Device and method for monitoring three-dimensional stress field of cubic rock samples based on distributed optical fiber

Abstract

The present invention discloses a device and method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber, wherein: distributed optical fiber sensing cables are arranged on six vertical surfaces of the rock sample to be tested, and are arranged in a regular rectangular grid according to the minimum bending radius of the distributed optical fiber sensing cables; the strain data of the surface of the rock sample to be tested is monitored by the distributed optical fiber sensing cables, and the strain data of the rock sample to be tested is processed by a distributed optical fiber strain demodulator and then sent to the optical fiber data acquisition processing software and a computer; calculations are performed based on the strain data, data grid processing is performed on each vertical surface, bilinear interpolation is used for the regular rectangular grid area, and inverse distance weighted average method is used for the discrete data of the surrounding optical fiber bending area, so as to realize interpolation inversion and three-dimensional stress field visualization. The present invention completes the all-round high-resolution monitoring of strain on the six surfaces of the rock, and can invert the three-dimensional spatial distribution of rock stress under the triaxial stress state.

Description

Technical Field

[0001] The present invention relates to the field of three-dimensional stress monitoring of geotechnical engineering, and in particular to a device and method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber. Background Art

[0002] The technical background of monitoring the three-dimensional stress field of rocks mainly stems from the need for in-depth research on the mechanical properties of rocks, especially in the fields of geological engineering, geotechnical engineering, mining engineering, etc. These fields usually require accurate evaluation of the deformation, stress distribution and stability of rocks under various external forces to ensure engineering safety and economic benefits.

[0003] Traditional rock stress field monitoring methods often rely on point or local measurement methods, such as resistance strain gauges, LVDT, extensometers, and Bragg gratings, which are currently considered to be reliable methods for rock surface strain measurement. They can only provide strain measurements at a single point or at a limited location, and rock mechanical properties can only be determined by limited strain data. In recent years, digital image correlation (DIC) has been introduced into strain field measurement as a non-invasive method because it can measure strain distribution within the monitoring window. At present, the development from two-dimensional (2D) to three-dimensional (3D) DIC has solved the 2D DIC measurement error caused by relatively small out-of-plane motion and realized full-field 3D surface strain measurement. However, DIC still has some limitations, such as dependence on strict external conditions, complex post-test data processing, and limited strain resolution of 50 με. Therefore, it is particularly important to develop a technology that can monitor the 3D stress field of rocks in a relatively simple, real-time, high strain and high temperature resolution.

[0004] Distributed fiber optic monitoring technology came into being in such a context. This technology uses optical fiber as a sensor to infer the temperature and strain distribution in the rock by measuring the changes in the optical signal in the optical fiber. Compared with traditional point or local measurement methods, distributed fiber optic monitoring technology has the advantages of continuous monitoring, high precision, high sensitivity, long-distance transmission and corrosion resistance, and can achieve comprehensive monitoring of the three-dimensional stress field of the rock. With the continuous development and improvement of fiber optic sensing technology, the application of distributed fiber optic monitoring technology in the monitoring of the three-dimensional stress field of the rock is becoming more and more extensive. Not only can rock samples be accurately measured in laboratory environments, but also long-term stability monitoring of structures such as rock slopes, tunnels, and mines can be performed in actual engineering. In addition, this technology can also be combined with other geomechanical analysis methods to conduct a more in-depth analysis and study of the three-dimensional stress field of the rock, providing an important reference for engineering design and disaster prevention.

[0005] Therefore, fiber optic sensing technology has been used to monitor the three-dimensional stress field of rocks. Distributed fiber optic sensing technology can monitor the three-dimensional stress field of rocks by direct measurement or indirect measurement according to the target to be monitored: ① Direct measurement is to lay the optical fiber directly in the monitored object. As the deformation and temperature of the monitored object change, the strain and temperature information obtained by the optical fiber is the strain and temperature distribution of the monitored object. The direct measurement method can effectively monitor the three-dimensional stress field of rocks, but it is not conducive to long-term monitoring. ② Indirect measurement is to lay the optical fiber on a certain support structure or use an optical fiber sensor probe. The deformation and temperature of the monitored object are converted into the corresponding parameter changes of the support structure or sensor, and then converted into the strain and temperature of the optical fiber. The deformation and temperature of the monitored object are indirectly obtained by detecting the strain and temperature on the optical fiber. The indirect measurement method can be used for long-term monitoring, but a special support structure must be designed, and the change of the support structure or sensor used must have a good correspondence with the strain and temperature of the optical fiber.

[0006] When monitoring stress fields in a laboratory environment, the monitoring object is often a cylindrical rock sample, which is usually laid out by spirally winding the cylindrical rock sample and directly pasting it to the rock surface. Stress field monitoring of cubic rock samples is rarely involved. When applied to cubic rock samples, since the optical fiber material is relatively brittle and has certain requirements for bending, it cannot be bent in the angular direction, and the spiral winding layout method is no longer suitable for cubic rock samples. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a device and method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber in view of the defects in the prior art.

[0008] The technical solution adopted by the present invention to solve its technical problem is:

[0009] The present invention provides a device for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber, the device comprising: a rock sample pressure loading component, a distributed optical fiber strain monitoring component, optical fiber data acquisition and processing software and a computer;

[0010] A rock sample pressure loading assembly is used to apply pressure to a cube-shaped rock sample to be tested;

[0011] A distributed optical fiber strain monitoring component includes a distributed optical fiber strain demodulator and a distributed optical fiber sensing cable. One end of the distributed optical fiber sensing cable is connected to the rock sample pressure loading component, and the other end is connected to the optical fiber data acquisition and processing software and a computer through the distributed optical fiber strain demodulator. The middle part of the distributed optical fiber sensing cable is arranged on six facades of the rock sample to be tested. According to the minimum bending radius of the distributed optical fiber sensing cable, one edge of the cube is used as the starting point, and the distributed optical fiber sensing cable is bent with the minimum bending radius so that it is arranged in a regular rectangular grid on each facade. The strain data of the surface of the rock sample to be tested is monitored by the distributed optical fiber sensing cable, and the strain data of the rock sample to be tested is processed by the distributed optical fiber strain demodulator and then sent to the optical fiber data acquisition and processing software and a computer.

[0012] Fiber optic data acquisition and processing software and computers are used to calculate based on strain data to obtain the three-dimensional spatial distribution of effective stress of the rock sample to be tested; and to divide the data into grids for each facade. For regular rectangular grid areas, bilinear interpolation is used, and for discrete data of the surrounding fiber optic bending areas, inverse distance weighted average method is used to achieve interpolation inversion and three-dimensional stress field visualization.

[0013] Furthermore, the rock sample pressure loading assembly of the present invention comprises: an axial pressure loading assembly, a confining pressure loading assembly, and a rock sample holder; wherein:

[0014] The rock sample to be tested, after the distributed optical fiber sensing cable is arranged and sealed, is placed in a rock sample holder, and the distributed optical fiber sensing cable is connected to the distributed optical fiber strain demodulator through the optical fiber through-piece on the rock sample holder; the confining pressure and axial pressure are applied to the rock sample to be tested through the confining pressure loading assembly and the axial pressure loading assembly respectively.

[0015] Furthermore, the distributed optical fiber sensing cable of the present invention is adhered to the rock sample to be tested by bonding with an epoxy resin adhesive.

[0016] Furthermore, the present invention determines the minimum bending radius according to the standard of the single-mode optical fiber used in the distributed optical fiber sensing cable; the grid spacing of the regular rectangular grid is twice the minimum bending radius.

[0017] The present invention provides a method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber, comprising the following steps:

[0018] Step 1: On the six vertical faces of the cube-shaped rock sample to be tested, the distributed optical fiber sensing cables are adhered to each face in a regular rectangular grid arrangement using an epoxy resin adhesive;

[0019] Step 2: Place the sealed rock sample to be tested in a rock sample holder, connect one end of the distributed optical fiber sensing cable to the rock sample pressure loading assembly, and connect the other end to the distributed optical fiber strain demodulator through the optical fiber penetration piece on the rock sample holder, and connect the distributed optical fiber strain demodulator to the optical fiber data acquisition and processing software and a computer;

[0020] Step 3: Start monitoring the rock sample to be tested, collect the surface strain data of the rock sample to be tested through the distributed optical fiber strain demodulator and transmit it to the optical fiber data acquisition and processing software and the computer;

[0021] Step 4: Perform three-dimensional stress field inversion on the strain data, pre-process the strain data, and divide the data into grids for each facade. For the regular rectangular grid area, use the bilinear interpolation method, and for the discrete data of the surrounding optical fiber bending area, use the inverse distance weighted average method to achieve interpolation inversion and three-dimensional stress field visualization.

[0022] Furthermore, the arrangement of the regular rectangular grid in step 1 of the present invention includes:

[0023] Select one side of the cube, and paste the distributed optical fiber sensor cable parallel to the side at a certain distance using epoxy resin adhesive;

[0024] The minimum bending radius r is determined according to the standard of the single-mode optical fiber used in the distributed optical fiber sensing cable. When the bending radius is less than the limit bending radius of the single-mode optical fiber, the distributed optical fiber sensing cable is reversely bent 180° and kept parallel. The distance between the two parallel distributed optical fiber sensing cables is 2r.

[0025] Repeat until the distributed optical fiber sensing cable approaches the boundary of the rock sample to be measured. The bending radius is r. The distributed optical fiber sensing cable is bent 90° to make it perpendicular to the previous distributed optical fiber sensing cable arrangement. Here, the tangent of the arc of the distributed optical fiber sensing cable is parallel to the boundary and is 2r away.

[0026] Repeatedly extend the distributed optical fiber sensing cable, then reversely bend the distributed optical fiber sensing cable 180° and keep it parallel until it is close to the boundary of the rock sample to be tested; repeat the above arrangement until the remaining three surfaces are arranged.

[0027] Furthermore, the method for performing three-dimensional stress field inversion on strain data in step 4 of the present invention includes:

[0028] Step 4.1, data preprocessing: for the collected strain data, the strain distribution ε of the rock sample under three-dimensional compression state monitored by the distributed optical fiber sensing cable is ijkt , calculate the three-dimensional stress distribution σ of the effective stress of the rock sample under the specified triaxial compression state ijkt :

[0029] σ ijkt =E•ε ijkt

[0030] In the above formula, i, j, k represent the physical components in the x, y, and z directions of the three-dimensional space at different times t; σ ijkt is the effective stress of rock, ε ijkt is the rock surface strain, E is the elastic modulus of the rock; each measurement point has a corresponding stress value and its position coordinates in three-dimensional space;

[0031] Step 4.2, divide the data grid: select a surface to be measured in the regular rectangular grid point area, the z coordinate is a fixed value, and the x and y coordinates vary; the horizontal coordinate x spacing is the fiber spacing 2r; the vertical coordinate y spacing is the fiber measurement point spacing, which is related to the distance resolution of the optical demodulator;

[0032] Step 4.3, determine the interpolation method: for the regular rectangular grid point area, use the bilinear interpolation method; for the data in the optical fiber bending area, divide it into discrete data and use the inverse distance weighted average method;

[0033] Step 4.4, interpolation calculation and output: use the selected interpolation method to perform interpolation calculation, output the interpolation stress values ​​of the calculated grid nodes, and form a new three-dimensional stress data field;

[0034] Step 4.5: Result verification and optimization: Verify the interpolation result to ensure that it is within a certain threshold range and remains consistent with the original data.

[0035] Furthermore, the bilinear interpolation method in step 4.3 of the present invention includes:

[0036] Assume that in a regular rectangular grid point area, the bilinear interpolation function is as follows:

[0037]

[0038] Among them, A, B, C, and D are undetermined coefficients, and their values ​​are determined using the undetermined coefficient method, that is, using the function values ​​of the function at the four vertices of the rectangle:

[0039] ,

[0040] ,

[0041] Substituting the bilinear interpolation function expression into four algebraic equations, the four unknown coefficients can be obtained by solving them.

[0042] Furthermore, the inverse distance weighted average method in step 4.3 of the present invention includes:

[0043] Calculate any measurement point (x k ,y k ) from the interpolation point (x, y):

[0044] , k=1,2,⋯,N;

[0045] Define the influence weight of the stress value of the kth measurement point on the stress value of the (x, y) point:

[0046] , k=1,2,⋯,N;

[0047] That is, the stress value at (x, y) is determined by the inverse distance weighted average of the known data according to the distance from the point, that is:

[0048]

[0049] in, represents the stress value at the kth point.

[0050] Further, the method of step 4.4 of the present invention comprises:

[0051] The selected interpolation method is used to perform interpolation calculations, and the calculated stress values ​​of each grid point are stored in a corresponding format. The stored data includes the coordinates of each grid point and the corresponding stress value; the interpolation stress value output of the grid point is called to form a new three-dimensional stress data field;

[0052] The method of step 4.5 includes:

[0053] Verify the interpolation result to determine whether it is within a certain threshold range; if not, optimize the interpolation method; the optimization adjustment method includes: optimizing the number of interpolation points; optimizing the interpolation order; if yes, output.

[0054] The beneficial effects produced by the present invention are:

[0055] 1. The present invention proposes a method for arranging a three-dimensional stress field of a cubic rock sample based on distributed optical fiber, which is connected to an optical fiber strain demodulator located outside a rock sample holder through an optical fiber penetration. This method can directly measure the three-dimensional spatial distribution of cubic rock strain under a triaxial stress state. Combined with the basic theory of rock elastic mechanics and the interpolation method, the three-dimensional spatial distribution of rock stress under a triaxial stress state can be further inverted, overcoming the difficulty that the current indoor optical fiber device cannot measure the three-dimensional stress field of a cubic rock sample under a triaxial stress state.

[0056] 2. The present invention proposes a method for arranging distributed optical fiber sensing cables in a regular rectangular grid. The arrangement method introduces a method for determining the minimum turning radius, which can effectively solve the problem that the optical fiber material is relatively brittle and cannot be excessively bent, making it impossible to perform winding arrangement monitoring on cubic rock samples. While ensuring low optical fiber light path loss, it completes all-round high-resolution monitoring of strain on the six surfaces of the rock.

[0057] 3. The present invention utilizes the characteristics of distributed optical fiber sensors that can sense the spatial distribution of rock surface strain on the transmission path and the information of its changes over time, and through the basic theory of rock elastic mechanics and interpolation method, effectively inverts the changes in the three-dimensional strain field of the rock surface.

[0058] 4. The present invention adopts different interpolation methods for different grid areas. For the regular rectangular grid point area, the bilinear interpolation method is adopted; for the data in the optical fiber bending area, it is divided into discrete data and the inverse distance weighted average method is adopted; and through the inversion method, the one-dimensional original strain data on the optical fiber can be converted into accurate, high-resolution, real-time three-dimensional stress field data, which can be used for subsequent stress analysis, visualization or further processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0060] Figure 1 It is a device for monitoring the three-dimensional stress field of rock based on distributed optical fiber according to an embodiment of the present invention;

[0061] Figure 2 This is an arrangement scheme for performing optical fiber monitoring of three-dimensional stress fields on six surfaces of a cubic rock sample (taking 80 cm×80 cm as an example) according to an embodiment of the present invention;

[0062] Figure 3 is a flow chart of rock three-dimensional stress field inversion according to an embodiment of the present invention;

[0063] Figure 4 It is a regular rectangular region mesh division according to an embodiment of the present invention. DETAILED DESCRIPTION

[0064] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0065] Example 1

[0066] like Figure 1As shown, the device for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to an embodiment of the present invention comprises: a rock sample pressure loading component, a distributed optical fiber strain monitoring component, optical fiber data acquisition and processing software and a computer 10; wherein:

[0067] A rock sample pressure loading assembly is used to apply pressure to a cube-shaped rock sample to be tested;

[0068] A distributed optical fiber strain monitoring component comprises a distributed optical fiber strain demodulator 8 and a distributed optical fiber sensing cable 7, wherein one end of the distributed optical fiber sensing cable 7 is connected to the rock sample pressure loading component, and the other end is connected to the optical fiber data acquisition and processing software and a computer 10 through the distributed optical fiber strain demodulator 8; wherein the middle part of the distributed optical fiber sensing cable 7 is arranged on six facades of the rock sample to be tested, and according to the minimum bending radius of the distributed optical fiber sensing cable 7, the distributed optical fiber sensing cable 7 is bent with one edge of the cube as the starting point and with the minimum bending radius, so that it is arranged in a regular rectangular grid on each facade;

[0069] In the preferred embodiment of the present invention, in order to monitor the three-dimensional stress field of rock under triaxial conditions, according to the standard of common single-mode optical fiber G.652D, the minimum bending radius is 30 mm, and the six vertical surfaces of the cubic rock sample are connected according to the following method: Figure 2 The arrangement shown is with a bending radius of 30 mm, which ensures low optical fiber path loss while completing all-round high-resolution monitoring of strain on the six surfaces of the rock.

[0070] The strain data of the surface of the rock sample to be tested is monitored by the distributed optical fiber sensing cable 7, and the strain data of the rock sample to be tested is processed by the distributed optical fiber strain demodulator 8 and then sent to the optical fiber data acquisition and processing software and the computer 10;

[0071] The optical fiber data acquisition and processing software and the computer 10 are used to calculate according to the strain data to obtain the three-dimensional spatial distribution of the effective stress of the rock sample to be tested; and to perform data grid processing on each facade, and to use the bilinear interpolation method for the regular rectangular grid area, and to use the inverse distance weighted average method for the discrete data of the surrounding optical fiber bending area, so as to realize interpolation inversion and three-dimensional stress field visualization.

[0072] In a preferred embodiment of the present invention, the distributed optical fiber sensor can sense the spatial distribution of rock surface strain on the transmission path and the information of its change over time, so as to preferentially monitor the changes of the three-dimensional stress field on the rock surface under triaxial stress conditions. Through the basic theory of rock elastic mechanics and data interpolation methods, the three-dimensional stress field distribution of the rock can be inverted, thus filling the gap that the three-dimensional stress field of the rock cannot be monitored in the laboratory.

[0073] Arrange the distributed optical fiber sensing cable on the surface of the cube rock sample with known elastic modulus E according to the design plan, and seal the rock sample with sealing silicone; put the arranged and sealed rock sample into the rock sample holder, connect the distributed optical fiber sensing cable to the distributed optical fiber strain demodulator through the optical fiber penetration; apply confining pressure and axial pressure to the rock sample through the confining pressure loading component and the axial pressure loading component respectively. Collect the surface strain data of the rock sample with the distributed optical fiber strain demodulator and transmit it to the optical fiber data acquisition and processing software and computer; use the collected three-dimensional spatial strain distribution of the rock sample.

[0074] Example 2

[0075] like Figure 1 As shown, in another specific embodiment of the present invention, a specific structure of a device for monitoring rock stress field is provided, including a distributed optical fiber strain monitoring component, a rock sample pressure loading component, an optical fiber data acquisition and processing software and a computer 10. The optical fiber data acquisition and processing software system is configured on a dedicated computer end, and a high-efficiency data transmission cable 9 is integrated on its side, and one end of the cable is directly connected to the distributed optical fiber strain monitoring module. The monitoring module is further seamlessly connected to the rock sample pressure loading system to form a complete experimental device. The core of the distributed optical fiber strain monitoring module is the optical fiber strain demodulator 8, which is precisely placed at the terminal of the data transmission cable and is responsible for parsing the signal from the optical fiber sensing network. The front end of the demodulator is tightly connected to the distributed optical fiber sensing cable 7, and the other end of the cable is directly connected to the rock sample pressure loading component to ensure the continuity and accuracy of the signal transmission.

[0076] The rock sample pressure loading assembly is composed of an axial pressure loading unit 4, a confining pressure loading unit and a precision rock sample holder 5, wherein the rock sample holder 5 is located at the terminal of the sensing optical cable, which not only firmly clamps the rock sample to be tested, but also serves as a key node for signal transmission. The bottom of the rock sample holder 5 is connected to the confining pressure loading unit through a dedicated pipeline to achieve precise control and regulation of the confining pressure. At the same time, the top of the rock sample holder 5 is designed with an optical fiber penetration 6, which is specially designed to provide a passage for the distributed optical fiber sensing cable 7 to protect the optical cable from mechanical damage. The core of the axial pressure loading assembly 4 is the rock sample to be tested, which is precisely placed in the internal space of the rock sample holder 5, and the distributed optical fiber sensing cable 7 is arranged on its surface according to this design scheme to achieve high-precision monitoring of the surface strain of the rock sample. The bottom of the rock sample to be tested is integrated with an axial pressure loading system, which is directly controlled by the axial pressure loading source below to ensure the accuracy and stability of the axial pressure application. The confining pressure loading assembly consists of a confining pressure power source 1 and a matching confining pressure loading metering pump 2. The confining pressure power source 1 is located outside the rock sample holder 5, and the pressure medium is delivered to the confining pressure loading unit through the precisely controlled confining pressure loading metering pump 3. At the same time, the confining pressure power source 1, the confining pressure loading metering pump 2 and the rock sample holder 5 are connected through a designed connecting pipeline 3 to ensure the smoothness and efficiency of the entire confining pressure loading process.

[0077] The distributed optical fiber strain demodulator 8 is used to collect and process the strain signal of the rock sample to be tested; the distributed optical fiber sensing cable 7 is used to measure the signal of deformation of the rock sample to be tested; the distributed optical fiber sensing cable 7 is arranged on the surface of the rock sample to be tested and is wrapped with epoxy resin to isolate the confining pressure medium. The distributed optical fiber sensing cable 7 can be deformed with the deformation of the rock sample to be tested, and is used to monitor the strain change on the surface of the rock sample to be tested, and is connected to the distributed optical fiber strain demodulator 8 to output the rock sample strain signal to the distributed optical fiber strain demodulator 8; the axial pressure loading component uses the axial pressure loading system 4 to be driven by the axial pressure loading source to load axial pressure on the rock sample to be tested; the confining pressure loading component is used to load confining pressure on the rock sample to be tested, and the component includes a confining pressure power source 1 and a confining pressure loading metering pump 2; it is connected to the rock sample holder 5 through the confining pressure injection hole; the confining pressure power source 1 is deionized water or oil; the confining pressure loading metering pump 2 is a high-precision metering pump. The rock sample holder 5 is used to fix the rock sample to be tested, and at the same time apply confining pressure and axial pressure to the rock sample to be tested, including sealing silica gel inside, the rock sample to be tested and the confining pressure injection hole on the outer wall; the distributed optical fiber sensing cable 7 is arranged on the rock sample to be tested, and is fixed in the rock sample holder 5 after being wrapped and sealed by epoxy resin; the distributed optical fiber strain monitoring component includes a distributed optical fiber strain demodulator 8 and a distributed optical fiber sensing cable 7, wherein the distributed optical fiber sensing cable 7 is connected to the distributed optical fiber strain demodulator 8 from the rock sample to be tested through the optical fiber penetration piece 6, and the collected data is connected to the optical fiber data acquisition and processing software and the computer 10 through the data transmission cable 9, and completes the three-dimensional spatial distribution output of the rock sample strain, the three-dimensional spatial distribution output of the rock sample effective stress, the interpolation inversion and the three-dimensional stress field visualization.

[0078] Example 3

[0079] The method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to an embodiment of the present invention comprises the following steps:

[0080] Step 1: On the six vertical faces of the cube-shaped rock sample to be tested, the distributed optical fiber sensing cable 7 is adhered to each face in a regular rectangular grid arrangement using an epoxy resin adhesive;

[0081] Step 2, place the sealed rock sample to be tested in the rock sample holder 5, connect one end of the distributed optical fiber sensing cable 7 to the rock sample pressure loading assembly, and connect the other end to the distributed optical fiber strain demodulator 8 through the optical fiber penetration 6 on the rock sample holder 5, and connect the distributed optical fiber strain demodulator 8 to the optical fiber data acquisition and processing software and the computer 10;

[0082] Step 3, start monitoring the rock sample to be tested, collect the surface strain data of the rock sample to be tested through the distributed optical fiber strain demodulator 8 and transmit it to the optical fiber data acquisition and processing software and the computer 10;

[0083] Step 4: Perform three-dimensional stress field inversion on the strain data, pre-process the strain data, and divide the data into grids for each facade. For the regular rectangular grid area, use the bilinear interpolation method, and for the discrete data of the surrounding optical fiber bending area, use the inverse distance weighted average method to achieve interpolation inversion and three-dimensional stress field visualization.

[0084] In a preferred embodiment of the present invention, a method for arranging distributed optical fiber sensing cables 7 in a regular rectangular grid is provided, and an optical fiber sensor is installed in the rock body on a cube rock sample with a side length of 80 cm. According to the standard of the commonly used single-mode optical fiber G.652D, the minimum bending radius is 30 mm, and the arrangement is carried out with a bending radius of 30 mm, while ensuring low optical fiber optical path loss, measuring as many points as possible, so that the subsequent interpolation calculation results are closer to the true value, thereby realizing all-round high-resolution monitoring of strain on the six surfaces of the rock.

[0085] First, select one side of the cube and stick the optical fiber to one surface of the rock sample 4 cm parallel to the side using an epoxy resin adhesive. Basic research on related adhesive materials shows that epoxy resin has a short curing time, which is convenient for fixing the optical fiber measurement line. At the same time, based on its physical properties, the coating of the epoxy resin rock surface will not affect the cracking of the sample surface, and its bonding force can enable the optical fiber to better cooperate with the deformation of the sample.

[0086] Then, at a bending angle slightly less than the fiber's limit, the fiber is bent 180° in the opposite direction and kept parallel, with a distance of 6 cm between the two parallel fibers.

[0087] Repeat the above steps until it approaches the rock boundary, with a bending radius of 3 cm, and bend the fiber optic cable 90° to make it perpendicular to the previous fiber optic arrangement. Here, the tangent of the fiber optic arc is parallel to the boundary and 6 cm away.

[0088] Repeat the process of extending the optical fiber, bending it 180° in the opposite direction and keeping it parallel until it is close to the rock boundary. Repeat all the steps on the remaining three surfaces.

[0089] Example 4

[0090] like Figure 3-4 As shown, an embodiment of the present invention provides a specific method for three-dimensional stress field inversion, including:

[0091] (1) Data preprocessing. Collect the three-dimensional strain data that need to be interpolated, and calculate the strain distribution ε of the rock under three-dimensional compression state monitored by the distributed optical fiber sensing cable. ijkt , calculate the three-dimensional stress distribution σ of the effective stress of the rock sample under the specified triaxial compression state ijkt :

[0092] σ ijkt =E•ε ijkt

[0093] In the above formula, i, j, k represent the physical components in the x, y, and z directions of the three-dimensional space at different times t, respectively; ijkt is the effective stress of rock, ε ijkt is the rock surface strain, and E is the elastic modulus of the rock. Each measurement point has a corresponding stress value and its position coordinates in three-dimensional space.

[0094] (2) Data grid division. Most of the strain data measured by this optical fiber arrangement scheme is interpolated by regular rectangular grid points. The data of the optical fiber bending part is discrete data. In the regular rectangular grid point area, for convenience, a surface to be measured is selected, the z coordinate is a constant, and the x and y coordinates vary. The specific rectangular grid is as follows Figure 4 The horizontal axis spacing is the optical fiber spacing of this design, which is 6 cm; the vertical axis spacing is the optical fiber measurement point spacing, which is related to the distance resolution of the optical demodulator.

[0095] (3) Determine the interpolation method. According to this optical fiber layout scheme, the data area is divided into two parts, such as Figure 1 As shown. For the regular rectangular grid point area, the bilinear interpolation method is used. Assume that in the area, the bilinear interpolation function is as follows:

[0096]

[0097] Among them, A, B, C, and D are undetermined coefficients, and their values ​​can be determined using the undetermined coefficient method, that is, using the function values ​​of the function at the four vertices (interpolation nodes) of the rectangle:

[0098] ,

[0099] ,

[0100] Substituting the above function expressions into four algebraic equations, we can get four unknown coefficients by solving them.

[0101] The data of the fiber bending area is divided into discrete data. To solve the interpolation problem of scattered data points, the inverse distance weighted average method, also known as the Shepard method, is used. The basic idea is: the function value at any non-observation point (x, y) is estimated by the observed value of the known data point, and the degree of influence varies according to the distance. The farther the distance, the lower the influence, and vice versa. Therefore, the influence of the function value of each observation point on the function value at (x, y) can be measured by the inverse of the square of the distance between the two points, that is, the inverse distance. The influence of all data points on the function value at (x, y) can be estimated in the form of weighted average.

[0102] First, calculate any measurement point (x k ,y k ) from the interpolation point (x, y):

[0103] , k=1,2,⋯,N;

[0104] Then define the influence weight of the stress value of the kth measurement point on the stress value of the (x, y) point:

[0105] , k=1,2,⋯,N;

[0106] That is, the stress value at (x, y) can be determined by taking the inverse distance weighted average of the known data according to the distance from the point, that is,

[0107]

[0108] in, represents the stress value at the kth point.

[0109] (4) Interpolation calculation and output. The selected interpolation method is used to perform interpolation calculations, and the interpolated stress values ​​of the calculated grid nodes are output to form a new three-dimensional stress data field. These data can be used for subsequent stress analysis, visualization or further processing.

[0110] In a preferred embodiment of the present invention, the interpolation calculation is performed using the selected interpolation method, and the calculated stress value of each grid point is stored in an appropriate format, such as a CSV file, a NumPy array, or a specialized scientific data format (such as HDF5). Ensure that the output data contains the coordinates of each grid point and the corresponding stress value. The interpolated stress values ​​of the calculated grid nodes are output to form a new three-dimensional stress data field. The new three-dimensional stress data field can be subsequently visualized using visualization tools (such as Matplotlib, ParaView, etc.) for easy analysis and understanding;

[0111] (5) Result verification and optimization: Verify the interpolation results, bring the interpolation points into the entire plane data set, check whether there are stress anomalies, ensure that they are within a reasonable range, and maintain a certain consistency with the original data. If necessary, the interpolation method can be optimized and adjusted, such as using other interpolation methods, combining local interpolation with global interpolation, and gradually adjusting the interpolation results through iterative optimization algorithms, etc., to improve the accuracy and stability of the interpolation.

[0112] In a preferred embodiment of the present invention, the interpolation result is verified to ensure that it is within a certain threshold range and is consistent with the original data. If necessary, the interpolation method can be optimized and adjusted to improve the accuracy and stability of the interpolation. Specifically, the number of interpolation points can be optimized: the number of interpolation points has a significant impact on the accuracy and computational efficiency of the interpolation result. Too many interpolation points will increase the amount of calculation, while too few interpolation points may lead to inaccurate interpolation results. Therefore, it is necessary to select a suitable number of interpolation points according to the characteristics of the data and the interpolation requirements. Optimization of interpolation order: For methods such as polynomial interpolation, the choice of interpolation order is crucial. Too high an order may lead to overfitting problems, that is, the interpolation function performs well near known data points, but may have large deviations in other areas. Therefore, it is necessary to find the optimal interpolation order through experiments or cross-validation methods. While improving accuracy, the amount of calculation will also increase.

[0113] It should be understood that the size of the serial numbers of the steps in the above embodiments does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0114] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should fall within the scope of protection of the appended claims of the present invention.

Claims

1. A device for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber, the device comprising: Rock sample pressure loading component, distributed optical fiber strain monitoring component, optical fiber data acquisition and processing software and computer; characterized in that: A rock sample pressure loading assembly is used to apply pressure to the cube-shaped rock sample to be tested; A distributed optical fiber strain monitoring component includes a distributed optical fiber strain demodulator and a distributed optical fiber sensing cable. One end of the distributed optical fiber sensing cable is connected to the rock sample pressure loading component, and the other end is connected to the optical fiber data acquisition and processing software and a computer through the distributed optical fiber strain demodulator. The middle part of the distributed optical fiber sensing cable is arranged on six facades of the rock sample to be tested. According to the minimum bending radius of the distributed optical fiber sensing cable, one edge of the cube is used as the starting point, and the distributed optical fiber sensing cable is bent with the minimum bending radius so that it is arranged in a regular rectangular grid on each facade. The strain data of the surface of the rock sample to be tested is monitored by the distributed optical fiber sensing cable, and the strain data of the rock sample to be tested is processed by the distributed optical fiber strain demodulator and then sent to the optical fiber data acquisition and processing software and a computer. Fiber optic data acquisition and processing software and computers are used to calculate based on strain data to obtain the three-dimensional spatial distribution of effective stress of the rock sample to be tested; and to divide the data into grids for each facade. For regular rectangular grid areas, bilinear interpolation is used, and for discrete data of the surrounding fiber optic bending areas, inverse distance weighted average method is used to achieve interpolation inversion and three-dimensional stress field visualization.

2. The device for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to claim 1, characterized in that: The rock sample pressure loading assembly includes: an axial pressure loading assembly, a confining pressure loading assembly, and a rock sample holder; wherein: The rock sample to be tested, after the distributed optical fiber sensing cable is arranged and sealed, is placed in a rock sample holder, and the distributed optical fiber sensing cable is connected to the distributed optical fiber strain demodulator through the optical fiber through-piece on the rock sample holder; the confining pressure and axial pressure are applied to the rock sample to be tested through the confining pressure loading assembly and the axial pressure loading assembly respectively.

3. The device for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to claim 1, characterized in that: The distributed optical fiber sensor cable is adhered to the rock sample to be tested by means of epoxy resin adhesive.

4. The device for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to claim 1, characterized in that: The minimum bending radius is determined according to the standard of the single-mode optical fiber used in the distributed optical fiber sensing cable; the grid spacing of the regular rectangular grid is twice the minimum bending radius.

5. A method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber, using the device according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1: On the six vertical faces of the cube-shaped rock sample to be tested, the distributed optical fiber sensing cables are adhered to each face in a regular rectangular grid arrangement using an epoxy resin adhesive; Step 2: Place the sealed rock sample to be tested in a rock sample holder, connect one end of the distributed optical fiber sensing cable to the rock sample pressure loading assembly, and connect the other end to the distributed optical fiber strain demodulator through the optical fiber penetration piece on the rock sample holder, and connect the distributed optical fiber strain demodulator to the optical fiber data acquisition and processing software and a computer; Step 3: Start monitoring the rock sample to be tested, collect the surface strain data of the rock sample to be tested through the distributed optical fiber strain demodulator and transmit it to the optical fiber data acquisition and processing software and the computer; Step 4: Perform three-dimensional stress field inversion on the strain data, pre-process the strain data, and divide the data into grids for each facade. For the regular rectangular grid area, use the bilinear interpolation method, and for the discrete data of the surrounding optical fiber bending area, use the inverse distance weighted average method to achieve interpolation inversion and three-dimensional stress field visualization.

6. The method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to claim 5, characterized in that: The arrangement of the regular rectangular grid in step 1 includes: Select one side of the cube, and paste the distributed optical fiber sensor cable parallel to the side at a certain distance using epoxy resin adhesive; The minimum bending radius r is determined according to the standard of the single-mode optical fiber used in the distributed optical fiber sensing cable. When the bending radius is less than the limit bending radius of the single-mode optical fiber, the distributed optical fiber sensing cable is reversely bent 180° and kept parallel. The distance between the two parallel distributed optical fiber sensing cables is 2r. Repeat until the distributed optical fiber sensing cable approaches the boundary of the rock sample to be measured. The bending radius is r. The distributed optical fiber sensing cable is bent 90° to make it perpendicular to the previous distributed optical fiber sensing cable arrangement. Here, the tangent of the arc of the distributed optical fiber sensing cable is parallel to the boundary and is 2r away. Repeatedly extend the distributed optical fiber sensing cable, then reversely bend the distributed optical fiber sensing cable 180° and keep it parallel until it is close to the boundary of the rock sample to be tested; repeat the above arrangement until the remaining three surfaces are arranged.

7. The method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to claim 6, characterized in that: The method for performing three-dimensional stress field inversion on strain data in step 4 includes: Step 4.1, data preprocessing: for the collected strain data, the strain distribution ε of the rock sample under three-dimensional compression state monitored by the distributed optical fiber sensing cable is ijkt , calculate the three-dimensional stress distribution σ of the effective stress of the rock sample under the specified triaxial compression state ijkt : s ijkt =E•e ijkt In the above formula, i, j, k represent the physical components in the x, y, and z directions of the three-dimensional space at different times t; σ ijkt is the effective stress of rock, ε ijkt is the rock surface strain, E is the elastic modulus of the rock; each measurement point has a corresponding stress value and its position coordinates in three-dimensional space; Step 4.2, divide the data grid: select a surface to be measured in the regular rectangular grid point area, the z coordinate is a fixed value, and the x and y coordinates vary; the horizontal coordinate x spacing is the fiber spacing 2r; the vertical coordinate y spacing is the fiber measurement point spacing, which is related to the distance resolution of the optical demodulator; Step 4.3, determine the interpolation method: for the regular rectangular grid point area, use the bilinear interpolation method; for the data in the optical fiber bending area, divide it into discrete data and use the inverse distance weighted average method; Step 4.4, interpolation calculation and output: use the selected interpolation method to perform interpolation calculation, output the interpolation stress values ​​of the calculated grid nodes, and form a new three-dimensional stress data field; Step 4.5: Result verification and optimization: Verify the interpolation result to ensure that it is within a certain threshold range and remains consistent with the original data.

8. The method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to claim 7, characterized in that: The bilinear interpolation method in step 4.3 includes: Assume that in a regular rectangular grid point area, the bilinear interpolation function is as follows: Among them, A, B, C, and D are undetermined coefficients, and their values ​​are determined using the undetermined coefficient method, that is, using the function values ​​of the function at the four vertices of the rectangle: ， ， Substituting the bilinear interpolation function expression into four algebraic equations, the four unknown coefficients can be obtained by solving them.

9. The method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to claim 7, characterized in that: The inverse distance weighted average method in step 4.3 includes: Calculate any measurement point (x k ,y k ) from the interpolation point (x, y): , k=1,2,⋯,N; Define the influence weight of the stress value of the kth measurement point on the stress value of the (x, y) point: , k=1,2,⋯,N; That is, the stress value at (x, y) is determined by the inverse distance weighted average of the known data according to the distance from the point, that is: in, represents the stress value at the kth point.

10. The method for monitoring the three-dimensional stress field of a cubic rock sample based on distributed optical fiber according to claim 7, characterized in that: The method of step 4.4 includes: The selected interpolation method is used to perform interpolation calculations, and the calculated stress values ​​of each grid point are stored in a corresponding format. The stored data includes the coordinates of each grid point and the corresponding stress value; the interpolation stress value output of the grid point is called to form a new three-dimensional stress data field; The method of step 4.5 includes: Verify the interpolation result to determine whether it is within a certain threshold range; if not, optimize the interpolation method; the optimization adjustment method includes: optimizing the number of interpolation points; optimizing the interpolation order; if yes, output.

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