Briquette coal pressing mold and method for measuring internal deformation and temperature of fractured coal body

Abstract

The invention discloses a briquette pressing mold and a method for measuring internal deformation and temperature of a fractured coal body. The method comprises the following steps: determining briquette coal pressing parameters and S-shaped optical fiber sensor embedding information; optimally designing and mounting a briquette coal pressing mold; preparing a briquette coal pressing material; pressing the briquette coal to obtain first briquette coal; burying an S-shaped optical fiber sensor in the first briquette coal to obtain second briquette coal; the second briquette coal is subjected to maintenance and treatment processing, and third briquette coal is obtained; performing a liquid nitrogen fracturing test on the third briquette, and collecting deformation and temperature data in real time through an S-shaped optical fiber sensor; processing the collected data to generate a strain field and temperature field three-dimensional space-time distribution diagram; and establishing a safety threshold prediction model based on the strain field and temperature field three-dimensional space-time distribution diagram. According to the device and the method, the limitation of a traditional local measurement means is overcome, reliable data support is provided for a coal seam liquid nitrogen fracturing mechanism, a fracture extension rule and coal mine safety management through a split type mold, sensor measurement and a multi-layer data acquisition technology, and the device and the method have remarkable experiment and engineering application values.

Description

Technical Field

[0001] The invention belongs to the technical field of coal body fracturing monitoring, sensors, and mineral geological exploration services using high and new technologies, and particularly relates to a method for measuring the internal deformation and temperature of a coal body fractured by liquid nitrogen based on the arrangement of S-shaped optical fibers. Background Art

[0002] With the continuous increase in the mining depth of coal mines, the internal stress state, deformation mechanism, and temperature field distribution of coal seams have become important research objects for coal mine safety and efficient mining. Traditional coal body monitoring mainly relies on strain gauges, thermocouples, or other discrete sensors locally arranged. This method not only has limited monitoring points, but also due to problems such as uneven installation positions, signal attenuation, and insufficient temperature compensation, it is difficult to achieve the goal of continuous distributed monitoring of the entire coal body interior. As a test sample that can simulate the physical and mechanical properties of raw coal, shaped coal is widely used in coal body test research due to its controllable manufacturing process and high homogeneity. However, the current traditional shaped coal preparation mainly adopts the method of integral filling and pressing, which cannot effectively reserve sensing channels, resulting in limited internal monitoring means, discontinuous data acquisition, and low positioning accuracy during low-temperature impact tests such as liquid nitrogen fracturing.

[0003] In recent years, as a new low-temperature impact technology, liquid nitrogen fracturing technology has been widely used in coal seam transformation and coalbed methane development. The injection of liquid nitrogen causes a drastic temperature gradient to form instantaneously in the coal body, inducing crack propagation, local deformation, and stress redistribution, thereby improving the permeability of the coal seam and coal mine safety. However, under the low-temperature impact of liquid nitrogen on the coal body, the internal strain and temperature changes show complex spatio-temporal distribution characteristics, and traditional local measurement methods are difficult to capture the overall response. There is an urgent need for a new technology that can achieve full-process, continuous, and distributed monitoring.

[0004] There are already relevant patents in China involving technologies such as shaped coal preparation, crack prefabrication, and automatic layout of optical fiber sensors, but none of them have proposed a targeted solution for continuous monitoring of the internal strain and temperature field distribution of the coal body during liquid nitrogen fracturing. Especially in aspects such as how to reserve S-shaped sensing channels inside the shaped coal, how to ensure the stable operation of optical fiber sensors in extremely low-temperature environments, and how to achieve three-dimensional measurement through multi-layer data acquisition, there are still large technical gaps.

[0005] Therefore, there is an urgent need for a method for measuring the internal deformation and temperature of a coal body fractured by liquid nitrogen based on the arrangement of S-shaped optical fibers, which can not only ensure that the structure of the shaped coal is similar to that of the raw coal, but also embed high-precision sensors to monitor the dynamic response during the liquid nitrogen fracturing process in real time. Summary of the Invention

[0006] In view of the above technical problems, the invention provides a shaped coal pressing mold and a method for measuring the internal deformation and temperature of a fractured coal body.

[0007] The technical solutions provided by this application are as follows: In the first aspect, a briquette pressing die is provided, including: A base; Side plates, arranged on the base; A top cover, forming a forming cavity with the base and the side plates; A guide groove rod, arranged at the bottom of the forming cavity and connected to the base, used to form a cylindrical spiral guide groove inside during the briquette pressing process; A channel rod, arranged at the upper part of the forming cavity and connected to the top cover, used to form a wellbore channel during the briquette pressing process; A pressing column, used to apply pressure to the top cover.

[0008] In a possible implementation manner, the cylindrical spiral guide groove spirally ascends from bottom to top, and the caliber gradually shrinks.

[0009] In the second aspect, a method for measuring the internal deformation and temperature of a fractured coal body based on the S-type optical fiber arrangement is provided, including: Determining the briquette pressing parameters and the S-type optical fiber sensor embedding information: The optimized design and installation of the briquette pressing die described in the first aspect; Preparing the briquette pressing material; Briquette pressing to obtain the first briquette; Embedding an S-type optical fiber sensor in the first briquette to obtain the second briquette; Curing and processing the second briquette to obtain the third briquette; Conducting a liquid nitrogen fracturing test on the third briquette, and collecting deformation and temperature data in real time through the S-type optical fiber sensor; Processing the collected data to generate a three-dimensional spatio-temporal distribution map of the strain field and the temperature field; Based on the three-dimensional spatio-temporal distribution map of the strain field and the temperature field, establishing a safety threshold prediction model.

[0010] In a possible implementation manner, the briquette pressing parameters include: briquette size and bedding structure; the S-type optical fiber sensor embedding information includes the laying path, embedding depth, and optical fiber diameter.

[0011] In a possible implementation manner, the S-type optical fiber sensor is a distributed optical fiber sensor with high sensitivity, low temperature resistance, and strain-temperature dual-parameter monitoring ability.

[0012] In a possible implementation manner, the optimized design method for the cylindrical spiral guide groove of the briquette pressing die includes: Using finite element simulation to optimize the curvature radius and embedding spacing of the cylindrical spiral guide groove to ensure that the strain transfer efficiency of the optical fiber in the liquid nitrogen environment is ≥95%; Locate the generatrix position of the cylindrical helical groove so that the coaxiality deviation from the axis of the shaft passage is ≤0.005 mm.

[0013] In a possible implementation, the method for preparing the briquette pressing material includes: Mix pulverized coal and coal tar binder in proportion, weigh a certain mass of water, add water while stirring, and mix thoroughly to obtain the briquette pressing material.

[0014] In a possible implementation, the method for embedding the S-shaped fiber optic sensor in the first briquette to obtain the second briquette includes: Feed the S-shaped fiber optic sensor along the path of the cylindrical helical groove using a robotic arm, and monitor the path deviation and fiber microbending loss in real time. Dynamically adjust the clamping force and feeding speed of the robotic arm until the S-shaped fiber optic sensor is completely embedded.

[0015] Furthermore, the tension control of the robotic arm adopts a PID closed-loop algorithm. Combining with the real-time path deviation feedback, dynamically adjust the clamping force and movement speed so that the path tracking error is ≤0.03 mm.

[0016] In a possible implementation, the method for processing the collected data to generate a three-dimensional spatio-temporal distribution map of the strain field and temperature field includes: performing wavelet filtering and baseline calibration processing on the collected continuous data, extracting the strain and temperature characteristic parameters of each monitoring point, reconstructing the three-dimensional strain field and temperature field spatio-temporal distribution model inside the briquette based on the finite element inversion algorithm, and drawing a dynamic evolution cloud map in combination with the phase field theory of coal body crack propagation.

[0017] In a possible implementation, the method for establishing a safety threshold prediction model based on the three-dimensional spatio-temporal distribution map of the strain field and temperature field includes: Based on the strain-temperature data collected in real time by the S-shaped fiber optic sensor and the COMSOL multi-physics field simulation model, establish a quantitative mapping relationship between the liquid nitrogen injection rate, pressure and crack parameters, and extract the crack propagation rate and spatio-temporal distribution characteristics through the three-dimensional spatio-temporal distribution map of the strain field and temperature field; Adopt regression analysis and multi-field coupling verification to quantify the laws of crack initiation, propagation and penetration under low-temperature impact, and form a safety threshold prediction model based on the strain-temperature dual parameters.

[0018] Advantages of the present invention: Through the S-shaped optical fiber path design, the present invention significantly expands the monitoring coverage. The spatial resolution is several times higher than that of the traditional straight layout, and it can accurately capture the strain and temperature gradient distribution in the core area of the coal body. Combining the briquette pressing and the sensor layout technology of the robotic arm, it realizes high-precision positioning with the deviation of the sensor burial position ≤0.1 mm, effectively ensuring the integrity of the coal body structure. The strain-temperature dual-parameter demodulation technology eliminates the cross-sensitivity between strain and temperature, and significantly improves the data reliability. In addition, the automated operation process of the robotic arm can effectively shorten the construction period compared with the traditional manual operation, providing efficient and accurate technical support for the research on the mechanism of coal and rock fracturing.

[0019] The method provided by the present invention utilizes a specific briquette pressing mold and an S-shaped optical fiber sensor, combined with finite element simulation, data processing methods, and COMSOL simulation, to be able to conduct geological exploration research on coal, providing a new method for mineral geological exploration services using high-tech. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is the flowchart of the method of the present invention; Figure 2 is the schematic diagram of the pressing device; Figure 3 is the schematic diagram of the briquette embedded with the S-shaped optical fiber sensor.

[0021] Figure 1 and Figure 3 In and : 1 - pressure column, 2 - side plate, 3 - briquette, 4 - bottom plate, 5 - top cover, 6 - guide groove rod, 7 - channel rod, 8 - shaft channel, 9 - S-shaped optical fiber sensor. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The content of the present invention will be further described below in combination with specific embodiments, and the content of the present invention is not limited thereto.

[0023] Embodiment 1 Refer to Figure 1 , a briquette pressing mold provided for this embodiment, including: Base 4; Side plate 2, arranged on the base 4; Top cover 5, forming a molding cavity with the base 4 and the side plate 2; Guide groove rod 6, arranged at the bottom of the molding cavity and connected to the base, used to form a cylindrical spiral guide groove during the briquette pressing process; Channel rod 7, arranged at the upper part of the molding cavity and connected to the top cover, used to form a shaft channel during the briquette pressing process, facilitating the subsequent installation of a stainless steel sleeve to introduce liquid nitrogen; Pressure column 1, used to apply pressure to the top cover.

[0024] In a possible implementation, the cylindrical spiral guide groove spirally ascends from bottom to top, and the aperture gradually shrinks.

[0025] It can be understood that the structure with gradually shrinking aperture can force the optical fiber to closely fit the coal body, reducing the interface gap.

[0026] Embodiment 2 See Figure 2 , a method for measuring the internal deformation and temperature of a coal body during liquid nitrogen fracturing based on the S-shaped optical fiber arrangement provided in this embodiment, the method includes the following steps: S201. Determine the briquette pressing parameters and the S-shaped optical fiber sensor embedding information.

[0027] In a possible implementation, the briquette pressing parameters include: briquette size and bedding structure; the S-shaped optical fiber sensor embedding information includes the laying path, embedding depth, and optical fiber diameter.

[0028] Optionally, the briquette parameter design determines that the briquette size is a cube with a side length of 100 mm. The curvature radius of the S-shaped path of the optical fiber laying plan is set to 50 mm, the optical fiber diameter is 5 mm, and the embedding spacing is 25 mm to ensure coverage of the core area of the coal body.

[0029] In a possible implementation, the S-shaped optical fiber sensor is a distributed optical fiber sensor with high sensitivity, low-temperature resistance, and strain-temperature dual-parameter monitoring capabilities.

[0030] Optionally, the S-shaped optical fiber sensor selects a distributed optical fiber grating sensor, with a wavelength range of 1520 - 1570 nm, a temperature sensitivity of 10 pm / °C, a strain sensitivity of 1.2 pm / με, and low-temperature resistance down to -196°C.

[0031] S202. Optimize the design and installation of the briquette pressing mold described in Embodiment 1.

[0032] In a possible implementation, the method for optimizing the design of the cylindrical spiral guide groove of the briquette pressing mold includes: Use finite element simulation to optimize the curvature radius and embedding spacing of the cylindrical spiral guide groove to ensure that the strain transfer efficiency of the optical fiber in the liquid nitrogen environment is ≥95%; Locate the position of the generatrix of the cylindrical spiral guide groove so that the coaxiality deviation from the axis of the shaft passage is ≤0.005 mm.

[0033] It should be noted that the embedding spacing refers to the vertical distance between the spiral lines of the cylindrical spiral guide groove.

[0034] Furthermore, the method for using finite element simulation to optimize the curvature radius and embedding spacing of the cylindrical spiral guide groove includes: Establish a three-dimensional parametric model including a forming chamber and a spiral guide groove structure, and define the elastic modulus and Poisson's ratio of the coal material; Simulate the temperature field and stress field distributions during liquid nitrogen injection through a multi-physics coupling module, conduct a combined analysis of the guide groove curvature radius and burial spacing using parametric scanning, and calculate the strain transfer efficiency and stress concentration coefficient at the fiber-coal interface; Through iterative optimization, select an optimal combination of the curvature radius and burial spacing to ensure that the strain transfer efficiency is ≥95% and the stress distribution is uniform; and based on the geometric constraint module, locate the position of the guide groove bus, and verify that the coaxiality deviation between the guide groove axis and the shaft passage is ≤0.005 mm through mesh refinement and sensitivity analysis; Calibrate the simulation model with experimental data to achieve the collaborative optimization of the guide groove structure and the fiber layout path.

[0035] Specifically, in this embodiment, an optimal combination with a curvature radius of 50 mm and a burial spacing of 25 mm is selected through iterative optimization.

[0036] Optionally, the forming chamber of the briquette pressing mold is 100 mm × 100 mm × 100 mm. The inner wall of the chamber is coated with a polytetrafluoroethylene coating and vaseline is coated during the experiment to facilitate demolding.

[0037] S203. Prepare the briquette pressing material.

[0038] In a possible implementation manner, the method for preparing the briquette pressing material includes: Mix pulverized coal and coal tar binder in proportion, weigh a certain mass of water, add water while stirring, and mix thoroughly to obtain the briquette pressing material.

[0039] Exemplarily, weigh 300 g of the selected good coal sample particles and put them into a stirring container; weigh a certain mass of water with a measuring cylinder, add water while stirring to fully mix the pulverized coal particles and water; then add a coal tar-water mixture preheated to 45°C and stir with a mixer for 35 minutes to obtain the briquette pressing material.

[0040] S204. Press the briquette to obtain the first briquette.

[0041] In a possible implementation manner, place the prepared coal sample mixed with pulverized coal and water in a mold, compact the coal sample in the mold with a compaction cylinder, and then place the pressure cylinder on the base of the press; then operate the press software according to the steps, set it to the pressure loading control mode, the pressure application speed is 0.5 kN / s, the final pressure is set to 15 MPa, and the briquette holding pressure time is about 70 min; demold the pressed and formed coal sample, and use a press for demolding to ensure a high success rate of briquette preparation.

[0042] Exemplarily, the molding press uses an X-type microcomputer-controlled electro-hydraulic servo experimental system. The test device mainly consists of two parts: a computer and a loading system. The computer is used for automatic control of the loading to make the hydraulic press reach the required hydraulic pressure. During the experiment, first turn on the power switch of the computer and the oil pump. After the hydraulic system has been preheated for a certain period of time, turn on the computer and control the entire system through the control software, and control the molding pressure through the oil pressure.

[0043] Furthermore, the control mode of demolding is set to pressure control to ensure stability during the demolding process and avoid the phenomena of briquette fracture and bottom damage, which may cause uneven end faces. Install a stainless steel casing with an inner diameter of 2 mm at the entrance of the shaft passage to prevent liquid nitrogen leakage.

[0044] S205: Embed an S-type fiber optic sensor in the first briquette to obtain the second briquette.

[0045] In a possible implementation, S205 includes: using a robotic arm to feed the S-type fiber optic sensor along the path of the cylindrical spiral groove, and real-time monitoring of the path deviation and fiber micro-bending loss, dynamically adjusting the clamping force and feeding speed of the robotic arm until the S-type fiber optic sensor is completely embedded.

[0046] Exemplarily, use the robotic arm to clamp the end of the fiber optic sensor with a clamping force of 2 N, embed it along the groove path with a constant tension of 7 N, and at the same time, use a laser displacement sensor to detect the path deviation (≤±0.05 mm) in real time and an OTDR tester to monitor the fiber micro-bending loss (≤0.1 dB / km).

[0047] Furthermore, the robotic arm tension control adopts a PID closed-loop algorithm, combined with real-time path deviation feedback, to dynamically adjust the clamping force and movement speed, so that the path tracking error ≤0.03 mm.

[0048] S206: Cure and process the second briquette to obtain the third briquette.

[0049] In a possible implementation, S206 includes: static curing, constant temperature curing, and cutting and grinding of the second briquette.

[0050] Furthermore, the static curing of the briquette includes: after the briquette is pressed, it is left to stand in an environment of 20°C and 60% humidity for 12 hours to preliminarily cure the coal body.

[0051] Furthermore, the constant temperature curing includes: transferring it to a constant temperature and humidity chamber and curing it for 7 days in an environment of 20±1°C and 95% humidity, and recording the shrinkage rate every day.

[0052] Furthermore, the cutting and grinding includes drilling a shaft passage in the center of the briquette and using a diamond wire cutting machine to remove the edge burrs, with a surface roughness Ra≤3.2 μm.

[0053] Optionally, install a stainless steel casing with an inner diameter of 6 mm at the entrance of the wellbore passage for injecting liquid nitrogen.

[0054] Figure 3 The schematic diagram of the briquette embedded with the S-shaped optical fiber sensor is shown.

[0055] S207. Conduct a liquid nitrogen fracturing test on the third briquette, and collect deformation and temperature data in real time through the S-shaped optical fiber sensor.

[0056] In a possible implementation manner, S207 includes: installing the prepared briquette at the central position of the fracturing chamber through a fixed fixture, and starting the test system after ensuring that the optical fiber connection cable has no interference.

[0057] Exemplarily, first, the liquid nitrogen injection system transports liquid nitrogen to the fracturing chamber through a cryogenic high-pressure pump via a double-layer vacuum adiabatic pipeline, and at the same time, the true triaxial pressurization system applies axial pressure and lateral pressure in stages; then inject liquid nitrogen through the wellbore passage, and the pressure is gradually increased to the set pressure and maintained for 30 minutes. During the process, the temperature in the fracturing chamber is monitored in real time through a quartz observation window and a platinum resistance sensor; the optical fiber demodulator synchronously collects the strain and temperature data of the S-shaped optical fiber sensor inside the briquette and sends them to the data acquisition system. After the test is terminated, the pressure is released to atmospheric pressure, and it is left standing until the temperature rises above -50 °C, and then the briquette is taken out for crack morphology and data matching analysis.

[0058] Optionally, the axial pressure is set to rise to 8 MPa at a rate of 0.5 MPa / min, and the lateral pressure is set to rise to 5 MPa at a rate of 0.3 MPa / min to simulate the formation stress environment.

[0059] Optionally, the liquid nitrogen injection rate is 0.8 L / min, and the set pressure is 8.0 ± 0.2 MPa.

[0060] Preferably, the sampling frequency of the optical fiber demodulator is 100 Hz, the resolution for strain is 1 με, and the resolution for temperature is 0.1 °C.

[0061] S208. Process the collected data to generate three-dimensional spatio-temporal distribution maps of the strain field and the temperature field.

[0062] In a possible implementation manner, S208 includes: performing wavelet filtering and baseline calibration processing on the collected continuous data, extracting the strain and temperature characteristic parameters of each monitoring point, reconstructing the three-dimensional strain field and temperature field spatio-temporal distribution model inside the briquette based on the finite element inversion algorithm, and drawing the three-dimensional spatio-temporal distribution maps of the strain field and the temperature field in combination with the phase field theory of coal body crack propagation.

[0063] S209. Establish a safety threshold prediction model based on the three-dimensional spatio-temporal distribution maps of the strain field and the temperature field.

[0064] In a possible implementation, S209 includes: S209a. Based on the strain-temperature data collected in real time by the S-type fiber optic sensor and the COMSOL multi-physics simulation model, establish a quantitative mapping relationship between the liquid nitrogen injection rate, pressure and fracture parameters (length, fractal dimension), and extract the fracture propagation rate and spatio-temporal distribution characteristics through the three-dimensional spatio-temporal distribution diagrams of the strain field and temperature field.

[0065] It should be noted that the liquid nitrogen injection rate directly affects the shrinkage stress distribution by regulating the internal temperature gradient and cooling rate of the coal body, showing the synchronous response characteristics of the strain peak and the sudden temperature drop.

[0066] S209b. Adopt regression analysis and multi-field coupling verification to quantify the laws of crack initiation, propagation and penetration under low-temperature impact, and form a safety threshold prediction model based on the strain-temperature dual parameters, providing an accurate quantitative basis for the optimization of the liquid nitrogen fracturing process and the safety assessment of the coal body.

[0067] The above is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any modification, equivalent replacement and improvement made by those skilled in the art within the technical scope disclosed by the present invention shall be included in the protection scope of the invention.

Claims

1. A briquette pressing mold, characterized in that, Comprising: Base; Side plates, arranged on the base; Top cover, forming a molding chamber together with the base and the side plates; Guide groove rod, arranged at the bottom of the molding chamber and connected to the base, used to form a cylindrical helical groove inside during the briquette pressing process; Channel rod, arranged at the upper part of the molding chamber and connected to the top cover, used to form a shaft channel during the briquette pressing process; Pressing column, used to apply pressure to the top cover.

2. The briquette pressing die according to claim 1, characterized in that, The cylindrical helical groove spirally ascends from bottom to top, and the caliber gradually shrinks.

3. Method for measuring internal deformation and temperature of fractured coal body based on S-shaped optical fiber arrangement, characterized in that, Comprising: Determine the briquette pressing parameters and the embedding information of the S-shaped fiber optic sensor: Optimized design and installation of the briquette pressing mold according to Claim 1 or 2; Prepare the briquette pressing material; Briquette pressing to obtain the first briquette; Embed the S-shaped fiber optic sensor in the first briquette to obtain the second briquette; Cure and process the second briquette to obtain the third briquette; Conduct a liquid nitrogen fracturing test on the third briquette, and collect deformation and temperature data in real time through the S-shaped fiber optic sensor; Process the collected data to generate a three-dimensional spatio-temporal distribution map of the strain field and the temperature field; Based on the three-dimensional spatio-temporal distribution map of the strain field and the temperature field, establish a safety threshold prediction model.

4. The method according to claim 3, characterized in that, The briquette pressing parameters include: briquette size and bedding structure; the embedding information of the S-shaped fiber optic sensor includes the laying path, embedding depth, and fiber diameter.

5. The method according to claim 3, characterized in that The S-shaped fiber optic sensor is a distributed fiber optic sensor with high sensitivity, low temperature resistance, and strain-temperature dual-parameter monitoring ability.

6. The method according to claim 3, characterized in that The optimized design method of the cylindrical helical groove of the briquette pressing mold includes: Use finite element simulation to optimize the curvature radius and embedding spacing of the cylindrical helical groove to ensure that the strain transfer efficiency of the optical fiber in the liquid nitrogen environment is ≥95%; Locate the generatrix position of the cylindrical helical groove to make the coaxiality deviation from the axis of the shaft channel ≤0.005 mm.

7. The method according to claim 3, characterized in that, The method for preparing the briquette pressing material includes: Mix pulverized coal and coal tar binder in proportion, weigh a certain mass of water, add water while stirring, and fully mix to obtain the briquette pressing material.

8. The method according to claim 3, characterized in that, The method for embedding the S-shaped fiber optic sensor in the first briquette to obtain the second briquette includes: Use a robotic arm to send the S-shaped fiber optic sensor along the path of the cylindrical helical groove, and monitor the path deviation and fiber microbending loss in real time, dynamically adjust the clamping force and feeding speed of the robotic arm until the S-shaped fiber optic sensor is completely embedded.

9. The method according to claim 3, characterized in that The method for processing the collected data to generate a three-dimensional spatio-temporal distribution map of the strain field and the temperature field includes: performing wavelet filtering and baseline calibration processing on the collected continuous data, extracting the strain and temperature characteristic parameters of each monitoring point, reconstructing the three-dimensional strain field and temperature field spatio-temporal distribution model inside the briquette based on the finite element inversion algorithm, and drawing a dynamic evolution cloud map in combination with the phase field theory of coal body crack propagation.

10. The method according to claim 3, wherein The method for establishing a safety threshold prediction model based on the three-dimensional spatio-temporal distribution map of the strain field and the temperature field includes: Based on the strain-temperature data collected in real time by the S-shaped fiber optic sensor and the COMSOL multi-physics field simulation model, establish a quantitative mapping relationship between the liquid nitrogen injection rate, pressure, and fracture parameters, and extract the fracture propagation rate and spatio-temporal distribution characteristics through the three-dimensional spatio-temporal distribution map of the strain field and the temperature field; By using regression analysis and multi-field coupling verification, the laws of crack initiation, propagation and penetration under low-temperature impact are quantified, and a safety threshold prediction model based on the strain-temperature dual parameters is formed.

Images