonic Lamb wave zero group velocity resonance according to claim 2 is characterized in that: The wavelength of the Nd:YAG laser is 1060nm and the pulse width is 7ns.

4. A computer device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the steps of the method described in claim 1 are implemented.

5. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method described in claim 1 are implemented.

6. A computer program product comprising a computer program, characterized in that When the computer program is executed by a processor, the steps of the method described in claim 1 are implemented.

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