Non-destructive testing method based on structural vibration - fiber optic acoustic waveguide sensing

Abstract

The present invention provides a non-destructive testing method based on structural vibration - fiber optic acoustic waveguide sensing. Among them, a section of the first pigtail of the FBG is adhered to one side of the component to be tested as the adhered section. The first pigtail, the FBG, and the second pigtail are placed in space without restraint, and the end of the second pigtail is placed in air or a reflective medium. A periodic excitation source or an impulse excitation source is applied to the other side of the component to be tested, and the vibration generated in the component to be tested serves as an acoustic wave source and is introduced into the FBG and the second pigtail in the forward direction through the first pigtail. The end of the second pigtail reflects the acoustic wave and transmits it back along the optical fiber in the reverse direction. The acoustic waves propagating in the forward and reverse directions form a standing wave in the second pigtail, the FBG, and the first pigtail of the FBG, and the FBG senses the standing wave signal. The standing wave signal on the FBG is demodulated into an electrical signal. The electrical signal is analyzed to judge the state of the component to be tested. The FBG of the present invention can be reused, is suitable for narrow spaces, has high sensitivity, is suitable for low-frequency excitation, and can prevent structural damage.

Description

Technical Field

[0001] The present invention relates to the field of non-destructive testing, and particularly to a non-destructive testing method based on structural vibration - fiber optic acoustic waveguide sensing. Background Art

[0002] Analysis of various structural dynamic characteristics, including modal analysis, harmonic response analysis, and transient dynamic analysis, can reflect the material characteristics and working state of the structure, and is a powerful means for structural health detection. The sensing device for dynamic characteristics, that is, the vibration sensor, is an essential equipment for such detection. Currently, the mainly used sensing systems include fiber optic sensing systems, piezoelectric sensing systems, and eddy current displacement sensing systems. Among them, the main advantages of the piezoelectric sensing system are high sensitivity, small size, light weight, stable performance, and wide frequency band, etc. Its disadvantage is that its sensing performance will be greatly affected in environments with high temperature, large temperature changes, humidity, or strong magnetic fields. The eddy current displacement sensor is suitable for testing the dynamic characteristics of rotating components and has the advantage of a wide response frequency band, but it is also greatly affected by temperature and electromagnetic fields.

[0003] Fiber optic sensing has outstanding advantages such as thin diameter, high sensitivity, anti-electromagnetic interference, light weight, and distributed measurement, and has become a prominent representative of the new generation of vibration sensors. The most successful representative among them is the Fiber Bragg Grating (FBG) vibration sensor. On the basis of the advantages of general fiber optic sensors, it also has advantages such as stable signals, multiplexing capabilities, and insensitivity to light source intensity and fiber bending. Currently, it has been widely used in the fields of petroleum, transportation, national defense, machinery, etc.

[0004] Currently, according to the FBG assembly method, FBG vibration sensors are mainly divided into three categories. One category is the full - paste assembly. That is, the FBG is directly pasted on the surface of the component to be measured or embedded in an elastic material to directly sense the vibration of the component to be measured. It can avoid damage to the sensor, but this also makes the sensor unable to be reused; on the other hand, due to the full - adhesive packaging method, the FBG may produce a chirping phenomenon due to uneven stress, causing the FBG spectrum to deform, and thus making the test results inaccurate.

[0005] The second category is the two - point assembly. That is, the pigtails at both ends of the fiber Bragg grating are fixed, and the FBG is in a suspended state. When the fixed points of the fiber Bragg grating pigtails generate displacement, it causes strain in the FBG, so the central wavelength of the FBG drifts, achieving the purpose of detecting vibration signals. This type of sensing structure requires additional auxiliary structures on the basis of the fiber Bragg grating sensing element, and the volume and mass are not suitable for narrow areas. Through appropriate structural design, vibration testing in the orthogonal direction can be achieved, but vibrations in non - orthogonal directions cannot be distinguished.

[0006] The third encapsulation method is floating assembly. That is, one end of the fiber optic grating pigtail is fixed in the sealing structure, and the other end pigtail of the fiber optic grating is extremely short and floats in the air. The length from the fixed pigtail of the FBG to the other end pigtail is controlled within 20 mm - 80 mm. This limitation is because if the length is too short, the vibration amplitude will be too small, affecting the sensitivity; while if the length is too long, the vibration frequency will be affected by the gravity of the optical fiber itself, resulting in inaccurate test results. Reasonable design of the fiber diameter and floating length can make this sensor have a relatively high sensitivity, and it is suitable for the measurement of micro-vibrations of large structures. However, this encapsulation requires a structure to enclose the FBG, which is not conducive to narrow spaces. All modes of vibration sensed by this encapsulation structure have complex FBG response signals, bringing great difficulties to signal analysis. Summary of the Invention

[0007] The present invention provides a non-destructive testing method and a testing system based on structural vibration - fiber optic acoustic waveguide sensing to solve one or several of the above problems.

[0008] According to one aspect of the present invention, a non-destructive testing method based on structural vibration - fiber optic acoustic waveguide sensing is provided. Among them, a section of the first pigtail of the FBG is pasted on one side of the component to be tested as the pasted section. The first pigtail, the FBG, and the second pigtail are placed in space without restraint, and the end of the second pigtail is placed in air or a reflective medium; a periodic excitation source or an impulse excitation source is applied to the other side of the component to be tested, and the vibration generated in the component to be tested serves as a sound wave source and is transmitted forward into the FBG and the second pigtail through the first pigtail. The end of the second pigtail reflects the sound wave and transmits it back along the optical fiber. The sound waves propagating forward and backward form a standing wave in the second pigtail, the FBG, and the first pigtail of the FBG. The FBG senses the standing wave signal; the standing wave signal on the FBG is demodulated into an electrical signal; the electrical signal is analyzed to judge the state of the component to be tested.

[0009] The advantages of the non-destructive testing method based on structural vibration - fiber optic acoustic waveguide sensing of the present invention are as follows: 1. It can be used for detecting the dynamic characteristics of the whole component. The pigtail of the FBG is attached to the component, and the spectral characteristics of the FBG itself will not generate chirping phenomenon. After the test, the FBG can be reused. 2. Since sound waves can be transmitted over a long distance in the optical fiber, it can avoid the adverse environment (such as high temperature) at the bonding point from directly affecting the characteristics of the FBG sensor, and is also suitable for monitoring the characteristics of components in narrow spaces, crowded spaces or those that cannot be directly observed. 3. The second pigtail of the FBG is suspended in the air (or reflection medium) with a certain length. The sound wave transmitted in it forms a backward wave reflection at the end of the pigtail, and then superimposes with the forward wave to form a standing wave, increasing the test sensitivity of the sensor. 4. This detection method is suitable for monitoring the dynamic characteristics of the whole structure under low-frequency excitation, reducing the high requirements for excitation equipment under high-frequency excitation. 5. Based on the sensing design of higher-order resonance, a small-energy excitation source can be used to generate larger vibrations, that is, a large sensing signal can be generated with a small excitation, reducing the difficulty of excitation, improving the signal-to-noise ratio of the sensor, and avoiding possible structural damage caused by vibrations at non-resonant frequencies.

[0010] FBG is most sensitive to axial strain. It has a certain sensitivity to radial strain, but the sensitivity to radial strain is less than that to axial strain. FBG does not sense torsional strain. Therefore, FBG has a good sensing ability for vibrations that generate linear strain along the axial direction of the optical fiber. And the vibrations that generate axial linear strain in the optical fiber can form longitudinal waves in the long optical fiber, which can be effectively sensed by the suspended FBG designed in the present invention. Therefore, the present invention indicates the mode that generates linear strain through finite element-based simulation analysis as the target mode, and determines the bonding direction according to the direction of the uniform linear strain indicated in the finite element analysis of the target mode. The bonding area should be in the area where the strain amplitudes of each particle indicated in the finite element analysis of the target mode are consistent.

[0011] Under the condition of periodic excitation, a damped structural component will cause vibrations with a relatively large but limited amplitude in the frequency band near the natural frequency. Within the safe vibration amplitude range, effectively detecting this vibration information can be used to detect the health status of the structural component. The natural vibration frequency of the structural component is not a fixed value, and it is affected by many factors such as the magnitude of the excitation force, the direction of the excitation force, the environmental temperature and even humidity. Therefore, the natural vibration frequency described in the present invention is actually a natural frequency band. Within this frequency band, the periodic excitation of the exciter can cause vibrations with a stable amplitude and a signal-to-noise ratio sufficient for effective structural health detection.

[0012] In some embodiments, the FBG of the present invention is located at the antinode of the standing wave, and the bonding section is located at the antinode of the standing wave.

[0013] In some embodiments, the present invention performs a finite element simulation modal analysis on the component to be measured, selects the mode that generates linear strain as the target mode, and uses the direction indicating the generation of uniform linear strain in the finite element simulation modal analysis as the direction in which the pasting section is pasted on the component to be measured; the pasting area where the pasting section is pasted on the component to be measured needs to be in the area where the strain amplitudes of each mass point indicated in the finite element simulation modal analysis of the target mode are consistent.

[0014] In some embodiments, the method for determining the excitation frequency of the periodic excitation source in the present invention includes: performing a finite element simulation modal analysis on the component to be measured, selecting the mode that generates linear strain as the target mode, obtaining the simulated natural frequency of the target mode, and making the frequency corresponding to the high-order harmonic of the periodic excitation near the simulated natural frequency of the target mode; experimental correction: along the direction indicating the generation of uniform linear strain in the finite element analysis of the target mode, paste the FBG on the area where the strain amplitudes of each mass point indicated in the finite element analysis of the target mode on the component to be measured are consistent, perform impulse excitation according to the action point and the direction of the force in actual non-destructive testing, observe the effective vibration frequency range and amplitude of the impulse response, and take the frequency corresponding to the maximum amplitude value in the spectrogram as the experimental natural frequency, and further make the integer multiple frequencies of the excitation frequency closer to the simulated natural frequency and the experimental natural frequency of the target mode.

[0015] For designing the frequency used for periodic excitation, the present invention uses the natural frequency of the structure as the design basis to obtain a larger sensing sensitivity and signal-to-noise ratio. There are two methods for the present invention to determine the natural frequency. One is to use the simulation modal analysis based on finite elements. This method can obtain the vibration modes and simulated natural frequency values of each order of modes. However, due to the inevitable errors between the assembly situation of the structure and the mechanical parameters of the material and the actual situation, the data obtained by the simulation cannot be absolutely accurate. The other is the method of obtaining the impulse response through experiments. Since theoretically the impulse excitation contains all frequencies, the frequency corresponding to the larger amplitude vibration indicated in the spectrum obtained from the impulse response is the actually possible excitation vibration frequency, and the frequency corresponding to the maximum amplitude value is the experimental natural frequency value under the impulse excitation condition. However, since the actual natural frequency is affected by the magnitude of the excitation force, if periodic excitation is used, there is a difference between the excitation intensity and the impulse excitation intensity, so there will also be a difference between its actual natural frequency and the experimental natural frequency obtained from the impulse excitation. Therefore, in view of the complexity of the natural frequency, the present invention comprehensively refers to the simulated natural frequency of the target mode obtained from the simulation modal analysis and the experimental natural frequency obtained through experiments.

[0016] Considering the case of periodic excitation, if the frequency of the periodic excitation signal x(t) is f and the period is T (f = 1 / T). According to the theory of Fourier series, it can be expanded into the sum of a DC component, a fundamental harmonic component with frequency f, a second harmonic component with frequency 2f, and an nth harmonic component with frequency nf. The fundamental harmonic is called the fundamental wave, and the second and higher harmonics are called higher harmonics. That is:

[0017]

[0018] where x0 is the DC component of the signal, a n is the amplitude of each harmonic, is the initial phase of each harmonic, that is, the periodic excitation actually contains components of each harmonic. Therefore, the frequencies of the higher harmonics of the periodic excitation are close to the simulation natural frequency and the experimental natural frequency, so as to effectively excite the target mode and can be effectively sensed by the FBG sensing system, and then effective non-destructive testing can be carried out.

[0019] In some embodiments, within the vibration frequency range where the FBG of the present invention can effectively sense and the vibration of the component to be measured can be effectively excited, among the integer multiple frequencies of the periodic excitation frequency, the frequency closest to the vibration frequency with the largest amplitude is selected as the reference frequency. The wavelength of the reference frequency is calculated at the constant wave speed in the optical fiber. The length D1 from the pasted section of the first pigtail to the fiber grating is an integer multiple of half the wavelength of the reference frequency, and the length D2 of the second pigtail is an integer multiple of half the wavelength of the reference frequency. Thus, the signal-to-noise ratio of the non-destructive testing signal can be effectively improved.

[0020] In some embodiments, the present invention selects the vibration frequency closest to the largest amplitude indicated by the simulation analysis of the target mode in the finite element simulation modal analysis of the component to be measured as the reference frequency.

[0021] In some embodiments, the present invention selects the frequency closest to the vibration frequency with the largest amplitude indicated by the impulse response as the reference frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic structural diagram of a non-destructive testing system based on structural vibration - fiber optic acoustic waveguide sensing according to an embodiment of the present invention;

[0023] Figure 2 is a schematic diagram of the reflection of the sound wave transmitted in the optical fiber at the interface between the end of the second pigtail and the air and the formation of a standing wave in the optical fiber;

[0024] Figure 3 is a schematic diagram of the structure of the component to be measured of a non-destructive testing system based on structural vibration - fiber optic acoustic waveguide sensing according to an embodiment of the present invention and the relationship between the pigtail of the FBG and the component to be measured;

[0025] Figure 4 For Figure 3 The finite element-based simulation modal analysis results of the first component to be measured shown in the figure;

[0026] Figure 5 For the frequency spectrum diagrams of impulse responses when FBGs are adhered to the first component to be measured shown in the figure in different directions; Figure 3 The frequency spectrum diagrams of impulse responses when FBGs are adhered to the first component to be measured shown in the figure in different directions;

[0027] Figure 6 For Figure 3 The response and frequency spectrum diagrams of the sensing system under the action of a 314 Hz periodic excitation in the assembled form of the first component to be measured and the FBG shown in the figure, where D1 and D2 are half wavelengths or integer multiples of half wavelengths corresponding to the reference frequency;

[0028] Figure 7 For Figure 3 The response and frequency spectrum diagrams of the sensing system under the action of a 314 Hz periodic excitation in the assembled form of the first component to be measured and the FBG shown in the figure, where D1 and D2 are not half wavelengths or integer multiples of half wavelengths corresponding to the reference frequency;

[0029] Figure 8 For Figure 3 The response waveform and frequency spectrum diagrams of the sensing system at 314 Hz periodic excitation under different pre-tightening torque conditions between the screws and the internal threaded holes on the first component to be measured shown in the figure;

[0030] Figure 9 For Figure 3 The response frequency spectrum of the sensing system at impulse excitation under different displacement conditions of the whole first component to be measured along the z direction shown in the figure;

[0031] Figure 10 For Figure 3 The response frequency spectrum of the sensing system at impulse excitation under the displacement condition of the first component to be measured in the z direction shown in the figure; where D1 and D2 are half wavelengths corresponding to the reference frequency;

[0032] Figure 11 Schematic diagram of the structure of the second component to be measured of the non-destructive testing system based on structural vibration - fiber optic acoustic waveguide sensing of an embodiment of the present invention, the relationship between the FBG and its pigtail fiber and the second component to be measured;

[0033] Figure 12 For Figure 11 The finite element-based simulation modal analysis results of the second component to be measured shown in the figure;

[0034] Figure 13 Frequency spectrum diagrams of impulse responses when FBGs are adhered to the second component to be measured in different directions;

[0035] Figure 14 ForFigure 11 Under the excitation of a 330 Hz periodic excitation, the response and spectrum of the sensing system in the form of the second component to be measured and the FBG assembly shown, where D1 and D2 are both half-wavelengths corresponding to the reference frequency;

[0036] Figure 15 For Figure 11 Under the excitation of a 330 Hz periodic excitation, the response and spectrum of the sensing system in the form of the second component to be measured and the FBG assembly shown, where D1 and D2 are not or one of them is not the half-wavelength or an integer multiple of the half-wavelength corresponding to the reference frequency;

[0037] Figure 16 For Figure 11 Under the excitation of a 330 Hz periodic excitation, the response and spectrum of the sensing system in the form of the second component to be measured and the FBG assembly shown, where D1 and D2 are both half-wavelengths corresponding to the reference frequency, and the hollow cylinder and the fastening screw are at different angles;

[0038] Figure 17 For Figure 11 Under the excitation of a 330 Hz periodic excitation, the response spectrum of the sensing system in the form of the second component to be measured and the FBG assembly shown, where D1 and D2 are both half-wavelengths corresponding to the reference frequency, and the solid cylinder has different displacements in the z direction;

[0039] Figure 18 For Figure 11 Under the excitation of a 330 Hz periodic excitation, the response spectrum of the sensing system in the form of the second component to be measured and the FBG assembly shown, where D1 and D2 are both half-wavelengths corresponding to the reference frequency, and the solid cylinder has a displacement in the z direction. Detailed implementation mode

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0041] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments may be combined with each other.

[0042] Finally, it should also be noted that in this article, relational terms such as first and second, front and back, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising" and "including" not only include those elements, but also other elements not explicitly listed, or elements inherent to such a process, method, article, or device. Without further limitation, an element defined by the statement "comprising..." does not exclude the existence of additional identical elements in the process, method, article, or device that includes the said element.

[0043] The present invention will be further described in detail below with reference to the accompanying drawings.

[0044] Figure 1 Schematically shows the structure of a non-destructive testing system based on structural vibration - fiber optic acoustic waveguide sensing according to an embodiment of the present invention.

[0045] Reference Figure 1 As shown, the non-destructive testing system based on structural vibration - fiber optic acoustic waveguide sensing includes an exciter 30, a first pigtail 11, a second pigtail 12, and a fiber Bragg grating (FBG) 10, as well as a demodulation device (including a tunable narrowband laser 21, an isolator 22, a circulator 23, and a photodetector 24) and an electrical signal analysis and processing system (including a data collector 25 and a data analyzer 26).

[0046] The exciter 30 can be an exciter that generates periodic excitation or an exciter that generates impulse excitation. The periodic excitation energy generated by the exciter 30 cancels the energy loss caused by the structural vibration damping in the structure, forming a vibration with a stable and finite amplitude.

[0047] A section of the first pigtail 11 of the FBG is adhered to one side of the component under test 40 to form an adhered section 13. The first pigtail, the FBG, and the second pigtail are placed in space without restraint, as long as it does not affect the propagation of the acoustic waveguide in the optical fiber. It can be freely placed on the table without restraint, or in a suspended state, but it cannot be placed in a liquid or solid to form a mechanical continuum with the optical fiber. The end of the second pigtail 12 is placed in air. The first pigtail 11 of the FBG can be directly adhered to the structure under test, or indirectly adhered. For example, the first pigtail 11 is first adhered to a small intermediate medium block, and the intermediate medium is then adhered to the component under test. In other embodiments, the end of the second pigtail 12 can also be placed in other reflective media.

[0048] The component 40 to be measured is fixed by the limiting member 50, and the exciter 30 acts on the other side of the component 40 to be measured, generating vibrations in the component 40 to be measured. The vibrations are transmitted as sound wave sources into the FBG and the second pigtail fiber 12 in the positive direction through the first pigtail fiber 11. The end of the second pigtail fiber 12 reflects the sound wave and transmits it in the reverse direction along the optical fiber. The sound waves propagating in the forward and reverse directions form a standing wave in the second pigtail fiber 12, the FBG, and the first pigtail fiber 11 of the FBG, and the FBG can sense the standing wave signal.

[0049] The demodulation device is used to demodulate the standing wave signal on the FBG into an electrical signal. The exciter 30 applies a force to the other side of the component 40 to be measured, and the end of the first pigtail fiber 11 of the FBG is connected to the circulator 4.

[0050] In this embodiment, the edge filtering method is used to demodulate the sound wave information on the FBG. The demodulation device includes an adjustable narrowband laser 21, an isolator 22, a circulator 23, and a photodetector 24. The light emitted by the adjustable narrowband laser 21 enters the FBG 10 through the isolator 22 and the circulator 23 at the end of the first pigtail fiber 11 to sense the sound wave signal on the FBG. The narrowband light with sound wave information in the FBG 10 is reflected by the FBG 10 and then enters the photodetector 24 through the circulator 23 to be converted into an electrical signal. The electrical signal is extracted by the data collector 25 and then enters the data processing device 26 for signal processing and analysis, and the result is used for non-destructive testing of the state of the component 40 to be measured. Among them, the wavelength of the adjustable narrowband laser 21 is set in the middle of the rising edge or the falling edge of the FBG spectrum.

[0051] In other embodiments, the matching grating method or other methods can also be used to demodulate the sound wave information on the FBG. The demodulation device of the matching grating method: a broadband light source, a matching grating, a circulator, and a photodetector. The broadband light emitted by the broadband light source enters the FBG through the circulator, the reflected light of the FBG enters the matching grating through the circulator again, and the reflected light of the matching grating enters the photodetector to convert the optical signal into an electrical signal.

[0052] The electrical signal analysis and processing system is used to analyze and process the electrical signal to judge the state of the component 40 to be measured.

[0053] In this embodiment, the electrical signal analysis and processing system includes a data collector 25 and a data analyzer 26. The electrical signal is imported into the data analyzer 26 through the data collector 25. After the data analyzer 26 processes the electrical signal through data processing software, a sensing system response and a spectrogram are obtained to judge the state of the component 40 to be measured. The state of the structural component 40 can also be judged manually by experience or by using artificial intelligence algorithms.

[0054] In other embodiments, the electrical signal analysis and processing system may also include an oscilloscope. The electrical signal is displayed as a time-domain waveform by the oscilloscope, and the state of the component 40 to be measured can be judged according to the time-domain waveform.

[0055] Due to the thinness of the optical fiber, sound waves can be transmitted over a long distance in the optical fiber. This detection method can avoid the direct influence of the harsh environment (such as high temperature) at the bonding point on the characteristics of the FBG sensor, and is also suitable for monitoring the characteristics of components in narrow spaces, crowded spaces or components that cannot be directly observed.

[0056] Figure 2 It is a schematic diagram of sound waves transmitted in the optical fiber being reflected at the interface between the end of the second pigtail and air or a reflective medium and forming a standing wave in the optical fiber.

[0057] Reference Figure 2 As shown, the end of the second pigtail suspended in air or a reflective medium reflects sound waves and transmits them along the reverse propagation direction. The sound waves propagating forward and backward form a standing wave in the second pigtail, the FBG, and the first pigtail. Since the vibration amplitude is the largest at the antinode of the standing wave, in order to improve the response sensitivity, the FBG and the bonded section 13 of the first pigtail are placed at the antinode of the standing wave.

[0058] Forward wave equation:

[0059]

[0060] Reverse wave equation:

[0061]

[0062] Standing wave equation expression:

[0063]

[0064] Where the amplitude is A, the angular frequency is ω, the propagation distance spatial coordinate is x, the time is t, and the wavelength is λ. From the above formula, it can be seen that the amplitude of the standing wave is twice the amplitude of the forward wave, and at the position of half the wavelength or an integer multiple of half the wavelength of the forward wave, that is, at the antinode of the standing wave, the amplitude reaches the maximum value.

[0065] Figure 3 Schematically shows the structure of the component to be measured of the non-destructive testing system based on structural vibration - fiber optic acoustic waveguide sensing according to an embodiment of the present invention and the relationship between the pigtail of the FBG and the component to be measured.

[0066] Reference Figure 3As shown, the first component to be measured 41 is a rigid polyurethane block embedded with a fastening structure (including a screw and an internal thread hole provided on the rigid polyurethane block and adapted to the screw), d is the length of the bonding section, D1 is the length from the bonding section on the first pigtail to the optical fiber of the FBG, and D2 is the length of the second pigtail 12 (i.e., the length from the FBG to the end of the second pigtail). Among them, the bonding direction of the bonding section is the y direction. The area with a horizontal line drawn at the lower part of the first component to be measured 41 is the area limited by the limiting member 50.

[0067] Figure 4 is Figure 3 the simulation modal analysis result based on finite element of the first component to be measured as shown.

[0068] For the convenience of bonding the FBG, the X plane facing the reader (i.e., the plane perpendicular to the X axis) is taken as the bonding plane to be considered. Refer to Figure 4 As shown, the vibration modes of each order obtained by the finite element simulation method. The third and fourth order modes are mainly torsional, but the fiber grating is insensitive to torsion. Therefore, the third and fourth order modes will not be sensed by this sensing device. The first, second and fifth order modes of the first component to be measured 41 generate relatively uniform linear displacements in the y direction on the x plane. Therefore, the y direction is the suitable bonding direction for the optical fiber. The displacements of each particle on the bonding length are the same, that is, the areas of the same color in the simulation diagram. The fifth order mode in the simulation is not easily excited in the actual test (the simulation natural frequency is 5086.5 Hz). Therefore, the first and second order modes are taken as the target modes. The simulation natural frequency of the first order mode is 1997.6 Hz, and the simulation natural frequency of the second order mode is 2616.0 Hz.

[0069] Based on the above analysis, the optical fiber bonding position of the first component to be measured 41 in this embodiment is as Figure 3 shown. According to the simulation modal analysis result, in order to make the sound waves in the optical fiber in the same phase, the bonding length d is taken as 8 mm to 10 mm (refer to the scale of this figure).

[0070] Further experimental calibration is carried out to obtain the effective vibration frequency range that can be actually excited and sensed by the FBG. The experimental calibration method is as follows: in the direction (the y direction in this embodiment) indicating the generation of uniform linear strain in the finite element analysis of the target mode, the FBG is bonded to the area where the strain amplitudes of each particle indicated in the finite element analysis of the target mode on the component to be measured are the same (the area shown in Figure 3 this embodiment, and the bonding length is 8 mm to 10 mm). According to the acting point of the excitation and the direction of the force in the actual non-destructive testing, impulse excitation is carried out, and the effective vibration frequency range and amplitude of the impulse response are observed, and the frequency corresponding to the maximum value of the amplitude in the spectrogram is taken as the experimental natural frequency. Figure 5 (a) is the obtained impulse response and spectrogram.

[0071] Figure 5 When the FBG is pasted on the test component 1 in different directions, the spectrogram of the impulse response. Figure 3 shown.

[0072] To detect Figure 4 the conclusion that the y-direction (the direction generating a relatively uniform linear displacement) shown in is the suitable pasting direction for the pigtail fiber, the FBG is directly pasted on the Figure 3 X-plane of the test component 1 shown in at other different angles, and the pasting directions are the z-direction ( Figure 5 (b)) and at a 45-degree angle to the z-direction ( Figure 5 (c)). The pasting area is the same as Figure 5 (a), and the length is still 8 mm to 10 mm. The specific pasting positions and the corresponding impulse response spectra under the excitation of the same impulse function are as shown in Figure 5 (b) and Figure 5 (c).

[0073] Referring to Figure 5 it can be seen that the signal amplitude of the pasting method along the y-direction is much larger than that of other frequency signals in the range of 1600 - 4000 Hz. The signal amplitude of the pasting method along the z-axis is greatly reduced, and the signals in the range of 500 - 2500 Hz are the main components. When pasted at a 45-degree angle to the Z-axis, there are certain signals in the frequency range less than 7000 Hz, and the signal amplitudes are all small, and the signals below 4000 Hz are the main ones. The measured signal-to-noise ratio of the system when pasted along the z-direction is 18.069; when pasted at a 45-degree angle to the z-axis, the signal-to-noise ratio of the system is 23.5958; when pasted along the y-direction, the signal-to-noise ratio of the system is 25.2305.

[0074] Therefore, the test results show that: Figure 5 (a) The signal-to-noise ratio of the impulse response obtained by the pasting method along the y-direction is higher than that of the responses in other pasting directions, and this pasting method is superior to other pasting methods. The fifth-order mode in the simulation is not easily excited in the actual test (the simulated natural frequency is 5086.5 Hz), so the first and second-order modes are used as the target modes.

[0075] Select the excitation method: In this embodiment, the impulse response can be repeated after multiple impulse experiments, so impulse excitation can be selected. Also, since the influence of low-energy periodic excitation on the test component may be very small, both excitation methods can be used.

[0076] Determine the excitation source frequency when performing periodic excitation: From Figure 5(a) From the spectral analysis of the impulse response obtained experimentally, it can be seen that the effective vibration frequency range that the FBG can sense for the component to be measured - 41 is around 1600 Hz to 3600 Hz. Among them, 1699 Hz, 1899 Hz, 1999 Hz, 2466 Hz, 2632 Hz, 2866 Hz, and 3632 Hz are the frequencies corresponding to the maximum values obtained from the impulse response, that is, the experimental natural frequencies. And the vibration amplitude corresponding to the frequency 2466 Hz is the largest, and relatively large-amplitude vibrations can be excited near the intervals of 1699 Hz - 1999 Hz and 2466 Hz - 2866 Hz. This range covers Figure 4 the simulated natural frequency of the first-order mode (1998 Hz) and the simulated natural frequency of the second-order mode (2616 Hz) in the target mode of the simulation.

[0077] Comprehensively examining the first-order and second-order simulated natural modes, and then combining with the test results of the impulse excitation experiment Figure 5 and the experimental natural frequencies indicated in (a), using a 314 Hz periodic signal as the excitation period, in the range of 1600 Hz to 3600 Hz, the high-order resonance frequencies of the 314 Hz periodic excitation are 1884 Hz, 2512 Hz, 2826 Hz, 3140 Hz, and 3454 Hz. Some of these frequencies are near the simulated natural frequencies and the experimental natural frequencies, and some of these frequencies are near the intervals of 1699 Hz - 1999 Hz and 2466 Hz - 2866 Hz. Therefore, it can be considered that vibrations with sufficient amplitude can be excited within this range.

[0078] Determine the length D1 from the bonding section of the first optical fiber pigtail to the FBG and the length D2 of the second optical fiber pigtail: According to the high-order resonance frequencies of 1884 Hz, 2198 Hz, 2512 Hz, 2826 Hz, 3140 Hz, and 3454 Hz of 314 Hz within the effective excitation range of 1600 Hz to 3600 Hz, calculate its wavelength, half-wavelength, quarter-wavelength, and three-quarter wavelength with the wave velocity in the optical fiber being the non-dispersive wave velocity of 3743.54 m / s (constant) as shown in Table 1. According to the standing wave theory, to make the signal-to-noise ratio of the FBG sensor optimal, the FBG and the bonding section are preferably placed at the half-wavelength or an integer multiple of the half-wavelength.

[0079] Table 1: Wavelength, half-wavelength, quarter-wavelength, and three-quarter wavelength at different frequencies

[0080]

[0081] From Figure 5 the excitation response spectrum in (a), it can be seen that the vibration amplitude is the largest near the experimental natural frequency of 2466 Hz. At Figure 4The direction of the second-order modal motion is consistent with the y direction of the pasting direction, which is the most likely modal to be sensed by the FBG. The simulated natural frequency indicated by the second-order mode is 2616 Hz. The vibration frequency closest to these two frequencies in Table 1 is 2512 Hz. Therefore, we select 2512 Hz in Table 1 as the reference frequency to design the length D1 from the pasting section of the first pigtail to the FBG and the length D2 of the second pigtail. Its half-wavelength is 745.1 mm. In other embodiments, the reference frequency can also be determined by sweeping the frequency.

[0082] Figure 6 For Figure 3 the response of the sensing system and the spectrogram of the first component to be measured under the periodic excitation of 314 Hz as shown.

[0083] The periodic excitation energy generated by the shaker cancels out the energy loss caused by the structural vibration damping in the structure, forming a vibration with stable energy and limited amplitude. According to Table 1, in order to place the FBG and the pasting section of the first pigtail at the antinode of the standing wave, two sets of data of D1 = 740 mm, D2 = 750 mm and D1 = 740 mm, D2 = 1495 mm are taken respectively, and their time-domain waveforms and spectra are tested as Figure 6 shown, and their signal-to-noise ratios are 33.4365 and 27.0675 respectively.

[0084] Figure 7 For Figure 3 the response of the sensing system and the spectrogram of the first component to be measured under the periodic excitation of 314 Hz as shown.

[0085] Among them, Figure 7 (a) D1: 2 mm, D2: 200 mm; Figure 7 (b) D1: 150 mm, D2: 2300 mm; Figure 7 (c) D1: 662 mm, D2: 1495 mm; Figure 7 (d) D1: 980 mm, D2: 1495 mm. The corresponding signal-to-noise ratios are 15.4913, 20.9960, 20.9138, and 23.4725 respectively.

[0086] It can be seen that Figure 6 the signal-to-noise ratio of the sensing signal shown is significantly greater than Figure 7 the signal-to-noise ratio of the sensing signal shown. Taking D1 and D2 as integer multiples of the half-wavelength or the half-wavelength can indeed improve the signal-to-noise ratio of the test signal.

[0087] Figure 8 For Figure 3 the time-domain waveform and spectrogram under different pre-tightening torque conditions between the screw on the first component to be measured and the mating internal thread hole as shown.

[0088] According to the relevant theory of screw fastening, the greater the pre-tightening torque, the smaller the vibration loss of the structural member 41. According to this characteristic, the fastening state between the screw and the mating internal thread hole can be judged.

[0089] A section of the first pigtail 11 of the FBG is adhered to one side of the first component to be measured 41 along the y direction. The FBG is in a suspended state, the second pigtail 12 of the FBG is in a suspended state, and the end of the second pigtail 12 is placed in the air. Among them, D1 = 740mm, D2 = 745mm, the length of the adhered section is 10mm, and the length of the FBG is 10mm.

[0090] The exciter that generates a periodic excitation of 314Hz is abutted against the other side of the first component to be measured 41. The vibration generated in the first component to be measured 41 is used as a sound wave source and is positively transmitted into the FBG 10 and the second pigtail 12 through the first pigtail 11. The end of the second pigtail 12 reflects the sound wave and transmits it back along the optical fiber in the reverse direction. The sound waves propagating in the forward and reverse directions form a standing wave in the second pigtail 12, the FBG 10 and the first pigtail 11 of the FBG, and the FBG senses the standing wave signal.

[0091] The standing wave signal on the FBG is demodulated into an electrical signal by the edge filtering method. The electrical signal is imported into the data analyzer 26 through the data collector 25. After the data analyzer 26 processes the electrical signal through the data processing software, the response and spectrum diagram of the sensing system are obtained, and the state of the first component to be measured can be judged according to the response and spectrum diagram of the sensing system.

[0092] Among them, the pre-tightening torques between the screw and the internal thread hole are respectively (a) 0 Ncm; (b) 35 Ncm; (c) 70 Ncm; (d) 100 Ncm

[0093] Table 2: Corresponding relationship between the power of the signal in one cycle and the torque

[0094]

[0095] The relationship between the signal power and the torque in one cycle obtained by calculation is shown in Table 2. It can be seen from Table 2 that as the pre-tightening force increases, the power of the test signal is stronger. Therefore, the pre-tightening state of the fastener can be judged by this test signal.

[0096] Figure 9 For Figure 3 the impulse response spectrum of the test system under different displacement conditions of the whole first component to be measured along the z direction as shown.

[0097] Monitor the translation of the whole first component to be measured 41 along the z-axis. The first component 1 may become loose from the bottom during operation (the surrounding constraints remain unchanged). In this non-destructive testing experiment, the same-intensity impulse excitation is applied to the first component to be measured to test the translation of the whole first component to be measured 41 along the z-axis.

[0098] A section of the first optical fiber pigtail 11 of the FBG is adhered to one side of the first component under test 41 along the y direction. The FBG is in a suspended state, the second optical fiber pigtail 12 of the FBG is in a suspended state, and the end of the second optical fiber pigtail 12 is placed in the air. Among them, D1 = 740 mm, D2 = 745 mm, the length of the adhered section is 10 mm, and the length of the FBG is 10 mm.

[0099] The shaker that generates the impulse excitation is abutted against the other side of the first component under test 41. The vibration generated in the first component under test 41 is used as a sound wave source and is positively transmitted into the FBG and the second optical fiber pigtail 12 through the first optical fiber pigtail 11. The end of the second optical fiber pigtail 12 reflects the sound wave and transmits it reversely along the optical fiber. The sound waves propagating forward and reversely form a standing wave in the second optical fiber pigtail 12, the FBG 10, and the first optical fiber pigtail 11 of the FBG, and the FBG senses the standing wave signal.

[0100] The standing wave signal on the FBG is demodulated into an electrical signal by the edge filtering method. The electrical signal is imported into the data analyzer 26 through the data collector 25. After the data analyzer 26 processes the electrical signal through the data processing software, the response and spectrum diagram of the sensing system are obtained, and the state of the first component under test can be judged according to the response and spectrum diagram of the sensing system.

[0101] Among them Figure 9 (a) Not detached from the bottom constraint; Figure 9 (b)-(d) Detached from the bottom constraint, the displacements in the z direction are (a) 0 mm; (b) 15.9 mm; (c) 35.9 mm; (d) 45.9 mm respectively.

[0102] It can be seen from Figure 9 that when the bottom of the first component under test is completely constrained (as shown in Figure 9 (a)), its impulse response spectrum shows multiple peaks below 4000 Hz. When its bottom is detached from the bottom constraint in the z direction, the main peak of the impulse response spectrum gradually moves to a lower frequency as the displacement increases. The spectrum shape also gradually transitions from multiple peaks to a single peak. The approximate displacement of the entire structure in the z direction can be evaluated from this characteristic.

[0103] Figure 10 For Figure 3 the impulse response spectra of the first component under test at three different displacements in the z direction shown.

[0104] For example, if the impulse response spectra of the first component under test are measured as Figure 10 (a), (b), and (c), according to the comparison of the spectrograms, its displacements in the z direction are approximately 16 mm, 36 mm, and 46 mm respectively.

[0105] Figure 11Schematic diagram of the structure of the second component to be measured of the non-destructive testing system based on structural vibration - fiber optic acoustic waveguide sensing according to an embodiment of the present invention, and the relationship between the FBG and its pigtail and the second component to be measured.

[0106] Refer to Figure 11 As shown, replace the first component to be measured 41 shown in Figure 3 with the second component to be measured 42 shown in Figure 11 It consists of two parts. The first part is a columnar cylinder 31 with a cavity inside. Its bottom is fixed to the limiting member 50, and the fixed part is the constrained part. The second part is a solid cylinder 34 with another hollow cylinder 32 at the top. The first part and the second part are fixed by bolts 33. The axial direction of the hollow cylinder 32 is the same as the axial direction of the bolts 33. All components of the entire second component to be measured 42 are made of aluminum alloy material.

[0107] Figure 12 For Figure 11 the simulation modal analysis results based on finite element of the second component to be measured shown in

[0108] Refer to Figure 12 As shown, in each order of mode, the strain modes of the first, second, third, fourth, sixth, seventh, and eighth orders are all linear displacements, only with different directions. The fifth and ninth orders are mainly torsional. Since the optical fiber is insensitive to torsion, the fifth and ninth order modes will not be sensed by this sensing device. The first, second, third, fourth, sixth, and eighth order modes of the second component to be measured 42 generate relatively uniform linear displacements along the z direction on the cylindrical surface. Therefore, the z direction is the suitable direction for pasting the optical fiber. According to the simulation results, in order to make the sound waves in the optical fiber in the same phase, the pasting length is taken as 8 - 15 mm (refer to the scale of this figure).

[0109] Figure 13 Spectrum diagram under the impulse response when the FBG is pasted on the second component to be measured in different directions.

[0110] Paste the FBG directly on the outer cylindrical surface of the columnar cylinder 31 with a cavity inside in the second component to be measured 42 shown in Figure 11 at different angles. The pasting directions are the z direction, the y direction, and an angle of 45 degrees with the z direction respectively. The specific pasting positions and the corresponding impulse response spectra under the excitation of the same impulse function are shown in Figure 13.

[0111] From Figure 13It can be seen that the amplitude of the pasting method in the z - direction is much larger than that of other pasting methods, and the signal frequency is in the range of 1600 - 7000 Hz; when pasted at an angle of 45 degrees with the z - axis, its signals are dispersed in the range of 200 - 12000 Hz, and the signal amplitudes of the natural frequencies of each experiment are basically the same; while the signal amplitude of the pasting method along the y - axis is greatly reduced, and the signals in the range of 200 - 8200 Hz are the main components. When actually measured and pasted along the z - direction, the signal - to - noise ratio of the system is 26.7107; when pasted along the direction at an angle of 45 degrees with the z - axis, the signal - to - noise ratio of the system is 24.5548; when pasted along the y - direction, the signal - to - noise ratio of the system is 18.9105.

[0112] Therefore, the test results show that: the signal - to - noise ratio of the impulse response obtained by the pasting method in the z - direction (the direction generating a relatively uniform linear displacement) is higher than that of the responses in other pasting directions, and this pasting direction is superior to other pasting directions.

[0113] Select the excitation method: After multiple impulse experiments, the impulse response can be repeated, so impulse excitation can be selected. Also, because the influence of low - energy periodic excitation on the component to be measured may be very small, both excitation methods can be adopted.

[0114] Determine the excitation source frequency for periodic excitation: From Figure 13 (a) Experimental analysis shows that the effective vibration frequency range of exciting the component to be measured two is around 1600 Hz - 7000 Hz. From Figure 12 The simulation analysis based on finite - element shows that since the first and second modes are not within this effective vibration frequency range, the overall displacements of the third, seventh, and eighth - order modes are bent, and the fifth and ninth - order vibration modes are torsional and not perceived by the FBG. Therefore, because the fourth and sixth orders correspond to uniform linear strains, the fourth order (simulation natural frequency 1994 Hz) and the sixth order (simulation natural frequency 2985 Hz) are selected as the target modes. From Figure 13 (a) The experimental natural frequencies corresponding to the maximum amplitude values of the measured FBG impulse response are around 1657 Hz, 3285 Hz, 5342 Hz, and 6428 Hz. From these four experimental natural frequencies and the simulation natural frequencies of the fourth and sixth orders, a 330 - Hz periodic signal is selected as the excitation, and the frequencies corresponding to its higher - order harmonics are 1650 Hz, 1980 Hz, 2970 Hz, 3300 Hz, 3630 Hz, 5280 Hz, 5610 Hz, 5940 Hz, 6270 Hz, and 6600 Hz. Some of the frequencies corresponding to these higher - order harmonics are near the simulation natural frequencies or experimental natural frequencies of the fourth and sixth orders. Calculating its wavelength, half - wavelength, quarter - wavelength, and three - quarter - wavelength with the non - dispersive wave speed of 3743.54 m / s in the optical fiber is shown in Table 3.

[0115] Table 3: Wavelength, half-wavelength, quarter-wavelength, and three-quarter wavelength at different frequencies

[0116]

[0117]

[0118] As can be seen from Figure 13 (a), the vibration amplitude corresponding to the experimental natural frequency of 3285 Hz is the largest. Among the frequencies corresponding to the higher harmonics of the 330 Hz periodic excitation designed in the present invention, 3300 Hz is the closest to this experimental natural frequency of 3285 Hz; 2970 Hz is the closest to the sixth-order simulated natural frequency of 2985 Hz. According to the principle of giving priority to experimental results, 3300 Hz is taken as the reference frequency for designing D1 and D2, and its half-wavelength is 567.2 mm.

[0119] Figure 14 For Figure 11 the time-domain waveform and spectrogram of the signal of the sensing system of the second component to be measured shown in under the action of a 330 Hz periodic excitation. To place the FBG and the bonding section at the antinode of the standing wave, according to Table 3, D1 = 567 mm and D2 = 570 mm are taken, and the measured signal-to-noise ratio is 19.2236.

[0120] Figure 15 For Figure 11 the time-domain waveform and spectrogram of the response signal of the sensing device of the second component to be measured shown in under the action of a 330 Hz periodic excitation.

[0121] Respectively select Figure 15 (a) D1: 430 mm, D2: 637 mm; Figure 15 (b) D1: 567 mm, D2: 637 mm; Figure 15 (c) D1: 567 mm, D2: 697 mm; Figure 15 (d) D1: 567 mm, D2: 727 mm; Figure 15 (e) D1: 567 mm, D2: 860 mm; Figure 15 (f) D1: 630 mm, D2: 647 mm; Figure 15 (g) D1: 630 mm, D2: 697 mm; Figure 15 (h) D1: 700 mm, D2: 637 mm, as Figure 15 (a)- Figure 15 (h) shown, and the corresponding signal-to-noise ratios are 9.4812, 10.7910, 13.7618, 16.8931, 17.7747, 18.0831, 15.0887, 17.2563 respectively.

[0122] On the contraryFigure 14 The signal-to-noise ratio is 19.2236, so Figure 14 the signal-to-noise ratio of the sensing signal of is significantly greater than that of the sensing signal in Figure 15. Taking D1 and D2 as half wavelengths or integer multiples of half wavelengths can indeed improve the response sensitivity and the signal-to-noise ratio of the effective test signal.

[0123] Figure 16 For Figure 11 the response and spectrogram of the detection system under the condition that the hollow cylinder and the fastening bolt on the second component to be measured shown in are at different included angles.

[0124] To illustrate the application of the present invention, this example is used to test the offset state between the axial direction of the top hollow cylinder and the axial direction of the bolt in the second component to be measured. Due to the structural geometric characteristics, when the axial directions of the top hollow cylinder and the bolt are the same or perpendicular, the whole structure is symmetric; among them, when the axial directions of the top hollow cylinder and the bolt are the same, the symmetry is higher. In this embodiment, the offset between the axial direction of the top hollow cylinder and the axial direction of the bolt is taken as the test target for non-destructive testing. When Figure 11 the second component to be measured shown in is working, the axial direction of the top hollow cylinder 32 and the axial direction of the bolt 33 may be offset.

[0125] Stick the first pigtail fiber 11 on the second component to be measured according to the direction shown in Figure 13 (a), the sticking length is 10 mm, D1 is 567 mm, and D2 is 570 mm, which is used for the non-destructive testing of the second component to be measured.

[0126] Press the exciter that generates a 330 Hz periodic excitation against the other side of the second component to be measured. The vibration generated in the second component to be measured 42 serves as a sound wave source and is positively transmitted into the FBG and the second pigtail fiber 12 by the first pigtail fiber 11. The end of the second pigtail fiber 12 reflects the sound wave and transmits it reversely along the optical fiber. The sound waves propagating forward and backward form a standing wave in the second pigtail fiber 12, FBG10 and the first pigtail fiber 11 of the FBG, and the FBG senses the standing wave signal.

[0127] Demodulate the standing wave signal on the FBG into an electrical signal by the edge filtering method. The electrical signal is imported into the data analyzer 26 through the data collector 25. After the data analyzer 26 processes the electrical signal through the data processing software, the response and spectrogram of the sensing system are obtained, and the state of the second component to be measured can be judged according to the response and spectrogram of the sensing system.

[0128] The included angles between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 are respectively Figure 16 (a) 0 degrees; Figure 16 (b) 90 degrees; Figure 16 (c) 10 degrees; Figure 16 (d) 20 degrees; Figure 16 (e) 30 degrees.

[0129] Reference Figure 16 As shown, when the angle between the axis of the hollow cylinder 32 and the axis of the bolt 33 is 0 degrees or 90 degrees, the periodic signal response spectrum is concentrated at 3300 Hz. When the angle is 0 degrees, the response amplitude is the largest; when the angle is 90 degrees, the amplitude is smaller, but larger than that at other angles. It can be seen from the spectrum that when the energy is concentrated at one frequency, the structure has symmetry (corresponding to the angle between the axis of the hollow cylinder 32 and the axis of the bolt 33 being 0 degrees or 90 degrees); and the stronger the signal, the better the symmetry (corresponding to the angle between the axis of the hollow cylinder 32 and the axis of the bolt 33 being 0 degrees). When the structure is not symmetric, the spectrum is dispersed and the signal amplitude is weak (corresponding to Figure 16 (the cases indicated by (c), (d), and (e)). Therefore, the symmetry state of the structural member can be judged by this feature.

[0130] Figure 17 For Figure 11 the impulse response spectrum of the test system under different displacement conditions of the solid cylinder of the second component to be tested in the z direction as shown.

[0131] Monitor the translation of the internal solid cylinder 34 of the second component to be tested 42 along the z-axis:

[0132] Let the initial working state of the second component to be tested be that there is a distance of 51.3 mm between the bottom of the top hollow cylinder 32 and the top of the columnar cylinder 31 with a cavity, and they are still fastened by the bolt 33. During the working process, due to the loosening of the fastening bolt 33, the top hollow cylinder 32 may slide downward, resulting in a decrease in the volume of the cavity part of the columnar cylinder 31. In this non-destructive testing example, the impulse excitation method is used to test the translation of the solid cylinder 34 in the second component to be tested 42 along the z-axis, and the change characteristics of its impulse response spectrum are obtained, as Figure 16 shown. Based on this as the detection basis, its relative position is evaluated.

[0133] Press the exciter that generates the impulse excitation against the other side of the second component to be tested, and the vibration generated in the second component to be tested is used as the sound wave source and is positively transmitted into the FBG and the second optical fiber 12 by the first optical fiber 11. The end of the second optical fiber 12 reflects the sound wave and transmits it reversely along the optical fiber. The sound waves propagating forward and reversely form a standing wave in the second optical fiber 12, the FBG 10, and the first optical fiber 11 of the FBG, and the FBG senses the standing wave signal.

[0134] Use the edge filtering method to demodulate the standing wave signal on the FBG into an electrical signal. The electrical signal is imported into the data analyzer 26 through the data collector 25. After the data analyzer 26 processes the electrical signal through the data processing software, the response and spectrum diagram of the sensing system are obtained, and the state of the solid cylinder of the second component to be tested can be judged according to the response and spectrum diagram of the sensing system.

[0135] Under the condition of different displacements of the solid cylinder in the z direction, the impulse response of the test system is measured. The fastening displacements in the z direction are respectively: Figure 17 (a) 51.3 mm; Figure 17 (b) 45.9 mm; Figure 17 (c) 26.7 mm; Figure 17 (d) 13 mm; Figure 17 (e) 2.9 mm; Figure 17 (f) 0 mm.

[0136] As can be seen from Figure 17 (a), the impulse response spectrum at the starting state of work mainly contains three main peaks, namely the signals around 1500 Hz, 2400 Hz and 3300 Hz. As the solid cylinder 34 moves downward along the z-axis direction, the peak values of the main peaks gradually move to higher frequencies. Figure 17 In (b), the signal with a frequency of 1500 Hz becomes weaker compared to Figure 17 (a), but the signal around 3300 Hz becomes stronger; Figure 17 In (c), the energy is mainly concentrated around 2400 Hz; Figure 17 The main energy of (d)-(e) becomes around 2400 Hz and around 3300 Hz; Figure 17 In (f), the main peak further changes to a higher frequency, and the main frequencies are around 1650 Hz, 3300 Hz, 5250 Hz and 6450 Hz. Therefore, through this feature, the degree of slip of the solid cylinder 34 in the cavity can be evaluated.

[0137] Figure 18 For Figure 11 the impulse response spectrum of the solid cylinder of the to-be-tested component two shown in the figure in the z-direction displacement.

[0138] For example, if the impulse response spectra of the structural components are measured as Figure 18 (a), (b), (c), it can be judged by comparing the spectrograms: Figure 18 (a) No slip occurs, Figure 18 (b) The slip amount is about 3 mm, Figure 18 (c) The slip amount is about 27 mm.

[0139] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting it; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A non-destructive testing method based on structural vibration - fiber optic acoustic waveguide sensing, wherein: A section of the first pigtail of the FBG is adhered to one side of the component to be tested as the adhered section. The first pigtail, the FBG, and the second pigtail are placed in space without restraint, and the end of the second pigtail is placed in air or a reflective medium; A periodic excitation source or an impulse excitation source is applied to the other side of the component to be tested. The vibration generated in the component to be tested serves as an acoustic wave source and is positively transmitted into the FBG and the second pigtail through the first pigtail. The end of the second pigtail reflects the acoustic wave and transmits it back along the optical fiber. The acoustic waves propagating forward and backward form a standing wave in the second pigtail, the FBG, and the first pigtail of the FBG, and the FBG senses the standing wave signal; Demodulate the standing wave signal on the FBG into an electrical signal; Analyze the electrical signal to judge the state of the component to be tested; The FBG is located at the antinode of the standing wave, and the adhered section is located at the antinode of the standing wave; Within the effective vibration frequency range where the FBG can sense and the vibration of the component to be tested can be excited, among the integral multiple frequencies of the periodic excitation frequency, select the frequency closest to the vibration frequency with the largest amplitude as the reference frequency. Calculate the wavelength of the reference frequency based on the constant wave speed in the optical fiber. The length D1 from the adhered section of the first pigtail to the fiber grating is an integer multiple of half the wavelength of the reference frequency, and the length D2 of the second pigtail is an integer multiple of half the wavelength of the reference frequency.

2. The non-destructive testing method according to claim 1, wherein, Perform a finite element simulation modal analysis on the component to be tested. Take the mode that generates linear strain as the target mode. Use the direction indicating the generation of uniform linear strain in the finite element simulation modal analysis as the direction in which the adhered section is adhered to the component to be tested; the adhered area where the adhered section is adhered to the component to be tested needs to be in the area where the strain amplitudes of each mass point indicated in the finite element simulation modal analysis of the target mode are consistent.

3. The non-destructive testing method according to claim 1, wherein, The method for determining the excitation frequency of the periodic excitation source includes: performing a finite element simulation modal analysis on the component to be tested, taking the mode that generates linear strain as the target mode, obtaining the simulated natural frequency of the target mode, and making the frequency corresponding to the higher harmonic of the periodic excitation near the simulated natural frequency of the target mode; Experimental calibration: Adhere the FBG to the area where the strain amplitudes of each mass point indicated in the finite element analysis of the target mode of the component to be tested in the direction indicating the generation of uniform linear strain in the finite element analysis of the target mode. According to the action point and the direction of the force in the actual non-destructive testing, perform impulse excitation, observe the effective vibration frequency range and amplitude of the impulse response, and take the frequency corresponding to the maximum amplitude value in the spectrogram as the experimental natural frequency, so that the integral multiple frequencies of the excitation frequency are further close to the simulated natural frequency of the target mode and the experimental natural frequency.

4. The non-destructive testing method according to claim 3, wherein, Select the frequency closest to the vibration frequency with the largest amplitude indicated in the finite element simulation modal analysis of the component to be tested as the reference frequency.

5. The non-destructive testing method according to claim 3, wherein, Select the frequency closest to the vibration frequency with the largest amplitude indicated by the impulse response as the reference frequency.

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