CN217980727U

CN217980727U - Nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing

Info

Publication number: CN217980727U
Application number: CN202222324123.3U
Authority: CN
Inventors: 滕林斌; 雷浩帆; 饶春芳; 姜子薇; 闫晓丽; 吴锴; 谌子滢; 阮一鸣; 左瑞阳
Original assignee: Jiangxi Normal University
Current assignee: Jiangxi Normal University
Priority date: 2022-09-01
Filing date: 2022-09-01
Publication date: 2022-12-06
Anticipated expiration: 2032-09-01

Abstract

The utility model provides a nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing, which comprises a vibration exciter, FBG, a demodulation device and an electric signal analysis and processing system; one section of the first tail fiber of the FBG is adhered to one side of the component to be tested to form an adhering section, the first tail fiber, the FBG and the second tail fiber are placed in a space without constraint, and the tail end of the second tail fiber is placed in air or a reflecting medium; the vibration exciter acts on the other side of the component to be detected, vibration is generated in the component to be detected, the vibration serves as a sound wave source and is transmitted into the FBG and the second tail fiber from the first tail fiber in the forward direction, the tail end of the second tail fiber reflects the sound wave and transmits the sound wave in the reverse direction along the optical fiber, the sound wave which is transmitted in the forward direction and the reverse direction forms standing waves in the second tail fiber, the FBG and the first tail fiber of the FBG, and the FBG senses standing wave signals; the demodulation device is used for demodulating the standing wave signal on the FBG into an electric signal; the electric signal analysis processing system is used for analyzing and processing the electric signal and judging the state of the component to be detected. The utility model discloses be suitable for narrow and small space, sensitivity height, available low frequency excitation, can prevent the structure damage.

Description

Nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing

Technical Field

The utility model relates to a nondestructive test field, in particular to nondestructive test system based on structural vibration-optic fibre acoustic guided wave sensing.

Background

The analysis of various structural dynamics characteristics, including modal analysis, harmonic response analysis and transient dynamics analysis, can reflect the material characteristics and the working state of the structure, and is a powerful means for carrying out structural health detection. A dynamic sensing device, i.e. a vibration sensor, is an essential device for performing such detection. The sensing systems mainly used at present include optical fiber sensing systems, piezoelectric sensing systems and eddy current type displacement sensing systems. The piezoelectric sensing system has the main advantages of high sensitivity, small volume, light weight, stable performance, wide frequency band and the like, and has the defect that the sensing performance is greatly influenced in the environment with high temperature, large temperature change, humidity or strong magnetic field. The eddy current type displacement sensor is suitable for testing the dynamic characteristics of a rotating component, has the advantage of wide response frequency band, and is greatly interfered by temperature and electromagnetic field.

The optical fiber sensor has the outstanding advantages of small diameter, high sensitivity, electromagnetic interference resistance, light weight, distributable measurement and the like, and becomes the outstanding representative of a new generation of vibration sensors. The most successful of the applications is represented by a Fiber Bragg Grating (FBG) vibration sensor, which has the advantages of stable signal, multiplexing capability, no influence of light source intensity and optical Fiber bending and the like on the basis of the advantages of a common optical Fiber sensor. The method is widely applied to the fields of petroleum, traffic, national defense, machinery and the like.

Currently, FBG vibration sensors are mainly classified into three categories according to the FBG assembly manner. One type is a full-face mount. Namely, the FBG is directly adhered to the surface of the component to be detected or embedded into the elastic material, so that the vibration of the component to be detected is directly sensed. It can avoid the sensor from being damaged, but it also makes the sensor unable to be reused; on the other hand, due to the fully-adhered packaging mode, the FBG may generate a chirp phenomenon due to non-uniform stress, so that the spectrum of the FBG is deformed, and further, the test result is inaccurate.

The second type is a two-point fitting. Namely, the tail fibers at two ends of the fixed fiber bragg grating are adopted, and the FBG is in a suspended state. When the fixed point of the fiber grating tail fiber is displaced, the FBG is strained, so that the central wavelength of the FBG drifts, and the purpose of detecting a vibration signal is achieved. The sensing structure needs additional auxiliary structures on the basis of the fiber grating sensing element, and the volume and the mass are not suitable for narrow areas. The vibration test in the orthogonal direction can be realized through proper structural design, but the vibration in the non-orthogonal direction cannot be distinguished.

The third packaging method is suspension assembly. The tail fiber at one end of the fiber bragg grating is fixed in the sealing structure, the tail fiber at the other end of the fiber bragg grating is extremely short and is suspended in the air, and the length from the tail fiber fixed by the FBG to the tail fiber at the other end is controlled to be 20-80 mm. This limitation is due to the fact that too short a length will make the vibration amplitude too small, affecting the sensitivity; and the vibration frequency is influenced by the gravity of the optical fiber due to too long length, and the test result is inaccurate. The reasonable design of the diameter and the suspension length of the optical fiber can ensure that the sensitivity of the sensor is higher, and the sensor is suitable for testing the micro-vibration of a large-scale structure. However, this package requires a structure for enclosing the FBG, and is not favorable for a narrow space. The packaging mode structure senses vibration of all modes, FBG response signals are complex, and great difficulty is brought to signal analysis.

SUMMERY OF THE UTILITY MODEL

The utility model provides a nondestructive test system based on structural vibration-optic fibre acoustic guided wave sensing to solve one or several kinds among the above-mentioned problem.

According to one aspect of the utility model, a nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing is provided, which comprises a vibration exciter generating periodic excitation or a vibration exciter generating impulse excitation, an FBG, a demodulating device and an electric signal analysis processing system; one section of the first tail fiber of the FBG is adhered to one side of the component to be detected to form an adhering section, the first tail fiber, the FBG and the second tail fiber are placed in a space without constraint, and the tail end of the second tail fiber is placed in air or a reflecting medium; the vibration exciter acts on the other side of the component to be detected, vibration is generated in the component to be detected, the vibration serves as a sound wave source and is transmitted into the FBG and the second tail fiber from the first tail fiber in the forward direction, the tail end of the second tail fiber reflects the sound wave and transmits the sound wave in the reverse direction along the optical fiber, the sound wave which is transmitted in the forward direction and the reverse direction forms standing waves in the second tail fiber, the FBG and the first tail fiber of the FBG, and the FBG senses standing wave signals; the demodulation device is used for demodulating the standing wave signal on the FBG into an electric signal; the electric signal analysis processing system is used for analyzing and processing the electric signal and judging the state of the component to be detected.

The utility model discloses a nondestructive test system based on structural vibration-optic fibre acoustic guided wave sensing's advantage has: 1. the FBG can be used for detecting the integral dynamic characteristics of the component, the tail fiber of the FBG is attached to the component, the spectral characteristics of the FBG cannot generate the chirp phenomenon, and the FBG can be repeatedly used after the test is finished; 2. because the sound wave can be at the longer distance of fiber transmission, can avoid the adverse circumstances (like high temperature) direct influence FBG sensor characteristic of pasting the point, be suitable for narrow and small space, crowded space or the monitoring of component characteristic that can not direct observation equally. 3. The second tail fiber of the FBG is suspended in the air (or a reflecting medium) in a certain length, and the sound wave transmitted in the second tail fiber forms backward wave reflection at the tail end of the tail fiber, so that standing waves are formed by superposition of the backward wave and the forward wave, and the test sensitivity of the sensor is increased; 4. the detection method is suitable for monitoring the dynamic characteristics of the whole structure under low-frequency excitation, and reduces the high requirement on excitation equipment under high-frequency excitation; 5. the sensing design based on the high-order resonance can generate larger vibration by using a small-energy excitation source, namely, can generate larger sensing signals by using small excitation, reduce the excitation difficulty, improve the signal-to-noise ratio of the sensor, and avoid possible structural damage caused by non-resonant frequency vibration.

FBGs are most sensitive to axial strain. There is some sensitivity to radial strain, but the sensitivity to radial strain is less than the sensitivity to axial strain. FBGs do not perceive torsional strain. Therefore, FBGs are well sensitive to vibrations that produce linear strain along the axial direction of the fiber. And the vibration that produces the axial linear strain in optic fibre can form the longitudinal wave in long optic fibre, can effectively by the utility model discloses a suspension type FBG perception. Therefore, the utility model discloses a mode that produces linear strain that simulation analysis based on finite element instructed is the target mode to confirm to paste the direction with the direction of the even line strain that instructs in the target mode finite element analysis, paste the region that the range of strain of each particle that the region need instruct in the finite element analysis of target mode is unanimous.

Damped structures will induce large but finite vibrations in a frequency band around the natural frequency under periodic excitation. Effectively detecting such vibration information within a safe vibration amplitude range would be useful for detecting the health of the structure. The natural vibration frequency of the structural member is not a fixed value, and is influenced by a plurality of 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 frequency is the natural frequency section really, in this section frequency, the periodic excitation of vibration exciter can arouse that vibration amplitude is stable, the SNR is enough to carry out the vibration that effective structure health detected.

In some embodiments, the FBG of the present invention is located at an antinode of the standing wave, and the adhesive segment is located at an antinode of the standing wave.

In some embodiments, the demodulation apparatus of the present invention comprises a narrowband light source, a circulator, and a photodetector; light emitted by the narrow-band light source enters the FBG from the tail end of the first tail fiber of the FBG, and a standing wave signal on the FBG is sensed; the photodetector is used to convert the standing wave signal reflected from the FBG through the circulator into an electrical signal.

In some embodiments, the utility model discloses a paste the section and paste the direction on the component that awaits measuring and be: carrying out the direction for generating uniform line strain indicated in finite element simulation modal analysis on the component to be detected; the pasting area of the pasting section on the component to be tested is as follows: and carrying out finite element simulation modal analysis on the component to be tested to generate a linear strain target mode in a region with consistent strain amplitude of each mass point indicated in the finite element simulation modal analysis. Thus, a nondestructive test signal with a high signal-to-noise ratio can be obtained.

In some embodiments, the length D1 of the fiber grating from the bonded section of the first pigtail is an integer multiple of the half-wavelength of the reference frequency, and the length D2 of the second pigtail is an integer multiple of the half-wavelength of the reference frequency; the reference frequency is the frequency closest to the vibration frequency with the maximum amplitude in the integral multiple frequency of the periodic excitation frequency in the vibration frequency range in which the FBG can effectively sense and the vibration of the component to be measured can be effectively excited. Thus, the signal-to-noise ratio of the nondestructive test signal can be improved.

In some embodiments, the utility model discloses a reference frequency is for carrying out the finite element simulation modal analysis to the component that awaits measuring, and the vibration frequency that the amplitude that is closest to target modal simulation analysis instruction is the biggest.

In some embodiments, the reference frequency of the present invention is the frequency closest to the vibration frequency at which the amplitude indicated by the impulse response is maximum.

Drawings

Fig. 1 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;

FIG. 2 is a schematic diagram of the sound wave transmitted in the optical fiber being reflected at the interface between the end of the second pigtail and the air and forming a standing wave in the optical fiber;

fig. 3 is a schematic diagram illustrating a structure of a component to be tested of the nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing and a relationship between a tail fiber of an FBG and the component to be tested according to an embodiment of the present invention;

FIG. 4 is a result of finite element-based simulation modal analysis of the first component under test shown in FIG. 3;

fig. 5 (a) - (c) are frequency spectrums of impulse responses when FBGs are attached to the first component to be tested shown in fig. 3 in different directions;

FIGS. 6 (a) - (b) are graphs of response and spectrum of the sensing system under 314Hz periodic excitation in the first form of the component to be tested and the FBG assembly shown in FIG. 3, wherein D1 and D2 are half-wavelengths or integral multiples of half-wavelengths corresponding to the reference frequency;

FIGS. 7 (a) - (D) are graphs of the response and spectrum of the sensing system under 314Hz periodic excitation for the first component under test and the FBG assembled form shown in FIG. 3, wherein D1 and D2 are not half-wavelength or integer multiples of half-wavelength corresponding to the reference frequency;

FIGS. 8 (a) - (d) are response waveforms and frequency spectrums of the sensing system under 314Hz periodic excitation under different pre-tightening torque conditions between the screw and the internal threaded hole on the first component to be tested shown in FIG. 3;

FIGS. 9 (a) - (d) are response frequency spectrums of the sensing system during impulse excitation under different displacement conditions along the z direction of the whole to-be-measured member shown in FIG. 3;

FIGS. 10 (a) - (c) are response frequency spectrums of a sensing system under impulse excitation of a component to be measured shown in FIG. 3 under a z-direction displacement condition; wherein D1 and D2 are half wavelengths corresponding to the reference frequency;

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

FIG. 12 is a result of finite element-based simulation modal analysis of the second component to be tested shown in FIG. 11;

FIGS. 13 (a) - (c) are impulse response frequency spectrums of FBGs attached to the second component to be tested in different directions;

FIG. 14 is a graph of response and spectrum of the sensing system under 330Hz periodic excitation in the assembled form of the second component to be tested and the FBG shown in FIG. 11, wherein D1 and D2 are both half wavelengths corresponding to the reference frequency;

FIGS. 15 (a) - (h) are graphs of response and spectrum of the sensing system under 330Hz periodic excitation in the assembled form of the second component to be tested and the FBG shown in FIG. 11, wherein neither D1 nor D2 is the half wavelength or the integral multiple of the half wavelength corresponding to the reference frequency;

FIGS. 16 (a) - (e) are graphs of response and spectrum of the sensing system under 330Hz periodic excitation under the conditions that D1 and D2 are both half wavelengths corresponding to the reference frequency and the hollow cylinder and the fastening screw are at different included angles under the assembling form of the second component to be tested and the FBG shown in FIG. 11;

fig. 17 (a) - (f) are response frequency spectrums of the sensing system during impulse excitation under the condition that D1 and D2 are both half wavelengths corresponding to the reference frequency and the solid cylinder has different displacements along the z direction under the assembly form of the second component to be measured and the FBG shown in fig. 11;

fig. 18 (a) - (c) show response spectrums of the sensing system during the impulse excitation of the solid cylinder in the z-direction displacement under the condition that D1 and D2 both correspond to half wavelength of the reference frequency in the second component to be measured and the FBG assembly form shown in fig. 11.

Detailed Description

To make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the attached drawings in the embodiments of the present invention are combined to clearly and completely describe the technical solution in the embodiments of the present invention, and obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments in the present invention, all other embodiments obtained by a person skilled in the art without creative work belong to the protection scope of the present invention.

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

Finally, it should also be noted that, in this document, relational terms such as first and second, and the like, front and back, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Furthermore, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrases "comprising 8230; \8230;" comprises 8230; "does not exclude the presence of additional like elements in a process, method, article, or apparatus that comprises the element.

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

Fig. 1 schematically shows the structure of a nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing according to an embodiment of the present invention.

Referring to fig. 1, the nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing comprises a vibration exciter 30, a first tail fiber 11, a second tail fiber 12, and a Fiber Bragg Grating (FBG) 10, as well as a demodulation device (comprising a tunable narrowband laser 21, an isolator 22, a circulator 23, and a photodetector 24) and an electric signal analysis processing system (comprising a data collector 25 and a data analyzer 26).

Exciter 30 may be either a periodic excitation or an impulse excitation producing exciter. The periodic excitation energy generated by exciter 30 cancels the energy loss caused by the vibration damping of the structure in the structure, and forms vibration with stable amplitude and limited amplitude.

One section of the first tail fiber 11 of the FBG is adhered to one side of the component 40 to be measured to form an adhered section 13, and the first tail fiber, the FBG and the second tail fiber are placed in a space without constraint as long as the propagation of the acoustic guided wave in the optical fiber is not influenced, and the FBG can be freely placed on a desktop without constraint or in a suspended state, but cannot be placed in liquid or solid to form a mechanical continuum with the optical fiber. The end of the second pigtail 12 is exposed to air. The first tail fiber 11 of the FBG can be directly adhered to the structure to be measured, or indirectly adhered, for example, the first tail fiber 11 is adhered to a small middle dielectric block, and the middle dielectric block is adhered to the member to be measured. In other embodiments, the end of the second pigtail 12 may be placed in other reflective media.

The component 40 to be measured is fixed by the limiting piece 50, the vibration exciter 30 acts on the other side of the component 40 to be measured, vibration is generated in the component 40 to be measured, the vibration is transmitted into the FBG and the second tail fiber 12 from the first tail fiber 11 in the forward direction as a sound wave source, the tail end of the second tail fiber 12 reflects the sound wave and transmits the sound wave along the optical fiber in the reverse direction, the sound wave which is transmitted in the forward direction and the reverse direction forms a standing wave in the second tail fiber 12, the FBG and the first tail fiber 11 of the FBG, and the FBG can sense a standing wave signal.

The demodulation device is used for demodulating the standing wave signal on the FBG into an electric signal. The exciter 30 applies force to the other side of the member to be measured 40, and the end of the first tail fiber 11 of the FBG is connected to the circulator 4.

In this embodiment, an edge filtering method is used to demodulate the acoustic information on the FBG, and the demodulation means includes a tunable narrowband laser 21, an isolator 22, a circulator 23 and a photodetector 24. The light emitted by the tunable narrow-band laser 21 enters the FBG10 through the isolator 22 and the circulator 23 through the end of the first pigtail 11, and the acoustic wave signal on the FBG is sensed. The narrow-band light with the acoustic wave information in the FBG10 is reflected by the FBG10 and then enters the photoelectric detector 24 through the circulator 23 to be converted into an electric signal, the electric signal is extracted by the data collector 25 and then enters the data processing device 26 for signal processing and analysis, and the result is used for performing nondestructive detection on the state of the component 40 to be detected. Wherein the wavelength of the tunable narrow band laser 21 is set in the middle of the rising or falling edge of the FBG spectrum.

In other embodiments, a matched grating method or the like may be used to demodulate the acoustic information on the FBG. Demodulation device of the matched grating method: the device comprises a broadband light source, a matched grating, a circulator and a photoelectric detector. Broadband light emitted by the broadband light source enters the FBG through the circulator, reflected light of the FBG enters the matching grating through the circulator, and the reflected light of the matching grating enters the photoelectric detector to convert an optical signal into an electric signal.

The electric signal analyzing and processing system is used for analyzing and processing the electric signal and judging the state of the component 40 to be measured.

In the present embodiment, the electric signal analyzing and processing system includes a data collector 25 and a data analyzer 26. The electrical signal is guided into the data analyzer 26 through the data acquisition device 25, and the data analyzer 26 processes the electrical signal through data processing software to obtain a sensing system response and a spectrogram, so as to determine the state of the component 40 to be measured. The status of the structural member 40 may also be determined manually, empirically, or using an artificial intelligence algorithm.

In other embodiments, the electrical signal analysis processing system includes and may also include an oscilloscope. The electric signal displays time domain waveform through an oscilloscope, and the state of the component to be detected 40 can be judged according to the time domain waveform.

Because the optical fiber is fine, the sound wave can be transmitted for a long distance through the optical fiber, the detection mode can avoid that the severe environment (such as high temperature) of the sticking point directly influences the characteristics of the FBG sensor, and the method is also suitable for monitoring the characteristics of the component in a narrow space, a crowded space or incapable of being directly observed.

Fig. 2 is a schematic diagram of the sound wave transmitted in the optical fiber being reflected at the interface of the second pigtail end and air or a reflective medium and forming a standing wave in the optical fiber.

Referring to fig. 2, the end of the second tail fiber suspended in the air or the reflective medium reflects the acoustic wave and transmits the acoustic wave along the reverse propagation direction, the forward and reverse propagating acoustic waves form a standing wave in the second tail fiber, the FBG and the first tail fiber, and the FBG and the first tail fiber attachment section 13 are placed at the antinode of the standing wave for improving the response sensitivity because the vibration amplitude at the antinode of the standing wave is the largest.

Forward wave equation:

inverse wave equation:

standing wave equation expression:

wherein the amplitude is A, the angular frequency is omega, the propagation distance space coordinate is x, the time is t, and the wavelength is lambda. As can be seen from the above equation, the amplitude of the standing wave is two parts of the amplitude of the forward wave, and the amplitude reaches the maximum value at the position of the half wavelength or an integral multiple of the half wavelength of the forward wave, i.e., at the antinode of the standing wave.

Fig. 3 schematically shows a structure of a first component to be tested of the nondestructive testing system based on structural vibration-optical fiber acoustic guided wave sensing and a relationship between a tail fiber of an FBG and the component to be tested according to an embodiment of the present invention.

Referring to fig. 3, the first component to be tested 41 is a rigid polyurethane block embedded with a fastening structure (including a screw and an internal threaded hole matched with the screw and formed on the rigid polyurethane block), D is the length of the adhesive section, D1 is the length from the adhesive section on the first pigtail to the optical fiber of the FBG, and D2 is the length of the second pigtail 12 (i.e., the length from the FBG to the end of the second pigtail). Wherein, the pasting direction of the pasting section is the y direction. The area of the lower part of the first component to be tested 41 for drawing a transverse line is the area limited by the limiting piece 50.

Fig. 4 is a simulation modal analysis result based on finite element of the first component to be tested shown in fig. 3.

For the convenience of attaching the FBG, the X-plane (i.e. the plane perpendicular to the X-axis) facing the reader is used as the attaching plane to be examined. Referring to fig. 4, the mode shape of each order mode obtained by the finite element simulation method, the third order mode and the fourth order mode are mainly torsion, but the fiber bragg grating is not sensitive to torsion, so the third order mode and the fourth order mode will not be sensed by the sensing device; the first, second, and fifth order modes of the first component 41 to be tested generate relatively uniform linear displacement in the y-direction in the x-plane, so that the y-direction is suitable for the bonding direction of the optical fiber. The displacement of each particle on the pasting length is consistent, namely the same color area is formed in the simulation diagram. The fifth order mode in the simulation is not easily excited in actual test (the simulated natural frequency is 5086.5 Hz), so the first and second order modes are taken 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.

In combination with the above analysis, the fiber attachment position of the first component to be tested 41 in this embodiment is shown in fig. 3. According to the simulation modal analysis result, in order to enable the sound waves in the optical fiber to have the same phase, the sticking length d is 8 mm-10 mm (refer to a scale of the figure).

Further experimental corrections were made to obtain the effective vibration frequency range that can actually be excited and perceived by the FBG. The experimental calibration method comprises the following steps: the FBG is attached to a region (the region shown in fig. 3 in this embodiment, the attachment length is 8mm to 10 mm) of the member to be measured, where the strain amplitudes of the respective mass points indicated in the finite element analysis of the target mode are consistent, in the direction (the y direction in this embodiment) of generating the uniform linear strain indicated in the finite element analysis of the target mode. And performing impulse excitation according to the action point and the direction of the excitation in the actual nondestructive testing, observing the effective vibration frequency range and amplitude of impulse response, and taking the frequency corresponding to the maximum value of the amplitude in the spectrogram as the experimental natural frequency. Fig. 5 (a) is a graph of the obtained impulse response and spectrum.

Fig. 5 is a frequency spectrum diagram of the impulse response when FBGs are attached to the first component to be tested shown in fig. 3 in different directions.

To detect that the y direction (direction generating more uniform linear displacement) shown in fig. 4 is suitable for the bonding direction of the tail fiber, FBGs are directly bonded to the X surface of the first component to be tested 41 shown in fig. 3 at different angles, i.e. the z direction (fig. 5 (b)) and the 45 degree angle with the z direction (fig. 5 (c)), respectively. The length of the pasting area is still 8 mm-10 mm as in FIG. 5 (a). The specific paste positions and corresponding impulse response spectra under excitation of the same impulse function are shown in fig. 5 (b) and 5 (c).

Referring to FIG. 5, the pasting mode signal along the y-direction has a much larger amplitude than other frequency signals at 1600-4000 Hz. The signal amplitude of the pasting mode along the z axis is greatly reduced, and the 500-2500Hz signal accounts for the main component. When the adhesive tape is adhered to a Z axis at an included angle of 45 degrees, certain signals exist in the frequency range smaller than 7000Hz, the signal amplitude is small, and the signals are mainly under 4000 Hz. Actually measuring the signal-to-noise ratio of the system when the system is pasted along the z direction to be 18.069; when the adhesive tape is adhered to the z-axis in the direction with the included angle of 45 degrees, the signal-to-noise ratio of the system is 23.5958; the signal-to-noise ratio of the system when pasted in the y-direction is 25.2305.

The test results thus demonstrate that: the pasting method along the y direction shown in fig. 5 (a) can obtain higher impulse response signal-to-noise ratio than other pasting methods, and this pasting method is superior to other pasting methods. The fifth-order mode in the simulation is not easy to excite in the actual test (the simulation natural frequency is 5086.5 Hz), so the first-order mode and the second-order mode are taken as target modes.

Selecting an excitation mode: in this embodiment, the impulse response may be repeated after multiple impulse experiments, so impulse excitation may be selected. And because the influence of low-energy periodic excitation on the component to be tested is small, both excitation modes can be adopted.

Determining the frequency of the excitation source at the time of periodic excitation: as can be known from the impulse response frequency spectrum analysis obtained by the experiment in FIG. 5 (a), the FBG can sense that the effective vibration frequency range of the first component 41 to be measured is near 1600Hz to 3600Hz, wherein 16999Hz, 1899Hz,1999Hz,2466Hz,2632Hz,2866Hz and 3632Hz are frequencies corresponding to the maximum values obtained by the impulse response, namely the experiment natural frequency. The vibration amplitude corresponding to the frequency 2466Hz is maximum, and the vibration with larger amplitude can be excited near the interval 1699Hz-1999Hz and the interval 2466Hz-2866Hz, and the range covers the simulated natural frequency (1998 Hz) of the first-order mode and the simulated natural frequency (2616 Hz) of the second-order mode in the target mode simulated by the graph 4.

First-order and second-order simulation natural modes are comprehensively considered, and then an experimental natural frequency indicated by an impulse excitation experimental test result figure 5 (a) is combined, a 314Hz periodic signal is used as an excitation period, and high-order resonant frequencies excited by the 314Hz periodic signal are 1884Hz, 2512Hz, 2826Hz, 3140Hz and 3454Hz in the range of 1600 Hz-3600 Hz. Some of these frequencies are around the simulated and experimental natural frequencies, and some of these frequencies are around the 1699Hz-1999Hz and 2466Hz-2866Hz intervals, so it is considered that sufficient amplitude vibration can be excited in this range.

For the frequency that the design cycle excitation used, the utility model discloses use the natural frequency of structure as the design foundation to obtain great sensing sensitivity and SNR. The utility model discloses there are two kinds to confirm the natural frequency method. One method is to use finite element-based simulation modal analysis, which can obtain the mode shape and the simulation inherent frequency value of each order of mode, but the assembly condition of the structure and the mechanical parameters of the material inevitably have errors with the reality, so the data obtained by simulation cannot be absolutely accurate. And the other method is an impulse response method obtained by experiments, because the impulse excitation contains all frequencies theoretically, the frequency corresponding to the vibration with larger amplitude indicated in the frequency spectrum obtained by the impulse response is the actual vibration frequency which can be excited, and the frequency corresponding to the maximum value of the amplitude is the experimental natural frequency value under the impulse excitation condition. However, since the actual natural frequency is influenced by the magnitude of the excitation force, if periodic excitation is used, the excitation intensity and the impulse excitation intensity differ, and thus the actual natural frequency and the experimental natural frequency obtained by the impulse excitation also differ. Therefore, in view of the complexity of the natural frequency, the present invention refers to the simulated natural frequency of the target mode obtained in the simulated mode analysis and the experimental natural frequency obtained by the experiment.

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

wherein x ₀ Is the direct current component of the signal, a _n For the amplitude of the respective harmonic(s),

the initial phase, i.e. the periodic excitation, of each harmonic in fact contains each harmonic component. Thus period of timeThe frequency of each order of high harmonic wave of excitation is close to the natural frequency of simulation and the natural frequency of experiment, can reach effective excitation target mode to can be effectively perceived by FBG sensing system, and then carry out effectual nondestructive test.

Determining the lengths D1 and D2 of the sticking sections of the first tail fiber and the FBG: the wavelength, half wavelength, quarter wavelength and three quarter wavelength are calculated by the wave speed 3743.54m/s (constant) of the optical fiber with the wave speed being non-dispersive according to the high-order resonant frequency 1884Hz, 2198Hz, 2512Hz, 2826Hz, 3140Hz and 3454Hz of 314Hz in the range of 1600Hz to 3600Hz of effective excitation, and are 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 adhesive segment are preferably disposed at a half wavelength or an integral multiple of the half wavelength.

Table 1: wavelength, half wavelength, quarter wavelength and three-quarter wavelength at different frequencies

As can be seen from the excitation response spectrum of fig. 5 (a), the amplitude of vibration is largest near the experimental natural frequency 2466 Hz. In fig. 4, the second-order modal motion direction is the same as the sticking direction y direction, i.e. the mode most likely to be sensed by the FBG. The second order mode indicates a simulated natural frequency of 2616Hz. The closest vibration frequency to these two frequencies in table 1 is 2512Hz, for which we choose 2512Hz in table 1 as the reference frequency to design the adhesive segment of the first pigtail to FBG length D1 and the second pigtail length D2 with half wavelength 745.1mm. In other embodiments, the reference frequency may be determined by frequency sweeping.

FIG. 6 is a graph of the response and spectrum of a sensing system of the component under test of FIG. 3 under a 314Hz periodic excitation.

The periodic excitation energy generated by the vibration exciter is used for counteracting energy loss caused by structural vibration damping in the structure, and stable-energy vibration with limited amplitude is formed. According to table 1, in order to place the FBG and the attached segment of the first tail fiber at the antinode of the standing wave, two sets of data, D1=740mm, D2=750mm and D1=740mm, and D2=1495mm, were taken, and the time domain waveform pattern and the frequency spectrum were tested and shown in fig. 6, and the signal-to-noise ratios thereof were 33.4365 and 27.0675, respectively.

FIG. 7 is a graph of the response and spectrum of a sensing system of the component under test of FIG. 3 under a 314Hz periodic excitation.

Fig. 7 (a) D1 is selected: 2mm, D2:200mm; fig. 7 (b) D1:150mm, D2:2300mm; FIG. 7 (c) D1:662mm, D2:1495mm; fig. 7 (D) D1:980mm, D2:1495mm. The corresponding signal-to-noise ratios are 15.4913, 20.9960, 20.9138, 23.4725, respectively.

It can be seen that the signal-to-noise ratio of the sensing signal shown in fig. 6 is significantly greater than that of the sensing signal shown in fig. 7, and the signal-to-noise ratio of the test signal can be indeed improved by taking the half wavelength or the integral multiple of the half wavelength for D1 and D2.

Fig. 8 is a time domain waveform and a frequency spectrum diagram of the screw on the first component to be tested shown in fig. 3 and the internal threaded hole matched with the screw under different pre-tightening torque conditions.

According to the theory related to screw tightening, the larger the pre-tightening torque, the smaller the vibration loss of the structural member 41. According to the characteristic, the fastening state between the screw and the matched internal threaded hole can be judged.

A section of the first tail fiber 11 of the FBG is adhered to one side of the first component 41 to be measured along the y direction, the FBG is in a suspension state, the second tail fiber 12 of the FBG is in a suspension state, and the tail end of the second tail fiber 12 is arranged in the air. Wherein, D1=740mm, D2=745mm, the length of the adhesive segment is 10mm, and the length of the FBG is 10mm.

The vibration exciter generating the periodic excitation of 314Hz is abutted against the other side of the first component to be tested 41, the vibration generated in the first component to be tested 41 is taken as a sound wave source and is transmitted into the FBG10 and the second tail fiber 12 from the first tail fiber 11 in the forward direction, the tail end of the second tail fiber 12 reflects the sound wave and transmits the sound wave along the optical fiber in the reverse direction, the sound wave which is transmitted in the forward direction and the reverse direction forms a standing wave in the second tail fiber 12, the FBG10 and the first tail fiber 11 of the FBG, and the FBG senses a standing wave signal.

And demodulating the standing wave signal on the FBG into an electric signal by using an edge filtering method. The electric signal is led into the data analyzer 26 through the data acquisition unit 25, the data analyzer 26 processes the electric signal through data processing software to obtain the sensing system response and the frequency spectrogram, and the state of the first component to be detected can be judged according to the sensing system response and the frequency spectrogram.

Wherein the pretension torque between the screw and the internally threaded hole is respectively (a) 0Ncm; (b) 35Ncm; (c) 70Ncm; (d) 100Ncm

Table 2: corresponding relation between power and torque of signal in one period

The calculated signal power versus torque for one cycle is applied as shown in table 2. As can be seen from Table 2, the test signal power is stronger with the increase of the pre-tightening force. Therefore, the pretensioning state of the fastener can be judged through the test signal.

Fig. 9 is an impulse response spectrum of the test system under different displacement conditions along the z direction of the whole to-be-tested member shown in fig. 3.

And monitoring the translation of the whole body of the member to be measured 41 along the z-axis. The component 1 to be tested may be loosened from the bottom (the surrounding constraint is unchanged) in the working process, and the impulse excitation with the same intensity is applied to the component to be tested in the nondestructive testing experiment so as to test the translation of the whole component to be tested 41 along the z axis.

A section of the first tail fiber 11 of the FBG is adhered to one side of the first component 41 to be measured along the y direction, the FBG is in a suspension state, the second tail fiber 12 of the FBG is in a suspension state, and the tail end of the second tail fiber 12 is arranged in the air. Wherein, D1=740mm, D2=745mm, the adhesive segment is 10mm long, and the FBG is 10mm long.

The vibration exciter generating impulse excitation is collided with the other side of the first component 41 to be detected, vibration generated in the first component 41 to be detected serves as a sound wave source and is transmitted into the FBG and the second tail fiber 12 from the first tail fiber 11 in the forward direction, the tail end of the second tail fiber 12 reflects the sound wave and transmits the sound wave in the reverse direction along the optical fiber, the sound wave which is transmitted in the forward direction and the reverse direction forms standing waves in the second tail fiber 12, the FBG10 and the first tail fiber 11 of the FBG, and the FBG senses a standing wave signal.

Wherein FIG. 9 (a) is not disengaged from the bottom constraint; FIGS. 9 (b) - (d) are out of the bottom constraint with z-direction displacements of (a) 0mm, respectively; (b) 15.9mm; (c) 35.9mm; (d) 45.9mm.

As can be seen from fig. 9, when a bottom of the component to be measured is completely constrained (as shown in fig. 9 (a)), the impulse response spectrum thereof shows a plurality of peaks below 4000 Hz. And when the bottom of the impulse response is separated from the bottom constraint along the z direction, the main peak of the frequency spectrum of the impulse response gradually moves to the low frequency along with the increase of the displacement. The spectral shape also gradually transitions from multiple peaks to a single peak. From this feature, the approximate displacement of the entire structure in the z-direction can be evaluated.

FIG. 10 is an impulse response spectrum of the component under test shown in FIG. 3 at three different displacements in the z-direction.

For example, if an impulse response spectrum of the to-be-measured member is shown in fig. 10 (a), (b), and (c), the z-direction displacement is about 16mm, 36mm, and 46mm, respectively, as can be seen from the comparison of the spectra.

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

Referring to fig. 11, the first component to be measured 41 shown in fig. 3 is replaced with the second component to be measured 42 shown in fig. 11. The device consists of two parts, wherein the first part is a cylindrical cylinder 31 with a cavity inside, the bottom of the cylindrical cylinder is fixed on a limiting piece 50, and the fixed part is a constraint part; the second part is a solid cylinder 34 with another hollow cylinder 32 at the top; the first part and the second part are fixed by bolts 33; the axial direction of the hollow cylinder 32 coincides with the axial direction of the bolt 33. All parts of the whole second component to be detected 42 are aluminum alloy materials.

Fig. 12 is a simulation modal analysis result based on finite elements of the second component to be tested shown in fig. 11.

Referring to fig. 12, in each order mode, the strain modes of the first order, the second order, the third order, the fourth order, the sixth order, the seventh order, and the eighth order are all linear displacements, and only the directions are different. The fifth and ninth order are predominantly torsional, and since the fibre is not sensitive to torsion, the fifth and ninth order modes will not be perceived by the present sensing apparatus. The first, second, third, fourth, sixth, and eighth-order modes of the second component 42 to be tested generate relatively uniform linear displacement along the z direction on the cylindrical surface, so that the z direction is suitable for the adhesion direction of the optical fiber. According to the simulation result, the pasting length is 8-15mm (refer to the scale of the figure) in order to make the sound wave in the optical fiber have the same phase.

Fig. 13 is a frequency spectrum diagram of the impact response when the FBGs are attached to the second component to be tested in different directions.

The FBGs are directly adhered to the outer cylindrical surface of the cylindrical cylinder 31 with the cavity inside in the second component to be measured 42 shown in fig. 11 at different angles, the adhering directions are respectively the z direction, and the y direction and the z direction form an included angle of 45 degrees. The specific paste position and the corresponding impulse response spectrum under the excitation of the same impulse function are shown in fig. 13.

As can be seen from fig. 13, the amplitude of the pasting manner in the z direction is much larger than that of other pasting manners, and the signal frequency is 1600-7000Hz; when the adhesive tape is adhered to the z-axis at an included angle of 45 degrees, signals of the adhesive tape are dispersed at 200-12000Hz, and the amplitudes of the signals of the inherent frequencies of the experiments are basically consistent; the signal amplitude of the y-axis pasting mode is greatly reduced, and the signal of 200-8200Hz accounts for the main component. Actually measuring the signal-to-noise ratio of the system when the adhesive tape is adhered along the z direction, wherein the signal-to-noise ratio of the system is 26.7107; when the adhesive tape is adhered along the direction with the included angle of 45 degrees with the z axis, the signal-to-noise ratio of the system is 24.5548; when pasted in the y-direction, the signal-to-noise ratio of the system is 18.9105.

The test results thus demonstrate that: the pasting mode in the z direction (the direction generating more uniform linear displacement) can obtain higher impulse response signal-to-noise ratio than the response signal-to-noise ratio in other pasting directions, and the pasting direction is superior to other pasting directions.

Selecting an excitation mode: the impulse response can be repeated after a plurality of impulse experiments, so that the impulse excitation can be selected. And because the influence of low-energy periodic excitation on the component to be tested is small, both excitation modes can be adopted.

Determining the frequency of the excitation source at the time of periodic excitation: as can be seen from the experimental analysis in FIG. 13 (a), the effective vibration frequency range of the excited second member to be tested is about 1600Hz-7000 Hz. From the finite element-based simulation analysis of fig. 12, since the first and second order modes are not in the effective vibration frequency range, the overall displacements of the third, seventh and eighth order mode structures are curved, and the fifth and ninth order modes are torsional and are not sensed by the FBG. The fourth order (simulated natural frequency 1994 Hz) and the sixth order (simulated natural frequency 2985 Hz) are selected as target modes, since they correspond to a uniform line strain. The experimental natural frequencies corresponding to the maximum amplitude values of the FBG impulse response were found to be around 1657Hz, 3285Hz, 5342Hz, and 6428Hz in fig. 13 (a). The four experimental natural frequencies and the simulated natural frequencies of the fourth-order and sixth-order modes select 330Hz periodic signals as excitation, and the frequencies corresponding to the obtained high-order harmonics are 1650Hz, 1980Hz, 2970Hz, 3300Hz, 3630Hz, 5280Hz, 5610 Hz, 5940Hz, 6270Hz and 6600Hz. Some of the frequencies corresponding to these higher order harmonics are near the fourth and sixth order simulated or experimental natural frequencies. The wavelength, half wavelength, quarter wavelength and three-quarter wavelength of the fiber are calculated at 3743.54m/s, where the wave velocity in the fiber is non-dispersive, as shown in Table 3.

TABLE 3 wavelength, half wavelength, quarter wavelength and three-quarter wavelength at different frequencies

As can be seen from fig. 13 (a), the vibration amplitude corresponding to the experimental natural frequency 3285Hz is the largest. In the frequency corresponding to the high-order harmonic wave of 330Hz periodic excitation designed by the utility model, 3300Hz is closest to the inherent frequency 3285Hz of the experiment; 2970Hz is closest to the sixth order artificial natural frequency 2985Hz. According to the principle of priority of experimental results, 3300Hz is taken as the reference frequency of the design D1 and D2, and the half wavelength is 567.2mm.

Fig. 14 is a time domain waveform and a frequency spectrum diagram of a signal of the sensing system of the second component to be measured shown in fig. 11 under the periodic excitation of 330Hz, in order to place the FBG and the adhesive segment at the antinode of the standing wave, according to table 3, D1=567mm, D2=570mm is taken, and the signal-to-noise ratio is measured to be 19.2236.

Fig. 15 is a time-domain waveform and a frequency spectrum diagram of a response signal of the sensing device of the second component under test shown in fig. 11 under the periodic excitation of 330 Hz.

Fig. 15 (a) D1 is selected: 430mm, D2:637mm; FIG. 15 (b) D1:567mm, D2:637mm; fig. 15 (c) D1:567mm, D2:697mm; fig. 15 (D) D1:567mm, D2:727mm; fig. 15 (e) D1:567mm, D2:860mm; FIG. 15 (f) D1:630mm, D2:647mm; fig. 15 (g) D1:630mm, D2:697mm; fig. 15 (h) D1:700mm, D2:637mm, as shown in FIGS. 15 (a) -15 (h), the corresponding SNR's are 9.4812, 10.7910, 13.7618, 16.8931, 17.7747, 18.0831, 15.0887, and 17.2563, respectively.

The signal-to-noise ratio of inverse view of fig. 14 is 19.2236, and thus the signal-to-noise ratio of the sensor signal of fig. 14 is significantly greater than the signal-to-noise ratio of the sensor signal of fig. 15. The half wavelength or integral multiple of the half wavelength of the D1 and the D2 can improve the response sensitivity and the signal to noise ratio of the test effective signal.

Fig. 16 is a response and frequency spectrum diagram of the detection system shown in fig. 11 when the hollow cylinder on the second member to be detected and the fastening bolt are in different included angles.

For the application of the present invention, this example is used to test the offset state of the top hollow cylinder and the bolt in the second component to be tested. Due to the geometrical characteristics of the structure, when the axial direction of the hollow cylinder at the top is consistent with the axial direction of the bolt or is vertical to the axial direction of the bolt, the whole structure is symmetrical; wherein the symmetry is higher when the axial direction of the hollow cylinder at the top is consistent with the axial direction of the bolt. In the embodiment, nondestructive testing is performed by taking the axial deviation of the hollow cylinder at the top and the axial deviation of the bolt as a testing target. When the second member to be measured shown in fig. 11 is in operation, the axial direction of the top hollow cylinder 32 and the axial direction of the bolt 33 may be deviated.

The first pigtail 11 is pasted on the second member to be tested in the direction shown in fig. 13 (a), and the pasted length is 10mm, d1 is 567mm, and d2 is 570mm, and is used for nondestructive testing of the second member to be tested.

A vibration exciter which generates 330Hz periodic excitation is collided with the other side of the second component to be detected, vibration generated in the second component to be detected 42 serves as a sound wave source and is transmitted into the FBG and the second tail fiber 12 from the first tail fiber 11 in the forward direction, the tail end of the second tail fiber 12 reflects the sound wave and transmits the sound wave in the reverse direction along the optical fiber, the sound wave which is transmitted in the forward direction and the reverse direction forms standing waves in the second tail fiber 12, the FBG10 and the first tail fiber 11 of the FBG, and the FBG perceives standing wave signals.

The standing wave signal on the FBG is demodulated to an electrical signal by edge filtering. The electric signal is led into the data analyzer 26 through the data acquisition unit 25, the data analyzer 26 processes the electric signal through data processing software to obtain the response and the spectrogram of the sensing system, and the state of the component two to be detected can be judged according to the response and the spectrogram of the sensing system.

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

Referring to fig. 16, when an included angle between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 is 0 degree or 90 degrees, the periodic signal response frequency spectrums are concentrated at 3300Hz, and the response amplitude is maximum when the included angle is 0 degree; the amplitude is smaller when the included angle is 90 degrees, but is larger when the included angle is relative to other angles; according to the frequency spectrum, when the energy is concentrated at one frequency, the structure has symmetry (corresponding to the included angle between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 is 0 degree or 90 degrees); and the stronger the signal, the better the symmetry (corresponding to the included angle between the axial direction of the hollow cylinder 32 and the axial direction of the bolt 33 being 0 degree); when the structure has no symmetry, the spectrum is dispersed and the signal amplitude is weak (corresponding to the cases indicated in fig. 16 (c), (d), (e)). Therefore, the symmetrical state of the structural member can be judged by this feature.

Fig. 17 is an impulse response frequency spectrum of the test system under the condition that the solid cylinder of the second component to be tested shown in fig. 11 is displaced along the z direction.

Monitoring the translation of the inner solid cylinder 34 of the second member to be tested 42 along the z-axis:

the second component to be tested is in a working initial state, namely, the distance of 51.3mm exists between the bottom of the top hollow cylinder 32 and the top of the cylindrical cylinder 31 with the cavity, the two components are still fastened by the bolt 33, and the top hollow cylinder 32 can slide downwards due to the looseness of the fastening bolt 33 in the working process, so that the volume of the cavity part of the cylindrical cylinder 31 is reduced. In the nondestructive testing embodiment, the solid cylinder 34 in the second component to be tested 42 is tested to translate along the z-axis in an impulse excitation manner, so as to obtain the impulse response frequency spectrum change characteristic, as shown in fig. 16. The relative position is evaluated based on the detection result.

The vibration exciter which generates impulse excitation is collided with the other side of the second component to be detected, vibration generated in the second component to be detected is taken as a sound wave source and is transmitted into the FBG and the second tail fiber 12 from the first tail fiber 11 in the forward direction, the tail end of the second tail fiber 12 reflects the sound wave and transmits the sound wave in the reverse direction along the optical fiber, the sound wave which is transmitted in the forward direction and the reverse direction forms standing waves in the second tail fiber 12, the FBG10 and the first tail fiber 11 of the FBG, and the FBG perceives standing wave signals.

And demodulating the standing wave signal on the FBG into an electric signal by using an edge filtering method. The electric signals are guided into the data analyzer 26 through the data acquisition unit 25, the data analyzer 26 processes the electric signals through data processing software to obtain sensing system response and a frequency spectrogram, and the state of the solid cylinder of the second component to be detected can be judged according to the sensing system response and the frequency spectrogram.

And testing the impulse response of the system under the condition that the solid cylinder displaces along the z direction in different ways. The fastening displacement in the z direction is respectively: FIG. 17 (a) 51.3mm; FIG. 17 (b) 45.9mm; FIG. 17 (c) 26.7mm; FIG. 17 (d) 13mm; FIG. 17 (e) 2.9mm; FIG. 17 (f) 0mm.

As can be seen from fig. 17 (a), the impulse response spectrum of the initial state of operation mainly includes three main peaks, i.e., signals of about 1500Hz, 2400Hz, and 3300Hz, and the peak of the main peak gradually moves to a higher frequency as the solid cylinder 34 moves downward along the z-axis direction. The 1500Hz frequency signal in fig. 17 (b) is weaker relative to fig. 17 (a), but the 3300Hz frequency accessory signal is stronger; the energy in FIG. 17 (c) is mainly concentrated around 2400 Hz; the main energies of FIGS. 17 (d) - (e) become around 2400Hz and 3300Hz accessories; in FIG. 17 (f), the main peak is further shifted to a high frequency, and the main frequencies are 1650Hz, 3300Hz,5250Hz, and 6450Hz or so. Therefore, the degree of slippage of the solid cylinder 34 in the cavity can be evaluated by this feature.

Fig. 18 is an impulse response spectrum of the z-direction displacement of the solid cylinder of the second member to be measured shown in fig. 11.

For example, if the impulse response spectrum of the structural member is shown in fig. 18 (a), (b), and (c), the following can be determined by comparing the spectra: no slippage occurred in FIG. 18 (a), the amount of slippage in FIG. 18 (b) was about 3mm, and the amount of slippage in FIG. 18 (c) was about 27mm.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present application.

Claims

1. The nondestructive detection system based on the structural vibration-optical fiber acoustic guided wave sensing is characterized by comprising a vibration exciter generating periodic excitation or a vibration exciter generating impulse excitation, an FBG (fiber Bragg Grating), a demodulation device and an electric signal analysis processing system;

one section of the first tail fiber of the FBG is adhered to one side of the component to be tested to form an adhering section, the first tail fiber, the FBG and the second tail fiber are placed in a space without constraint, and the tail end of the second tail fiber is placed in air or a reflecting medium;

the vibration exciter acts on the other side of the component to be detected, vibration is generated in the component to be detected, the vibration serves as a sound wave source and is transmitted into the FBG and the second tail fiber from the first tail fiber in the forward direction, the tail end of the second tail fiber reflects the sound wave and transmits the sound wave in the reverse direction along the optical fiber, the sound wave which is transmitted in the forward direction and the reverse direction forms standing waves in the second tail fiber, the FBG and the first tail fiber of the FBG, and the FBG senses standing wave signals;

the demodulation device is used for demodulating the standing wave signal on the FBG into an electric signal;

the electric signal analyzing and processing system is used for analyzing and processing the electric signal and judging the state of the component to be detected.

2. The nondestructive inspection system of claim 1, wherein the FBG is located at an antinode of the standing wave and a section of the first pigtail attached to the component under test is located at the antinode of the standing wave.

3. The nondestructive inspection system of claim 1, wherein the demodulation means comprises a narrowband light source, a circulator, and a photodetector;

light emitted by the narrow-band light source enters the FBG from the tail end of the first tail fiber of the FBG, and a standing wave signal on the FBG is sensed;

the photodetector is used for converting the standing wave signal reflected from the FBG through the circulator into an electric signal.

4. The nondestructive testing system of claim 1, wherein the adhering direction of the adhering section on the member to be tested is: carrying out the direction for generating uniform line strain indicated in finite element simulation modal analysis on the component to be detected;

the pasting area of the pasting section on the component to be tested is as follows: and in the finite element simulation modal analysis of the component to be measured, generating a region with uniform linear strain of the target mode, wherein the strain amplitudes of all particles indicated in the finite element simulation modal analysis are consistent.

5. The nondestructive testing system of any of claims 1-4, wherein the length D1 of the attached section of the first pigtail to the FBG is an integer multiple of half wavelength of the reference frequency, and the length D2 of the second pigtail is an integer multiple of half wavelength of the reference frequency;

the reference frequency is the frequency which is closest to the vibration frequency with the maximum amplitude in the integral multiple frequency of the periodic excitation frequency within the vibration frequency range in which the FBG can be effectively sensed and the vibration of the component to be detected can be effectively excited.

6. The nondestructive testing system of claim 5, wherein the reference frequency is a vibration frequency closest to a maximum amplitude indicated by a target modal simulation analysis in a finite element simulation modal analysis of the component under test.

7. The nondestructive inspection system of claim 5, wherein the reference frequency is the frequency closest to the vibration frequency at which the amplitude indicated by the impulse response is greatest.