A defect imaging method of double-layer material based on improved laser ultrasonic full focusing algorithm

Abstract

The application provides a kind of defect imaging method of double-layer material based on improved laser ultrasonic full focusing algorithm, and relates to laser ultrasonic nondestructive testing technical field;Steps are as follows: in double-layer material, obtain ultrasonic B scanning signal by laser excitation ultrasonic wave, and then constitute full matrix capture data;By optimizing the sound propagation time calculation method, laser ultrasonic directivity coefficient, the laser ultrasonic full focusing algorithm is improved;Through the laser ultrasonic directivity full focusing algorithm, the full matrix capture data is calculated, the directivity full focusing imaging amplitude corresponding to all pixel points in the imaging area is obtained, and the final defect image is constructed.Compared with prior art, the signal-to-noise ratio of defect imaging result graph is greatly improved by improving the full focusing algorithm, and clear defect image is generated.

Description

Technical Field

[0001] This invention relates to the field of laser ultrasonic nondestructive testing, and in particular to a defect imaging method for bilayer materials based on an improved laser ultrasonic total focusing algorithm. Background Technology

[0002] In the field of nondestructive testing, ultrasonic testing is a common method widely used to detect delamination and internal porosity in composite materials. Laser-induced ultrasound is a common excitation method. This technology, based on the thermoelastic effect, excites ultrasonic signals within the material and has the following significant advantages: non-contact and the ability to obtain strong ultrasonic signals. Using a laser as the ultrasonic excitation source can improve the success rate of ultrasonic testing.

[0003] In fields such as aerospace, renewable energy, and civil construction, bilayer materials composed of composite materials and metals are being used more and more widely. Therefore, there is a large demand for internal defect detection of bilayer materials containing anisotropic media.

[0004] However, when ultrasound propagates in a bilayer material, it is refracted at the interface, causing the propagation path of the ultrasound to be deflected. As a result, existing imaging algorithms applicable to single-medium materials cannot image internal defects in bilayer materials.

[0005] Therefore, a laser ultrasonic imaging algorithm suitable for bilayer materials containing anisotropic media is needed to solve the problem of increased imaging error caused by the deviation of the ultrasonic propagation path at the material interface. Summary of the Invention

[0006] This invention discloses a defect imaging method for bilayer materials based on an improved laser-ultrasound full-focusing algorithm. By using a directional coefficient to perform a weighted summation of the full-focusing algorithm, and accurately solving the sound propagation time according to the propagation theory of ultrasound in bilayer materials and Snell's law, the algorithm can be applied to the imaging of internal defects in bilayer materials, solving the problem of increased imaging error caused by the deviation of the ultrasound propagation path at the material interface.

[0007] The technical solution of this invention is: a defect imaging method for bilayer materials based on an improved laser-ultrasound total focusing algorithm, comprising the following steps:

[0008] Step 1: In the bilayer material, ultrasonic waves are excited by laser to obtain ultrasonic B-scan signals, which are then used to form full matrix capture data.

[0009] The bilayer material is a bilayer containing an anisotropic medium. Ultrasonic waves are excited sequentially at excitation positions on the surface of the bilayer material using a pulsed laser. An imaging region is defined within the bilayer material, and ultrasonic waves are reflected at defects within the imaging region. During each excitation, a laser interferometer is used to sequentially detect the ultrasonic signal at detection positions on the surface of the bilayer material. The A-scan ultrasonic signals acquired by the laser interferometer constitute the B-scan ultrasonic signals. The B-scan ultrasonic signals from all excitation positions are then combined to form a full matrix acquisition data set.

[0010] The excitation positions consist of N positions spaced Δd on the sample surface, and the detection positions consist of M positions spaced Δd on the sample surface. The thickness of the imaging region is the same as the sample thickness, and the length is the same as the total length of the M positions.

[0011] Ultrasonic waves are excited at the first excitation position. The ultrasonic waves generate ultrasonic reflection waves at the defects in the imaging area. Ultrasonic A-scan signals are acquired in sequence at all detection positions. The ultrasonic B-scan signals at the first excitation position are composed of the A-scan signals at all detection positions.

[0012] Ultrasonic waves are excited at the second excitation position. The ultrasonic waves generate ultrasonic reflection waves at the defects in the imaging area. Ultrasonic A-scan signals are acquired in sequence at all detection positions. The ultrasonic B-scan signals at the second excitation position are composed of the A-scan signals at all detection positions.

[0013] This process continues until ultrasound waves are excited at the last excitation position in sequence. The ultrasound waves generate reflected ultrasound waves at the defects in the imaging area. Ultrasonic A-scan signals are acquired sequentially at all detection positions. The A-scan signals from all detection positions constitute the ultrasonic B-scan signal at the last excitation position. The ultrasonic B-scan signals from all excitation positions constitute the full matrix acquisition data.

[0014] Step 2: Improve the laser ultrasound full-focusing algorithm by optimizing the sound propagation time calculation method and the laser ultrasound directionality coefficient.

[0015] Based on the propagation law of ultrasound in bilayer materials and Snell's law, the expression for sound propagation time is derived. A laser ultrasound directional coefficient is constructed according to the laser ultrasound directional characteristics. The full focusing algorithm is then improved using the directional coefficient to obtain a directional optimized full focusing algorithm.

[0016] Step 2.1: Based on the propagation law of ultrasound in bilayer materials and Snell's law, derive the sound propagation time, specifically as follows:

[0017] Based on the propagation theory of ultrasound in a bilayer material containing anisotropic media and Snell's law, an expression for sound propagation time is derived. This expression is used to calculate the sound propagation time of any pixel in the imaging region within the full matrix capture data. The sound propagation time expression is:

[0018] ,

[0019] Where t is the sound propagation time, d G1 d represents the distance the ultrasonic wave travels from the excitation location to a pixel location in the imaging region within the first layer of the medium. D1 d represents the distance the ultrasonic wave travels in the second layer of the medium from the excitation location to a pixel location in the imaging region. G2 d represents the distance the ultrasonic wave travels in the first layer of medium from a pixel location in the imaging region to the detection location. D2 This represents the propagation distance of an ultrasonic wave from a pixel location in the imaging region to the detection location in the second medium, where G represents the excitation location, D represents the detection location, c1 represents the ultrasonic wave propagation velocity in the first isotropic medium, and v... g (θ1) represents the ultrasonic energy velocity at a propagation angle of θ1 in the second medium, where θ1 is the refraction angle of the ultrasonic wave in the second medium during its propagation from the excitation position to a pixel position in the imaging region. g (θ2) represents the ultrasonic energy velocity when the propagation angle is θ2 in the second medium. θ2 is the refraction angle in the first medium when the ultrasonic wave propagates from a pixel position in the imaging area to the detection position.

[0020] Step 2.2: Based on the elastic wave theory in anisotropic media, derive the ultrasonic energy velocity in the second layer of media.

[0021] Based on the elastic wave theory in anisotropic media, an expression for the ultrasonic energy velocity in the second layer of media is derived, which is used to calculate the ultrasonic energy velocity in any propagation direction in the imaging region.

[0022] Based on the elastic wave theory in anisotropic media, the ultrasonic phase velocity v in the second layer of media is derived. p ,for:

[0023] ,

[0024] Where ρ is the density of the second layer medium, and Г ijLet i represent the Christoffel tensor, where i represents the row number of the matrix element in the Christoffel tensor, with values ​​of 1, 2, and 3, and j represents the column number of the matrix element in the Christoffel tensor, with values ​​of 1, 2, and 3.

[0025] The expression for Christoffel's voice tensor is:

[0026] ,

[0027] Where C mn The `voigt` marker represents the element value of the elasticity matrix of the material. `m` represents the row number of the element in the elasticity matrix, with values ​​of 1, 2, 3, 4, 5, and 6. `n` represents the column number of the element in the elasticity matrix, with values ​​of 1, 2, 3, 4, 5, and 6. x This represents the direction vector of ultrasound propagation in the second medium, where x represents the direction order and takes values ​​of 1, 2, and 3. The expression for the direction vector is:

[0028] ,

[0029] Where θ represents the angle between the direction vector and the normal to the material surface.

[0030] Based on the theory of ultrasonic wave propagation, the velocity of ultrasonic energy in the second medium is derived. The expression is:

[0031] ,

[0032] in, Let C represent the y-th element in the ultrasonic energy velocity vector. yzab Let represent the fourth-order elastic tensor of the material, and y, z, a, b represent the indices of the spatial dimensions, with values ​​of 1, 2, 3, 4, 5, and 6. z and°u b This represents the z-th and b-th elements in the normalized ultrasonic phase velocity eigenvector, where n a v represents the a-th element in the propagation direction vector. p This is the phase velocity of the ultrasonic wave. From the energy velocity expression, the magnitude of the energy velocity can be obtained as:

[0033] .

[0034] Among them, v g (θ) represents the speed of ultrasonic energy when the propagation angle is θ in the second medium. This represents the first element in the ultrasonic energy velocity vector. This represents the second element in the ultrasonic energy velocity vector. This represents the third element in the ultrasonic energy velocity vector.

[0035] The magnitude and direction of ultrasonic energy velocity during propagation can be obtained through geometric relationships and path-time relationships. Based on the theory of ultrasonic propagation in anisotropic media, the ultrasonic phase velocity corresponding to any ultrasonic energy velocity during propagation can be obtained from a table.

[0036] Step 2.3: Based on the phase velocity and energy velocity of ultrasound in ultrasound propagation, obtain the quantitative evaluation standard of phase velocity, further obtain the evaluation standard of refraction point, and then obtain the ultrasound propagation path of the refraction point.

[0037] According to Snell's law, there are refraction points along the propagation path of ultrasound. In a certain ultrasound excitation and detection process, the interface between the two media between the excitation position and a certain pixel position in the imaging area is discretized into equally spaced points. Each discrete point is used as a refraction point for quantitative evaluation.

[0038] A quantitative evaluation standard for phase velocity is constructed based on the relationship between ultrasonic phase velocity and ultrasonic energy velocity during ultrasonic propagation to determine the accuracy of table lookup. The phase velocity evaluation standard is as follows:

[0039] ,

[0040] Where f1 represents the phase velocity quantification evaluation standard, v p The phase velocity of an ultrasonic wave, v g (θ) represents the velocity of ultrasonic energy, θ diff This indicates the angle between the direction of the ultrasonic phase velocity and the direction of the ultrasonic energy velocity.

[0041] At each refraction point, the direction and magnitude of the phase velocity that minimizes the phase velocity quantification evaluation standard are found, and a refraction point quantification evaluation standard is constructed based on Snell's law. The refraction point evaluation standard is as follows:

[0042] ;

[0043] Where f2 represents the quantitative evaluation standard for the refraction point, θ I This represents the angle of incidence of the ultrasonic wave in the first medium during ultrasonic refraction.

[0044] For each refraction point, calculate the corresponding refraction point quantization standard, find the refraction point that minimizes the refraction point quantization standard, and approximate this refraction point as the refraction point when the ultrasound propagates from the excitation position to a certain pixel in the imaging region. Based on the position of the refraction point and the two-point distance formula, calculate the propagation distance d of the ultrasound in the first layer of the medium when it propagates from the excitation position to a certain pixel in the imaging region. G1And the propagation distance d of the ultrasonic wave in the second medium from the excitation position to a certain pixel position in the imaging region. D1 ;

[0045] Similarly, calculate the refraction point of the ultrasonic reflected wave when it propagates from a pixel in the imaging region to the ultrasonic detection position, and calculate the propagation distance d of the ultrasonic wave in the first medium when it propagates from a pixel in the imaging region to the detection position based on the distance formula between the two points according to the refraction point position result. G2 And the propagation distance d of the ultrasonic wave in the second medium when it travels from a pixel position in the imaging region to the detection position. D2 .

[0046] Step 2.4: Combine the laser ultrasound directional coefficient and the total focusing imaging amplitude to obtain the directional total focusing imaging amplitude, thus obtaining the improved laser ultrasound total focusing algorithm.

[0047] (1) The method for constructing the full focusing algorithm based on the sound propagation time is as follows:

[0048] The sound propagation time expression is fused using the delay superposition rule to construct a full-focus imaging amplitude expression. This amplitude expression is used to calculate the ultrasonic wave reflection signal amplitude corresponding to each pixel in the region under test. The ultrasonic wave reflection signal amplitude expression is the full-focus imaging amplitude expression, which is as follows:

[0049] ,

[0050] Where I is the total focusing imaging amplitude, t is the sound propagation time, and A GD This represents the ultrasound A-scan signal excited at position G and detected at position D. G represents the excitation position, with values ​​of 1, 2, 3...N, and D represents the detection position, with values ​​of 1, 2, 3...M.

[0051] (2) The full-focusing algorithm is optimized using the laser-ultrasound directivity coefficient to obtain a directionally optimized full-focusing algorithm, specifically:

[0052] Combining the laser ultrasound directivity coefficient and the total focusing imaging amplitude, we obtain the directional total focusing imaging amplitude. The expression for the directional total focusing imaging amplitude is then:

[0053] ,

[0054] Among them, I ND Z represents the amplitude of the directional full-focus imaging. g and Z d denoted as the laser-ultrasonic directionality coefficient, g represents the laser excitation process, and d represents the laser detection process.

[0055] Expression for laser-ultrasonic directivity coefficient:

[0056] ,

[0057] ,

[0058] Among them, Z g Z is the laser excitation directionality coefficient. d G is the laser detection directionality coefficient. T (θ I ) represents the angle of ultrasonic wave propagation as θ I The directivity of laser-excited ultrasound, G T (θ1) represents the directivity of laser-excited ultrasound when the ultrasound propagation angle is θ1, D T (θ I ) represents the angle of ultrasonic propagation as θ I The directivity of laser detection, D T (θ2) represents the directivity of laser detection when the ultrasonic propagation angle is θ2, d G1 d represents the acoustic propagation distance of the ultrasonic wave from the excitation location to a pixel in the test area within the first layer of material. D1 d represents the acoustic propagation distance of the ultrasonic wave from the excitation location to a pixel in the test area within the second layer of material. G2 The acoustic propagation distance d of an ultrasonic wave from a pixel in the test area to the location to be detected in the first layer of material. D2 The distance the ultrasonic wave travels in the second layer of material from a pixel in the area to be tested to the location to be detected.

[0059] Step 3: Based on the improved laser ultrasound total focusing algorithm, image the full matrix capture data.

[0060] The ultrasonic detection signal corresponding to all pixels in the imaging region in the full matrix capture data is calculated by using a directional optimized full-focusing algorithm to obtain the directional full-focusing imaging amplitude corresponding to all pixels, and a defect image is constructed based on the directional full-focusing imaging amplitude corresponding to all pixels.

[0061] Compared with the prior art, the beneficial effects of the present invention are:

[0062] (1) Based on the propagation theory of ultrasound in a double-layer material containing anisotropic media and Snell's law, the expression for sound propagation time is derived, which makes the calculation of the propagation time of ultrasound in the double-layer material more accurate, and thus obtains a more accurate location of the defect reflection wave.

[0063] (2) Since the propagation path is deflected due to acoustic refraction when the ultrasonic wave propagates in the double-layer material, the full focusing algorithm is improved based on the laser ultrasonic directionality coefficient to accurately calculate the weight of the laser ultrasonic directionality in the propagation, which greatly improves the signal-to-noise ratio of the defect imaging result map and effectively improves the imaging quality. Attached Figure Description

[0064] Figure 1 This is a flowchart illustrating a defect imaging method for bilayer materials based on an improved laser-ultrasound total focusing algorithm, provided as an embodiment of the present invention.

[0065] Figure 2 This is a schematic diagram of the equipment connection for laser ultrasonic excitation and detection experiments provided in an embodiment of the present invention;

[0066] Figure 3 A schematic diagram illustrating the principle of directional full-focusing imaging of dual-layer materials provided in an embodiment of the present invention;

[0067] Figure 4 This is a defect imaging result image provided in an embodiment of the present invention;

[0068] Figure 5 The imaging results of defects in the aluminum / CFRP bilayer material are obtained by the method described in this invention.

[0069] In the attached diagram, the component names represented by each label are as follows:

[0070] 1. Oscilloscope; 2. Computer; 3. Pulsed laser; 4. Laser interferometer; 5. Cylindrical mirror; 6. Mirror; 7. Double-layer specimen. Detailed Implementation

[0071] like Figure 1 As shown in the figure, an embodiment of the present invention provides a laser-ultrasonic directional full-focusing algorithm defect imaging method applicable to bilayer materials containing anisotropic media, comprising the following steps:

[0072] Select a CFRP plate with a length of 250 mm, a width of 50 mm, and a thickness of 10 mm and a 7075 aluminum alloy plate with a length of 250 mm, a width of 50 mm, and a thickness of 5 mm. Bond the two together with epoxy resin adhesive. During bonding, place a polytetrafluoroethylene film with a length of 50 mm, a width of 2 mm, and a thickness of 0.01 mm between the two plates to simulate delamination defects in the double-layer material.

[0073] like Figure 2As shown, the pulsed laser 3 is connected to the computer 2, and the laser interferometer 4 is connected to the interferometer 1 and then to the computer 2. Fifty-one laser excitation positions are set on the aluminum side surface of the sample, spaced 0.2 mm apart (N=51, Δd=0.2 mm), ranging from -5 mm to 5 mm. Ten hundred and one laser detection positions are set on the sample surface, spaced 0.2 mm apart (M=101, Δd=0.2 mm), ranging from -10 mm to 10 mm. The pulsed laser has a wavelength of 1064 nm and a repetition frequency of 1 kHz. The excitation light is focused into a line source by the reflector 6 and cylindrical mirror 5 and then irradiates the sample surface.

[0074] Following step 1, a laser ultrasound experiment was conducted to obtain ultrasound A-scan signals. In Matlab, the 101 ultrasound A-scan signals detected at one excitation location were assembled into a 1500×101 column two-dimensional matrix, with the number of rows representing the number of experimental sampling time points. The ultrasound B-scan signals from all 51 excitation locations were then assembled into a 51×101×1500 three-dimensional matrix in Matlab; this three-dimensional matrix represents the full matrix capture data.

[0075] The sound propagation time is calculated based on step 2, specifically as follows:

[0076] Based on step 2.1, select any pixel in the imaging region to obtain its sound propagation time expression.

[0077] Based on step 2.2, and according to the sample material parameters, the ultrasonic phase velocity and energy velocity are calculated, and the results are as follows. Figure 3 As shown.

[0078] Based on step 2.3, discrete points are selected between the excitation position and the detection position as refraction points, and f1 and f2 corresponding to each refraction point are calculated. Once the refraction points are found, the specific values ​​of the acoustic wave time are further obtained.

[0079] Based on step 2.4, calculate the directional full-focus imaging amplitude corresponding to all pixels in the imaging region, as shown in step 2.4. Figure 4 As shown.

[0080] Based on step 3, the imaging results of defects in the aluminum / CFRP bilayer material calculated using the method described in this invention are obtained, such as... Figure 5 As shown in the image, pixels with a brightness level higher than the surrounding pixels are identified as defects.

Claims

1. A defect imaging method for bilayer materials based on an improved laser-ultrasound total focusing algorithm, characterized in that, The specific steps are as follows: Step 1: In the bilayer material, ultrasonic waves are excited by laser to obtain ultrasonic B-scan signals, which are then used to form full matrix capture data; Step 2: Improve the laser ultrasound total focusing algorithm by optimizing the sound propagation time calculation method and the laser ultrasound directivity coefficient; Step 3: Based on the improved laser ultrasound total focusing algorithm, calculate the ultrasound detection signal of all pixels in the full matrix capture data in the imaging area to obtain the directional total focusing imaging amplitude of all pixels, and construct the defect image based on the directional total focusing imaging amplitude of all pixels.

2. The defect imaging method for bilayer materials based on the improved laser-ultrasound total focusing algorithm according to claim 1, characterized in that, The excitation positions consist of multiple uniformly spaced locations on the sample surface, and the detection positions also consist of multiple uniformly spaced locations on the sample surface. The thickness of the imaging region is the same as the sample thickness, and its length is the same as the total length of the detection positions. Ultrasonic waves are excited at the first excitation position, generating reflected ultrasonic waves at defects in the imaging region. A-scan signals are acquired sequentially at all detection positions, and these A-scan signals constitute the ultrasonic B-scan signal for the first excitation position. Similarly, ultrasonic waves are excited at the second excitation position, generating reflected ultrasonic waves at defects in the imaging region. A-scan signals are acquired sequentially at all detection positions, and these A-scan signals constitute the ultrasonic B-scan signal for the second excitation position. This process continues until ultrasonic waves are excited sequentially at the last excitation position, generating reflected ultrasonic waves at defects in the imaging region. A-scan signals are acquired sequentially at all detection positions, and these A-scan signals constitute the ultrasonic B-scan signal for the last excitation position. The ultrasonic B-scan signals from all excitation positions constitute the full matrix acquisition data.

3. The defect imaging method for bilayer materials based on the improved laser-ultrasound total focusing algorithm according to claim 1, characterized in that, Step 2, the specific steps are as follows: Step 2.1: Based on the propagation law of ultrasound in bilayer materials and Snell's law, derive the sound propagation time; Step 2.2: Based on the elastic wave theory in anisotropic media, derive the ultrasonic energy velocity in the second layer of media; Step 2.3: Based on the phase velocity and energy velocity in ultrasonic propagation, obtain the quantitative evaluation standard of phase velocity, further obtain the evaluation standard of refraction point, and then obtain the ultrasonic propagation path of the refraction point; Step 2.4: Combine the laser ultrasound directional coefficient and the total focusing imaging amplitude to obtain the directional total focusing imaging amplitude, thus obtaining the improved laser ultrasound total focusing algorithm.

4. The defect imaging method for bilayer materials based on the improved laser-ultrasound total focusing algorithm according to claim 3, characterized in that, Step 2 In .1, The sound propagation time expression is used to calculate the sound propagation time of any pixel in the imaging region within the full matrix capture data. The sound propagation time expression is: ， Where T is the sound propagation time, d g1 d represents the distance the ultrasonic wave travels from the excitation location to a pixel location in the imaging region within the first layer of the medium. d1 d represents the distance the ultrasonic wave travels in the second layer of the medium from the excitation location to a pixel location in the imaging region. g2 d represents the distance the ultrasonic wave travels in the first layer of medium from a pixel location in the imaging region to the detection location. d2 c represents the distance the ultrasonic wave travels from a pixel location in the imaging region to the detection location in the second medium, and c1 represents the speed of ultrasonic wave propagation in the first medium. g (θ r1 ) represents the propagation angle θ in the second medium. r1 The speed of ultrasonic energy at time θ r1 It is the angle of refraction in the second medium when ultrasound propagates from the excitation position to a pixel position in the imaging region, c. g (θ r2 ) represents the propagation angle θ in the second medium. r2 The speed of ultrasonic energy at time θ r2 It is the angle of refraction of an ultrasonic wave in the first layer of medium as it propagates from a pixel location in the imaging region to the detection location.

5. The defect imaging method for bilayer materials based on the improved laser-ultrasound total focusing algorithm according to claim 3, characterized in that, Step 2 In .2, Based on the elastic wave theory in anisotropic media, the ultrasonic phase velocity v in the second layer of media is derived. p ,for: ， Where ρ is the density of the second layer medium, and Г ij Let i represent the Christoffel tensor, where i represents the row number of the matrix element in the Christoffel tensor, and j represents the column number of the matrix element in the Christoffel tensor.

6. The defect imaging method for bilayer materials based on the improved laser-ultrasound total focusing algorithm according to claim 5, characterized in that, Step 2 In .2, Derivation of ultrasonic energy velocity in the second medium The expression is: ； in, C represents the y-th element in the ultrasonic energy velocity vector. yzab Let represent the fourth-order elastic tensor of the material, and y, z, a, b represent the indices of the spatial dimensions, °u z and°u b This represents the z-th and b-th elements in the normalized ultrasonic phase velocity eigenvector, where n a v represents the a-th element in the propagation direction vector. p It is the phase velocity of the ultrasonic wave.

7. The defect imaging method for bilayer materials based on the improved laser-ultrasound total focusing algorithm according to claim 4, characterized in that, In step 2.3, according to Snell's law, there are refraction points on the propagation path of ultrasound. During a certain ultrasound excitation and detection process, the interface between the two media between the excitation position and a certain pixel position in the imaging area is discretized into equally spaced points. Each discrete point is used as a refraction point for quantitative evaluation. A quantitative evaluation standard for phase velocity is constructed based on the relationship between phase velocity and energy velocity in ultrasonic propagation to determine the accuracy of table lookup. At each refraction point, find the phase velocity direction and magnitude that minimizes the phase velocity quantification evaluation standard, and construct the refraction point quantification evaluation standard based on Snell's law; For each refraction point, calculate the corresponding refraction point quantization standard, find the refraction point that minimizes the refraction point quantization standard, and approximate this refraction point as the refraction point when the ultrasound propagates from the excitation position to a certain pixel in the imaging region. Based on the position of the refraction point and the two-point distance formula, calculate the propagation distance of the ultrasound in the first layer of the medium when it propagates from the excitation position to a certain pixel in the imaging region and the propagation distance of the ultrasound in the second layer of the medium when it propagates from the excitation position to a certain pixel in the imaging region.

8. The defect imaging method for bilayer materials based on the improved laser-ultrasound total focusing algorithm according to claim 3, characterized in that, In step 2.4, the full-focusing algorithm is optimized using the laser-ultrasound directivity coefficient to obtain a directionality-optimized full-focusing algorithm, the expression of which is: ， Among them, I ND Z represents the amplitude of the directional full-focus imaging. g and Z d Let g be the laser-ultrasonic directionality coefficient, d be the laser excitation process, and A be the laser detection process. GD This represents the ultrasound A-scan signal excited at position G and detected at position D. G represents the excitation position, N represents the total number of excitation positions, D represents the detection position, and M represents the total number of detection positions.

9. The defect imaging method for bilayer materials based on the improved laser-ultrasound total focusing algorithm according to claim 8, characterized in that, Expression for laser-ultrasonic directivity coefficient: ， ， Among them, Z g Z is the laser excitation directionality coefficient. d G is the laser detection directionality coefficient. T (θ I ) represents the angle of ultrasonic wave propagation as θ. I The directivity of laser-excited ultrasound, G T (θ1) represents the directivity of laser-excited ultrasound when the ultrasound propagation angle is θ1, D T (θ I ) represents the angle of ultrasonic propagation as θ I The directivity of laser detection, D T (θ2) represents the directivity of laser detection when the ultrasonic propagation angle is θ2, d G1 d represents the acoustic propagation distance of the ultrasonic wave from the excitation location to a pixel in the test area within the first layer of material. D1 d represents the acoustic propagation distance of the ultrasonic wave from the excitation location to a pixel in the test area within the second layer of material. G2 The acoustic propagation distance d of an ultrasonic wave from a pixel in the test area to the location to be detected in the first layer of material. D2 The distance the ultrasonic wave travels in the second layer of material from a pixel in the area to be tested to the location to be detected.

Images