CN109187194B - OFDR-based soil body tension mechanical property optical fiber monitoring and testing method and device - Google Patents

Abstract

The invention relates to an OFDR-based optical fiber monitoring and testing method and device for soil tension mechanical properties. The strain sensing optical fiber is laid horizontally in the soil beam; the tester mainly consists of a reaction support and a loading system. It is composed of an OFDR signal demodulation and processing module and a digital image acquisition and analysis device. The OFDR signal demodulation and processing module is connected to the strain sensing optical fiber in the soil beam to collect the strain distribution data inside the soil beam in real time and display the light wave amount and soil Beam strength, etc., the digital image acquisition and analysis device tracks the position changes on the surface of the soil beam, obtains the strain field and displacement field after the soil beam is stressed and deformed, and checks each other with the data measured by the optical fiber. The invention can real-time monitor the cracking deformation information on the surface and inside of the soil beam as well as the spatiotemporal evolution law of tension and compression strain during the four-point bending test, measure the tensile strength of the soil under different moisture content, dry density and other conditions, and grasp the soil stress. Elasto-plastic stress-strain constitutive relations after tension.

Description

An OFDR-based optical fiber monitoring and testing method for soil tension mechanical properties and device

Technical field

The invention relates to the field of stress deformation and strength testing technology of rock and soil, as well as the technical field of distributed optical fiber monitoring engineering. Specifically, it relates to an OFDR-based optical fiber monitoring and testing method and device for the tensile mechanical properties of soil.

Background technique

Tensile mechanical properties are one of the basic mechanical properties of geotechnical media. They play a very important role in the process of deformation and destruction of rock and soil masses, which also generates many related geotechnical engineering problems. The tensile strength of soil, like compressive strength, shear strength and other indicators, are important parameters to measure its mechanical properties. During the formation process of the soil, the integrity and integrity of the original rock were damaged to varying degrees. The mechanical properties still have a certain degree of compressive strength and shear strength, but the strength value has been greatly reduced, and the tensile strength has been greatly reduced. The strength is mostly or almost completely lost.

In the past engineering practice, the mechanical properties or damage resistance under load were mainly measured based on the compressive or shear strength of the soil, because compared with the compressive or shear strength, the tensile strength is numerically smaller. Much more, and difficult to measure accurately, this strength indicator is generally ignored in engineering. This neglect is manifested as 0 matrix suction and 0 tensile stress load in most cases. It is a relatively conservative estimate of soil strength and needs to be improved in contemporary geotechnical engineering design.

In addition, the soil failure mode encountered in actual engineering is mainly shear failure, such as landslides and foundation soil instability. However, it is also common for soil to undergo tensile failure and cracks under the action of tensile stress, such as tension cracks at the trailing edge of slopes, tensile cracks in the core walls of earth-rock dams under the action of soil arches, and soil cracks in dry environments. The phenomenon of cracks appearing on the body and the development of ground fissures, etc., some transmission line towers and wind power towers can also easily cause tensile damage to the surrounding soil under the action of horizontal loads. The existence of cracks will greatly destroy the integrity of the soil structure, weaken the mechanical properties, reduce stability, increase permeability, aggravate evaporation, aggravate soil erosion and weathering on slopes, etc., which will bring problems to geotechnical engineering and environmental geotechnical engineering. series of negative impacts. The reason why tensile cracks appear in soil is because the tensile stress exceeds the tensile strength of the soil itself. Therefore, it is of great engineering significance to monitor the stress and strain changes during the tension and cracking process of soil and measure its tensile strength. On this basis, it is of great engineering significance to systematically understand the tension failure mechanism of soil and prevent soil cracking, which can effectively improve the relevant The level of prevention and control of geological disasters has been improved, saving a lot of manpower and material resources.

Since the tensile mechanical properties of soil have not been taken seriously in the field of geotechnical engineering in the past, there are relatively few research reports, especially in China. Most of the existing soil tensile strength testing methods are based on rocks, concrete, etc. Other materials fields. Test methods for soil tensile strength can be divided into two categories: direct method and indirect method. The direct method is divided into uniaxial tensile and triaxial tensile tests. The indirect method mainly includes soil beam bending test and axial fracturing test. , radial fracturing test and air pressure splitting test. Uniaxial tensile and triaxial tensile tests apply tensile stress to the sample under unconfined and triaxial stress conditions respectively, and directly measure the peak tensile stress to obtain the tensile strength. The indirect method is based on certain theoretical assumptions, using fracturing, bending and other methods to conduct tests, and finally calculates the tensile strength through the corresponding theoretical formula. Compared with the problem of stress concentration in the three-point bending test, in the four-point bending test of the soil beam, the bending moment experienced by the soil is constant between the load application points, so the tensile stress distribution is relatively uniform, which is a more ideal test methods. In these tests, tensile stress can be obtained by certain means, but obtaining soil strain information faces many challenges. Because the strain is unevenly distributed along the tensile direction and there are many uncertainties, the tensile strain generally concentrates near the failure surface. However, the traditional displacement monitoring method can only calculate the average strain of the sample, which is different from the real situation. Far from it. If you choose resistance strain gauges commonly used in civil engineering, there are problems such as difficulty in installation and large disturbance to the in-situ soil. It is precisely because of the lack of soil strain monitoring technology that people currently have unclear understanding of the soil tension stress-strain constitutive relationship, which seriously restricts theoretical research and engineering practice in this field.

Distributed optical fiber monitoring (DFOS) technology has developed rapidly in recent years and has been successfully used in detecting cracks in concrete, asphalt and other materials. With the help of monitoring technologies such as quasi-distributed fiber Bragg grating (FBG), fully distributed Brillouin optical time domain reflectometry (BOTDR), and Brillouin optical time domain analysis (BOTDA), the strain and strain along the length of the entire fiber can be automatically obtained. Distribution of monitoring information such as temperature. However, this technology has not been widely used in soil cracking monitoring due to limitations in monitoring accuracy (generally dozens of microstrains), spatial resolution (generally meter level) and sampling time (generally taking more than ten to tens of minutes). Good use. OFDR (Optical Frequency Domain Reflectometer) technology is a cutting-edge sensing technology that has emerged in recent years with millimeter-level spatial resolution and 1 microstrain accuracy. Compared with other monitoring methods, OFDR has the advantages of large data collection volume, high signal-to-noise ratio, small sampling intervals, high accuracy of results, and is suitable for long-distance monitoring and high-frequency collection. Therefore, it has broad application in the field of soil tension cracking measurement. application prospects.

Recently, some researchers at home and abroad have tried to bury strain-sensing optical fibers into the soil to be monitored, and analyze the deformation characteristics of the soil based on fiber-optic sensing data, or monitor whether it shrinks and cracks. Since these studies did not use special, standardized, and integrated test equipment, they were unable to control the entire test process and boundary conditions. Therefore, they could only draw some qualitative conclusions, which were of little significance for engineering reference. Due to the long test time, fiber readings are also affected by ambient temperature, humidity, etc., making the analysis results less reliable. In addition, the direct burial method is more convenient in construction, but the interaction mechanism and coordinated deformation between the soil and the strain-sensing optical fiber cannot be guaranteed. At the same time, there is no scientific basis for the selection of strain-sensing optical fiber and the setting of anchor points. Therefore, there is great uncertainty in whether the optical fiber strain monitoring results are effective or not, which greatly restricts the promotion and application of this technology in engineering. Based on OFDR technology, micro-deformation can be monitored with high precision and high spatial resolution during the soil cracking process, and the coordination characteristics of the interface deformation between the soil and the strain-sensing optical fiber can be refined. On this basis, Further optimize the sensor layout process and improve monitoring reliability.

Contents of the invention

In view of the shortcomings of the existing technology, the purpose of the present invention is to provide an OFDR-based optical fiber monitoring and testing method and device for soil tension mechanical properties.

The present invention adopts the following technical solution: an OFDR-based optical fiber monitoring and testing device for soil tension mechanical properties, including a test soil beam, a tester, and a strain sensing optical fiber; the tester includes a chassis shell and a reaction bracket , loading board, speed regulating drive device, OFDR signal demodulation and processing module, digital image acquisition and analysis device, the described chassis shell is provided with a speed regulating drive device, and the speed regulating driving device is connected to the dynamometer and the loading plate, The loading plate can move up and down along the vertical direction of the inner wall of the chassis shell. A test soil beam is placed between the loading plate and the reaction bracket. The strain sensing optical fiber passes through the test soil beam in the horizontal direction. The strain sensing optical fiber communicates with OFDR through the signal transmission optical fiber. The signal demodulation and processing modules are connected, and the digital image acquisition window of the digital image acquisition and analysis device corresponds to the test observation surface of the test soil beam.

The speed-regulating drive device includes a stepper motor and a gearbox.

Digital image acquisition and analysis equipment includes high-speed cameras and computers.

The loading plate is provided with rollers at both ends.

A method using optical fiber monitoring and testing devices for soil tension mechanical properties based on OFDR, including the following steps:

The first step is to prepare the test soil beam: press the soil beam in layers according to the given dry density in the soil beam pressing mold. When the soil beam is pressed to the layout position of the strain sensing optical fiber, the strain sensing optical fiber is passed through the soil in sequence. The fiber optic penetration holes on the side of the beam pressing mold box are laid in the soil, and heavy objects are hung appropriately to put them in a slight tension state, and then the soil is filled and pressed according to the given dry density;

The second step is to mark the soil sample: take out the pressed soil beam, prick pinholes densely on the test observation surface, and use this as the surface texture of the soil beam, then naturally air-dry or dry it to the required moisture content, and then cover it with film ;

The third step is to connect the OFDR signal demodulation and processing module: uncover the soil beam coating, place it horizontally on the loading plate of the tester, connect all the strain sensing optical fibers to each other in parallel or series and then connect them to the OFDR signal demodulation module. On the interface of the adjustment and processing module;

The fourth step is to start the test: turn on the switches of the OFDR signal demodulation and processing module, digital image acquisition and analysis device, and speed-regulating drive device in sequence. The speed-regulating drive device pushes the loading plate to move downward at the set speed, and the test soil beam follows. Four-point bending occurs; the OFDR signal demodulation and processing module acquires and presents the strain distribution inside the soil beam in real time; the digital image acquisition and analysis device tracks the texture changes on the surface of the soil beam in real time, and obtains the strain field and displacement after the soil beam is deformed by force. field;

The fifth step, data processing: Based on the measured data, establish the tensile stress-strain constitutive relationship of the soil and obtain the tensile strength and cracking strain value parameters of the soil.

The digital image acquisition and analysis device collects and analyzes based on the digital image coherence method or the particle image velocimetry method.

The OFDR signal demodulation and processing module can customize the resolution in the range of 1mm-10cm according to test accuracy and denoising requirements.

The soil beam pressing mold is composed of four side plates fixed on a base plate, and the two side plates at both ends are provided with optical fiber penetration holes.

The strain sensing optical fiber has the sheath threaded through electrode engraving and electrical discharge machining technology.

The strain sensing optical fiber is fixed in the soil using a tube-type or plate-type anchoring device.

Beneficial effects: Using the optical fiber monitoring and testing method and device for soil tension mechanical properties based on OFDR of the present invention, the cracking deformation information on the surface and inside of the soil beam during the four-point bending test can be monitored in real time as well as the spatio-temporal evolution rules of tension and compression strains. , determine the tensile strength of soil under different conditions such as moisture content and dry density, and on this basis, grasp the elastic-plastic stress-strain constitutive relationship of soil after tension. It has the advantages of economical reliability, accurate testing, and high degree of automation.

Description of the drawings

Figure 1 is a schematic diagram of the device in the optical fiber monitoring and testing method for soil tension mechanical properties according to a preferred embodiment of the present invention.

These include: 1. OFDR signal demodulation and processing module, 2. Signal transmission optical fiber, 3. Strain sensing optical fiber, 4. Test soil sample, 5. Loading plate, 6. Gearbox, 7. Stepper motor, 8. Camera, 9. Computer, 10. Soil beam pressing mold, 11. Optical fiber penetration hole, 12. Chassis shell, 13. Reaction bracket, 14. Tester, 15. Digital image acquisition and analysis device, 16. Dynamometer .

Figure 2 is a schematic structural diagram of a digital image acquisition and analysis device according to an embodiment of the present invention.

Figure 3 is a schematic diagram of the soil beam pressing mold.

Figure 4 is a diagram showing the strain distribution along the length direction of a strained optical fiber measured using a method according to an embodiment of the present invention.

Figure 5 is a comparison diagram of the strain measured by optical fiber and the strain measured by PIV.

Figure 6 is a PIV processing result of the midpoint of the bottom of the soil beam measured during four-point bending using a method according to an embodiment of the present invention.

Figure 7 shows the recording results of several typical cracking states of soil using a method according to an embodiment of the present invention.

Figure 8 is a diagram showing changes in soil tensile strength under different moisture content and dry density conditions measured using a method according to an embodiment of the present invention.

Detailed ways

The present invention will be described in more detail below with reference to the accompanying drawings and preferred embodiments.

An OFDR-based fiber optic monitoring and testing method for soil tension mechanical properties, including the following steps:

The first step is to prepare the test soil beam. The soil beam is pressed in layers according to the given dry density in the soil beam pressing mold. When the soil beam is pressed to the layout position of the strain sensing optical fiber, the strain sensing optical fiber is sequentially passed through the fiber optic penetration on the side of the soil beam pressing mold box. The holes are laid in the soil, and heavy objects are hung appropriately to keep them in a slight tension state.

The second step is to label the soil sample. Take out the pressed soil beam, use steel needles with a diameter of 0.1mm to intensively punch pinholes on the test observation surface, and use this as the surface texture of the soil beam. Then it is naturally air-dried (or dried) to the required moisture content, and then covered with film;

The third step is to uncover the coating of the soil beam, place it horizontally on the loading plate of the tester, connect all the strain sensing optical fibers to each other in parallel or series, and then connect them to the interface of the OFDR signal demodulation and processing module. .

Step four, start experimenting. Open the switches of the OFDR signal demodulation and processing module, digital image acquisition and analysis device, stepper motor and gearbox in sequence. The stepper motor pushes the loading plate to move downward at a specific speed, and the soil beam bends at four points; OFDR signal The demodulation and processing module acquires and presents the strain distribution state inside the soil beam in real time; the digital image acquisition and analysis device tracks the texture changes on the surface of the soil beam in real time and obtains the strain field and displacement field after the soil beam is deformed by force.

The fifth step is to establish the tensile stress-strain constitutive relationship of the soil based on the measured data, and obtain the tensile strength and cracking strain values of the soil.

The digital image processing system is based on the digital image coherence method or the particle image velocimetry method.

The OFDR signal demodulation and processing module includes an optical fiber demodulator, terminal computing processing and visualization system. Among them, the OFDR signal demodulation and processing module can customize the resolution in the range of 1mm-10cm according to test accuracy and denoising requirements.

The device used in the optical fiber monitoring and testing method of the soil tension mechanical properties includes a soil beam pressing mold 10, a tester 14, and a strain sensing optical fiber 3; the soil beam pressing mold 10 is composed of five steel plates. It is embedded and assembled; the four side panels are connected with bolts, and then the whole is embedded in the notch of the bottom plate and connected to the bottom plate through bolts. Two of the side panels have small round holes 11; the strain The sensing optical fiber 3 passes horizontally through the soil beam pressing mold 10 and is laid in the soil beam 4; the tester 14 mainly consists of a reaction bracket 13, a chassis shell 12, a loading plate 5, a stepper motor 7, and a gearbox 6 , OFDR signal demodulation and processing module 1, digital image acquisition and analysis device 15. The stepper motor 7 adjusted by the gearbox 6 is connected to the dynamometer (16) to push the loading plate 5 with balls at both ends to move downward along the inner wall of the chassis shell 12. The OFDR signal demodulation and processing module 1 is connected to the strain sensing optical fiber 3 in the soil beam 4 to collect the internal strain data of the soil beam 4 in real time, and displays parameters such as light wave volume and soil beam intensity in real time. The digital image acquisition The analysis device 15 is installed before and after the tester 14. By identifying the texture of the soil beam 4 and tracking the position changes on the surface of the soil beam 4, the strain field and displacement field of the soil beam after force deformation are obtained, and the optical fiber strain data is calibrated or verified. The digital image acquisition and analysis device includes a high-pixel digital camera and a computer. The loading system includes a stepper motor, a gearbox, and a loading plate; the strain-sensing optical fiber has its sheath threaded through electrode engraving and electrical discharge processing technology; the strain-sensing optical fiber uses heat shrink tubing and Small discs are anchored in the soil to ensure coordination of the deformation of the strain sensing optical fiber and the soil.

As a further optimization of the above solution, the rigid side plate of the soil beam pressing mold 10 is provided with optical fiber penetration holes 11 for passing through transversely arranged optical fibers. The strain sensing optical fiber 3 is processed through electrode engraving and electric discharge processing technology. The sheath is threaded; the OFDR signal demodulation and processing module can customize the resolution in the range of 1mm-10cm according to test accuracy and denoising requirements.

Further, the digital image acquisition and analysis device 15 also includes:

(1) Mark the textured test soil beam 4; on the test observation surface, use steel needles with a diameter of 0.1mm to intensively punch pinholes, and use this as the surface texture of the soil beam 4;

(2) High-pixel camera 8; the high-pixel camera is placed in front of the test soil beam (4), and by identifying the texture of the soil beam (4) and tracking the position changes on the surface of the soil beam 4, the strain field and the strain field after the soil beam is deformed by force are obtained. Displacement field, and calibrate or verify fiber strain data. ;

(3) Digital image processing software; the digital image processing software is based on Digital Image Correlation (DIC for short) or Particle Image Velocimetry (PIV for short), etc.

Example

As shown in Figures 1 and 2, an OFDR-based optical fiber monitoring and testing device for soil tension mechanical properties includes a soil beam pressing mold 10, a tester 14, and a strain sensing optical fiber 3; the soil beam pressing The mold 10 is made up of five steel plates embedded and assembled into each other; the four side plates are connected with bolts, and then the whole is embedded in the notch of the bottom plate and connected to the bottom plate through bolts. Two of the side panels have small openings. The circular hole 11; the strain sensing optical fiber 3 passes through the soil beam pressing mold 10 in the horizontal direction and is laid in the soil beam 4; the tester 14 mainly consists of a reaction bracket 13, a chassis shell 12, a loading plate 5, and a step It is composed of an inlet motor 7, a gearbox 6, an OFDR signal demodulation and processing module 1, and a digital image acquisition and analysis device 15. The OFDR signal demodulation and processing module 1 is connected to the strain sensing optical fiber 3 in the soil beam 4 in real time. The internal strain data of the soil beam 4 is collected, and parameters such as the amount of light waves and the strength of the soil beam are displayed in real time. The digital image acquisition and analysis device 15 is located at the front and rear of the tester 14 and tracks the surface of the soil beam 4 by identifying the texture of the soil beam 4 The position changes, the strain field and displacement field of the soil beam after force deformation are obtained, and the optical fiber strain data is calibrated or verified.

The rigid side plate of the soil beam pressing mold 10 is provided with an optical fiber penetration hole 11 for passing through the transversely arranged optical fiber; the strain sensing optical fiber 3 has its sheath threaded through electrode engraving and electric discharge processing technology. Processing; the OFDR signal demodulation and processing module can customize the resolution in the range of 1mm-10cm according to test accuracy and denoising requirements. The digital image acquisition and analysis device also includes: (1) a test soil beam marked with texture; on the test observation surface, steel needles with a diameter of 0.1mm are used to intensively punch pinholes, and use this as the surface texture of the soil beam; (2) High-pixel camera; the high-pixel camera is placed in front of the test soil beam. By identifying the texture of the soil beam and tracking the position changes on the surface of the soil beam, the strain field and displacement field of the soil beam after force deformation are obtained, and the optical fiber strain data is analyzed Calibration or verification; (3) Digital image processing software; the digital image processing software is based on Digital Image Correlation (DIC) or Particle Image Velocimetry (PIV), etc.

The testing method of the above-mentioned soil tension mechanical properties optical fiber monitoring and testing device provided in this embodiment includes the following steps:

1) Prepare test soil beam 4. The soil beam is pressed in layers according to a given dry density in the soil beam pressing mold 10. When the soil beam is pressed to the layout position of the strain sensing optical fiber 3, the strain sensing optical fiber is passed through the side of the box of the soil beam pressing mold 10 in sequence. The optical fiber penetration holes 11 are laid in the soil, and weights are appropriately hung to put them in a slight tension state.

2) Mark the soil sample. Take out the pressed soil beam, use steel needles with a diameter of 0.1mm to intensively punch pinholes on the test observation surface, and use this as the surface texture of the soil beam. Then it is naturally air-dried (or dried) to the required moisture content, and then covered with film;

3) Uncover the soil beam coating, place it horizontally on the loading plate 5 of the tester, connect all the strain sensing optical fibers (3) to each other in parallel or series and then connect them to the OFDR signal demodulation and processing module 1 on the interface.

4) Start testing. Open the switches of the OFDR signal demodulation and processing module 1, the digital image acquisition and analysis device 15, the stepper motor 7, and the gearbox 6 in sequence. The stepper motor 7 pushes the loading plate 5 to move downward at a specific speed, and the soil beam 4 follows. Four-point bending occurs; the OFDR signal demodulation and processing module 1 acquires and displays the strain distribution state inside the soil beam 4 in real time; the digital image acquisition and analysis device 15 tracks the texture changes on the surface of the soil beam in real time and obtains the strain field after the soil beam is deformed by force. and displacement field.

5) Based on the measured data, establish the tensile stress-strain constitutive relationship of the soil and obtain the tensile strength and cracking strain values of the soil. Specifically, the cracking strain is obtained from the optical fiber monitoring data, and the tensile strength can be obtained by combining the dynamometer data: the Ft curve is made from the dynamometer readings, and the soil tensile stress F is determined to finally approach the stable soil tension. The tensile stress value F ₀ corresponds to the bending moment M ₀ , which is given by The tensile strength σ _t is calculated, where I is the moment of inertia of the soil beam, h is the height of the soil beam, and t is time.

When the optical fiber monitoring and testing device for the soil tension mechanical properties of this embodiment is actually used, soft kaolin with a moisture content of 32% is first configured, and then a soil beam is used to press the mold 10 (the dimensions are 50cm×15cm×length×width×height) 15cm) layered and compacted (average density is 1.87g/cm ³ ). During the pressing process, 6 strain sensing optical fibers (3) (OF1-1, OF1-2, OF2-1, OF2-1, OF3-1, OF3-2) were installed horizontally in three layers and two rows inside the soil beam. The heights from the bottom of the soil beam are 3cm (OF1-1, OF1-2), 6cm (OF2-1, OF2-1), and 9cm (OF3-1, OF3-2), and the lateral spacing is 5cm. Take out the pressed soil beam, use steel needles with a diameter of 0.1mm to intensively punch pinholes on the test observation surface, and use this as the surface texture of the soil beam. Place the prepared soil beam on the corresponding position of the loading plate 5, then connect the six strain sensing optical fibers 3 to the OFDR signal demodulation and processing module 1 interface, and open the OFDR signal demodulation and processing module 1 and digital image in sequence. Collection and analysis device 15, stepper motor 7, and gearbox 6 switches. The stepper motor 7 pushes the loading plate 5 to move downward at a speed of 0.28mm/min. The soil beam begins to bend at four points. The OFDR signal demodulation and processing module 1 obtains and presents the strain distribution state inside the soil beam in real time. ; The digital image acquisition and analysis device 15 tracks the texture changes on the surface of the soil beam in real time, and obtains the strain field and displacement field after the soil beam is deformed by force through the built-in PIV digital image processing software; at the same time, the OFDR signal demodulation and processing module 1 also monitors The strain time history curves of soil at different parts of the specimen during four-point bending are shown.

It should be noted that, in addition to the above-mentioned embodiments, the patent of the present invention may also have other implementation modes. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope of the patent claims of the present invention.

Claims (5)

1. A method of using optical fiber monitoring and testing devices for soil tension mechanical properties based on OFDR, which is characterized by including the following steps: The first step is to prepare the test soil beam: press the soil beam in layers according to the given dry density in the soil beam pressing mold. When the soil beam is pressed to the layout position of the strain sensing optical fiber, the strain sensing optical fiber is passed through the soil in sequence. The fiber optic penetration holes on the side of the beam pressing mold box are laid in the soil, and heavy objects are hung appropriately to put them in a slight tension state, and then the soil is filled and pressed according to the given dry density; The second step is to mark the soil sample: take out the pressed soil beam, prick pinholes densely on the test observation surface, and use this as the surface texture of the soil beam, then naturally air-dry or dry it to the required moisture content, and then cover it with film ; The third step is to connect the OFDR signal demodulation and processing module: uncover the soil beam coating, place it horizontally on the loading plate of the tester, connect all the strain sensing optical fibers to each other in parallel or series and then connect them to the OFDR signal demodulation module. On the interface of the adjustment and processing module; The fourth step is to start the test: turn on the switches of the OFDR signal demodulation and processing module, digital image acquisition and analysis device, and speed-regulating drive device in sequence. The speed-regulating drive device pushes the loading plate to move downward at the set speed, and the test soil beam follows. Four-point bending occurs; the OFDR signal demodulation and processing module acquires and presents the strain distribution inside the soil beam in real time; the digital image acquisition and analysis device tracks the texture changes on the surface of the soil beam in real time, and obtains the strain field and displacement after the soil beam is deformed by force. field; The fifth step, data processing: Based on the measured data, establish the tensile stress-strain constitutive relationship of the soil and obtain the tensile strength and cracking strain value parameters of the soil; The OFDR-based optical fiber monitoring and testing device for soil tension mechanical properties includes a test soil beam (4), a tester (14), and a strain sensing optical fiber (3); the tester (14) includes a chassis Shell (12), reaction force bracket (13), loading plate (5), speed regulating drive device, OFDR signal demodulation and processing module (1), digital image acquisition and analysis device (15), the chassis shell ( 12) There is a speed-adjusting drive device inside. The speed-adjusting drive device is connected to the dynamometer (16) and the loading plate (5). The loading plate (5) can move up and down along the vertical direction of the inner wall of the chassis shell (12). When measuring The test soil beam (4) is placed between the force meter (16) and the reaction support (13). The strain sensing optical fiber (3) passes through the test soil beam (4) in the horizontal direction, and the strain sensing optical fiber (3) passes the signal. The transmission optical fiber (2) is connected to the OFDR signal demodulation and processing module (1), and the digital image acquisition window of the digital image acquisition and analysis device (15) corresponds to the test observation surface of the test soil beam (4); the speed-regulating drive device includes a step Enter the motor (7) and gearbox (6); the digital image acquisition and analysis device (15) includes a high-speed camera (8) and a computer (9); the loading plate (5) is provided with rollers at both ends; The signal demodulation and processing module includes fiber optic demodulator, terminal computing processing and visualization system. 2. The method of using an optical fiber monitoring and testing device for soil tension mechanical properties based on OFDR according to claim 1, characterized in that the digital image acquisition and analysis device is based on a digital image coherence method or a particle image velocimetry method. to collect and analyze. 3. The method of using an optical fiber monitoring and testing device for soil tension mechanical properties based on OFDR according to claim 1, characterized in that the soil beam pressing mold (10) has four side plates fixed on a base plate. It consists of two side panels at both ends with optical fiber penetration holes (11). 4. The method of using an optical fiber monitoring and testing device for soil tension mechanical properties based on OFDR according to claim 1, characterized in that the strain sensing optical fiber threads the sheath through electrode engraving and electric discharge processing technology. Chemical treatment, the strain sensing optical fiber is fixed in the soil using a tube or plate anchoring device. 5. The method of using optical fiber monitoring and testing device for soil tension mechanical properties based on OFDR according to claim 1, characterized in that the OFDR signal demodulation and processing module is within 1 mm according to the testing accuracy and denoising requirements. -Customized resolution within 10cm range.