Method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites

Abstract

The application belongs to the technical field of stress detection, and discloses an air coupling ultrasonic stress detection method for carbon fiber reinforced ceramic matrix composite materials, which comprises the following steps: according to an acoustic elasticity principle model and a tensile deformation model, a sound velocity and stress relationship model is established; an air coupling ultrasonic stress detection system is built, and a standard sample with the same material and process as a target component is used to calibrate the stress-sound velocity relationship; the calibrated stress-sound velocity model is applied to online actual component monitoring, so that quantitative evaluation of the internal stress of the carbon fiber reinforced ceramic matrix composite material is realized. The air coupling ultrasonic stress detection method for carbon fiber reinforced ceramic matrix composite materials is used, so that non-contact quantitative evaluation of the internal stress of the ceramic matrix composite material is realized.

Description

Technical Field

[0001] This invention relates to the field of stress detection technology, and in particular to an air-coupled ultrasonic stress detection method for carbon fiber reinforced ceramic matrix composites. Background Technology

[0002] Carbon fiber reinforced ceramic matrix composites (such as C / SiC and C / C) have become core materials for critical high-temperature components in aerospace, defense, and nuclear power equipment due to their high strength, high modulus, high temperature resistance, and excellent thermal shock resistance. During material preparation, such as in the chemical vapor infiltration or reactive melt infiltration stages, and under subsequent complex service environments, factors such as the mismatch in thermal expansion coefficients between the matrix and fibers, interfacial residual stress, and external mechanical loads can lead to residual or service stresses within the material. These stresses are key causes of microcrack initiation and propagation, interfacial debonding, fiber breakage, and even overall structural failure. Therefore, achieving quantitative monitoring of the internal stress of composite materials is of paramount engineering significance for the health assessment, life prediction, and safety assurance of high-end equipment.

[0003] Currently, traditional non-destructive testing (NDT) methods have significant limitations in stress measurement of ceramic matrix composites. While drilling can directly measure residual stress, it is a destructive method and cannot be applied to online monitoring or NDT of critical components. X-ray diffraction (XRD), although capable of accurately measuring stress in crystalline materials, has limited penetration depth, making it difficult to obtain volumetric stress information within the material, and it is poorly applicable to amorphous or composite materials. Acoustic emission technology, while capable of capturing transient signals during loading, is more suitable for dynamic monitoring of damage processes and struggles to achieve quantitative stress assessment under static or quasi-static conditions. Conventional contact ultrasonic testing requires coupling agents such as water or glycerin, which can contaminate porous ceramic matrix composites and potentially alter their hygroscopicity, thermal conductivity, and other physical properties, thus affecting their performance in high-temperature or vacuum environments. Furthermore, contact probes require high surface flatness, making them unsuitable for complex curved surfaces.

[0004] In summary, existing technologies have significant shortcomings in detecting the internal stress of carbon fiber reinforced ceramic matrix composites. There is an urgent need to propose a non-contact, pollution-free air-coupled ultrasonic stress detection method suitable for complex working conditions to meet the pressing needs of high-end fields such as aerospace for health monitoring of critical components. Summary of the Invention

[0005] The purpose of this invention is to provide an air-coupled ultrasonic stress detection method for carbon fiber reinforced ceramic matrix composites, which enables non-contact quantitative assessment of the internal stress of ceramic matrix composites without interfering with the material surface and working conditions. It can capture the changes in the overall elastic properties of the material caused by stress and transform the complex mechanical state into an intuitive acoustic signal for interpretation.

[0006] To achieve the above objectives, the present invention provides a method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites, comprising the following steps: Step S1: Based on the acoustoelasticity principle model and the tensile deformation model, establish a model for the relationship between sound velocity and stress; Step S2: Build an air-coupled ultrasonic stress testing system and use a standard sample that is completely consistent with the material and process of the target component to calibrate the stress-sound velocity relationship. Step S3: Apply the calibrated stress-sound velocity model to online monitoring of actual components to achieve quantitative assessment of internal stress in carbon fiber reinforced ceramic matrix composites.

[0007] Preferably, in step S1, for the carbon fiber reinforced ceramic matrix composite sample, the fixed total spacing between the transmitting and receiving transducers is set to... Total sound It consists of the air segment sound time and the material segment sound time, as shown below: ; in, The speed of sound in ambient air; This refers to the real-time thickness of the material. The speed at which ultrasound propagates in a material is as follows: ; During the tensile process, the Poisson effect leads to the thickness of the material. Due to shrinkage, the thickness changes as shown below according to the tensile deformation model: ; in, This is the initial thickness; Poisson's ratio; Young's modulus; Due to the total spacing Fixed, reduced material thickness This is automatically converted into an equal air path increment; combined with the stretching deformation model, the propagation velocity expression is obtained: .

[0008] Preferably, under the natural state of zero stress and no load, the wave equation inside the material follows the Christoffel equation, as follows: ; in, For the linear elastic stiffness tensor of the material; and The direction cosine of the wavefront normal vector; The density of the material; The initial sound velocity of the material under stress-free conditions; This is the displacement vector of the resonant plane wave; The Kronecker function is shown below: ; When a material is subjected to external loads that induce internal stress, its equivalent stiffness changes, causing... Let be the effective elastic stiffness tensor under stress, where Let be the Cauchy stress tensor under stress. Then, according to the principle of acoustoelasticity, the wave equation evolves as follows: .

[0009] Preferably, since the condition for the wave equation to have a non-zero solution is that the determinant of the coefficient matrix is ​​zero, the first-order solution of the characteristic equation yields: ; For uniaxial load The speed of longitudinal waves propagating perpendicular to the direction of the force. The following linear relationship is satisfied: ; in, The acoustoelastic coefficient of the material; This represents the stress value.

[0010] Preferably, the expression for the change in propagation speed and thickness is substituted into the total acoustic time formula to obtain the total acoustic time. With stress The relational model is shown below: ; Based on the linear relationship between sound speed and stress, combined with the expression for propagation speed The air-coupled ultrasonic stress detection model is calibrated by adjusting the acoustoelastic coefficient. With the initial sound velocity of the material This enables accurate stress inversion.

[0011] Preferably, in step S2, an air-coupled ultrasonic stress detection system is constructed, comprising a mechanical loading unit, a detection unit, and a data acquisition unit; The mechanical loading unit includes: a DDL100 electronic universal testing machine, a controller, and a main control computer; the data acquisition and processing unit is responsible for synchronously acquiring stress data and ultrasonic signals, and extracting the acoustic time corresponding to the ultrasonic wave peak with the highest amplitude through an algorithm, and finally completing the modeling and inversion calculation of the relationship between stress and sound velocity.

[0012] Preferably, the detection unit uses a pair of air-coupled ultrasonic probes arranged opposite each other in a penetration mode, along with a dedicated ultrasonic pulse transmitting and receiving device, and uses a set of multi-degree-of-freedom precision adjustment fixtures to achieve the adjustment of the distance and angle between the air-coupled ultrasonic transducers; The relative positions of the transmitter and receiver of the air-coupled ultrasonic transducer are adjusted and locked using a dovetail groove displacement slide and a height adjustment slide rail, ensuring that the center lines of the sound beams of the two transducers coincide and the total spacing is equal. The angle of incidence of the transducer relative to the sample surface is adjusted by the fixture-integrated angle rotating slide and turntable connector to ensure that the sound wave is incident perpendicularly. The transducer clamp and support base plate are used to ensure the long-term stability of the sensor position under experimental conditions.

[0013] Preferably, a graded tensile load is applied to the carbon fiber reinforced ceramic matrix composite specimen on the mechanical loading unit, and at each stress stabilization state, the known stress value is recorded and the corresponding ultrasonic signal is sampled. Record multiple sets of stress values With the corresponding ultrasonic peak time And in conjunction with the initial thickness of the sample Young's modulus Compared to Poisson Initial sound velocity obtained from data under stress-free conditions The acoustoelastic coefficient of the material was extracted by fitting using the nonlinear least squares method. The final calibrated stress-velocity model is shown below: .

[0014] Preferably, the calibrated stress and sound velocity model is applied to online monitoring of actual components to achieve quantitative assessment of the internal stress of carbon fiber reinforced ceramic matrix composites. The specific process is as follows: The actual thickness of the component at the detection point is measured. Air-coupled ultrasonic transducers are installed on both sides of the component under test in the same configuration, serving as the transmitter and receiver respectively. The component is fixed by a special fixture with angle adjustment function to ensure that the surface of the component is perpendicular to the line connecting the two probes, and the perpendicularity is calibrated. The fixture design should take into account the shape, size and material properties of the component to avoid introducing additional stress. Then, ultrasonic signals are acquired, and the peak sound time is extracted from the signal. The sound velocity is calculated by using the sound time, component thickness and probe spacing. The sound velocity is then input into the established stress and sound velocity model to retrieve the current internal stress value or stress change of the component. Ultimately, a system based on total volume was established. , Sample thickness probe spacing The stress inversion model with independent variables is shown below: .

[0015] Therefore, the present invention employs the above-mentioned air-coupled ultrasonic stress detection method for carbon fiber reinforced ceramic matrix composites, which realizes non-contact quantitative assessment of internal stress in ceramic matrix composites without interfering with the material surface and working conditions. It can capture the changes in the overall elastic properties of the material caused by stress and transform the complex mechanical state into intuitive acoustic signals for interpretation.

[0016] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the air-coupled ultrasonic testing of carbon fiber reinforced ceramic matrix composite samples according to the present invention; Figure 2 This is a schematic diagram of the structure of the air-coupled ultrasonic testing fixture of the present invention; Figure 3 This is a schematic flowchart of the air-coupled ultrasonic stress detection method for carbon fiber reinforced ceramic matrix composites in an embodiment of the present invention; Figure 4 This is a schematic diagram of the ultrasonic signal transmission of the sample under stress and without stress.

[0018] Figure Labels 1. Air-coupled ultrasonic transducer; 2. Transducer clamping component; 3. Turntable connector; 4. Angle rotation slide; 5. Slider; 6. Dovetail groove displacement slide; 7. Height adjustment slide rail; 8. Support base plate; 9. Sample. Detailed Implementation

[0019] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0020] The present invention relates to an air-coupled ultrasonic stress detection method for carbon fiber reinforced ceramic matrix composites. Its core is based on the acoustoelastic effect, which indicates that the propagation speed of ultrasonic waves in a solid medium changes slightly with variations in the internal stress state of the material. For example... Figure 1 As shown, this invention achieves non-contact stress assessment by establishing a linear mapping model between stress and sound velocity. The invention encompasses three core components: theoretical modeling, system calibration, and online measurement.

[0021] Step S1: Based on the acoustoelasticity principle model and the tensile deformation model, establish a model for the relationship between sound velocity and stress.

[0022] Step S11: For the carbon fiber reinforced ceramic matrix composite material sample involved in this invention, the fixed total distance between the transmitting and receiving transducers is set as follows: Total sound It consists of the air segment sound time and the material segment sound time, as shown below: ; in, The speed of sound in ambient air; This refers to the real-time thickness of the material. The speed at which ultrasound propagates in a material is as follows: ; During the tensile process, the Poisson effect leads to the thickness of the material. Due to shrinkage, the thickness changes as shown below according to the tensile deformation model: ; in, This is the initial thickness; Poisson's ratio; Young's modulus; Due to the total spacing Fixed, reduced material thickness This is automatically converted into an equal air path increment; combined with the stretching deformation model, the propagation velocity expression is obtained: .

[0023] Step S12: Under the natural state of zero stress and no load, the wave equation inside the material follows the Christoffel equation, as shown below: ; in, For the linear elastic stiffness tensor of the material; and The direction cosine of the wavefront normal vector; The density of the material; The initial sound velocity of the material under stress-free conditions; This is the displacement vector of the resonant plane wave; The Kronecker function is shown below: .

[0024] When a material is subjected to external loads that induce internal stress, its equivalent stiffness changes, causing... Let be the effective elastic stiffness tensor under stress, where Let be the Cauchy stress tensor under stress. Then, according to the principle of acoustoelasticity, the wave equation evolves as follows: ; The condition for this equation to have a non-zero solution is that the determinant of the coefficient matrix is ​​zero. By solving this characteristic equation using a first-order approximation, we can obtain: ; Because the change in sound velocity caused by stress is much smaller than the initial sound velocity. , can item Approximately simplified to 2 For uniaxial loads The speed of longitudinal waves propagating perpendicular to the direction of the force. The following linear relationship is satisfied: ; in, The acoustoelastic coefficient of the material; This represents the stress value.

[0025] Step S13: Substitute the expressions for propagation speed and thickness change into the total acoustic time formula to obtain the total acoustic time. With stress The relational model is shown below: ; Based on the linear relationship between sound speed and stress, combined with the expression for propagation speed The air-coupled ultrasonic stress detection model is calibrated by adjusting the acoustoelastic coefficient. With the initial sound velocity of the material This enables accurate stress inversion.

[0026] Step S2: Build an air-coupled ultrasonic stress testing system and use a standard sample that is completely consistent with the material and process of the target component to calibrate the stress-sound velocity relationship.

[0027] Step S21: Build an air-coupled ultrasonic stress detection system, including a mechanical loading unit, a detection unit, and a data acquisition unit.

[0028] The mechanical loading unit includes: DDL100 electronic universal testing machine, controller and main control computer.

[0029] The detection unit employs a pair of air-coupled ultrasonic probes arranged opposite each other in a penetration mode, along with dedicated ultrasonic pulse transmitting and receiving equipment. A multi-degree-of-freedom precision adjustment fixture allows for the adjustment of the distance and angle between the air-coupled ultrasonic transducers. Figure 2 As shown.

[0030] A pair of air-coupled ultrasonic transducers 1, serving as the transmitter and receiver respectively, are arranged opposite each other in a penetration mode. A transducer clamp 2 is used to fix the transducers, ensuring mechanical stability during the experiment and preventing positional displacement due to vibration. A turntable connector 3 serves as the connection structure between the transducer clamp and the adjustment mechanism, used to transmit displacement for angle adjustment. An angle rotation slide 4 allows for fine-tuning of the transducer's angle relative to the sample surface 9; this component is used during testing to ensure perpendicular incidence of sound waves, eliminating refraction errors caused by tilt angles. A slider 5 is used for translation and locking of the components. A dovetail groove displacement slide 6 is used for precise horizontal displacement adjustment; the total distance between the transmitting and receiving transducers can be precisely adjusted via the dovetail groove displacement slide 6, ensuring a constant total sound path throughout the loading process. A height adjustment slide rail 7 provides vertical freedom; by translating the horizontal beam up and down on the slide rail, multi-point stress monitoring can be performed at different height positions of the sample 9. A support base plate 8 provides support for the entire fixture.

[0031] The relative positions of the transmitting and receiving ends of the air-coupled ultrasonic transducer 1 are adjusted and locked using the dovetail groove displacement slide 6 and the height adjustment slide rail 7, ensuring that the center lines of the sound beams of the two transducers coincide and the total spacing is equal. The angle of incidence of the transducer relative to the surface of the sample 9 must be kept constant during the detection process. The fixture integrates an angle-rotating slide 4 and a turntable connector 3 to adjust the incident angle of the transducer relative to the sample 9 surface, ensuring that the sound wave is incident perpendicularly. The transducer holder 2 and the support base plate 8 are used to ensure the long-term stability of the sensor position under experimental conditions.

[0032] The data acquisition and processing unit is responsible for synchronously acquiring stress data and ultrasonic signals, and extracting the acoustic time corresponding to the ultrasonic wave peak with the highest amplitude through algorithms, and finally completing the modeling and inversion calculation of the relationship between stress and sound velocity.

[0033] Step S22: Based on the air-coupled ultrasonic stress detection system, standard samples with the same material and process as the target component are used to calibrate the stress-sound velocity relationship.

[0034] On the mechanical loading unit, graded tensile loads were applied to the carbon fiber reinforced ceramic matrix composite specimens, and the known stress values ​​and corresponding ultrasonic times were recorded at each stress stabilization state.

[0035] Record multiple sets of stress values With the corresponding ultrasonic peak time And in conjunction with the initial thickness of the sample Young's modulus Compared to Poisson Initial sound velocity obtained from data under stress-free conditions The acoustoelastic coefficient of the material was extracted by fitting using the nonlinear least squares method. The final calibrated stress-velocity model is shown below: .

[0036] Step S3: Apply the calibrated stress-sound velocity model to online monitoring of actual components to achieve quantitative assessment of internal stress in carbon fiber reinforced ceramic matrix composites.

[0037] The actual initial thickness of the component at the detection point is measured by mounting probes on both sides of the component with the same configuration. The component is fixed with a dedicated clamp, which should have an angle adjustment function to ensure that the surface of the component is perpendicular to the line connecting the two probes, and the perpendicularity is calibrated. The clamp design should consider the shape, size, and material properties of the component to avoid introducing additional stress. Ultrasonic signals are then acquired, and the peak acoustic time is extracted from the signal. The sound velocity is calculated using the acoustic time, component thickness, and probe spacing. The sound velocity is then input into the established stress and sound velocity model to retrieve the current internal stress value or stress change of the component, as shown below: .

[0038] Example This embodiment uses the internal stress detection of a silicon carbide ceramic matrix composite (C / SiC) component as an example to detail the specific implementation process of the present invention, such as... Figure 3 As shown.

[0039] First, a detection system is constructed according to the technical solution described in this invention. A pair of air-coupled ultrasonic transducers are symmetrically arranged on both sides of the component under test, ensuring that the center lines of the sound beams coincide and are perpendicular to the surface of the component. The total spacing between the two transducers is locked using the slide table fasteners. The transmitting probe is connected to the output of the ultrasonic pulse transmitter, and the receiving probe is connected to the preamplifier, which is then connected to the data acquisition card.

[0040] Then, before formal monitoring, the stress-sound velocity relationship needs to be calibrated using standard specimens that are completely identical in material and manufacturing process to the target component. Under no-load conditions ( Under these conditions, the reference total acoustic time of ultrasonic waves penetrating the entire path of air-sample-air is measured. Tensile loads are applied to the specimen in stages using a mechanical testing machine, for example, by... Increase stress to the step size Set the stretching rate to... To ensure a quasi-static loading environment.

[0041] After each stress level stabilizes, ultrasonic waveform signals are acquired, and multiple sets of stress values ​​are recorded. With the corresponding specified ultrasonic peak sound time ,like Figure 4 As shown. And in conjunction with the initial thickness of the sample. Young's modulus Compared to Poisson The initial velocity of sound was obtained from data under stress-free conditions. The acoustoelastic coefficient of the material was extracted by fitting using the nonlinear least squares method. Establish a quantitative calibration model for stress and sound velocity. .

[0042] Finally, the calibrated model is applied to actual component monitoring: the component is fixed using a dedicated clamp with fine-tuning capabilities to ensure the component surface is perpendicular to the probe's acoustic beam. The clamp material should be lightweight and highly rigid to avoid introducing additional vibration or stress. During component service, the system periodically transmits and receives air-coupled ultrasonic signals. The data processing unit extracts the peak amplitude time based on the signal, calculates the sound velocity using the sound time, component thickness, and probe spacing, and then inputs the sound velocity into the established stress and sound velocity model. The system directly outputs the current stress change within the component relative to the baseline state.

[0043] Therefore, this invention employs the aforementioned air-coupled ultrasonic stress detection method for carbon fiber reinforced ceramic matrix composites, achieving non-contact quantitative assessment of the internal stress of ceramic matrix composites. This method does not interfere with the material surface or operating conditions, captures changes in the overall elastic properties of the material caused by stress, and transforms the complex mechanical state into intuitive acoustic signals for interpretation. This complete solution, from principle calibration to engineering application, provides innovative technical means for the safety monitoring and life prediction of key components in high-end equipment during service.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for detecting air-coupled ultrasonic stress in carbon fiber reinforced ceramic matrix composites, characterized in that, Includes the following steps: Step S1: Based on the acoustoelasticity principle model and the tensile deformation model, establish a model for the relationship between sound velocity and stress; Step S2: Build an air-coupled ultrasonic stress testing system and use a standard sample that is completely consistent with the material and process of the target component to calibrate the stress-sound velocity relationship. Step S3: Apply the calibrated stress-sound velocity model to online monitoring of actual components to achieve quantitative assessment of internal stress in carbon fiber reinforced ceramic matrix composites.

2. The method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites according to claim 1, characterized in that, In step S1, for the carbon fiber reinforced ceramic matrix composite sample, the fixed total spacing between the transmitting and receiving transducers is set to... Total sound It consists of the air segment sound time and the material segment sound time, as shown below: ； in, The speed of sound in ambient air; This refers to the real-time thickness of the material. The speed at which ultrasound propagates in a material is as follows: ； During the tensile process, the Poisson effect leads to the thickness of the material. Due to shrinkage, the thickness changes as shown below according to the tensile deformation model: ； in, This is the initial thickness; Poisson's ratio; Young's modulus; Due to the total spacing Fixed, reduced material thickness This is automatically converted into an equal air path increment; combined with the stretching deformation model, the propagation velocity expression is obtained: 。 3. The method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites according to claim 2, characterized in that, Under natural conditions of zero stress and no load, the wave equation within the material follows the Christoffel equation, as shown below: ； in, For the linear elastic stiffness tensor of the material; and The direction cosine of the wavefront normal vector; The density of the material; The initial sound velocity of the material under stress-free conditions; The displacement vector of the resonant plane wave; The Kronecker function is shown below: ； When a material is subjected to external loads that induce internal stress, its equivalent stiffness changes, causing... Let be the effective elastic stiffness tensor under stress, where Let be the Cauchy stress tensor under stress. Then, according to the principle of acoustoelasticity, the wave equation evolves as follows: 。 4. The method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites according to claim 3, characterized in that, Since the wave equation has a non-zero solution if the determinant of the coefficient matrix is ​​zero, we can obtain the following by solving the first-order characteristic equation: ； For uniaxial load The speed of longitudinal waves propagating perpendicular to the direction of the force. The following linear relationship is satisfied: ； in, The acoustoelastic coefficient of the material; This represents the stress value.

5. The method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites according to claim 4, characterized in that, Substituting the expressions for propagation speed and thickness changes into the total acoustic time formula, we obtain the total acoustic time. With stress The relational model is shown below: ； Based on the linear relationship between sound speed and stress, combined with the expression for propagation speed... The air-coupled ultrasonic stress detection model is calibrated by adjusting the acoustoelastic coefficient. With the initial sound velocity of the material This enables accurate stress inversion.

6. The method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites according to claim 1, characterized in that, In step S2, an air-coupled ultrasonic stress detection system is built, which includes a mechanical loading unit, a detection unit, and a data acquisition unit. The mechanical loading unit includes: a DDL100 electronic universal testing machine, a controller, and a main control computer; the data acquisition and processing unit is responsible for synchronously acquiring stress data and ultrasonic signals, and extracting the acoustic time corresponding to the ultrasonic wave peak with the highest amplitude through an algorithm, and finally completing the modeling and inversion calculation of the relationship between stress and sound velocity.

7. The method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites according to claim 6, characterized in that, The detection unit uses a pair of air-coupled ultrasonic probes arranged opposite each other in a penetration mode, along with a dedicated ultrasonic pulse transmitting and receiving device. Through a set of multi-degree-of-freedom precision adjustment fixtures, the distance and angle between the air-coupled ultrasonic transducers can be adjusted. The relative positions of the transmitter and receiver of the air-coupled ultrasonic transducer are adjusted and locked using a dovetail groove displacement slide and a height adjustment slide rail, ensuring that the center lines of the sound beams of the two transducers coincide and the total spacing is equal. The angle of incidence of the transducer relative to the sample surface is adjusted by the fixture-integrated angle rotating slide and turntable connector to ensure that the sound wave is incident perpendicularly. The transducer clamp and support base plate are used to ensure the long-term stability of the sensor position under experimental conditions.

8. The method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites according to claim 7, characterized in that, On the mechanical loading unit, graded tensile loads were applied to the carbon fiber reinforced ceramic matrix composite specimens, and the known stress values ​​were recorded and the corresponding ultrasonic signals were sampled at each stress stabilization state. Record multiple sets of stress values With the corresponding ultrasonic peak time And in conjunction with the initial thickness of the sample Young's modulus Compared to Poisson Initial sound velocity obtained from data under stress-free conditions The acoustoelastic coefficient of the material was extracted by fitting using the nonlinear least squares method. The final calibrated stress-velocity model is shown below: 。 9. The method for air-coupled ultrasonic stress detection of carbon fiber reinforced ceramic matrix composites according to claim 8, characterized in that, The calibrated stress and sound velocity model was applied to online monitoring of actual components to achieve quantitative assessment of the internal stress of carbon fiber reinforced ceramic matrix composites. The specific process is as follows: The actual thickness of the component at the detection point is measured. Air-coupled ultrasonic transducers are installed on both sides of the component under test in the same configuration, serving as the transmitter and receiver respectively. The component is fixed by a special fixture, which should have an angle adjustment function to ensure that the surface of the component is perpendicular to the line connecting the two probes, and the perpendicularity is calibrated. The fixture design should take into account the shape, size and material properties of the component to avoid introducing additional stress. Subsequently, ultrasonic signals are acquired, peak acoustic time is extracted from the signals, and sound velocity is calculated using acoustic time, component thickness and probe spacing. The sound velocity is then input into the established stress and sound velocity model to retrieve the current internal stress value or stress change of the component. Ultimately, a system based on total volume was established. , Sample thickness probe spacing The stress inversion model with independent variables is shown below: 。

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