Laser ultrasonic nondestructive testing device and method based on compressed sensing computational imaging

Abstract

The invention belongs to the technical field of laser ultrasonic nondestructive testing, and provides a laser ultrasonic nondestructive testing device based on compressed sensing calculation imaging. Comprising a pulse laser, a half-wave plate, a polarization beam splitting cube, a photoelectric detector, a beam expanding system, a diaphragm, a reflecting mirror, a homogenizing system, a digital micromirror array, a 4F imaging system, a sample, a reflecting mirror, a laser barrel vibration meter and an industrial personal computer. The invention further discloses an inversion imaging algorithm used for compressed sensing calculation imaging, the compressed sensing theory is introduced into laser ultrasound, the number of scanning points in traditional field scanning can be effectively reduced, the field scanning time is shortened, and meanwhile, due to the adoption of the digital micromirror array, the imaging accuracy is improved. Noise caused by movement of a mechanical system in a traditional scanning system can be effectively reduced, in addition, a frequency domain synthetic aperture algorithm is introduced into compressed sensing calculation imaging, the defect that only two-dimensional images can be obtained through compressed sensing imaging is overcome, and three-dimensional information of the defects can be reconstructed.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser ultrasonic non-destructive testing, and specifically relates to a laser ultrasonic non-destructive testing device and method based on compressive sensing computational imaging. Background Art

[0002] Non-destructive testing technology is a technology for identifying and detecting internal defects of an object without damaging the object to be detected. Ultrasonic non-destructive testing utilizes the transmission, reflection, and scattering characteristics of ultrasonic waves in a medium, and realizes the detection and measurement of macroscopic defects, organizational structures, and geometric characteristics inside the object to be detected by receiving and analyzing ultrasonic echoes. Laser ultrasonic, as an ultrasonic testing technology developed in recent years, uses a pulsed laser to excite ultrasonic waves and a laser interferometer to receive ultrasonic waves. Ultrasonic imaging is a method of obtaining a visible image of an object using ultrasonic waves. Since ultrasonic waves can penetrate many opaque objects, information on the acoustic characteristics of the internal structure of these objects can be obtained using ultrasonic waves. Ultrasonic imaging technology converts this information into an image visible to the human eye. Although laser ultrasonic has the advantages of non-contact and high spatial resolution compared with traditional ultrasonic, due to its characteristic of using point focusing to excite sound waves, the imaging of laser ultrasonic still mainly remains in the stage of point-by-point scanning imaging.

[0003] Therefore, there is an urgent need to design a laser ultrasonic non-destructive testing device and method based on compressive sensing computational imaging. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a laser ultrasonic non-destructive testing device and method based on compressive sensing computational imaging to solve the problems proposed in the prior art.

[0005] A laser ultrasonic non-destructive testing device and method based on compressive sensing computational imaging includes: a pulsed laser, a half-wave plate, a polarization beam splitting cube, a photodetector, a beam expander system, a diaphragm, a mirror, a homogenization system, a digital micromirror device array, a 4F imaging system, a sample, a mirror, a laser barrel vibration meter, and an industrial control computer.

[0006] The pulsed laser emitted by the pulsed laser passes through a half-wave plate and a polarization beam splitter cube and is divided into two polarized light beams with different energies. One of the polarized light beams irradiates on a photodetector, and the generated voltage is transmitted to an industrial control computer as the trigger of the entire system. The other pulsed laser beam continues to propagate backward, enters an expanding beam system and is amplified, then passes through a diaphragm and is reflected by a mirror into a homogenizing system for light field homogenization. Then, a rectangular light field with uniform spatial energy is projected onto a digital micromirror array. The pattern displayed by the digital micromirror array controlled by the industrial control computer reflects the pulsed laser into a 4F imaging system and projects the pattern onto the surface of the sample at a set scaling ratio. Ultrasonic waves are excited through the interaction between the pulsed laser and the substance on the surface of the sample to be measured and propagate inside the sample. The acoustic waves excited by the encoded light field are affected by defects inside and carry the information of the defects to propagate to the other side of the sample;

[0007] A detection light beam emitted by a laser barrel vibration meter is reflected by a mirror to the surface of the sample, and carries the overall vibration information of the detection area back to the laser barrel vibration meter along the original path and transmits the result to the industrial control computer. The industrial control computer processes this part of the data and forms an image.

[0008] Preferably, the single pulse width of the pulsed laser can be ns level, ps level, fs level, and different pulse widths correspond to different excitation modes of photoacoustic waves.

[0009] Preferably, the laser at the ns level mainly generates acoustic waves excited by thermal expansion.

[0010] Preferably, the expanding beam system is composed of multiple lenses, and its function is to expand a small collimated light spot into a large collimated light spot with a larger size. They are composed of two convex-convex lens combinations, concave-convex lens combinations, or more lens combinations.

[0011] Preferably, the digital micromirror array is composed of very small mirrors arranged together, and their sizes are as small as 13.8um or smaller.

[0012] Preferably, each mirror can deflect plus or minus 12 degrees on the diagonal.

[0013] Preferably, a method for laser ultrasonic nondestructive testing based on compressive sensing computational imaging includes the following steps:

[0014] SS001: Turn on the pulsed laser and the laser barrel vibration meter, preheat for a period of time to make the two instruments reach the optimal working temperature range, turn on the industrial control computer, and set the encoding matrix I=(i 1 (x,y),i 2 (x,y),…i m(x, y)), where m is the total number of measurements;

[0015] SS002: Adjust the rotation angle of the half-wave plate so that the light intensity on the photodetector emitted by the polarization beam splitter cube is less than its tolerance threshold. At the same time, the output of the photodetector when its impedance is matched should be between 100 - 500 mv, and connect the output of the photodetector to the industrial control computer as the trigger of the entire system;

[0016] SS003: Adjust the magnification of the beam expander system, and use the aperture at the back end to intercept the place where the energy of the expanded light spot is more uniform. At the same time, the transmission area of the aperture should be larger than the incident window area of the homogenization system to ensure that the homogenization system can be fully utilized;

[0017] SS004: Adjust the distance between the two microlens arrays in the homogenization system so that the size of the homogenized light spot on the focal plane of the homogenization system is as large as possible and the same as that of the digital micromirror array. At the same time, measure the total energy of this light spot to ensure that it is below the damage threshold of the digital micromirror array, and this threshold is 10 mj / cm2;

[0018] SS005: Control the digital micromirror array by the industrial control computer to display the specified coding matrix pattern. At the same time, adjust the angle of the light spot emitted by the homogenization system irradiating on the digital micromirror array so that the incident direction has a 24-degree angle with the normal direction of the digital micromirror array;

[0019] SS006: Select the appropriate lens size, focal length, and lens spacing in the 4F imaging system. The selected lens diameter should be greater than twice the diagonal of the reflected light spot, which can reduce the aberration at the edge of the pattern. In addition, the appropriate lens focal length and lens spacing can be selected according to the invention space and the size of the light spot desired to be projected on the sample;

[0020] SS007: Polish the surface of the aluminum alloy test block containing defects inside and clamp it on a three-dimensional displacement platform. Adjust the position of the sample before and after the imaging focal plane of the 4F imaging system to find the position where the clearest projection pattern can be presented on the sample surface;

[0021] SS008: Make the outgoing light of the laser barrel vibration meter perpendicularly irradiate the sample surface through a mirror, and ensure that the excitation light and the detection light are at the same position in the front and back planes of the sample. Adjust the size of the outgoing light spot so that the detection light can completely cover the possible excitation sound field area. At the same time, finely adjust the displacement stage and the mirror of the sample so that the detection light reflected from the sample surface can return completely along the original path;

[0022] SS009: The result B = [b 1 (t), b 2 (t), … b m(t) is transmitted back to the industrial control computer and undergoes computational imaging with the coding matrix to obtain the internal defect imaging result T(x, y) within a certain range.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The use of laser ultrasonic technology can achieve defect imaging inside the sample in a completely non-contact manner, avoiding the coupling process in traditional acoustic detection and can be applied under some extreme conditions;

[0025] 2. Compared with traditional C-scan imaging, due to the use of the compressive sensing imaging algorithm, the number of scans can be significantly reduced, saving the time required for imaging;

[0026] 3. Compared with traditional swept-field laser ultrasound, the present invention uses spatial light modulation to achieve acoustic field coding, eliminating the need to introduce additional mechanical structures, reducing the vibration and error generated during mechanical movement, and improving the signal-to-noise ratio of the collected signals;

[0027] 4. By introducing the frequency-domain synthetic aperture algorithm in compressive sensing computational imaging, the defect of compressive sensing computational imaging that can only obtain two-dimensional images is compensated, and the three-dimensional information of the defect can be reconstructed. Description of the Drawings

[0028] Figure 1 is a schematic structural diagram of a laser ultrasonic non-destructive testing device based on compressive sensing computational imaging according to the present invention;

[0029] Figure 2 is an ultrasonic scan imaging diagram of the sample to be tested according to the present invention;

[0030] Figure 3 is a workflow diagram of the present invention;

[0031] Figure 4 is a hardware control interface diagram of the present invention;

[0032] Figure 5 is a schematic diagram of the coding matrix used in the present invention;

[0033] Figure 6 is a barrel detection acoustic field diagram of the present invention;

[0034] Figure 7 is a compressive sensing imaging diagram of the internal defect of the present invention;

[0035] Figure 8 is a defect depth inversion diagram based on the imaging result of the present invention.

[0036] In the figure:

[0037] 1. Pulse laser; 2. Half-wave plate; 3. Polarizing beam splitting cube; 4. Photoelectric detector; 5. Beam expander system; 6. Aperture; 7. Mirror; 8. Homogenization system; 9. Digital micromirror array; 10. 4F imaging system; 11. Sample; 12. Mirror; 13. Laser barrel vibration meter; 14. Industrial control computer. Detailed implementation mode

[0038] The following further describes the implementation mode of the present invention in detail in conjunction with the drawings and embodiments. The following embodiments are used to illustrate the present invention, but cannot be used to limit the scope of the present invention.

[0039] As shown in the attached Figure 1 to the attached Figure 8 figure:

[0040] Embodiment 1:

[0041] The present invention provides a laser ultrasonic non-destructive testing device based on compressive sensing computational imaging, including: pulse laser 1, half-wave plate 2, polarizing beam splitting cube 3, photoelectric detector 4, beam expander system 5, aperture 6, mirror 7, homogenization system 8, digital micromirror array 9, 4F imaging system 10, sample 11, mirror 12, laser barrel vibration meter 13 and industrial control computer 14;

[0042] The pulsed laser emitted by the pulse laser 1 is split into two polarized light beams with different energies through the half-wave plate 2 and the polarizing beam splitting cube 3. One of the polarized light beams is irradiated on the photoelectric detector 4, and the generated voltage is transmitted to the industrial control computer 14 as the trigger of the entire system. The other pulsed laser beam continues to propagate backward, is amplified after entering the beam expander system 5, is reflected by a mirror 7 after passing through an aperture 6, and enters the homogenization system 8 for light field homogenization. Then, the rectangular light field with uniform spatial energy is projected onto the digital micromirror array 9. The pattern displayed by the digital micromirror array 9 controlled by the industrial control computer 14 reflects the pulsed laser into a 4F imaging system 10, and projects the pattern onto the surface of the sample 11 at a set scaling ratio. Ultrasonic waves are excited through the interaction between the pulsed laser and the substance on the surface of the sample 11 to be measured and propagate inside the sample 11. The acoustic waves excited by the encoded light field are affected by defects inside and carry the information of the defects to propagate to the other side of the sample 11;

[0043] A beam of detection light emitted by the laser barrel vibration meter 13 is reflected by a mirror 12 onto the surface of the sample 11, carries the overall vibration information of the detection area and returns to the laser barrel vibration meter 13 along the original path, and the result is transmitted to the industrial control computer 14. The industrial control computer 14 processes this part of the data and forms an image;

[0044] A method for laser ultrasonic non-destructive testing based on compressive sensing computational imaging includes the following steps:

[0045] SS001: Turn on the pulsed laser 1 and the laser barrel vibration measuring instrument 13, preheat for a period of time to allow the two instruments to reach the optimal working temperature range, turn on the industrial control computer 14, and set the coding matrix I = (i 1 (x,y), i 2 (x,y), … i m (x,y)) according to the expected imaging area and imaging resolution of the current invention, where m is the total number of measurements;

[0046] SS002: Adjust the rotation angle of the half-wave plate 2 so that the light intensity of the light exiting from the polarization beam splitting cube 3 on the photodetector 4 is less than its tolerance threshold. At the same time, the output of the photodetector 4 when its impedance is matched should be between 100 - 500 mv, and connect the output of the photodetector 4 to the industrial control computer 14 as the trigger for the entire system;

[0047] SS003: Adjust the magnification of the beam expander system 5, and use the aperture 6 at the back end to intercept the area where the energy of the expanded light spot is more uniform. At the same time, the transmission area of the aperture 6 should be larger than the incident window area of the homogenization system 8 to ensure that the homogenization system 8 can be fully utilized;

[0048] SS004: Adjust the distance between the two microlens arrays in the homogenization system 8 so that the size of the homogenized light spot on the focal plane of the homogenization system 8 is as large as possible and the same as that of the digital micromirror array 9. At the same time, measure the total energy of this light spot to ensure that it is below the damage threshold of the digital micromirror array 9, and this threshold is 10 mj / cm2;

[0049] SS005: Control the digital micromirror array 9 by the industrial control computer 14 to display the specified coding matrix pattern. At the same time, adjust the angle of the light spot exiting from the homogenization system 8 irradiating on the digital micromirror array 9 so that the incident direction has a 24-degree angle with the normal direction of the digital micromirror array 9;

[0050] SS006: Select appropriate lens size, focal length, and lens spacing in the 4F imaging system 10. The selected lens diameter should be greater than twice the diagonal of the reflected light spot, which can reduce the aberration at the pattern edge. In addition, appropriate lens focal length and lens spacing can be selected according to the invention space and the size of the light spot expected to be projected on the sample 11;

[0051] SS007: Polish the surface of the aluminum alloy test block containing defects inside to be smooth, and clamp it on a three-dimensional displacement platform. Adjust the position of the sample 11 before and after the imaging focal plane of the 4F imaging system 10 to find the position where the clearest projected pattern can be presented on the surface of the sample 11;

[0052] SS008: The outgoing light of the laser barrel vibrometer 13 is vertically irradiated onto the surface of the sample 11 through the mirror 12, and it is ensured that the excitation light and the detection light are at the same position within the front and back planes of the sample 11. Adjust the spot size of the outgoing light so that the detection light can completely cover the possible excitation sound field area. At the same time, finely adjust the displacement stage of the sample 11 and the mirror 12 so that the detection light reflected from the surface of the sample 11 can return along the original path completely;

[0053] SS009: Transmit the result B = [b 1 (t), b 2 (t), … b m (t)] measured by the laser barrel vibrometer 13 to the industrial control computer 14, and perform computational imaging with the encoding matrix to obtain the internal defect imaging result T(x, y) within a certain range.

[0054] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings:

[0055] The result of the immersion ultrasonic imaging of the sample to be measured is as Figure 2 shown. The sample is made of two pieces of 6061 aluminum alloy with dimensions of 160mm * 160mm * 0.6mm by vacuum diffusion welding. The surface of one piece of aluminum alloy is machined by a CNC machine tool, and four columns of defects with different sizes and shapes are designed. The outer contour size of each column of defects is 7.2mm, 5.4mm, 3.6mm, and 6.4mm. When the welding is completed, no trace can be seen on the surface of the sample;

[0056] The overall process of the invention is as Figure 3 shown;

[0057] The hardware system of the invention is written in labview, and its graphical interface is as Figure 4 shown;

[0058] Based on the above laser ultrasonic non-destructive testing device, the invention sample is detected and imaged according to the implementation steps given in the flowchart as follows:

[0059] Turn on the pulsed laser and the laser barrel vibrometer, and preheat for a period of time to make the two instruments reach the optimal working temperature range; turn on the industrial control computer. For the pentagram in the second column of the object of the present invention, determine to use an encoding matrix with an area of 8 * 8mm. At the same time, in order to improve the imaging resolution, an encoding matrix with 40 * 40 pixel sizes is used, and the size of each pixel is 200 * 200um. The specific pattern is as Figure 5 shown, and a total of 600 matrix patterns are generated by random generation;

[0060] Adjust the rotation angle of the half-wave plate so that the light intensity output from the polarization beam splitting cube outputs a voltage of 400 mV under the impedance matching of the photodetector, which is used as the system trigger;

[0061] Adjust the distance between the two microlens arrays in the homogenization system so that the size of the homogenized light spot on the focal plane of the homogenization system is 14 * 10 mm, which is the same size as the digital micromirror array chip. At the same time, the single-pulse energy of this light spot is measured to be 5 mj / cm2, which is less than the damage threshold of the digital micromirror array chip;

[0062] Adjust the output light spot of the homogenization system so that its incident direction has an angle of 24 degrees with the normal direction of the digital micromirror array chip. Then, the industrial control computer inputs the 600 encoded matrix patterns generated just now into the memory of the DMD system;

[0063] Use two lenses with a focal length of 50 mm and a diameter of 50.8 mm to form a 4F system, and place it horizontally on the normal direction of the digital micromirror array chip. At the same time, the distance between the front lens of the 4F system and the surface of the digital micromirror array chip is 50 mm;

[0064] Polish the surface of the above aluminum alloy sample smoothly and clamp it on a three-dimensional displacement platform so that it falls before and after the focal position of the rear lens of the 4F system. By finely adjusting the displacement stage, the image of the encoded matrix can be clearly presented on the sample surface;

[0065] Adjust the incident angle of the laser barrel detector so that the detection light spot and the excitation light spot are at the same position in the front and back planes of the sample. At the same time, adjust the size of the output light spot so that it can completely cover the excitation sound field area;

[0066] Fine-tune the displacement stage and the mirror of the sample so that the barrel detector can receive the best signal. Then, trigger through the photodetector control system. After each light pulse, the encoded pattern is changed synchronously. At the same time, the high-speed acquisition card collects the sound signals measured by the barrel detector and transmits them to the industrial control computer for storage. Among them, the sound field signals collected each time are as Figure 6 shown;

[0067] Select and record the minimum value of the amplitude of the direct wave in the interval of 0.4 us to 0.6 us in the results of 600 acquisitions, and calculate the imaging of the internal defects in the detection range through compressed sensing with the corresponding encoded matrix. The imaging results are as Figure 7 shown. The left figure is the calculation imaging result without any processing, and the right figure is the result obtained by quadruple interpolation of the left image. It can be found that it is not clear and sharp enough compared with the internal real defects, but shows a result similar to diffraction;

[0068] Next, perform frequency-domain synthetic aperture inversion on the interpolated computational imaging results. Invert the images at intervals of 0.1 mm for the possible depths where the defects may exist. The results are as shown in Figure 8 shown. It can be seen that when the assumed depth is 0.6 mm, the pentagram result is relatively the clearest, and at this time, it also coincides with the designed depth of the actual defect. This is the depth of the defect at this time, and at the same time, the clearest two-dimensional image result is also obtained.

[0069] Working principle: The present invention emits pulsed laser by a pulsed laser, and through a series of optical components, projects the encoded matrix pattern on the digital micromirror array onto the sample in an appropriate proportion. Through the photoacoustic interaction between the pulsed laser and the sample surface, a special encoded sound field is generated and propagates inside the medium. When there are defects inside the sample, the defects will change the intensity distribution and phase relationship of the encoded sound field. On the other side of the sample, an optical bucket detector emits a collimated laser beam with a certain width and irradiates the other side of the sample. The total displacement of the encoded sound field within the laser coverage can be collected through the laser interference principle, and it and the encoded matrix are jointly input into the imaging algorithm to obtain the three-dimensional information of the defect.

[0070] There are two algorithms used in the invention, namely the compressive sensing imaging algorithm based on orthogonal matching pursuit and the frequency-domain synthetic aperture algorithm. The former utilizes the characteristics of compressive sensing to achieve compression during the sampling process, greatly reducing the number of scans required for imaging, and performs two-dimensional imaging on defects with a certain depth inside. The latter, according to the propagation characteristics of sound waves in solids, performs inversion calculation on the two-dimensional image obtained by compressive sensing, and can not only obtain the depth where the defect is located, but also obtain a clearer defect image. Therefore, the three-dimensional information of the defect can be obtained by combining the two.

[0071] The process of the compressive sensing algorithm can be written as the following expression:

[0072]

[0073] where B = [b 1 , b 2 , … b m T is the bucket detection result, and S(s 1 , s 2 , … s p×q ) are respectively the original encoded matrix I(x, y) and the defect pattern S(x, y) rearranged in the form of column vectors. φ is a sparse transformation matrix in a certain domain, and the spatial pattern in this domain is transformed into a sparse vector When the encoded matrix I(x, y) satisfies the restricted isometry property condition, the orthogonal matching pursuit algorithm can be used to solve the sparse vector

[0074] The frequency-domain synthetic aperture algorithm in the present invention is based on the angular spectrum propagation theory. The idea of the angular spectrum propagation theory is that assuming the distribution of the incident sound field in the z0 plane in the real space is P(x, y, t; z 0 ), due to the Huygens principle, the distribution in the z1 plane can be obtained through the propagation law of the sound field, that is, P(x, y, t; z 1 ). The specific method is to perform a two-dimensional Fourier transform on the incident sound field P(x, y, t; z 0 ) to obtain the angular spectrum P(k x , k y , ω; z 0 ), multiply it by the angular spectrum propagation factor H(k x , k y , ω; z 0 , z 1 ) through a simple dot product operation, and finally obtain the sound field distribution P(x, y, t; z 1 ) in the z1 plane of the real space through an inverse Fourier transform. Based on the inverse process of this concept, the frequency-domain synthetic aperture algorithm in the present invention can be obtained. The sound field p[x, y; z 1 , t 1 calculated by compressive sensing can be used to inversely obtain the defect sound field p[x, y; z 0 , and at the same time, using the multi-layer depth tomography method, its location depth can also be estimated.

[0075] The barrel vibration meter of the present invention is not limited to using the laser barrel vibration meter (13), and other barrel measurement methods also belong to the content of the present invention;

[0076] The directions of the pulse laser (1) and the laser barrel vibration meter (13) provided in the present invention are not limited to being arranged relatively, and other arranged directions also belong to the content of the present invention.

[0077] The embodiments of the present invention are given for the purpose of illustration and description. Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A laser ultrasonic nondestructive testing device based on compressed sensing computational imaging, characterized in that: include: Pulse laser (1), half-wave plate (2), polarization beam splitter cube (3), photodetector (4), beam expansion system (5), aperture (6), reflector (7), homogenization system (8), digital micromirror array (9), 4F imaging system (10), sample (11), reflector (12), laser barrel vibrometer (13) and industrial computer (14); The pulse laser emitted by the pulse laser (1) passes through a half-wave plate (2) and a polarization beam splitter cube (3) and is split into two polarized light beams with different energies. One of the polarized light beams is irradiated on a photodetector (4), and the voltage generated by the polarized light beam is transmitted to an industrial computer (14) as a trigger for the entire system. The other pulse laser beam continues to propagate backwards, enters a beam expansion system (5) and is amplified, and then passes through an aperture (6) and is reflected by a reflector (7) and enters a homogenization system (8) for light field homogenization. Then, a rectangular light field with uniform spatial energy is projected onto a digital micromirror array (9), wherein the industrial computer (14) controls the pattern displayed by the digital micromirror array (9) to reflect the pulse laser into a 4F imaging system (10), and the pattern is projected onto the surface of a sample (11) at a set scaling ratio. Ultrasonic waves are excited by the interaction between the pulse laser and the surface material of the sample (11) to be tested and propagate inside the sample (11). The sound waves excited by the coded light field are affected by defects inside the sample and propagate to the other side of the sample (11) carrying information about the defects. A beam of detection light is emitted by a laser barrel vibrometer (13), reflected by a reflector (12) to the surface of a sample (11), and returns to the laser barrel vibrometer (13) along the original path with the overall vibration information of the detection area, and transmits the result to an industrial control computer (14), which processes this part of the data and forms an image.

2. The laser ultrasonic nondestructive testing device based on compressed sensing computational imaging according to claim 1, characterized in that: The single pulse width of the pulse laser (1) can be at the ns level, ps level, or fs level, wherein different pulse widths correspond to different excitation modes of photoacoustic waves.

3. The laser ultrasonic nondestructive testing device based on compressed sensing computational imaging as claimed in claim 2, characterized in that: The nanosecond-level laser is mainly an acoustic wave excited by thermal expansion.

4. The laser ultrasonic nondestructive testing device based on compressed sensing computational imaging as claimed in claim 3, characterized in that: The beam expansion system (5) is composed of a plurality of lenses.

5. The laser ultrasonic nondestructive testing device based on compressed sensing computational imaging as claimed in claim 4, characterized in that: The digital micromirror array (9) is composed of very small reflective mirrors arranged together, and their size is as small as 13.8 um.

6. The laser ultrasonic nondestructive testing device based on compressed sensing computational imaging as claimed in claim 5, characterized in that: Each of the reflectors can be deflected diagonally by plus or minus 12 degrees.

7. A method for laser ultrasonic nondestructive testing based on compressed sensing computational imaging according to any one of claims 1 to 6, characterized in that: The following steps are involved: SS001: Turn on the pulse laser (1) and the laser barrel vibrometer (13), preheat for a period of time, let the two instruments reach the optimal operating temperature range, turn on the industrial computer (14), and set the encoding matrix I = (i1(x, y), i2(x, y), ... i m (x,y)), where m is the total number of measurements; SS002: Adjust the rotation angle of the half-wave plate (2) so that the light intensity emitted by the polarization beam splitter cube (3) on the photodetector (4) is less than its tolerance threshold. At the same time, the output of the photodetector (4) should be between 100-500mv when its impedance is matched. The output of the photodetector (4) is connected to the industrial computer (14) as a trigger for the entire system. SS003: Adjust the magnification of the beam expansion system (5), and use the aperture (6) at the rear end to intercept the area where the energy of the beam spot is more uniform after the beam expansion. At the same time, the transmission area of ​​the aperture (6) should be larger than the incident window area of ​​the homogenization system (8) to ensure that the homogenization system (8) can be fully utilized; SS004: Adjust the distance between the two microlens arrays in the homogenization system (8) so that the homogenized light spot size on the focal plane of the homogenization system (8) is as large as that of the digital micromirror array (9) as possible, and measure the total energy of the light spot to ensure that it is below the damage threshold of the digital micromirror array (9), which is 10 mj / cm2; SS005: The industrial computer (14) controls the digital micromirror array (9) to display the specified coding matrix pattern, and at the same time adjusts the angle at which the light spot emitted by the homogenization system (8) irradiates the digital micromirror array (9) so that the incident direction and the normal direction of the digital micromirror array (9) have an angle of 24 degrees; SS006: Select the appropriate lens size, focal length and lens spacing in the 4F imaging system (10). The selected lens diameter should be greater than twice the diagonal of the reflected light spot to reduce the aberration at the edge of the pattern. Select the appropriate lens focal length and lens spacing according to the invention space and the light spot size that is expected to be projected on the sample (11); SS007: The surface of an aluminum alloy test block with internal defects is polished smooth and clamped on a three-dimensional displacement platform. The position of the sample (11) is adjusted before and after the imaging focal plane of the 4F imaging system (10) to find the position that can present the clearest projection pattern on the surface of the sample (11); SS008: The output light of the laser barrel vibrometer (13) is vertically irradiated onto the surface of the sample (11) through the reflector (12), and the excitation light and the detection light are ensured to be at the same position in the front and back planes of the sample (11). The spot size of the output light is adjusted so that the detection light can completely cover the possible excitation sound field area. At the same time, the displacement stage and the reflector (12) of the sample (11) are fine-tuned so that the detection light reflected from the surface of the sample (11) can return completely along the original path; SS009: The result B obtained by the laser barrel vibrometer (13) is [b1(t), b2(t), ... b m (t)] is transmitted back to the industrial computer (14) and is imaged by calculation with the encoding matrix, thereby obtaining the internal defect imaging result T(x, y) within a certain range.

Images