A method for improving the signal-to-noise ratio of nondestructive testing signals based on structural vibration-fiber acoustic guided wave sensing

Abstract

The present invention provides a method for improving the signal-to-noise ratio of a nondestructive testing signal based on structural vibration-fiber acoustic guided wave sensing, wherein a section of a first pigtail of a FBG is pasted on one side of a component to be tested as a pasting section, the first pigtail, the FBG and the second pigtail are placed in space without constraints, and the end of the second pigtail is placed in air or a reflective medium; within the effective vibration frequency range that the FBG can effectively sense and the vibration of the component to be tested can be 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 using the constant wave velocity in the optical fiber, the length D1 from the pasting 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. The present invention designs the length D1 from the pasting section of the first pigtail to the fiber grating and the length D2 of the second pigtail by the reference frequency, so as to effectively improve the signal-to-noise ratio of the nondestructive testing signal.

Description

Technical Field

[0001] The present invention relates to the field of nondestructive testing, and in particular to a method for improving the signal-to-noise ratio of a nondestructive testing signal based on structural vibration-optical fiber acoustic guided wave sensing and a nondestructive testing method. Background Art

[0002] Various structural dynamic characteristics analysis, including modal analysis, harmonic response analysis, and transient dynamic analysis, can reflect the material characteristics and working status of the structure, and is a powerful means of structural health detection. The sensing device of dynamic characteristics, namely vibration sensor, is an essential equipment for such detection. The main sensing systems currently used include optical fiber sensing system, piezoelectric sensing system and eddy current displacement sensing system. Among them, the main advantages of piezoelectric sensing system are high sensitivity, small size, light weight, stable performance and wide bandwidth. Its disadvantage is that the sensing performance will be greatly affected in high temperature, large temperature change, humid or strong magnetic field environment. Eddy current displacement sensor is suitable for testing the dynamic characteristics of rotating parts and has the advantage of a wide response frequency band, but it is also greatly affected by temperature and electromagnetic field.

[0003] Optical fiber sensing has the outstanding advantages of 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 is the Fiber Bragg Grating (FBG) vibration sensor, which has the advantages of stable signals, multiplexing, and is not affected by light source intensity and fiber bending. It has been widely used in the fields of petroleum, transportation, national defense, machinery, etc.

[0004] At present, according to the FBG assembly method, FBG vibration sensors are mainly divided into three categories. One is full-stick assembly. That is, the FBG is directly pasted on the surface of the component to be tested or embedded in the elastic material to directly sense the vibration of the component to be tested. It can prevent the sensor from being damaged, but it also makes the sensor unable to be reused; on the other hand, due to the full-stick packaging method, the FBG may produce chirp due to uneven stress, causing the FBG spectrum to deform, and thus making the test results inaccurate.

[0005] The second type is a two-point assembly. That is, the FBG is suspended by fixing the pigtails at both ends of the FBG. When the fixed point of the FBG pigtail is displaced, the FBG is strained, 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 FBG sensing element, and the volume and mass are not suitable for small areas. Vibration testing in orthogonal directions can be achieved through appropriate structural design, but vibrations in non-orthogonal directions cannot be distinguished.

[0006] The third packaging method is suspended assembly. That is, the pigtail at one end of the fiber grating is fixed in a sealed structure, and the pigtail at the other end of the fiber grating is extremely short and suspended in the air. The length from the fixed pigtail of FBG to the other end of the pigtail is controlled at 20mm-80mm. This limitation is due to the fact that if the length is too short, the vibration amplitude will be too small, affecting the sensitivity; if the length is too long, the vibration frequency will be affected by the gravity of the optical fiber itself, and the test result will be inaccurate. Reasonable design of the optical fiber diameter and suspension length can make this sensor more sensitive, and it is suitable for testing micro-vibrations of large structures. However, this packaging requires a closed FBG structure, which is not conducive to small spaces. This packaging method senses all modes of vibration, and the FBG response signal is complex, which brings great difficulty to signal analysis.

[0007] When performing nondestructive testing on the component to be tested, if the signal-to-noise ratio of the nondestructive testing signal is too low, it will affect the accuracy of the nondestructive testing effect. Summary of the invention

[0008] The present invention provides a method for improving the signal-to-noise ratio of a nondestructive testing signal based on structural vibration-optical fiber acoustic guided wave sensing and a nondestructive testing method to solve one or more of the above problems.

[0009] According to one aspect of the present invention, a method for improving the signal-to-noise ratio of a nondestructive testing signal based on structural vibration-fiber acoustic wave sensing is provided, wherein a section of a first pigtail of FBG is pasted to one side of a component to be tested as a pasted section, the first pigtail, FBG and the second pigtail are placed in space without constraints, and the end of the second pigtail is placed in air or a reflective medium; within the effective vibration frequency range that the FBG can effectively sense and the vibration of the component to be tested can be excited, among the integer multiples 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 using the constant wave velocity 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.

[0010] The present invention can effectively improve the signal-to-noise ratio of the nondestructive testing signal by determining the effective vibration frequency range of the component to be tested, selecting the reference frequency to design the length D1 of the sticking section of the first pigtail to the fiber grating and the length D2 of the second pigtail.

[0011] In some embodiments, the present invention selects the frequency of the vibration with the largest amplitude indicated by the target modal simulation analysis in the finite element simulation modal analysis of the component to be tested as the reference frequency; or, selects the frequency of the vibration with the largest amplitude indicated by the impulse response as the reference frequency.

[0012] According to another aspect of the present invention, a nondestructive testing method based on structural vibration-fiber acoustic guided wave sensing is provided, comprising the above-mentioned method for improving the signal-to-noise ratio of nondestructive testing signals based on structural vibration-fiber acoustic guided wave sensing, wherein: 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 is used as a sound wave source and is transmitted forwardly from the first pigtail to the FBG and the second pigtail, the end of the second pigtail reflects the sound wave and transmits it backward along the optical fiber, the sound waves propagating forward and backward form standing waves in the second pigtail of the FBG, the FBG and the first pigtail, and the FBG senses the standing wave signal; the standing wave signal on the FBG is demodulated into an electrical signal; and the electrical signal is analyzed to determine the state of the component to be tested.

[0013] The advantages of the nondestructive testing method based on structural vibration-fiber acoustic guided wave sensing of the present invention are: 1. It can be used to detect the dynamic characteristics of the entire component. Attached to the component is the FBG pigtail, and the spectral characteristics of the FBG itself will not produce chirping phenomena. The FBG can be reused after the test. 2. Since the sound wave can be transmitted over a long distance in the optical fiber, the harsh environment (such as high temperature) at the bonding point can be avoided to directly affect the FBG sensor characteristics. It is also suitable for monitoring the characteristics of small spaces, crowded spaces or components that cannot be directly observed. 3. The second pigtail of FBG is suspended in the air (or reflective medium) at a certain length, and the sound wave transmitted therein forms a backward wave reflection at the end of the pigtail, which is then superimposed with the forward wave to form a standing wave, thereby increasing the sensor test sensitivity; 4. This detection method is suitable for monitoring the dynamic characteristics of the entire structure under low-frequency excitation, reducing the high requirements for the excitation equipment under high-frequency excitation; 5. The sensing design based on high-order resonance can generate larger vibrations with a small energy excitation source, that is, a larger perception 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 non-resonant frequency vibrations.

[0014] FBG is most sensitive to axial strain. It has a certain sensitivity to radial strain, but its sensitivity to radial strain is lower than that to axial strain. FBG does not sense torsional strain. Therefore, FBG has a good perception of vibrations that produce linear strain along the axial direction of the optical fiber. And the vibrations that produce 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 by the present invention. Therefore, the present invention uses the mode that produces linear strain indicated by the simulation analysis based on finite element as the target mode, and determines the pasting direction by the direction of the uniform linear strain indicated in the finite element analysis of the target mode. The pasting area needs to be in the area where the strain amplitude of each particle indicated in the finite element analysis of the target mode is consistent.

[0015] Under the condition of periodic excitation, damped structural parts will cause large but limited vibrations in the frequency band near the natural frequency. Within the safe vibration amplitude range, effective detection of this vibration information can be used to detect the health status of the structural parts. The natural vibration frequency of a structural part is not a fixed value. It is affected by many factors such as the magnitude of the exciting force, the direction of the exciting force, the ambient temperature and even the humidity. Therefore, the natural vibration frequency described in the present invention is actually a natural frequency range. Within this frequency range, the periodic excitation of the exciter can cause vibrations with stable vibration amplitude and a signal-to-noise ratio sufficient for effective structural health detection.

[0016] In some embodiments, the FBG of the present invention is located at the antinode of the standing wave, and the adhesive section is located at the antinode of the standing wave.

[0017] In some embodiments, the present invention performs finite element simulation modal analysis on the component to be measured, takes the mode that produces linear strain as the target mode, and uses the direction of producing uniform linear strain indicated 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 amplitude of each particle indicated in the finite element simulation modal analysis of the target mode is consistent.

[0018] In some embodiments, the method of determining the excitation frequency of a periodic excitation source of the present invention includes: performing finite element simulation modal analysis on the component to be measured, taking the mode that produces linear strain as the target mode, obtaining the simulated natural frequency of the target mode, and making the frequency corresponding to the high-order harmonics of the periodic excitation near the simulated natural frequency of the target mode;

[0019] Experimental calibration: follow the direction of uniform linear strain indicated in the finite element analysis of the target modal, and stick the FBG to the area on the component to be tested where the strain amplitude of each particle is consistent with that indicated in the finite element analysis of the target modal. Perform impulse excitation according to the excitation point and force direction in the actual nondestructive testing, observe the effective vibration frequency range and amplitude of the impulse response, and take the frequency corresponding to the maximum amplitude in the spectrum diagram as the experimental natural frequency; make the integer multiple frequency of the excitation frequency closer to the simulated natural frequency and experimental natural frequency of the target modal.

[0020] In order to design the frequency used for periodic excitation, the present invention uses the natural frequency of the structure as the design basis to obtain a larger sensor sensitivity and signal-to-noise ratio. There are two methods for determining the natural frequency in the present invention. One is to use simulation modal analysis based on finite elements. This method can obtain the vibration mode and simulation natural frequency value of each order mode, but due to the assembly of the structure and the mechanical parameters of the material, there are inevitably errors with the actual data, so the data obtained by simulation cannot be absolutely accurate. The other is the experimental impulse response method. Since the impulse excitation contains all frequencies in theory, the frequency corresponding to the larger amplitude vibration indicated in the spectrum obtained by the impulse response is the actual possible vibration frequency, and the frequency corresponding to the maximum amplitude is the experimental natural frequency value under the impulse excitation condition. However, since the actual natural frequency is affected by the magnitude of the exciting force, if periodic excitation is used, the intensity of its excitation is different from the intensity of the impulse excitation, so its actual natural frequency will be different from the experimental natural frequency obtained by the impulse excitation. Therefore, in view of the complexity of the natural frequency, the present invention comprehensively refers to the simulation natural frequency of the target mode obtained in the simulation modal analysis and the experimental natural frequency obtained by the experiment.

[0021] Consider 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 accumulation of DC component, first harmonic component with frequency f, second harmonic component with frequency 2f and nth harmonic component with frequency nf. The first harmonic is called the fundamental wave, and the second and higher harmonics are called higher harmonics. That is:

[0022]

[0023] 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 each harmonic component. Therefore, the frequency of each order of high-order harmonics of the periodic excitation is close to the simulation natural frequency and the experimental natural frequency, which can effectively excite the target mode and can be effectively sensed by the FBG sensor system, thereby performing effective non-destructive testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic structural diagram of a nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing according to an embodiment of the present invention;

[0025] Figure 2 A schematic diagram of an acoustic wave transmitted in an optical fiber being reflected by the interface with air at the end of the second pigtail and forming a standing wave in the optical fiber;

[0026] Figure 3A schematic diagram of the structure of a component to be tested and the relationship between the FBG pigtail and the component to be tested in a nondestructive testing system based on structural vibration-fiber acoustic guided wave sensing according to an embodiment of the present invention;

[0027] Figure 4 for Figure 3 The finite element-based simulation modal analysis results of the component to be tested are shown;

[0028] Figure 5 Paste FBG in different directions Figure 3 The frequency spectrum of the impulse response when the component under test is put on is shown;

[0029] Figure 6 for Figure 3 The sensor system response and spectrum diagram under 314Hz periodic excitation in the test component 1 and FBG assembly form shown, where D1 and D2 are half wavelength or integer multiples of half wavelength corresponding to the reference frequency;

[0030] Figure 7 for Figure 3 The sensor system response and spectrum diagram under 314Hz periodic excitation in the test component 1 and FBG assembly form shown, where D1 and D2 are not half wavelength or integer multiples of half wavelength corresponding to the reference frequency;

[0031] Figure 8 for Figure 3 The sensor system response waveform and spectrum diagram under 314Hz periodic excitation under different pre-tightening torque conditions between the screw and the internal threaded hole on the test component 1 shown;

[0032] Fig. 9 for Figure 3 The response spectrum of the sensor system under impulse excitation under different displacement conditions of the component to be tested as a whole along the z direction is shown;

[0033] Fig.10 for Figure 3 The sensor system response spectrum of the component under test 1 under the condition of displacement in the z direction when the impulse is excited; wherein D1 and D2 are half wavelengths corresponding to the reference frequency;

[0034] Fig.11 A schematic diagram of the structure of a second component to be tested and the relationship between the FBG and its pigtail and the second component to be tested in a nondestructive testing system based on structural vibration-fiber acoustic guided wave sensing according to an embodiment of the present invention;

[0035] Fig.12 for Fig.11 The finite element-based simulation modal analysis results of the second component to be tested are shown;

[0036] Fig.13The impulse response spectrum diagram when the FBG is pasted on the second component to be tested in different directions;

[0037] Fig.14 for Fig.11 The sensor system response and spectrum diagram under 330Hz periodic excitation in the second component under test and FBG assembly shown, where D1 and D2 are both half wavelengths corresponding to the reference frequency;

[0038] Fig.15 for Fig.11 The sensor system response and spectrum diagram under 330Hz periodic excitation in the second component to be tested and the FBG assembly form shown, where neither D1 nor D2 or one of them is a half wavelength or an integer multiple of a half wavelength corresponding to the reference frequency;

[0039] Fig.16 for Fig.11 Under the conditions of the second component to be tested and the FBG assembly shown, D1 and D2 are both half wavelengths corresponding to the reference frequency, the hollow cylinder and the fastening screw are at different angles, and the sensor system response and spectrum diagram under 330Hz periodic excitation;

[0040] Fig.17 for Fig.11 Under the conditions of the second component to be tested and the FBG assembly shown, D1 and D2 are both half-wavelength corresponding to the reference frequency, and the response spectrum of the sensor system under impulse excitation under different displacement conditions of the solid cylinder along the z direction;

[0041] Fig.18 for Fig.11 In the shown component under test 2 and FBG assembly form, D1 and D2 are both the sensor system response spectra under impulse excitation of the solid cylinder displaced in the z direction under the half-wavelength condition corresponding to the reference frequency. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application may be combined with each other.

[0044] Finally, it should be noted that, in this article, relational terms such as first and second, front and back 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 "include" and "comprise" include not only those elements, but also other elements not explicitly listed, or also include elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "include..." do not exclude the existence of other identical elements in the process, method, article or device that includes the elements.

[0045] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0046] Figure 1 The structure of a nondestructive testing system based on structural vibration-fiber acoustic guided wave sensing according to an embodiment of the present invention is schematically shown.

[0047] refer to Figure 1 As shown, the nondestructive testing system based on structural vibration-fiber acoustic guided wave sensing includes an exciter 30, a first pigtail 11, a second pigtail 12 and a Bragg fiber grating (FBG) 10, as well as a demodulation device (including an adjustable 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).

[0048] The exciter 30 may be an exciter that generates periodic excitation or an exciter that generates impulse excitation. The periodic excitation energy generated by the exciter 30 offsets the energy loss caused by structural vibration damping in the structure, forming a vibration with a stable amplitude and limited vibration.

[0049] A section of the first pigtail 11 of the FBG is pasted on one side of the structure to be tested 40 to form a pasting section 13. The first pigtail, FBG and the second pigtail are placed in the space without constraints. As long as the propagation of the acoustic guided wave in the optical fiber is not affected, they can be placed freely and unconstrained on the desktop or in a suspended state, but they 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 the air. The first pigtail 11 of the FBG can be directly pasted on the structure to be tested, or it can be pasted indirectly, such as first pasting the first pigtail 11 on a small intermediate medium block, and then pasting the intermediate medium on the structure to be tested. In other embodiments, the end of the second pigtail 12 can also be placed in other reflective media.

[0050] The component to be tested 40 is fixed by the limiting member 50, and the exciter 30 acts on the other side of the component to be tested 40 to generate vibration in the component to be tested 40. The vibration is used as a sound wave source and is transmitted positively from the first pigtail 11 to the FBG and the second pigtail 12. The end of the second pigtail 12 reflects the sound wave and transmits it backward along the optical fiber. The sound waves propagating forward and backward form standing waves in the second pigtail 12 of the FBG, the FBG and the first pigtail 11, and the FBG can sense the standing wave signal.

[0051] The demodulation device is used to demodulate the standing wave signal on the FBG into an electrical signal. The exciter 30 applies force to the other side of the component to be tested 40 , and the end of the first pigtail 11 of the FBG is connected to the circulator 4 .

[0052] In this embodiment, the edge filtering method is used to demodulate the acoustic information on the FBG, and 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 through the end of the first pigtail 11 to sense the acoustic signal on the FBG. The narrowband light with acoustic 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 acquisition device 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 to be tested 40. Among them, the wavelength of the adjustable narrowband laser 21 is set at the middle of the rising edge or the falling edge of the FBG spectrum.

[0053] In other embodiments, the matched grating method and other methods can also be used to demodulate the acoustic wave information on the FBG. The demodulation device of the matched grating method includes a broadband light source, a matched grating, a circulator and a photodetector. The broadband light emitted by the broadband light source enters the FBG through the circulator, and the reflected light of the FBG enters the matched grating through the circulator. The reflected light of the matched grating enters the photodetector to convert the optical signal into an electrical signal.

[0054] The electrical signal analysis and processing system is used to analyze and process the electrical signal and determine the state of the component to be tested 40 .

[0055] In this embodiment, the electrical signal analysis and processing system includes a data collector 25 and a data analyzer 26. The electrical signal is introduced into the data analyzer 26 through the data collector 25. The data analyzer 26 processes the electrical signal through data processing software to obtain the sensor system response and spectrum diagram to determine the state of the component 40 to be tested. The state of the structural member 40 can also be determined manually based on experience or by using an artificial intelligence algorithm.

[0056] 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 to be tested 40 can be determined according to the time domain waveform.

[0057] Since the optical fiber is thin, sound waves can be transmitted over a long distance in the optical fiber. This detection method can avoid the harsh environment (such as high temperature) at the bonding point from directly affecting the characteristics of the FBG sensor. It is also suitable for monitoring the characteristics of small spaces, crowded spaces or components that cannot be directly observed.

[0058] Figure 2 This is a schematic diagram of the acoustic wave transmitted in the optical fiber being reflected at the interface between the end of the second pigtail and the air or the reflective medium and forming a standing wave in the optical fiber.

[0059] refer to Figure 2 As shown, the end of the second pigtail suspended in the air or reflective medium reflects the sound wave and transmits it in the reverse propagation direction. The sound waves propagating in the forward and reverse directions form standing waves in the second pigtail, FBG and the first pigtail. Since the vibration amplitude is the largest at the antinode of the standing wave, the FBG and the first pigtail adhesive section 13 are placed at the antinode of the standing wave to improve the response sensitivity.

[0060] Forward wave equation:

[0061]

[0062] Inverse wave equation:

[0063]

[0064] Standing wave equation expression:

[0065]

[0066] 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, we can see that the amplitude of the standing wave is twice the amplitude of the forward wave, and the amplitude reaches the maximum value at the half wavelength or integer multiple of the half wavelength of the forward wave, that is, at the antinode of the standing wave.

[0067] Figure 3 The structure of a component to be tested in a nondestructive testing system based on structural vibration-fiber acoustic guided wave sensing according to an embodiment of the present invention and the relationship between the FBG pigtail and the component to be tested are schematically shown.

[0068] refer to Figure 3 As shown, the component to be tested 41 is a rigid polyurethane block embedded with a fastening structure (including screws and internal threaded holes on the rigid polyurethane block that are adapted to the screws), d is the length of the pasting section, D1 is the length of the optical fiber from the pasting section on the first pigtail to 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 pasting direction of the pasting section is the y direction. The area with horizontal lines at the bottom of the component to be tested 41 is the area where the limiting member 50 is limited.

[0069] Figure 4 for Figure 3 The finite element-based simulation modal analysis results of the component to be tested are shown.

[0070] To facilitate pasting FBG, the X plane facing the reader (i.e. the plane perpendicular to the X axis) is used as the pasting plane to be examined. Figure 4 As shown in the figure, the vibration modes of each mode obtained by the finite element simulation method are mainly torsion in the third and fourth order modes, but the fiber Bragg grating is not sensitive to torsion, so the third and fourth order modes will not be sensed by this sensor device; the first, second and fifth order modes of the component to be tested 41 produce relatively uniform linear displacements along the y direction on the x plane, so the y direction is suitable for the pasting direction of the optical fiber. The displacements of each particle on the pasting length are consistent, that is, the same color area in the simulation figure. The fifth order mode in the simulation is not easy to be excited in the actual test (the simulated natural frequency is 5086.5Hz), so the first and second order modes are used as target modes. The simulated natural frequency of the first order mode is 1997.6Hz, and the simulated natural frequency of the second order mode is 2616.0Hz.

[0071] Based on the above analysis, the optical fiber pasting position of the component to be tested 41 in this embodiment is as follows: Figure 3 According to the simulation modal analysis results, in order to make the acoustic waves in the optical fiber in the same phase, the bonding length d is set to 8mm~10mm (refer to the scale of the figure).

[0072] Further experimental calibration is performed to obtain the effective vibration frequency range that can actually be excited and sensed by the FBG. The experimental calibration method is: in the direction of uniform linear strain indicated in the finite element analysis of the target modal (the y direction in this embodiment), the FBG is attached to the region of the target modal finite element analysis on the component to be tested where the strain amplitude of each particle point is consistent (the y direction in this embodiment). Figure 3 In the area shown, the pasting length is 8mm to 10mm). According to the excitation point and force direction in the actual non-destructive testing, impulse excitation is performed, the effective vibration frequency range and amplitude of the impulse response are observed, and the frequency corresponding to the maximum amplitude in the spectrum diagram is taken as the experimental natural frequency. Figure 5 (a) is the impulse response and spectrum obtained.

[0073] Figure 5 Paste FBG in different directions Figure 3 The spectrum diagram of the impulse response when the component under test is installed is shown in the figure.

[0074] For detection Figure 4 The conclusion that the y direction (the direction that produces a more uniform linear displacement) is suitable for the pigtail bonding direction is that the FBG is directly bonded to the Figure 3The X-plane of the component to be tested 41 is pasted in the z direction ( Figure 5 (b)) and at an angle of 45 degrees to the z direction ( Figure 5 (c)). The pasting area is the same as Figure 5 (a), the length is still 8mm~10mm. The specific pasting position and the corresponding impulse response spectrum under the same impulse function excitation are shown in Figure 5 (b) and Figure 5 (c) as shown.

[0075] refer 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 at 1600-4000Hz. The signal amplitude of the pasting method along the z axis is greatly reduced, and the 500-2500Hz signal is the main component. When pasting at an angle of 45 degrees with the z axis, there are certain signals in the frequency range less than 7000Hz, and the signal amplitude is small, and the signal below 4000Hz is the main component. The measured signal-to-noise ratio of the system when pasting along the z direction is 18.069; when pasting at an angle of 45 degrees with the z axis, the signal-to-noise ratio of the system is 23.5958; when pasting along the y direction, the signal-to-noise ratio of the system is 25.2305.

[0076] Therefore, the test results show that: Figure 5 The impulse response signal-to-noise ratio obtained by the pasting method along the y direction shown in (a) is higher than the response signal-to-noise ratio of other pasting directions. This pasting method is better than other pasting methods. The fifth-order mode in the simulation is not easy to excite in the actual test (the simulated natural frequency is 5086.5Hz), so the first and second-order modes are used as target modes.

[0077] Selecting the excitation method: In this embodiment, the impulse response can be repeated after multiple impulse experiments, so impulse excitation can be selected. Since low-energy periodic excitation may have little effect on the component to be tested, both excitation methods can be used.

[0078] Determine the frequency of the excitation source for periodic excitation: Figure 5 (a) The impulse response spectrum analysis obtained in the experiment shows that the effective vibration frequency range of the component under test 41 that FBG can sense is around 1600Hz~3600Hz, among which 1699Hz, 1899Hz, 1999Hz, 2466Hz, 2632Hz, 2866Hz and 3632Hz are the frequencies corresponding to the maximum values ​​obtained by the impulse response, that is, the experimental natural frequency. The vibration amplitude corresponding to the frequency 2466Hz is the largest, and vibrations with large amplitudes can be excited near the 1699Hz-1999Hz interval and the 2466Hz-2866Hz interval. This range covers Figure 4The simulated natural frequency of the first-order mode (1998 Hz) and the simulated natural frequency of the second-order mode (2616 Hz) in the simulated target mode.

[0079] Comprehensively examine the first-order and second-order simulated natural modes, combined with the impulse excitation experimental test results Figure 5 (a) The experimental natural frequency indicated uses a 314Hz periodic signal as the excitation period. In the range of 1600Hz to 3600Hz, the high-order resonant frequencies of 314Hz periodic excitation are 1884Hz, 2512Hz, 2826Hz, 3140Hz and 3454Hz. Some of these frequencies are near the simulated natural frequency and the experimental natural frequency, and some of these frequencies are near the 1699Hz-1999Hz range and the 2466Hz-2866Hz range. Therefore, it can be considered that vibrations of sufficient amplitude can be excited within this range.

[0080] Determine the length D1 of the first pigtail to the FBG and the length D2 of the second pigtail: According to the high-order resonant frequencies of 1884Hz, 2198Hz, 2512Hz, 2826Hz, 3140Hz and 3454Hz in the effective excitation range of 1600Hz to 3600Hz, the wavelength, half wavelength, quarter wavelength and three-quarter wavelength are calculated with the wave speed in the optical fiber as the non-dispersive wave speed of 3743.54m / s (constant) as shown in Table 1. According to the standing wave theory, in order to optimize the signal-to-noise ratio of the FBG sensor, the FBG and the glue section are preferably placed at half a wavelength or an integer multiple of a half wavelength.

[0081] Table 1: Wavelength, half wavelength, quarter wavelength and three quarter wavelength at different frequencies

[0082]

[0083] Depend on Figure 5 (a) The excitation response spectrum shows that the vibration amplitude is the largest near the experimental natural frequency of 2466 Hz. Figure 4 The motion direction of the second-order mode is consistent with the y direction of the pasting direction, which is the mode most likely to be sensed by the FBG. The simulated natural frequency indicated by the second-order mode is 2616Hz. The vibration frequency closest to these two frequencies in Table 1 is 2512Hz. For this reason, we select 2512Hz in Table 1 as the reference frequency to design the first pigtail pasting section to the FBG length D1 and the second pigtail length D2, and its half wavelength is 745.1mm. In other embodiments, the reference frequency can also be determined by frequency sweeping.

[0084] Figure 6 for Figure 3 The sensor system response and spectrum diagram of the component under test are shown under 314Hz periodic excitation.

[0085] The periodic excitation energy generated by the exciter offsets 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 adhesive section of the FBG and the first pigtail at the antinode of the standing wave, two sets of data were taken, D1 = 740mm, D2 = 750mm and D1 = 740mm, D2 = 1495mm, and their time domain wave graphs and spectra were tested as follows: Figure 6 As shown, the signal-to-noise ratios are 33.4365 and 27.0675 respectively.

[0086] Figure 7 for Figure 3 The sensor system response and spectrum diagram of the component under test are shown under 314Hz periodic excitation.

[0087] Among them, select Figure 7 (a) D1: 2 mm, D2: 200 mm; Figure 7 (b)D1: 150mm, D2: 2300mm; Figure 7 (c)D1: 662mm, D2: 1495mm; 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.

[0088] visible, Figure 6 The sensor signal-to-noise ratio is significantly greater than Figure 7 As shown in the sensing signal noise ratio, D1 and D2 taking half wavelength or integer multiples of half wavelength can indeed improve the signal-to-noise ratio of the test signal.

[0089] Figure 8 for Figure 3 The time domain waveform and spectrum diagram of the screw and the matching internal threaded hole on the tested component under different pre-tightening torque conditions are shown.

[0090] According to the screw tightening related theory, the greater the pre-tightening torque, the smaller the vibration loss of the structural member 41. Based on this feature, the tightening state between the screw and the internal threaded hole adapted thereto can be judged.

[0091] A section of the first pigtail 11 of the FBG is pasted to one side of the component to be tested 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 pasted section length is 10mm, and the FBG length is 10mm.

[0092] An exciter that generates a periodic excitation of 314 Hz is brought into contact with the other side of the component to be tested 41. The vibration generated in the component to be tested 41 is used as a sound wave source and is transmitted forwardly from the first fiber pigtail 11 to FBG 10 and the second fiber pigtail 12. The end of the second fiber pigtail 12 reflects the sound wave and transmits it backward along the optical fiber. The forward and reverse propagating sound waves form standing waves in the second fiber pigtail 12, FBG 10 and the first fiber pigtail 11 of the FBG, and the FBG senses the standing wave signal.

[0093] The standing wave signal on the FBG is demodulated into an electrical signal by edge filtering. The electrical signal is introduced into the data analyzer 26 through the data acquisition device 25. The data analyzer 26 processes the electrical signal through the data processing software to obtain the sensor system response and the spectrum diagram. The state of the component to be tested can be determined based on the sensor system response and the spectrum diagram.

[0094] The pre-tightening torque between the screw and the internal threaded hole is (a) 0Ncm; (b) 35Ncm; (c) 70Ncm; (d) 100Ncm

[0095] Table 2: Corresponding relationship between power and torque of a signal in one cycle

[0096]

[0097] The relationship between the signal power and torque in one cycle obtained by application calculation is shown in Table 2. As can be seen from Table 2, as the preload increases, the test signal power becomes stronger. Therefore, the preload state of the fastener can be judged by this test signal.

[0098] Fig. 9 for Figure 3 The impulse response spectrum of the test system is shown under different displacement conditions of the component to be tested as a whole along the z direction.

[0099] Monitor the overall translation of the component under test 41 along the z-axis. The component under test 1 may be loosened from the bottom during operation (the surrounding constraints remain unchanged). This non-destructive testing experiment uses the same intensity impulse excitation to act on the component under test to test the overall translation of the component under test 41 along the z-axis.

[0100] A section of the first pigtail 11 of the FBG is pasted to one side of the component to be tested 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 pasted section length is 10mm, and the FBG length is 10mm.

[0101] The exciter that generates impulse excitation is pressed against the other side of the component to be tested 41. The vibration generated in the component to be tested 41 is used as a sound wave source and is transmitted forwardly from the first pigtail 11 to the FBG and the second pigtail 12. The end of the second pigtail 12 reflects the sound wave and transmits it backward along the optical fiber. The forward and reverse propagating sound waves form standing waves in the second pigtail 12 of the FBG, FBG10 and the first pigtail 11, and the FBG senses the standing wave signal.

[0102] The standing wave signal on the FBG is demodulated into an electrical signal by edge filtering. The electrical signal is introduced into the data analyzer 26 through the data acquisition device 25. The data analyzer 26 processes the electrical signal through the data processing software to obtain the sensor system response and the spectrum diagram. The state of the component to be tested can be determined based on the sensor system response and the spectrum diagram.

[0103] in Fig. 9 (a) Not free from the bottom restraint; Fig. 9 (b)-(e) After being freed from the bottom constraint, the displacements in the z direction are (a) 0 mm; (b) 15.9 mm; (c) 35.9 mm; and (d) 45.9 mm.

[0104] Depend on Fig. 9 It can be seen that when the bottom of the component to be tested is fully constrained (such as Fig. 9 (a) shows that the impulse response spectrum shows multiple peaks below 4000Hz. When the bottom is freed from the bottom constraint along the z direction, the main peak of the impulse response spectrum gradually moves to low frequency as the displacement increases. The spectrum shape also gradually changes from multiple peaks to a single peak. This feature can be used to evaluate the approximate displacement of the entire structure along the z direction.

[0105] Fig.10 for Figure 3 The impulse response spectra of the component under test at three different displacements in the z direction are shown.

[0106] For example, if the impulse response spectrum of the component under test is Fig.10 According to the comparison of the spectra for (a), (b) and (c), the displacements in the z direction are approximately 16 mm, 36 mm and 46 mm, respectively.

[0107] Fig.11 This is a schematic diagram of the structure of a second component to be tested and the relationship between the FBG and its pigtail and the second component to be tested in a nondestructive testing system based on structural vibration-fiber acoustic guided wave sensing according to an embodiment of the present invention.

[0108] refer to Fig.11 As shown, Figure 3 The component to be tested 41 shown in the figure is replaced by Fig.11The second component to be tested 42 is shown. It consists of two parts. The first part is a cylindrical cylinder 31 with a cavity inside, and its bottom is fixed to a limiter 50, and the fixed part is a constraint part; the second part is a solid cylinder 34 with another hollow cylinder 32 on the top; the first part and the second part are fixed by bolts 33; the axial direction of the hollow cylinder 32 is consistent with the axial direction of the bolts 33. All parts of the second component to be tested 42 are made of aluminum alloy.

[0109] Fig.12 for Fig.11 The finite element-based simulation modal analysis results of the second component to be tested are shown.

[0110] refer to Fig.12 As shown, in each order mode, the strain modes of the first order, second order, third order, fourth order, sixth order, seventh order, and eighth order are all linear displacements, with only different directions. The fifth order and ninth order are mainly torsion. Since the optical fiber is insensitive to torsion, the fifth order and ninth order modes will not be sensed by this sensing device. The first, second, third, fourth, sixth, and eighth order modes of the component 2 42 to be tested produce relatively uniform linear displacements along the z direction on the cylindrical surface, so the z direction is a suitable pasting direction for 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 8-15mm (refer to the scale of the figure).

[0111] Fig.13 It is the spectrum diagram under the action of impulse response when FBG is pasted on the second component to be tested in different directions.

[0112] Paste FBG directly at different angles Fig.11 The outer cylindrical surface of the cylindrical cylinder 31 with a cavity inside the second test component 42 is pasted in the z direction, the y direction and the angle of 45 degrees with the z direction. The specific pasting position and the corresponding impulse response spectrum under the same impulse function excitation are as follows: Fig.13 shown.

[0113] Depend on Fig.13 It can be seen that the amplitude of the z-direction pasting method is much larger than that of other pasting methods, and the signal frequency is 1600-7000Hz; when pasting at an angle of 45 degrees to the z-axis, the signal is scattered at 200-12000Hz, and the signal amplitude of each experimental natural frequency is basically the same; while the signal amplitude of the y-axis pasting method is greatly reduced, and the signal of 200-8200Hz is the main component. When pasting along the z direction, the signal-to-noise ratio of the system is 26.7107; when pasting along the direction with an angle of 45 degrees to the z-axis, the signal-to-noise ratio of the system is 24.5548; when pasting along the y direction, the signal-to-noise ratio of the system is 18.9105.

[0114] Therefore, the test results show that the impulse response signal-to-noise ratio obtained by the sticking method in the z direction (the direction that produces a relatively uniform linear displacement) is higher than the response signal-to-noise ratio of other sticking directions, and this sticking direction is superior to other sticking directions.

[0115] Select the excitation method: After multiple impulse tests, the impulse response can be repeated, so impulse excitation can be selected. Since low-energy periodic excitation may have little effect on the component to be tested, both excitation methods can be used.

[0116] Determine the frequency of the excitation source for periodic excitation: Fig.13 (a) Experimental analysis shows that the effective vibration frequency range of the excited component 2 is around 1600Hz-7000Hz. Fig.12 From the simulation analysis based on finite element, it can be seen that since the first and second order modes are not in this effective vibration frequency range, the overall displacement of the third, seventh and eighth order modes is bending, and the fifth and ninth order modes are torsion, which are not sensed by FBG. Therefore, since the fourth and sixth orders correspond to uniform linear strain, the fourth order (simulated natural frequency 1994Hz) and the sixth order (simulated natural frequency 2985Hz) are selected as the target modes. Fig.13 (a) The experimental natural frequencies corresponding to the maximum amplitude values ​​of the measured FBG impulse response are around 1657Hz, 3285Hz, 5342Hz, and 6428Hz. Based on the four experimental natural frequencies and the simulated natural frequencies of the fourth and sixth order modes, a 330Hz periodic signal is selected as the excitation, and the frequencies corresponding to the obtained high-order harmonics are 1650Hz, 1980Hz, 2970Hz, 3300Hz, 3630Hz, 5280Hz, 5610Hz, 5940Hz, 6270Hz, and 6600Hz. Some of the frequencies corresponding to these high-order harmonics are near the fourth and sixth order simulated natural frequencies or experimental natural frequencies. The wavelength, half wavelength, quarter wavelength, and three-quarter wavelength are calculated with the wave speed in the optical fiber being 3743.54m / s, which is non-dispersive, as shown in Table 3.

[0117] Table 3: Wavelength, half wavelength, quarter wavelength and three quarter wavelength at different frequencies

[0118]

[0119]

[0120] Depend on Fig.13(a) It can be seen that the vibration amplitude corresponding to the experimental natural frequency of 3285Hz is the largest. Among the frequencies corresponding to the high-order harmonics of the 330Hz periodic excitation designed by the present invention, 3300Hz is closest to the experimental natural frequency of 3285Hz; 2970Hz is closest to the sixth-order simulation natural frequency of 2985Hz. According to the principle of giving priority to experimental results, 3300Hz is taken as the reference frequency for designing D1 and D2, and its half-wavelength is 567.2mm.

[0121] Fig.14 for Fig.11 The signal time domain waveform and spectrum diagram of the sensor system of the second component to be tested under 330Hz periodic excitation are shown. In order to place the FBG and the adhesive section at the antinode of the standing wave, according to Table 3, D1=567mm, D2=570mm are taken, and the measured signal-to-noise ratio is 19.2236.

[0122] Fig.15 for Fig.11 The time domain waveform and spectrum diagram of the response signal of the sensor device of the second component to be tested under 330Hz periodic excitation are shown.

[0123] Select Fig.15 (a) D1: 430 mm, D2: 637 mm; Fig.15 (b) D1: 567 mm, D2: 637 mm; Fig.15 (c) D1: 567 mm, D2: 697 mm; Fig.15 (d) D1: 567 mm, D2: 727 mm; Fig.15 (e) D1: 567 mm, D2: 860 mm; Fig.15 (f) D1: 630 mm, D2: 647 mm; Fig.15 (g) D1: 630 mm, D2: 697 mm; Fig.15 (h) D1: 700mm, D2: 637mm, e.g. Fig.15 (a)- Fig.15 As shown in (h), the corresponding signal-to-noise ratios are 9.4812, 10.7910, 13.7618, 16.8931, 17.7747, 18.0831, 15.0887, and 17.2563, respectively.

[0124] On the other hand Fig.14 The signal-to-noise ratio is 19.2236, so Fig.14 The signal-to-noise ratio of the sensor signal is significantly greater than Fig.15 The signal-to-noise ratio of the sensing signal. D1 and D2 taking half wavelength or integer multiples of half wavelength can indeed improve the response sensitivity and improve the signal-to-noise ratio of the effective test signal.

[0125] Fig.16 for Fig.11The response and spectrum diagram of the detection system when the hollow cylinder and the fastening bolt on the second component to be tested are at different angles.

[0126] To illustrate the application of the present invention, this example is used to test the offset state of the top hollow cylinder axial direction and the bolt axial direction in the second component to be tested. According to the geometric characteristics of the structure, when the axial direction of the top hollow cylinder is consistent with the axial direction of the bolt or perpendicular to it, the entire structure is symmetrical; the symmetry is higher when the axial direction of the top hollow cylinder is consistent with the axial direction of the bolt. This embodiment uses the offset of the axial direction of the top hollow cylinder and the axial direction of the bolt as the test target for non-destructive testing. When Fig.11 When the second component to be tested is in operation, the axial direction of the top hollow cylinder 32 and the axial direction of the bolt 33 may be offset.

[0127] Press the first pigtail 11 Fig.13 It is pasted on the second component to be tested in the direction shown in (a), with a pasting length of 10 mm, D1 of 567 mm, and D2 of 570 mm, for non-destructive testing of the second component to be tested.

[0128] The exciter that generates 330Hz periodic excitation is pressed against the other side of the second component to be tested. The vibration generated in the second component to be tested 42 is used as the sound wave source and is transmitted forwardly from the first pigtail 11 to the FBG and the second pigtail 12. The end of the second pigtail 12 reflects the sound wave and transmits it backward along the optical fiber. The forward and reverse propagating sound waves form standing waves in the second pigtail 12 of the FBG, FBG10 and the first pigtail 11, and the FBG senses the standing wave signal.

[0129] The standing wave signal on the FBG is demodulated into an electrical signal by edge filtering. The electrical signal is introduced into the data analyzer 26 through the data acquisition device 25. The data analyzer 26 processes the electrical signal through the data processing software to obtain the sensor system response and the spectrum diagram. The state of the second component to be tested can be determined based on the sensor system response and the spectrum diagram.

[0130] The angles between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 are Fig.16 (a) 0 degrees; Fig.16 (b) 90 degrees; Fig.16 (c) 10 degrees; Fig.16 (d) 20 degrees; Fig.16 (e) 30 degrees.

[0131] refer to Fig.16As shown in the figure, when the angle between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 is 0 degrees or 90 degrees, the spectrum of the periodic signal response is concentrated at 3300Hz, and the response amplitude is the largest when the angle is 0 degrees; the amplitude is smaller when the angle is 90 degrees, but the amplitude is larger than that at other angles; it can be seen from the spectrum that when the energy is concentrated at one frequency, the structure is symmetrical (corresponding to the angle between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 is 0 degrees or 90 degrees); and the stronger the signal, the better the symmetry (corresponding to the angle between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 is 0 degrees); when the structure is not symmetrical, the spectrum is dispersed and the signal amplitude is weak (corresponding to Fig.16 (c), (d), and (e)). Therefore, this feature can be used to determine the symmetry of a structural component.

[0132] Fig.17 for Fig.11 The impulse response spectrum of the test system under different displacement conditions of the solid cylinder of the second component to be tested along the z direction is shown.

[0133] Monitor the internal solid cylinder 34 of the second component 42 to be tested to translate along the z-axis:

[0134] The working initial state of the second component to be tested is that there is a distance of 51.3 mm between the bottom of the top hollow cylinder 32 and the top of the cylindrical cylinder 31 with a cavity, and they are still fastened by bolts 33. During the working process, the top hollow cylinder 32 may slide downward due to the loosening of the fastening bolts 33, thereby causing the volume of the cavity part of the cylindrical cylinder 31 to decrease. This non-destructive testing embodiment uses an impulse excitation method to test the translation of the solid cylinder 34 in the second component to be tested 42 along the z-axis, and obtains its impulse response spectrum change characteristics, such as Fig.16 This is used as a test basis to evaluate its relative position.

[0135] The exciter that generates impulse excitation is pressed against the other side of the second component to be tested. The vibration generated in the second component to be tested is used as a sound wave source and is transmitted forwardly from the first pigtail 11 to the FBG and the second pigtail 12. The end of the second pigtail 12 reflects the sound wave and transmits it backward along the optical fiber. The forward and reverse propagating sound waves form standing waves in the second pigtail 12 of the FBG, FBG10 and the first pigtail 11, and the FBG senses the standing wave signal.

[0136] The standing wave signal on the FBG is demodulated into an electrical signal by edge filtering. The electrical signal is introduced into the data analyzer 26 through the data acquisition device 25. The data analyzer 26 processes the electrical signal through the data processing software to obtain the sensing system response and the spectrum diagram. The state of the solid cylinder of the second component to be tested can be judged according to the sensing system response and the spectrum diagram.

[0137] The impulse response of the test system is tested under different displacement conditions of the solid cylinder along the z direction. The fastening displacements in the z direction are: Fig.17 (a) 51.3 mm; Fig.17 (b) 45.9 mm; Fig.17 (c) 26.7 mm; Fig.17 (d) 13 mm; Fig.17 (e) 2.9 mm; Fig.17 (f)0mm.

[0138] Depend on Fig.17 (a) It can be seen that the impulse response spectrum of the initial working state mainly includes three main peaks, namely signals around 1500 Hz, 2400 Hz and 3300 Hz. As the solid cylinder 34 moves downward along the z-axis direction, the peaks of the main peaks gradually move to higher frequencies. Fig.17 (b) The signal with a frequency of 1500 Hz is Fig.17 (a) becomes weaker, but the signal near the frequency of 3300 Hz becomes stronger; Fig.17 (c) The energy is mainly concentrated around 2400 Hz; Fig.17 The main energy of (d)-(e) becomes around 2400Hz and around 3300Hz; Fig.17 The main peaks in (f) further change to high frequencies, with the main frequencies being around 1650 Hz, 3300 Hz, 5250 Hz and 6450 Hz. Therefore, this feature can be used to evaluate the degree of slippage of the solid cylinder 34 in the cavity.

[0139] Fig.18 for Fig.11 The impulse response spectrum of the solid cylinder of the second component to be tested when displaced in the z direction is shown.

[0140] For example, if the impulse response spectrum of the structural component is measured to be Fig.18 By comparing the spectra of (a), (b), and (c), we can determine: Fig.18 (a) No slip occurs, Fig.18 (b) The slip is about 3 mm. Fig.18 (c) The slippage is about 27 mm.

[0141] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for improving the signal-to-noise ratio of nondestructive testing signals based on structural vibration-fiber acoustic guided wave sensing, wherein: A section of the first pigtail of the FBG is pasted to one side of the component to be tested as a pasted section, the first pigtail, the FBG and the second pigtail are placed in the space without constraints, and the end of the second pigtail is placed in the air or a reflective medium; In the effective vibration frequency range that FBG can effectively sense and the vibration of the component to be tested can be 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, and the wavelength of the reference frequency is calculated using the constant wave velocity in the optical fiber. The length D1 from the bonding 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 method for improving the signal-to-noise ratio according to claim 1, wherein: The vibration frequency with the largest amplitude indicated by the finite element simulation modal analysis of the component to be tested is selected as the reference frequency.

3. The method for improving the signal-to-noise ratio according to claim 1, wherein: The frequency closest to the vibration frequency with the largest amplitude indicated by the impulse response is selected as the reference frequency.

4. A nondestructive testing method based on structural vibration-optical fiber acoustic guided wave sensing, comprising the method for improving the signal-to-noise ratio of a nondestructive testing signal based on structural vibration-optical fiber acoustic guided wave sensing according to any one of claims 1 to 3, wherein: 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 is used as an acoustic wave source and transmitted forward from the first pigtail to the FBG and the second pigtail. The end of the second pigtail reflects the acoustic wave and transmits it backward along the optical fiber. The forward and reverse propagating acoustic waves form standing waves in the second pigtail of the FBG, the FBG and the first pigtail, and the FBG senses the standing wave signal. Demodulating the standing wave signal on the FBG into an electrical signal; The electrical signal is analyzed to determine the state of the component to be tested.

5. The nondestructive testing method according to claim 4, wherein: The FBG is located at the antinode of the standing wave, and the adhesive section is located at the antinode of the standing wave.

6. The nondestructive testing method according to claim 5, wherein: Finite element simulation modal analysis is performed on the component to be tested, the mode that produces linear strain is taken as the target mode, and the direction of producing uniform linear strain indicated in the finite element simulation modal analysis is used as the direction in which the pasting section is pasted on the component to be tested; the pasting area where the pasting section is pasted on the component to be tested needs to be in the area where the strain amplitude of each particle indicated in the finite element simulation modal analysis of the target mode is consistent.

7. The nondestructive testing method according to claim 6, wherein: The method for determining the excitation frequency of the periodic excitation source includes: performing finite element simulation modal analysis on the component to be tested, taking the mode that produces linear strain as the target mode, obtaining the simulated natural frequency of the target mode, and making the frequency corresponding to the high-order harmonics of the periodic excitation near the simulated natural frequency of the target mode; Experimental calibration: follow the direction of uniform linear strain indicated in the finite element analysis of the target modal, and stick the FBG to the area where the strain amplitude of each particle indicated in the finite element analysis of the target modal on the component to be tested is consistent. According to the excitation point and force direction 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 in the spectrum diagram as the experimental natural frequency; The integer multiple frequencies of the excitation frequency are made closer to the simulated natural frequencies and experimental natural frequencies of the target mode.

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