An OFDR distributed optical fiber-based concrete crack simulation width calibration system and method

Abstract

The application discloses a kind of concrete crack simulation width calibration system and method based on OFDR distributed optical fiber, it is related to concrete structure health monitoring technical field, including crack control mechanism, distributed optical fiber sensing system and displacement calibration mechanism, accurate control simulation crack opening degree is passed through the base with brake device, sensing optical fiber is laid out using comprehensive pasting or fixed-point pasting mode and control its intersection angle and fixed-point distance with crack, utilize displacement meter to provide crack width true value benchmark.The application uses the modified strain integration method of introducing comprehensive strain transfer coefficient to calculate crack width, the influence law of optical fiber layout parameter on strain response is proved, can effectively separate the strain superposition interference of dense crack, realizes the high-precision quantitative monitoring and calibration of concrete tiny crack, provides test basis for the application of OFDR distributed optical fiber technology in engineering.

Description

Technical Field

[0001] This invention relates to the field of concrete structure health monitoring technology, and more specifically to a concrete crack simulation width calibration system based on OFDR distributed optical fiber. Background Technology

[0002] In shield tunnel and large concrete structure engineering projects, due to factors such as complex geological conditions, adjacent engineering activities, and train vibrations, the structure inevitably suffers from varying degrees of damage, among which segment cracking is one of the most common. Cracks not only disrupt the continuity and integrity of the lining but also easily lead to water leakage and steel corrosion. Therefore, real-time, high-precision quantitative monitoring of cracks is of great significance for ensuring the long-term safe operation of the project.

[0003] Traditional crack detection methods mainly include manual visual inspection, machine vision-based inspection technology, and 3D laser scanning technology. Manual inspection relies on the experience and judgment of inspectors, which is inefficient and highly subjective. It also poses a high risk in harsh environments such as long tunnels, making it difficult to achieve routine monitoring. Machine vision technology is greatly affected by environmental factors such as insufficient light, dust interference, and limited shooting distance in tunnels, resulting in insufficient accuracy in identifying small cracks and a high risk of missed detections. 3D laser scanning technology has high equipment costs, complex data processing, and difficulty in achieving continuous dynamic monitoring of crack development.

[0004] In recent years, distributed fiber optic sensing technology has been introduced into health monitoring. However, commonly used technologies such as BOTDR, BOFDA, and BOTDA are limited by spatial resolution, resulting in significant errors in the accurate identification and quantitative analysis of minute cracks. Optical frequency domain reflectometer (OFDR) technology, with a spatial resolution of up to 1 mm, offers a significant advantage in quantitative crack analysis. However, fiber strain is affected by many factors, including spatial resolution, fiber deployment method, fixed-point distance, and the angle between the fiber and the crack. Currently, the industry lacks a standardized crack simulation test platform capable of systematically and accurately controlling these variables, and a high-precision calibration method for crack width considering the coupling effects of multiple factors has not yet been developed. This severely restricts the application of OFDR technology in the quantitative monitoring of concrete cracks. Summary of the Invention

[0005] In view of this, the present invention provides a concrete crack simulation width calibration system and method based on OFDR distributed optical fiber, and provides a standardized crack simulation test platform that can accurately control the above variables, thereby realizing the accurate calibration of quantitative monitoring of concrete cracks.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A concrete crack simulation width calibration system based on OFDR distributed optical fiber includes: The crack control mechanism includes a fixed steel plate, a movable steel plate, and a base with a rotating handle and an internal braking device. The fixed steel plate is horizontally fixed, and the movable steel plate is connected to the base and driven by the rotating handle to achieve horizontal displacement. The gap between the two steel plates forms a simulated crack. A distributed optical fiber sensing system includes a distributed sensing optical fiber and an OFDR distributed optical fiber demodulator. The sensing optical fiber is deployed on the surface of a fixed steel plate and a moving steel plate and spans a simulated crack. The U-shaped bend between adjacent straight segments of the sensing optical fiber is fixed without adhesive. The end of the sensing optical fiber is connected to the OFDR distributed optical fiber demodulator. A displacement calibration mechanism is installed at the edge of the simulated crack to obtain the actual crack width as a calibration benchmark.

[0007] Preferably, the sensing optical fiber is a distributed strain sensing optical fiber with an outer diameter of 0.9 mm and a polyurethane high-transmission tight-pack sheath structure. When the sensing optical fiber is laid out in a fully bonded manner, all straight sections of the optical fiber are sealed and fixed with epoxy resin AB glue.

[0008] Preferably, when the sensing optical fiber is laid out by fixed-point bonding, adhesive is applied only at the preset fixed-point positions for fixation, and no adhesive is applied to the free section of the optical fiber between the two fixed points.

[0009] Preferably, the fixed steel plate and the movable steel plate are placed horizontally side by side, and the internal braking device of the base is used to prevent the movable steel plate from sliding in the opposite direction during the test.

[0010] A method for calibrating the simulated width of concrete cracks based on OFDR distributed optical fibers, comprising: S1. Fix the fixed steel plate horizontally, and connect the moving steel plate to the base with a rotating handle and a braking device to form a simulated crack between the two steel plates with a precisely controllable opening. S2. The sensing fiber is laid on the surface of the fixed steel plate and the moving steel plate and spans the simulated crack. The U-shaped bend between adjacent straight segments of the sensing fiber is fixed without glue. S3. Set up a displacement calibration device at the edge of the simulated crack to establish a true reference for the crack width; S4. Expand the simulated crack width step by step by rotating the handle. After each loading step, wait for the strain to stabilize and simultaneously collect the strain data from the OFDR distributed fiber demodulator and the measured data from the displacement calibration mechanism. S5. The fiber optic strain data is processed using the modified strain integral method that introduces the comprehensive strain transfer coefficient to calculate the crack width, and then compared and calibrated with the true value of the displacement calibration mechanism.

[0011] As can be seen from the above technical solutions, compared with the prior art, this invention discloses a concrete crack simulation width calibration system and method based on OFDR distributed optical fiber. By constructing a precise and controllable crack simulation test platform with a braking device, it provides a modified strain integral method crack width calculation model that incorporates a comprehensive strain transfer coefficient, significantly improving the accuracy and reliability of quantitative crack width calculation. Through gradient tests with different fiber-crack intersection angles and different fixed-point distances, the influence of fiber layout parameters on strain response sensitivity is explored, providing theoretical guidance for optimized fiber layout in engineering projects. Utilizing the high symmetry of the strain peak distribution on both sides of a single crack under high spatial resolution, the interference of adjacent cracks or structural joints on strain superposition is effectively separated and eliminated, solving the strain decoupling problem in dense crack scenarios. In addition, the device adopts an optimized design with a U-shaped bending section and a fixed-point free section without adhesive coating, effectively avoiding fiber breakage due to local stress concentration during the test, ensuring the continuity and stability of the test, and providing a scientific experimental basis and standardized calibration method for the engineering application of OFDR distributed optical fiber technology in quantitative monitoring of concrete structure cracks. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0013] Figure 1 A schematic diagram of an example crack control device provided by the present invention; Figure 2 A schematic diagram of the fully pasted optical fiber deployment scheme 1 provided by the present invention; Figure 3 This is a schematic diagram of the fixed-point fiber optic cable deployment scheme 2 provided by the present invention; Figure 4 This is a schematic diagram of the displacement gauge installation provided by the present invention; Figure 5 The fiber strain distribution diagram provided by this invention; Figure 6 A schematic diagram showing the measured values ​​of the crack width displacement gauge and the calculated values ​​of the fiber optic strain provided by this invention. Detailed Implementation

[0014] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] This invention discloses a concrete crack simulation width calibration system based on OFDR distributed optical fiber, such as... Figure 1 As shown, it includes: The crack control mechanism includes a fixed steel plate, a movable steel plate, and a base with a rotating handle and an internal braking device. The fixed steel plate is horizontally fixed, and the movable steel plate is connected to the base and driven by the rotating handle to achieve horizontal displacement. The gap between the two steel plates forms a simulated crack. The distributed optical fiber sensing system includes distributed sensing optical fibers and an OFDR distributed optical fiber demodulator. The sensing optical fibers are laid on the surface of a fixed steel plate and a moving steel plate and cross a simulated crack. The U-shaped bend between adjacent straight segments of the sensing optical fibers is fixed without adhesive. The ends of the sensing optical fibers are connected to the OFDR distributed optical fiber demodulator. The displacement calibration mechanism is set up at the edge of the simulated crack to obtain the actual crack width as a calibration benchmark.

[0016] In one specific embodiment, the sensing fiber is a distributed strain sensing fiber with an outer diameter of 0.9 mm and a polyurethane high-transmission tight-fitting sheath structure. When the sensing fiber is laid out in a fully bonded manner, all straight sections of the fiber are sealed and fixed with epoxy resin AB glue.

[0017] In one specific embodiment, when the sensing optical fiber is laid out by fixed-point bonding, adhesive is applied only at the preset fixed points for fixation, and no adhesive is applied to the free section of the optical fiber between the two fixed points.

[0018] In one specific embodiment, the fixed steel plate and the movable steel plate are placed horizontally side by side, and the internal braking device of the base is used to prevent the movable steel plate from sliding in the opposite direction during the test.

[0019] A method for calibrating the simulated width of concrete cracks based on OFDR distributed optical fibers, comprising: S1. Fix the fixed steel plate horizontally, and connect the moving steel plate to the base with a rotating handle and a braking device to form a simulated crack between the two steel plates with a precisely controllable opening. S2. The sensing fiber is laid on the surface of the fixed steel plate and the moving steel plate and spans the simulated crack. The U-shaped bend between adjacent straight segments of the sensing fiber is fixed without glue. S3. Set up a displacement calibration device at the edge of the simulated crack to establish a true reference for the crack width; S4. Expand the simulated crack width step by step by rotating the handle. After each loading step, wait for the strain to stabilize and simultaneously collect the strain data from the OFDR distributed fiber demodulator and the measured data from the displacement calibration mechanism. S5. The fiber optic strain data is processed using the modified strain integral method that introduces the comprehensive strain transfer coefficient to calculate the crack width, and then compared and calibrated with the true value of the displacement calibration mechanism.

[0020] In specific embodiment 1

[0021] 1. Crack test simulation test steps: First, it should be noted that the cracked substrate used in this embodiment consists of two steel plates measuring 360mm × 180mm × 10mm. A 0.9mm polyurethane high-transmission tight-packed sheathed distributed strain sensing optical fiber manufactured by Suzhou Nanzhi Sensing Technology Co., Ltd. is used. The demodulator is an OSI-S type OFDR distributed fiber optic demodulator, with a maximum spatial resolution of 1mm and a strain measurement accuracy of ±1.0με. The procedure is as follows: A. Arrange the crack control device by placing two 360mm×180mm×10mm steel plates side by side horizontally, fixing one plate and connecting the other plate to a base with a rotating handle and braking effect to form a crack control device. B. Deploying Distributed Sensing Fiber Optics: In "Comprehensive Adhesion Scheme 1," a single fiber optic cable passes through the crack eight times: segments 1, 2, 7, and 8 (from top to bottom) are perpendicular to the crack at 90°, with a straight length of 200mm; segments 3 and 4 are obliquely perpendicular to the crack at 80°, with a length of 203mm; segments 5 and 6 are obliquely perpendicular to the crack at 70°, with a length of 213mm. After the fiber optic cable is positioned with adhesive, the straight segments are completely sealed with epoxy resin AB glue. In "Point-based Adhesion Scheme 2," a single fiber optic cable passes through the crack six times: segments 1, 2, 3, and 4 are perpendicular to the crack at 90°, with fixed-point distances of 140mm, 100mm, 60mm, and 30mm respectively; segment 5 is obliquely perpendicular to the crack at 80°, with a fixed-point distance of 30mm; segment 6 is obliquely perpendicular to the crack at 70°, with a fixed-point distance of 30mm. Adhesive and AB glue are used only at the fixed points for fixation; the free segments between two fixed points are not glued. In all designs, the U-shaped bends (approximately 200mm in length) outside adjacent straight segments are not glued. After removing the coating from the fiber optic patch cord ends with fiber optic strippers and wiping with alcohol, they are spliced ​​using a fiber optic fusion splicer, controlling the splice loss rate to be less than or equal to 0.02dB. C. The testing instruments are set up with a displacement gauge with a range of 0~30mm and an accuracy of 0.01mm at the edge of the moving steel plate corresponding to the simulated crack. The handle is rotated to move the moving steel plate horizontally, so that each fiber segment is slightly taut, which is taken as the initial state with a crack width of 0.00mm.

[0022] D. Crack Propagation and Data Acquisition: Continue rotating the handle horizontally to the right, starting from 0.00 mm, controlling the crack propagation step size to 0.05 mm, simulating a maximum crack width of 0.40 mm. After each propagation, wait two minutes for the fiber strain to stabilize. The OFDR demodulator then reads the strain data at three spatial resolutions: 1 mm, 2 mm, and 5 mm.

[0023] To investigate the effects of spatial resolution, fiber optic deployment method, fixed-point distance, and the angle between the fiber and the crack on the crack test results, groups were set up as shown in Table 1.

[0024] Table 1 Crack Test Groups

[0025] 2. Simulation test crack test results and analysis: A. Crack identification and location analysis: as follows Figure 5 The figure shows the strain distribution of optical fibers in the fully bonded fiber optic test group L-1.1 and the fixed-point fiber optic test group L-2.1. When the crack width is 0.40 mm, the peak strain of the L-1.1 fiber is large and has too much noise, so it is not shown in the figure. As the crack width gradually increases, the fiber strain also shows an increasing trend, and the fiber strain curves at the corresponding locations are basically consistent. Therefore, OFDR distributed optical fiber can more realistically reflect the gradual expansion of the crack. The strain peak of the fully bonded fiber shown in the left figure is a "sharp peak", while the strain peak of the fixed-point fiber shown in the right figure is a "horizontal straight line". This is because the fixed-point fiber can distribute the concentrated deformation caused by the crack to a fixed length of sensing fiber. The two fixed points contain multiple nearly equal fiber strain values, which can improve the test range.

[0026] Because the fixed-point distance of the fixed-point fiber is greater than the spatial resolution used in the test, only the test results of the fully bonded fiber optic cables L-1.1, L-1.2, and L-1.3 were selected to compare the actual crack location with the measured fiber strain peak location. The test used high spatial resolution, and the requirement for crack location accuracy was also relatively high. Determining the actual location of the first crack using manual measurement methods was difficult to meet the accuracy requirements. To reduce manual measurement errors, the first strain peak location measured by the fiber optic cable was taken as the actual location of the first crack C1-1. The actual locations of cracks C1-2 to C1-8 were calculated according to the fiber optic cable layout scheme and then compared with the measured fiber strain peak locations. The results are shown in Table 2 below. The measured fiber optic locations and the actual crack locations are basically consistent. The maximum fiber crack location errors at spatial resolutions of 1mm, 2mm, and 5mm are 2mm, 4mm, and 7mm, respectively, with average errors of 0.88mm, 1.25mm, and 3.50mm, respectively.

[0027] Table 2 Comparison of measured locations

[0028] In the table, "spr" represents spatial resolution, and the unit of location data is mm.

[0029] Crack width calculation: In concrete structure crack monitoring based on OFDR distributed optical fiber technology, four key issues need to be considered when calculating crack width using the strain integration method: the slip zone length, the concrete strain near the crack, the angle between the optical fiber and the crack, and the strain transfer coefficient from the concrete to the fiber core. The slip zone length can be directly obtained from the optical fiber strain curve, and the total deformation of the cracked matrix and the optical fiber within the slip zone is consistent. In this simulated crack test, a steel plate was used as the cracked matrix, and the strain of the steel plate itself can be ignored. Simultaneously, test groups where the optical fiber is perpendicular to the crack were selected for crack width calculation, ignoring the influence of the angle. When the optical fiber is orthogonal to the crack, the crack width is ω, the strain slip zone length is L, and the left endpoint of the strain slip zone is taken as the starting point for strain integration. Based on existing research, the strain transfer coefficient α = 0.95 is selected. Integrating the optical fiber strain within the slip zone yields the simulated crack width, and the actual crack width is compared with the displacement gauge measurement. The results are shown below. Figure 6 As shown, the measured values ​​of the crack width displacement gauge and the calculated values ​​of the fiber optic strain are quite close. The maximum error of the fully bonded test group is 8.56%, and the average error is 5.78%. The maximum error of the fixed-point test group is 7.70%, and the average error is 4.82%, which is slightly lower than the error of the fully bonded test group.

[0030] In specific embodiment 2, such as Figure 2 As shown, the distributed sensing fiber optic arrangement in "Full Adhesion Scheme 1" is as follows: A single fiber optic cable passes through the crack eight times, designated C1-1 to C1-8. Segments 1, 2, 7, and 8 (from top to bottom) are perpendicular to the crack at 90°, with a straight length of 200mm; segments 3 and 4 are obliquely perpendicular to the crack at 80°, with a length of 203mm; and segments 5 and 6 are obliquely perpendicular to the crack at 70°, with a length of 213mm. After the fiber optic cable is positioned with adhesive, the straight segments are completely sealed with epoxy resin AB glue.

[0031] In specific embodiment 3, such as Figure 3As shown, in "Fixed-Point Bonding Scheme 2", a single optical fiber passes through the crack six times, labeled C2-1 to C2-6 (no markings are provided due to space limitations in the diagram): Segments 1, 2, 3, and 4 are perpendicular to the crack at 90° angles, with fixed-point distances of 140mm, 100mm, 60mm, and 30mm respectively; segment 5 is obliquely perpendicular to the crack at 80° angles, with a fixed-point distance of 30mm; segment 6 is obliquely perpendicular to the crack at 70° angles, with a fixed-point distance of 30mm. Adhesive and AB glue are used only at the fixed points for fixation; the free sections between the two fixed points are not glued.

[0032] In specific embodiment 4, such as Figure 4 As shown, the testing instrument is set up with a displacement gauge with a range of 0~30mm and an accuracy of 0.01mm, mounted on the edge of the moving steel plate corresponding to the simulated crack. Rotating the handle moves the moving steel plate horizontally, slightly tauting each fiber segment, thus setting the initial state with a crack width of 0.00mm.

[0033] This invention provides a corresponding quantitative calculation model for crack width. The method uses a modified strain integral method formula, and the calculated crack width is in high agreement with the measured values ​​from the displacement gauge. The measured crack width values ​​from the displacement gauge and the calculated fiber optic strain values ​​are very close. The maximum error in the fully bonded deployment test group is 8.55%, and the average error is 5.77%. The maximum error in the fixed-point deployment test group is 7.70%, and the average error is 4.82%, slightly lower than the error in the fully bonded test group. The influence of complex deployment parameters on the sensing response has been clarified. Through finely designed tests with different intersection angles and fixed-point gradients, it has been verified that under fixed-point deployment, the smaller the fixed-point distance and the larger the intersection angle between the fiber and the crack, the higher the sensitivity of the fiber to the strain response of micro-cracks. The problem of strain superposition in the slip zone of dense cracks has been overcome. This simulation test platform verifies that, at spatial resolutions of 1mm or 2mm, the high symmetry of the strain peak distribution on both sides of a single crack can effectively separate and eliminate the interference of adjacent cracks or structural joint grooves on the strain superposition in the slip zone, improving the detection capability under complex working conditions. The device of this invention has a simple and controllable structure. The design of not applying glue to the "U"-shaped bending section and the fixed-point free section effectively avoids the fiber breaking due to local stress concentration during the test, ensuring the continuous conduct of the test.

[0034] The working principle and process of this invention are as follows: A control device with an internal braking effect is used to move a steel plate horizontally by rotating a handle, accurately simulating the crack propagation process between two steel plates. The braking device prevents the steel plates from sliding back during the test. Optical fibers arranged on the surface at specific angles (90°, 80°, 70°) and fixed distances (30mm and 140mm) generate axial strain as the spacing between the steel plates increases. The strain peak value is captured in real time using an OFDR demodulator with adjusted high spatial resolution (1mm, 2mm, 5mm). When calculating the crack width, because the encapsulation layer and colloid cause asynchronous deformation between the fiber core and the structure, a comprehensive strain transfer coefficient is introduced into the strain integral method, and the strain data is processed using a modified strain integral method formula. Finally, the calculated crack width is compared and verified with the measured data from the displacement gauge.

[0035] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. The methods disclosed in the embodiments are described simply because they correspond to the methods disclosed in the embodiments; relevant parts can be found in the method section.

[0036] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A concrete crack simulation width calibration system based on OFDR distributed optical fiber, characterized in that, include: The crack control mechanism includes a fixed steel plate, a movable steel plate, and a base with a rotating handle and an internal braking device. The fixed steel plate is horizontally fixed, and the movable steel plate is connected to the base and driven by the rotating handle to achieve horizontal displacement. The gap between the two steel plates forms a simulated crack. A distributed optical fiber sensing system includes a distributed sensing optical fiber and an OFDR distributed optical fiber demodulator. The sensing optical fiber is deployed on the surface of a fixed steel plate and a moving steel plate and spans a simulated crack. The U-shaped bend between adjacent straight segments of the sensing optical fiber is fixed without adhesive. The end of the sensing optical fiber is connected to the OFDR distributed optical fiber demodulator. A displacement calibration mechanism is installed at the edge of the simulated crack to obtain the actual crack width as a calibration benchmark.

2. The concrete crack simulation width calibration system based on OFDR distributed optical fiber according to claim 1, characterized in that, The sensing fiber is a distributed strain sensing fiber with an outer diameter of 0.9 mm and a polyurethane high-transmission tight-pack sheath structure. When the sensing fiber is laid out in a fully bonded manner, all straight sections of the fiber are sealed and fixed with epoxy resin AB glue.

3. The concrete crack simulation width calibration system based on OFDR distributed optical fiber according to claim 1, characterized in that, When the sensing optical fiber is laid out by fixed-point bonding, adhesive is applied only at the preset fixed points for fixation, and no adhesive is applied to the free section of the optical fiber between the two fixed points.

4. The concrete crack simulation width calibration system based on OFDR distributed optical fiber according to claim 1, characterized in that, The fixed steel plate and the movable steel plate are placed horizontally side by side, and the internal braking device of the base is used to prevent the movable steel plate from sliding in the opposite direction during the test.

5. A method for calibrating the simulated width of concrete cracks based on OFDR distributed optical fiber, applied to the concrete crack simulation width calibration system based on OFDR distributed optical fiber as described in any one of claims 1-4, characterized in that, include: S1. Fix the fixed steel plate horizontally, and connect the moving steel plate to the base with a rotating handle and a braking device to form a simulated crack between the two steel plates with a precisely controllable opening. S2. The sensing fiber is laid on the surface of the fixed steel plate and the moving steel plate and spans the simulated crack. The U-shaped bend between adjacent straight segments of the sensing fiber is fixed without glue. S3. Set up a displacement calibration device at the edge of the simulated crack to establish a true reference for the crack width; S4. Expand the simulated crack width step by step by rotating the handle. After each loading step, wait for the strain to stabilize and simultaneously collect the strain data from the OFDR distributed fiber demodulator and the measured data from the displacement calibration mechanism. S5. The fiber optic strain data is processed using the modified strain integral method that introduces the comprehensive strain transfer coefficient to calculate the crack width, and then compared and calibrated with the true value of the displacement calibration mechanism.

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