Stress microscopic tomography device and method based on laser ultrasonic nondestructive testing technology

Abstract

The invention discloses a stress microtomography device and method based on a laser ultrasonic nondestructive testing technology. The device comprises a pulse laser, a half-wave plate, a polarization beam splitter, a photoelectric detector, a beam expanding system, a diaphragm, a reflector, a homogenizing system, a digital micromirror array, a sleeve lens, a dichroscope, a beam splitting film, an objective lens, a sample, a three-dimensional mobile platform, a laser vibration meter, a collimation system, an imaging system and an industrial personal computer. Stress scanning field detection can be realized by combining the three-dimensional mobile platform, and the limitation that only single-point detection can be realized in the traditional stress detection technology is solved. Periodic grating patterns with different space constants are generated by adopting the digital micromirror array, and ultrasonic waves with different wavelengths can be excited to detect stress defects with different depths. Meanwhile, by measuring the single-frequency surface wave generated by the periodic grating with different space grating constants, frequency deviation occurs in the gradient stress material, so that the corresponding stress can be inverted through the surface wave velocity.

Description

Technical Field

[0001] The present invention relates to the technology of laser ultrasonic non-destructive testing, and particularly to a stress microscopic tomography imaging device and method based on the laser ultrasonic non-destructive testing technology. Background Art

[0002] During the manufacturing, processing, treatment or use of materials, as well as under various factors such as external loads and environmental factors, residual stresses often exist inside the materials. Residual stresses can weaken the strength, toughness and stability of materials, reduce the material properties and structural integrity, thus affecting assembly and connection and increasing maintenance costs. Therefore, stress detection of materials is very important. However, the generation of residual stresses is very common and difficult to effectively detect. Traditional stress detection methods include three types: destructive, semi-destructive and non-destructive. Among them, the destructive detection method and the semi-destructive detection method are the contour method, the slotting method and the blind hole method, the ring core method respectively. Both of these two methods cause damage to the workpiece to be measured, which is not suitable for in-service workpieces and high-precision and sophisticated equipment. In addition, the non-destructive detection methods are X-ray diffraction method, neutron diffraction method, magnetic measurement method, nano-indentation method, ultrasonic method. These non-destructive detection methods are restricted by the detected materials, application scenarios, expensive equipment, susceptibility to detection environment and single detection type. The laser ultrasonic non-destructive testing technology uses an excitation laser to irradiate on the material to generate ultrasonic waves, and uses a laser interferometer to detect the ultrasonic echoes, and converts the ultrasonic waves into electrical signals and transmits them to an industrial control computer to analyze the information carried by the ultrasonic waves. The laser ultrasonic detection technology has technical advantages such as non-damage, non-contact, no need for a coupling agent, strong anti-interference ability, good stability, long-distance detection, high detection resolution and fast field scanning, and is applied to defect detection in special environments such as high temperature, high pressure, corrosion and radiation. In addition, traditional stress detection methods have great difficulties in detecting and calibrating the stress gradient in the depth direction of materials, and are insufficient to meet the requirements of stress detection.

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

[0004] Object of the Invention: Aiming at the problems existing in the traditional stress detection system, the present invention proposes a stress microscopic tomography imaging device and method based on the laser ultrasonic non-destructive testing technology to solve the problems proposed in the prior art.

[0005] Technical Solution: A stress microscopic tomography imaging device based on the laser ultrasonic non-destructive testing technology includes: a pulsed laser, a half-wave plate, a polarization beam splitter, a photodetector, a beam expander system, a diaphragm, a mirror, a homogenization system, a digital micromirror device, a sleeve lens, a dichroic mirror, a beam splitting film, an objective lens, a sample, a three-dimensional moving platform, a laser vibrometer, a collimation system, an imaging system, and an industrial control computer.

[0006] The stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology

[0007] (1) Path of the excitation light: The pulsed laser emitted by the pulsed laser passes through a half-wave plate and then through a polarization beam splitter and is divided into two polarized lights with different energies. One of the polarized lights is converted from an optical signal to an electrical signal after passing through a photodetector and the signal is transmitted to the industrial control computer to trigger the entire system; the other pulsed laser continues to propagate backward, is amplified after passing through an expanding system, is reflected by a mirror after passing through a diaphragm, and then enters a homogenizing system for optical field homogenization. The rectangular optical field with uniform spatial energy after homogenization is projected onto the digital micromirror array. The industrial control computer controls the digital micromirror array to generate periodic grating patterns with different spatial grating constants, and after reflecting the pulsed laser, it passes through a sleeve lens, a dichroic mirror, a beam splitting film, and finally is scaled and projected onto the sample placed on the three-dimensional moving stage through an objective lens;

[0008] (2) Path of the detection light: A beam of detection light emitted by a laser vibrometer passes through a collimation system and propagates perpendicular to the surface of the sample. After passing through a dichroic mirror and a beam splitting film, it is focused on the surface of the sample by an objective lens, and the overall vibration information of the detection area is collected;

[0009] (3) The excitation light and the detection light return together through the objective lens, reach the imaging system after being selected and distributed by the beam splitting film, and then the industrial control computer performs data processing and tomographic imaging.

[0010] For the stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology, the single pulse width of the pulsed laser includes ns level, ps level, and fs level.

[0011] For the stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology, the expanding system is composed of 2 - 5 lenses, and its function is to expand a small collimated light spot to a larger collimated light spot with a larger size;

[0012] The digital micromirror array is composed of a series of mirrors arranged together; each mirror can deflect by plus or minus 8 - 15 degrees on the diagonal;

[0013] The collimation system is composed of 2 - 5 reflecting mirrors and a diaphragm, and its function is to control the direction of the laser beam perpendicular to the surface of the sample, and is composed of one or more reflecting mirrors and a diaphragm;

[0014] The imaging system is composed of a long working distance objective lens and a charge-coupled device (CCD).

[0015] For the stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology, the digital micromirror array (DMD) can be replaced by a spatial light modulator (SLM).

[0016] The stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology, wherein the beam expanding system is composed of a combination of 2 - 5 convex lenses or concave lenses.

[0017] The stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology,

[0018] The periodic pulsed laser grating excites ultrasonic waves with a single frequency component on the sample and propagates along its surface; and this single-frequency surface wave is affected by the residual stress in the sample, resulting in a shift in the center frequency; the acoustic surface waves excited by the periodic grating patterns with different spatial grating constants have a gradient change in wavelength under the action of the stress defect with a depth gradient; by measuring the single-frequency surface waves generated by the periodic gratings with different spatial grating constants, a frequency shift occurs in the gradient stress material, enabling the corresponding residual stress to be inversely calculated through the surface wave velocity; the surface waves excited by the periodic gratings with different spatial grating constants have different frequencies and different penetration depths of the surface waves, and the residual stress at different depths inside the sample is inversely calculated from the penetration depth of the surface waves.

[0019] The detection method using the stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology includes the following steps:

[0020] (1) Turn on the pulsed laser and the laser vibrometer;

[0021] (2) Turn on the industrial control computer, set the periodic grating patterns with different resolution spatial grating constants to obtain surface waves with different wavelengths;

[0022] (3) Adjust the rotation angle of the half-wave plate so that the light intensity of the light emerging from the polarization beam splitter on the photodetector is less than its tolerance threshold, and 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 for the entire system;

[0023] (4) Adjust the magnification of the beam expanding system, and use the diaphragm at the back end to intercept the place where the energy of the expanded light spot is relatively uniform. At the same time, the transmission area of the diaphragm should be larger than the incident window area of the homogenization system to ensure that the homogenization system can be fully utilized;

[0024] (5) 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;

[0025] (6) The industrial control computer controls the digital micromirror array to display a specified coded matrix pattern, and at the same time adjusts the angle of the homogenized system's outgoing light spot irradiating the digital micromirror array so that the incident direction has an angle of 20 - 30 degrees with the normal of the digital micromirror array;

[0026] (7) Select the lens of the sleeve lens, and select the lens focal length and lens spacing according to the required light spot size irradiated on the sample;

[0027] (8) Select a dichroic mirror according to the wavelength of the selected laser;

[0028] (9) Adjust the mirror in the collimation system so that the direction and position of the light beam pass through two diaphragms and are accurately perpendicular to the sample target area;

[0029] (10) Clamp the sample on the three - dimensional moving platform, and adjust the imaging focal plane position of the sleeve lens so that a clear pattern is projected on the sample surface;

[0030] (11) The detection light emitted by the laser vibrometer passes through the mirror in the collimation system after reflection and calibration, then reaches the beam - splitting film through the dichroic mirror. The detection light then projects through the objective lens onto the sample on the three - dimensional moving platform; adjust the size of the detection light spot so that it can completely cover the area that may excite the sound field; and make the excitation light and the detection light overlap and reach the imaging system by adjusting the angle of the beam - splitting film;

[0031] (12) Transmit the results measured by the laser vibrometer back to the industrial control computer, and perform computational imaging with periodic grating patterns of different spatial grating constants, so as to obtain a three - dimensional map of the depth - gradient stress of the sample.

[0032] The detection method of the stress microscopic tomography imaging device based on the laser ultrasonic non - destructive testing technology,

[0033] The formulas for calculating the residual stress include: the formula (1) for calculating the frequency of the surface wave and the formula (2) for calculating the penetration depth of the surface wave:

[0034] c = λf (1)

[0035] h = vf -0.96 (2)

[0036] In formula (1), c is the surface wave velocity, λ is the surface wave wavelength, and f is the surface wave frequency; in formula (2), h is the penetration depth of the surface wave, v is the surface wave velocity, and f is the surface wave frequency;

[0037] Calculate the relationship between the residual stress and the depth from formula (3):

[0038]

[0039] In formula (3), σ is stress, h is the penetration depth of the surface wave, i and j represent the i-th and j-th detections, the value of i is (1, 2, 3…n), the value of j is (1, 2, 3…n), and i is greater than j;

[0040] Combining formula (2) and formula (3), we can get the relationship between residual stress gradient depth and surface wave frequency as shown in formula (4):

[0041]

[0042] In formula (4), σ is stress, f is the surface wave frequency, i and j represent the i-th and j-th detections, the value of i is (1, 2, 3...n), the value of j is (1, 2, 3...n), and i is greater than j; according to the formula acoustic elastic theory, the gradient stress and penetration depth can be inverted;

[0043] Wave vector k, wavelength λ, spatial grating constant d, angular velocity ω, frequency f, spectral variation rate ΔR w , and the period T are shown in the following formula:

[0044]

[0045] ω=2*π*f(7)

[0046]

[0047] The application of the stress micro-tomography device based on laser ultrasonic non-destructive testing technology in stress detection.

[0048] Further preferably, the pulsed laser emits pulsed laser light which, after passing through a half-wave plate and then a polarization beam splitter, is split into two polarized light beams with different energies. One of the polarized light beams is converted from an optical signal to an electrical signal after passing through a photodetector and the signal is transmitted to an industrial control computer as a trigger for the entire system. The other pulsed laser beam continues to propagate backward, is amplified after passing through an expanding system, is reflected by a mirror after passing through a diaphragm, and then enters a homogenizing system for optical field homogenization. The homogenized rectangular optical field with uniform spatial energy is projected onto a digital micromirror array. The industrial control computer controls the digital micromirror array to generate periodic grating patterns with different spatial grating constants and reflects the pulsed laser light, which then passes through a sleeve lens, a dichroic mirror, a beam-splitting film, and finally passes through an objective lens to be projected onto a sample (the sample is placed on a three-dimensional moving stage) at a set scaling ratio. The periodic pulsed laser grating excites ultrasonic waves with a single frequency component on the sample and propagates along its surface. The single-frequency surface wave is affected by the residual stress in the sample and undergoes a shift in the central frequency. The wavelengths of the surface acoustic waves excited by periodic grating patterns with different spatial grating constants change in a gradient under the action of stress defects with a depth gradient. By measuring the single-frequency surface waves generated by periodic gratings with different spatial grating constants, a frequency shift occurs in the gradient stress material, enabling the corresponding residual stress to be inversely calculated through the surface wave velocity. The surface waves excited by periodic gratings with different spatial grating constants have different frequencies and different penetration depths, and the residual stress at different depths inside the sample can be inversely calculated from the surface wave penetration depth.

[0049] Meanwhile, a beam of detection light emitted by a laser vibrometer passes through a collimation system and propagates perpendicular to the surface of the sample. After passing through a dichroic mirror and a beam-splitting film, it is focused on the surface of the sample by an objective lens, and overall vibration information of the detection area is collected.

[0050] Subsequently, the excitation light and the detection light return together through the objective lens, reach the imaging system after being selected and distributed by the beam-splitting film, and then data processing and tomographic imaging are performed by the industrial control computer.

[0051] After a set of periodic grating patterns with different spatial grating constants in a resolution detection area are all scanned, the industrial control computer controls the three-dimensional moving platform to move the sample to the next detection area. The three-dimensional moving platform realizes stress scanning field detection.

[0052] Preferably, the single pulse width of the pulsed laser can be in the ns level, ps level, or fs level, and different pulse widths correspond to different excitation modes of photoacoustic waves.

[0053] Preferably, the laser in the ns level mainly excites acoustic waves by thermal expansion.

[0054] Preferably, the beam expander system is composed of multiple lenses, whose function is to expand a small collimated light spot into a large collimated light spot of a larger size. They are composed of two convex-convex lenses, in the combination mode of concave-convex lenses, and are composed of more lens combinations.

[0055] Preferably, the digital micromirror array can be replaced by a spatial light modulator or the like.

[0056] Preferably, the digital micromirror array is composed of very small mirrors arranged together, and their sizes are as small as 13.8 um or smaller.

[0057] Preferably, each mirror can be deflected by plus or minus 12 degrees on the diagonal.

[0058] Preferably, the collimation system is composed of multiple reflecting mirrors and diaphragms, whose function is to control the direction of the laser beam perpendicular to the sample surface. It is composed of one or more reflecting mirrors and diaphragms, and there are various combination modes.

[0059] Preferably, the dichroic mirror can select a suitable dichroic mirror according to the wavelength of the selected laser.

[0060] Preferably, the imaging system is composed of a long working distance objective lens and a CCD.

[0061] Preferably, a method for stress microscopic tomography based on laser ultrasonic non-destructive testing technology includes the following steps:

[0062] Turn on the pulsed laser and the laser vibrometer, preheat for a period of time to make the two instruments reach the optimal working temperature range. Turn on the industrial control computer, set the periodic grating pattern with different resolution spatial grating constants to obtain surface waves of different wavelengths;

[0063] Adjust the rotation angle of the half-wave plate so that the light intensity of the light emerging from the polarization beam splitter on the photodetector 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;

[0064] Adjust the magnification of the beam expander system, and use the rear diaphragm to intercept the place where the energy of the expanded light spot is more uniform. At the same time, the transmission area of the diaphragm should be larger than the incident window area of the homogenization system to ensure that the homogenization system can be fully utilized;

[0065] 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 as that of the digital micromirror array. At the same time, measure the total energy of the light spot to ensure that it is below the damage threshold of the digital micromirror array, and this threshold is 10 mj / cm2;

[0066] The industrial control computer controls the digital micromirror array to display a specified coded matrix pattern, and at the same time adjusts the angle of the homogenized system's outgoing light spot 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;

[0067] Select a suitable lens for the sleeve lens, and select a suitable lens focal length and lens spacing according to the required light spot size irradiated on the sample;

[0068] Select a suitable dichroic mirror according to the wavelength of the selected laser;

[0069] Adjust the mirror in the collimation system so that the direction and position of the light beam make it pass through two diaphragms and accurately vertically align with the sample target area;

[0070] Wipe the surface and the inside of the aluminum alloy sample with residual stress defects with alcohol, and clamp it on the three-dimensional moving platform. Adjust the imaging focal plane position of the sleeve lens so that a clear pattern is projected on the sample surface;

[0071] The detection light emitted by the laser vibrometer passes through the mirror in the collimation system for reflection and calibration, then reaches the beam splitting film through the dichroic mirror, and the detection light is projected on the sample on the three-dimensional moving platform through the objective lens. Adjust the size of the detection light spot so that it can completely cover the area that may excite the sound field. And by adjusting the angle of the beam splitting film, the excitation light and the detection light overlap and reach the imaging system.

[0072] Transmit the results measured by the laser vibrometer back to the industrial control computer, and perform computational imaging with periodic grating patterns of different spatial grating constants to obtain a three-dimensional map of the depth gradient stress of the sample.

[0073] The structure of a stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology according to the present invention is as Figure 1 shown.

[0074] Advantages: Compared with the prior art, the present invention has the following advantages: (1) Based on the laser ultrasonic non-destructive testing technology, the present invention can detect stress defects of a sample without contacting and damaging the sample to be tested. Compared with traditional testing technologies, the present invention does not require a coupling agent and will not cause environmental pollution, and can be used for testing under extreme environmental conditions such as high temperature and high pressure, corrosion, radiation, and toxicity. (2) Compared with traditional stress detection technologies that can only perform single-point detection, the present invention combines a three-dimensional moving platform to achieve stress scanning field detection. (3) Compared with the way of adjusting the light beam in traditional stress detection technologies, which cannot achieve complex optical field regulation, the present invention adopts the method of spatial light modulation, uses a digital micromirror array to generate periodic grating patterns with different spatial grating constants, and can generate ultrasonic waves with different wavelengths for stress defect detection at different depths. (4) Compared with traditional stress detection technologies that have great difficulties in detecting and calibrating stress gradients in the depth direction of materials, the present invention can perform stress microtomography imaging, making up for the deficiency of traditional stress detection technologies that cannot image the depth gradient stress field. Description of the Drawings

[0075] Figure 1 FIG. is a schematic structural diagram of a stress microtomography imaging device based on the laser ultrasonic non-destructive testing technology of the present invention;

[0076] Figure 2 FIG. is a diagram of the sample to be tested of the present invention;

[0077] Figure 3 FIG. is a flowchart of the operation of the present invention;

[0078] Figure 4 FIG. is a diagram of the hardware control interface of the present invention;

[0079] Figure 5 FIG. is a schematic diagram of periodic grating patterns with different spatial grating constants adopted by the present invention;

[0080] Figure 6 FIG. is a surface wave velocity-stress diagram of the material of the present invention;

[0081] Figure 7 FIG. is a diagram of gradient stress depth detection of the present invention;

[0082] In the figures: 1. Pulse laser; 2. Half-wave plate; 3. Polarizing beam splitter; 4. Photoelectric detector; 5. Beam expander system; 6. Diaphragm; 7. Mirror; 8. Homogenization system; 9. Digital micromirror array; 10. Sleeve lens; 11. Dichroic mirror; 12. Beam splitting film; 13. Objective lens; 14. Sample; 15. Three-dimensional moving platform; 16. Laser vibrometer; 17. Collimation system; 18. Imaging system; 19. Industrial control computer. Detailed Embodiments

[0083] The following further describes the embodiments of the present invention in conjunction with the accompanying 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.

[0084] Embodiment 1

[0085] The present invention provides a stress microscopic tomography imaging device based on laser ultrasonic non-destructive testing technology, including: a pulsed laser 1, a half-wave plate 2, a polarization beam splitter 3, a photodetector 4, a beam expander system 5, a diaphragm 6, a mirror 7, a homogenization system 8, a digital micromirror array 9, a sleeve lens 10, a dichroic mirror 11, a beam splitting film 12, an objective lens 13, a sample 14, a three-dimensional moving platform 15, a laser vibrometer 16, a collimation system 17, an imaging system 18, and an industrial control computer 19;

[0086] The pulsed laser 1 emits pulsed laser light. After passing through the half-wave plate 2 and then through the polarization beam splitter 3, it is divided into two polarized lights with different energies. One of the polarized lights is converted from an optical signal to an electrical signal after passing through the photodetector 4 and the signal is transmitted to the industrial control computer 19 as the trigger for the entire system. The other pulsed laser light continues to propagate backward, is amplified after passing through the beam expander system 5, is reflected by a mirror 7 after passing through a diaphragm 6 and enters the homogenization system 8 for light field homogenization. The rectangular light field with uniform spatial energy after homogenization is projected onto the digital micromirror array 9. The industrial control computer 19 controls the digital micromirror array 9 to generate periodic grating patterns with different spatial grating constants and reflects the pulsed laser light. Then, it passes through the sleeve lens 10, the dichroic mirror 11, the beam splitting film 12, and finally is projected onto the sample through the objective lens 13 at a set scaling ratio (the sample is placed on the three-dimensional moving stage 15). The periodic pulsed laser grating excites ultrasonic waves with a single frequency component on the sample and propagates along its surface. And this single-frequency surface wave is affected by the residual stress in the sample, resulting in a shift in the center frequency. The wavelengths of the surface waves excited by the periodic grating patterns with different spatial grating constants change in a gradient under the action of the stress defects with a depth gradient. The surface wave velocity changes in a gradient under the influence of the gradient stress, enabling the corresponding residual stress to be inversely calculated through the surface wave velocity. The surface waves excited by the periodic gratings with different spatial grating constants have different frequencies and different penetration depths of the surface waves. The residual stresses at different depths inside the sample 14 are inversely calculated from the penetration depth of the surface waves.

[0087] At the same time, a beam of detection light emitted by the laser vibrometer 16 passes through the collimation system 17 and propagates perpendicular to the surface of the sample. After passing through the dichroic mirror 11 and the beam splitting film 12, it is focused on the surface of the sample by the objective lens 13, and the overall vibration information of the detection area is collected.

[0088] Subsequently, the excitation light and the detection light return together through the objective lens 13, reach the imaging system 18 after being selected and distributed by the beam splitting film 12, and then the industrial control computer 19 performs data processing and tomographic imaging.

[0089] After all the periodic grating patterns with different spatial grating constants in a resolution detection area are scanned, the industrial control computer 19 controls the three-dimensional moving platform 15 to move the sample to the next detection area. The three-dimensional moving platform realizes stress field scanning detection.

[0090] A method for stress microscopic tomography based on laser ultrasonic non-destructive testing technology includes the following steps:

[0091] Turn on the pulsed laser and the laser vibrometer, preheat for a period of time to let the two instruments reach the optimal working temperature range, turn on the industrial control computer, and set periodic grating patterns with different spatial grating constants at different resolutions to obtain surface waves with different wavelengths;

[0092] Adjust the rotation angle of the half-wave plate 2 so that the light intensity of the light emitted by the polarization beam splitter 3 on the photodetector 4 is less than its tolerance threshold, and at the same time, the output of the photodetector 4 when its impedance is matched should be between 100 - 500 mv. Connect the output of the photodetector 4 to the industrial control computer 19 as the trigger of the entire system;

[0093] Adjust the magnification of the beam expander system 5, and use the diaphragm 6 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 diaphragm 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;

[0094] 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;

[0095] The industrial control computer 19 controls the digital micromirror array 9 to display a specified coding matrix pattern. At the same time, adjust the angle of the light spot emitted by the homogenization system 8 irradiating on the digital micromirror array 9 so that its incident direction has a 24-degree angle with the normal direction of the digital micromirror array 9;

[0096] Select a suitable lens for the sleeve lens 10, and select a suitable lens focal length and lens spacing according to the required light spot size irradiated on the sample;

[0097] Select a suitable dichroic mirror 11 according to the wavelength of the selected laser;

[0098] Adjust the mirror in the collimation system 17 so that the direction and position of the light beam pass through the two diaphragms and are accurately aligned perpendicular to the sample target area;

[0099] Wipe the surface and the interior of the aluminum alloy sample 14 with stress defects with alcohol, and clamp it on the three-dimensional moving platform 15. Adjust the imaging focal plane position of the sleeve lens 10 so that a clear pattern is projected on the sample surface;

[0100] The detection light emitted by the laser vibrometer 16 passes through the collimation system 17 and is reflected and calibrated by the mirror in it, then passes through the dichroic mirror 11 and reaches the beam splitting film 12. The detection light then passes through the objective lens 13 and is projected onto the sample on the three-dimensional moving platform 15. Adjust the spot size of the detection light so that it can completely cover the area that may excite the sound field. And by adjusting the angle of the beam splitting film 12, the excitation light and the detection light overlap and reach the imaging system 18.

[0101] The results measured by the laser vibrometer 16 are transmitted back to the industrial control computer 19 and are calculated and imaged with periodic grating patterns with different spatial grating constants, so as to obtain the three-dimensional depth gradient stress map T(x, y, z) of the sample.

[0102] The formulas required for calculating the residual stress include: the formula (1) for calculating the frequency of the surface wave and the formula (2) for calculating the penetration depth of the surface wave.

[0103] c = λf (1)

[0104] h = v / f -0.96 (2)

[0105] In formula (1), c is the surface wave velocity, λ is the surface wave wavelength, and f is the surface wave frequency. In formula (2), h is the penetration depth of the surface wave, v is the surface wave velocity, and f is the surface wave frequency.

[0106] The relationship between the residual stress and the depth is calculated by formula (3)

[0107]

[0108] In formula (3), σ is the stress, h is the penetration depth of the surface wave, i takes values of (1, 2, 3…n), and j takes values of (1, 2, 3…n).

[0109] Combining formula (2) and formula (3) can obtain the relationship between the residual stress gradient depth and the surface wave frequency as shown in formula (4)

[0110]

[0111] In formula (4), σ is the stress, f is the surface wave frequency, i takes values of (1, 2, 3…n), and j takes values of (1, 2, 3…n). According to the acoustoelastic theory, the gradient stress and the penetration depth can be inversely calculated.

[0112] Wave vector k, wavelength λ, spatial grating constant d, angular velocity ω, frequency f, spectral resolution ΔRw , and the period T are shown in the following formula:

[0113]

[0114] ω=2*π*f (7)

[0115]

[0116] Because of the one thousandth change in the speed of sound, laser ultrasound can be effectively measured. The above formulas are combined to calculate the grating period to be 1000. In actual experiments, due to the overall size of the DMD (10*14mm), a grating with a period of 200 can only be generated on a surface of 10mm at most. The resolution can be improved by interpolation and zero filling.

[0117] The technical solution of the present invention is further described below in conjunction with the accompanying drawings:

[0118] The samples to be tested are Figure 2 As shown, the sample size is 25*1.5mm aluminum alloy, which is laser shot peening strengthened to contain stresses of 0MPa, 200MPa, 400MPa, 600MPa, 800MPa, 1000MPa, and no trace can be seen on the surface of the sample:

[0119] The overall flow chart of the invention is as follows Figure 3 As shown:

[0120] The invented hardware system is written by Labview, and its graphical interface is as follows Figure 4 As shown:

[0121] Based on the above laser ultrasonic nondestructive testing device, the invention sample is tested and imaged according to the implementation steps given in the flow chart, as follows:

[0122] Turn on the pulse laser and laser vibrometer, preheat for a while, and let the two instruments reach the optimal operating temperature range; turn on the industrial computer, and determine the use of a coding matrix with an area of ​​10*10mm for the grating center frequency of the present invention is 40MHz, the grating is 200 periods, and the spatial grating constant is 0.075mm. The specific pattern is as follows Figure 5 shown.

[0123] By changing the angle of the half-wave plate, the light intensity output by the polarization beam splitter is adjusted so that a 400mv voltage signal is generated under the impedance matching condition of the photodetector. This signal will be used for the synchronous triggering of the system.

[0124] Adjust the distance between the two microlens arrays in the homogenization system to ensure that a uniform light spot with a size of 14*10 mm is formed on the focal plane of the homogenization system, which matches the size of the digital micromirror device (DMD) chip. In addition, the single-pulse energy of the measured light spot is 5 mj / cm2, which is lower than the damage threshold of the DMD chip;

[0125] Adjust the homogenizing system to ensure that the incident direction of the output light spot forms an accurate angle of 24 degrees with respect to the normal direction of the digital micromirror device (DMD) chip. Subsequently, the industrial control computer loads the encoded matrix image generated just now into the memory of the DND system;

[0126] Perform surface polishing on the aluminum alloy specimen to ensure its surface is flat. Subsequently, fix the specimen on a three-dimensional adjustable displacement stage. Adjust the displacement stage so that the specimen is accurately positioned on the imaging system plane. By finely adjusting the displacement stage, ensure that the encoded matrix pattern can be clearly imaged on the specimen surface;

[0127] Adjust the incident angle of the laser vibrometer so that the detection light spot and the excitation light spot are at the same position in the sample plane. At the same time, adjust the spot size of the outgoing light so that it can completely cover the excitation sound field area;

[0128] Fine-tune the positions of the sample displacement stage and the mirror so that the laser vibrometer receives the best signal quality. Subsequently, use the photodetector control unit to trigger the synchronous change of the encoded pattern after each light pulse. At the same time, the high-speed data acquisition card is responsible for collecting the acoustic wave signals measured by the laser vibrometer and transmitting these data to the industrial control computer for storage. Figure 6 Shows the surface wave velocity relationship diagram corresponding to the gradient stress.

[0129] The frequencies of the surface waves excited by the periodic grating patterns with different spatial grating constants generated by the digital micromirror device (DMD) are f 1 、f 2 、f 3 , the surface excitation wavelengths are λ 1 、λ 2 、λ 3 , and according to formulas (2), (3), and (4), the measured residual stress detection values of aluminum alloy sample 14 are σ 1 、σ 2 、σ 3 , and the depths are h 1 、h2、h 3 . By performing the successive difference processing on the three residual stresses, h 1 、h 2 -h 1 、h 3 -h 2The residual stresses at each gradient depth are σ 1 , σ 2 - σ 1 , σ 3 - σ 2 , as shown in Figure 7 .

[0130] 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 a suitable proportion. The periodic pulsed laser grating excites ultrasonic waves with a single frequency component on the sample and propagates along its surface. And this single-frequency surface wave is affected by the residual stress in the sample, resulting in a shift in the central frequency. The wavelengths of the surface waves excited by the periodic grating patterns with different spatial grating constants change in a gradient under the action of the stress defects in the depth gradient. The surface wave velocity changes in a gradient under the influence of the gradient stress, and gradually decreases as the stress increases, enabling the corresponding stress to be inversed through the surface wave velocity. The laser vibrometer emits a collimated laser beam with a certain width, irradiates the surface of the sample, collects the overall vibration information of the detection area, and transmits the information to the industrial control computer, which generates the three-dimensional distribution information of the stress distribution.

[0131] The present invention uses a digital micromirror array to generate periodic grating patterns with different spatial grating constants, so as to obtain different wavelengths, and combined with the acoustoelastic theory, it can realize the calculation of the magnitude and depth of the stress. And stress inversion can be realized through the relationship between the surface wave velocity and the stress magnitude. Similarly, the stress depth gradient distribution can be inversed through the relationship between the surface wave penetration depth and the stress magnitude.

[0132] The present invention uses a three-dimensional moving platform to realize the material stress distribution scanning field and generate stress microtomography through the industrial control computer.

[0133] The laser vibrometer of the present invention is not limited to using the laser vibrometer (16), and other vibration measurement methods also belong to the content of the present invention;

[0134] The directions of the pulsed laser (1) and the laser vibrometer (16) of the present invention are not limited to being relatively arranged, and other arranged directions also belong to the content of the present invention.

[0135] The digital micromirror array (9) of the present invention is not limited to DMD, and optical modulators such as SLM also belong to the content of the present invention;

[0136] 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 stress microtomography device based on laser ultrasonic nondestructive testing technology, characterized in that: include: Pulse laser (1), half-wave plate (2), polarization beam splitter (3), photodetector (4), beam expansion system (5), aperture (6), reflector (7), homogenization system (8), digital micromirror array (9), set of simple lenses (10), dichroic mirror (11), beam splitting film (12), objective lens (13), sample (14), three-dimensional mobile platform (15), laser vibrometer (16), collimation system (17), imaging system (18), industrial computer (19).

2. The stress microtomography device based on laser ultrasonic nondestructive testing technology according to claim 1 is characterized in that: (1) Path of excitation light: The pulsed laser emitted by the pulsed laser passes through a half-wave plate and then a polarization beam splitter to be split into two beams of polarized light with different energies. One beam of polarized light is converted from an optical signal to an electrical signal after passing through a photodetector and the signal is transmitted to an industrial computer to trigger the entire system; the other beam of pulsed laser continues to propagate backward, is amplified after passing through a beam expansion system, and then is reflected by a reflector after passing through an aperture and enters a homogenization system for light field homogenization. The rectangular light field with uniform spatial energy after homogenization is projected onto a digital micromirror array. The industrial computer controls the digital micromirror array to generate periodic grating patterns with different spatial grating constants and reflects the pulsed laser. After passing through a sleeve lens, a dichroic mirror, and a beam splitting film, the pulsed laser is finally zoomed and projected onto a sample placed on a three-dimensional moving stage through an objective lens; (2) Path of the detection light: A beam of detection light emitted by the laser vibrometer passes through the collimation system in a propagation direction perpendicular to the sample surface, passes through the dichroic mirror and the beam splitting film, and is focused on the sample surface by the objective lens, and the overall vibration information of the detection area is collected; (3) The excitation light and the detection light return through the objective lens together, and after being selectively distributed by the beam splitting film, they reach the imaging system, and then the industrial computer processes the data and produces tomographic images.

3. The stress microtomography device based on laser ultrasonic nondestructive testing technology according to claim 2 is characterized in that: The single pulse width of the pulse laser includes ns level, ps level and fs level.

4. The stress microtomography device based on laser ultrasonic nondestructive testing technology according to claim 2 is characterized in that: The beam expansion system is composed of 2-5 lenses, and its function is to expand a small collimated light spot to a larger collimated light spot; The digital micromirror array is composed of mirrors arranged together; each mirror can be deflected by plus or minus 8-15 degrees on the diagonal line; The collimation system is composed of 2-5 reflecting mirrors and diaphragms, and its function is to control the direction of the laser beam to be perpendicular to the sample surface, and is composed of one or more reflecting mirrors and diaphragms; The imaging system consists of a long working distance objective lens and a charge coupled device.

5. The stress microtomography device based on laser ultrasonic nondestructive testing technology according to claim 2 is characterized in that: The digital micromirror array may be replaced by a spatial light modulator.

6. The stress microtomography device based on laser ultrasonic nondestructive testing technology according to claim 4 is characterized in that: The beam expansion system is composed of a combination of 2-5 convex lenses or concave lenses.

7. The stress microtomography device based on laser ultrasonic nondestructive testing technology according to claim 1 is characterized in that: The periodic pulsed laser grating excites ultrasonic waves with a single frequency component on the sample and propagates along its surface; the single-frequency surface wave is affected by the residual stress in the sample and the center frequency is shifted; the acoustic surface waves excited by periodic grating patterns with different spatial grating constants undergo a gradient change in wavelength under the action of stress defects with a depth gradient; by measuring the single-frequency surface waves generated by periodic gratings with different spatial grating constants, a frequency shift occurs in the gradient stress material, making it possible to invert the corresponding residual stress through the surface wave velocity; the surface waves excited by periodic gratings with different spatial grating constants have different frequencies and different surface wave penetration depths, and the residual stresses at different depths inside the sample are inverted from the surface wave penetration depth.

8. A detection method using the stress microtomography device based on laser ultrasonic nondestructive testing technology according to claim 1, characterized in that: The following steps are involved: (1) Turn on the pulse laser and laser vibrometer; (2) Turn on the industrial computer and set the periodic grating patterns with different spatial grating constants to obtain surface waves of different wavelengths; (3) Adjust the rotation angle of the half-wave plate so that the light intensity emitted by the polarization beam splitter on the photodetector is less than its tolerance threshold. At the same time, the output of the photodetector should be between 100-500mv when its impedance is matched. Connect the output of the photodetector to the industrial computer as a trigger for the entire system. (4) Adjust the magnification of the beam expansion system and use the aperture at the rear end to intercept the area where the energy of the beam spot is more uniform after beam expansion. 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; (5) 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 that of the digital micromirror array. At the same time, measure the total energy of the light spot to ensure that it is below the damage threshold of the digital micromirror array, which is 10 mj / cm2. (6) The industrial computer controls the digital micromirror array to display the specified coding matrix pattern, and at the same time adjusts the angle of the light spot emitted by the homogenization system to irradiate the digital micromirror array so that the incident direction and the normal direction of the digital micromirror array have an angle of 20-30 degrees; (7) Select the lens of the tube lens, and select the lens focal length and lens spacing according to the required spot size irradiated on the sample; (8) Selecting a dichroic mirror according to the selected laser wavelength; (9) adjusting the reflector in the collimation system so that the direction and position of the light beam are precisely aligned vertically with the sample target area through the two apertures; (10) Clamp the sample on a three-dimensional moving platform, and adjust the imaging focal plane position of the tube lens so that a clear pattern is projected on the sample surface; (11) The laser vibrometer emits a detection light which is reflected and calibrated by a reflector in a collimation system and then reaches a beam splitting film through a dichroic mirror. The detection light is then projected onto a sample on a three-dimensional moving platform through an objective lens. The spot size of the detection light is adjusted so that it can completely cover an area where the acoustic field may be excited. The angle of the beam splitting film is adjusted so that the excitation light and the detection light overlap and reach the imaging system. (12) The results obtained by the laser vibrometer are transmitted back to the industrial computer and are imaged by computation with periodic grating patterns with different spatial grating constants to obtain a three-dimensional map of the depth gradient stress of the sample.

9. The detection method of the stress microtomography device based on laser ultrasonic nondestructive testing technology according to claim 8 is characterized in that: The formulas required for calculating the residual stress include: the frequency formula for calculating the surface wave (1) and the penetration depth formula for calculating the surface wave (2): c=λf (1) h=vf -0.96 (2) In formula (1), c is the surface wave velocity, λ is the surface wave wavelength, and f is the surface wave frequency; in formula (2), h is the penetration depth of the surface wave, v is the surface wave velocity, and f is the surface wave frequency; The relationship between residual stress and depth is calculated by formula (3): In formula (3), σ is stress, h is the penetration depth of the surface wave, i and j represent the i-th and j-th detections, the value of i is (1, 2, 3…n), the value of j is (1, 2, 3…n), and i is greater than j; Combining formula (2) and formula (3), we can get the relationship between residual stress gradient depth and surface wave frequency as shown in formula (4): In formula (4), σ is stress, f is the surface wave frequency, i and j represent the i-th and j-th detections, the value of i is (1, 2, 3...n), the value of j is (1, 2, 3...n), and i is greater than j; according to the formula acoustic elastic theory, the gradient stress and penetration depth can be inverted; Wave vector k, wavelength λ, spatial grating constant d, angular velocity ω, frequency f, spectral variation rate ΔR w , the relationship between the period T is shown below: ω=2*π*f (7) 10. Application of the stress micro-tomography device based on laser ultrasonic non-destructive testing technology as claimed in claim 1 in stress detection.

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