Laser nondestructive detection method and device for surface defects based on dynamic speckle illumination

Abstract

The present invention relates to a surface defect laser nondestructive testing method and apparatus based on dynamic speckle illumination. This method utilizes the coherent despeckle effect of the photoacoustic effect to achieve highly sensitive detection of surface microcracks. Photoacoustic signals within the speckle pattern cancel each other, resulting in a strong signal only at the speckle's circular boundary. However, surface microcracks prevent the coherent cancellation of the speckle photoacoustic signals within the circular spot, resulting in a single speckle photoacoustic signal generated by the microcrack. Multiple detection points around the speckle pattern are then used to generate multiple photoacoustic signals. Ultimately, an algorithm is used to generate a two-dimensional image of the microcrack.

Description

Technical Field

[0001] The present invention belongs to the field of laser ultrasonic nondestructive testing, relates to nondestructive defect detection of parts, and specifically relates to a surface defect laser nondestructive detection method and device based on dynamic speckle illumination. Background Art

[0002] Components inevitably develop defects during the production and processing process. Microcracks, in particular, extending from the interior to the surface, can reflect a variety of important indicators, including a component's airtightness, fatigue resistance, and mechanical strength. Similarly, during use, components can be subjected to mechanical forces, high-temperature cycling, impact, and prolonged heavy loads, which can cause structural tears. This manifests as the development and growth of microcracks, ultimately leading to component failure.

[0003] Early detection of surface microcracks in components without damaging them is crucial for improving production processes, replacing and repairing in-service components, ensuring system stability, and maintaining production safety. Currently, a variety of nondestructive testing methods are used, including radiographic testing (RT), ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT), eddy current testing (ECT), acoustic emission testing (AE), thermal imaging / infrared (TIR), leak testing (LT), alternating current field measurement technology (ACFMT), and magnetic flux leakage (MFL). Information is primarily carried in the form of ultrasound, X-rays, magnetic fields, electric fields, and infrared. High-energy radiation like X-rays has strong ionizing effects on the human body and requires specific environmental requirements for use. Magnetic and electric field testing methods often require the use of magnetic particle or point-by-point scanning probes, which can be inefficient and inconvenient. For defects smaller than 100 microns, the resolution of infrared thermal imaging is insufficient for high-precision measurement.

[0004] Ultrasonic nondestructive testing offers advantages such as ease of use, safety, and a wide variety of methods. Traditional ultrasonic nondestructive testing uses an ultrasonic transducer placed in close proximity to a component. However, due to the short distance between surface defects and the transducer, accurate detection is difficult. Contact measurement also requires a certain level of operator proficiency. While there have been reports of non-contact air-coupled probe ultrasonic nondestructive testing, the results are still inferior to those achieved with contact ultrasonic probes. Using lasers to excite ultrasonic waves and laser vibrometers to detect them allows for non-contact, high-precision nondestructive testing.

[0005] Although the above ultrasonic detection methods have been put into practical use, for micro-crack defects on the surface of components, whether using the attenuation of the transmitted wave amplitude of the sound wave or detecting the reflected echo, when the defect scale is close to the detected ultrasonic length, the diffraction and diffraction of the sound wave cause the existing methods to be insufficient in defect detection accuracy. Summary of the Invention

[0006] The purpose of the present invention is to provide a surface defect laser nondestructive detection method and device based on dynamic speckle illumination, which solves the problem of how to effectively detect microcracks.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] The present invention provides a surface defect laser nondestructive detection method based on dynamic speckle illumination, which comprises:

[0009] S1, the laser beam emitted by the pulsed laser is processed by optical components to obtain a flat-top beam. After the flat-top beam passes through the scattering plate, a randomly scattered laser beam is obtained. The laser speckle hits the workpiece and generates high-frequency, broadband ultrasonic waves. The ultrasonic waves are measured as electrical signals by the laser vibrometer set outside the speckle area. in Indicates the position of the laser vibrometer detection point;

[0010] S2, rotate the scattering plate to a certain angle and measure the new electrical signal Where i represents the signal obtained by the i-th measurement, and a total of N measurements are made, so the average of N times is taken as the effective signal

[0011] S3, adjust the laser vibrometer detection point to rotate a certain angle θ relative to the workpiece. At this time, the position of the laser vibrometer detection point is Repeat step S2 to get a new measurement signal

[0012] S4, repeat step S3 to obtain the measurement signal at different positions j The image of its surface microcrack defects is generated by the algorithm Get, among them They are the constant, the coordinates of the defect and the central angle of a certain laser vibrometer detection point relative to the entire circular laser speckle detection point trajectory.

[0013] Preferably, the plurality of laser vibrometer detection points are evenly distributed around the circular laser speckle area.

[0014] Furthermore, the laser vibrometer detection point is scanned to a corresponding position by the galvanometer system to form a plurality of laser vibrometer detection points.

[0015] Furthermore, a plurality of laser vibrometer detection points are formed by rotating the motor workpiece.

[0016] Preferably, the laser uses a wavelength of 532 nm, a pulse width of 6-10 ns, and a laser energy of 50 mJ per single pulse.

[0017] Furthermore, the maximum vibration frequency measured by the laser vibrometer reaches 20 MHz.

[0018] Preferably, the laser speckle pattern is achieved using 600-mesh ground glass, so that the size of a single speckle is about 10-150 microns, and the increment of the rotation angle θ measured each time is 2.5 degrees.

[0019] Furthermore, the laser speckle distribution has a diameter of about 2.5 cm, and the laser vibrometer detection points are distributed on a circle with a diameter of 5 cm.

[0020] A surface defect laser nondestructive testing device based on dynamic speckle illumination is also provided: it includes a pulsed laser, a scattering plate arranged in the pulsed laser beam, and a laser vibrometer for detecting the surface vibration of the workpiece, and the detection point of the laser vibrometer can be rotated and adjusted relative to the workpiece, and the laser vibrometer is used to measure the electrical signal. in Indicates the position of the laser vibrometer detection point, and rotates the scattering plate a certain angle to measure a new electrical signal again After multiple measurements, the average value is taken as the effective signal Then adjust the laser vibrometer detection point to rotate a certain angle θ relative to the workpiece, and repeat the above steps to obtain a new measurement signal. Get the measurement signal at different positions j The image of surface microcrack defects is generated by the algorithm It is obtained that α, r, and θ are constants, the coordinates of the defect, and the central angle of a laser vibrometer detection point relative to the entire circular laser speckle detection point trajectory, respectively.

[0021] Preferably, it further comprises a galvanometer system or a rotary motor, wherein the galvanometer system cooperates with the laser vibrometer to change the position of the detection point of the laser vibrometer, and the rotary motor is used to drive the workpiece to rotate.

[0022] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0023] The present invention's surface defect laser nondestructive testing method and device based on dynamic speckle illumination utilizes the coherent speckle elimination effect of the photoacoustic effect. Ultrasonic waves generated by lasers with identical excitation laser parameters and spatial distribution are coherent waves, identical in waveform and amplitude. Furthermore, under low-intensity excitation, the photoacoustic signals are symmetrical bipolar signals. Because the speckle sizes are uniformly and randomly distributed, the photoacoustic signals within the speckle cancel each other out. When a laser vibrometer detects the signal, a strong signal is generated by coherent superposition at the circular boundary of the speckle, while the ultrasonic signal within the circular speckle is zero. However, when surface microcracks are present within the workpiece, the coherent cancellation between the speckles within the circular spot is disrupted, ultimately resulting in the signal collected by the laser vibrometer's detection point having a finite value within the circular spot. Furthermore, the resolution limit is independent of the ultrasonic frequency generated by the laser and depends solely on the size of the laser speckle. The presence of surface microcracks causes the speckle photoacoustic signal to be unable to coherently cancel out inside the circular spot, resulting in a speckle photoacoustic signal generated by the microcrack. The laser vibrometer detection point is adjusted to rotate relative to the workpiece, the signal is measured multiple times, and finally a two-dimensional image of the microcrack is calculated through an algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Hereinafter, some specific embodiments of the present invention will be described in detail in an exemplary and non-limiting manner with reference to the accompanying drawings. The same reference numerals in the accompanying drawings indicate the same or similar components or parts. It should be understood by those skilled in the art that these drawings are not necessarily drawn to scale. In the accompanying drawings:

[0025] Figure 1 Schematic diagram of a surface defect laser nondestructive testing device based on dynamic speckle illumination according to the present invention;

[0026] Figure 2 It is a schematic diagram of the workpiece surface speckle, defects and detection point positions;

[0027] Figure 3 It is the ultrasonic signal detected by the laser vibrometer arranged outside the circular excitation light;

[0028] Figure 4 It is the ultrasonic signal detected by the laser vibrometer when there are micro cracks in the workpiece;

[0029] The description of the accompanying drawings is as follows:

[0030] 1: scattering sheet;

[0031] 2: Pulsed laser;

[0032] 3: Laser vibrometer detection point;

[0033] 4: Workpiece;

[0034] 5: Rotating motor;

[0035] 6: Laser speckle;

[0036] 7: micro cracks;

[0037] 8: Speckle photoacoustic signal generated by microcracks. DETAILED DESCRIPTION

[0038] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0039] The present invention proposes a surface defect laser nondestructive detection technology and device based on dynamic speckle illumination, and the core is to achieve high-sensitivity detection of surface microcracks based on the coherent despeckle effect of the photoacoustic effect. Figure 1 shown.

[0040] After passing through diffuser plate 1, pulsed laser light 2 creates a randomly distributed spot on workpiece 4, illuminating the entire area with a circular pattern. Rotating diffuser plate 1 randomly creates different speckle patterns, improving the spatial uniformity of the speckle pattern. When the laser speckle pattern impinges on workpiece 4, it absorbs the laser energy and converts it into heat, ultimately causing thermal expansion and the generation of ultrasonic waves. This phenomenon is known as the photoacoustic effect.

[0041] According to the photoacoustic effect, the ultrasonic waves excited by the same excitation laser parameters and the same spatial distribution are coherent waves, which are exactly the same in waveform and amplitude. And under low light intensity excitation, the photoacoustic signals are symmetrical bipolar signals. Figure 3 As shown in the figure, since the speckle sizes are uniformly and randomly distributed, the photoacoustic signals within the speckle will cancel each other out. The ultrasonic signal detected by the laser vibrometer arranged outside the circular excitation light will be as follows Figure 3 As shown in the lower part, a stronger signal is obtained by coherent superposition on the circular boundary, while the ultrasonic signal is zero inside the circular speckle.

[0042] When a workpiece 4 is illuminated by a circular spot with randomly distributed laser speckles 6 within it, the photoacoustic waves generated by a significant portion of these laser speckles 6 coherently superimpose, increasing the intensity of the photoacoustic signal at the boundary and weakening the signal within. Rotating the scattering plate 1 to alter the pattern of the laser speckle 6 and superimpose the photoacoustic signals generated by the different speckle laser patterns increases the randomness of the overall distribution of the laser speckle 6. In other words, the speckle distribution at every point within the circular spot is strictly uniform. Therefore, the more the laser spot pattern is altered by rotating the scattering plate 1, the more pronounced the corresponding speckle effect.

[0043] like Figure 4 When surface microcracks 7 are present within workpiece 4, the coherent cancellation between the scattered speckles within the circular spot is disrupted, ultimately causing the signal collected by the laser vibrometer's detection point to have a finite value within the circular spot. Furthermore, its resolution limit is independent of the ultrasonic frequency generated by the laser and depends solely on the size of the laser speckle pattern 6. The presence of surface microcracks 7 prevents the coherent cancellation of the speckle photoacoustic signal within the circular spot, resulting in a speckle photoacoustic signal 8 generated by the microcrack.

[0044] If only one laser vibrometer detection point 3 is used, it is not enough to obtain a two-dimensional image of the microcrack 7. The present invention proposes to detect the entire circular laser speckle. The more points are taken, the finer the image of the microcrack 7 is. In this case, the microcrack 7 can be regarded as the source of the speckle photoacoustic signal 8 generated by the microcrack. The photoacoustic signals measured at different positions of the circular speckle are recorded as in is the position of the laser vibrometer detection point 3. Spatial distribution of internal surface microcracks 7 It can be solved by the following formula:

[0045]

[0046] in are the constant, the coordinates of the defect, and the central angle of a laser vibrometer detection point 3 relative to the entire circular laser speckle detection point trajectory. After the above analysis, the overall concept of the present invention has become clear. The specific operation process of the proposed surface defect laser non-destructive detection technology and device based on dynamic speckle illumination is as follows:

[0047] (1) The laser beam emitted by the pulsed laser is processed by optical components to obtain a flat-top beam. After the flat-top beam passes through the scattering plate 1, a randomly scattered laser beam is obtained. The laser speckle hits the workpiece 4 to generate high-frequency, broadband ultrasonic waves, which are measured by the laser vibrometer as electrical signals. in Indicates the position of point 3 detected by the laser vibrometer.

[0048] (2) Rotate the scattering plate 1 to a certain angle and measure a new electrical signal Where i represents the signal obtained by the i-th measurement. A total of N measurements are made, so the average of N times is taken as the effective signal

[0049] (3) Use the rotary motor 5 to rotate the workpiece 4 by a certain angle θ, or use the galvanometer system to scan the laser vibrometer detection point 3 to the corresponding position. At this time, the position of the laser vibrometer detection point 3 Repeat step (2) to get a new measurement signal.

[0050] (4) Repeat step (3) to obtain the measurement signal at different positions j The image of its surface microcrack defects can be obtained by the following algorithm

[0051] In this implementation example, an aluminum alloy plate was used as workpiece 4. The plate contained a 50-micron-wide, 5-mm-long, arc-shaped microcrack with a depth of approximately 30-80 microns. Traditional laser nondestructive testing (NDT) methods are difficult to detect because the crack is shallow, allowing surface waves to easily diffract through it, resulting in no discernible change in amplitude. Therefore, the method and apparatus proposed in this invention were used for detection.

[0052] The laser used has a wavelength of 532nm, a pulse width of 6-10ns, and a laser energy of 50mJ per pulse. The specific laser energy can be controlled using external optical components; the maximum measurement vibration frequency of the laser vibrometer reaches 20MHZ.

[0053] Laser speckle patterning was achieved using 600-mesh frosted glass. Imaging of the speckle pattern revealed that individual speckles ranged from 10 to 150 microns in size, sufficient to cover the defect area. The rotation angle θ was measured in increments of 2.5 degrees each time, with data from 144 positions measured. The laser speckle pattern had a diameter of approximately 2.5 cm, and the laser vibrometer detection points were distributed on a 5 cm diameter circle.

[0054] Through comparative tests with intact samples, the results confirm that the technology proposed by the present invention can not only detect the corresponding defects, but also image the shapes of the defects.

[0055] The above embodiments are only for illustrating the technical concept and features of the present invention. Their purpose is to enable people familiar with this technology to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A surface defect laser nondestructive detection method based on dynamic speckle illumination, characterized in that: It includes: S1, the laser beam emitted by the pulsed laser is processed by optical components to obtain a flat-top beam. After the flat-top beam passes through the scattering plate, a randomly scattered laser beam is obtained. The laser speckle hits the workpiece and generates high-frequency, broadband ultrasonic waves. The ultrasonic waves are measured as electrical signals by the laser vibrometer set outside the speckle area. ,in Indicates the position of the laser vibrometer detection point; S2, rotate the scattering plate to a certain angle and measure the new electrical signal , where i represents the signal obtained by the i-th measurement, and a total of N measurements are made, so the average of N times is taken as the effective signal ; S3, adjust the laser vibrometer detection point to rotate a certain angle θ relative to the workpiece. At this time, the position of the laser vibrometer detection point is , repeat step S2 to get a new measurement signal ; S4, repeat step S3 to obtain the measurement signal at different positions j , the image of its surface microcrack defect is generated by the algorithm We get, where α, , θ are the constant, the coordinates of the defect and the central angle of a laser vibrometer detection point relative to the entire circular laser speckle detection point trajectory.

2. The surface defect laser nondestructive testing method based on dynamic speckle illumination according to claim 1, characterized in that: Multiple laser vibrometer detection points are evenly distributed around the circular laser speckle area.

3. The surface defect laser nondestructive testing method based on dynamic speckle illumination according to claim 2, characterized in that: The laser vibrometer detection point is scanned to the corresponding position by the galvanometer system to form multiple laser vibrometer detection points.

4. The surface defect laser nondestructive testing method based on dynamic speckle illumination according to claim 2, characterized in that: The motor workpiece is rotated to form multiple laser vibrometer detection points.

5. The surface defect laser nondestructive testing method based on dynamic speckle illumination according to claim 1, characterized in that: The laser uses a wavelength of 532nm, a pulse width of 6-10ns, and a laser energy of 50mJ per pulse.

6. The surface defect laser nondestructive testing method based on dynamic speckle illumination according to claim 5, characterized in that: The maximum vibration frequency that can be measured by the laser vibrometer is 20 MHZ.

7. The surface defect laser nondestructive testing method based on dynamic speckle illumination according to claim 1, characterized in that: Laser speckle patterning was achieved using 600-mesh ground glass, resulting in a single speckle size of 10–150 μm, with the rotation angle θ increment of 2.5 deg for each measurement.

8. The surface defect laser nondestructive testing method based on dynamic speckle illumination according to claim 7, characterized in that: The laser speckle distribution has a diameter of 2.5 cm, and the laser vibrometer detection points are distributed on a circle with a diameter of 5 cm.

9. A surface defect laser nondestructive testing device based on dynamic speckle illumination, characterized in that: It includes a pulsed laser, a scattering plate set in the pulsed laser beam, and a laser vibrometer for detecting the vibration of the workpiece surface. The detection point of the laser vibrometer can be rotated and adjusted relative to the workpiece, and the laser vibrometer is used to measure the electrical signal. ,in Indicates the position of the laser vibrometer detection point, and rotates the scattering plate a certain angle to measure a new electrical signal again , after multiple measurements, the average value is taken as the effective signal ; Then adjust the laser vibrometer detection point to rotate a certain angle θ relative to the workpiece, and repeat the above steps to obtain a new measurement signal , get the measurement signal at different positions j , the image of surface microcrack defects is generated by the algorithm We get, where α, , θ are the constant, the coordinates of the defect and the central angle of a laser vibrometer detection point relative to the entire circular laser speckle detection point trajectory.

10. The surface defect laser nondestructive testing device based on dynamic speckle illumination according to claim 9, characterized in that: It also includes a galvanometer system or a rotary motor, wherein the galvanometer system cooperates with the laser vibrometer to change the position of the detection point of the laser vibrometer, and the rotary motor is used to drive the workpiece to rotate.

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