Centered loading method for rock specimens in true triaxial tests

Abstract

The present invention provides a centering and loading method for a true triaxial test rock sample. The centering and loading of the sample is achieved by first performing sample preloading and centering, and then performing sample loading and centering. A threshold is set according to the actual situation of the centering and loading, and the threshold is used as an evaluation standard for the centering. At the same time, in the sample loading stage, an active end and an adjustment end are first adjusted to be equal to the distance between the sample and a laser sensor in the horizontal direction and the vertical direction by a stress control method. When the displacement difference between the upper end pressure sensor and the lower end pressure sensor or the displacement difference between the left end pressure sensor and the right end pressure sensor is greater than a threshold value A2, the actuation mode of the adjustment end is adjusted to displacement control, and the center position of the sample is adjusted. When the displacement difference between the upper end pressure sensor and the lower end pressure sensor or the displacement difference between the left end pressure sensor and the right end pressure sensor is less than the threshold value A2, the adjustment end is restored to stress control or deformation control until the sample is center loaded.

Description

Technical Field

[0001] The invention relates to the technical field of rock indoor loading test, in particular to a true triaxial test rock sample centering loading method. Background Art

[0002] Rock mechanics experiments are an important means of studying the various mechanical properties of rocks and are fundamental to the development of rock mechanics. True triaxial rock testing machines are commonly used in these studies. True triaxial testing machines can apply unequal compressive stresses to rock specimens in three directions, simulating the actual stress conditions experienced by rocks in strata. To ensure uniform loading during true triaxial testing, the specimen's geometric center must be aligned with the collective center of the loading system. Common methods for achieving this alignment include using a hand pump to adjust the actuator or setting up a master-slave actuator system to adjust the specimen's geometric center. The hand pump actuator adjusts the specimen's position by manually adjusting the hydraulic oil pressure within the actuator cylinder. However, manual control cannot accurately control the pressure, making it difficult to accurately control the test position. In the master-slave actuator system, the active actuator controls deformation or stress, while the passive actuator follows through displacement. However, friction between the actuator, the hydraulic cylinder, and the specimen pressure head can lead to cumulative errors or loss of control, resulting in eccentric loading of the specimen.

[0003] For example, in the prior art, a patent document entitled "A Method for Precise Centering Control of Rock Sample Positions in Indoor True Triaxial Tests" with application number CN202011495764.4 and publication date June 7, 2022, provides a method for precise centering control of rock sample positions in indoor true triaxial tests. The method first coaxially arranges two symmetrical actuator pistons around the sample to be centered, one as the active end and the other as the passive end. After auxiliary centering is completed before the test, preloading is performed. During the preloading process, the active end is force-controlled, while the passive end actuator follows the displacement. Due to the friction between the various components of the actuator and the loading frame, the actual load borne by the sample is different from the predetermined load, and the accuracy of the auxiliary centering stage described in the early stage cannot be guaranteed.

[0004] In view of this, it is necessary to design an improved centering loading method for rock specimens in true triaxial tests to solve the above problems. Summary of the Invention

[0005] The purpose of the present invention is to provide a true triaxial test rock sample centering loading method.

[0006] To achieve the above-mentioned object, the present invention provides a method for centering and loading a rock specimen in a true triaxial test, comprising the following steps:

[0007] S1, sample preloading and centering stage: control the left end actuator and the right end actuator in the horizontal direction and the upper end actuator and the lower end actuator in the vertical direction to move the sample to the center position of the loading frame in a displacement control manner, and use the left end pressure sensor, the right end pressure sensor, the upper end pressure sensor and the lower end pressure sensor arranged on the left end actuator, the right end actuator, the upper end actuator and the lower end actuator close to one end of the sample to monitor the pressure value. When the pressure value monitored by the left end pressure sensor or the right end pressure sensor reaches the threshold value A1, the pressure value is adjusted. The left actuator and the right actuator stop moving; when the pressure value monitored by the upper pressure sensor or the lower pressure sensor reaches a threshold value A1, the upper actuator and the lower actuator stop moving; using the left laser sensor, the right laser sensor, the upper laser sensor and the lower laser sensor fixed to the inner wall of the loading frame corresponding to the left pressure sensor, the right pressure sensor, the upper pressure sensor and the lower pressure sensor, respectively, to measure the distance between the sample and the loading frame in the vertical direction and the horizontal direction;

[0008] Wherein, step S1 includes the following steps:

[0009] S11. After placing the sample on the lower pressure sensor, use the upper laser sensor to measure the distance between the sample and the inner wall of the loading frame as L2', use the lower laser sensor to measure the distance between the sample and the inner wall of the loading frame as L1', use the left laser sensor to measure the distance between the sample and the inner wall of the loading frame as L3', and use the right laser sensor to measure the distance between the sample and the inner wall of the loading frame as L4'. Based on this, determine the initial position of the sample;

[0010] S12. Under displacement control, the lower laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L1, the upper laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L2, the upper laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L3, and the right laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L4.

[0011] S2, sample loading and centering stage: after preloading is completed, the pressure values ​​monitored by all pressure sensors are cleared to zero, and normal loading begins; one of the left end actuator and the right end actuator in the horizontal direction and the upper end actuator and the lower end actuator in the vertical direction is used as the active end, and the other is used as the adjustment end, the active end and the adjustment end are synchronously stress-controlled or deformation-controlled, and the control system monitors the values ​​of the upper laser sensor, the lower laser sensor, the left laser sensor, and the right laser sensor. When the distance difference between the sample and the left and right inner walls of the loading frame or the distance difference between the sample and the upper and lower inner walls of the inner wall of the loading frame is greater than the threshold value A2, the movement mode of the adjustment end is changed to displacement control, and the center position of the sample is adjusted until the distance difference between the sample and the left and right inner walls of the loading frame or the distance difference between the sample and the upper and lower inner walls of the inner wall of the loading frame is less than the threshold value A2, and then the adjustment end is restored to stress control or deformation control to achieve centering loading of the sample;

[0012] Wherein, step S2 includes the following steps:

[0013] S21. After the sample preloading is completed, the values ​​of the upper pressure sensor and the lower pressure sensor are cleared, and one of the upper actuator and the lower actuator is used as the active end, and the other is used as the adjustment end. During the sample loading phase, the upper actuator and the lower actuator are both loaded in a stress-controlled or deformation-controlled manner, so that L1 = L2;

[0014] S22. After the sample preloading is completed, the values ​​of the left-end pressure sensor and the right-end pressure sensor are cleared, and one of the left-end actuator and the right-end actuator is used as the active end, and the other is used as the adjustment end. During the sample loading stage, the left-end actuator and the right-end actuator are both loaded using stress control or deformation control to make L3=L4.

[0015] Preferably, in step S21, when L1≠L2 and |L1-L2|≥A2, the actuation mode of the lower actuator or the upper actuator is adjusted to displacement control to adjust the position of the sample so that |L1-L2|≤A2.

[0016] Preferably, in step S22, when L3≠L4 and |L3-L4|≥A2, the actuation mode of the left end actuator or the right end actuator is adjusted to displacement control to adjust the position of the sample so that |L3-L4|≤A2.

[0017] Preferably, in step S1, the threshold A1 is set according to the type of sample and the test environment.

[0018] The beneficial effects of the present invention are:

[0019] The present invention proposes a true triaxial test rock specimen centering loading method, which realizes centering loading of the specimen by first performing specimen pre-loading centering and then performing specimen loading centering, and sets a threshold value according to the actual situation of centering loading as a centering evaluation standard; at the same time, in the specimen loading stage, the active end and the adjustment end are first adjusted to be equal in the horizontal and vertical distances between the specimen and the laser sensor by a stress control method, and when the displacement difference between the upper pressure sensor and the lower pressure sensor or the displacement difference between the left pressure sensor and the right pressure sensor is greater than the threshold value A2, the actuation mode of the adjustment end is adjusted to displacement control, and the center position of the specimen is adjusted so that the displacement difference between the upper pressure sensor and the lower pressure sensor or the displacement difference between the left pressure sensor and the right pressure sensor is less than the threshold value A2, the actuation mode of the adjustment end is restored to stress control, until the distances between the specimen and the two pressure sensors in the vertical direction are equal or the distance between the left actuator and the left laser sensor is equal to the distance between the right actuator and the right laser sensor. Through the above method, taking into account the factors such as friction that cause eccentric loading of the rock specimens during the actual centering loading of the true triaxial test, a combination of multiple control methods is adopted to make the eccentric loading factors generated by horizontal and vertical loading offset each other, thereby effectively controlling the eccentric loading of the specimens within the standard range of rock mechanics testing, achieving uniform loading of the specimens, and avoiding local stress concentration during the specimen loading process. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the adjustment of the sample preload centering stage in Example 1 of the present invention;

[0021] Figure 2 Schematic diagram of the adjustment during the sample loading and centering stage in Example 1 of the present invention;

[0022] The reference numerals are as follows:

[0023] 1. Lower end laser sensor; 2. Lower end actuator; 3. Lower end pressure sensor. DETAILED DESCRIPTION

[0024] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] It should also be noted here that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the solutions of the present invention are shown in the drawings, while other details that are not closely related to the present invention are omitted.

[0026] In addition, it should be noted that the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0027] See also Figure 1 and Figure 2 As shown, the present invention provides a method for centering and loading a rock specimen in a true triaxial test, comprising the following steps:

[0028] S1, sample preloading and centering stage: control the left and right actuators in the horizontal direction and the upper and lower actuators 2 in the vertical direction to move the sample to the center position of the loading frame in a displacement-controlled manner, and use the left pressure sensor, right pressure sensor, upper pressure sensor and lower pressure sensor arranged on the left actuator, right actuator, upper actuator and lower actuator 2 near one end of the sample to monitor the pressure value. When the pressure value monitored by the left pressure sensor or the right pressure sensor reaches the threshold value A1, the left actuator and the right actuator stop moving; when the pressure value monitored by the upper pressure sensor or the lower pressure sensor reaches the threshold value A1, the upper actuator and the lower actuator 2 stop moving; use the left laser sensor, right laser sensor, upper laser sensor and lower laser sensor fixed on the inner wall of the loading frame corresponding to the left pressure sensor, right pressure sensor, upper pressure sensor and lower pressure sensor to respectively measure the distance between the sample and the loading frame in the vertical and horizontal directions;

[0029] Specifically, step S1 includes the following steps:

[0030] S11. After placing the sample on the lower pressure sensor 3, the upper laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L2', the lower laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L1', the left laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L3', and the right laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L4'. Based on this, the initial position of the sample is determined;

[0031] S12. Under displacement control, the distance between the sample and the inner wall of the loading frame is measured as L1 by the lower pressure sensor 3, the distance between the sample and the inner wall of the loading frame is measured as L2 by the upper laser sensor, the distance between the sample and the inner wall of the loading frame is measured as L3 by the upper laser sensor, and the distance between the sample and the inner wall of the loading frame is measured as L4 by the right laser sensor.

[0032] S2, Specimen Loading and Centering Stage: After preloading is completed, the pressure values ​​monitored by all pressure sensors are reset to zero, and normal loading begins. One of the horizontal and vertical actuators is used as the active end, and the other as the adjustment end. The active and adjustment ends are synchronously subjected to stress control or deformation control. Simultaneously, the control system monitors the values ​​of the upper and lower laser sensors and the left and right laser sensors. When the distance difference between the specimen and the left and right inner walls of the loading frame, or the distance difference between the specimen and the upper and lower inner walls of the loading frame, exceeds a threshold value A2, the movement mode of the adjustment end is changed to displacement control, and the center position of the specimen is adjusted until the distance difference between the specimen and the left and right inner walls, or the distance difference between the specimen and the upper and lower inner walls of the loading frame, is less than the threshold value A2. At this point, the adjustment end is restored to stress control or deformation control, achieving centering loading of the specimen. It should be noted that threshold value A1 is set as needed during actual loading, based on the sample type and test environment, and threshold value A2 is set as needed during centering loading, and this is not a limitation herein.

[0033] Furthermore, step S2 includes the following steps:

[0034] S21. Method for upper and lower centering of the specimen during the loading phase: After the specimen is preloaded, the values ​​of the upper and lower pressure sensors are cleared. One of the upper actuator and the lower actuator 2 is used as the active end, and the other is used as the adjustment end. During the specimen loading phase, both the upper and lower actuators are loaded using stress control or deformation control, so that L1 = L2. As loading progresses, when L1 ≠ L2 and |L1 - L2| ≥ A2, the actuation mode of the lower actuator 2 or the upper actuator is adjusted to displacement control, and the position of the specimen is adjusted so that |L1 - L2| ≤ A2.

[0035] S22. Method for left and right centering of the specimen during the loading phase: After the specimen is preloaded, the values ​​of the pressure sensors at the left and right ends are cleared, and one of the left and right actuators is used as the active end, and the other as the adjustment end. During the specimen loading phase, both the left and right actuators use stress control or deformation control to adjust the geometric center position of the specimen so that L3 = L4; as the loading progresses, when L3 ≠ L4 and |L3-L4| ≥ A2, both the left and right actuators are loaded using stress control or deformation control so that |L3-L4| ≤ A2.

[0036] The following is a further description of the centering loading method for a true triaxial test rock specimen according to the present invention with reference to specific embodiments:

[0037] Example 1

[0038] This embodiment provides a method for centering and loading a rock specimen in a true triaxial test, comprising the following steps:

[0039] S1, sample preloading and centering stage: control the left and right actuators in the horizontal direction and the upper and lower actuators 2 in the vertical direction to move the sample to the center position of the loading frame in a displacement-controlled manner, and use the left pressure sensor, right pressure sensor, upper pressure sensor and lower pressure sensor arranged on the left actuator, right actuator, upper actuator and lower actuator 2 near one end of the sample to monitor the pressure value. When the pressure value monitored by the left pressure sensor or the right pressure sensor reaches the threshold value A1, the left actuator and the right actuator stop moving; when the pressure value monitored by the upper pressure sensor or the lower pressure sensor reaches the threshold value A1, the upper actuator and the lower actuator 2 stop moving; use the left laser sensor, right laser sensor, upper laser sensor and lower laser sensor fixed on the inner wall of the loading frame corresponding to the left pressure sensor, right pressure sensor, upper pressure sensor and lower pressure sensor to respectively measure the distance between the sample and the loading frame in the vertical and horizontal directions;

[0040] Specifically, step S1 includes the following steps:

[0041] S11. After placing the sample on the lower pressure sensor 3, the upper laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L2'. The lower laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L1'. The left laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L3'. The right laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L4'. Based on this, the initial position of the sample is determined.

[0042] S12. Under displacement control, the distance between the sample and the inner wall of the loading frame is measured as L1 by the lower pressure sensor 3, the distance between the sample and the inner wall of the loading frame is measured as L2 by the upper laser sensor, the distance between the sample and the inner wall of the loading frame is measured as L3 by the upper laser sensor, and the distance between the sample and the inner wall of the loading frame is measured as L4 by the right laser sensor.

[0043] S2, Specimen Loading and Centering Stage: After preloading is completed, the pressure values ​​monitored by all pressure sensors are reset to zero, and normal loading begins. One of the horizontal and vertical actuators is used as the active end, and the other as the adjustment end. The active and adjustment ends are synchronously subjected to stress control or deformation control. At the same time, the control system monitors the values ​​of the upper and lower laser sensors and the left and right laser sensors. When the distance difference between the specimen and the left and right inner walls of the loading frame, or the distance difference between the specimen and the upper and lower inner walls of the loading frame, exceeds a threshold value A2, the movement mode of the adjustment end is changed to displacement control, and the center position of the specimen is adjusted until the distance difference between the specimen and the left and right inner walls, or the distance difference between the specimen and the upper and lower inner walls of the loading frame, is less than the threshold value A2. At this time, the adjustment end is restored to stress control or deformation control, achieving centering loading of the specimen. It should be noted that in this embodiment, A1 = 5 kN and A2 = 5 μm. In other embodiments, A1 and A2 can be set according to the needs of the centering loading process, and this is not limited here.

[0044] Furthermore, step S2 includes the following steps:

[0045] S21. Method for upper and lower centering during the specimen loading phase: After the specimen is preloaded, the values ​​of the upper and lower pressure sensors are cleared. One of the upper actuator and the lower actuator 2 is used as the active end, and the other is used as the adjustment end. During the specimen loading phase, both the upper and lower actuators are loaded using stress control or deformation control, so that L1 = L2. As loading progresses, when L1 ≠ L2 and |L1 - L2| ≥ 5μm, the actuation mode of the lower actuator 2 or the upper actuator is adjusted to displacement control, and the position of the specimen is adjusted so that |L1 - L2| ≤ 5μm.

[0046] S22. Method for left and right centering of the specimen during the loading phase: After the specimen is preloaded, the values ​​of the pressure sensors at the left and right ends are cleared, and one of the left and right actuators is used as the active end, and the other as the adjustment end. During the specimen loading phase, both the left and right actuators use stress control or deformation control to adjust the geometric center position of the specimen so that L3 = L4; as the loading progresses, when L3 ≠ L4 and |L3-L4| ≥ 5μm, both the left and right actuators are loaded using stress control or deformation control so that |L3-L4| ≤ 5μm.

[0047] In summary, the centering loading method for a true triaxial test rock specimen proposed in the present invention realizes centering loading of the specimen by first performing sample preloading centering and then performing sample loading centering, and sets a threshold value according to the actual situation of centering loading as an evaluation standard for centering; at the same time, in the sample loading stage, the active end and the adjustment end are first adjusted to be equal to the distance between the specimen and the laser sensor in the horizontal and vertical directions by stress control, and when the displacement difference between the upper pressure sensor and the lower pressure sensor or the displacement difference between the left pressure sensor and the right pressure sensor is greater than the threshold A2, the actuation mode of the adjustment end is adjusted to displacement control, and the center position of the specimen is adjusted so that the displacement difference between the upper pressure sensor and the lower pressure sensor or the displacement difference between the left pressure sensor and the right pressure sensor is less than the threshold A2, the actuation mode of the adjustment end is restored to stress control until the distance between the specimen and the two pressure sensors in the vertical direction is equal or the distance between the left actuator and the left laser sensor is equal to the distance between the right actuator and the right laser sensor, thereby realizing centering loading of the specimen.

[0048] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A true triaxial test rock specimen centering loading method, characterized in that: The steps include: S1, sample preloading and centering stage: control the left end actuator and the right end actuator in the horizontal direction and the upper end actuator and the lower end actuator in the vertical direction to move the sample to the center position of the loading frame in a displacement control manner, and use the left end pressure sensor, the right end pressure sensor, the upper end pressure sensor and the lower end pressure sensor arranged on the left end actuator, the right end actuator, the upper end actuator and the lower end actuator close to one end of the sample to monitor the pressure value. When the pressure value monitored by the left end pressure sensor or the right end pressure sensor reaches the threshold value A1, the pressure value is adjusted. The left actuator and the right actuator stop moving; when the pressure value monitored by the upper pressure sensor or the lower pressure sensor reaches a threshold value A1, the upper actuator and the lower actuator stop moving; using the left laser sensor, the right laser sensor, the upper laser sensor and the lower laser sensor fixed to the inner wall of the loading frame corresponding to the left pressure sensor, the right pressure sensor, the upper pressure sensor and the lower pressure sensor, respectively, to measure the distance between the sample and the loading frame in the vertical direction and the horizontal direction; Wherein, step S1 includes the following steps: S11. After placing the sample on the lower pressure sensor, use the upper laser sensor to measure the distance between the sample and the inner wall of the loading frame as L2', use the lower laser sensor to measure the distance between the sample and the inner wall of the loading frame as L1', use the left laser sensor to measure the distance between the sample and the inner wall of the loading frame as L3', and use the right laser sensor to measure the distance between the sample and the inner wall of the loading frame as L4'. Based on this, determine the initial position of the sample; S12. Under displacement control, the lower laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L1, the upper laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L2, the upper laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L3, and the right laser sensor is used to measure the distance between the sample and the inner wall of the loading frame as L4. S2, sample loading and centering stage: after preloading is completed, the pressure values ​​monitored by all pressure sensors are cleared to zero, and normal loading begins; one of the left actuator and the right actuator in the horizontal direction and the upper actuator and the lower actuator in the vertical direction is used as the active end, and the other is used as the adjustment end, and the active end and the adjustment end are synchronously subjected to stress control or deformation control, and at the same time, the control system monitors the values ​​of the upper laser sensor, the lower laser sensor, the left laser sensor, and the right laser sensor. When the distance difference between the sample and the left and right inner walls of the loading frame or the distance difference between the sample and the upper and lower inner walls of the inner wall of the loading frame is greater than a threshold value A2, the movement mode of the adjustment end is changed to displacement control, and the center position of the sample is adjusted until the distance difference between the sample and the left and right inner walls of the loading frame or the distance difference between the sample and the upper and lower inner walls of the inner wall of the loading frame is less than the threshold value A2, and then the adjustment end is restored to stress control or deformation control to achieve centering loading of the sample; Wherein, step S2 includes the following steps: S21. After the sample preloading is completed, the values ​​of the upper pressure sensor and the lower pressure sensor are cleared, and one of the upper actuator and the lower actuator is used as the active end, and the other is used as the adjustment end. During the sample loading phase, the upper actuator and the lower actuator are both loaded in a stress-controlled or deformation-controlled manner, so that L1 = L2; In step S21, when L1≠L2 and |L1-L2|≥A2, the actuation mode of the lower actuator or the upper actuator is adjusted to displacement control to adjust the position of the sample so that |L1-L2|≤A2; S22. After the sample preloading is completed, the values ​​of the left-end pressure sensor and the right-end pressure sensor are cleared, and one of the left-end actuator and the right-end actuator is used as the active end, and the other is used as the adjustment end. During the sample loading phase, the left-end actuator and the right-end actuator are both loaded using a stress control or deformation control method, so that L3 = L4; In step S22, when L3≠L4 and |L3-L4|≥A2, the actuation mode of the left end actuator or the right end actuator is adjusted to displacement control to adjust the position of the sample so that |L3-L4|≤A2.

2. The centering loading method for true triaxial test rock specimens according to claim 1 is characterized in that: In step S1 , the threshold A1 is set according to the type of sample and the test environment.

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