Metal internal defect rapid imaging detection method based on frequency domain interpolation and laser ultrasound

Abstract

The invention discloses a metal internal defect rapid imaging detection method based on frequency domain interpolation and laser ultrasound, and belongs to the field of laser ultrasound nondestructive testing. The method comprises the following steps: firstly, carrying out large-interval rapid scanning on a local area of the surface of a tested piece by utilizing laser ultrasound, obtaining a group of ultrasonic body wave time domain signals with the number of x at different positions of the surface of a material, and then carrying out two-dimensional Fourier transform on the obtained time domain signals to obtain a frequency-wave number two-dimensional signal matrix; zero filling is carried out on a high-frequency part in the frequency domain, and then two-dimensional inverse Fourier transform is carried out to obtain interpolation to obtain a group of n time domain signal matrixes (n is gt; and finally, carrying out synthetic aperture imaging processing on the interpolated time domain signal to obtain a high-resolution synthetic aperture imaging result. According to the invention, for metal internal defect detection and imaging, the problems of long time consumption, large required data volume and the like of traditional laser ultrasonic synthetic aperture detection can be effectively solved, and the laser ultrasonic synthetic aperture detection efficiency and the imaging resolution are greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of non-destructive testing, and particularly relates to a rapid imaging detection method for metal internal defects based on frequency domain interpolation and laser ultrasound, which can achieve rapid detection and high-resolution imaging of metal internal defects, and effectively improves the detection efficiency of laser ultrasound synthetic aperture. Background Art

[0002] In the past thirty years, with the steady installation and commissioning of nuclear power, offshore wind power, etc. in China, the problem of long-term service safety of core equipment in extreme environments such as high temperature, irradiation, and salt spray has become increasingly prominent. Conventional detection methods are difficult to achieve real-time in-situ detection, and there is an urgent need to develop high-precision real-time in-situ detection technologies and equipment that meet the requirements of the above extreme environments. Ultrasonic synthetic aperture detection has the advantages of high sensitivity and good adaptability to complex components, and has been widely used in non-destructive testing of key equipment. Laser ultrasound has the advantages of long distance, non-contact, no need for coupling agent, both excitation and detection can be scanned mobilely and the number is not limited, etc., and can be applied to harsh environments such as high temperature, high pressure, and high corrosion. Therefore, laser ultrasound synthetic aperture detection can meet the requirements of good accessibility and high detection accuracy for the parts prone to structural damage under extreme environmental conditions.

[0003] In order to improve the detection ability of laser ultrasound synthetic aperture for internal defects of core equipment materials and realize the visualization of internal defects, scholars have proposed various defect imaging algorithms. Laser ultrasound synthetic aperture technology is a technology that combines synthetic aperture imaging and laser ultrasound. Laser ultrasound synthetic aperture technology combines the advantages of laser ultrasound and synthetic aperture imaging, such as: obtaining imaging data at a long distance, exciting various modes of ultrasonic waves, etc. In order to obtain high-spatial-resolution and high-signal-to-noise-ratio laser ultrasound synthetic aperture defect images, traditional laser ultrasound synthetic aperture usually requires high-spatial-resolution scanning on the sample surface to obtain enough scanning point information, which requires a very long scanning time, thus severely limiting the application of this technology in engineering. Therefore, a fast laser ultrasound imaging detection technology is needed to effectively improve the detection efficiency. Summary of the Invention

[0004] In order to solve the problem that the data acquisition process of the current laser ultrasound synthetic aperture (SAFT) detection technology takes a long time, the purpose of the present invention is to provide a rapid imaging detection method for metal internal defects based on frequency domain interpolation and laser ultrasound, which greatly improves the detection efficiency and imaging resolution of laser ultrasound synthetic aperture.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A rapid imaging detection method for metal internal defects based on frequency domain interpolation and laser ultrasound, comprising the following steps:

[0007] S1. Coincide the excitation laser and the detection laser on the surface of the test piece, set the scanning pitch as Δx and the total scanning length as L, and move the test piece along the scanning direction to perform large-pitch rapid one-dimensional scanning detection;

[0008] S2. Obtain the ultrasonic body wave time-domain signals S1(t), S2(t)…S x (t) of x measurement points through one-dimensional scanning, and sequentially form a two-dimensional signal data matrix S(t, x);

[0009] S3. Perform two-dimensional Fourier transform on the two-dimensional signal data matrix S(t, x) to obtain a frequency-wavenumber two-dimensional signal matrix U(f, k);

[0010] S4. In the frequency-wavenumber two-dimensional signal matrix, fill the high-frequency part with zeros in the frequency domain to obtain an interpolated frequency-wavenumber two-dimensional signal matrix U'(f, k);

[0011] S5. Perform inverse two-dimensional Fourier transform on the interpolated frequency-wavenumber two-dimensional signal matrix U'(f, k) to obtain a time-domain signal matrix S'(t, n) with increased virtual scanning points, where n is an integer greater than 2*x;

[0012] S6. Calculate the virtual scanning step according to the number of scanning points of the time-domain signal matrix S'(t, n) with increased virtual scanning points, and perform virtual focusing imaging processing on the internal defects of the test piece through the laser ultrasonic synthetic aperture imaging method to obtain the imaging of the internal defects of the metal.

[0013] The reason for filling the high-frequency part with zeros in the frequency domain described in S4 is that after the two-dimensional Fourier transform of the time-domain signal, the frequency-wavenumber two-dimensional signal matrix shows a centrosymmetric bilateral spectrum, that is, the low-frequency and low-wavenumber signal components are distributed at the edge of the matrix, and the high-frequency and high-wavenumber components are distributed at the center of the matrix; since the scanning of the two-dimensional signal fully satisfies the sampling theorem, the amplitude of the signal at the center of the matrix approaches zero, and all the information of the time-domain signal is already included in the original spectrum. Therefore, without destroying the time-domain signal, only the high-frequency part of the spectrum needs to be filled with zeros to improve the resolution of the time-domain signal.

[0014] When detecting by the method of the present invention, large-step scanning is performed to obtain detection signals of a small number of sparse scanning points, and then the detection signals of dense scanning points are reconstructed through frequency-wavenumber domain interpolation signal processing, so that while greatly reducing the detection time, problems such as low imaging resolution caused by fewer acquired signals will not occur.

[0015] In step S6, the laser ultrasonic synthetic aperture imaging method is a virtual focusing post-processing imaging technology based on full matrix acquisition, with high algorithm accuracy and flexibility. The synthetic aperture imaging method is based on the principle of delay summation to achieve focusing at each point in a predefined region of interest. According to the principle of wave superposition, the signal intensity of any point p(x, y) in the measured region can be obtained using the full matrix data to achieve image characterization within the region. A rectangular coordinate system is established with the center of the excitation laser as the origin. Point P is an arbitrary point within the test piece, with coordinates (x, y). The distance from point P to the detection laser is calculated to obtain the amplitude of point P in each column of the echo signal. The signals are superimposed to obtain the acoustic wave amplitude I(x, y) representing the information of this point;

[0016]

[0017] Where: S ij —— Amplitude information of point P in the echo signal excited by excitation laser i and received by detection laser j ij —— The entire process time from when the acoustic wave is emitted to point P until its echo is received by the detection laser, defined as:

[0018]

[0019] Where: c—— The propagation speed of the acoustic wave inside the test piece

[0020] I(x, y) is the acoustic wave amplitude after superposition of the pixel points inside the test piece, which can be used to characterize the defect situation inside the test piece.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention discloses a rapid imaging detection method for internal defects of metals based on frequency domain interpolation and laser ultrasonics. For the internal defects of the metal materials of core equipment, laser ultrasonics is used to perform one-dimensional scanning of a local area on the surface of the measured object, and a set of ultrasonic body wave time domain signals are obtained at different positions on the material surface. Then, a two-dimensional Fourier transform is performed on the obtained time domain signals to obtain the frequency - wavenumber spectrum. Next, the high-frequency part is zero-filled in the frequency domain, and then an inverse two-dimensional Fourier transform is performed to obtain the interpolated time domain signals. Finally, synthetic aperture imaging processing is performed on the interpolated time domain signals to obtain a high-resolution synthetic aperture imaging result. For the detection and imaging of internal defects of metal materials, when detecting, large-step scanning is performed to obtain detection signals of a small number of sparse scanning points, and then the signals of the unscanned points are reconstructed and restored through frequency - wavenumber domain interpolation signal processing to generate detection signals of dense scanning points, which can effectively solve problems such as long detection time and large amount of required data in traditional laser ultrasonic synthetic aperture detection, and greatly improve the detection efficiency and imaging resolution of laser ultrasonic synthetic aperture. Description of the Drawings

[0023] Figure 1 Schematic diagram of laser ultrasonic sparse scanning detection related to the present invention;

[0024] Figure 2 Flow chart of the specific implementation steps of the present invention;

[0025] Figure 3 Schematic diagram of the signal processing process related to the present invention;

[0026] Figure 4 Schematic diagram of laser ultrasonic synthetic aperture (SAFT) imaging. Specific implementation mode

[0027] The technical solution of the present invention will be further described in detail below in conjunction with the specific embodiments of the present invention and the corresponding drawings. The described embodiments are only part of the embodiments of the present invention, not all embodiments. All other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present invention.

[0028] The steps of the embodiment are as follows Figure 1 、 Figure 2 and Figure 3 shown, and are described in detail as follows:

[0029] S1. As shown in Figure 1 , the excitation laser and the detection laser are made to coincide on the surface of the test piece, and the test piece is moved along the scanning direction shown in the figure for large-spacing rapid one-dimensional scanning detection. The scanning pitch is set to Δx = 4 mm, the total scanning length is L = 60 mm, 15 groups of signals are scanned, and the scanning time is t = 1 min;

[0030] S2. The body wave time-domain signals S1(t), S2(t)... S x (t) of x measurement points are obtained by one-dimensional scanning, and are sequentially composed into a two-dimensional signal data matrix S(t, x);

[0031] S3. Perform two-dimensional Fourier transform on the two-dimensional signal data matrix S(t, x) to obtain a frequency-wave number two-dimensional signal matrix U(f, k);

[0032] S4. In the frequency-wave number two-dimensional signal matrix, the high-frequency part is filled with zeros in the frequency domain so that k' = 8k, and the interpolated frequency-wave number two-dimensional signal matrix U'(f, k) is obtained;

[0033] S5. Perform two-dimensional inverse Fourier transform on the interpolated frequency-wave number two-dimensional signal matrix U'(f, k) to obtain a time-domain signal matrix S'(t, n) with increased virtual scanning points, where n is an integer greater than 2*x;

[0034] S6. Calculate the virtual scanning step according to the number of scanning points of the time-domain signal matrix S'(t,n) with increased virtual scanning points, and perform virtual focusing imaging processing on the interior of the test piece through the laser ultrasonic synthetic aperture imaging method to obtain the imaging of metal internal defects.

[0035] As Figure 4 shown, the excitation laser and the detection laser perform equidistant scanning according to the scanning step of Δx to obtain signal data. d is the sound path from the excitation point to the measured point P. P is any point inside the test piece, with coordinates (x p , z p ), and (x i , z0) is the position where the laser moves. N is the number of scanning points.

Claims

1. A rapid imaging detection method for internal defects of metals based on frequency domain interpolation and laser ultrasound, characterized in that It includes the following steps: S1. Coincide the excitation laser and the detection laser on the surface of the test piece, set the scanning pitch as Δx and the total scanning length as L, and move the test piece along the scanning direction for large-pitch rapid one-dimensional scanning detection; S2. One-dimensional scanning to obtain the ultrasonic body wave time-domain signals S1(t), S2(t)…S x (t) of x measurement points, and sequentially form a two-dimensional signal data matrix S(t, x); S3. Perform two-dimensional Fourier transform on the two-dimensional signal data matrix S(t, x) to obtain the two-dimensional signal matrix U(f, k) of frequency - wavenumber; S4. In the two-dimensional signal matrix of frequency - wavenumber, fill the high-frequency part with zeros in the frequency domain to obtain the interpolated two-dimensional signal matrix U'(f, k) of frequency - wavenumber; S5. Perform inverse two-dimensional Fourier transform on the interpolated two-dimensional signal matrix U'(f, k) of frequency - wavenumber to obtain the time-domain signal matrix S'(t, n) with increased virtual scanning points, where n is an integer greater than 2*x; S6. Calculate the virtual scanning step according to the number of scanning points of the time-domain signal matrix S'(t, n) with increased virtual scanning points, and perform virtual focusing imaging processing on the internal defects of the test piece by the laser ultrasonic synthetic aperture imaging method to obtain the imaging of internal defects of the metal.

2. A rapid imaging detection method for internal defects in metals based on frequency domain interpolation and laser ultrasound according to claim 1, characterized in that, The filling of the high-frequency part with zeros in the frequency domain in S4 is because after the two-dimensional Fourier transform of the time-domain signal, the two-dimensional signal matrix of frequency - wavenumber shows a centrosymmetric bilateral spectrum, that is, the low-frequency and low-wavenumber signal components are distributed at the edge of the matrix, and the high-frequency and high-wavenumber components are distributed at the center of the matrix; since the scanning of the two-dimensional signal fully satisfies the sampling theorem, the amplitude of the signal at the center of the matrix approaches zero, and all the information of the time-domain signal is already included in the original spectrum. Therefore, without destroying the time-domain signal, only the high-frequency part of the spectrum needs to be filled with zeros to improve the resolution of the time-domain signal.

3. A rapid imaging detection method for internal defects of metals based on frequency domain interpolation and laser ultrasound according to claim 1, characterized in that During detection, large-step scanning is performed to obtain detection signals of a small number of sparse scanning points, and then the detection signals of dense scanning points are reconstructed through frequency - wavenumber domain interpolation signal processing, so that while greatly reducing the scanning time, the problem of low imaging resolution caused by obtaining fewer signals will not occur.

4. A rapid imaging detection method for internal defects of metals based on frequency domain interpolation and laser ultrasound according to claim 1, characterized in that, The laser ultrasonic synthetic aperture imaging method described in step S6 is a virtual focusing post-processing imaging technique. The laser ultrasonic synthetic aperture imaging method is based on the principle of delay summation to achieve focusing at each point in a predefined region of interest. According to the principle of wave superposition, the signal amplitude at any point p(x p ,z p ) in the measured region is obtained using all the scan data to achieve image characterization within the region; a rectangular coordinate system is established with the center of the excitation laser as the origin. Point P is an arbitrary point within the test specimen, with coordinates (x p ,z p ). The distance from point P to the detection laser is calculated to obtain the amplitude of point P in each column of echo signals, and the signals are superimposed to obtain the acoustic wave amplitude I(x,z) characterizing the information of this point; Where: S i —— The amplitude information of point P in the echo signal excited by the excitation laser and received by the detection laser, t i —— The total time from when the sound wave is emitted to when its echo is received by the detection laser after reaching point P, defined as: In the formula: c—the speed of sound propagation inside the test piece I(x, z) is the acoustic wave amplitude after superposition of the internal pixel points of the test piece, and thus characterizes the defect situation inside the test piece.

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