A monitoring method for dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring

Abstract

The present invention relates to the technical field of distributed optical fiber monitoring, and discloses a method for monitoring the dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring, comprising the following steps: Step 1, preparation of rock samples: After injecting the mixed slurry into the mold, let it stand, demold, and maintain it, and drill cylindrical rock samples at four angles of 0°, 30°, 45°, and 90° respectively; Step 2, preparation and pretreatment of optical fibers; Step 3, combined layout of optical fibers and acoustic emission sensors: Each cylindrical rock sample is attached with four optical fibers, and the attachment method is: The optical fibers are attached horizontally along the rock sample at 0°, 90°, 180°, and 270°, and acoustic emission probes are installed at both the upper and lower ends of the rock sample; Step 4, stress loading and detection: Perform compression loading and simultaneous analysis of optical fiber data and acoustic emission data. By reasonably selecting the drilling angle and monitoring the crack propagation direction, the present invention can optimize the design and construction plan of geothermal wells and ensure the long-term stability and safety of geothermal wells.

Description

Technical Field

[0001] The invention relates to the technical field of distributed optical fiber monitoring, and in particular to a method for monitoring dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring. Background Art

[0002] As a basic component of geological engineering and building structures, the mechanical behavior of rock materials is crucial to understanding and predicting the stability of engineering structures. Although rocks have high intrinsic strength, they are still prone to cracking and fracture under mechanical forces such as uniaxial compression. These damage processes directly affect the safety and durability of engineering structures. Therefore, it is necessary to accurately monitor the strain and crack extension of rocks in uniaxial compression experiments. Although traditional rock mechanics experimental methods such as direct observation and acoustic emission monitoring can provide certain crack information, they are limited in capturing the formation and extension process of fine cracks inside rocks.

[0003] Optical Frequency Domain Reflectometry (OFDR) is an advanced monitoring method based on the detection of the optical fiber Rayleigh back scattering spectra (RBS) with the advantages of high measurement accuracy, high sensing spatial resolution, and long measurement distance. The high spatial resolution and sensitivity of OFDR technology enable it to capture tiny strain changes, which is of great significance for early crack detection and rock damage warning. In the field of distributed optical fiber sensing, although the temperature, vibration, stress and strain sensing technology based on OTDR technology has made significant progress in test distance, spatial resolution, sensitivity, accuracy and dynamic response, these indicators still need to be improved to meet the needs of a wider range of applications, prompting researchers to explore new methods and mechanisms. As a new technology, OFDR is superior to traditional OTDR in spatial resolution and accuracy, but its test distance is short, especially in sensing applications, it can only cover tens of meters, which is difficult to meet the needs of long-distance sensing. Summary of the invention

[0004] The present invention aims to provide a method for monitoring the dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring, so as to solve the problem in the prior art that the OFDR test distance is difficult to meet the long-distance sensing requirements.

[0005] To achieve the above object, the present invention adopts the following technical solution: a method for monitoring the dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring, comprising the following steps:

[0006] Step 1. Preparation of rock samples: Mix cement with sandstones of different particle sizes to obtain a mixed slurry. After injecting the mixed slurry into a mold, let it stand, demold, and cure, and then drill cylindrical rock samples at four angles of 0°, 30°, 45°, and 90° respectively;

[0007] Step 2. Preparation and pretreatment of optical fibers: Use single-mode optical fibers with a diameter of 125 microns and a coating diameter of 250 microns, and perform fusion splicing and protection treatment on the optical fibers;

[0008] Step 3. Joint layout of optical fibers and acoustic emission sensors: Four optical fibers are attached to each cylindrical rock sample, and the attachment method is: attach the optical fibers horizontally along the cylindrical body of the rock sample at 0°, 90°, 180°, and 270°; install acoustic emission probes at both the upper and lower ends of the rock sample respectively;

[0009] Step 4. Stress loading and detection: First, perform preloading to ensure that the rock sample and the experimental equipment are in a stable state, and then perform compression loading and simultaneous analysis of optical fiber data and acoustic emission data.

[0010] Preferably, as an improvement, in Step 1, the mass ratio of each raw material in the mixed slurry is cement: 260-mesh sandstone: 150-mesh sandstone: 80-mesh sandstone: 60-mesh sandstone: water = 40: 6: 2: 20: 12: 15.

[0011] Preferably, as an improvement, in Step 2, the fusion splicing operation is: Use an optical fiber fusion splicer to fuse one end of four optical fibers. Before fusion splicing, remove the coating on the optical fiber, expose a section of the optical fiber, and perform a vertical cut on the optical fiber cutting machine.

[0012] Preferably, as an improvement, in Step 2, the pretreatment operation is: After optical fiber fusion splicing, put a transparent sleeve on the fusion splicing point and perform heat sealing treatment.

[0013] Preferably, as an improvement, in Step 2, after fusion splicing, perform a fusion splicing test. Insert a laser pen into one end of the signal receiver and emit. If red light emits from the other end, it proves that the fusion splicing is successful, otherwise the fusion splicing fails.

[0014] Preferably, as an improvement, in Step 3, the installation directions of the acoustic emission probes are 0° to 180° and 90° to 270° respectively.

[0015] Preferably, as an improvement, in Step 4, before detection, test the sensitivity of the optical fiber and the acoustic emission probe. First, press the single-mode optical fiber and turn on the single measurement mode of the optical fiber demodulator to check whether the optical fiber can receive signals normally; then tap different positions of the rock sample to determine whether the probe position can accurately monitor the acoustic emission signal.

[0016] Preferably, as an improvement, in step three, it further includes dividing the rock sample into six equal parts in the 0° direction and additionally attaching two optical fibers to achieve strain monitoring of the six equal parts.

[0017] The principle and advantages of this solution are as follows: In practical applications, in this technical solution, aiming at the problem that the OFDR test distance in the prior art is difficult to meet the long-distance sensing requirements, the distributed optical fiber monitoring technology is applied to the uniaxial compression test of rock samples, and combined with the preparation of rock samples, multi-angle drilling and sampling, and multi-angle laying of optical fibers on the rock samples, the strain distribution and failure mode of the rock samples in the uniaxial compression test are systematically analyzed. During the technology research and development period, considering the bedding of the artificial rock samples cast in layers, multi-angle sampling is carried out on the artificial rock samples; in the optical fiber laying stage, a multi-angle laying method is adopted to improve the monitoring accuracy; in the result evaluation stage, the combined analysis of optical fiber data inversion, acoustic emission and failure mode is carried out to ensure the accuracy of result analysis. The experimental results show that the application of the distributed optical fiber monitoring technology in the uniaxial compression test of rock samples is feasible and effective. The optical fiber sensor can collect the strain changes of the rock samples during the compression process in real time and accurately, and through data analysis, clearly display the strain distribution law of the rock samples under the compression loading conditions. This technology provides reliable data support for in-depth understanding of the stress situation and failure mechanism of rock samples.

[0018] In summary, by using the distributed optical fiber monitoring technology, this study detailedly analyzed the strain distribution and failure mode of rock samples at different angles in the uniaxial compression test, thus verifying the application feasibility of the optical fiber monitoring technology in the uniaxial mechanics of rocks. The research results show that the changes in stress and strain during the uniaxial failure process of rocks are significantly related to the drilling angle. It has important reference value for understanding the mechanical behavior and failure mechanism of rocks under failure conditions during shale gas development. This technical solution can optimize the construction plan for shale gas development and ensure the long-term stability and safety of shale development by reasonably selecting the drilling angle and monitoring the crack propagation direction. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of rock sample drilling in an embodiment of the present invention.

[0020] Figure 2 It is a schematic diagram of cracks in rock samples at 0°, 30°, 45°, and 90° in an embodiment of the present invention.

[0021] Figure 3 It is a three-dimensional acoustic emission positioning map 0° (left) 30° (right) in an embodiment of the present invention.

[0022] Figure 4 It is a three-dimensional acoustic emission positioning map 45° (left) 90° (right) in an embodiment of the present invention.

[0023] Figure 5It is the Y-1-0° optical fiber strain waterfall diagram in the embodiment of the present invention.

[0024] Figure 6 It is the Y-1-0° optical fiber strain diagram in the embodiment of the present invention.

[0025] Figure 7 It is the strain diagram of the 45° rock sample at the 1st second in the embodiment of the present invention.

[0026] Figure 8 It is the strain diagram of the 45° rock sample at the 23rd second in the embodiment of the present invention.

[0027] Figure 9 It is the three-dimensional acoustic emission positioning diagram of the 45° rock sample (left), the 45° optical fiber cylindrical strain diagram at the 55th second (middle), and the initial crack diagram (right) in the embodiment of the present invention.

[0028] Figure 10 It is the 45° optical fiber cylindrical strain diagram of the 45° rock sample at the 65th second (left) and the 76th second (right) in the embodiment of the present invention.

[0029] Figure 11 It is the crack diagram of the 45° rock sample (left), the 45° optical fiber cylindrical strain diagram at the 79th second (middle), and the 81st second (right) in the embodiment of the present invention.

[0030] Figure 12 It is the 45° optical fiber cylindrical strain diagram of the 45° rock sample at the 84th second (left) and the 87th second (right) in the embodiment of the present invention.

[0031] Figure 13 It is the final development diagram of the crack diagram of the 45° rock sample (left), the 45° optical fiber cylindrical strain diagram at the 98th second (middle), and the 101st second (right) in the embodiment of the present invention. Specific embodiments

[0032] The following is a further detailed description through specific embodiments, but the embodiments of the present invention are not limited thereto. If not specifically specified, the technical means used in the following embodiments are conventional means well-known to those skilled in the art; the experimental methods used are all conventional methods; the materials, reagents, etc. used can all be obtained through commercial channels.

[0033] Example 1

[0034] A method for monitoring the dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring, comprising the following steps:

[0035] Step 1. Preparation of rock samples: Prepare the mold, and evenly apply No. 50 mechanical general lubricating oil on the inner wall of the mold to prevent affecting the surface flatness of the formed specimens. When applying the lubricating oil, first disassemble the mold to ensure uniform application. When reinstalling the mold, the gaps should be aligned with each other;

[0036] Place in a cement mortar mixer according to the mass ratio of cement: 260-mesh sandstone: 150-mesh sandstone: 80-mesh sandstone: 60-mesh sandstone: water being 40:6:2:20:12:15, and stir slowly. During the stirring process, first put in all the raw materials, then slowly add water using a measuring cylinder, and stir for 2 - 3 minutes;

[0037] After the stirring is completed, pour the cement slurry into the mold, place it on a concrete vibrator and vibrate for 2 - 3 minutes until the water surface becomes flat; after standing for 24 hours, demold and spray water for maintenance;

[0038] Then drill cylindrical rock samples of 50×100 mm at four angles of 0°, 30°, 45°, and 90° respectively, as Figure 1 shown.

[0039] Step 2: Joint layout of optical fiber and acoustic emission sensor: Use 22 single-mode fibers (SMF) with a length of 1 m, a diameter of 125 microns, and a coating diameter of 250 microns. Its diameter has a certain influence on the mechanical strength and installation method of the optical fiber. Its spatial resolution theoretically reaches 0.1 mm or even higher. Use AB-type coupling agent to horizontally attach four optical fibers along the cylindrical column of the rock sample at 0°, 90°, 180°, and 270°. The fifth rock sample will set the variable at 0° and attach six optical fibers according to six equal parts. Install acoustic emission probes at both ends of the rock sample respectively, and the installation angle (direction) is 0° - 180°, 90° - 270°.

[0040] Pretreatment operation: Before attachment, perform fusion splicing and protection treatment on the optical fiber. First, use an optical fiber fusion splicer to fuse one end of the four optical fibers. Before fusion splicing, remove the coating on the optical fiber, expose about 3 cm of the optical fiber, and perform vertical cutting on the optical fiber cutting machine. Then, perform the same operation on the optical fiber connected to the signal receiver. Next, place the two optical fibers in the optical fiber fusion splicer, make them in a horizontal docking position, and fuse them together. Finally, put a transparent sleeve on the fusion splicing point and perform heat sealing treatment to protect the optical fiber connection from external interference.

[0041] Insert a laser pen and emit at one end of the signal receiver. If red light emits from the other end, it proves that the fusion splicing is successful; otherwise, the fusion splicing fails.

[0042] Step 3: Uniaxial compression test: Use uniaxial compression equipment for testing. This equipment includes an axial compression loading system, a confining pressure loading system, a seepage system, an acoustic emission system, a temperature system, etc. (prior art). The specific test steps are as follows:

[0043] S1. Cut the rock samples at four angles of 0°, 30°, 45°, and 90°, and number them as Y-1-0°, Y-1-30°, Y-1-45°, and Y-1-90° respectively. Additionally, the fifth rock sample will be divided into six equal parts in the 0° direction and numbered as Y-2 to explore the influence of the loading direction on the mechanical properties of the rock;

[0044] S2. Fiber optic layout: Four optical fibers are attached to each rock sample, corresponding to the directions of 0°, 90°, 180°, and 270° respectively, to achieve strain monitoring at the corresponding positions. For the rock sample numbered Y-2, two additional optical fibers will be attached to achieve strain monitoring of the six equal parts; Acoustic emission probes are installed at the upper and lower ends of the rock sample respectively, and the installation angles (directions) are 0° to 180° and 90° to 270°;

[0045] S3. Calibration and marking: Calibrate the acoustic emission probes and optical fibers, and mark their positions for subsequent data analysis. Before the experiment starts, test the sensitivity of the optical fibers and acoustic emission probes. First, gently press the single-mode optical fiber and turn on the single measurement mode of the optical fiber demodulator to check whether the optical fiber can receive signals normally. Secondly, tap different positions of the rock sample to determine whether the position of the acoustic emission probe can accurately detect the acoustic emission signal;

[0046] S4. Preloading stage: Perform preloading before the experiment starts to ensure that the rock sample and the experimental equipment are in a stable state. Record the strain and acoustic emission signals during the preloading stage;

[0047] S5. Compression loading: Compressively load the rock sample through a uniaxial loading device, and record the strain and acoustic emission signals during the loading process. The loading rate should be adjusted according to the experimental requirements to ensure the accuracy and reliability of data acquisition, and keep applying pressure until the rock sample breaks;

[0048] S6. Fiber optic detection: Insert a laser pen at one end of the signal receiver and emit. If red light emits from the other end, it can indicate that the entire optical fiber participates in the detection and is not damaged by stress;

[0049] S7. Data recording: Real-time record the strain and acoustic emission signal data during the compression loading process, and store and back up according to the specified frequency for subsequent analysis and research.

[0050] Results and Analysis

[0051] 1. Force analysis of rock samples and stress law

[0052] Adopting this technical solution, most optical fibers can detect data, and relatively complete rock sample fragments are obtained. The corresponding force analysis and stress law can be obtained from the recombination of the broken fragments of the rock sample ( Figure 2)It can be seen that after uniaxial compression, the rock samples mainly fail in axial splitting. The crack direction is basically parallel to the stress loading direction and the generated cracks are relatively developed. In addition, the presence of weak planes will induce the crack direction.

[0053] (1) 0-degree sample: ( Figure 2 Left 1)

[0054] ① Longitudinal through-cracks were observed at the center of the sample, and the cracks penetrated the entire rock sample.

[0055] ② The crack initiation position was close to the top of the sample and gradually extended downward until the bottom.

[0056] ③ This indicates that obvious axial tensile failure occurred during the compression of this sample.

[0057] (2) 30-degree sample: ( Figure 2 Left 2)

[0058] ① Obvious longitudinal cracks appeared at the top of the sample, starting from the top and extending downward.

[0059] ② The bottom of the sample remained intact, but the top cracked.

[0060] ③ This indicates that the top of the sample was subjected to a large compressive stress, resulting in the failure of the top material and the generation of cracks.

[0061] (3) 45-degree sample: ( Figure 2 Left 3)

[0062] ① Obvious oblique cracks were observed on the surface of the sample, showing a certain angle.

[0063] ② The overall structure of the sample remained, but the cracks penetrated the surface of the sample.

[0064] ③ This indicates that shear failure may have occurred during the compression of this sample, resulting in the formation of oblique cracks.

[0065] (4) 90-degree sample: ( Figure 2 Right 1)

[0066] ① The failure of this sample was relatively severe, and the cracks started from the top of the sample, showing multiple intersecting cracks.

[0067] ② The sample was broken into multiple small pieces, indicating that multiple fractures occurred during the compression of this sample.

[0068] ③ It may be due to the presence of initial microcracks inside the sample, making the stress concentrate at multiple points, resulting in the generation of multiple cracks.

[0069] 2. Acoustic emission data analysis

[0070] In this study, the crack initiation and development of rocks under uniaxial compression were monitored and analyzed by acoustic emission technology. Acoustic emission technology is a non-destructive testing method that can monitor the dynamic changes of internal defects and cracks in materials in real time. In the experiment, four channels, namely 1, 2, 5, and 6, were set up to collect acoustic emission signals at different angles. By recording parameters such as arrival time and amplitude, the distribution law of acoustic emission events during the compression process of the rock was analyzed. In the experiment, we obtained data through acoustic emission technology and performed three-dimensional positioning by calculating the arrival time of sound waves such as Figure 3 and Figure 4 , so as to obtain information about the internal cracks of the rock sample. Subsequently, we compared these three-dimensional positioning results with the crack information obtained by distributed fiber optic monitoring to verify the accuracy and effectiveness of distributed fiber optic monitoring.

[0071] From Figure 3 (a), the crack distribution of the 0-degree rock sample can be seen. The red dots represent the crack occurrence positions, and the blue dots represent the sensor positions. It can be seen that the cracks are mainly concentrated in the upper and middle parts of the rock sample, and these areas are stress concentration points. The cracks propagate along the axial and lateral directions, showing complex bending and branching paths, indicating the stress distribution and transmission inside the rock. The branching and complex paths of these cracks indicate that there are multiple stress concentration points inside the rock sample, resulting in the cracks expanding in different directions.

[0072] Figure 3 (b) shows the crack distribution of the 30-degree rock sample. Compared with the 0-degree rock sample, the cracks of the 30-degree rock sample are also mainly concentrated in the upper and middle regions, but the crack distribution is more dispersed. The crack orientation and morphology show the stress concentration and transmission characteristics of the rock sample at a 30-degree angle. The cracks propagate along the axial and lateral directions, forming complex paths, further illustrating the stress distribution inside the rock.

[0073] Figure 4 (a) shows the crack distribution of the 45-degree rock sample. It can be seen that the cracks are mainly concentrated in the upper and middle parts of the rock sample, and these areas are stress concentration points. The cracks propagate along the axial and lateral directions, showing complex bending and branching paths, indicating the stress distribution and transmission inside the rock. The crack distribution at a 45-degree angle is more dispersed, indicating the stress transmission characteristics in different directions.

[0074] Figure 4 (b) shows the crack distribution of the 90-degree rock sample. Compared with the 45-degree rock sample, the cracks of the 90-degree rock sample are more concentrated in the lower and middle regions of the rock sample. The crack orientation and morphology show the stress concentration and transmission characteristics of the rock sample at a 90-degree angle. The cracks propagate along the axial and lateral directions, forming complex paths.

[0075] Through three-dimensional positioning, it can be found that 0°, 30°, and 45° are all monitoring the top of the main crack development, with only slight crack development at the bottom, and 90° has obvious crack development both above and below.

[0076] 3. Combined with fiber optic monitoring data analysis

[0077] The distributed optical fiber monitoring system is used to collect strain data in all directions on the surface of the rock sample in real time. The collected data are analyzed and visualized through the data processing software MATLAB to obtain the strain distribution law of the rock under different loading directions.

[0078] 3.1Y-1-0° rock sample analysis

[0079] In the first fiber, it is possible to Figure 5 It can be clearly seen in the figure that the optical fiber can monitor the gradually increasing strain over time. The original data shows that the strain gradient of each increase is 25, which is consistent with the uniform pressure applied by the uniaxial compressor. This preliminarily verifies the feasibility of distributed optical fiber monitoring uniaxial strain. Since this optical fiber is not laid on the cracks of the broken rock sample, it only monitors the strain caused by the uniform pressure of the rock sample by the uniaxial compressor. It can be observed from the figure that when the axial strain reaches 4200 microstrain, a jump point appears in the curve, and when the sample is damaged, the strain drops sharply to nearly 0; the second optical fiber monitors tensile stress instead of compressive stress in the initial stage, which may be due to position errors in the pre-stressing stage, or cracks from the inside of the rock sample are monitored. Subsequently, the optical fiber begins to monitor the gradually increasing strain until the rock is damaged when the axial strain reaches 2600 microstrain. From this point on, the monitored strain gradually decreases and finally approaches 0; the third optical fiber fails to monitor effective data, which may be due to the unevenness of the upper and lower ends of the rock sample or the failure to firmly stick the optical fiber when it is laid; in the fourth optical fiber, uniformly increasing strain can be detected in the first 20 seconds. After 20 seconds, an abnormal strain suddenly increased at 30-50 mm. This is because under the action of axial compression, cracks in the rock sample began to develop until the rock sample was severely damaged. At the moment of rupture, local stress was released, which reduced the axial pressure.

[0080] In summary, the fourth optical fiber first detected the crack at 30-50 mm at 20 seconds. Figure 6 As shown in Figure 1, this indicates that the weak plane of the rock sample is located at the fourth optical fiber. The second optical fiber is located opposite the fourth optical fiber and is affected by the stress in the longitudinal direction of the crack, and the tensile stress is monitored.

[0081] Therefore, the information of this crack was also detected at 20 seconds. However, since the installation location did not rupture, the uniform pressure of the single-axis compressor was continuously monitored until the rock sample was damaged. The first optical fiber was installed on the left and right sides of the second and fourth optical fibers, and the detected strain increase was very uniform, with minimal influence from the crack. Therefore, it was concluded that the crack started to generate at the position of 30 - 50 mm of the fourth optical fiber at 20 seconds, propagated horizontally towards the first and second optical fibers, and spread vertically towards the upper and lower ends.

[0082] According to the above process, the Y-1-30° rock sample, Y-1-45° rock sample, Y-1-90° rock sample, and Y-2-0° rock sample were analyzed. The results showed that the general rule was that the strain detected by the optical fiber showed a stable and gentle trend of uniform increase from 0 to 50 s at the optical fiber position. As the pressure of the compressor increased, the strain detected by the optical fiber also increased significantly, possibly because multiple cracks in the rock sample were detected by the optical fiber. In the later stage, the detected stress showed a decreasing trend, which was more scattered and less concentrated than at the beginning and decayed faster. This was because after the rock sample ruptured in the later stage, the rock blocks fell off, so the stress change inside the rock sample could not be detected. It can be seen that at the middle position, from 50 - 80 s, the strain changed from downward pressure to upward pressure. Thus, it can be concluded that when fracturing at this position, no upward pressure was received.

[0083] (1) Initial expansion stage (0 - 50 seconds)

[0084] Within the first 50 seconds, all optical fibers showed a trend of uniform strain increase, indicating that the rock sample was uniformly stressed during this stage and the cracks had not spread to each monitoring position. Starting from around 50 seconds, the strain curves of the optical fibers in all directions showed a significant decrease, indicating that the cracks began to spread and affected the optical fiber monitoring points in all four directions.

[0085] (2) Further expansion stage (50 - 80 seconds)

[0086] After 50 seconds, the strain curves of each optical fiber showed a further decrease, indicating that the cracks continued to extend outward after the initial expansion. At 80 seconds, the strain of each optical fiber decreased significantly again, indicating that the cracks further expanded and reached a new monitoring range.

[0087] 1. Visualization processing of distributed optical fiber monitoring data

[0088] In this study, four groups of rock samples with different sampling angles were used, and uniaxial compression tests were carried out on them respectively. During the experiment, the distributed fiber optic sensing technology was used to monitor the strain distribution of the rock samples in real time, and acoustic emission events were recorded to assist in analyzing the crack propagation process. To visualize and analyze these data, we wrote MATLAB code to generate dynamic GIF images, showing the strain distribution on the surface of the rock samples at each moment. The following are the key steps and explanations of this MATLAB model:

[0089] (1) Data loading and processing: First, load the strain data of the four groups of rock samples with different sampling angles to ensure that each group of data has the same size at the same height points. This step provides the basis for the subsequent visualization of the strain distribution.

[0090] (2) Create a cylindrical model: Define the cylindrical parameters (radius 25 mm, height 100 mm), and create a cylindrical model. Use linear interpolation to map the fiber optic data to specific angular regions on the cylindrical surface.

[0091] (3) Dynamic visualization: By looping through the data per second, generate the strain distribution maps at each moment and save them as GIF files in sequence. The images show the distribution of strain on the cylindrical surface and its change over time, providing an intuitive visualization tool for analyzing crack propagation as shown in the figure.

[0092] Through the above method, we can visually observe the dynamic changes in the strain distribution of the rock samples during uniaxial compression. These changes reveal the initiation, propagation, and penetration processes of the cracks, thus providing reliable data support and visualization tools for analyzing the influence of sampling angles on crack propagation.

[0093] 4.1 Crack initiation and propagation at different sampling angles

[0094] The 45° rock sample was tested. At the initial stage of compression, as Figure 7 shown, the strain distribution of the 45° rock sample was relatively uniform, and no obvious strain concentration area appeared. At this time, the microcracks inside the rock sample had not started to propagate. As the compression force increased, the strain in some areas gradually increased, forming a strain concentration area. As can be seen from the right figure, obvious strain concentration occurred in the middle area of the sample (yellow area), indicating that the material in this area was under greater pressure, while tensile strain occurred in the upper and lower end areas (dark red areas).

[0095] It can be seen that at the beginning of uniaxial compression, the upper and lower ends are often first subjected to tensile strain, and the middle part is under compressive strain. As time increases, the strain also increases. As Figure 8 shown, at this time, the first crack can already be monitored, which is located in the 90 - 100 mm area of the first optical fiber and spreads towards the central area. As Figure 13As shown, from the 45° acoustic emission three-dimensional positioning diagram ( Figure 9 In the figure (left), it can be observed that there is a stress concentration curve between the sixth and fifth probes, so there is a crack between the two probes. Then, under the optical fiber monitoring, the optical fiber cylindrical strain diagram ( Figure 9 At 60 seconds, the first and fourth optical fibers simultaneously detected tensile strain. The fifth probe is located between the first and fourth optical fibers. Therefore, the initial crack map ( Figure 9 Right) illustrates the crack initiation location and extension direction.

[0096] like Figure 10 As shown in the figure, at 65 seconds, the strain of the fourth optical fiber was abnormal. The initial crack was offset and spread to the third optical fiber, and it extended from top to bottom. At 76 seconds, the second and third cracks began to develop at about 20 mm from the fourth optical fiber. According to subsequent analysis, one of the two cracks extended upward and the other extended downward.

[0097] like Figure 11 As shown in the figure, at 79 seconds, it can be clearly monitored that the second crack has extended to the 30mm position of the fourth optical fiber. At the same time, it can be observed that the first crack is detected at 60 to 100mm on the upper side of the third optical fiber. Therefore, the crack starts from the acoustic emission probe No. 5 and passes through the fourth optical fiber. After reaching the monitoring range of the third optical fiber, the crack begins to spread to the upper and lower ends. At the 81st second, the third optical fiber detects the second crack from the fourth optical fiber, from which it can be inferred that the crack begins to develop horizontally to the third optical fiber.

[0098] like Figure 12 As shown, at 84 seconds, the second crack at the fourth optical fiber began to grow upward and was also expanding. The first crack at the third optical fiber monitoring location continued to grow downward to 60 mm, while the first crack developed horizontally to 80 mm at the second optical fiber. At 87 seconds, the second crack began to grow downward, the third crack had grown downward to the bottom, and the first crack grew downward to 40 mm at the third optical fiber and continued to grow upward at the second optical fiber.

[0099] like Figure 13 As shown in the figure, it has entered the late stage. From 98 seconds to 101 seconds, the second crack develops downward while the third crack develops upward, and finally the two cracks converge together. At the same time, the crack at the third optical fiber continues to develop downward, and finally converges with the second and third cracks. At this point, all cracks have been developed.

[0100] Through the analysis of the crack propagation of rock uniaxial compression for the remaining three groups, it is finally concluded that the failure mode of the sample is closely related to the drilling angle. The strain distributions and failure conditions of rock samples at different angles in the uniaxial compression test are different, indicating that the drilling angle has a significant impact on the stress and failure mechanism of rock samples. The failure modes of the 0-degree and 30-degree samples mainly show axial tensile failure, with cracks starting from the top and gradually expanding downward. Fiber optic monitoring shows that at these angles, the strain is mainly concentrated longitudinally, and the failure mode is relatively simple. The 45-degree sample shows shear failure, with cracks presenting obliquely. The fiber optic monitoring data shows that the strain is evenly distributed on the surface of the sample, but there is a sharp change during shear failure, indicating that the sample is more vulnerable to shear stress at this angle. The 90-degree sample has the most serious failure. The fiber optic monitoring data shows the generation of multiple cracks, with a complex strain distribution, and the failure mode is multiple fractures. This indicates that at this angle, there are more initial microcracks inside the sample, causing stress concentration at multiple points, resulting in multiple fractures in the sample during compression.

[0101] The above are only embodiments of the present invention. Specific technical solutions and / or common knowledge such as characteristics well known in the art are not described in detail herein. It should be noted that for those skilled in the art, without departing from the technical solution of the present invention, several deformations and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to interpret the content of the claims.

Claims

1. A method for monitoring the dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring, characterized in that: The steps include: Step 1: Preparation of rock samples: Mix cement with sandstone of different particle sizes to obtain a mixed slurry, inject the mixed slurry into a mold, let it stand, demould, and maintain it, and then drill cylindrical rock samples at four angles of 0°, 30°, 45°, and 90°; Step 2, preparation and pretreatment of optical fiber: use single-mode optical fiber with a diameter of 125 microns and a coating diameter of 250 microns, and perform fusion splicing and protection treatment on the optical fiber; the fusion splicing operation is: use an optical fiber fusion splicer to fusion splice one end of four optical fibers, before fusion splicing, remove the coating on the optical fiber to expose a section of optical fiber, and perform vertical cutting on the optical fiber cutting machine; the pretreatment operation is: after the optical fiber is fusion spliced, put a transparent sleeve on the fusion point and perform heat sealing treatment; Step 3: Joint deployment of optical fiber and acoustic emission sensors: Four optical fibers are attached to each cylindrical rock sample. The attachment method is: attach the optical fiber horizontally along the cylindrical body of the rock sample at 0°, 90°, 180°, and 270°; install acoustic emission probes at the upper and lower ends of the rock sample respectively; Step 4: Stress loading and testing: Pre-stress to ensure that the rock sample and experimental equipment are in a stable state, and then perform compression loading and joint analysis of optical fiber data and acoustic emission data.

2. According to claim 1, a method for monitoring dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring is characterized by: In step 1, the mass ratio of each raw material in the mixed slurry is cement: 260 mesh sandstone: 150 mesh sandstone: 80 mesh sandstone: 60 mesh sandstone: water = 40:6:2:20:12:

15.

3. The method for monitoring dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring according to claim 2 is characterized by: In step 2, after the welding is completed, a welding test is performed by inserting a laser pen into one end of the signal receiver to emit light. If the other end emits red light, it proves that the welding is successful, otherwise it fails.

4. The method for monitoring dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring according to claim 3 is characterized by: In step three, the installation directions of the acoustic emission probes are 0° to 180° and 90° to 270° respectively.

5. The method for monitoring dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring according to claim 4 is characterized by: In step 4, the sensitivity of the optical fiber and the acoustic emission probe is tested before detection. First, the single-mode optical fiber is pressed, the single-shot measurement mode of the optical fiber demodulator is turned on, and the optical fiber is checked to see if it can receive the signal normally. Then, different positions of the rock sample are tapped to determine whether the acoustic emission probe position can accurately monitor the acoustic emission signal.

6. The method for monitoring dynamic characteristics of uniaxial failure of artificial shale based on distributed optical fiber and acoustic emission monitoring according to claim 5 is characterized by: Step three also includes dividing the rock sample into six equal parts in the 0° direction and attaching two additional optical fibers to achieve strain monitoring of the six equal parts.

Images