FBG-based crack monitoring optical fiber and method

Abstract

The invention discloses an FBG-based crack monitoring optical fiber and method, and belongs to the technical field of optical fiber sensing monitoring, and the FBG-based crack monitoring method comprises the following steps: fixing a monitoring optical fiber on the surface of a to-be-detected substrate; bragg wavelength is obtained through the multiple groups of Bragg gratings, and when the Bragg wavelength changes, a wavelength change value is obtained; calculating a fiber core strain change value according to the wavelength change value; calculating a substrate strain change value according to the fiber core strain change value and the calibrated strain conductivity; and substituting the substrate strain change value into a preset crack-strain transfer model, and calculating to obtain a crack opening amount and a crack center position. According to the invention, factors of substrate strain redistribution and coating / fiber core interface slippage are comprehensively considered, so that the crack displacement of the tension member can be monitored more accurately.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber sensing monitoring, and particularly relates to a crack monitoring optical fiber and method based on FBG. BACKGROUND

[0002] In the engineering application of infrastructure construction, external load may induce cracking of engineering structures, causing structural damage. These damages can significantly reduce the carrying capacity of the structure. If not discovered and repaired in time, the cracks will develop and expand, leading to a decrease in the effective carrying area of the structure, and even may cause catastrophic accidents. For example, in reinforced concrete structures, steel cracking not only reduces the integrity and durability of the concrete structure, but also, under the influence of the external environment, corrosion is prone to occur at the steel crack, which will further accelerate the instability of the engineering structure. If the engineering structure can be monitored in real time at an early stage, cracks can be discovered in time, and the structure can be repaired and improved before the cracks further expand and affect the safety of the structure, serious consequences can be effectively avoided, and economic losses can be minimized. Therefore, efficient and accurate monitoring of cracks has become a problem to be solved in the field of infrastructure construction engineering.

[0003] Traditional monitoring methods mainly rely on manual visual inspection, but this method consumes a lot of manpower and time, and it is particularly difficult to identify small cracks, and the detection results are prone to inaccuracy due to human factors. In recent years, with the development of optoelectronic technology and computer fields, non-destructive monitoring technologies based on machine vision, acoustic emission, ultrasonic waves, etc. have been applied in the crack monitoring of engineering structures. Although these monitoring methods can achieve long-term monitoring of engineering structure cracks through non-contact means, they have problems such as being susceptible to environmental interference, limited ability to monitor cracks in materials, and being less sensitive to small or hidden cracks.

[0004] Fiber optic sensing technology is a new monitoring method that can sense the basic physical quantities such as temperature and strain along the entire length of the optical fiber in real time by taking light as the carrier and optical fiber as the medium. Compared with traditional monitoring methods, fiber optic sensing technology has the advantages of small size, long-term durability, resistance to electromagnetic interference, long-distance large-range signal transmission, and continuous sensing in space, and is very suitable for application in the monitoring of engineering structures. At present, distributed fiber optic sensors (DFOS) have attracted wide research interest in the laboratory and field monitoring of concrete and steel structure cracks. It does not need to know the location of the crack in advance, and multiple cracks can be monitored simultaneously. For example, optical time domain reflectometry (OTDR) is used for distributed sensing of reinforced concrete structure cracks. Brillouin-based sensors, including BOTDR and BOTDA technology, also appear in structural health monitoring, and frequency domain optical frequency domain reflectometry (OFDR) is used to monitor the cracking behavior of the base material. Although distributed fiber optic sensors have many advantages in structural monitoring, the high cost, low spatial resolution, and low accuracy further limit their application in structural health monitoring, especially for the monitoring of key parts of engineering structures. As a discrete fiber optic sensor, FBG sensors have the advantages of small size, light weight, high sensitivity, high measurement accuracy, corrosion resistance, and gradually become a research hotspot in the field of crack monitoring. By measuring the change of the wavelength of light, high-precision measurement of small structural deformation changes can be achieved, and early structural deformation monitoring can be achieved. However, FBG sensors still face some challenges in practical application of structural monitoring. The spatial resolution and measurement accuracy of FBG sensors are affected by the installation process of the optical fiber, which will directly affect the stability and accuracy of the test results. At the same time, FBG sensors are also affected by factors such as interface slip and base strain redistribution in crack monitoring, and there is currently little research on this. The influence of external load on structural monitoring needs to be further improved. SUMMARY

[0005] Based on the technical problems existing in the prior art, the present application provides a FBG-based crack monitoring optical fiber and method.

[0006] According to the first aspect of the technical scheme of the present application, the present application provides a FBG-based crack monitoring optical fiber, which comprises a plurality of groups of Bragg gratings, the plurality of groups of Bragg gratings are uniformly arranged along the axial direction, the plurality of groups of Bragg gratings are sleeved with a fiber core on the outside, the fiber core is sleeved with an epoxy resin layer on the outside, and the epoxy resin layer is provided with a coating layer on the outside. The material of the fiber core is ultraviolet-sensitive germanium-doped single-mode quartz glass, and the diameter is 8.2pm±0.3pm; the refractive index is 1.468, the grating region length of the Bragg grating is less than or equal to 10mm, the grating period is 535.5nm, and the grating reflectivity is 70%. The material of the epoxy resin layer is ultraviolet curing acrylate, and the outer diameter is 165 mu m±5 mu m; the coating layer is ultraviolet curing acrylate, and the outer diameter is 245 mu m±5 mu m.

[0007] The further improvement of the present application is that the grating pitch of the two adjacent groups of Bragg gratings is 60 mm.

[0008] According to the second aspect of the technical scheme of the present application, the present application provides a crack monitoring method based on FBG, which is based on a crack monitoring optical fiber based on FBG, and comprises the following steps: Step S1, fixing the monitoring optical fiber to the surface of the substrate to be detected; Step S2, obtaining the Bragg wavelength through the plurality of groups of Bragg gratings, and obtaining the wavelength change value when the Bragg wavelength changes; Step S3, calculating the core strain change value according to the wavelength change value; Step S4, calculating the substrate strain change value according to the core strain change value and the calibrated strain conductivity; Step S5, substituting the substrate strain change value into the preset crack-strain transfer model to calculate the crack opening amount and the crack center position.

[0009] The further improvement of the present application is that the calculation of the strain conductivity comprises the following steps: respectively establishing a first balance equation of the core, a second balance equation of the epoxy resin layer and a third balance equation of the coating layer; establishing a first shear hysteresis model and a second shear hysteresis model according to the second balance equation and the third balance equation; calculating the strain transfer rate according to the first shear hysteresis model, the first balance equation and the second shear hysteresis model.

[0010] The further improvement of the present application is that in the step of calculating the substrate strain change value according to the core strain change value and the calibrated strain conductivity, the substrate strain change value is the quotient of the core strain change value and the strain conductivity.

[0011] The further improvement of the present application is that the establishment of the crack-strain transfer model comprises the following steps: establishing a function formula with the coordinate of the crack center position along the core axis as the slope and the crack opening amount as the intercept according to the substrate strain change value; setting boundary conditions for the function formula; solving the function formula after setting the boundary conditions to obtain the crack-strain transfer model.

[0012] The further improvement of the present application is that the boundary conditions include that the axial displacement of the fiber core at the fiber core symmetry axis is zero and the strain at both ends of the fiber core is zero.

[0013] The further improvement of the present application is that the monitoring optical fiber is fixed on the surface of the substrate by using a cyanoacrylate adhesive.

[0014] The further improvement of the present application is that in the step of obtaining the Bragg wavelength, the sampling interval is less than or equal to 1 pm.

[0015] The further improvement of the present application is that the method further comprises the following steps: The crack opening amount is compared with a preset alarm threshold value, when the crack opening amount exceeds the alarm threshold value, an audible and visual alarm signal is sent, and the crack opening amount and the crack center position are uploaded to an operation and maintenance center.

[0016] Compared with the prior art, the above technical scheme of the present application has the following beneficial technical effects: 1. The crack monitoring optical fiber and method based on FBG of the present application carry out monitoring of the substrate crack based on the FBG sensor.

[0017] 2. The present application is based on a crack-strain transmission model, which is used to describe the influence of the cracking displacement of the tension member crack on the strain transmission of the Bragg grating, and comprehensively considers the factors of substrate strain redistribution and coating / fiber core interface slip, so as to more accurately monitor the action mechanism of the cracking displacement of the tension member crack on the strain transmission of the FBG sensor.

[0018] 3. The present application utilizes the high-precision measurement capability of the FBG sensor to monitor the strain change in the crack opening process in real time. Through experimental verification, it is found that the FBG sensor can more accurately reflect the position, width and depth of the crack, and at the same time avoids the limitations of traditional FBG technology in spatial resolution and strain transmission efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0019] The accompanying drawings serve to better understand the present application and do not constitute an improper limitation thereof. Among them: Figure 1 is a structural schematic view of a crack monitoring optical fiber based on FBG of the present application; Figure 2 is a top view of the crack monitoring optical fiber based on FBG of the present application installed on a substrate; Figure 3 is a side view of the crack monitoring optical fiber based on FBG of the present application installed on a substrate; Figure 4 is a strain transmission rate curve under different adhesive lengths in Example 3 of the present application; Figure 5is a strain transfer rate curve of different bonding widths in the embodiment 3 of the present application; Figure 6 is a partial enlarged view of Figure 5 Figure 7 is a strain transfer rate curve of different bonding layer thicknesses in the embodiment 3 of the present application; Figure 8 is a partial enlarged view of Figure 7 Figure 9 is a comparison chart of wavelength changes corresponding to the optimal parameter group and the other three groups in the embodiment 3 of the present application; Figure 10 is a relationship chart of tensile rate and wavelength change in the embodiment 3 of the present application; Figure 11 is a relationship chart of crack depth and wavelength change in the embodiment 3 of the present application; Figure 12 is a relationship chart of tensile force and wavelength change in the embodiment 3 of the present application. DETAILED DESCRIPTION

[0020] Exemplary embodiments of the present application are described herein below with reference to the accompanying drawings, in which various details of the present application are set forth to facilitate an understanding, and should be considered in connection with the summary. Thus, it will be appreciated that those skilled in the art will be able to devise various arrangements and modifications of the embodiments described herein without departing from the scope and spirit of the application. Also, for the purpose of clarity and the brevity, the description below omits the description of well-known functions and structures.

[0021] The present application provides a FBG-based crack monitoring optical fiber and a method thereof, the FBG-based crack monitoring optical fiber comprises a plurality of groups of Bragg gratings, the plurality of groups of Bragg gratings are uniformly arranged along an axial direction, a fiber core is arranged outside the plurality of groups of Bragg gratings, an epoxy resin layer is arranged outside the fiber core, and a coating layer is arranged outside the epoxy resin layer; the fiber core is made of ultraviolet light sensitive germanium-doped single-mode quartz glass, has a diameter of 8.2 μm±0.3 μm, and has a refractive index of 1.468; the Bragg grating has a grating region length of less than or equal to 10 mm and a grating period of 535.5 nm; the grating reflectivity is 70%; the epoxy resin layer is made of ultraviolet curing acrylate, has an outer diameter of 165 μm±5 μm, and the coating layer is ultraviolet curing acrylate, has an outer diameter of 245 μm±5 μm.

[0022] In another aspect, the present application provides a FBG-based crack monitoring method, which is based on the FBG-based crack monitoring optical fiber described above.

[0023] The technical solutions of the present application are further described below with reference to the accompanying drawings and specific embodiments.​​

[0024] Example One The application provides a FBG-based crack monitoring optical fiber, as shown in the figure, which comprises a plurality of groups of Bragg gratings uniformly arranged along the axial direction, a fiber core arranged outside the plurality of groups of Bragg gratings, an epoxy resin layer arranged outside the fiber core, and a coating layer arranged outside the epoxy resin layer. Figures 1-3

[0025] Specifically, the fiber core is made of ultraviolet light sensitive germanium-doped single-mode quartz glass, has a diameter of 8.2 μm ± 0.3 μm, a refractive index of about 1.468, a grating area length of less than or equal to 10 mm, and a grating period of about 535.5 nm; and the grating reflectivity is about 70%, which ensures that the signal-to-noise ratio is greater than 30 dB under 1 pm sampling resolution.

[0026] Specifically, the epoxy resin layer is made of ultraviolet curing acrylate, preferably low-modulus ultraviolet curing acrylate, and has an outer diameter of 165 μm ± 5 μm; and the coating layer is made of ultraviolet curing acrylate, preferably high-modulus ultraviolet curing acrylate, and has an outer diameter of 245 μm ± 5 μm.

[0027] ​Specifically, the ultraviolet photosensitive germanium-doped single-mode quartz glass is selected to ensure the grating writing efficiency required for 70% reflectivity, without hydrogen loading or high-temperature annealing, and the process is simple and has good long-term stability. The 8.2 µm core diameter ensures good compatibility, and special alignment is not required for on-site installation, reducing the difficulty of engineering, avoiding the "multiple peak splitting" caused by multimode interference, and ensuring the uniqueness of crack-wavelength demodulation. The grating length of less than or equal to 10 mm can lock the spatial resolution to the centimeter level, and can distinguish mutation areas with a crack of less than 5 mm; the grating length is too long, which can smooth the high gradient strain of the crack tip, resulting in underestimation of the opening amount. The grating period of 535.5 nm is matched with the order of magnitude of the subsequent strain coefficient, which is convenient for subsequent calculation. The reflectivity of 70% ensures that the signal-to-noise ratio is greater than 30 dB under a 1 pm sampling interval, and avoids the grating-induced loss and multiple reflection crosstalk caused by the reflectivity greater than 90%; at the same time, it allows multiple 60 mm interval series, and a single optical fiber can be connected to more than 30 points, covering a 2 m monitoring area. The size limitation of the epoxy resin layer makes the theoretical shear transfer length 4-5 mm, which forms an 8-10 times safety factor with the subsequent "adhesion length ≥ 40 mm", ensuring that the fiber core can still capture a 1 pm wavelength shift when the crack opening is 20 µm. The low-modulus ultraviolet-cured acrylate is between soft silicone rubber and hard epoxy, neither completely shielding the substrate strain nor causing interface peeling due to excessive hardness. The size of the coating layer is limited to the standard communication fiber outer diameter, which can be directly connected to the demodulator using commercial FC / APC connectors without the need to re-strip, improving the efficiency of on-site laying. The outer high-modulus acrylate is scratch-resistant, avoiding micro-bending loss caused by early deployment, and ensuring 0.1 pm long-term stability.

[0028] Specifically, the grating pitch is set to 60 mm, which is twice the half-width of the crack affected area (about 30 mm), and can completely capture the strain gradient caused by crack opening, while avoiding excessive grating density that causes spectral overlap. A 1 m length of a single optical fiber can be deployed with 16 grating points, which is the optimal cost-coverage area.

[0029] Specifically, the FBG sensor is a fiber Bragg grating sensor, which is a combination of a fiber core and a plurality of Bragg gratings in the embodiment. The fiber Bragg grating is formed by exposing the fiber core to a strong ultraviolet light interference pattern to induce a periodic refractive index modulation of the grating structure. The fiber using the fiber Bragg grating can directly convert the measured parameter into an optical wavelength. The basic principle is to detect the wavelength shift of the reflected "Bragg" light as a function of the measurement value. When the light of a broadband light source passes through the fiber Bragg grating, the specific wavelength light that satisfies the Bragg condition is reflected, and other wavelengths of light continue to transmit. The reflected Bragg wavelength ( ) is determined by the effective core refractive index ( ) of the fiber and the grating period ( ), and the reflected Bragg wavelength is expressed as:

[0030] The Bragg wavelength is sensitive to both strain and temperature. Strain causes a change in grating period due to physical elongation of the fiber and a change in fiber refractive index due to photoelastic effect, while temperature causes thermal expansion of the fiber and a change in fiber refractive index due to thermo-optic effect. For single-mode silica fiber, the Bragg wavelength shift due to strain change ( ) and temperature change ( ) is given by:

[0031] where and are the initial Bragg wavelength and its shift, is the strain change, is the temperature change. Pe, and are the photoelastic coefficient (theoretical value = 0.22), thermo-optic coefficient and thermal expansion coefficient of the fiber, respectively, and denote the strain and temperature coefficients, respectively, with typical values of 0.78 and 6.67 . When using the strain characteristics of fiber gratings for pressure measurement, the temperature cross-sensitivity effect needs to be eliminated, i.e. When

[0032] The further improvement of the present application is that the grating pitch of the adjacent two groups of Bragg gratings is 60 mm, and the grating length of each group of Bragg gratings is less than or equal to 10 mm.

[0033] Preferably, the FBG sensor is composed of a single-mode fiber and three Bragg gratings, and the three grating points (λ 1530 ,λ 1533 , λ 1536 ) are strain sensors. The fiber containing the Bragg grating is parallelly pasted on the surface of the aluminum alloy. The fiber is bonded to the surface of the aluminum alloy substrate by epoxy resin, and the bonding length of the fiber is 40 cm. The surface width of the sample is directly used as the bonding width. In terms of thickness control, the epoxy resin adhesive is dropped at a fixed interval of 4 mm by using the doctor blade method, and then uniformly coated by using the doctor blade. The 9903 acrylic epoxy resin is selected as the experimental adhesive based on its high shear modulus.

[0034] Example Two According to the technical scheme of the present application, the present application provides a crack monitoring method based on FBG, which comprises the following steps: ​Step S1: Fix the monitoring optical fiber to the surface of the substrate to be tested; Step S2: Obtain the Bragg wavelength through the several sets of Bragg gratings; when the Bragg wavelength changes, obtain the wavelength change value. Step S3: Calculate the fiber core strain change value based on the wavelength change value; Step S4: Calculate the substrate strain change value based on the fiber core strain change value and the calibrated strain conductivity. Step S5: Substitute the base strain change value into the preset crack-strain transfer model to calculate the crack opening and crack center position.

[0035] Specifically, in the step of obtaining the Bragg wavelength through the aforementioned sets of Bragg gratings, and obtaining the wavelength change value when the Bragg wavelength changes, the wavelength change value is... When calculating the fiber core strain change value based on the wavelength change value, the formula is as follows:

[0036] In the formula, P is the Bragg wavelength when there is no crack. e The photoelasticity coefficient of the optical fiber (theoretical value = 0.22). The strain change is represented by the fiber core strain value. This strain change is converted into a change in Bragg wavelength. When deformation occurs on the metal surface where the Bragg grating sensor is located, it can be used to monitor and record the change in light wavelength in real time. In this study, multiple Bragg grating sensors were deployed to achieve distributed monitoring of metal deformation and rust.

[0037] Specifically, the calculation of the strain transmissibility is based on several assumptions, including: The bare fibers and coating between the epoxy resin layer and the substrate are all linearly elastic. Assume there is no relative slippage at the interfaces; The axial normal stress at the interface between the exposed fiber core and the substrate is ignored.

[0038] Specifically, the calculation of the strain transmissibility includes the following steps: For the epoxy resin layer, there is a second equilibrium equation: (1) In the formula, Where is the radius of the coating layer. 'a' represents the coating layer on the horizontal axis. , radius is Shear stress at the point, For the epoxy resin layer on the horizontal axis , radius is the shear stress at the location of the coating, .

[0039] from the shear lag model, the shear stress (2) where is the shear modulus of the epoxy.

[0040] gives (3) By integration, and using in place of , the first shear lag model is obtained: (4) where is the displacement of the epoxy at the location of the coating with the abscissa and the radius , is the displacement of the coating at the location of the coating with the abscissa and the radius , is the shear modulus of the coating, and is the radius of the epoxy.

[0041] For the coating, there is a third equilibrium equation: (5) where is the core radius, is the shear stress of the core at the location of the coating with the abscissa and the radius . is the shear stress of the coating at the location of the coating with the abscissa and the radius , .

[0042] The second shear lag model is obtained in the same way: (6) where is the displacement of the core at the location of the coating with the abscissa and the radius .

[0043] Taking : (7) Equation (7) in equation (4) gives: (8) The first equilibrium equation for the core along the axial direction: ​​ (9) wherein, is the stress of the fiber core at the x-coordinate .

[0044] Adding equation (6) and equation (8) together: (10) Differentiating equation (10) gives: (11) wherein,

[0045] Solving equation (11) gives: (12) wherein, D is an integral constant, which can be obtained according to the boundary conditions. Since the optical fiber does not bear load at both ends, the boundary conditions are when, and the general solution considering that the strain of the structure is linearly distributed is: (13) wherein, is the length of the optical fiber.

[0046] Then, the strain transfer rate is:

[0047] Specifically, in the step of calculating the substrate strain change value according to the fiber core strain change value and the calibrated strain transfer rate, the substrate strain change value is the quotient of the fiber core strain change value and the strain transfer rate.

[0048] Specifically, the establishment of the crack-strain transfer model includes the following steps: According to the substrate strain change value, a function is established with the coordinate of the crack center position along the fiber core axis as the slope and the crack opening as the intercept;

[0049] (z) = z + δ; wherein, is the deformation of the bare fiber core, is the deformation of the coating layer; the overall deformation of the substrate material caused by the crack and the strain itself is represented as (z , i.e., the function G c is the shear modulus of the coating layer, z is the crack center position, and δ is the crack opening.

[0050] According to the function, a simplified differential equation of the core deformation is obtained:

[0051] A =

[0052]

[0053] Specifically, boundary conditions are set for the function; The function after setting the boundary conditions is solved to obtain the crack-strain transfer model.

[0054] The boundary conditions include that the axial displacement of the core at the core symmetry axis is zero, and the strain at both ends of the core is zero, .

[0055] The differential equation is solved to obtain a general solution:

[0056]

[0057] In the formula, Z is the displacement along the z direction (the core axis direction).

[0058] According to the boundary conditions, we have:

[0059] In the formula, L is the distance from the base crack end to the base edge.

[0060] The solution is:

[0061] Substituting the final result, i.e., the crack-strain transfer model, is obtained:

[0062] Specifically, the steps further include: The crack opening is compared with a preset alarm threshold, and when the crack opening exceeds the alarm threshold, an audible and visual alarm signal is issued, and the crack opening and the crack center position are uploaded to an operation and maintenance center, which is an information acquisition center of personnel handling cracks.

[0063] Example Three Based on the experiment of the FBG-based crack monitoring optical fiber in Example 1, an aluminum alloy Ti-6A1-4V material is selected as the base, the composition is Ti-6Al-4V, and it belongs to (α+β The titanium alloy has good comprehensive mechanical properties, and the size of the aluminum alloy substrate is 500 mm 50 mm 4 mm. A 0.2 mm wide crack was cut in the middle of one side using wire cutting technology. To study the effect of crack depth on fiber strain distribution, a total of 6 crack depths were selected, including 2 mm, 5 mm, 8 mm, 11 mm, 14 mm, and 17 mm. The number of Bragg gratings is 3, and the number of single-mode light is 1. The single-mode optical fiber is composed of an acrylic ester coating, a fiber cladding, and a core, corresponding to the coating layer, the epoxy resin layer, and the core, respectively. A 193 nm excimer laser combined with a phase mask is used to make a fiber grating with a grating spacing of 6 cm. The main reason for using acrylic ester coating fiber is its good strain transfer rate and high measurement accuracy. In the experiment, a fiber containing a grating is parallelly pasted on the surface of the aluminum alloy. The fiber is bonded to the surface of the red copper plate using cyanoacrylate resin, and the fiber bonding length is 40 cm. To facilitate monitoring, a 2-meter long tail fiber is reserved and directly spliced with the jumper wire through fiber splicing technology and connected to the input port of the grating demodulator. The aluminum alloy plate is fixed at both ends by the upper and lower clamps of the testing machine, and the length of the fixed area at both ends is about 50 mm. The stress range of the steel plate is from 0 kN to 23 kN. To monitor the strain distribution of the fiber under different load levels during the tensile test, two constant tension values of 200 N and 2000 N, three tensile rates of 1 mm / min, 3 mm / min, and 5 mm / min are adopted, and the fiber is connected with the grating demodulator to ensure that the data is recorded synchronously during the tensile process. The whole process is completed by the tensile testing machine to ensure the accuracy of the test results. Before and after the tensile test, the crack width and depth are measured using an optical microscope.

[0064] In this experiment, HG-FBG optical fiber grating sensing system is used. It can transmit the light from the broadband light source to the external FBG sensor through the optical splitter, the sensor reflects the specific wavelength of light back, and then transmits it to the photoelectric conversion system through the optical splitter. Then the photoelectric conversion module converts the FBG emitted light wavelength signal into an electrical signal through the photoelectric detector. Finally, the electrical signal is transmitted to the data acquisition module for processing, and finally converted into a digital signal, which is transmitted to the software system through the serial port.

[0065] As Figures 4-8 shown, when the bonding length is 5 mm, the maximum strain transfer rate is still relatively small. With the increase of the bonding length, the proportion of high strain transfer is also higher. Therefore, according to the length of the substrate, the longer the bonding length is within a reasonable range, the better. Figure 4 The curve of strain transfer rate with the change of bonding width on the fiber with a bonding length of 200 mm is shown in Figure 5 The local enlargement of position-95 mm to-90 mm is shown in accordance withFigure 5 It can be seen that the larger the width of the same position, the greater the transfer rate. Among them, the width of 1mm, 2mm, 3mm, the difference of transfer rate is particularly obvious, and 4 to 7mm, although there is an increasing change, but overall, the gap can be ignored. Because the width of the alloy plate is 4mm, 4mm is selected as the best parameter. It can be seen from the figure that the closer to the critical point of the optical fiber, the greater the influence of thickness on the transfer rate, so the best parameter of the thickness of the cement layer should be selected from 0.1mm, 0.2mm, 0.3mm, 0.4mm. Considering the technical limitations, it is impossible to apply too thin glue, plus the gap of the four numbers is not very large, so 0.4mm is selected as the best parameter of the thickness of the cement layer.

[0066] As shown in Figures 9-12 , when the crack occurs, the crack opening rate also has a significant effect on the wavelength change of the optical fiber. Under the condition of constant tension given by the tensile testing machine, a vertical crack tensile rate is introduced. Let the crack produce different degrees of deformation under different tensile rates, so as to reflect on the optical fiber sensor. In this experiment, under the premise of keeping F=200N constant, the tensile rate of 1mm / min, 3mm / min and 5mm / min is set respectively to test. The greater the tensile rate, the greater the wavelength change reflected on the optical fiber sensor, and the greater the corresponding strain. At a rate of 5mm / min, the data is unstable. In the actual experiment, too fast rate will cause the clamp of the tensile testing machine to slip with the alloy plate, resulting in inaccurate data. Therefore, in the subsequent experiment, the rate of 1mm / min is used for tensile test.

[0067] The depth of the crack has the greatest influence on the wavelength change of the optical fiber sensor. With the deepening of the crack depth, the optical fiber experiences the elastic stage, the softening stage and the debonding stage, and the wavelength change obtained in different stages is different. In this experiment, four aluminum alloy plates with crack depths of 3mm, 7mm, 11mm and 15mm were prepared. Under the same tension (F=200N) and the same tensile rate (1mm / min), the substrate was stretched.

[0068] It can be seen from the figure that the deeper the depth of the crack, the greater the wavelength change. Among them, the 7mm experimental group appeared equipment loosening in the initial stretching stage, resulting in the force unable to be normally transmitted, but it recovered normally after 5s, resulting in the wavelength change being small, but the overall still conforms to the dynamic change law.

[0069] When different forces act on the two ends of the substrate, the crack will also deform, and when the pulling force is large, the wavelength change can be seen. When the crack depth (selecting three groups of 3mm, 7mm, 11mm) and the stretching rate (1mm / min) are unchanged, the pulling force of 200N and 2000N is respectively used on the two ends of the substrate. Under the premise that the microscope observation confirms that the crack depth and width do not change, the wavelength change is obtained Figures 11-12 . It can be seen from Figures 11-12 that under the same pulling force, the wavelength change increases with the increase of the crack depth; under the same crack depth, the greater the pulling force, the greater the wavelength change.

[0070] Those skilled in the art can clearly understand that, for the convenience and brevity of description, only the above-mentioned division of each functional unit and module is exemplified, and in actual application, the above-mentioned functions can be completed by different functional units and modules according to needs, that is, the internal structure of the device is divided into different functional units or modules to complete all or part of the functions described above. Each functional unit and module in the embodiment can be integrated in one processing unit, or each unit can exist physically, or two or more units can be integrated in one unit. The above-mentioned integrated unit can be realized in the form of hardware or in the form of software functional unit. In addition, the specific name of each functional unit and module is only for convenient distinction, and does not limit the protection scope of the present application. The specific working process of the unit and module in the above system can refer to the corresponding process in the foregoing method embodiment, which will not be repeated here.

[0071] The above specific embodiments do not constitute a limitation on the protection scope of the present application. Those skilled in the art should understand that various modifications, combinations, sub-combinations and substitutions can occur depending on design requirements and other factors. Any modification, equivalent replacement and improvement within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A crack monitoring optical fiber based on FBG, characterized in that, It includes several sets of Bragg gratings, which are uniformly arranged along the axial direction. A fiber core is sleeved on the outside of the several sets of Bragg gratings, an epoxy resin layer is sleeved on the outside of the fiber core, and a coating layer is provided on the outside of the epoxy resin layer. The fiber core is made of ultraviolet-sensitive germanium-doped single-mode quartz glass with a diameter of 8.2 μm ± 0.3 μm and a refractive index of 1.

468. The grating region length of the Bragg grating is less than or equal to 10 mm, the grating period is 535.5 nm, and the grating reflectivity is 70%. The epoxy resin layer is made of UV-curable acrylate with an outer diameter of 165μm±5μm, and the coating layer is made of UV-curable acrylate with an outer diameter of 245μm±5μm.

2. The fiber optic cable for crack monitoring based on FBG according to claim 1, characterized in that, The pitch between two adjacent sets of Bragg gratings is 60 mm.

3. A crack monitoring method based on FBG, wherein the crack monitoring fiber based on FBG is described in any one of claims 1-2, characterized in that, It includes the following steps: Step S1: Fix the monitoring optical fiber to the surface of the substrate to be tested; Step S2: Obtain the Bragg wavelength through the several sets of Bragg gratings; when the Bragg wavelength changes, obtain the wavelength change value. Step S3: Calculate the fiber core strain change value based on the wavelength change value; Step S4: Calculate the substrate strain change value based on the fiber core strain change value and the calibrated strain conductivity. Step S5: Substitute the base strain change value into the preset crack-strain transfer model to calculate the crack opening and crack center position.

4. The crack monitoring method based on FBG according to claim 3, characterized in that, The calculation of the strain transmissibility includes the following steps: The first equilibrium equation for the fiber core, the second equilibrium equation for the epoxy resin layer, and the third equilibrium equation for the coating layer are established respectively. A first shear lag model and a second shear lag model are established based on the second equilibrium equation and the third equilibrium equation. The strain transmissibility is calculated based on the first shear lag model, the first equilibrium equation, and the second shear lag model.

5. The crack monitoring method based on FBG according to claim 3, characterized in that, In the step of calculating the substrate strain change value based on the fiber core strain change value and the calibrated strain conductivity, the substrate strain change value is the quotient of the fiber core strain change value and the strain conductivity.

6. The crack monitoring method based on FBG according to claim 3, characterized in that, The establishment of the crack-strain transfer model includes the following steps: Based on the strain change value of the substrate, a function is established with the coordinate of the crack center position along the fiber core axis as the slope and the crack opening amount as the intercept. Set boundary conditions for the function; The crack-strain transfer model is obtained by solving the function after setting boundary conditions.

7. The crack monitoring method based on FBG according to claim 6, characterized in that, The boundary conditions include zero axial displacement of the fiber core at the fiber core's axis of symmetry and zero strain at both ends of the fiber core.

8. The crack monitoring method based on FBG according to claim 3, characterized in that, The monitoring optical fiber is fixed to the surface of the substrate using ethyl cyanoacrylate adhesive.

9. The crack monitoring method based on FBG according to claim 3, characterized in that, In the step of obtaining the Bragg wavelength, the sampling interval is less than or equal to 1 pm.

10. The crack monitoring method based on FBG according to claim 3, characterized in that, It also includes the following steps: The crack opening size is compared with a preset alarm threshold. When the crack opening size exceeds the alarm threshold, an audible and visual alarm signal is issued, and the crack opening size and the crack center location are uploaded to the operation and maintenance center.

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