Near-surface defect positioning method based on laser ultrasonic surface wave

Abstract

The invention relates to a near-surface defect positioning method based on a laser ultrasonic surface wave, which comprises the following steps of: extracting a defect tiny inverse signal and calculating reflection time delay by utilizing a high-frequency surface wave signal excited by laser through a wavenumber domain method to determine a defect position, and determining the burial depth of the defect through a reflection wave amplitude value of the defect; comprising the steps of S1, laser ultrasonic finite element simulation; s2, acquiring high-frequency laser ultrasonic signals; s3, direct wave and reflected wave extraction and time delay calculation; s4, determining defect space coordinates; and S5, determining the burial depth of the defect. The method makes full use of the advantages of high frequency and long propagation distance of surface waves, can effectively solve the problem of defect detection and positioning of the tested piece, can quickly detect the submillimeter defects, is high in positioning precision, and is suitable for positioning and detecting the submillimeter defects on the near surface of a structure.

Description

Technical Field

[0001] The invention belongs to the technical field of nondestructive testing, and in particular relates to a near-surface defect positioning method based on laser ultrasonic surface waves. Background Art

[0002] As the main raw material for mechanical equipment and its key components, metal materials are widely used in various important fields such as railway tracks, aerospace, petrochemicals, deep-sea exploration, medical equipment, etc. Due to processing or long-term use, large or small defects may appear inside metal workpieces. These defects change the mechanical properties of the workpieces to a certain extent, causing quality problems and even safety hazards. Accurate detection of defects is very critical.

[0003] At present, the conventional internal defect detection methods are mainly the reflection method and transmission method of shear wave or longitudinal wave, which are complex in calculation, poor in detection accessibility, small in detection range and low in detection efficiency. In order to make up for the limitations of traditional ultrasonic detection technology, new non-contact non-destructive testing methods represented by electromagnetic ultrasonic testing, air-coupled ultrasonic testing and laser ultrasonic testing have emerged in recent years:

[0004] (1) Electromagnetic ultrasonic testing: Based on the principle of electromagnetic induction, various modes of ultrasonic waves can be excited in the medium being tested, which can be used to detect surface and internal defects of metal materials. This method does not require a coupling medium and can also be used to detect metal materials under high temperature conditions. However, the conductivity and geometric shape of the test piece have a great influence on the electromagnetic ultrasonic testing results. Compared with conventional ultrasonic methods, the efficiency and sensitivity of electromagnetic ultrasonic testing are lower.

[0005] (2) Air-coupled ultrasonic testing: Air is used as a coupling medium for non-destructive testing of materials and characterization of material properties. It can be used for non-contact non-destructive testing in high and low temperature environments. Due to the huge acoustic impedance difference between air and the test piece, and the large absorption rate of air to high-frequency sound waves, compared with traditional ultrasonic technology, the ultrasonic waves excited by this method are severely attenuated in the air, with low detection sensitivity and poor signal-to-noise ratio. In addition, this method has high processing accuracy requirements and processing costs for air-coupled ultrasonic sensors.

[0006] (3) Laser ultrasonic testing method: The emergence of laser ultrasonic testing technology, with its advantages of high resolution and easy integration, has become a hot topic in non-destructive testing research at home and abroad, providing a new means for non-destructive testing and evaluation of materials. Laser ultrasonic testing uses laser as the excitation source of ultrasonic waves. It can detect changes in material structure (such as small defects, hardness, residual stress, elastic modulus, grain size, etc.) without destroying the structural characteristics and physical properties of the material. Compared with traditional ultrasonic testing methods, laser ultrasonic technology has the advantages of rich excitation modes, wide bandwidth, high sensitivity to small cracks, high spatial resolution, and suitable for completing large-area rapid scanning and detection of test pieces. Laser ultrasonic testing of defects is generally divided into two methods: pulse echo method and transmission capture method. By detecting various modes of ultrasonic waves (longitudinal waves, transverse waves, surface waves, etc.) excited by laser pulses, the presence of defects is determined by the reflected or transmitted signals propagating to the surface defects. Among the various ultrasonic propagation modes excited by lasers, surface waves only propagate along the surface of the plate. The movement of the wave is limited to an area near the surface. Its propagation depth is equivalent to the wavelength, the excitation efficiency is high, and it is easy to detect. It is suitable for the detection of near-surface defects.

[0007] Therefore, the present invention proposes a near-surface defect positioning method based on laser ultrasonic surface waves. Summary of the invention

[0008] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a near-surface defect locating method based on laser ultrasonic surface waves, which can solve the problem of defect detection and locating in a metal material plate specimen structure.

[0009] The present invention solves the technical problem by the following technical solutions:

[0010] A near-surface defect positioning method based on laser ultrasonic surface waves, wherein the defect positioning system adopted by the method comprises a test piece, a laser excitation module, an ultrasonic receiving module, an optical path integration unit and a computer; a pulse laser probe and an ultrasonic probe are arranged on the defect near-surface side of the test piece, the pulse laser probe fixes the laser excitation point, and the ultrasonic detection point of the ultrasonic probe can move along the x and y directions; the test piece is fixed on a fixture, the pulse laser emitted by the laser excitation module is integrated by the optical path integration unit and focused into a point or line source laser to irradiate the surface of the test piece and stimulate laser ultrasonic waves, and the ultrasonic receiving module receives the laser ultrasonic signal and transmits the collected data to the computer;

[0011] The steps of the method are:

[0012] S1. Laser ultrasonic finite element simulation: Use finite element simulation software to establish a laser ultrasonic thermal-mechanical coupling model, determine the frequency range and detection depth of the surface wave energy under the material and structure of the test piece, and obtain the amplitude-burial depth curve of the laser ultrasonic under the model;

[0013] S2, high-frequency laser ultrasonic signal acquisition: The ultrasonic receiving module receives the laser ultrasonic signal of the test piece, and moves the ultrasonic detection point in the x (y) direction for multiple times to measure multiple groups of data. The high-frequency laser ultrasonic signal with sub-millimeter defect information is obtained through the frequency band range obtained by S1 simulation with a high-pass filter or a band-pass filter;

[0014] S3. Extraction and delay calculation of direct wave and reflected wave: Assuming the propagation direction of the surface wave R is the positive direction of the x(y) axis, the wavenumber domain filtering method is used to extract the direct high-frequency surface wave signal propagating along the positive direction of x(y) and the reflected surface wave signal propagating along the negative direction of x(y):

[0015] The collected wave field data is transformed into the wave number domain through three-dimensional Fourier transform, and its expression is:

[0016]

[0017] Where: w(x,y,t) is the out-of-plane displacement of a single measurement point;

[0018] W(kx,ky,f) is the wavefield data in the frequency-wavenumber domain;

[0019] When the propagation direction of the surface wave is the positive direction of the x(y) axis, the k x >0(k y >0) to obtain the direct surface wave signal, and use k x <0(k y <0) to obtain the reflected surface wave signal, implement filtering in the wave number domain, and obtain a single direct wave and reflected wave signal, expressed as:

[0020]

[0021] Where: W d (kx,ky,f)W r (kx,ky,f) are the wavefield data in the frequency-wavenumber domain of the direct surface wave and the reflected surface wave respectively;

[0022] Perform three-dimensional Fourier inverse transform on the wave number wave field data of the direct wave and the reflected wave to return to the spatial domain. The expression is:

[0023]

[0024] Get the direct wave signal w in the spatial domain d(x,y,t) and the reflected wave w r (x, y, t) signal, the waveform cross-correlation algorithm is used to extract the waveform delay, the expression is:

[0025]

[0026] B(τ) is the normalized coefficient of the cross-correlation calculation. When the correlation coefficient is the largest, the value τ at this time is the time delay Δt;

[0027] S4. Determination of defect spatial coordinates: The surface wave velocity is measured experimentally or calculated by the following formula:

[0028]

[0029] Where: C R represents the surface wave velocity; E is the elastic modulus of the material; ρ is the material density; σ is the Poisson's ratio of the material;

[0030] The defect is located in the x(y) direction by the time difference between the direct wave reaching the ultrasonic probe and the surface wave signal reflected from the defect reaching the ultrasonic probe.

[0031] Repeat operations S1 to S4, and move the detection point along the y direction while fixing the x coordinate to locate the defect in the y direction;

[0032] S5. Determination of defect burial depth: For sub-millimeter defects, the amplitude of the reflected wave is almost unaffected by the defect size, but has an approximately linear relationship with the defect burial depth. By comparing with the amplitude-burial depth curve of S1, the burial depth of the defect is determined, the depth of the defect is located, and the three-dimensional coordinates of the defect are determined.

[0033] The advantages and beneficial effects of the present invention are:

[0034] Compared with the prior art, the near-surface defect positioning method of the present invention is based on laser ultrasonic surface waves, which fully utilizes the advantages of high frequency and long propagation distance of surface waves, can effectively solve the defect detection and positioning problems of the tested parts, and can quickly detect sub-millimeter defects with high positioning accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a flow chart of the present invention;

[0036] Figure 2 It is the finite element simulation sound field diagram of the present invention;

[0037] Figure 3 It is a reflection amplitude-burial depth curve diagram of the present invention;

[0038] Figure 4 A diagram of a defect location system of the present invention;

[0039] Figure 5 It is the positioning principle diagram of the present invention. DETAILED DESCRIPTION

[0040] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0041] like Figure 1 As shown, a near-surface defect positioning method based on laser ultrasonic surface waves is innovative in that the steps of the method are:

[0042] (1) Taking the internal defects of aluminum plate as an example, a laser ultrasonic propagation model of aluminum plate with thickness h = 10 mm is established. The laser is incident vertically to the center of the material, the center of the light source is at the origin of the coordinate system, and the radius of the light source is R a is 0.5mm, and the laser pulse light rise time t 0 The time duration is 8ns and the defect size is 1mm.

[0043] The laser ultrasonic propagation sound field is obtained as Figure 2 As shown, it can be seen that obvious surface wave R signals, defect reflection echo rR signals and projection wave tR signals are stimulated. Through acoustic field analysis, the laser ultrasonic frequency band is selected to be 1MHz-2MHz, and the buried depth detection range is below 1mm.

[0044] By changing the defect burial depth through parametric scanning, the relationship curve between amplitude and burial depth is obtained, such as Figure 3 shown.

[0045] (2) Figure 4 As shown, a laser ultrasonic defect positioning system is built. The specifications of the aluminum alloy specimen are 260mm*150mm*17mm, and the size of the defect to be measured is 5mm*0.1mm*0.5mm. The pulse laser and the vibration laser probe are on the near-surface side of the specimen defect. The Nd:YAG laser excites a pulse laser with a wavelength of 532nm, a pulse width of 10ns, a repetition frequency of 20Hz, and a voltage value of 625V, corresponding to a single pulse energy of 14.7mJ. After the optical path is integrated with the focusing lens, a 0.2mm point light source is formed. The laser irradiates the surface of the workpiece to stimulate surface acoustic waves. The measuring laser performs non-contact detection on the high-frequency laser ultrasonic signal of the measuring point. The sampling rate is 62.5MH. The ultrasonic signal is obtained by averaging 20 times and displayed on the computer.

[0046] Keep the laser excitation position fixed, adjust the detection laser position as needed through electron microscope scanning, so as to complete the detection of large areas, select several suitable detection points in the x direction, read the distance L between the excitation laser and the detection laser, and store the laser ultrasonic signal on the computer.

[0047] Since the defect size is sub-millimeter, low-frequency signals cannot be effectively detected and characterized, which will affect the detection accuracy. A 1MHz-2MHz bandpass filter is designed to filter the laser ultrasonic signal to obtain a high-frequency laser ultrasonic signal w(x, y, t).

[0048] (3) In order to extract the direct surface waves and reflected surface waves, the wave field data is transformed into the wave number domain by three-dimensional Fourier transform:

[0049]

[0050] Where: w(x,y,t) refers to the out-of-plane displacement of a single measurement point;

[0051] W(kx,ky,f) is the wavefield data in the frequency-wavenumber domain.

[0052] Define the propagation direction of the wave as the positive direction of the x-axis, then we can use k x >0 to characterize and extract the direct surface wave signal, using k x <0 to characterize the extracted reflected surface wave signal. After filtering, the inverse Fourier transform is performed on it respectively. The expression is:

[0053]

[0054] The direct wave signal w in the spatial domain is obtained respectively d (x,y,t) and the reflected wave w r (x, y, t) signal, the waveform cross-correlation algorithm is used to extract the waveform delay, the expression is:

[0055]

[0056] B(τ) is the normalized coefficient of the cross-correlation calculation. When the correlation coefficient is the largest, the value τ at this time is the time delay Δt. The time delay values ​​of all scanning points are calculated one by one for each detection point.

[0057] (4) From the above process, we can get the flight time delay of the ultrasonic wave and the defect echo as Δt, as follows: Figure 5 The distance between the ultrasonic excitation end and the receiving end is L, then the ultrasonic surface wave speed C is calculated or measured. R , the distance between the ultrasonic receiving end and the defect can be estimated, that is, d = C R ×Δt / 2, establish a coordinate system with the excitation position as the origin, and obtain the x coordinate of the defect as a=L+d, and take the average value after multiple measurements Under the measured x-coordinate, change the y-coordinate of the detection point and repeat the above steps (1) to (4) to achieve the y-coordinate of the defect. Determination of the defect coordinates

[0058] (5) In order to quantitatively analyze the relationship between the reflected surface wave and the crack burial depth, the collected reflected wave and the fitting curve of the amplitude specimen reflected wave amplitude-burial depth are compared to determine the defect burial depth and complete the three-dimensional positioning of the defect. Figure 5 As shown, the expression of the buried depth obtained by simulation in this method is y=-0.49x+1.08, where y is the normalized amplitude and x is the buried depth, thereby obtaining the buried depth of the test piece.

[0059] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A near-surface defect location method based on laser ultrasonic surface waves, characterized in that: The defect positioning system adopted by the method includes a test piece, a laser excitation module, an ultrasonic receiving module, an optical path integration unit and a computer; a pulse laser probe and an ultrasonic probe are arranged on the defect-proximal surface side of the test piece, the pulse laser probe fixes the laser excitation point, and the ultrasonic detection point of the ultrasonic probe can move along the x and y directions; the test piece is fixed on a fixture, the pulse laser emitted by the laser excitation module is integrated by the optical path integration unit and focused into a point or line source laser to irradiate the surface of the test piece and stimulate laser ultrasonic waves, and the ultrasonic receiving module receives the laser ultrasonic signal and transmits the collected data to the computer; The steps of the method are: S1. Laser ultrasonic finite element simulation: Use finite element simulation software to establish a laser ultrasonic thermal-mechanical coupling model, determine the frequency range and detection depth of the surface wave energy under the material and structure of the test piece, and obtain the amplitude-burial depth curve of the laser ultrasonic under the model; S2, high-frequency laser ultrasonic signal acquisition: The ultrasonic receiving module receives the laser ultrasonic signal of the test piece, and moves the ultrasonic detection point in the x (y) direction for multiple times to measure multiple groups of data. The high-frequency laser ultrasonic signal with sub-millimeter defect information is obtained through the frequency band range obtained by S1 simulation with a high-pass filter or a band-pass filter; S3. Extraction and delay calculation of direct wave and reflected wave: Assuming the propagation direction of the surface wave R is the positive direction of the x(y) axis, the wavenumber domain filtering method is used to extract the direct high-frequency surface wave signal propagating along the positive x(y) direction and the reflected surface wave signal propagating along the x(y) direction respectively: The collected wave field data is transformed into the wave number domain through three-dimensional Fourier transform, and its expression is: Where: w(x,y,t) is the out-of-plane displacement of a single measurement point; W(kx,ky,f) is the wavefield data in the frequency-wavenumber domain; When the propagation direction of the surface wave is the positive direction of the x(y) axis, the k x >0(k y >0) to obtain the direct surface wave signal, and use k x <0(k y <0) to obtain the reflected surface wave signal, implement filtering in the wave number domain, and obtain a single direct wave and reflected wave signal, expressed as: Where: W d (kx,ky,f)W r (kx,ky,f) are the wavefield data in the frequency-wavenumber domain of the direct surface wave and the reflected surface wave respectively; Perform three-dimensional Fourier inverse transform on the wave number wave field data of the direct wave and the reflected wave to return to the spatial domain. The expression is: Get the direct wave signal w in the spatial domain d (x,y,t) and the reflected wave w r (x, y, t) signal, the waveform cross-correlation algorithm is used to extract the waveform delay, the expression is: B(τ) is the normalized coefficient of the cross-correlation calculation. When the correlation coefficient is the largest, the value τ at this time is the time delay Δt; S4. Determination of defect spatial coordinates: The surface wave velocity is measured experimentally or calculated by the following formula: Where: C R represents the surface wave velocity; E is the elastic modulus of the material; ρ is the material density; σ is the Poisson's ratio of the material; The defect is located in the x(y) direction by the time difference between the direct wave reaching the ultrasonic probe and the surface wave signal reflected from the defect reaching the ultrasonic probe. Repeat operations S1 to S4, and move the detection point along the y direction while fixing the x coordinate to locate the defect in the y direction; S5. Determination of defect burial depth: For sub-millimeter defects, the amplitude of the reflected wave is almost unaffected by the defect size, but has an approximately linear relationship with the defect burial depth. By comparing with the amplitude-burial depth curve of S1, the burial depth of the defect is determined, the depth of the defect is located, and the three-dimensional coordinates of the defect are determined.

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