WO2017076343A1

WO2017076343A1 - Biaxial testing system to examine the kinetic behavior of particulate media

Info

Publication number: WO2017076343A1
Application number: PCT/CN2016/104641
Authority: WO
Inventors: Quan Yuan; Yu-Hsing WANG
Original assignee: The Hong Kong University Of Science And Technology
Priority date: 2015-11-06
Filing date: 2016-11-04
Publication date: 2017-05-11
Also published as: CN108351286A

Abstract

A biaxial cell includes an upper cap; a sample cell; at least one observation window attached to the sample cell; two membrane holders, with flexible membranes attached therein, attached to opposite sides of the sample cell; two lateral cells, each containing a liquid reservoir, attached to the sample cell and surrounding the two membrane holders; and a base cap.

Description

BIAXIAL TESTING SYSTEM TO EXAMINE THE KINETIC BEHAVIOR OF PARTICULATE MEDIA

BACKGROUND

Soils are particulate media and the relevant physical principles that govern their macro-scale engineering properties originate from the particle interactions. Micromechanical studies have been extensively carried out to explain the underlying mechanisms of soil behavior using numerical simulations, predominantly by the discrete element method (DEM) and through experimentation. Although DEM simulations have been widely used and resulted in fruitful findings, it is worth noting that the predictions from DEM simulations are determined from the input particle-scale geometries and preset contact models, both of which are idealizations/simplifications of the physical world. Hence, the results provided by simulations can be limited.

A two-dimensional analogue for approximating granular materials in the form of a rod assembly is known in the art. Such a two-dimensional rod assembly can render responses analogous to the behavior of a real soil sample and, most importantly, can ease the observations of the particle motion. The majority of the tests on rod assemblies are performed using a biaxial apparatus. However, biaxial shearing devices are typically custom designed, costly, and not easy to be reproduced. In addition, the conventionally used devices only allow testing using a sample cell with a rigid boundary, which is expected to affect the features of the sample deformation, e.g., restricting the development of shear banding. In addition, the conventional measurement techniques for tracing particle motion can be further improved for higher resolution and accuracy by means of sophisticated techniques (i.e., particle image velocimetry (PIV) and close-range photogrammetry) in order to reveal the movement of the sample not only at the particle level but also at the level of particle contact. The particle shapes used in the experiment are also limited to regular shapes and most of them are round. Hence, a new design and manufacture of a new biaxial testing system that is able to resolve all of the problems indicated above is desirable.

The advent of 3D printing techniques, also known as additive manufacturing, has made significant impacts and changes in many fields, including biomedical engineering and healthcare, manufacturing, and even food production. Although such techniques have become more and more popular, it has not been widely used in geotechnical engineering related applications. Hence, in one or more embodiments the present disclosure uses 3D printing to manufacture at least some components of the biaxial testing system.

SUMMARY OF INVENTION

In one aspect, embodiments disclosed herein relate to a biaxial cell that includes an upper cap； a sample cell； at least one observation window attached to the sample cell； two membrane holders, with flexible membranes attached therein, attached to opposite sides of the sample cell； two lateral cells, each containing a liquid reservoir, attached to the sample cell and surrounding the two membrane holders； and a base cap.

In another aspect, embodiments disclosed herein relate to a biaxial testing system, that includes a loading frame； a biaxial cell, including: an outer cap； a sample cell； at least one observation window attached to the sample cell； two membrane holders, with flexible membranes attached therein, attached to opposite sides of the sample cell； two lateral cells, each containing a liquid reservoir, attached to the middle cell and surrounding the two membrane holders； and a base cap； a volume measuring device； and a control unit.

In yet another aspect, embodiments disclosed herein relate to a method of using a biaxial testing apparatus to test the kinetic behavior of particulate media that includes loading a particulate media into a sample cell of a biaxial cell, the biaxial cell including: an outer cap； the sample cell； at least one observation window attached to the sample cell； two membrane holders, with flexible membranes attached therein, attached to opposite sides of the sample cell； two lateral cells, each containing a liquid reservoir, attached to the sample cell and surrounding the two membrane holders； and a base cap； attaching the biaxial cell into a loading frame； applying a confining pressure to the particulate media； applying a varying axial load to the particulate media； and acquiring vertical displacement data from the particulate media during the application of the axial load.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an illustration of a biaxial testing system according to one or more embodiments of the present disclosure.

FIG. 2 shows an illustration of a biaxial cell according to one or more embodiments of the present disclosure.

FIG. 3 shows an exploded view of a biaxial cell according to one or more embodiments of the present disclosure.

FIGS. 4-10 show a progression of the sample preparation and assembly process of a biaxial cell according to one or more embodiments of the present disclosure.

DETAILED DESCRIPTION

Embodiments disclosed herein relate generally to biaxial testing systems and biaxial sample cells used therein. More specifically, embodiments of the present disclosure relate to biaxial sample cells having a flexible boundary and their use in biaxial testing systems. As discussed above, using numerical simulations to explain the underlying mechanisms of particulate media, such as soils, can be limited and oversimplified making physical tests in micromechanical studies an important supplement. Among different physical tests that are designed to obtain experimental evidence, a biaxial apparatus is typically used because of the ease of characterizing the involved micromechanical properties during testing.

In one or more embodiments, a biaxial testing system according to the present disclosure generally includes a loading frame for a biaxial cell, a volume measuring device, and a control unit. In one or more embodiments, at least some components of the biaxial testing system may be appropriated from more common triaxial testing systems with no modifications, or only slight modifications, necessary for these components to function appropriately. For example, the loading frame and the control unit of a triaxial testing system may be appropriated for use with the biaxial testing system according to the present disclosure with only simple and routine modification.

The loading frame is used to hold the biaxial cell in place and includes a mechanism to deliver a load to the biaxial cell and a sample held therein. In one or more embodiments, the mechanism to deliver a load to the biaxial cell may be a piston rod and the load may be applied by a load unit controlled by the control unit. In one or more embodiments, the axial load applied by the load unit may be monitored by a load cell and the vertical displacement of the piston rod (indicative of vertical displacement of the sample by the applied axial load) may be measured by a linear variable differential transformer (LVDT) . The volume measuring device is used to measure the volume of a liquid (e.g., water) as it changes in response to pressure increases on the sample. In one or more embodiments, the volume measuring device may use two pressure transducers to, respectively, detect the confining pressure (σ₃) in the biaxial cell and to detect the water level height, which is used to infer the sample volume change) . The control unit includes electronic equipment that, as mentioned above, controls the application of a load on the sample during the testing. Specifically, the control unit may be connected to a dual channel pneumatic loading unit for applying loads to the sample by way of deviatoric stress (σ_d) and confining pressure (σ₃) , a signal conditioning unit, a process interface unit, and a computer. In one or more embodiments, the biaxial testing system may also include a digital camera positioned and configured in such a way as to continuously capture high-resolution images of the sample in the biaxial cell during a test. These images may be used to perform Particle Image Velocimetry (PIV) , which traces and analyzes the movement of particulates and particulate contact during a test.

For example, FIG. 1 shows an illustration of a biaxial testing system 100 according to one or more embodiments of the present disclosure. The loading frame 124 and biaxial cell (the biaxial cell itself will be shown in more detail in FIGS. 2-3) of the biaxial testing system are located within section (1) of FIG. 1, the sections being delineated by dashed lines. The volume measuring device of the biaxial testing system is located in section (2) of FIG. 1. The control unit of the biaxial testing system is located in section (3) of FIG. 1.

Many particular components of the loading frame and the biaxial cell depicted in section (1) are more specifically identified in FIG. 1. For example, an outer piston rod 102 is shown connected at an upper end to a loading unit 126 and as entering the upper cap 104 of the biaxial cell and engaging with an inner piston 106. The inner piston 106 has an inner piston rod 108 that is connected to a loading plate 110. The inner piston 106, inner piston rod 108, and loading plate 110 all enter a sample cell 112 of the biaxial cell, with the loading plate 110 interfacing with the upper surface of a particulate sample 114 therein. Also shown, are lateral cells 116 on opposite sides of sample cell 112, the lateral cells 116 include liquid reservoirs 118 that are used to put pressure on, and otherwise stress, a flexible membrane 120 that separates the liquid and the particulate sample 114. Located at the bottom of the biaxial cell is a base cap 122. A load cell 128 may be disposed, as shown in FIG. 1, within the system in order to measure deviatoric stress (σ_d) . A linear variable differential transformer (LVDT) 130 may also be provided, as shown in FIG. 1, to measure axial strain (ε_a) .

Pressure transducers

132 and 134 may be provided to measure cell pressure and volume changes, respectfully.

FIG. 2 shows an illustration of a biaxial cell 200 according to one or more embodiments of the present disclosure. Some components depicted in FIG. 1 are also shown in this illustration including: the upper cap 104, the two lateral cells 116, the sample cell 112, and the base cap 122. In FIG. 2, on top of the upper cap 104 is a piston rod seal 202 that is used to seal and guide the outer piston rod 102 into the inner piston 106, both previously shown in FIG. 1. Also shown is observation window 204, which provides a clear view of the particulate sample when loaded into the sample cell 112, allowing for the capture of images by a digital camera described above. FIG. 2 shows that the upper cap 104 and the base cap 122 of the biaxial cell 200 are connected by columns 206, which serve to maintain the stability of the sample cell 112 and otherwise maintain its solidity during testing.

FIG. 3 shows a more detailed, and exploded, view of a biaxial cell 200 according to the present disclosure. Some components depicted in FIG. 1 and/or FIG. 2 are also shown in this illustration including: the upper cap 104, the two lateral cells 116, the sample cell 112, the base cap 122, the inner piston 106, the inner piston rod 108, the loading plate, the sample cell, piston rod seal 202, observation windows 204, and columns 206 (only one out of four shown is labeled) . Newly shown in FIG. 3, the piston holder 300 is used to hold the inner piston 106 in place by fitting into a recess in the sample cell 112. Also depicted are membrane holders 302, which are used to hold flexible membranes that separate the particulate sample within the sample cell 112 from the liquid in the liquid reservoirs of the lateral cells 116. The liquid reservoirs of the lateral cells 116 are clearly visible although they are not labeled for clarity purposes. The membrane holders 302 in FIG. 3 each have an opening where the flexible membrane is held, the openings match corresponding openings on the lateral cells 116 and the openings directly interface each other when the biaxial cell 200 is assembled. The blind nuts 304 (only one of the two shown is labeled) are used to reversibly attach the upper cap 104 to the columns 206, the columns themselves being reversibly attached to the base cap 122.

In one or more embodiments, and as depicted in FIG. 1, the upper cap and lateral cells of a biaxial cell according to the present disclosure are utilized to form a closed space where a confining pressure σ₃ can be applied by applying compressed air onto the liquid contained within the liquid reservoirs of the lateral cells and then onto the sample via the flexible boundary separating the liquid from the sample. In addition to the confining pressure (σ₃) a deviatoric stress (σ_d) is also applied to the sample from the loading plate and inner piston. During testing, because of the flexible membrane separating the sample and the liquid, when a sample deforms laterally (i.e., protrudes into the water reservoir space) during shearing the water level in the two lateral cells is changed accordingly. This change of the water level may be detected by the volume-measuring device, as the liquid reservoirs of the lateral cells and the volume-measuring device are fluidly connected (see Fig. 1) , and the change of water level may be used to infer the volume change of the sample. In order to achieve an accurate measurement of the volume change, the air pressure acting in the lateral cells and in the volume-measuring device has to be identical.

In one or more embodiments, the sample cell, as shown in FIG. 3, may have a hole centrally located in its upper portion, which is used to accommodate a piston holder. The piston holder is used to minimize abrasion between the inner piston assembly and the inner wall of the hole when axial loading (i.e., deviatoric stress (σ_d) ) is applied during the testing. The top of the inner piston assembly may be connected to the outer piston rod and then to the loading frame/system. The bottom of the inner piston assembly is linked to the loading plate to transfer the axial loading (i.e., deviatoric stress σ_d) onto the sample. In one or embodiments, the circular area of the inner piston, excluding the area occupied by the outer piston rod, is identical to the cross sectional area of the loading plate and thus it may be ensured that during testing an equal stress is transmitted from the piston to the loading plate and then to the sample. In one or more embodiments, the sample cell has a cuboid inner void which accommodates the testing sample.

In one or more embodiments, and shown most clearly in FIG. 3, the sample cell may have attached thereto two opposing observation windows and two opposing flexible membrane holders. The flexible membrane holders allow for flexible sample boundaries, achieved by first attaching a flexible membrane onto each flexible membrane holder and then installing/attaching the whole flexible membrane holders onto opposing sides of the sample cell. The material used for the flexible membrane is not particularly limited. However, in one or more embodiments, the flexible membrane may be a membrane of latex or of any other type of polymeric membrane that is otherwise flexible and substantially impermeable to liquid. The transparent observation windows facilitate image capture for subsequent particle image velocimetry (PIV) analyses. In one or more embodiments, there may be at least two reference dots with known coordinates marked on the observation windows, which may be used to convert the pixel coordinates of captured images into spatial coordinates for the PIV analyses discussed above. For example, the distances between two adjacent reference dots may be set to a known value, both in the horizontal and vertical directions making, thereby rendering the pixel dimensions of a captured image known and allowing for detailed PIV analyses.

In one or more embodiments, each of the components of the biaxial cell may be fabricated using 3D printing. For example, it may be particularly beneficial to fabricate the sample cell using 3D printing due to its complex geometry, which renders it difficult to be fabricated using conventional methods including working with lathes, drills, stamping presses, and moulding machines. Thus, in one or more embodiments, at least the sample cell is fabricated using 3D printing, while the other components of the biaxial cell may be conventionally fabricated from aluminum or stainless steel, the exception being the observation windows which may be fabricated from a transparent plastic or glass material.

While the sample cell has a complex geometry, it is desirable for it to be fabricated as a single piece to ensure that it is seamless and leak-proof. In general, 3D printing refers to processes used to synthesize three-dimensional objects in which successive layers of the consolidated object are formed from unconsolidated starting materials that may include a variety of polymeric materials, ceramics, and metals/alloys. In one or more embodiments, the sample cell (or any other component of the biaxial testing system) may be fabricated by 3D printing of acrylonitrile-butadiene-styrene (ABS) polymers and copolymers, or other similar polymers that can form an object with high stiffness and strength. In one or more embodiments, the sample cell may be fabricated from another high stiffness and strength material such as stainless steel. The fabrication of the object takes place under computer control, whereby the computer controls the fabrication by utilizing a digital model of the object to be created. The digital model may be created with a computer-aided design (CAD) package, via a 3D scanner, or by applying photogrammetric methods and software to images of an existing object to be replicated.

The sample preparation and assembly process of the biaxial cell according to one or more embodiment of the present disclosure are illustrated in FIGS. 4-10. First, and as shown in FIG. 4, a sample cell is placed on the top of the base cap. Note that the membrane holders and the inner piston assembly together with the loading plate were installed into the sample cell prior to this stage. As shown in FIG. 5, support plates are placed in the hole of the membrane holder to serve as temporary lateral support to facilitate sample preparation. In one or more embodiments, the testing sample will be in the form of a rod assembly, with the rods making up the assembly used to approximate a particulate material. In one or more embodiments, the rods may be made from metallic, ceramic, or polymeric materials. In some embodiments, sand or any other granular material may be used as the testing sample. During the sample loading as shown in FIG. 5, the rods were randomly put layer by layer through the front side of the sample cell. In one or more embodiments, all the rods may be carefully aligned in a single plane to ease the focus adjustment during image taking. After the rods are filled and the sample packing is completed, the inner piston assembly together with the loading plate is pushed into position to make a firm touch with the upper surface of the rod assembly and the front and rear observation windows are placed and fixed in position as shown in FIG. 6. In one or more embodiments, a slight vacuum pressure (e.g. ～15 kPa) may then be applied to hold the sample in place for the subsequent replacement of the lateral support plates by the membrane holder with the flexible membrane attached therein, as shown in FIG. 7. The two lateral cells are then connected to the sample cell, as shown in FIG 8. The upper cap may be mounted next and tightly connected to the sample cell and lateral cells through the four columns, and the outer piston rod is fixed to the top of the inner piston assembly, as shown in FIG. 9. The assembled biaxial cell of FIG. 9 may then be connected to the loading frame, and the lateral cells may be connected to a water container (shown on the left of FIG. 10) filled with water, as shown in FIG. 10. Once connected to the water container, the lateral cells may be filled with water from the water container. Finally, the volume-measuring device and the control unit may be connected to arrive at the biaxial testing system depicted in FIG. 1. FIG. 10 also shows that a back-pressure pipe may be connected to the upper cap (e.g., the inlet/outlet shown on right of upper cap) with the purpose to keep the air pressure in the lateral cells and the volume-measuring device identical, as noted.

Once the biaxial testing system is assembled, a static confining pressure, σ₃ (e.g., up to ～200 kPa) may be applied to the sample； the axial loading (or deviatoric stress σ_d) applied during the shearing stage may be in the strain-controlled mode to obtain a complete stress-strain curve with a speed of 0.08％/min that is strain-adjustable. In other words, during the test the strain rate is fixed and the stress is servo-controlled to keep the strain rate constant. When a digital camera is used to take images during the testing, the sample images may be set to automatically be collected at intervals of about 1 minute. During the testing, the deviatoric stress σ_d and axial strain ε_a may be measured by the load cell and the LVDT, respectively.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

A biaxial cell, comprising:

an upper cap；

a sample cell；

at least one observation window attached to the sample cell；

two membrane holders, with flexible membranes attached therein, attached to opposite sides of the sample cell；

two lateral cells, each containing a liquid reservoir, attached to the sample cell and surrounding the two membrane holders； and

a base cap.
The biaxial cell of claim 1, wherein at least one component of the biaxial cell is fabricated by 3D printing.
The biaxial cell of claim 2, wherein the sample cell is fabricated by 3D printing.
The biaxial cell of claim 1, wherein the at least one observation window has at least two reference dots thereon.
The biaxial cell of claim 1, wherein the flexible membranes are latex.
A biaxial testing system, comprising:

a loading frame；

a biaxial cell, comprising:

an outer cap；

a sample cell；

at least one observation window attached to the sample cell；

two membrane holders, with flexible membranes attached therein, attached to opposite sides of the sample cell；

two lateral cells, each containing a liquid reservoir, attached to the middle cell and surrounding the two membrane holders； and

a base cap；

a volume measuring device； and

a control unit.
The biaxial testing system of claim 6, wherein the control unit is connected to a loading unit and comprises a signal conditioning unit, a process interface unit, and a computer.
The biaxial testing system of claim 6, further comprising a digital camera.
The biaxial testing system of claim 6, wherein at least one component of the biaxial cell is fabricated by 3D printing.
The biaxial testing system of claim 8, wherein the sample cell is fabricated by 3D printing.
The biaxial testing system of claim 6, wherein the at least one observation window has at least two reference dots thereon.
The biaxial testing system of claim 6, wherein the flexible membranes are latex.
A method of using a biaxial testing apparatus to test the kinetic behavior of particulate media, comprising:

loading a particulate media into a sample cell of a biaxial cell, the biaxial cell comprising:

an outer cap；

the sample cell；

at least one observation window attached to the sample cell；

two membrane holders, with flexible membranes attached therein, attached to opposite sides of the sample cell；

two lateral cells, each containing a liquid reservoir, attached to the sample cell and surrounding the two membrane holders； and

a base cap；

attaching the biaxial cell into a loading frame；

applying a confining pressure to the particulate media；

applying a varying axial load to the particulate media； and

acquiring vertical displacement data from the particulate media during the application of the axial load.
The method of claim 13, wherein applying the confining pressure is accomplished by applying compressed air to a liquid in the liquid reservoir of the lateral cells.
The method of claim 14, wherein the confining pressure is static.
The method of claim 13, wherein the varying axial load is a steadily increasing load.
The method of claim 13, wherein the vertical displacement data is acquired by capturing images of the particulate media at specific intervals during application of the axial load.
The method of claim 13, wherein the particulate media is a rod assembly.