Laser ultrasonic defect measuring device and method suitable for seawater high-pressure environment

Abstract

The invention discloses a laser ultrasonic defect measuring device suitable for a seawater high-pressure environment, which comprises a seawater high-pressure environment pressure tank and a laser ultrasonic defect detection module, the seawater high-pressure environment pressure tank is provided with a pressure tank, and the laser ultrasonic defect measuring device is characterized in that a plurality of optical windows are arranged on the wall of an outer cavity of the pressure tank; the laser ultrasonic defect detection module is mounted on a scanning module, the scanning module has six-axis motion freedom degrees, and the scanning module controls the irradiation positions and directions of two laser beams emitted by the laser ultrasonic defect detection module, so that the laser beams vertically enter the optical window and irradiate the surface of a sample to be detected. According to the invention, online defect detection of the to-be-detected sample in a high-water-pressure environment is realized, and the method has the advantages of being online, non-destructive, suitable for large-size samples and the like.

Description

Technical Field

[0001] This invention relates to a laser ultrasonic testing device and method, specifically to a laser ultrasonic testing device and method suitable for high-pressure seawater environments, and is particularly suitable for defect measurement. Background Technology

[0002] Marine equipment forms the backbone of research and development in three major areas: marine safety assurance, marine resource development, and marine scientific research. Before entering formal service, marine equipment needs to undergo testing and verification using ground simulation devices, and its design is optimized and improved during ground trials. For example, Chinese utility model patent CN221925951U discloses a marine pressure environment simulation device, including a simulation chamber with at least one open end. A chamber cover is provided at the open end, and a sealing component and multiple locking components are provided at the connection between the open end and the cover. Each locking component includes a screw and a hanging hole, with the hanging hole corresponding to the screw. A nut is engaged on the screw. The purpose of this design is to provide stronger sealing and stability to the chamber cover while making it easier to open, allowing marine instruments to be placed inside the simulation chamber from the open end for performance testing. While the document does not mention specific performance testing methods, the phrase "testing inside the simulation chamber" suggests that only the working state of the marine instruments is tested inside the chamber, and not through the simulation chamber itself.

[0003] Currently, there is a lack of in-situ characterization technology for seawater land simulation devices. The main in-situ characterization method is to use high-speed cameras to capture the surface processes of the samples under test, but it is not possible to measure and obtain the internal material information of the samples during the evaluation process in the seawater land simulation device. Therefore, it is urgent to develop in-situ characterization technology suitable for seawater land simulation devices to provide more in-situ characterization data for the upgrading and improvement of seawater equipment.

[0004] Laser ultrasound, a method of ultrasonic testing, combines the advantages of both lasers and ultrasound. It is a non-contact, high-resolution, and non-destructive in-situ characterization technique. Laser ultrasound generates ultrasonic waves through the thermoelastic effect when an excitation laser beam is irradiated onto the surface of the sample. A probe laser beam, after irradiating the sample surface, enters an interferometer to obtain surface wave displacement. By analyzing the detected surface wave displacement, information about the material's internal structure, such as residual stress and defects, can be obtained. Currently, laser ultrasound is used in high-precision industrial applications such as steel pipe production lines and aircraft composite material inspection. There are no publicly reported applications of laser ultrasound testing systems suitable for seawater and ground simulation devices. For example, Chinese invention application CN 111257236 A discloses a dual-pulse laser ultrasonic testing device and its testing method. The device comprises a pulsed laser emitting device and a continuous laser emitting device respectively arranged on the front and rear sides of a sample. The pulsed laser emitting device has a first beam splitter and a reflector directly in front of it to split the pulsed laser beam into two beams perpendicular to the front surface of the sample. A lens is provided between the first beam splitter and the sample surface. A photorefractive two-wavelength mixing interferometer and a photodetector are also provided on one side of the rear surface of the sample. The continuous laser emitting device has a second beam splitter in front of it, which splits the continuous laser beam into two beams. One beam is angled towards the rear surface of the sample and then reflected into the photorefractive two-wavelength mixing interferometer, while the other beam is directly directed towards the photorefractive two-wavelength mixing interferometer. This invention has the advantages of reducing damage and improving measurement accuracy.

[0005] However, when applied to simulate high-pressure seawater environments, existing laser ultrasonic systems have the following limitations: low signal-to-noise ratio, severe laser energy attenuation in underwater environments leading to low ultrasonic excitation efficiency; and limited scanning accuracy, with multi-layered media (optical window-water) causing optical path offset and affecting detection and positioning accuracy. Therefore, there is a need to develop new devices and methods for defect measurement using laser ultrasound suitable for seawater surface simulation devices. Summary of the Invention

[0006] The purpose of this invention is to provide a laser ultrasonic defect measurement device suitable for high-pressure seawater environments, so as to achieve high-precision, non-destructive testing of internal defects in materials under high-pressure seawater environments, and improve environmental adaptability, signal-to-noise ratio and scanning accuracy.

[0007] Another objective of this invention is to provide a laser ultrasonic defect measurement method suitable for high-pressure seawater environments.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by this invention is: a laser ultrasonic defect measurement device suitable for high-pressure seawater environments, comprising a high-pressure seawater environment pressure tank and a laser ultrasonic defect detection module. Multiple optical windows are provided on the outer wall of the high-pressure seawater environment pressure tank. The laser ultrasonic defect detection module is mounted on a scanning module, which has six degrees of freedom of motion. The scanning module controls the irradiation position and direction of the two laser beams emitted by the laser ultrasonic defect detection module, so that the laser beams are perpendicularly incident on the optical windows and irradiate the surface of the sample to be tested.

[0009] In the above technical solution, by providing multiple optical windows on the outer chamber wall of the pressure tank, a laser ultrasonic defect measurement module can be installed outside the pressure tank in a high-pressure seawater environment to achieve in-situ characterization of the material to be tested inside the pressure tank. Installing the laser ultrasonic defect detection module on a scanning module with six-axis motion degrees of freedom allows for perpendicular incidence of the laser through the optical windows, avoiding optical positioning problems caused by optical propagation through multiple media, while also achieving a large field of view scanning.

[0010] In a preferred embodiment, the outer chamber of the pressure vessel is cylindrical, and multiple optical windows on the outer chamber wall are arranged in a ring around the outer chamber wall. The geometric center of each optical window is located in the same plane perpendicular to the central axis of the cylinder, and the optical windows are made of sapphire material.

[0011] The use of sapphire material balances light transmittance and pressure resistance. Sapphire can withstand the high pressure of seawater environments and has ideal transmittance in this system's wavelength band, ensuring that the laser intensity attenuation rate remains within an acceptable range. The laser ultrasonic module emits two laser beams, which, after passing through the optical window, illuminate the surface of the sample under test, exciting ultrasonic waves and detecting surface wave signals. The laser ultrasonic testing system is integrated into the scanning module. The scanning system achieves a large field of view scanning through six-axis displacement, while maintaining the laser perpendicular to the surface of the annular optical window, eliminating the influence of the multi-layered medium (optical window-water) on the optical propagation path.

[0012] In a preferred embodiment, there are 12 to 18 optical windows, wherein the optical windows are circular windows with a diameter of not less than 50 mm.

[0013] A further technical solution includes a laser ultrasonic defect detection module comprising an Nd:YAG Q-switched laser, a polarization beam splitter, a quarter-wave plate, a spatial light modulator, a first electrically driven focusing lens, a dichroic mirror, a second electrically driven focusing lens, and a two-wave mixing interferometer. The Nd:YAG Q-switched laser emits a linear pulsed laser that illuminates the polarization beam splitter. The s-polarized pulsed laser separated by the polarization beam splitter passes through the quarter-wave plate and enters the spatial light modulator. The spatial light modulator modulates the Gaussian intensity pulsed laser into a grating intensity pulsed laser, reflects it, and passes it again through the quarter-wave plate, converting it into a p-polarized pulsed laser. This pulse is then focused by the first electrically driven focusing lens, passes through the optical window of the high-pressure environment tank, and illuminates the surface of the sample to be tested, exciting ultrasonic waves. Simultaneously, the two-wave mixing interferometer emits a continuous laser, which is focused by the second electrically driven focusing lens, passes through the optical window of the high-pressure environment tank, illuminates the surface of the sample, and is reflected, carrying surface ultrasonic displacement information. This reflected light passes through the dichroic mirror and the second electrically driven focusing lens and re-enters the two-wave mixing interferometer to obtain surface wave displacement information.

[0014] In a preferred embodiment, the scanning module includes a six-axis robotic arm and a control unit. The six-axis robotic arm has a robotic arm fixture that cooperates with the laser ultrasonic defect detection module. The laser ultrasonic defect detection module is encapsulated on the robotic arm fixture. The control unit controls the movement of the robotic arm to adjust the relative position of the laser ultrasonic defect detection module and the optical window, and adjusts the laser incident angle so that the laser beam remains perpendicular to the optical window and irradiates the sample surface.

[0015] To achieve another objective of this invention, a laser ultrasonic defect measurement method suitable for high-pressure seawater environments is provided. This method utilizes the aforementioned laser ultrasonic defect measurement device suitable for high-pressure seawater environments and includes the following steps: (1) Place the sample to be tested inside the pressure tank of the high-pressure environment pressure tank of seawater, seal the pressure tank, apply a predetermined pressure intensity and keep the pressure stable through the pressurization system and hydraulic control system in the high-pressure environment pressure tank of seawater, and simulate the environmental pressure of the sample to be tested at the expected seawater depth. (2) The control unit of the scanning module controls the laser ultrasonic defect detection module to move to the optical window of the pressure tank, and controls the laser beam of the laser ultrasonic defect detection module to remain perpendicular to the plane of the optical window; (3) A laser ultrasonic defect detection module is used to detect defects in the sample to be tested inside the pressure vessel using the laser ultrasonic detection method; (4) Repeat steps (2) and (3) to change the detection area of ​​the sample to be tested for defect detection until all areas are tested.

[0016] The preferred technical solution, step (3) specifically includes the following steps: (3-1) A linearly polarized pulsed laser emitted from an Nd:YAG Q-switched laser enters a polarization beam splitter with the same polarization direction and is reflected as s-polarized light into a quarter-wave plate. After passing through the quarter-wave plate, the linearly polarized pulsed laser becomes circularly polarized light. (3-2) A circularly polarized pulsed laser is incident on a spatial light modulator. The spatial light modulator is used to adjust the intensity distribution of the pulsed laser into a grid distribution. After being reflected by the modulated circularly polarized pulsed laser, it passes through a quarter-wave plate with the polarization direction in the p direction. After being transmitted through a polarization beam splitter, it passes through a first motorized focusing lens. The focusing position is adjusted by controlling the first motorized focusing lens with a motor, and the grid-shaped pulsed laser is focused onto the surface of the sample to be tested in the pressure vessel. Ultrasonic waves are generated on the surface of the sample to be tested through the thermoelastic effect. (3-3) The dual-wave mixing interferometer generates a continuous laser, which is then irradiated onto the sample surface after passing through the second motorized focusing lens. The laser carries the ultrasonic information at the irradiation position and is reflected back into the dual-wave mixing interferometer. The ultrasonic displacement signal is converted into a change in light intensity signal through the dual-wave mixing principle. The change in light intensity information is processed by computer to obtain the defect information of the surface / inside of the sample under test.

[0017] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: 1. This invention places the sample to be tested in a high-pressure environment pressure tank of seawater, and uses a laser ultrasonic defect detection module in conjunction with a scanning module to detect surface wave signals through a dual-wave mixing interferometer. This enables online defect detection of the sample under high water pressure, and has the advantages of being online, non-destructive, and suitable for large-size samples.

[0018] 2. This invention transforms the laser spot into a grid-like distribution using a spatial light modulator, enhancing the energy efficiency of the excited ultrasonic waves. Each stripe of the grid generates a broadband pulse signal, effectively improving signal interference, and through filtering, generates narrowband excitation, optimizing the frequency of the surface acoustic waves.

[0019] 3. This invention achieves focusing of lasers at different distances through an electric focusing lens, thus maintaining high efficiency in exciting ultrasonic waves.

[0020] 4. By using a six-axis robotic arm to keep the laser perpendicular to the incident light, the problems of light propagation and positioning in multi-layered media are eliminated. At the same time, through the cooperation of the robotic arm and multiple optical windows, a large field of view detection is achieved. It can inspect the samples to be tested in different areas of the seawater pressure chamber one by one, ensuring high-precision detection in each area and realizing laser ultrasonic defect measurement in seawater pressure environment. Attached Figure Description

[0021] Figure 1 This is a structural schematic diagram of an embodiment; Figure 2 This is a schematic diagram of the optical path of the laser ultrasonic defect detection module in the embodiment; Figure 3 This is a schematic diagram of a grid-like excitation pulsed laser.

[0022] The components include: 1. Nd:YAG Q-switched laser; 2. Quarter-wave plate; 3. Spatial light modulator; 4. Polarizing beam splitter; 5. First motorized focusing lens; 6. Dichroic mirror; 7. Two-wave mixer interferometer; 8. Second motorized focusing lens; 9. Reflector; 10. Sample to be tested inside the pressure tank; 11. Six-axis robotic arm; 12. Laser ultrasonic defect detection module; 13. Sapphire optical window; 14. Seawater pressure tank. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments: Example: See Figure 1 As shown, a laser ultrasonic defect measurement device suitable for high-pressure seawater environments includes a six-axis robotic arm 11, a laser ultrasonic defect detection module 12, a sapphire optical window 13, a seawater pressure tank 14, and a sample to be tested 10 located inside a pressure simulation device.

[0024] The seawater pressure tank 14 applies seawater pressure to the sample 10 to simulate the hydrostatic pressure the sample would experience at the expected ocean depth. Only a portion of the pressure tank 14 is shown in the figure. The pressure tank 14 is equipped with a sapphire optical window 13, and the sample 10 is located inside the pressure tank 14.

[0025] In this embodiment, 16 Φ50 mm sapphire optical windows 13 are designed on the outer periphery of the pressure tank 14. The sapphire optical windows can withstand the pressure intensity of the high-pressure environment of seawater and have ideal transmittance in the system band, which can ensure that the laser intensity attenuation rate is within an acceptable range.

[0026] The laser ultrasonic defect detection module 12 uses pulsed excitation laser to generate ultrasonic waves on the surface of the sample to be tested, and uses a dual-wave mixing interferometer to continuously detect the surface wave signal. The surface wave signal is then processed and analyzed by a computer to finally realize the online detection of surface / internal defects of the sample to be tested.

[0027] To efficiently utilize the optical intensity of the laser, it is necessary to select underwater surfaces with good transmission performance in the wavelength range. Pure water has good penetration for the optics in the 400-700 nm wavelength range. In order to better distinguish between pulsed excitation laser and continuous probe laser, in this example, the wavelength of pulsed excitation laser is selected as 532 nm and the wavelength of continuous probe laser is selected as 632 nm to ensure a good signal-to-noise ratio.

[0028] The selection of the pulsed laser is related to the size of the defect to be detected. Generally speaking, the smaller the defect size to be detected, the shorter the pulse width of the laser needs to be. In this example, the minimum defect size to be detected is Φ100 μm. In order to ensure the signal-to-noise ratio, the pulse width of the pulsed excitation laser is selected as 10 ns and the pulse energy is 20 mJ.

[0029] The scanning module controls the movement of the six-axis robotic arm 11 via a control unit, ensuring that the laser beam from the laser ultrasonic defect detection module is perpendicularly incident on the optical window. In this way, the scanning module effectively avoids optical transmission and positioning problems caused by multi-layered media, thereby achieving high-precision scanning defect detection of samples inside the seawater pressure chamber.

[0030] Figure 2 This is a schematic diagram of a laser ultrasonic defect detection module, which includes an Nd:YAG Q-switched laser 1, a polarization beam splitter 4, a quarter-wave plate 2, a spatial light modulator 3, a first motorized focusing lens 5, a dichroic mirror 6, a dual-wave mixing interferometer 7, a second motorized focusing lens 8, a reflector 9, and a sample to be tested 10 inside a pressure vessel.

[0031] The operation steps are as follows: The Nd:YAG Q-switched laser 1 emits a linear pulsed laser beam that illuminates the polarization beam splitter 4. The polarization beam splitter reflects the s-polarized light and transmits the p-polarized light. The resulting s-polarized pulsed laser beam passes through a quarter-wave plate 2 and is converted into circularly polarized light. It then enters the spatial light modulator 3, which modulates the Gaussian intensity pulsed laser beam into a grating intensity pulsed laser beam and reflects it back through the quarter-wave plate 2. At this point, the circularly polarized light is converted into a p-polarized pulsed laser beam. This beam is then focused by the first motorized focusing lens 5 and further passes through the dichroic mirror 6, which transmits through the optical window of the high-pressure environment tank to the surface of the sample 10 to excite ultrasonic waves. Meanwhile, the dual-wave mixing interferometer 7 emits a continuous laser beam, which is focused by the second motorized focusing lens 6, reflected by the mirror 9, passes through the dichroic mirror 6, and then through the optical window of the high-pressure environment tank to illuminate the surface of the sample 10. The reflected beam carries the surface ultrasonic wave displacement information and enters the dual-wave mixing interferometer 7 to obtain surface wave displacement information. The dichroic mirror is used to make the probe beam and the excitation beam coincide, forming a pulse echo condition.

[0032] The specific steps for using it are as follows: Step 1: The Nd:YAG Q-switched laser 1 emits linearly polarized pulsed laser light, which enters the polarization beam splitter 2 with the same polarization direction and is reflected into the quarter-wave plate 3. After passing through the quarter-wave plate 3, the linearly polarized pulsed laser light becomes circularly polarized light. The fast axis of the quarter-wave plate needs to be at 45° with the polarization direction of the polarization beam splitter.

[0033] Step 2: Circularly polarized pulsed laser light is incident on spatial light modulator 4. Since the grid-shaped light spot excites ultrasound more efficiently, the intensity distribution of the pulsed laser is adjusted to a grid distribution using spatial light modulator 4. The modulated circularly polarized pulsed laser light then passes through a quarter-wave plate with the polarization direction p, and continues to propagate along the original optical path at polarization beam splitter 2. After passing through an electrically driven focusing lens, the position of the focusing lens is adjusted by a motor to focus the grid-shaped pulsed laser light onto the surface of the sample to be tested in the seawater pressure simulation chamber. Ultrasonic waves are generated on the sample through the thermoelastic effect. Step 3: Before scanning, a test block of the same material is used to measure and obtain the waveform for adjusting the laser pulse energy and detection scheme; Step 4: A continuous probe laser inside the dual-wave mixing interferometer illuminates the sample surface, carrying surface wave information from the illuminated location. This information is reflected into the interferometer, where the surface wave displacement signal is converted into a light intensity signal change through the dual-wave mixing principle. The light intensity change information is processed by a computer, and a Gaussian filter (center frequency 15MHz, bandwidth 8MHz) is selected to reduce the influence of surface waves and guided waves, extracting the volume wave signal. A confocal imaging algorithm is then used to reconstruct and acquire the defect information on the surface / internal part of the sample under test.

[0034] Figure 3 This diagram illustrates a grid-like excitation pulsed laser. After passing through a spatial light modulator, the pulsed laser transforms from a point source (Gaussian intensity distribution) into a grid pattern. The spacing between each grid fringe is λ. Each fringe generates a broadband pulse signal. Due to interference between signals, the acoustic wave frequencies far from the center frequency fc are dephased and disrupted within a few fringes. In effect, the grating pattern acts as a filter, generating narrowband excitation from a broadband ultrasonic source. Therefore, the resulting surface acoustic wave propagates at frequency fc, and its velocity can be calculated using v = fc × λ. The frequency fc is determined by performing a Fourier transform on the acquired ultrasonic signal and identifying the location of its maximum value in the frequency domain.

[0035] The apparatus of this embodiment is used to rapidly measure internal defects in underwater samples, comprising the following steps: Step 1: The control unit in the scanning module controls the robotic arm to place the laser ultrasonic module in front of the optical window of the pressure tank, keeping the incident direction of the laser vertical to the plane of the optical window. The purpose is to eliminate the optical positioning problem caused by the incident angle in the optical propagation problem of multi-layer media.

[0036] Step 2: A pulsed laser is generated using an Nd:YAG Q-switched laser in the laser ultrasound module. This pulsed laser is then converted into a grid-like spot by a spatial light modulator and focused onto the surface of the sample by an electrically driven focusing lens, exciting surface waves. The purpose is to utilize laser energy more effectively to excite surface acoustic waves with large displacement, thereby improving the signal-to-noise ratio of the detection signal.

[0037] Step 3: After the continuous laser emitted by the dual-wave mixing interferometer in the laser ultrasound module reaches the surface of the sample to be tested, the laser carrying surface wave information is reflected into the dual-wave mixing interferometer. The surface wave displacement information in the optical signal is extracted through the principle of dual-wave mixing. After the data is processed by the computer, the defect information of the test area of ​​the sample is reconstructed using the confocal imaging algorithm.

[0038] Step 4: Change the detection area of ​​the sample to be tested corresponding to the next optical window, and repeat steps 1-3 to realize laser ultrasonic defect measurement under seawater pressure environment.

Claims

1. A laser ultrasonic defect measurement device suitable for high-pressure seawater environments, comprising a high-pressure seawater environment pressure tank and a laser ultrasonic defect detection module, characterized in that: The outer chamber wall of the pressure vessel is provided with multiple optical windows; the laser ultrasonic defect detection module is installed on a scanning module, the scanning module has six-axis motion degrees of freedom, the scanning module controls the irradiation position and direction of the two laser beams emitted by the laser ultrasonic defect detection module, so that the laser beams are perpendicularly incident on the optical windows and irradiate the surface of the sample to be tested.

2. The laser ultrasonic defect measurement device suitable for high-pressure seawater environments according to claim 1, characterized in that: The outer chamber of the pressure vessel is cylindrical in shape, and multiple optical windows on the outer chamber wall are distributed in a ring around the outer chamber wall. The geometric center of each optical window is located in the same plane perpendicular to the central axis of the cylinder, and the optical windows are made of sapphire material.

3. The laser ultrasonic defect measurement device suitable for high-pressure seawater environments according to claim 2, characterized in that: It has 12 to 18 optical windows, and the optical windows are circular windows with a diameter of not less than 50 mm.

4. The laser ultrasonic defect measurement device suitable for high-pressure seawater environments according to claim 1, characterized in that: The laser ultrasonic defect detection module includes an Nd:YAG Q-switched laser, a polarization beam splitter, a quarter-wave plate, a spatial light modulator, a first motorized focusing lens, a dichroic mirror, a second motorized focusing lens, and a two-wave mixing interferometer. The Nd:YAG Q-switched laser emits a linear pulse laser that illuminates the polarization beam splitter. The s-polarized pulse laser separated by the polarization beam splitter passes through the quarter-wave plate and enters the spatial light modulator. The spatial light modulator modulates the Gaussian intensity pulse laser into a grating intensity pulse laser and reflects it, passing it again through the quarter-wave plate to convert it into a p-polarized pulse laser. This pulse laser is then focused by the first motorized focusing lens, passes through the optical window of the high-pressure environment tank, and illuminates the surface of the sample to be tested, exciting ultrasonic waves. Simultaneously, the two-wave mixing interferometer emits a continuous laser, which is focused by the second motorized focusing lens, passes through the optical window of the high-pressure environment tank, illuminates the surface of the sample inside the tank, and is reflected, carrying surface ultrasonic displacement information. This reflected light passes through the dichroic mirror and the second motorized focusing lens and re-enters the two-wave mixing interferometer to obtain surface wave displacement information.

5. The laser ultrasonic defect measurement device suitable for high-pressure seawater environments according to claim 1, characterized in that: The scanning module includes a six-axis robotic arm and a control unit. The six-axis robotic arm has a robotic arm fixture that cooperates with the laser ultrasonic defect detection module. The laser ultrasonic defect detection module is encapsulated on the robotic arm fixture. The control unit controls the movement of the robotic arm, adjusts the relative position of the laser ultrasonic defect detection module and the optical window, and adjusts the laser incident angle so that the laser beam is perpendicular to the optical window and irradiates the sample surface.

6. A laser ultrasonic defect measurement method suitable for high-pressure seawater environments, implemented using the laser ultrasonic defect measurement device suitable for high-pressure seawater environments as described in any one of claims 1 to 5, comprising the following steps: (1) Place the sample to be tested inside the pressure tank of the high-pressure environment pressure tank of seawater, seal the pressure tank, apply a predetermined pressure intensity and keep the pressure stable through the pressurization system and hydraulic control system in the high-pressure environment pressure tank of seawater, and simulate the environmental pressure of the sample to be tested at the expected seawater depth. (2) The control unit of the scanning module controls the laser ultrasonic defect detection module to move to the optical window of the pressure tank, and controls the laser beam of the laser ultrasonic defect detection module to remain perpendicular to the plane of the optical window; (3) A laser ultrasonic defect detection module is used to detect defects in the sample to be tested inside the pressure vessel using the laser ultrasonic detection method; (4) Repeat steps (2) and (3) to change the detection area of ​​the sample to be tested for defect detection until all areas are tested.

7. The laser ultrasonic defect measurement method suitable for high-pressure seawater environments according to claim 6, characterized in that, Step (3) specifically includes the following steps: (3-1) A linearly polarized pulsed laser emitted from an Nd:YAG Q-switched laser enters a polarization beam splitter with the same polarization direction and is reflected as s-polarized light into a quarter-wave plate. After passing through the quarter-wave plate, the linearly polarized pulsed laser becomes circularly polarized light. (3-2) A circularly polarized pulsed laser is incident on a spatial light modulator. The spatial light modulator is used to adjust the intensity distribution of the pulsed laser into a grid distribution. After being reflected by the modulated circularly polarized pulsed laser, it passes through a quarter-wave plate with the polarization direction in the p direction. After being transmitted through a polarization beam splitter, it passes through a first motorized focusing lens. The focusing position is adjusted by controlling the first motorized focusing lens with a motor, and the grid-shaped pulsed laser is focused onto the surface of the sample to be tested in the pressure vessel. Ultrasonic waves are generated on the surface of the sample to be tested through the thermoelastic effect. (3-3) The dual-wave mixing interferometer generates a continuous laser, which is then irradiated onto the sample surface after passing through the second motorized focusing lens. The laser carries the ultrasonic information at the irradiation position and is reflected back into the dual-wave mixing interferometer. The ultrasonic displacement signal is converted into a change in light intensity signal through the dual-wave mixing principle. The change in light intensity information is processed by computer to obtain the defect information of the surface / inside of the sample under test.

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