Multi-module biaxial testing apparatus and method for meso-scale analysis of granular materials

Abstract

This invention discloses a multi-module biaxial testing apparatus and method for the microscopic analysis of granular materials, relating to the field of geotechnical engineering laboratory testing. The aim is to achieve loading control under various stress paths and acquire microscopic images of the granular material during loading. The testing apparatus includes a base and a base plate mounted on the base, as well as an imaging device. The top surface of the base plate has four side plates forming a cuboid-shaped sample loading cavity. One side plate is fixedly mounted to the base plate, while the other three are movably mounted and respectively equipped with a loading device, a displacement measuring device, and a force measuring device. The testing method involves first filling the sample loading cavity with granular material to form a sample, then consolidating the sample, determining the loading method, and performing a biaxial test on the sample. Finally, the images and data are analyzed. This invention is used to obtain macroscopic stress-strain information of the sample, as well as microscopic information such as particle position, contact, and porosity changes within the sample, realizing the correlation between macroscopic mechanical behavior and microscopic structural evolution.

Description

Technical Field

[0001] This invention relates to the field of testing or analyzing materials by measuring their chemical or physical properties, particularly to the field of laboratory testing in geotechnical engineering, specifically to a testing apparatus and method for studying the macroscopic mechanical behavior and microstructure evolution of particulate materials. Background Technology

[0002] In geotechnical engineering, saturated granular materials (such as sand) are prone to liquefaction under cyclic loading. This liquefaction occurs when the accumulated excess pore water pressure leads to a sharp decrease in effective stress, loss of shear strength and modulus, resulting in serious engineering disasters such as foundation instability, building tilting, or failure. Laboratory tests are crucial for studying saturated sand liquefaction, primarily including cyclic triaxial tests and cyclic torsional shear tests. These tests effectively obtain the macroscopic mechanical response of the samples. However, conventional three-dimensional soil element tests are limited to measuring macroscopic mechanical parameters and cannot directly observe microscopic information such as particle contact, movement, and pore structure evolution within the sample. To further reveal the microscopic mechanism of liquefaction, discrete element numerical simulation methods are currently used to simulate particle-scale behavior. However, there is a lack of corresponding effective physical experimental verification methods, making it difficult to experimentally verify the correspondence between microscopic particle changes and macroscopic dynamic responses under different stress paths (such as consolidation, monotonic drained shear, monotonic undrained shear, and cyclic loading).

[0003] An invention patent with publication number CN113293807A and publication date of August 24, 2021, discloses a microscopic test model of sand structure. This invention patent uses cylindrical aluminum rods as the sand particle simulation material, achieves vertical loading through a static loading mechanism, and uses an imaging system combined with PIV (particle image velocimetry) technology to obtain the velocity and displacement fields of the particles. It tests the displacement of particles inside the sand structure at each moment, thereby clarifying the internal particle motion law and the failure mechanism of the sand structure, and simultaneously obtaining the model's macroscopic mechanical performance. However, this invention patent is limited to unidirectional static loading and cannot achieve stress paths such as consolidation, drainage / non-drainage condition control, and cyclic loading. The sand particle simulation material (aluminum rod) was placed on the model frame and a two-dimensional test was conducted in the xz plane (i.e. the plane corresponding to the x-axis and z-axis, where the x-axis is horizontal and the z-axis is vertical). The sand particle simulation material is significantly affected by its own weight, and the test cannot effectively simulate the "liquefaction phenomenon" where the average effective stress drops to zero. Therefore, it is impossible to study the microscopic mechanism of saturated sand under complex working conditions such as sand liquefaction.

[0004] The invention patent with publication number CN102081030A and publication date of June 1, 2011 discloses a geotechnical mechanics model test system and a refined test method based on macro- and micro-mechanical systems. This invention patent utilizes a digital camera to capture images of soil sample changes, a stereomicroscope to obtain microscopic images of the soil sample's microstructure, and combines continuous-discrete coupled numerical simulation to achieve macro- and micro-analysis. However, the test method is complex and cumbersome to analyze, limited to simple loading, and cannot achieve multiple stress paths such as cyclic loading. Furthermore, it can only obtain micro-particle information close to the surface of the observation surface, making it difficult to effectively characterize the correlation between macroscopic response and micro-behavior across the entire sample volume.

[0005] Overall, existing technologies lack an integrated testing system that can be applied to multiple stress paths, such as isobaric consolidation, monotonic drained shear, monotonic undrained shear, and cyclic biaxial loading. Existing technologies cannot perform non-contact analysis of microscopic information such as distance, contact, and porosity changes between particles throughout the entire testing process, and cannot combine macroscopic mechanical response with microstructure evolution. Summary of the Invention

[0006] This invention first provides a multi-module biaxial testing device for microscopic analysis of particulate materials, with the aim of achieving loading control under multiple stress paths and simultaneously acquiring microscopic images of particulate materials during the loading process.

[0007] The technical solution adopted by the present invention to achieve the above-mentioned objective is as follows: a multi-module biaxial testing device for microscopic analysis of particulate materials, comprising a base and a bottom plate fixedly installed on the base. The top surface of the bottom plate is horizontal, and four vertically arranged side plates are provided on the top surface of the bottom plate, forming a rectangular sample loading cavity. The length, width, and height directions of the sample loading cavity are set as the x-axis, y-axis, and z-axis, respectively. The center of the bottom surface of the sample loading cavity is the origin. The side plate located in the positive x-axis direction is the first side plate, the side plate located in the negative x-axis direction is the third side plate, the side plate located in the positive y-axis direction is the second side plate, and the side plate located in the negative x-axis direction is the fourth side plate. The first side plate is fixedly installed on the bottom plate, and the top surface of the bottom plate... The device is equipped with one set of x-axis slide rails and two sets of y-axis slide rails. The third side plate is slidably mounted on the x-axis slide rail, the second side plate is slidably mounted on one set of y-axis slide rails, and the fourth side plate is slidably mounted on another set of y-axis slide rails. Loading devices are fixedly mounted on the bottom plate or bracket on the side of the second, third, and fourth side plates that are opposite to the sample loading cavity. The loading devices are used to drive the side plates to move in a horizontal straight line. The multi-module biaxial testing device also includes a displacement measuring device, a force measuring device, and an imaging device. The displacement measuring device is used to detect the displacement of the side plates, the force measuring device is used to detect the force exerted by the loading device on the side plates, and the imaging device is arranged directly above and facing the sample loading cavity.

[0008] In order to allow the position of the loading devices of the second and fourth side plates to be adjusted along the x-axis and to ensure that the force exerted by the loading devices on the second and fourth side plates passes through the center of the sample loading cavity, furthermore, the base or bottom plate of the second and fourth side plates facing away from the sample loading cavity is provided with mounting holes arranged at intervals along the x-axis, and the loading devices of the second and fourth side plates are fixedly installed in the mounting holes.

[0009] In order to accurately control the displacement of each side plate and the force between the loading device and the side plate, the loading device is a servo motor, the displacement measuring device is an encoder built into the servo motor, the force measuring device is a force sensor, and the force sensor is set between the loading device and the side plate.

[0010] To facilitate leveling of the top surface of the base plate, an adjustment support is provided at the bottom of the base, and a spirit level is provided on the top surface of the base plate.

[0011] To ensure the safety of the test, a transparent cover plate is provided on the top of the four side plates, and the bottom plate, side plates and cover plate form a closed sample loading cavity.

[0012] To ensure the imaging device can capture high-quality images, the base plate is made of metal, with a matte black coating on the top surface of the base plate in the area of ​​the sample loading cavity. A frame is also provided on the outside of the base, and the imaging device is suspended and mounted on the frame. The imaging device can move along the x, y, and z axes. The imaging device is equipped with a ring-shaped supplementary light, which is positioned directly above and facing the sample loading cavity. The camera of the imaging device is positioned on a vertical line passing through the center of the supplementary light.

[0013] For example, the frame is in the shape of a cuboid, with each edge of the cuboid being a rod. The two ends of each rod are fixedly connected at the vertices of the cuboid. Two rods arranged along the x-axis or y-axis at the top of the frame are respectively mounted on first slide rails. A crossbar is provided between the two first slide rails, and the crossbar can slide along the length of the first slide rail. A second slide rail is mounted on the crossbar. A vertical rod is mounted on the second slide rail, and the vertical rod can slide along the length of the second slide rail. A third slide rail is mounted on the vertical rod. The shooting device is mounted at the lower end of the third slide rail, and the vertical rod can slide along the length of the third slide rail. A light stand is also mounted on the vertical rod, and a fill light is mounted on the light stand.

[0014] This invention also provides a multi-module biaxial testing method for microscopic analysis of particulate materials, with the aim of achieving loading control under multiple stress paths and simultaneously acquiring microscopic images of the particulate materials during loading. The technical solution adopted by this invention is: a multi-module biaxial testing method for microscopic analysis of particulate materials, which uses any of the aforementioned multi-module biaxial testing devices for microscopic analysis of particulate materials to test the particulate materials, including the following steps.

[0015] S1. Fill the sample loading cavity with particulate material to form a sample. The particulate material is a cylindrical test rod with a vertical central axis and equal height.

[0016] The density of the aluminum rod is close to that of sand, and the circular cross-section of the aluminum rod can simulate two-dimensional granular materials. Specifically, in step S1, the test rod is an aluminum rod with a height of 30 mm. Aluminum rods with a diameter of 15 mm and a diameter of 8 mm are randomly placed in the sample loading cavity at a ratio of 6:1.

[0017] S2. Apply confining pressure to the sample using a loading device to consolidate the sample.

[0018] S3. Determine the loading method and conduct a biaxial test on the specimen, including applying stress to the specimen through the loading device and side plates, using a displacement measuring device to detect the displacement of the second, third, and fourth side plates respectively, using a force measuring device to detect the stress of the second, third, and fourth side plates on the specimen respectively, and using an imaging device to photograph the specimen.

[0019] Specifically, in step S3, the loading method is one of the following four methods.

[0020] Loading Method 1: Cyclic non-drain shearing, including the following three steps.

[0021] S3-1a, the third side plate moves along the positive x-axis at a velocity v x The second and fourth side plates move simultaneously at speeds equal in size and opposite in direction, while maintaining a constant horizontal cross-sectional area of ​​the sample loading cavity.

[0022] S3-1b, When the shear stress q on the specimen reaches the preset shear stress q max At that time, the third side plate changed to a speed of -v x The second and fourth side plates move at speeds equal in magnitude and opposite in direction, maintaining a constant horizontal cross-sectional area of ​​the sample loading cavity. The formula for calculating the shear stress q is: q = (σ... x –σ y ) / 2, σ x Let σ be the stress experienced by the specimen in the x-axis direction. y The stress is the stress experienced by the specimen in the y-axis direction.

[0023] S3-1c, When the shear stress q on the specimen reaches the preset shear stress -q max Then, repeat steps S3-1a and S3-1b, and cycle until the average effective stress p′ of the specimen decreases to near 0, where the formula for calculating the average effective stress p′ is: p′ = (σ x +σ y ) / 2.

[0024] Loading Method 2: Monotonic conventional drainage shear, the third side plate moves along the positive x-axis at a velocity v x The specimen is moved and compressed, while the second and fourth side plates maintain the confining pressure in the y-axis direction during the consolidation stage until the strain of the specimen in the x-axis reaches the preset value.

[0025] Loading Method 3: Monotonic equal effective stress drainage shear, the third side plate moves along the positive x-axis at a velocity v x Motion, real-time detection of the stress σ exerted on the specimen by the third side plate. x And calculate the stress σ in the y-axis direction based on the target effective stress p0. y Both the second and fourth side plates apply stress σ to the specimen in real time. y Maintain the average effective stress of the specimen during loading. p 'constant.

[0026] Loading Method 4: Monotonic undrained shear, the third side plate moves along the positive x-axis at a velocity v x The second and fourth side plates move simultaneously at speeds equal in size and opposite in direction, while maintaining a constant horizontal cross-sectional area of ​​the sample loading cavity, until the strain of the sample in the x-axis direction reaches the preset value.

[0027] To ensure that the experiment is quasi-static and to avoid the influence of inertial effects on the test results, furthermore, in step S3, the velocity of each side plate satisfies the inertial number I < 0.001.

[0028] S4. Analyze the images and data obtained in step S3.

[0029] Specifically, in step S4, firstly, macroscopic mechanical curves, including stress-strain curves, are plotted to analyze the macroscopic deformation characteristics, strength, and liquefaction behavior of the specimen; secondly, the captured images are preprocessed, including filtering, contrast enhancement, binarization, and edge detection. Image processing algorithms are used to identify the center position, radius, and contour of each test bar, and the microstructure parameters of the specimen are calculated and statistically analyzed, as well as the evolution of the microstructure parameters with the loading process. The microstructure parameters include the spacing between the test bars, the number of contacts, the contact direction, the coordination number, and the porosity; finally, the macroscopic mechanical response curves and the microstructure evolution data are subjected to time-series correlation analysis.

[0030] The beneficial effects of this invention are: it can achieve loading through various stress paths, including isobaric consolidation, cyclic undrained shear, monotonic conventional drained shear, monotonic effective stress drained shear, and monotonic undrained shear. During loading, by conducting biaxial tests on the specimen, macroscopic stress-strain information and microscopic information such as particle position, contact, and porosity changes within the specimen can be obtained, thereby effectively realizing the correlation between macroscopic mechanical behavior and microscopic structural evolution. This invention is feature-rich, employs a modular design, and is easy to operate and expand. Attached Figure Description

[0031] Figure 1 This is a three-dimensional structural schematic diagram of an embodiment of the multi-module biaxial testing device for microscopic analysis of particulate materials according to the present invention.

[0032] Figure 2 yes Figure 1 The illustrated embodiment is a top view with the cover plate, frame, and other structures mounted on the frame hidden.

[0033] Figure 3 yes Figure 1 A schematic diagram of the imaging system in the embodiment shown.

[0034] Figure 4 This is a flowchart of the multi-module biaxial test method for microstructure analysis of particulate materials according to the present invention.

[0035] Reference numerals in the attached drawings: base 1, bottom plate 2, first side plate 3-1, second side plate 3-2, third side plate 3-3, fourth side plate 3-4, x-axis slide rail 4-1, y-axis slide rail 4-2, loading device 5, force measuring device 6, imaging device 7, mounting hole 8, adjusting support 9, cover plate 10, frame 11, supplementary light 12, first slide rail 13-1, horizontal bar 13-2, second slide rail 13-3, vertical bar 13-4, third slide rail 13-5, light holder 13-6, test bar 14. Detailed Implementation

[0036] The invention will now be further described with reference to the accompanying drawings.

[0037] like Figure 1 and Figure 2As shown, the multi-module biaxial experimental apparatus for microscopic analysis of particulate materials of the present invention includes a base 1 and a base plate 2 fixedly installed on the base 1. The base 1 serves to support and fix the base plate 2. To ensure the strength of the base 1 and the base plate 2, both the base 1 and the base plate 2 are made of metal, for example, the base 1 is a steel frame and the base plate 2 is a steel plate. To facilitate leveling of the base plate 2 and ensure that the top surface of the base plate 2 is horizontal, the bottom of the base 1 is provided with an adjusting support 9 for height adjustment. For example, the base 1 is a rectangular metal frame, and an adjusting support 9 is provided at each of the four corners of the base 1. A bubble level is provided on the top surface of the base plate 2, which allows observation of whether the top surface of the base plate 2 is horizontal.

[0038] The top surface of the base plate 2 is horizontal. Four vertically arranged side plates surround a rectangular sample loading cavity on the top surface of the base plate 2. This cavity is used to fill particulate material and form a sample. The length, width, and height directions of the sample loading cavity are defined as the x-axis, y-axis, and z-axis, respectively. The center of the rectangle on the top surface of the base plate 2 within the sample loading cavity is taken as the origin. The side plate located in the positive x-axis direction is the first side plate 3-1, the side plate located in the negative x-axis direction is the third side plate 3-3, the side plate located in the positive y-axis direction is the second side plate 3-2, and the side plate located in the negative x-axis direction is the fourth side plate 3-4. The first side plate 3-1 is fixedly installed on the base plate 2. For example, the first side plate 3-1 is fixedly installed on the base plate 2 by a connecting plate with a right-angled cross-section and bolts. A set of x-axis slide rails 4-1 is installed on the top surface of the base plate 2. Each set of x-axis slide rails 4-1 includes at least two rails, all arranged along the x-axis. A third side plate 3-3 is slidably mounted on the x-axis slide rails 4-1, and can slide along the x-axis. Two sets of y-axis slide rails 4-2 are also installed on the top surface of the base plate 2. Each set of y-axis slide rails 4-2 includes at least two rails, all arranged along the y-axis. A second side plate 3-2 is slidably mounted on one set of y-axis slide rails 4-2, and a fourth side plate 3-4 is slidably mounted on the other set of y-axis slide rails 4-2. Both the second side plate 3-2 and the fourth side plate 3-4 can slide along the y-axis. For example, see... Figure 2 The top surface of the base plate 2 is equipped with two x-direction slide rails 4-1 and four y-direction slide rails. The four y-direction slide rails are divided into two groups, with two rails in each group.

[0039] The base plate 2 and four side plates form an open-top sample loading cavity. To ensure test safety, a transparent cover plate 10 is provided on the top of the four side plates, that is, a transparent cover plate 10 is provided on the top of the sample loading cavity, and the base plate 2, side plates, and cover plate 10 form a closed sample loading cavity. The cover plate 10 is preferably a transparent plastic plate.

[0040] The side plates can be in one set or multiple sets. When there is one set, the side plates consist of four pieces: side plate 3-1, side plate 3-2, side plate 3-3, and side plate 3-4. The length, width, and height of the four side plates are preferably equal; for example, the height of all four side plates is 30mm, meaning the dimension of each side plate in the z-axis direction is 30mm. When there are multiple sets, each set includes four side plates of the same size, but the dimensions of the side plates in each set are different, thus forming sample loading cavities of different sizes. For example, there are four sets of side plates, each set containing four side plates of equal length, width, and height, with lengths of 350mm, 400mm, 450mm, and 500mm respectively. Each set of side plates can form sample loading areas with different dimensions in the x-axis and y-axis directions, used to investigate the influence of sample size on shear response.

[0041] Loading devices 5 are fixedly installed on the bottom plate 2 or bracket on the side of the second side plate 3-2, the third side plate 3-3, and the fourth side plate 3-4 facing away from the sample loading cavity. The loading devices 5 are used to drive the corresponding side plates to move in a horizontal straight line. In order to accurately control the force exerted by the loading devices on the side plates, as well as control the displacement and speed of the side plates, the loading devices 5 are servo motors.

[0042] The multi-module biaxial testing apparatus for microscopic analysis of particulate materials also includes a displacement measuring device and a force measuring device 6. The displacement measuring device detects the displacement of the side plates, specifically the second side plate 3-2, the third side plate 3-3, and the fourth side plate 3-4. For example, the displacement measuring device can be an encoder integrated into a servo motor. The displacement of the third side plate 3-3 is the displacement of the sample loading cavity in the x-axis direction, reflecting the deformation of the sample in the x-axis direction. The displacements of the third side plate 3-3 and the fourth side plate 3-4 reflect the displacement of the sample loading cavity in the y-axis direction, and also reflect the deformation of the sample in the y-axis direction. The force measuring device 6 detects the force exerted by the loading device 5 on the side plates. The force measuring device 6 corresponds one-to-one with the loading device 5, reflecting the force exerted by the side plates on the sample within the sample loading cavity. For example, the force measuring device 6 can be a force sensor, positioned between the loading device 5 and the side plates.

[0043] Since the first side plate 3-1 is fixedly installed on the base plate 2, the loading device 5 of the third side plate 3-3 is also fixedly installed on the base plate 2 or the bracket. Even if the size of the sample loading area changes in the y-axis direction, the loading device 5 can always be located on the z-axis centerline of the third side plate 3-3. To allow adjustment of the position of the loading devices 5 on the second side plate 3-2 and the fourth side plate 3-4 along the x-axis, ensuring that the force exerted by the loading device 5 on the second side plate 3-2 and the fourth side plate 3-4 passes through the center of the sample loading cavity, such as... Figure 2As shown, the base 1 or bottom plate 2 on the side of the second side plate 3-2 and the fourth side plate 3-4 facing away from the sample loading cavity are provided with mounting holes 8 spaced apart along the x-axis. The loading devices 5 of the second side plate 3-2 and the fourth side plate 3-4 are fixedly installed in the mounting holes 8. By fixing the loading devices 5 of the second side plate 3-2 and the fourth side plate 3-4 in the corresponding mounting holes 8, even if the size of the sample loading area changes in the x-axis direction, it can be ensured that the force exerted by the loading devices 5 on the second side plate 3-2 and the fourth side plate 3-4 passes approximately through the center of the sample loading cavity.

[0044] The multi-module biaxial testing apparatus for microscopic analysis of particulate materials also includes an imaging device 7, which is positioned directly above and facing the sample loading chamber. To ensure high-quality imaging, the top surface of the base plate 2 in the area of ​​the sample loading chamber is coated with a matte black coating. For example, the entire top surface of the base plate 2 may have a matte black coating. Alternatively, the top surface of the base plate 2 may have a black powder coating. The imaging device 7 features a high-resolution camera, for example, with a resolution of 9504 × 6336 pixels, capable of capturing high-precision two-dimensional images of the particulate sample in real time during loading.

[0045] To facilitate the installation of the imaging device 7, a frame 11 is provided on the outer side of the base 1. The frame 11 and the base 1 are not directly connected. The imaging device 7 is suspended on the frame 11 and can move along the x-axis, y-axis, and z-axis to adjust its position according to the shape of the sample loading cavity. The imaging device 7 is equipped with a ring-shaped supplementary light 12, which is positioned directly above and facing the sample loading cavity. The camera of the imaging device 7 is positioned on a vertical line passing through the center of the supplementary light 12. For example, see... Figure 1 and Figure 3The frame 11 is rectangular in shape, with each edge of the cuboid serving as a rod. The two ends of each rod are fixedly connected at the vertices of the cuboid. For example, all rods of the frame 11 are aluminum rods, with their ends fixedly connected. Supports are provided at the four corners of the bottom of the frame 11 to ensure its stability. Two rods arranged along the x-axis or y-axis at the top of the frame 11 are respectively fitted with first slide rails 13-1. For example, the first slide rails 13-1 are fixedly installed on the top surface of the rods with screws. A crossbar 13-2 is provided between the two first slide rails 13-1, and the crossbar 13-2 can slide along the length of the first slide rails 13-1. A second slide rail 13-3 is installed on the crossbar 13-2, and the second slide rail 13-3 is fixedly installed on the crossbar 13-2, for example, with screws. A vertical rod 13-4 is mounted on the second slide rail 13-3. The vertical rod 13-4 is vertically oriented and can slide along the length of the second slide rail 13-3. A third slide rail 13-5 is mounted on the vertical rod 13-4. The shooting device 7 is mounted on the lower end of the third slide rail 13-5. The vertical rod 13-4 can slide along the length of the third slide rail 13-5, allowing for the suspended mounting of the shooting device 7, and the suspension height of the shooting device 7 is adjustable. A light stand 13-6 is also mounted on the vertical rod 13-4, and a fill light 12 is mounted on the light stand 13-6. To allow adjustment of the position of the fill light 12, such as... Figure 1 and Figure 3 As shown, the lamp holder 13-6 is movable, and the position of the supplementary light 12 can be adjusted in the x-axis, y-axis and z-axis directions.

[0046] The present invention also provides a multi-module biaxial test method for microscopic analysis of particulate materials. The method uses the above-mentioned multi-module biaxial test device for microscopic analysis of particulate materials to test particulate materials, and includes the following steps.

[0047] S1. Fill the sample loading cavity with particulate material to form a sample. The particulate material is a cylindrical test rod 14. The central axis of the test rod 14 is vertical. The central axis of the test rod 14 is arranged along the z-axis in the sample loading cavity. The height of each test rod 14 is equal and the height of each test rod 14 does not exceed the height of the sample loading cavity.

[0048] Test rod 14 is used to simulate granular materials. Since the density of aluminum rods is close to that of sand, and the circular cross-section of the aluminum rod can simulate two-dimensional granular materials, test rod 14 is preferably made of aluminum. Specifically, the height of the aluminum rod is 30 mm, that is, the dimension of the aluminum rod along its central axis is 30 mm. Aluminum rods with a diameter of 15 mm and a diameter of 8 mm are randomly placed in the sample loading cavity at a ratio of 6:1. The central axis of test rod 14 is arranged along the z-axis direction in the sample loading cavity, which can eliminate the influence of gravity on the two-dimensional sample and facilitate the observation of liquefaction phenomena under cyclic loading.

[0049] S2. Apply confining pressure to the sample using loading device 5 to consolidate the sample.

[0050] In step S2, a consolidation pressure is set. Three loading devices 5 apply pressure to the three side plates respectively, and the four side plates apply consolidation pressure to the sample to achieve consolidation. Consolidation is generally equal consolidation pressure. Specifically, the force exerted by the three loading devices 5 on the side plates increases uniformly by 0.5N per second. The force measuring device 6 measures the force on the side plates in real time. The stress is calculated based on the contact area between the loading plate and the sample. When the stress reaches the target consolidation pressure, it is maintained stably for 30 minutes to complete consolidation.

[0051] S3. Determine the loading method and conduct a biaxial test on the specimen, including applying stress to the specimen through the loading device 5 and the side plate, using the displacement measuring device to detect the displacement of the second side plate 3-2, the third side plate 3-3 and the fourth side plate 3-4 respectively, using the force measuring device 6 to detect the stress on the specimen by the second side plate 3-2, the third side plate 3-3 and the fourth side plate 3-4 respectively, and using the imaging device 7 to photograph the specimen.

[0052] The loading method can be one of the following four methods.

[0053] Loading Method 1: Cyclic Undrained Shear. Loading Method 1 is used to simulate the liquefaction behavior of saturated particulate materials under cyclic loading. It can effectively observe the correspondence between microscopic mechanisms such as interparticle distance and the evolution of pores indirect contact with particles under cyclic loading and macroscopic liquefaction response. It includes the following three steps.

[0054] S3-1a, the third side plate 3-3 moves along the positive x-axis at a velocity v x Motion, velocity v x Generally, the second side plate 3-2 and the fourth side plate 3-4 move simultaneously at speeds that are equal in size, opposite in direction, and keep the horizontal cross-sectional area of ​​the sample loading cavity constant.

[0055] First, determine the initial area A0 of the sample, which is the area of ​​the sample on the horizontal plane after consolidation. The calculation formula is: A0 = L x0 ×L y0 , where: L x0 L is the distance between the first side plate 3-1 and the third side plate 3-3. y0 Let v be the distance between the second side plate 3-2 and the fourth side plate 3-4. Then, the third side plate 3-3 moves along the positive x-axis at a constant velocity v. x During motion, the sample is compressed in the x-axis direction. After a motion time t, the length of the sample in the x-axis direction becomes L. x L x =L x0 -v·t. To maintain a constant sample area (simulating undrained conditions), the length L of the sample in the y-axis direction is calculated in real time. y L y=A0 / L x Based on this, the relative velocity v between the second side plate 3-2 and the fourth side plate 3-4 is set. y v y =(L y0 -L y ) / t, the second side plate 3-2 and the fourth side plate 3-4 are of equal size (v y / 2) Stretching outwards simultaneously at opposite speeds. When t is sufficiently small, near-continuous, real-time speed updates can be achieved, thereby precisely maintaining a constant sample area during the loading process.

[0056] S3-1b, When the shear stress q on the specimen reaches the preset shear stress q max At that time, the third side plate 3-3 changed to a speed of -v x The motion, specifically, involves the third side plate 3-3 changing its direction of motion while maintaining a fixed velocity, moving along the negative x-axis. Correspondingly, the second side plate 3-2 and the fourth side plate 3-4 move at velocities of equal magnitude but opposite directions, maintaining a constant horizontal cross-sectional area of ​​the sample loading cavity. The formula for calculating the shear stress q is: q = (σ... x –σ y ) / 2, σ x Let σ be the stress experienced by the specimen in the x-axis direction. y The stress is the stress experienced by the specimen in the y-axis direction.

[0057] S3-1c, When the shear stress q on the specimen reaches the preset shear stress -q max Then, repeat steps S3-1a and S3-1b, and cycle until the average effective stress p′ of the specimen decreases to near 0, where the formula for calculating the average effective stress p′ is: p′ = (σ x +σ y ) / 2. When the average effective stress p′ of the specimen decreases to near 0, the specimen enters a "liquefied" state, at which point the load-bearing capacity of the particle skeleton is significantly lost. The preset number of cycles can then be completed, or loading can be stopped once the shear stress q on the specimen reaches a specified value.

[0058] Loading Method 2: Monotonic Conventional Drainage Shear, used to simulate monotonic shear behavior under drainage conditions. The third side plate 3-3 moves along the positive x-axis at a velocity v. x The specimen is moved and compressed, while the second side plate 3-2 and the fourth side plate 3-4 maintain the confining pressure in the y-axis direction during the consolidation stage until the strain of the specimen in the x-axis reaches the preset value.

[0059] After consolidation, the third side plate 3-3 moves along the positive x-axis at a velocity v x Motion, velocity v xGenerally, the confining pressure is kept constant. The sample is compressed in the x-axis direction, and the second side plate 3-2 and the fourth side plate 3-4 are adjusted in real time to maintain the confining pressure in the y-axis direction during the consolidation stage. During loading, the strain in the x-axis direction is continuously monitored, and loading is stopped when the strain reaches a preset value. Under this loading method, the volume of the sample can change, which can be used to study the dilatation or contraction characteristics of granular materials and the corresponding microstructure evolution.

[0060] Loading Method 3: Monotonic equal effective stress drained shear. Loading Method 3 aims to maintain the average effective stress p = (σ...) of the specimen. x +σ y ) / 2 is constant.

[0061] After consolidation, the third side plate 3-3 moves along the positive x-axis at a velocity v x Motion, velocity v x Generally, the stress σ exerted on the specimen by the third side plate 3-3 is kept constant and compressed in the x-axis direction. x And calculate the stress σ in the y-axis direction based on the target effective stress p0. y Both the second side plate 3-2 and the fourth side plate 3-4 apply stress σ to the specimen in real time. y The two loading devices 5 in the y-axis direction adjust the displacement of the second side plate 3-2 and the fourth side plate 3-4 in real time to achieve σ. y Dynamic tracking control is used to maintain a constant average effective stress p in the sample throughout the shearing process. Loading method three is used to study the shear characteristics and microstructural response of granular materials under constant average stress conditions.

[0062] Loading Method 4: Monotonic undrained shear, used for comparative studies of strength characteristics and microscopic mechanisms under undrained monotonic shear. The third side plate 3-3 moves along the positive x-axis at a velocity v. x Motion, velocity v x Generally, the second side plate 3-2 and the fourth side plate 3-4 move at the same speed, with equal magnitude and opposite direction, while keeping the horizontal cross-sectional area of ​​the sample loading cavity constant in real time, until the strain of the sample in the x-axis direction reaches the preset value.

[0063] After consolidation, the initial area of ​​the sample is A0 = L. x0 ×L y0 L x0 L is the distance between the first side plate 3-1 and the third side plate 3-3. y0 This is the distance between the second side plate 3-2 and the fourth side plate 3-4. The third side plate 3-3 moves along the positive x-axis at a velocity v. x Motion, velocity v x Generally kept constant, the dimension of the sample in the y-axis direction is L. x Real-time calculation of the sample's dimension L in the y-axis direction yL y =A0 / L x And control the second side plate 3-2 and the fourth side plate 3-4 to a size of v y / 2. Move at opposite speeds to maintain a constant sample area (simulating undrained conditions), and stop loading when the strain in the x-axis direction reaches a preset value.

[0064] The four loading methods described above can each be designed as four loading modules. Step S3 simply requires selecting the loading module as needed. Among the four loading methods, drainage and non-drainage are achieved by precisely controlling the area of ​​the specimen in the two-dimensional plane (equivalent to the volume under three-dimensional conditions) to be constant or variable, in order to simulate the corresponding stress paths in soil mechanics.

[0065] All four loading methods are driven by the second side plate 3-2, the third side plate 3-3, and the fourth side plate 3-4 and their corresponding loading devices 5. Simultaneously, based on real-time feedback from the displacement measuring device and the force measuring device 6, the loading device 5 is adjusted to complete the entire biaxial test process. To ensure the test is quasi-static and avoid the influence of inertial effects on the test results, the velocity of each side plate satisfies the inertia number I < 0.001. , ρ is the shear strain rate, d is the characteristic size of the particle, ρ is the particle density, and p is the average effective stress.

[0066] The specimen is photographed using imaging device 7. The ring-shaped supplementary light 12, along with the transparent cover plate 10 and the black base plate 2, significantly enhances the contrast between the circular cross-section of the test rod 14 and the background, facilitating subsequent image processing. Imaging device 7 records the cross-sectional changes of the test rod 14 within the entire specimen area in real time during the loading process at specified time intervals (e.g., one frame every 10 seconds, depending on the loading speed). Imaging device 7 is mounted on frame 11, and is separated from loading device 5, effectively isolating it from interference from loading vibrations, ensuring image clarity and stability, and thus achieving precise synchronous acquisition of macroscopic loading processes and microscopic images.

[0067] S4. Analyze the images and data obtained in step S3.

[0068] First, based on stress, displacement, and strain data, macroscopic mechanical curves, including stress-strain curves and effective stress paths, are plotted to analyze the macroscopic deformation characteristics, strength, and liquefaction behavior of the specimens. Second, the captured images are preprocessed, including filtering, contrast enhancement, binarization, and edge detection. Image processing algorithms are used to identify the center position, radius, and contour of each test rod 14, and the microstructure parameters of the specimens are calculated and statistically analyzed, along with the evolution of these parameters with the loading process. The microstructure parameters include the spacing, number of contacts, contact direction, coordination number, and porosity of the test rods 14. Finally, a time-series correlation analysis is performed between the macroscopic mechanical response curves and the microstructure evolution data to quantitatively reveal the multi-scale mechanical mechanisms of particulate materials under different stress paths.

[0069] The experimental apparatus of this invention can perform loading tests under various stress paths, including isobaric consolidation, monotonic drained shear, monotonic undrained shear, and cyclic undrained shear. It can flexibly combine different loading modules and specimen sizes according to research needs, and supports continuous, intuitive, and non-contact acquisition of microscopic information such as the position, contact, and porosity changes of the test rod 14 during the entire loading process. This effectively correlates macroscopic mechanical response with microscopic fabric evolution, providing an effective physical testing method for the study of multi-scale mechanical behavior in geotechnical engineering.

Claims

1. A multi-module biaxial experimental apparatus for microscopic analysis of particulate materials, characterized in that: Includes a base (1) and a base plate (2) fixedly installed on the base (1). The top surface of the base plate (2) is horizontal. The top surface of the base plate (2) is provided with four vertically arranged side plates that form a rectangular sample loading cavity. The length, width and height directions of the sample loading cavity are set as x-axis, y-axis and z-axis, respectively. The center of the bottom surface of the sample loading cavity is the origin. The side plate located in the positive x-axis direction is the first side plate (3-1), the side plate located in the negative x-axis direction is the third side plate (3-3), the side plate located in the positive y-axis direction is the second side plate (3-2), and the side plate located in the negative x-axis direction is the fourth side plate (3-4). The first side plate (3-1) is fixedly installed on the base plate (2). The top surface of the base plate (2) is equipped with a set of x-axis slide rails (4-1) and two sets of y-axis slide rails (4-2). The third side plate (3-3) slides. The second side plate (3-2) is slidably mounted on a set of y-direction slide rails (4-2), and the fourth side plate (3-4) is slidably mounted on another set of y-direction slide rails (4-2). Loading devices (5) are fixedly mounted on the base (1) or bottom plate (2) on the side of the second side plate (3-2), the third side plate (3-3), and the fourth side plate (3-4) facing away from the sample loading cavity. The loading device (5) is used to drive the side plate to move in a horizontal straight line. The multi-module biaxial test device also includes a displacement measuring device, a force measuring device (6), and an imaging device (7). The displacement measuring device is used to detect the displacement of the side plate, the force measuring device (6) is used to detect the force exerted by the loading device (5) on the side plate, and the imaging device (7) is arranged directly above the sample loading cavity and facing the sample loading cavity.

2. The multi-module biaxial experimental apparatus for microscopic analysis of particulate materials as described in claim 1, characterized in that: The second side plate (3-2) and the fourth side plate (3-4) have mounting holes (8) spaced apart along the x-axis on the base (1) or bottom plate (2) on the side opposite to the sample loading cavity. The loading device (5) of the second side plate (3-2) and the fourth side plate (3-4) is fixedly installed in the mounting holes (8).

3. The multi-module biaxial experimental apparatus for microscopic analysis of particulate materials as described in claim 1, characterized in that: The loading device (5) is a servo motor, the displacement measuring device is an encoder built into the servo motor, and the force measuring device (6) is a force sensor, which is located between the loading device (5) and the side plate.

4. The multi-module biaxial experimental apparatus for microscopic analysis of particulate materials as described in claim 1, characterized in that: The bottom of the base (1) is provided with an adjustment support (9), and the top surface of the base plate (2) is provided with a level bubble; the top of the four side plates is provided with a transparent cover plate (10), and the base plate (2), side plates and cover plate (10) form a closed sample loading cavity.

5. The multi-module biaxial experimental apparatus for microscopic analysis of particulate materials as described in any one of claims 1 to 4, characterized in that: The base plate (2) is a metal plate, and the top surface of the base plate (2) is provided with a black matte coating in the area of ​​the sample loading cavity; the outer side of the base (1) is also provided with a frame (11), the shooting device (7) is suspended on the frame (11), and the shooting device (7) can move along the x-axis, y-axis and z-axis. The shooting device (7) is equipped with a ring-shaped supplementary light (12), which is arranged directly above the sample loading cavity and facing the sample loading cavity. The camera of the shooting device (7) is arranged on a vertical line passing through the center of the supplementary light (12).

6. The multi-module biaxial experimental apparatus for microscopic analysis of particulate materials as described in claim 5, characterized in that: The frame (11) is in the shape of a cuboid, with each edge of the cuboid being a rod. The two ends of each rod are fixedly connected at the vertices of the cuboid. Two rods arranged along the x-axis or y-axis at the top of the frame (11) are respectively equipped with first slide rails (13-1). A crossbar (13-2) is provided between the two first slide rails (13-1). The crossbar (13-2) can slide along the length of the first slide rails (13-1). A second slide rail (13-3) is installed on the crossbar (13-2). The vertical rod (13-4) is installed on the second slide rail (13-3). The vertical rod (13-4) can slide along the length direction of the second slide rail (13-3). The third slide rail (13-5) is installed on the vertical rod (13-4). The shooting device (7) is installed at the lower end of the third slide rail (13-5). The vertical rod (13-4) can slide along the length direction of the third slide rail (13-5). The lamp holder (13-6) is also installed on the vertical rod (13-4). The fill light (12) is installed on the lamp holder (13-6).

7. A multi-module biaxial experimental method for microstructure analysis of particulate materials, characterized in that, The multi-module biaxial testing apparatus for microscopic analysis of particulate materials as described in any one of claims 1 to 6 is used to test particulate materials, comprising the following steps: S1. Fill the sample loading cavity with particulate material to form a sample. The particulate material is a cylindrical test rod (14). The central axis of the test rod (14) is vertical and the height of the test rod (14) is equal. S2. Apply confining pressure to the sample using the loading device (5) to consolidate the sample; S3. Determine the loading method and conduct a biaxial test on the specimen, including applying stress to the specimen through the loading device (5) and the side plate, using the displacement measuring device to detect the displacement of the second side plate (3-2), the third side plate (3-3) and the fourth side plate (3-4) respectively, using the force measuring device (6) to detect the stress on the specimen by the second side plate (3-2), the third side plate (3-3) and the fourth side plate (3-4) respectively, and using the imaging device (7) to photograph the specimen. S4. Analyze the images and data obtained in step S3.

8. The multi-module biaxial test method for microstructure analysis of particulate materials as described in claim 7, characterized in that: In step S1, the test rod (14) is an aluminum rod with a height of 30 mm. Aluminum rods with a diameter of 15 mm and a diameter of 8 mm are randomly placed in the sample loading cavity at a ratio of 6:

1.

9. The multi-module biaxial test method for microstructure analysis of particulate materials as described in claim 7 or 8, characterized in that, In step S3, the loading method is one of the following four methods: Loading Method 1: Looping without draining water shearing, including the following three steps, S3-1a, the third side plate (3-3) moves along the positive direction of the x-axis with a speed v x Movement, the second side plate (3-2) and the fourth side plate (3-4) move at the same time with equal size, opposite direction and keep the horizontal sectional area of the sample loading cavity unchanged S3-1b, when the shear stress q of the sample reaches the preset shear stress q max , the third side plate (3-3) moves at a speed -v x , the second side plate (3-2) and the fourth side plate (3-4) move at a speed equal in magnitude but opposite in direction, and the horizontal cross-sectional area of the sample loading cavity remains unchanged, wherein the calculation formula of the shear stress q is: q = (σ x - σ y ) / 2, σ x is the stress of the sample in the x-axis direction, and σ y is the stress of the sample in the y-axis direction; S3-1c, When the shear stress q on the specimen reaches the preset shear stress -q max Then, repeat steps S3-1a and S3-1b, and cycle until the average effective stress p′ of the specimen decreases to near 0, where the formula for calculating the average effective stress p′ is: p′ = (σ x +σ y ) / 2; Loading Method 2: Monotonic conventional drainage shear, the third side plate (3-3) moves along the positive x-axis at a velocity v x The specimen is moved and compressed, and the second side plate (3-2) and the fourth side plate (3-4) maintain the confining pressure in the y-axis direction during the consolidation stage until the strain of the specimen in the x-axis reaches the preset value. Loading Method 3: Monotonic equal effective stress drainage shear, the third side plate (3-3) moves along the positive x-axis at a velocity v x Motion, real-time detection of the stress σ exerted on the specimen by the third side plate (3-3) x And calculate the stress σ in the y-axis direction based on the target effective stress p0. y The second side plate (3-2) and the fourth side plate (3-4) both apply stress σ to the specimen in real time. y Maintain the average effective stress of the specimen during loading. p 'constant; Loading Method 4: Monotonic undrained shear, the third side plate (3-3) moves along the positive x-axis at a velocity v x The second side plate (3-2) and the fourth side plate (3-4) move simultaneously at speeds equal in magnitude and opposite in direction, while maintaining a constant horizontal cross-sectional area of ​​the sample loading cavity in real time, until the strain of the sample in the x-axis direction reaches the preset value.

10. The multi-module biaxial test method for microstructure analysis of particulate materials as described in claim 9, characterized in that: In step S4, firstly, macroscopic mechanical curves, including stress-strain curves, are plotted to analyze the macroscopic deformation characteristics, strength, and liquefaction behavior of the specimens; secondly, the captured images are preprocessed, including filtering, contrast enhancement, binarization, and edge detection. Image processing algorithms are used to identify the center position, radius, and contour of each test bar (14), and the microstructure parameters of the specimens are calculated and statistically analyzed, as well as the evolution of the microstructure parameters with the loading process. The microstructure parameters include the spacing, number of contacts, contact direction, coordination number, and porosity of the test bars (14); finally, the macroscopic mechanical response curves and microstructure evolution data are analyzed in a time series correlation.

Images