Sample preparation method of uniform discrete element sample

Abstract

The invention relates to a sample preparation method of a uniform discrete element sample. The sample preparation method comprises the steps of generating a wall boundary, generating sample particles layer by layer, compressing the sample to a target void ratio, applying a stress boundary, repeatedly generating the sample, compressing, applying stress and finally completely splicing. Introducing a particle boundary on a sample layering interface so as to ensure that all layers of samples are in close contact at the boundary; during compression, the top and bottom boundaries are moved at the same time, so that the uniformity of the sample is improved; when the confining pressure is applied, a servo mechanism suitable for irregular particle boundaries is adopted; and after the sample is spliced, the confining pressure is continuously adjusted through the servo mechanism until the confining pressure is balanced again. By introducing particle boundaries, providing a servo mechanism of irregular particle boundaries and adopting a compression mode of synchronous movement of upper and lower boundaries, the defect of sample nonuniformity caused by unidirectional compression in a traditional layered sample preparation method is improved, and the uniformity of void ratio distribution and stress distribution of discrete element samples can be remarkably improved; and the accuracy and reliability of a geotechnical test discrete element digital-analog result are improved.

Description

Technical Field

[0001] This invention relates to numerical modeling techniques for geotechnical engineering, and in particular to a method for preparing uniform discrete element samples. Background Technology

[0002] The Discrete Element Method (DEM) is a newly emerging meshless numerical simulation method that offers unique advantages in studying the micromechanical properties of granular materials such as sand and rock. This method allows relative displacement between elements without being limited by displacement continuity and deformation compatibility conditions, making it particularly suitable for solving complex problems involving large deformations and nonlinearities. It can accurately simulate the compressibility, dilatation, anisotropy, stress path dependence, and state dependence of granular materials. Due to its significant advantages in the micromechanical study of granular materials, coupled with the rapid development of computer science in recent years, the DEM has been widely applied to the study of discontinuous media problems such as seismic liquefaction, shear zone deformation, and piping failure.

[0003] However, when using the discrete element method (DEM) to simulate geotechnical element tests, the first step is specimen preparation, and the uniformity of specimen preparation significantly affects the test results, such as shear strength and failure mode. Considering that the size and number of particles in DEM specimens are usually much smaller than those in laboratory element tests, this places more stringent requirements on the uniformity of the DEM specimen preparation method.

[0004] Existing discrete element method (DEM) sample preparation methods have certain applicability and limitations. For example, isobaric methods and radial expansion methods are suitable for preparing uniform and dense samples, but less effective for loose samples. Gravity deposition methods, due to stress gradients, exhibit a porosity distribution that is loose at the top and dense at the bottom, and an anisotropic initial structure. In contrast, traditional layering methods have certain advantages, such as higher horizontal uniformity and simulation of laboratory sample preparation processes. The cause of vertical non-uniformity is relatively clear, namely, a unilateral, asymmetric wall boundary movement pattern.

[0005] Based on this, this paper proposes a sample preparation method for uniform discrete element samples. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing uniform discrete element samples, so as to improve the uniformity of discrete element samples and ensure the accuracy of subsequent simulation results.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A method for preparing a uniform discrete element sample includes the following steps: S1. Generate the boundary of the four walls; S2. Generate the first layer of sample particles within the space enclosed by the wall boundary; S3. Simultaneously move the top wall boundary and the bottom wall boundary to compress the sample to the target porosity; S4. Apply vertical stress to the top and bottom wall boundaries via a servo mechanism; S5. Define the set of sample particles located within a preset thickness below the top wall boundary as the particle boundary; S6. Move the top wall boundary upward to create a new space between it and the particle boundary, and generate a new layer of sample particles in this space; S7. Simultaneously move the top wall boundary and the bottom particle boundary to compress the new layer of sample to the target porosity; S8. Apply vertical stress to the top wall boundary and bottom particle boundary through servo mechanism two; S9. Repeat steps S5 to S8 until all sample layers are prepared; S10. Assemble all sample layers to complete sample preparation.

[0009] Furthermore, the preset thickness in step S5 is twice the average particle size of the sample particles.

[0010] Furthermore, when moving the particle boundaries, an overall movement method is adopted.

[0011] Furthermore, when preparing a new layer of sample particles, the sample particles that have already been prepared in the remaining layers are fixed in place.

[0012] Furthermore, step S10 includes the following process: After all the sample layers are prepared, the sample layers are translated to achieve splicing and assembly according to the gap distance between two adjacent sample layers. Remove all movement restrictions on the sample particles; A servo mechanism is used to apply stress to the top and bottom wall boundaries. The stress applied to the top wall boundary is downward and the stress applied to the bottom wall boundary is upward, until the actual stress applied is consistent with the preset target stress and the equilibrium state is reached, and then the process stops.

[0013] Furthermore, the equilibrium state refers to a state where the unbalanced force is less than one ten-thousandth of the average contact force.

[0014] Furthermore, the servo mechanism firstly applies a moving speed to the top and bottom wall boundaries, causing the actual stress to eventually approach the target stress. The expression for this moving speed v is as follows: ; in, For servo relaxation factor, For the wall area, For the target stress value, This is the actual stress value. To calculate the step size, For contact normal stiffness.

[0015] Furthermore, the second servo mechanism applies a moving speed to the top wall boundary and particle boundary, causing the actual stress to eventually approach the target stress. The expression for this moving speed v is as follows: ; in For servo relaxation factor, For the wall area, For the target stress value, This is the actual stress value. To calculate the step size, and These are the contact normal stiffness and tangential stiffness, respectively. and These are the sample particle radius and the boundary particle radius, respectively. It is the angle between the contact direction and the horizontal direction.

[0016] The advantages of this invention are as follows: It proposes improved measures for the traditional layered sample preparation method in discrete element simulation, retaining its advantage of good horizontal uniformity while employing three measures to improve its poor vertical uniformity, particularly at the interlayer boundaries: First, by introducing particle boundaries at the layer boundaries, their irregular geometric edge shapes ensure close contact between layered samples; second, a servo mechanism suitable for irregular boundaries is proposed, enabling the application of confining pressure at the particle boundaries; finally, a compression method involving synchronous movement of the top and bottom walls is adopted, improving upon the localized inhomogeneities within the sample layer caused by the traditional layered method which only moves the top single-sided wall. Combining these three optimization techniques significantly improves the uniformity of porosity and stress distribution in discrete element samples, enhancing the accuracy and reliability of discrete element numerical simulation results in geotechnical tests. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the sample preparation process in the embodiment;

[0018] Figure 2 This is a schematic diagram of the wall boundary and particle boundary in the embodiment;

[0019] Figure 3 This is a schematic diagram of the first layer of sample particles in the initial state in the embodiment;

[0020] Figure 4 This is a schematic diagram of the first layer of the sample after servo compression in the embodiment.

[0021] Figure 5 This is a schematic diagram of the second layer of sample particles in the initial state in the embodiment;

[0022] Figure 6 This is a schematic diagram of the second layer of the sample after servo compression in the embodiment.

[0023] Label Explanation 1-1. Top wall boundary; 1-2. Bottom wall boundary; 1-3. Side wall boundary; 2. Particle boundary; 3. Sample particles; 4. Void distance between sample layers. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to embodiments. It should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., used in this document indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the present invention.

[0025] This embodiment proposes a method for preparing uniform discrete element specimens. The main process includes generating the wall boundary of the first layer of specimens, generating the first layer of specimen particles 3, simultaneously moving the top and bottom walls to compress the specimen to the target porosity, applying stress through a servo mechanism, selecting particle boundary 2, raising the wall to generate a new layer of specimens, repeating the compression and servo steps until all specimen layers are prepared, and then splicing all specimen layers together. Figure 1 As shown, the specific steps are explained in detail below.

[0026] S1. Generate wall boundaries, including top wall boundary 1-1, bottom wall boundary 1-2, and side wall boundaries 1-3. (Refer to...) Figure 2 The shape of the wall boundary is relatively flat, and the surrounding particles are arranged relatively regularly near the wall. However, the particle boundary 2 mentioned above has irregular edges, which causes the particle boundary 2 and the sample particles 3 to interlock and maintain close contact.

[0027] S2. For example Figure 3 As shown, the first layer of sample particles 3 is generated within the space enclosed by the wall boundary. The area enclosed by the wall boundary is large enough that the initially generated first layer of sample particles 3 do not overlap or contact each other.

[0028] S3. Simultaneously move the top wall boundary 1-1 and the bottom wall boundary 1-2, with the top wall boundary 1-1 moving downward and the bottom wall boundary 1-2 moving upward, so that the space where the first layer of sample particles 3 are located is compressed. By adopting the method of simultaneously moving the top wall boundary 1-1 and the bottom wall boundary 1-2, the defect of the traditional layering method, which only moves the top wall and causes vertical unevenness of the sample, is improved.

[0029] Compression is stopped when the porosity of the sample particles 3 in the space reaches the target value. During the compression process, the sample particles 3 will be driven to move. After the sample particles 3 come into contact with each other, they continue to move and slip and rearrange. Therefore, when preparing a sample with a large target porosity, it is necessary to assign a larger particle friction coefficient to the sample particles 3 to improve the anti-slip ability between particles.

[0030] S4. For example Figure 4 As shown, stress is applied to the top wall boundary 1-1 and the bottom wall boundary 1-2 through a servo mechanism. The stress applied to the top wall boundary 1-1 is downward and the stress applied to the bottom wall boundary 1-2 is upward, until the applied actual stress tends to be consistent with the preset target stress and the equilibrium state is reached and then the process stops.

[0031] The servo mechanism works by applying a movement speed to the top wall boundary 1-1 and the bottom wall boundary 1-2, causing the actual stress to eventually approach the target stress. The expression for this movement speed v is as follows: ; in, For servo relaxation factor, For the wall area, For the target stress value, This is the actual stress value. To calculate the step size, For contact normal stiffness.

[0032] S5. The set of sample particles 3 located within a preset thickness below the top wall boundary 1-1 is defined as particle boundary 2. The preset thickness is the thickness of particle boundary 2. In this embodiment, the preset thickness is twice the average particle size of sample particles 3, so as to ensure that sample particles 3 will not pass through particle boundary 2.

[0033] S6. For example Figure 5 As shown, the top wall boundary 1-1 is moved upward to form a new space between it and the particle boundary 2. A new layer of sample particles 3 is generated in this new space. When preparing the new layer of sample particles 3, the remaining layers of sample particles 3 that have been prepared are fixed in place.

[0034] S7. Simultaneously move the top wall boundary 1-1 and the particle boundary 2, with the top wall boundary 1-1 moving downwards and the particle boundary 2 moving upwards, compressing the space containing the new layer of sample particles 3. When moving the particle boundary 2, a whole-body movement is used to limit the mutual displacement and rotation between particles within the particle boundary 2, maintaining the shape of the particle boundary 2 and ensuring that adjacent sample layers on both sides of the particle boundary 2 can fit tightly together during subsequent sample splicing. This demonstrates that by introducing the particle boundary 2 at the layering interface, the wall boundary effect can be eliminated, ensuring a tight contact between the sample layers at the layering interface.

[0035] When the porosity of sample particles 3 in the space reaches the target value, compression is stopped.

[0036] S8. For example Figure 6 As shown, stress is applied to the top wall boundary 1-1 and the particle boundary 2 through the second servo mechanism. The stress applied to the top wall boundary 1-1 is downward and the stress applied to the particle boundary 2 is upward, until the actual stress applied is consistent with the preset target stress and the equilibrium state is reached and then the process stops.

[0037] The second servo mechanism applies a moving speed to the top wall boundary 1-1 and the particle boundary 2, causing the actual stress to eventually approach the target stress. The expression for this moving speed v is as follows: ; in For servo relaxation factor, For the wall area, For the target stress value, This is the actual stress value. To calculate the step size, and These are the contact normal stiffness and tangential stiffness, respectively. and These are the sample particle radius and the boundary particle radius, respectively. It is the angle between the contact direction and the horizontal direction. The angle setting allows the second servo mechanism to adapt to stress application on irregular boundaries, adjusting the magnitude of the boundary stress by moving the boundary position. Simultaneously, this mechanism is also suitable for stress application on regular boundaries. When the angle is equal to 90°, the movement speed formula of Servo Mechanism 2 can be simplified to the movement speed formula of Servo Mechanism 1.

[0038] S9. Repeat steps S5 to S8 until all sample layers are prepared.

[0039] S10. After preparing all sample layers, determine the porosity distance 4 between adjacent sample layers (refer to Appendix). Figure 6 The sample layer is translated to achieve splicing and assembly, and then the movement restrictions of all sample particles 3 are released; at the same time, a servo mechanism is used to apply stress to the top wall boundary 1-1 and the bottom wall boundary 1-2, where the stress applied to the top wall boundary 1-1 is downward and the stress applied to the bottom wall boundary 1-2 is upward, until the actual stress applied is consistent with the preset target stress and the equilibrium state is reached and then the process stops.

[0040] In this embodiment, the equilibrium state refers to an unbalanced force being less than one ten-thousandth of the average contact force.

[0041] The above embodiments are only used to explain the concept of the present invention, and are not intended to limit the protection of the present invention. Any non-substantial modifications made to the present invention using this concept should fall within the protection scope of the present invention.

Claims

1. A method for preparing a uniform discrete element sample, characterized in that, Includes the following steps: S1. Generate the boundary of the four walls; S2. Generate the first layer of sample particles within the space enclosed by the wall boundary; S3. Compress the sample by simultaneously moving the top and bottom wall boundaries until the sample reaches the target porosity; S4. Apply vertical stress to the top and bottom wall boundaries via a servo mechanism; S5. Define the set of sample particles located within a preset thickness below the top wall boundary as the particle boundary; S6. Move the top wall boundary upward to create a new space between it and the particle boundary, and generate a new layer of sample particles in this space; S7. Simultaneously move the top wall boundary and the bottom particle boundary to compress the new layer of sample to the target porosity; S8. Apply vertical stress to the top wall boundary and bottom particle boundary through servo mechanism two; S9. Repeat steps S5 to S8 until all sample layers are prepared; S10. Assemble all sample layers to complete sample preparation.

2. The sample preparation method for a uniform discrete element sample as described in claim 1, characterized in that, The preset thickness in step S5 is twice the average particle size of the sample.

3. The sample preparation method for a uniform discrete element sample as described in claim 1, characterized in that, When moving the particle boundary, the entire particle is moved.

4. The sample preparation method for a uniform discrete element sample as described in claim 1, characterized in that, When preparing a new layer of sample particles, fix the remaining sample particles that have already been prepared.

5. The sample preparation method for a uniform discrete element sample as described in claim 1, characterized in that, Step S10 includes the following process: After all the sample layers are prepared, the sample layers are translated to achieve splicing and assembly according to the gap distance between two adjacent sample layers. Remove all movement restrictions on all sample particles; A servo mechanism is used to apply stress to the top and bottom wall boundaries. The stress applied to the top wall boundary is downward and the stress applied to the bottom wall boundary is upward, until the actual stress applied is consistent with the preset target stress and the equilibrium state is reached, and then the process stops.

6. A method for preparing a uniform discrete element sample as described in claim 1 or 5, characterized in that, The equilibrium state refers to a state where the unbalanced force is less than one ten-thousandth of the average contact force.

7. A method for preparing a uniform discrete element sample as described in claim 1 or 5, characterized in that, The servo mechanism is one that applies a movement speed to the top and bottom wall boundaries, causing the actual stress to eventually approach the target stress. The expression for this movement speed v is as follows: ； in, For servo relaxation factor, For the wall area, For the target stress value, This is the actual stress value. To calculate the step size, For contact normal stiffness.

8. The sample preparation method for a uniform discrete element sample as described in claim 1, characterized in that, The second servo mechanism applies a moving speed to the top wall boundary and particle boundary, causing the actual stress to eventually approach the target stress. The expression for this moving speed v is as follows: ； in For servo relaxation factor, For the wall area, For the target stress value, This represents the actual stress value. To calculate the step size, and These are the contact normal stiffness and tangential stiffness, respectively. and These are the sample particle radius and the boundary particle radius, respectively. It is the angle between the contact direction and the horizontal direction.

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