Nondestructive testing signal screening method and nondestructive testing method based on structural vibration-fiber acoustic guided wave sensing

Abstract

This invention provides a method for screening nondestructive testing signals based on structural vibration-fiber acoustic guided wave sensing. Finite element simulation modal analysis is performed on the component under test. The mode that produces linear strain is selected as the target mode. The direction of uniform linear strain indicated in the finite element simulation modal analysis is used as the direction in which the sensing device is attached to the component under test. The area where the sensing device is attached to the component under test must be within an area where the strain amplitudes of each particle indicated in the finite element simulation modal analysis of the target mode are consistent. This method, based on the finite element simulation modal analysis of the component under test, determines the attachment direction and area of the sensing device, and can produce a nondestructive testing signal with a high signal-to-noise ratio.

Description

Technical Field

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

[0002] Various types of 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 are a powerful means of structural health detection. Dynamic characteristics sensing devices, namely vibration sensors, are essential equipment for such detection. The main sensing systems currently used 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 bandwidth. Its disadvantage is that the sensing performance will be greatly affected in high temperature, large temperature changes, humid or strong magnetic field environments. Eddy current displacement sensors are suitable for testing the dynamic characteristics of rotating parts and have the advantage of a wide response frequency band, but they are also greatly affected by temperature and electromagnetic fields.

[0003] Fiber optic sensors, with their outstanding advantages such as thin diameter, high sensitivity, immunity to electromagnetic interference, light weight, and distributed measurement capabilities, have become a prominent representative of the new generation of vibration sensors. The most successful example is the Fiber Bragg Grating (FBG) vibration sensor. In addition to the advantages of conventional fiber optic sensors, it also offers signal stability, multiplexing capabilities, and immunity to light source intensity and fiber bending. It is currently widely used in the petroleum, transportation, defense, and machinery industries.

[0004] Currently, FBG vibration sensors are primarily categorized into three types based on their assembly method. The first type is fully bonded assembly. This involves directly attaching the FBG to the surface of the component under test or embedding it within an elastic material, directly sensing the component's vibration. This approach protects the sensor from damage, but also limits its reusability. Furthermore, due to the fully bonded packaging method, the FBG may experience uneven stress, resulting in chirping, distorting the FBG spectrum and leading to inaccurate test results.

[0005] The second type is a two-point assembly. This involves fixing the fiber Bragg grating (FBG) pigtails at both ends, leaving the FBG suspended in the air. Displacement of the FBG pigtail's fixing points causes strain in the FBG, shifting the FBG's center wavelength and detecting vibration signals. This type of sensing structure requires additional auxiliary structures in addition to the FBG sensor element, and its size and weight make it unsuitable for confined areas. While appropriate structural design can enable vibration testing in orthogonal directions, it cannot distinguish vibrations in non-orthogonal directions.

[0006] The third packaging method is suspended assembly. This involves securing one end of the FBG pigtail within a sealed structure, while the other end is extremely short and suspended in mid-air. The length from the fixed FBG pigtail to the other end is controlled between 20mm and 80mm. This limitation is due to the fact that if the length is too short, the vibration amplitude will be too small, affecting sensitivity; while if the length is too long, the vibration frequency will be affected by the gravity of the fiber itself, resulting in inaccurate test results. Reasonable design of the fiber diameter and suspension length can make this sensor highly sensitive, making it suitable for testing micro-vibrations of large structures. However, this packaging method requires a sealed FBG structure, which is not suitable for confined spaces. This packaging method senses all modes of vibration, and the FBG response signals are complex, making signal analysis very difficult.

[0007] When performing nondestructive testing on a component to be tested, signals will be emitted in all directions of the component to be tested. Excessive signals will interfere with the nondestructive testing effect. Summary of the Invention

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

[0009] According to one aspect of the present invention, a method for screening nondestructive testing signals based on structural vibration-fiber acoustic guided wave sensing is provided, wherein a 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 sensing device is attached to the component to be tested; the attachment area of the sensing device on the component to be tested needs to be an area where the strain amplitudes of each particle indicated in the finite element simulation modal analysis of the target mode are consistent.

[0010] The present invention performs finite element simulation modal analysis on the component to be tested, and the determined adhesion direction and area of the sensing device can obtain a non-destructive testing signal with a high signal-to-noise ratio.

[0011] According to another aspect of the present invention, a nondestructive testing method based on structural vibration-fiber acoustic guided wave sensing is also provided, which includes the above-mentioned screening method for nondestructive testing signals based on structural vibration-fiber acoustic guided wave sensing, wherein: the sensing device includes an FBG, and a first pigtail and a second pigtail of the FBG; a section of the first pigtail 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 constraints, 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 is used as an acoustic wave source and is transmitted forward from the first pigtail to the FBG and the second pigtail, and the end of the second pigtail reflects the acoustic wave and transmits it in the reverse direction along the optical fiber, and the forward and reverse propagating acoustic waves form standing waves 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; and the electrical signal is analyzed to determine the state of the component to be tested.

[0012] The advantages of the nondestructive testing method based on structural vibration and fiber-optic acoustic guided wave sensing of the present invention are as follows: 1. It can be used to detect the dynamic characteristics of the entire component. Attached to the component is the FBG pigtail, the spectral characteristics of the FBG itself will not produce chirping, and the FBG can be reused after the test. 2. Because sound waves can be transmitted over long distances through the optical fiber, the harsh environment (such as high temperature) at the attachment point can be prevented from directly affecting the FBG sensor characteristics. It is also suitable for monitoring the characteristics of small and crowded spaces or components that cannot be directly observed. 3. The second fiber pigtail of FBG is suspended in the air (or reflective medium) at a certain length. The sound wave transmitted therein forms a backward wave reflection at the end of the fiber 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.

[0013] FBG is most sensitive to axial strain. It has a certain sensitivity to radial strain, but its sensitivity to radial strain is lower than its sensitivity to axial strain. FBG does not sense torsional strain. Therefore, FBG has a good sense for 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 finite element simulation analysis as the target mode, and determines the pasting direction based on 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.

[0014] Under periodic excitation, damped structural components will experience large but limited vibrations in a frequency band near their natural frequency. Effectively detecting this vibration information within a safe vibration amplitude range can be used to monitor the health of the structural component. The natural vibration frequency of a structural component is not a fixed value; it is affected by numerous factors, including the magnitude and direction of the excitation force, ambient temperature, and even humidity. Therefore, the natural vibration frequency described in this invention is actually a natural frequency range within which periodic excitation by the exciter can induce vibrations with a stable amplitude and a signal-to-noise ratio sufficient for effective structural health monitoring.

[0015] 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.

[0016] 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 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 higher harmonics of the periodic excitation near the simulated natural frequency of the target mode; experimental correction: in the direction of producing uniform linear strain indicated in the finite element analysis of the target mode, the FBG is pasted to the area where the strain amplitude of each particle indicated in the finite element analysis of the target mode on the component to be tested is consistent, and impulse excitation is performed according to the excitation point and force direction in actual non-destructive testing, observing the effective vibration frequency range and amplitude of the impulse response, and taking the frequency corresponding to the maximum amplitude in the spectrum diagram as the experimental natural frequency; making the integer multiple frequency of the excitation frequency further close to the simulated natural frequency and experimental natural frequency of the target mode.

[0017] To design the frequency for periodic excitation, the present invention uses the natural frequency of the structure as a design basis to achieve greater sensor sensitivity and signal-to-noise ratio. The present invention uses two methods to determine the natural frequency. One is to use finite element-based simulation modal analysis. This method can obtain the vibration mode shape and simulated natural frequency values of each mode. However, due to the inevitable errors between the assembly conditions of the structure and the mechanical parameters of the materials and the actual data, the data obtained by simulation cannot be absolutely accurate. The other is to experimentally obtain the impulse response method. Since impulse excitation theoretically contains all frequencies, the frequency corresponding to the larger amplitude vibration 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 conditions. However, since the actual natural frequency is affected by the magnitude of the excitation force, if periodic excitation is used, the excitation intensity will differ from the impulse excitation intensity, so its actual natural frequency will also differ from the experimental natural frequency obtained by impulse excitation. Therefore, given the complexity of the natural frequency, the present invention comprehensively refers to the simulated natural frequency of the target mode obtained by simulation modal analysis and the experimental natural frequency obtained by experiment.

[0018] 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:

[0019]

[0020] Where x0 is the DC component of the signal, a n is the amplitude of each harmonic, = is the initial phase of each harmonic, meaning that the periodic excitation actually contains harmonic components. Therefore, the frequencies of the various harmonics of the periodic excitation are close to the simulated and experimental natural frequencies, effectively exciting the target mode and enabling effective sensing by the FBG sensor system, enabling effective nondestructive testing.

[0021] In some embodiments, the present invention selects a vibration frequency closest to the maximum amplitude as a reference frequency among the integer multiples of the periodic excitation frequency within the vibration frequency range that the FBG can effectively sense and the vibration of the component to be measured can be effectively excited, and calculates the wavelength of the reference frequency using the constant wave velocity in the optical fiber. The length D1 of the adhesive section of the first pigtail to the optical 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.

[0022] In some embodiments, the present invention selects the vibration frequency with the largest amplitude indicated by the finite element simulation modal analysis of the component to be tested as the reference frequency; or, selects the vibration frequency with the largest amplitude indicated by the impulse response as the reference frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic structural diagram of a nondestructive testing system based on structural vibration-fiber acoustic guided wave sensing according to an embodiment of the present invention;

[0024] Figure 2 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;

[0025] Figure 3 A 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 one embodiment of the present invention;

[0026] Figure 4 for Figure 3 The finite element-based simulation modal analysis results of the component to be tested are shown;

[0027] Figure 5 Attach FBG in different directions Figure 3 The spectrum of the impulse response when the component under test is placed on the surface is shown in the figure.

[0028] Figure 6 for Figure 3 The sensor system response and spectrum diagram of the component under test and FBG assembly under 314Hz periodic excitation, where D1 and D2 are half-wavelength or integer multiples of half-wavelength corresponding to the reference frequency;

[0029] Figure 7 for Figure 3 The sensor system response and spectrum diagram of the test component 1 and FBG assembly under 314Hz periodic excitation, where D1 and D2 are not half-wavelength or integer multiples of half-wavelength corresponding to the reference frequency;

[0030] 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 are shown;

[0031] Figure 9 for Figure 3 The sensor system response spectrum under impulse excitation under different displacement conditions of the entire component under test along the z direction is shown;

[0032] Figure 10 for Figure 3 The sensor system response spectrum of the component under test 1 under the condition of z-direction displacement under impulse excitation is shown; where D1 and D2 are half wavelengths corresponding to the reference frequency;

[0033] Figure 11 Schematic diagram of the structure of the 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 one embodiment of the present invention;

[0034] Figure 12 for Figure 11 The finite element-based simulation modal analysis results of the second component to be tested are shown;

[0035] Figure 13 The impulse response spectra of the FBG when it is attached to the second component under test in different directions are shown in Figure 2.

[0036] Figure 14 for Figure 11 The sensor system response and spectrum diagram of the second component under test and the FBG assembly form under 330Hz periodic excitation, where D1 and D2 are half the wavelength corresponding to the reference frequency;

[0037] Figure 15 for Figure 11 The sensor system response and spectrum diagram of the second component under test and the FBG assembly form under 330Hz periodic excitation, where neither D1 nor D2 is equal to or one of the half-wavelengths corresponding to the reference frequency is equal to or equal to an integer multiple of a half-wavelength.

[0038] Figure 16 for Figure 11 The sensor system response and spectrum diagram under 330Hz periodic excitation are shown for the second component under test and the FBG assembly, with D1 and D2 at half the wavelength corresponding to the reference frequency, and the hollow cylinder and the fastening screw at different angles.

[0039] Figure 17 for Figure 11 The sensor system response spectrum under impulse excitation under different displacement conditions of the solid cylinder along the z direction under the condition that D1 and D2 are both half the wavelength corresponding to the reference frequency under the condition of the second component under test and the FBG assembly shown;

[0040] Figure 18 for Figure 11 In the shown test component 2 and FBG assembly form, D1 and D2 are both the sensor system response spectra under impulse excitation of the solid cylinder displacement in the z direction under the half-wavelength condition corresponding to the reference frequency. DETAILED DESCRIPTION

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only 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 making creative efforts shall fall within the scope of protection of the present invention.

[0042] It should be noted that, unless there is any conflict, the embodiments and features in the embodiments of this application can be combined with each other.

[0043] Finally, it should be noted that, in this document, relational terms such as first and second, front and back are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any 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 elements inherent to such process, method, article or device. In the absence of further limitations, the elements defined by the sentence "include..." do not exclude the presence of other identical elements in the process, method, article or device that includes the elements.

[0044] The present invention will be further described in detail below with reference to the accompanying drawings.

[0045] 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.

[0046] 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 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).

[0047] The exciter 30 can be a periodic excitation exciter or an impulse exciter. 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 and limited amplitude.

[0048] The first pigtail 11, the second pigtail 12 and the Fiber Bragg Grating (FBG) 10 are sensing devices. A section of the first pigtail 11 of the FBG is adhered to one side of the component to be measured 40 to form an adhered section 13. The first pigtail, FBG and second pigtail are placed in space without constraints. As long as they do not affect the propagation of acoustic guided waves in the optical fiber, they can be placed freely and unconstrained on a table 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 adhered directly to the structure to be measured, or it can be adhered indirectly, such as first adhering the first pigtail 11 to a small intermediate medium block, and then adhering the intermediate medium to the component to be measured. In other embodiments, the end of the second pigtail 12 can also be placed in other reflective media.

[0049] 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 acts as a sound wave source and is transmitted forward from the first fiber pigtail 11 to the FBG and the second fiber pigtail 12. The end of the second fiber pigtail 12 reflects the sound wave and transmits it in the reverse direction along the optical fiber. The forward and reverse propagating sound waves form standing waves in the second fiber pigtail 12 of the FBG, the FBG and the first fiber pigtail 11, and the FBG can sense the standing wave signal.

[0050] The demodulator 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 .

[0051] In this embodiment, an edge filtering method is used to demodulate the acoustic wave information on the FBG. The demodulation device includes a tunable narrowband laser 21, an isolator 22, a circulator 23, and a photodetector 24. The light emitted by the tunable narrowband laser 21 passes through the isolator 22 and the circulator 23 and the end of the first pigtail 11 into the FBG 10 to sense the acoustic wave signal on the FBG. The narrowband light carrying the acoustic wave information in the FBG 10 is reflected by the FBG 10 and then passes through the circulator 23 into the photodetector 24 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. The results are used to perform non-destructive testing on the state of the component to be tested 40. The wavelength of the tunable narrowband laser 21 is set to the middle of the rising edge or the falling edge of the FBG spectrum.

[0052] In other embodiments, a matched grating method or other methods can also be used to demodulate the acoustic wave information on the FBG. The demodulation device for the matched grating method includes a broadband light source, a matched grating, a circulator, and a photodetector. Broadband light emitted by the broadband light source enters the FBG through the circulator. The reflected light from the FBG then passes through the circulator and enters the matched grating. The reflected light from the matched grating enters the photodetector, which converts the optical signal into an electrical signal.

[0053] 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 .

[0054] In this embodiment, the electrical signal analysis and processing system includes a data collector 25 and a data analyzer 26. The electrical signal is fed into the data analyzer 26 via the data collector 25. The data analyzer 26 then processes the signal using data processing software to generate a sensor system response and a frequency spectrum, which are used to determine the state of the component 40 under test. The state of the component 40 can also be determined manually based on experience or using an artificial intelligence algorithm.

[0055] 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 based on the time domain waveform.

[0056] Because the optical fiber is thin, sound waves can be transmitted over a long distance in the optical fiber. This detection method can prevent 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.

[0057] Figure 2 This is a schematic diagram of an acoustic wave transmitted in an optical fiber being reflected at the interface between the end of the second pigtail and the air or reflective medium, thereby forming a standing wave in the optical fiber.

[0058] 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 forward and reverse propagating sound waves 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, in order to improve the response sensitivity, the FBG and the first pigtail adhesive section 13 are placed at the antinode of the standing wave.

[0059] Forward wave equation:

[0060]

[0061] Inverse wave equation:

[0062]

[0063] Standing wave equation expression:

[0064]

[0065] Where the amplitude is A, the angular frequency is ω, the propagation distance in space is x, the time is t, and the wavelength is λ. As can be seen from the above formula, the amplitude of the standing wave is twice the amplitude of the forward wave, and the amplitude reaches its maximum at half the wavelength of the forward wave or an integer multiple of half the wavelength, that is, at the antinode of the standing wave.

[0066] Figure 3 The figure schematically shows 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.

[0067] refer to Figure 3 As shown, component 1 to be tested (41) is a rigid polyurethane block embedded with a fastening structure (including screws and internally threaded holes adapted for the screws). d is the length of the adhesive section, D1 is the length from the adhesive section on the first pigtail to the fiber optic cable (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). The adhesive section adheres in the y-direction. The horizontally lined area below component 1 to be tested (41) is the area constrained by the position limiting member 50.

[0068] Figure 4 for Figure 3 The finite element-based simulation modal analysis results of the component to be tested are shown.

[0069] 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 insensitive 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 bonding direction of the optical fiber. The displacements of each particle along the bonding length are consistent, that is, the same color area in the simulation diagram. 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. 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.

[0070] 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 to 10mm (refer to the scale of the figure).

[0071] Further experimental calibration is performed to obtain the effective vibration frequency range that can be actually excited and sensed by the FBG. The experimental calibration method is: in the direction of uniform linear strain indicated by the finite element analysis of the target modal (in this embodiment, the y direction), the FBG is attached to the area on the component to be tested where the strain amplitude of each particle indicated by the finite element analysis of the target modal is consistent (in this embodiment, the y direction). Figure 3In the area shown, the adhesive length is 8mm to 10mm. According to the excitation point and force direction in actual nondestructive testing, impulse excitation is performed, and the effective vibration frequency range and amplitude of the impulse response are observed. The frequency corresponding to the maximum amplitude in the spectrum is taken as the experimental natural frequency. Figure 5 (a) is the impulse response and spectrum obtained.

[0072] Figure 5 Attach 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.

[0073] For detection Figure 4 The conclusion that the y direction (the direction that produces a more uniform linear displacement) is the most suitable direction for attaching the pigtail is that the FBG is directly attached to the Figure 3 The X-surface 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 as follows Figure 5 (b) and Figure 5 (c) shown.

[0074] refer to Figure 5 As can be seen, the signal amplitude for pasting along the y-axis is much greater at 1600-4000Hz than at other frequencies. However, the signal amplitude for pasting along the z-axis is significantly reduced, with the 500-2500Hz signal being the primary component. When pasting at a 45-degree angle to the z-axis, there is a small signal present in the frequency range below 7000Hz, with the signal amplitude being dominant below 4000Hz. The measured signal-to-noise ratio for pasting along the z-axis is 18.069; when pasting at a 45-degree angle to the z-axis, the system's signal-to-noise ratio is 23.5958; and when pasting along the y-axis, the system's signal-to-noise ratio is 25.2305.

[0075] Therefore, the test results show that: Figure 5 The impulse response signal-to-noise ratio (SNR) obtained for the y-direction attachment method (a) is higher than that for the responses in other attachment directions, indicating that this attachment method is superior to the others. The fifth-order mode in the simulation is difficult to excite in the actual test (the simulated natural frequency is 5086.5 Hz), so the first and second-order modes are selected as the target modes.

[0076] Selecting an excitation method: In this embodiment, the impulse response is repeatable after multiple impulse tests, so impulse excitation can be selected. Since low-energy periodic excitation may have little impact on the component under test, both excitation methods can be used.

[0077] Determine the excitation source frequency during 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 large amplitude vibrations can be excited near the 1699Hz-1999Hz range and the 2466Hz-2866Hz range. 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 simulated target mode.

[0078] 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 frequencies indicated by the signal are 314 Hz. Within the range of 1600 Hz to 3600 Hz, the high-order resonant frequencies for 314 Hz excitation are 1884 Hz, 2512 Hz, 2826 Hz, 3140 Hz, and 3454 Hz. Some of these frequencies are near both the simulated and experimental natural frequencies, and some are also near the 1699 Hz to 1999 Hz and 2466 Hz to 2866 Hz ranges. Therefore, it can be assumed that vibrations of sufficient amplitude can be excited within this range.

[0079] Determine the length D1 from the bonding section of the first pigtail to the FBG and the length D2 of the second pigtail: Based on the high-order resonant frequencies of 1884 Hz, 2198 Hz, 2512 Hz, 2826 Hz, 3140 Hz, and 3454 Hz within the effective excitation range of 1600 Hz to 3600 Hz, and the non-dispersive wave velocity in the fiber of 3743.54 m / s (a constant), the wavelength, half-wavelength, quarter-wavelength, and three-quarter-wavelength are calculated as shown in Table 1. According to standing wave theory, to optimize the signal-to-noise ratio of the FBG sensor, the FBG and bonding section should be placed at half a wavelength or an integer multiple of a half wavelength.

[0080] Table 1: Wavelength, half wavelength, quarter wavelength, and three-quarter wavelength at different frequencies

[0081]

[0082] Depend on Figure 5 (a) The excitation response spectrum shows that the vibration amplitude is the largest near the experimental natural frequency 2466Hz. Figure 4The second-order mode's motion direction aligns with the y-direction of the attachment, making it the mode most likely 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 selected 2512 Hz from Table 1 as the reference frequency for designing the first pigtail's attachment section to the FBG length D1 and the second pigtail's length D2, with a half-wavelength of 745.1 mm. In other embodiments, the reference frequency can also be determined through a frequency sweep.

[0083] Figure 6 for Figure 3 The sensor system response and spectrum diagram of the component under test under 314Hz periodic excitation are shown.

[0084] The periodic excitation energy generated by the exciter offsets the energy loss caused by the structural vibration damping in the structure, forming a stable vibration with 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, respectively, and their time domain wave graphs and spectra were tested as shown in the following figure. Figure 6 As shown, the signal-to-noise ratios are 33.4365 and 27.0675 respectively.

[0085] Figure 7 for Figure 3 The sensor system response and spectrum diagram of the component under test under 314Hz periodic excitation are shown.

[0086] 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.

[0087] visible, Figure 6 The signal-to-noise ratio of the sensor signal shown is significantly greater than Figure 7 As shown in the sensor 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.

[0088] Figure 8 for Figure 3 The time domain waveform and spectrum diagram of the screw and the corresponding internal threaded hole on the tested component under different pre-tightening torque conditions are shown.

[0089] According to screw tightening 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 determined.

[0090] A section of the first FBG pigtail 11 was attached to one side of the component under test 41 along the y-direction. The FBG was suspended. The second FBG pigtail 12 was also suspended, with the end of the second pigtail 12 exposed to air. D1 = 740 mm, D2 = 745 mm, the attached section length was 10 mm, and the FBG length was 10 mm.

[0091] An exciter that generates a periodic excitation of 314 Hz is pressed against the other side of the component to be tested 41. The vibration generated in the component to be tested 41 acts as a sound wave source and is transmitted forwardly from the first fiber pigtail 11 to the FBG 10 and the second fiber pigtail 12. The end of the second fiber pigtail 12 reflects the sound wave and transmits it in the reverse direction 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.

[0092] The standing wave signal on the FBG is demodulated into an electrical signal using edge filtering. The electrical signal is then fed into a data analyzer 26 via a data acquisition unit 25. The data analyzer 26 then processes the electrical signal using data processing software to obtain a sensor system response and a frequency spectrum. The state of the component under test can be determined based on the sensor system response and frequency spectrum.

[0093] The pre-tightening torque between the screw and the internal threaded hole is (a) 0Ncm; (b) 35Ncm; (c) 70Ncm; (d) 100Ncm

[0094] Table 2: Corresponding relationship between power and torque of the signal in one cycle

[0095]

[0096] The relationship between signal power and torque within a cycle, obtained through calculation, is shown in Table 2. As can be seen from Table 2, the test signal power increases with increasing preload force. Therefore, the preload status of the fastener can be determined using this test signal.

[0097] Figure 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.

[0098] Monitor the overall translation of the component under test 41 along the z-axis. The component under test 1 may become loose from its base during operation (with the surrounding constraints remaining unchanged). This nondestructive testing experiment applies impulse excitation of the same intensity to the component under test to test the overall translation of the component under test 41 along the z-axis.

[0099] A section of the first FBG pigtail 11 was attached to one side of the component under test 41 along the y-direction. The FBG was suspended. The second FBG pigtail 12 was also suspended, with the end of the second pigtail 12 exposed to air. D1 = 740 mm, D2 = 745 mm, the attached section length was 10 mm, and the FBG length was 10 mm.

[0100] 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 the sound wave source and is transmitted forward from the first fiber pigtail 11 to the FBG and the second fiber pigtail 12. The end of the second fiber pigtail 12 reflects the sound wave and transmits it in the reverse direction along the optical fiber. The forward and reverse propagating sound waves form standing waves in the second fiber pigtail 12, FBG10 and the first fiber pigtail 11 of the FBG, and the FBG senses the standing wave signal.

[0101] The standing wave signal on the FBG is demodulated into an electrical signal using edge filtering. The electrical signal is then fed into a data analyzer 26 via a data acquisition unit 25. The data analyzer 26 then processes the electrical signal using data processing software to obtain a sensor system response and a frequency spectrum. The state of the component under test can be determined based on the sensor system response and frequency spectrum.

[0102] in Figure 9 (a) Not free from the bottom restraint; Figure 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.

[0103] Depend on Figure 9 It can be seen that when the bottom of the component to be tested is fully constrained (such as Figure 9 As shown in (a), the impulse response spectrum exhibits multiple peaks below 4000 Hz. However, when the base of the structure is freed from its bottom constraint in the z-direction, the main peak of the impulse response spectrum gradually shifts to lower frequencies as displacement increases. The spectrum also gradually transitions from multiple peaks to a single peak. This characteristic can be used to estimate the approximate displacement of the entire structure in the z-direction.

[0104] Figure 10 for Figure 3 The impulse response spectra of the component under test at three different displacements in the z direction are shown.

[0105] For example, if the impulse response spectrum of the component under test is Figure 10 According to the spectrum comparison for (a), (b) and (c), the z-direction displacements are approximately 16 mm, 36 mm and 46 mm, respectively.

[0106] Figure 11This is a schematic diagram of the structure of the 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.

[0107] refer to Figure 11 As shown, Figure 3 The component to be tested 41 shown is replaced by Figure 11 The second component under test 42 is shown. It consists of two parts. The first part is a cylindrical cylinder 31 with a hollow interior. Its bottom is fixed to a stopper 50, where it serves as a restraining element. The second part is a solid cylinder 34 with another hollow cylinder 32 on top. The first and second parts are fixed by bolts 33. The axial direction of the hollow cylinder 32 is aligned with the axial direction of the bolts 33. All components of the second component under test 42 are made of aluminum alloy.

[0108] Figure 12 for Figure 11 The finite element-based simulation modal analysis results of the second component to be tested are shown.

[0109] refer to Figure 12 As shown, in each mode, the strain modes of the first, second, third, fourth, sixth, seventh, and eighth orders are all linear displacements, differing only in direction. The fifth and ninth orders are primarily torsion, and since optical fibers are 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 under test 42 produce relatively uniform linear displacements along the z direction on the cylindrical surface, making the z direction a suitable direction for attaching the optical fiber. According to the simulation results, to ensure that the acoustic waves in the optical fiber are in the same phase, the attachment length is 8-15 mm (see the scale of the figure).

[0110] Figure 13 The spectrum diagrams are of impulse response when the FBG is attached to the second component under test in different directions.

[0111] Paste the FBG directly at different angles Figure 11 The outer surface of the cylindrical cylinder 31 with a cavity inside the second component 42 to be tested is attached in the z direction, the y direction, and a 45-degree angle with the z direction. The specific attachment positions and the corresponding impulse response spectrum under the same impulse function excitation are shown as follows: Figure 13 shown.

[0112] Depend on Figure 13It can be seen that the amplitude of the z-direction bonding method is much greater than that of other bonding methods, with a signal frequency range of 1600-7000Hz. When bonding at a 45-degree angle to the z-axis, the signal is dispersed between 200-12000Hz, and the signal amplitudes of the natural frequencies of each experiment are basically consistent. The signal amplitude of the y-axis bonding method is significantly reduced, with the 200-8200Hz signal being the main component. The measured signal-to-noise ratio of the system is 26.7107 when bonding along the z-direction; 24.5548 when bonding at a 45-degree angle to the z-axis; and 18.9105 when bonding along the y-direction.

[0113] Therefore, the test results show that the impulse response signal-to-noise ratio obtained by the bonding 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 bonding directions, and this bonding direction is superior to other bonding directions.

[0114] Selecting an excitation method: After multiple impulse tests, the impulse response is repeatable, so impulse excitation can be used. Since low-energy periodic excitation is likely to have little impact on the component under test, both excitation methods can be used.

[0115] Determine the excitation source frequency during periodic excitation: Figure 13 (a) Experimental analysis shows that the effective vibration frequency range of the excited component 2 is around 1600Hz-7000Hz. Figure 12 Finite element-based simulation analysis shows that since the first and second order modes are not within this effective vibration frequency range, the third, seventh and eighth order modes are bending displacements, and the fifth and ninth order modes are torsional, which are not sensed by the 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. Figure 13 (a) The experimental natural frequencies corresponding to the maximum amplitudes of the measured FBG impulse response are near 1657 Hz, 3285 Hz, 5342 Hz, and 6428 Hz. Based on these four experimental natural frequencies and the simulated natural frequencies of the fourth and sixth modes, a 330 Hz periodic signal was selected as the excitation. The resulting higher-order harmonics correspond to frequencies of 1650 Hz, 1980 Hz, 2970 Hz, 3300 Hz, 3630 Hz, 5280 Hz, 5610 Hz, 5940 Hz, 6270 Hz, and 6600 Hz. Some of these higher-order harmonics correspond to frequencies near the simulated or experimental fourth and sixth natural frequencies. The wavelength, half-wavelength, quarter-wavelength, and three-quarter-wavelength values were calculated, assuming a non-dispersive optical fiber velocity of 3743.54 m / s, as shown in Table 3.

[0116] Table 3: Wavelength, half wavelength, quarter wavelength and three-quarter wavelength at different frequencies

[0117]

[0118]

[0119] Depend on Figure 13 (a) It can be seen that the vibration amplitude corresponding to the experimental natural frequency of 3285 Hz is the largest. Among the frequencies corresponding to the higher-order harmonics of the 330 Hz periodic excitation designed by the present invention, 3300 Hz is closest to this experimental natural frequency of 3285 Hz; 2970 Hz is closest to the sixth-order simulated natural frequency of 2985 Hz. Based on the principle of prioritizing experimental results, 3300 Hz is selected as the reference frequency for the designs of D1 and D2, with a half-wavelength of 567.2 mm.

[0120] Figure 14 for Figure 11 The signal time domain waveform and spectrum of the sensing system of the second component under test 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, and the measured signal-to-noise ratio is 19.2236.

[0121] Figure 15 for Figure 11 The time domain waveform and spectrum of the response signal of the sensing device of the second component to be tested under 330Hz periodic excitation are shown.

[0122] Select separately 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: 700mm, D2: 637mm, e.g. Figure 15 (a)- Figure 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.

[0123] On the other hand Figure 14 The signal-to-noise ratio is 19.2236, so Figure 14The signal-to-noise ratio of the sensor signal is significantly greater than Figure 15 Taking half wavelength or integer multiples of half wavelength for D1 and D2 can indeed improve the response sensitivity and the signal-to-noise ratio of the effective test signal.

[0124] Figure 16 for Figure 11 The response and spectrum of the detection system when the hollow cylinder and the fastening bolt on the second component to be tested are at different angles are shown.

[0125] 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. Due to the geometric characteristics of the structure, when the axial direction of the top hollow cylinder is consistent with or perpendicular to the axial direction of the bolt, 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 Figure 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.

[0126] Press the first pigtail 11 Figure 13 The direction shown in (a) is pasted on the second component to be tested, the pasting length is 10mm, D1 is 567mm, and D2 is 570mm, which is used for non-destructive testing of the second component to be tested.

[0127] An exciter generating a 330 Hz periodic excitation is brought into contact with the other side of the second component to be tested. The vibration generated in the second component to be tested 42 acts 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 in the reverse direction along the optical fiber. The forward and reverse propagating sound waves form standing waves in the second pigtail 12, FBG 10 and the first pigtail 11 of the FBG, and the FBG senses the standing wave signal.

[0128] The standing wave signal on the FBG is demodulated into an electrical signal using edge filtering. The electrical signal is then fed into a data analyzer 26 via a data acquisition unit 25. The data analyzer 26 then processes the electrical signal using data processing software to obtain a sensor system response and a frequency spectrum. The state of the second component under test can be determined based on the sensor system response and frequency spectrum.

[0129] The included angles between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 are 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.

[0130] refer to Figure 16As shown in FIG, 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 3300 Hz, 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 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); and the stronger the signal, the better the symmetry (corresponding to when 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 when the angle between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 is 0 degrees). Figure 16 (c), (d), and (e) indicate the situation). Therefore, this feature can be used to determine the symmetry of the structural component.

[0131] 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 along the z direction is shown.

[0132] Monitor the internal solid cylinder 34 of the second component to be tested 42 to translate along the z-axis:

[0133] Assume that the starting state of the second component under test 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 with a cavity 31. They are still fastened by bolts 33. During operation, the top hollow cylinder 32 may slide downward due to the loosening of the fastening bolts 33, thereby reducing the volume of the cavity portion of the cylindrical cylinder 31. This non-destructive testing embodiment uses impulse excitation to test the translation of the solid cylinder 34 along the z-axis in the second component under test 42, and obtains its impulse response spectrum change characteristics, such as Figure 16 This is used as a test basis to evaluate its relative position.

[0134] 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 the sound wave source and is transmitted forward from the first fiber pigtail 11 to the FBG and the second fiber pigtail 12. The end of the second fiber pigtail 12 reflects the sound wave and transmits it in the reverse direction along the optical fiber. The forward and reverse propagating sound waves form standing waves in the second fiber pigtail 12, FBG10 and the first fiber pigtail 11 of the FBG, and the FBG senses the standing wave signal.

[0135] The standing wave signal on the FBG is demodulated into an electrical signal using edge filtering. The electrical signal is then fed into a data analyzer 26 via a data acquisition unit 25. Data analyzer 26 then processes the electrical signal using data processing software to generate a sensor system response and a frequency spectrum. This information can be used to determine the state of the solid cylinder in component 2 under test.

[0136] The impulse response of the test system is tested under different displacement conditions of the solid cylinder along the z direction. The tightening displacements in the z direction are: 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)0mm.

[0137] Depend on Figure 17 As shown in (a), 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, the peaks of the main peaks gradually move to higher frequencies. Figure 17 (b) The signal with a frequency of 1500 Hz is relative to Figure 17 (a) becomes weaker, but the signal near the frequency of 3300 Hz becomes stronger; Figure 17 (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 peaks shift further toward higher frequencies, with the main frequencies being around 1650 Hz, 3300 Hz, 5250 Hz, and 6450 Hz. Therefore, this feature can be used to assess the degree of slippage of the solid cylinder 34 within the cavity.

[0138] Figure 18 for Figure 11 The impulse response spectrum of the solid cylinder of the second component to be tested when displaced in the z direction is shown.

[0139] For example, if the impulse response spectrum of the structural component is measured to be Figure 18 (a), (b), (c), we can judge by comparing the spectra: Figure 18 (a) No slip occurs, Figure 18 (b) The slippage is about 3 mm, Figure 18 (c) The slippage is about 27 mm.

[0140] 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 them. 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. Nondestructive testing method based on structural vibration-fiber acoustic guided wave sensing, where: The invention relates to a method for screening nondestructive testing signals based on structural vibration and fiber-optic guided acoustic wave sensing. The method includes: performing finite element simulation modal analysis on the component to be tested, taking the mode that produces linear strain as the target mode, and using the direction of uniform linear strain indicated in the finite element simulation modal analysis as the direction in which the sensing device is attached to the component to be tested; the attachment area of the sensing device to the component to be tested must be within an area where the strain amplitudes of each particle indicated in the finite element simulation modal analysis of the target mode are consistent; The sensing device includes an FBG, and a first pigtail and a second pigtail of the FBG; a section of the first pigtail is adhered to one side of the component to be measured as the adhered 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; A periodic excitation source or an impulse excitation source is applied to the other side of the component under test. The vibration generated in the component under test acts as an acoustic wave source and is 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 in the reverse direction 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. The FBG senses the standing wave signal. Demodulating the standing wave signal on the FBG into an electrical signal; Analyzing the electrical signal to determine the state of the component to be tested; The FBG is located at the antinode of the standing wave, and the adhesive section is located at the antinode of the standing wave; Within the effective vibration frequency range that the FBG can sense and the vibration of the component to be measured can be excited, among the integer multiples of the periodic excitation frequency, the vibration frequency closest to the maximum 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 adhesive 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 nondestructive testing method according to claim 1, 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 close to the simulated natural frequency of the target mode; Experimental calibration: Using the direction of uniform linear strain indicated in the finite element analysis of the target modal, attach the FBG to the area on the component to be tested where the strain amplitude of each particle indicated in the finite element analysis of the target modal is consistent. Perform impulse excitation according to the excitation point and force direction in 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 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.

3. The nondestructive testing method according to claim 1, wherein: The vibration frequency with the largest amplitude indicated by the finite element simulation modal analysis closest to the target modal analysis in the finite element simulation modal analysis of the component to be tested is selected as the reference frequency.

4. The nondestructive testing method according to claim 2, wherein: The vibration frequency with the largest amplitude closest to the impulse response is selected as the reference frequency.

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