Non-contact laser testing method for acoustic emission of ceramic matrix composite material in extreme thermal environment

Abstract

The invention provides a non-contact laser testing method for acoustic emission of a ceramic-based composite material in an extreme thermal environment, and relates to the technical field of nondestructive testing, the non-contact laser testing method comprises the following steps: S1, preparing a glass bead composite coating, and coating the surface of a composite material test piece with the glass bead composite coating; s2, building a non-contact monitoring system; s3, carrying out a thermal coupling experiment and signal acquisition; and S4, performing multi-stage noise reduction processing on the non-contact signal. According to the method, the surface of the composite material test piece is coated with the quartz glass beads, and the laser reflectivity of the surface of the ceramic-based composite material can be improved to 60% or above from 10% or below in an environment of 1500 DEG C by utilizing the directional regression reflection characteristic of the quartz glass beads, so that the amplitude of a laser vibration measurement signal is remarkably enhanced. And meanwhile, aiming at the characteristics that the noise of the laser vibration meter is large and the noise is nonlinearly increased along with the frequency and the like, the signal noise level can be effectively reduced and the characterization capability of the acoustic emission waveform can be improved through median filtering, least square smoothing filtering, self-correlation filtering and other methods.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing, specifically to a non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments. Background Technology

[0002] Acoustic emission monitoring (AE) technology, as a dynamic non-destructive testing method, enables real-time damage tracking by capturing elastic wave signals generated by the propagation of material damage. However, high-temperature environments pose significant challenges to traditional contact-based AE monitoring: on the one hand, contact sensors are limited by their high-temperature resistance, and are prone to sensitivity decay and hardware failure at temperatures above 1000℃. Furthermore, the coupling agent between the sensor and the specimen will fail at high temperatures, leading to signal transmission interruption. On the other hand, the strong thermal noise generated by material thermal vibration and electromagnetic radiation from induction heating devices in high-temperature environments can severely mask the true damage signal, reducing the signal-to-noise ratio.

[0003] To address the high-temperature adaptability issues of contact monitoring, non-contact laser vibration measurement technology has emerged as an important alternative. Based on the Doppler effect, it captures minute vibrations on the surface of the specimen (corresponding to the vibration response of acoustic emission signals) using a laser beam, eliminating the need for direct contact with the specimen and effectively avoiding damage to the sensor from high temperatures. However, this technology faces a core bottleneck in monitoring high-temperature composite materials—insufficient laser reflectivity on the specimen surface. At high temperatures, composite material surfaces are prone to forming oxide layers. For example, C / SiC composite materials form a SiO2 oxide layer in air at 1500℃, and thermal deformation exacerbates surface roughness, leading to significant absorption or scattering of the laser beam and a substantial reduction in reflection efficiency. The surface reflectivity of C / SiC composite materials is approximately 35% at room temperature, but drops to below 10% at 1500℃, resulting in weak laser vibration measurement signal amplitude or even signal loss, failing to meet the sensitivity requirements for damage monitoring.

[0004] Meanwhile, even with non-contact laser vibration measurement signals, noise interference in high-temperature environments remains difficult to eliminate. This noise primarily includes: random thermal noise generated by the thermal vibration of material molecules (frequency 10-100kHz), power frequency electromagnetic interference from the induction heating device (50-60Hz), and pulsed mechanical noise generated by the friction of the tensile testing machine's fixtures (1-10kHz). Without effective noise reduction, this noise will superimpose with the actual damage signal, leading to distortion in subsequent signal analysis and making it impossible to accurately extract damage-related information.

[0005] In existing technologies, solutions for improving the reflectivity of laser vibration signals generally involve surface polishing and metal coating. However, surface polishing cannot resist high-temperature oxidation, and the reflectivity will decrease again in a short period of time. Metal coating is prone to chemical reaction with the composite matrix at high temperatures, which will change the surface mechanical properties of the material and affect the tensile test results. In addition, noise reduction methods for high-temperature non-contact signals are mostly single algorithms, which are difficult to suppress multiple types of noise at the same time, resulting in limited noise reduction effects. Summary of the Invention

[0006] To address the shortcomings of the prior art, the present invention aims to provide a non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments. This method can stably improve laser reflectivity at high temperatures and efficiently eliminate non-contact signal noise, thus solving the problems of insufficient laser reflectivity on the test specimen surface and the difficulty in eliminating noise interference in existing non-contact laser testing methods.

[0007] Specifically, on the one hand, the present invention provides a non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments, comprising the following steps: S1: Prepare a glass microsphere composite coating and apply it to the surface of the composite material specimen to form a stable high-reflectivity layer; S2: Set up a non-contact monitoring system, install the specimen into the tensile testing machine, adjust the laser vibration meter to align with the coating area, and set up the induction heating device and signal acquisition system; S3: Perform thermo-coupling experiments and signal acquisition, which includes the following sub-steps: S31: High-temperature heating and holding: The temperature of the specimen is raised to the first preset temperature at a constant heating rate and held at that temperature; S32: Apply tensile load: Under the heat preservation condition, apply axial tensile load to the specimen at a constant tensile speed, and monitor the load-displacement curve in real time during the tensile process. Determine the damage stage of the specimen based on the curve characteristics. Simultaneously, during the application of tensile load, laser vibration measurement signals, tensile load signals, and specimen temperature signals are acquired. S4: Perform multi-stage noise reduction processing on non-contact signals, which includes the following sub-steps: S41: Level 1 Noise Reduction: Median Filtering to Suppress Pulse Mechanical Noise: A median filter is used to process the acquired raw laser vibration signal, and the filtered signal... The calculation formula is: ; In the formula: The original laser vibration measurement signal is represented by n; n is the sampling point number. This is for median operations; S42: Second-stage noise reduction: Least squares smoothing filter to suppress power frequency electromagnetic interference: Least squares smoothing filter is used to smooth the signal after the first-stage noise reduction. The signal is processed and filtered. The calculation formula is: ; In the formula: m is the half width of the smooth window; The time for the (n+j)th sampling point; t is time (s); coefficients Fitting within the window [nm, n+m] using the least squares method get; S43: Three-stage noise reduction: Autocorrelation filtering suppresses random thermal noise: Autocorrelation filtering is used to suppress random thermal noise in the signal after two-stage noise reduction. After processing, a clean acoustic emission signal is obtained: ; In the formula: For delayed sampling points; For signal The autocorrelation function, This represents a mapping operator from the autocorrelation function to the reconstructed time-domain signal.

[0008] Further: Step S1 includes the following sub-steps: S11: Coating material selection: Quartz glass microspheres and high-temperature inorganic adhesives are selected as the core materials for the coating; S12: Composite coating preparation: According to the mass ratio of quartz glass microspheres to high-temperature inorganic adhesive of 3:1, quartz glass microspheres are slowly added to high-temperature inorganic adhesive, while high-speed disperser is used to stir to ensure that the quartz glass microspheres are uniformly dispersed in the adhesive to form a composite coating slurry with no agglomeration and good fluidity. S13: Coating and Curing: The composite coating slurry obtained in step S12 is uniformly coated onto the surface of the specimen using a spraying method, ensuring that the coating area covers the acoustic emission signal monitoring area. After coating, the specimen is placed in an oven and cured according to the curing regime of 80℃ / 2h→200℃ / 1h→500℃ / 1h to form a high-reflectivity composite coating that is firmly bonded to the surface of the specimen and is stable at high temperature.

[0009] Further: In step S11, the diameter of the quartz glass microspheres is 50-100 μm.

[0010] Further: In step S13, the coating thickness of the coated area is 100-150 μm.

[0011] Further: Step S2 includes the following sub-steps: S21: Specimen Installation and Positioning: Install the ceramic matrix composite specimen coated with glass microsphere composite coating onto the fixture of the thermocoupled tensile testing machine, ensuring that the specimen axis is consistent with the tensile direction; adjust the specimen position so that the monitoring area of ​​the coating is directly in front of the transparent observation window of the tensile testing machine; S22: Laser Vibration Meter Setup: Fix the laser vibration meter on the optical platform, adjust the angle and distance of the laser vibration meter so that the laser beam is perpendicularly incident on the composite coating area on the surface of the specimen; the distance d between the laser vibration meter and the specimen is set to 1-1.5m; S23: Arrangement of induction heating device: The induction heating coil is wrapped around the tensile section of the specimen, and the temperature of the specimen is collected in real time by thermocouples arranged on the surface of the specimen to ensure that the temperature of the specimen is stable at the second preset temperature during the test. S24: Signal acquisition system connection: Connect the signal output terminal of the laser vibrometer to the computer, and connect the load sensor and displacement sensor signals of the tensile testing machine to the data acquisition system to realize the synchronous acquisition of temperature load, displacement and acoustic emission signals.

[0012] Further: In step S22, when the laser is incident on the composite coating area on the surface of the specimen, the diameter of the laser spot is 2-3 mm.

[0013] Further: In step S23, the second preset temperature is 500-1500℃.

[0014] Further: In step S32, the method for acquiring the acoustic emission signal is as follows: The offset of the reflected laser frequency caused by damage to the specimen under tensile load is obtained. : ; In the formula: v is the vibration velocity of the specimen surface; λ is the laser wavelength; Laser vibrometers detect frequency shifts. By deducing the surface vibration velocity v of the specimen, the time-domain waveform of the acoustic emission signal is obtained, thus realizing non-contact acoustic emission signal acquisition.

[0015] Further: In step S41, the sampling window of the median filter has 5 sampling points.

[0016] Furthermore, the specimen was stretched using a thermo-coupled tensile testing machine at a stretching speed of 2 mm / min.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention coats the surface of a composite material specimen with quartz glass microspheres. Utilizing the directional retroreflection characteristics of the quartz glass microspheres, when a laser beam is incident on the surface of the quartz glass microspheres, the light is first refracted through the outer surface of the microspheres and enters the interior of the microspheres. Multiple reflections occur on the inner surface of the microspheres, and finally the light is refracted out from the outer surface of the microspheres. The outgoing light is parallel and opposite to the incident light. This structure can increase the laser reflectivity of the surface of the ceramic matrix composite material from below 10% to above 60% at 1500℃, significantly enhancing the amplitude of the laser vibration measurement signal.

[0018] 2. This invention addresses the characteristics of laser vibrometers, such as high noise and nonlinear increase in noise with frequency. By employing methods such as median filtering, least squares smoothing filtering, and autocorrelation filtering, the signal noise level can be effectively reduced, thereby improving the characterization capability of acoustic emission waveforms.

[0019] 3. This invention uses non-contact Doppler laser vibration meter signals as the basis for analysis. Doppler vibration meter testing is not only simple and accurate, but also more applicable to structural health monitoring under extreme high temperature environments.

[0020] 4. Based on the testing principle of laser vibration meters, this invention has developed a method for enhancing the acquisition of vibration meter signals under high temperature conditions and a method for enhancing signals under structural surface treatment. This method achieves high-quality acquisition of acoustic emission signals under high temperature conditions. Compared with traditional acoustic emission signals, this method has better testing capabilities for key acoustic emission signals.

[0021] 5. This invention provides a non-contact testing and diagnostic method for ceramic matrix composites under high-temperature conditions, forming a relatively complete sensing, testing and experimental scheme that can be applied in the aerospace field, providing theoretical support for non-contact measurement of composite materials. Attached Figure Description

[0022] Figure 1 This is a physical diagram of the experimental apparatus of the present invention; Figure 2 This is a schematic diagram of the experimental system of the present invention; Figure 3 This is a schematic diagram illustrating the principle of retroreflection of laser light through quartz glass microspheres in this invention. Figure 4 This is a test signal diagram of the in-situ measurement laser vibration meter of the present invention; Figure 5 This is a test signal diagram of a laser vibrometer after coating quartz glass microspheres according to the present invention; Figure 6 This is the method for comparing the original signal with the denoised signal in this invention. Detailed Implementation

[0023] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0024] Example 1: The entire testing system includes a laser Doppler vibration meter, an induction heating device, a tensile testing machine, the composite material beam / plate structure to be tested, an acoustic emission sensor probe, an acoustic emission signal acquisition device, and a computer, such as... Figure 1 and Figure 2 As shown.

[0025] Experimental tensile testing machine and sample: A thermocoupled tensile testing machine was used. The machine stretched the graphite sample at a set speed of 2 mm / min. Simultaneously, an induction heating system was applied to the center of the graphite sample to heat the stretched area at a stable temperature of 1500℃. A waveguide rod was used to connect to an acoustic emission (AE) detection system to collect AE signals. A mixture of green high-temperature adhesive and glass microspheres was applied to the laser vibrometer test location for observation. Signals were collected in real time during the thermocouple loading process.

[0026] Specifically, the testing steps are as follows: S1: Preparation and application of glass microsphere composite coating: S11: Coating Raw Material Selection: Quartz glass microspheres and high-temperature inorganic adhesives are selected as the core raw materials for the coating. The quartz glass microspheres have a diameter controlled between 50-100 μm, are composed of SiO2 with a purity ≥99%, and have a melting point as high as 1670℃. They can maintain a stable spherical structure and optical properties even at extreme high temperatures of 1500℃, avoiding damage to reflective properties due to high-temperature deformation. The high-temperature inorganic adhesive is an aluminosilicate-based adhesive with a room temperature shear strength ≥5MPa and a shear strength ≥2MPa at 1500℃. It also exhibits good interfacial compatibility with both the ceramic matrix composite matrix and the quartz glass microspheres, eliminating the risk of high-temperature chemical reactions.

[0027] S12: Composite coating preparation: According to the mass ratio of quartz glass microspheres to high-temperature inorganic adhesive of 3:1, slowly add quartz glass microspheres to high-temperature inorganic adhesive, and at the same time use a high-speed disperser with a speed of 2000r / min to stir for 30min to ensure that the quartz glass microspheres are uniformly dispersed in the adhesive to form a composite coating slurry with no agglomeration and good fluidity.

[0028] S13: Coating Application and Curing: The composite coating slurry is uniformly applied to the surface of the ceramic matrix composite specimen using a spraying method. The coating area must cover the acoustic emission signal monitoring area, with an area of ​​not less than 50mm × 50mm. The coating thickness is controlled at 100-150μm, matching the diameter of the glass microspheres to ensure that the microspheres are partially exposed on the coating surface, thus maximizing retroreflection. After coating, the specimen is placed in an oven and cured according to the following curing regime: 80℃ / 2h (low temperature pre-curing) → 200℃ / 1h (medium temperature curing) → 500℃ / 1h (high temperature curing) to form a high-reflectivity composite coating that is firmly bonded to the specimen surface and stable at high temperatures. The core principle of this step is the directional retroreflection characteristics of the glass microspheres: such as... Figure 3 As shown, when a laser beam is incident on the surface of quartz glass microspheres, the light is first refracted by the outer surface of the microspheres and then enters the interior of the microspheres. The angle of refraction is... Satisfy the law of refraction ,in The refractive index of air, The refractive index is that of quartz glass. Multiple reflections occur on the inner surface of the microspheres, which are then refracted out from the outer surface, with the outgoing light rays parallel and opposite to the incident light rays. According to optical simulation calculations, this structure can increase the laser reflectivity from less than 10% on the surface of the ceramic matrix composite material at 1500℃ to over 60%, significantly enhancing the amplitude of the laser vibration measurement signal. The reflectivity R increases by 6 times, and the received signal amplitude A increases by a simultaneous 6 times.

[0029] S2: Construction of a non-contact monitoring system: S21: Specimen Installation and Positioning: Install the ceramic matrix composite specimen coated with glass microsphere composite coating onto the fixture of the thermocoupled tensile testing machine, ensuring that the specimen axis is consistent with the tensile direction to avoid eccentric loading during the tensile process; adjust the specimen position so that the monitoring area of ​​the coating is directly in front of the transparent observation window of the tensile testing machine to facilitate laser beam incidence.

[0030] S22: Laser Vibration Meter Setup: Select a laser Doppler vibration meter with a resolution ≤0.1nm and a sampling frequency ≥2MHz, and fix it on the optical platform. Adjust the angle and distance of the laser vibration meter so that the laser beam is perpendicularly incident on the composite coating area on the surface of the specimen. Ensure that the incident angle deviation is ≤5° and the laser spot diameter is controlled at 2-3mm to ensure that the spot is smaller than the area of ​​the coating monitoring area and that the spot falls completely on the coating. The distance d between the laser vibration meter and the specimen is set to 1-1.5m. According to the laser vibration signal intensity formula (1) shown below, the decrease in reflectivity will directly lead to a significant attenuation of the received signal amplitude, or even signal loss, which cannot meet the sensitivity requirements of damage monitoring. (1); In the formula: A is the amplitude of the laser vibration received signal; k is a system constant, which is related to the model of the laser vibration meter and the optical path loss; R is the laser emission power; R is the surface reflectivity of the specimen; d is the distance between the laser vibrometer and the specimen.

[0031] At this distance, the signal attenuation is within an acceptable range, while avoiding the impact of high temperatures on the instrument caused by excessively close proximity.

[0032] S23: Arrangement of induction heating device: The induction heating coil is wrapped around the tensile section of the specimen, with a distance of ≥10mm from the coating monitoring area to avoid the coil blocking the laser beam path. The temperature control accuracy of the induction heating device is set to ±5℃. The temperature of the specimen is collected in real time by thermocouples placed on the surface of the specimen and 5mm away from the coating area to form a closed-loop temperature control, ensuring that the temperature of the specimen remains stable at the second preset temperature, i.e., 500-1500℃, during the test.

[0033] S24: Signal acquisition system connection: Connect the signal output terminal of the laser vibrometer to a data acquisition card with a sampling rate ≥2MHz and a resolution ≥16bit. The data acquisition card communicates with the computer via a USB 3.0 interface. At the same time, connect the load sensor and displacement sensor signals of the tensile testing machine to the data acquisition system to achieve synchronous acquisition of "temperature-load-displacement-acoustic emission signals". The trigger threshold of the acquisition system is determined according to the pre-experiment to ensure effective capture of damage signals and avoid false triggering. In this embodiment, it is set to 0.1V.

[0034] S3: Thermocoupling Experiment and Signal Acquisition: S31: High-temperature heating and holding: Start the induction heating device and raise the temperature of the specimen to the first preset temperature, such as 800℃, 1200℃, or 1500℃, at a heating rate of 5℃ / min. After reaching the first preset temperature, hold for 30 minutes to ensure uniform internal temperature of the specimen and eliminate the influence of temperature gradient on the mechanical properties and acoustic emission signal of the material.

[0035] S32: Tensile load application: Under the heat preservation condition, start the tensile testing machine and apply an axial tensile load to the specimen at a tensile speed of 2-5 mm / min. During the tensile process, monitor the load-displacement curve in real time and determine the damage stage of the material based on the curve characteristics. The damage stages specifically include elastic deformation, plastic deformation, crack initiation, crack propagation, and fracture.

[0036] S33: Multi-parameter synchronous acquisition: Throughout the tensile test, the laser vibration signal (sampling frequency 2MHz), tensile load signal (sampling frequency 100Hz), specimen temperature signal (sampling frequency 10Hz), and displacement signal (sampling frequency 100Hz) are synchronously acquired through the data acquisition system. The acquired data is stored in the computer in binary format to ensure the integrity and traceability of the data.

[0037] In this step, the principle of acoustic emission signal generation and acquisition is as follows: When the ceramic matrix composite material is damaged under tensile load, such as fiber breakage, matrix cracking, or interface debonding, it releases elastic wave energy. The elastic wave propagates to the surface of the specimen, causing minute surface vibrations. The laser beam emitted by the laser vibrometer irradiates the vibrating surface. According to the Doppler effect, the frequency of the reflected laser will shift, and the amount of shift is... Satisfying the formula: (2) In the formula: v is the surface vibration velocity of the specimen (m / s); λ is the laser wavelength, typically 632.8 nm. The laser vibrometer detects the frequency shift. This allows for the deduction of the surface vibration velocity *v* of the specimen, thus obtaining the time-domain waveform of the acoustic emission signal and achieving non-contact acoustic emission signal acquisition. For example... Figure 4 , Figure 5 The image shows the signal collected by a traditional acoustic emission probe and the signal measured by a non-contact laser vibrometer. It can be seen that under high temperature conditions, the non-contact probe can collect a relatively obvious acoustic emission signal.

[0038] S4: Multi-stage noise reduction processing for non-contact signals: S41: Level 1 noise reduction: Median filtering suppresses pulse mechanical noise. A median filter with a window size of 5 is used to filter the acquired raw laser vibration signal. The processing involves 5 sampling points. The core principle of median filtering is to sort all data within the signal sliding window by size and take the median value as the output value at the center of the window. This method can effectively suppress the pulse mechanical noise generated by the friction of the tensile testing machine fixtures. The pulse width is typically 1-5 sampling points, and it does not cause smoothing distortion to the sharp peaks of the acoustic emission signal. The filtered signal... The calculation is shown in the following formula: (3) In the formula: n is the sampling point number; This is for median operations.

[0039] S42: Second-stage noise reduction: Least squares smoothing filter suppresses power frequency electromagnetic interference. For the 50-60Hz power frequency electromagnetic interference from the induction heating device, which exhibits periodic sinusoidal fluctuations, a third-order least squares smoothing filter is used to smooth the signal after the first-stage noise reduction. The method involves constructing a quadratic polynomial within a sliding window, as shown in formula (4), to minimize the sum of squared residuals between the polynomial fitting value and the signal value within the window, thereby eliminating periodic low-frequency interference and improving the filtered signal. The calculation is shown in formula (5): (4) In the formula: y is the fitted signal value; t is time (s); represents the fitting coefficient.

[0040] (5) In the formula: m is the half width of the smooth window, and in this example, m=10; The time for the (n+j)th sampling point; coefficients a0, a1, a2 are fitted within the window [nm, n+m] using the least squares method. get.

[0041] S43: Three-stage noise reduction: Autocorrelation filtering suppresses random thermal noise. For high-frequency random thermal noise (10-100kHz) generated by molecular thermal vibrations, autocorrelation filtering is used to suppress random thermal noise in the signal after the second-stage noise reduction. The process involves processing the signal. The core principle of autocorrelation filtering is to utilize the periodicity of acoustic emission signals, such as the continuous acoustic emission signal generated by crack propagation, and the weak autocorrelation of random thermal noise. This is achieved by calculating the autocorrelation function of the signal. As shown in formula (6), the signal component corresponding to the peak value in the autocorrelation function is extracted, random noise is suppressed, and finally a pure acoustic emission signal is obtained. : (6) (7) In the formula: The delay time (sampling points) is N; N is the signal length. For signal The autocorrelation function, This represents a mapping operator from the autocorrelation function to the reconstructed time-domain signal. A concrete implementation of this operator could be used for identification... The significant peak value is obtained and a matched filter is constructed, or the autocorrelation function is converted into a power spectrum through the Wiener-Khinchin theorem, filtered in the frequency domain, and then inversely transformed back to the time domain.

[0042] like Figure 6 The image shows a comparison between the original signal and the signal after noise reduction. Through the above three-stage noise reduction process, the signal-to-noise ratio of the laser vibration measurement signal can be improved from the original 5-8dB to 20-25dB, meeting the accuracy requirements of subsequent damage feature extraction, such as peak frequency, signal energy, and rise time.

[0043] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments, characterized in that: It includes the following steps: S1: Prepare a glass microsphere composite coating and apply it to the surface of the composite material specimen to form a stable high-reflectivity layer; S2: Set up a non-contact monitoring system, install the specimen into the tensile testing machine, adjust the laser vibration meter to align with the coating area, and set up the induction heating device and signal acquisition system; S3: Perform thermo-coupling experiments and signal acquisition, which includes the following sub-steps: S31: High-temperature heating and holding: The temperature of the specimen is raised to the first preset temperature at a constant heating rate and held at that temperature; S32: Apply tensile load: Under the heat preservation condition, apply axial tensile load to the specimen at a constant tensile speed, and monitor the load-displacement curve in real time during the tensile process. Determine the damage stage of the specimen based on the curve characteristics. Simultaneously, during the application of tensile load, laser vibration measurement signals, tensile load signals, and specimen temperature signals are acquired. S4: Perform multi-stage noise reduction processing on non-contact signals, which includes the following sub-steps: S41: Level 1 Noise Reduction: Median Filtering to Suppress Pulse Mechanical Noise: A median filter is used to process the acquired raw laser vibration signal, and the filtered signal... The calculation formula is: ； In the formula: The original laser vibration measurement signal is represented by n; n is the sampling point number. This is for median operations; S42: Second-stage noise reduction: Least squares smoothing filter to suppress power frequency electromagnetic interference: Least squares smoothing filter is used to smooth the signal after the first-stage noise reduction. The signal is processed and filtered. The calculation formula is: ； In the formula: m is the half width of the smooth window; The time for the (n+j)th sampling point; t represents time; coefficient Fitting within the window [nm, n+m] using the least squares method get; S43: Three-stage noise reduction: Autocorrelation filtering suppresses random thermal noise: Autocorrelation filtering is used to suppress random thermal noise in the signal after two-stage noise reduction. After processing, a clean acoustic emission signal is obtained: ； In the formula: For delayed sampling points; For signal The autocorrelation function, This represents a mapping operator from the autocorrelation function to the reconstructed time-domain signal.

2. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments according to claim 1, characterized in that: Step S1 includes the following sub-steps: S11: Coating material selection: Quartz glass microspheres and high-temperature inorganic adhesives are selected as the core materials for the coating; S12: Composite coating preparation: According to the mass ratio of quartz glass microspheres to high-temperature inorganic adhesive of 3:1, quartz glass microspheres are slowly added to high-temperature inorganic adhesive, while high-speed disperser is used to stir to ensure that the quartz glass microspheres are uniformly dispersed in the adhesive to form a composite coating slurry with no agglomeration and good fluidity. S13: Coating and Curing: The composite coating slurry obtained in step S12 is uniformly coated onto the surface of the specimen using a spraying method, ensuring that the coating area covers the acoustic emission signal monitoring area. After coating, the specimen is placed in an oven and cured according to the curing regime of 80℃ / 2h→200℃ / 1h→500℃ / 1h to form a high-reflectivity composite coating that is firmly bonded to the surface of the specimen and is stable at high temperature.

3. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments as described in claim 2, characterized in that: In step S11, the diameter of the quartz glass microspheres is 50-100 μm.

4. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments as described in claim 2, characterized in that: In step S13, the coating thickness of the coated area is 100-150 μm.

5. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments as described in claim 1, characterized in that: Step S2 includes the following sub-steps: S21: Specimen Installation and Positioning: Install the ceramic matrix composite specimen coated with glass microsphere composite coating onto the fixture of the thermocoupled tensile testing machine, ensuring that the specimen axis is consistent with the tensile direction; adjust the specimen position so that the monitoring area of ​​the coating is directly in front of the transparent observation window of the tensile testing machine; S22: Laser Vibration Meter Setup: Fix the laser vibration meter on the optical platform, adjust the angle and distance of the laser vibration meter so that the laser beam is perpendicularly incident on the composite coating area on the surface of the specimen; the distance d between the laser vibration meter and the specimen is set to 1-1.5m; S23: Arrangement of induction heating device: The induction heating coil is wrapped around the tensile section of the specimen, and the temperature of the specimen is collected in real time by thermocouples arranged on the surface of the specimen to ensure that the temperature of the specimen is stable at the second preset temperature during the test. S24: Signal acquisition system connection: Connect the signal output terminal of the laser vibrometer to the computer, and connect the load sensor and displacement sensor signals of the tensile testing machine to the data acquisition system to achieve synchronous acquisition of temperature load, displacement and acoustic emission signals.

6. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments as described in claim 5, characterized in that: In step S22, when the laser is incident on the composite coating area on the surface of the specimen, the diameter of the laser spot is 2-3 mm.

7. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments as described in claim 5, characterized in that: In step S23, the second preset temperature is 500-1500℃.

8. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments as described in claim 1, characterized in that: In step S32, the method for acquiring the acoustic emission signal is as follows: The offset of the reflected laser frequency caused by damage to the specimen under tensile load is obtained. : ； In the formula: v is the vibration velocity of the specimen surface; λ is the laser wavelength; Laser vibrometers detect frequency shifts. By deducing the surface vibration velocity v of the specimen, the time-domain waveform of the acoustic emission signal is obtained, thus realizing non-contact acoustic emission signal acquisition.

9. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments as described in claim 1, characterized in that: In step S41, the sampling window of the median filter has 5 sampling points.

10. The non-contact laser testing method for acoustic emission of ceramic matrix composites in extreme thermal environments as described in any one of claims 1-9, characterized in that: The specimens were stretched using a thermo-coupled tensile testing machine at a speed of 2 mm / min.

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