Abstract:
A device for determining material properties includes a bottom panel extending from a first end to a second end of the device, for supporting a material to be tested. A closed loop flexible wall extends upwardly from the panel and is in operational engagement with a flexible wall idle means towards the first end of the device and a flexible wall driving means in spaced relation with the flexible wall idle means. The flexible wall driving means causes movement of the closed loop flexible wall along a predetermined closed loop path. The flexible wall driving means and the means of driving it are arranged at a position outside of a material containment area of the device.

Description:
[0001]    This application claims benefit of Serial No. 2012901433, filed 12 Apr. 2012 in Australia and which application is incorporated herein by reference. To the extent appropriate, a claim of priority is made to each of the above disclosed applications. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a device for determining material properties. The device can be used to measure the mechanical properties, such as shear strength and viscosity, of any flowable material, including complex materials such as soils, granular materials, powders, suspensions, emulsions, colloids and melted polymers. 
         [0003]    The device has applications in many industries such as mining, geotechnical engineering and civil engineering projects in which it is desired to know the properties and failure behaviour of e.g. soil or sand under certain loading conditions. The information gathered using the device is important to the design of building foundations, pipework, tunnel construction, and also for use in landslide and earthquake management structures. 
         [0004]    The device also has application in the food industry, agriculture, bio engineering, chemical engineering, mechanical engineering, material processing, petroleum engineering, pharmaceutics, as well as the cosmetics and paint industries. The information gathered using the device is then useful to improving efficiency and optimising processes. 
       BACKGROUND OF THE INVENTION 
       [0005]    The testing of flowable solid materials such as soil, sand and liquids for their mechanical properties such as shear strength and viscosity has long been an important aspect in the design of, for example, building foundations, earthworks, underground pipe installations, new liquid products such as paints, cosmetics and foodstuffs. This is usually done by deforming samples of the materials and measuring the corresponding bulk relationships connecting nominal stress, strain, and their rates of change. Bulk properties such as the material strength and viscosity are then obtained from these relationships. Several devices with various geometries already exist and are in routine use. For example, the tri-axial apparatus, the Couette rheometer and the plane-Couette rheometer are described below. 
         [0006]      FIG. 1  shows a known device for determining material properties, being a tri-axial apparatus  1  in which the material  2  to be tested is supported on a base  3  and is sealed within a rubber membrane  7  that is surrounded by water  4  in a confining cylinder  5 . A load  6  is applied to the material from above. A disadvantage of this device is that the material  2 , for example soil, bulges within the rubber membrane  3  as it is compressed. Furthermore, the soil tends to fail along a localised failure zone, or plane, within the bulk material causing the rubber membrane  3  to further shift and deform. As a result, the movement of the rubber membrane  3  interacts with the soil deformation. As a result, the material behaviour along the localised failure cannot be isolated and measured and bulk measurements of the material sample cannot provide reliable data about the properties determining the emergence of the localised failure. A disadvantage of this device is thus that it is limited to only measuring material properties undergoing low levels of deformation that do not cause material failure. 
         [0007]      FIG. 2  shows another known device for material properties, being a cylindrical Couette shear device  10 . The Couette shear device  10  comprises an inner cylinder  11 , a concentric outer cylinder  12  and an annular testing space  13  formed between the two cylinders. The material to be tested is placed in the annular space  13  and the outer cylinder  12  is rotated relative to the inner cylinder  11 . The rotation creates a continuous shear strain within the material. However, due to the cylindrical geometry, the stresses produced within the material diminish radially from the inner cylinder  11  towards the outer cylinder  12 . As such, the stresses and the strain rate are intrinsically heterogeneous. The device thus has the same disadvantage as the tri-axial apparatus, in that the bulk measurement of strain cannot accurately reflect the true strain gradients within the bulk material due to the non-homogeneity of the material deformation. 
         [0008]    A further known device is a plane Couette device (not shown), which comprises a pair of rotating cylinders and a flexible belt arranged around the cylinders. The belt and cylinders are placed inside a tank and the entire apparatus is filled with water. The belt is driven by one of the cylinders to create a shear flow inside the belt. This device produces a continuous shear flow inside the belt, but it has the disadvantage that the fluid sample is allowed to flow both inside and outside the belt. Any measurement of the shear strength and viscosity of the fluid is therefore not representative for a fixed material sample. This ‘open’ system also makes it almost impossible to control the confining stresses applied to the fluid. This is a major issue for most of the complex materials mentioned above, since the confining stresses play a crucial role in determining their mechanical behaviour and thus interpreting their properties. 
       OBJECT OF THE INVENTION 
       [0009]    It is the object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages. 
       SUMMARY OF THE INVENTION 
       [0010]    There is disclosed herein a device for determining material properties, the device including: 
         [0011]    a first end; 
         [0012]    a second end; 
         [0013]    a bottom panel, extending from the first end to the second end, for supporting a material to be tested; 
         [0014]    a closed loop flexible wall arranged to extend upwardly from the bottom panel; 
         [0015]    a flexible wall idle means disposed towards the first end of the device; 
         [0016]    a flexible wall driving means disposed in spaced relation with the flexible wall idle means; 
         [0017]    the closed loop flexible wall being arranged in operational engagement with the flexible wall idle means and the flexible wall driving means so that movement of the flexible wall driving means causes movement of the closed loop flexible wall along a pre-determined closed loop path; 
         [0018]    means to drive the flexible wall driving means; and 
         [0019]    means for measuring one or more properties of the material to be tested; 
         [0020]    wherein the bottom panel and the closed loop flexible wall together form a material containment area therewithin, and wherein the flexible wall idle means, the flexible wall driving means and the means for driving the flexible wall driving means are each arranged at a position outside of the material containment area. 
         [0021]    An advantage of the device is that a continuous shear strain can be applied to a material to be tested simply by applying a torque to the flexible wall driving means such that it causes movement of the flexible wall in a pre-determined path. The device is capable of shearing a material sample continuously and as homogeneously as possible, whilst also providing the means to impose normal ‘confining’ stresses on the material sample if desired. 
         [0022]    The flexible wall and bottom panel forms a material containment area whose bulk geometry is substantially fixed—it does not substantially deform as the material to be tested deforms under load. As such, the bulk material deformation is not adversely affected by changes to the device geometry, in contrast to the known triaxial apparatus. 
         [0023]    Furthermore, the arrangement of the flexible wall idle means, the flexible wall driving means and the means for driving the flexible wall driving means outside of the material containment area allows the material to flow inside the flexible wall, unhindered by the proximity of the flexible wall driving means and flexible wall idle means, for as long as a torque is applied to the flexible wall driving means. The result is a near homogeneous deformation of the material; the strain gradients across the material at any point within the material containment area are substantially homogeneous throughout. This has the advantage that there are no localised failures and the bulk measurement of a property such as shear strain within the material will be representative for the bulk material sample. Tests can therefore be conducted at higher loads than is the case with either the triaxial apparatus or the cylindrical Couette shear device. 
         [0024]    In a preferred embodiment, the flexible wall driving means is any one of a driving wheel, pulley, gear or cog arranged in operational engagement with the closed loop flexible wall. 
         [0025]    Preferably, the flexible wall is arranged to wrap around the flexible wall driving means towards the second end of the device. Preferably, the position of at least one of the flexible wall driving means and the flexible wall idle means is adjustable along an axis defined between the first end and the second end of the device. Alternatively, the flexible wall driving means is disposed outside of the closed loop flexible wall. 
         [0026]    In a preferred embodiment, the flexible wall idle means is any one of a passive wheel, pulley, gear, or cog. 
         [0027]    Preferably, the closed loop flexible wall is arranged to wrap around the flexible wall idle means towards the first end of the device. In the alternative embodiment in which the flexible wall driving means is disposed outside of the flexible wall, a second flexible wall idle means is preferably disposed towards the second end of the device and the closed loop flexible wall is arranged to wrap around both the first and the second flexible wall idle means. In either embodiment, the flexible wall preferably moves along a rectangular-oval, obround or ‘stadium’ shaped path. 
         [0028]    Preferably, the closed loop flexible wall is notched for engagement with the flexible wall driving means and the flexible wall idle means. 
         [0029]    Preferably, the flexible wall driving means and the flexible wall idle means each comprise one or more teeth at the circumference thereof for engagement with the closed loop flexible wall. 
         [0030]    In a preferred embodiment, the flexible wall comprises a belt. Alternatively, the flexible wall comprises a chain or a set of articulated panels. 
         [0031]    Preferably, the closed loop flexible wall is arranged to include two generally parallel straight sections extending between the first end and the second end of the device and semi-circular recirculation zones at each of the first end and the second end thereof. In a preferred embodiment, a means of applying a lateral inward force to the closed loop flexible wall is provided at at least one location along at least one of the straight sections. 
         [0032]    Preferably, a friction reducing means is provided between the closed loop flexible wall and the means of applying a lateral inward force to the closed loop flexible wall. 
         [0033]    More preferably, the friction reducing means includes any one of at least one roller, at least one ball bearing or a lubricant. 
         [0034]    Preferably, the means for driving the flexible wall driving means includes a motor for applying torque to the belt driving means. More preferably, the motor is a variable speed motor. 
         [0035]    Preferably, the material containment area includes a pre-determined maximum fill level for the material to be tested. 
         [0036]    Preferably, the device includes a means of applying a controlled load to the material from above the device. 
         [0037]    Preferably, the device includes a top panel adapted to be positioned over the material to be tested within the material containment area. 
         [0038]    Preferably, the means for measuring one or more properties of the material to be tested includes at least one of a torque metre, a strain gauge, a load cell, a proximity sensor, a particle image velocimetry apparatus, a thermal camera or an accelerometer. 
         [0039]    Preferably, at least one of the top panel and the bottom panel is made of a transparent material. 
         [0040]    Preferably, the device is housed in an outer sealed enclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    A preferred embodiment of the present invention will now be described, by way of an example only, with reference to the accompanying drawings wherein: 
           [0042]      FIG. 1  is a schematic representation of a prior art device; 
           [0043]      FIG. 2  is a schematic representation of another prior art device; 
           [0044]      FIG. 3  is a schematic representation of a device in accordance with the present invention; 
           [0045]      FIG. 4  is a schematic representation of the device of  FIG. 3  along a section A-A; 
           [0046]      FIG. 5  is a schematic representation of the device of  FIG. 3  along a section B-B; 
           [0047]      FIG. 6  is a schematic representation of the device of  FIG. 3 , including a top panel; 
           [0048]      FIG. 7  is a schematic representation of the device of  FIG. 6  along a section A-A; 
           [0049]      FIG. 8  is a schematic representation of the device of  FIG. 6  along a section B-B, shown enclosed in a confining case (lid removed); 
           [0050]      FIG. 9  is a schematic representation of the device of  FIG. 8  shown with the lid in place; and 
           [0051]      FIG. 10  is a photograph of an embodiment of the device shown without a top panel or confining case. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0052]      FIGS. 3 ,  4 ,  5  and  10 , show a first embodiment of a device  100  for determining material properties. The device  100  has a first end  107  and a second end  109 . The device  100  includes a rigid, elongate fixed bottom panel  105 , seen in  FIGS. 3 ,  4  and  5 . The fixed bottom panel  105  is made of a material that is rigid enough that it does not bend under load. In the embodiment shown, the fixed bottom panel is made of transparent material such as glass. This allows material measurements to be taken through the transparent panel using particle image velocimetry or a thermal imaging camera or the like. 
         [0053]    A closed loop flexible wall  130  extends upwardly from the bottom panel  105 . The flexible wall  130  is moveable along a pre-determined path and is driven by a flexible wall driving means  110 . 
         [0054]    The flexible wall driving means is in the form of a driving wheel  110  that is suspended on a gantry  112  (seen only in  FIG. 10 ) above the first end  107  of the device  100 . The driving wheel  110  has a central axis  115  that is perpendicular to the plane of the fixed bottom panel  105  such that the plane of the wheel  110  itself is parallel to the fixed panel  105 . The driving wheel  110  is operationally connected to a motor  117  (shown in  FIG. 10 ) via one or more gearboxes (not shown). The motor  117  is an electric or thermic motor that is operable at a desired variable speed, or under a desired torque. 
         [0055]    A flexible wall idle means, in the form of an idle wheel  120  having the same diameter as the driving wheel  110 , is suspended on a gantry  122  (not shown) above the second end  109  of the device  100  at the same height as the driving wheel  110 . The idle wheel  120  has a central axis  125  that is perpendicular to the plane of the fixed bottom panel  105  such that the plane of the wheel  120  itself is parallel to the fixed panel  105 . 
         [0056]    The closed loop flexible wall  130  has an upper edge  135  and a lower edge  138 . In this embodiment, the flexible wall  130  is a belt. The upper edge  135  of the belt  130  wraps around each of the driving wheel  110  and the idle wheel  120  such that the closed loop wall  130  has two parallel straight sides and a semi-circular recirculation portion at each of the first end and the second end where the belt engages with the driving wheel  110  and the idle wheel  120 . In use, the belt therefore moves along a rectangular-oval, obround or ‘stadium’ shaped path. 
         [0057]    The area within the belt  130 , above the fixed bottom panel  105  and below the driving wheel  110  and idle wheel  120  forms a material containment area  145 . The material containment area  145  has a maximum fill level, below the driving wheel  110  and the idle wheel  120 , for a material to be tested marked thereon. The maximum fill level is set such that the material within the containment area  145  will be allowed to dilate, without touching the driving wheel  110  and idle wheel  120 . The positioning of the flexible wall idle means, flexible wall driving means and the motor outside the material containment area means that the drive system does not disturb the material to be tested and the material can freely recirculate within the moving flexible wall 
         [0058]    Referring now to  FIGS. 6 to 9 , the device  100  also includes a removable top panel  150 . The top panel  150  is insertable into the material containment area  145  beneath the driving wheel  110  and the idle wheel  120  such that it completely covers the material  101 . The top panel  150  provides a cover for the material and can also be used to impart a normal confining vertical stress to the material. In the embodiment shown, the top panel  150  is made of a transparent material such as glass to facilitate material measurement using particle image velocimetry or a thermal imaging camera or the like. 
         [0059]    The flexible wall or belt  130  is made of a flexible material that is chosen to adequately support the material laterally within the containment area  145  without its geometry altering substantially under load. The belt  130  must however be flexible enough to allow for minor deformation in order to function to transfer a normal lateral force applied to it to the material  101  itself. In the example of the device shown in  FIG. 10 , the belt  130  is a metallic automotive belt. The belt  130  is notched for engagement with the driving wheel  110  and idle wheel  120 . The driving wheel  110  and idle wheel  120  are each provided with a plurality of teeth  118  (see in  FIGS. 6 and 10 ) around the circumference thereof for engagement with the belt  130 . 
         [0060]    In an embodiment, the material of the flexible wall is chosen to have a surface roughness similar to the surface roughness of the tested material. For example, the material of the flexible wall may be chosen to have a roughness similar to a specific material such as concrete or stone when the tested material is sand, gravel, clay or silt. 
         [0061]    In the embodiment shown, a distance between the two straight parallel sections is between about 1 mm to 1 m, although it may be more or less than this. 
         [0062]    The distance between the two re-circulation portions is between about 1 cm to 5 m, although it may be more or less than this. 
         [0063]    The height of the flexible wall is about 1 cm to 1 m, although it may be shorter than 1 cm or taller than 1 m. 
         [0064]    The device  100  dimensions are selected according to the material to be tested. For instance, if a very expensive material is to be tested, a small device is appropriate. A large device is appropriate for testing granular materials having large grains. 
         [0065]    The aspect ratio may be selected to reduce the relative contribution of the flexible wall  130  on the measurements to be taken. A tall flexible wall  130  may limit the relative effect of any friction at the surface of the bottom panel  105  and/or the top panel  150  on the material sample. A small distance between the parallel sections should limit the relative effect of the two re-circulation portions on the material sample. 
         [0066]    In an alternative embodiment, the bottom panel  105  and/or the top panel  150  is made of stainless steel or other material that is not subject to chemical reaction whilst it is in contact with the material to be tested. The device  100  may include more than one idle wheel  120 , more than one driving wheel  110  and more than one motor  117 , if desired. 
         [0067]      FIGS. 4 and 5  show schematically how the device  100  is loaded to create deformation of the material  101  to be tested. 
         [0068]    A torque is applied to the driving wheel  110  via motor  117 . The motor can be operated at constant or variable speed. The torque needed to maintain the constant or variable speed can then be measured by a torque metre. Alternatively, the motor can be operated at a constant or variable torques and the corresponding speed of the belt can be measured via a tachometer. The applied torque causes the driving wheel  110  to rotate (clockwise in the embodiment shown) which in turn causes the belt  130  to move clockwise along the oval/obround shaped path. The belt  130  moves continuously in the oval/obround shaped loop whilst the motor  117  is switched on and is connected to the driving wheel  110 . The parallel motion of the belt walls between the driving wheel  110  and the idle wheel  120  applies a shear strain to the material  101  that is proportional to the speed of the motor  117 . Accordingly, the shear strain rate within the material can be measured or controlled by dividing the velocity difference of the parallel belt walls by their distance. The shear force is measured or controlled by equating the torque measured by the torque metre to the moment from the coupled shear forces exerted on the wheels by the parallel belt walls. Alternatively, strain gauges and a load cell may be applied directly to the parallel belt walls in order to take strain measurements. 
         [0069]    A normal lateral load is applied to the exterior of the belt  130  via lateral load members  140   a ,  140   b  along the elongate sides of the belt between the driving wheel  110  and the idle wheel  120 . Application of the normal lateral force to the belt places the material under normal lateral stress and creates a normal lateral strain in the test material. The normal lateral force is applied using known experimental techniques that are standard in the art. The lateral stress acting normal to the belt walls is obtained by dividing the force applied on the walls by their area. The corresponding normal strain rate is obtained or controlled by dividing the normal relative velocity of the lateral belt walls by their distance. 
         [0070]    Application of a normal lateral load to the belt  130  may generate undesirable friction that will affect the true application of the load to the test material. The friction at the interface between the load members  140   a ,  140   b  and the belt  130  is reduced using one or more rollers  147  positioned externally of the belt as shown in  FIG. 10 , between the belt and the loading members  140   a ,  140   b . In alternative embodiments, the rollers  147  may be replaced with ball bearings, lubricants or other friction reducing means as will be apparent to the skilled person in the art. Where rollers or ball bearings are positioned externally of the belt  130 , they provide an additional rigidity to the belt  130  in functioning to contain the material  101  within the material containment area  145  when the device  100  is under load. 
         [0071]    The top panel  150  itself provides a compressive load above the material  101 . An additional vertical normal force is applied from above the device via a vertical load member  155  attached to the top panel  150  ( FIGS. 7 to 9 ). The application of a normal vertical force places the material under normal vertical stress. As with the lateral load members  140   a  and  140   b , the normal vertical force is applied using known experimental techniques that are standard in the art. Normal vertical stress can be measured or controlled by dividing the normal vertical force by the area of the top panel. Normal vertical strain rate can be measured or controlled by dividing the velocity of the top panel by the sample height. 
         [0072]      FIGS. 8 and 9  show the device  100  housed in an optional confining case  160  that surrounds the entire device  100  such that it is sealed other than for the application of normal lateral, normal vertical and torque loads. In an embodiment, water is added into the casing  160  to generate an isotropic stress on the belt  130  and top panel  150 . Such an experiment can be useful for the testing of materials that are intended for underwater use. 
         [0073]    Furthermore, the confining case  160  provides an extra rigidity to the device  100  and it functions to contain the material  101  being tested in the event that the belt  130  should buckle under load. 
         [0074]    The device  100  may be operated as follows. Firstly, it must be determined as to which mechanical property of the material is to be measured. A corresponding force or deformation is then applied to the material in order to place the material under stress. For example, if it is desired to measure shear strain rate, a torque is applied to the driving wheel to apply a designated shear stress to the material via the belt  130 . Continuous movement of the belt is measured and takes an oval, obround or stadium-like path that causes corresponding continuous flow of the material such that a measurable, near-homogeneous, bulk strain rate develops throughout the bulk material. 
         [0075]    The bulk material boundary formed by the belt  130  does not substantially change under load, therefore measurement of the bulk shear strain will not be affected by changes in the belt geometry. The overall result is a significant improvement in the reliability of the bulk shear strain measurement within the material. 
         [0076]    Similarly, if it is desired to measure lateral strain rate then a designated normal lateral force is applied to the belt  130  using load members  140   a ,  140   b.    
         [0077]    If a normal vertical strain rate is to be measured then a designated compressive vertical force is applied to the material via the top panel  150  and the vertical load member  155 , in such a way that the normal vertical stress is controlled and strain rate is measured. Alternatively, if a normal vertical stress is to be measured then a compressive vertical deformation is applied to the material via controlled motion of the top panel  150  using the vertical load member  155 , and then measuring the corresponding compressive vertical force. 
         [0078]    Each of the forces applied to the material is controllable via adjustment of either the speed of the motor  117  to adjust the torque or adjustment of the normal load applied laterally or vertically to the material. 
         [0079]    A combination of forces can also be applied simultaneously to the material to simulate a desired real life loading situation. As such, the device is very versatile and has widespread applications in many industrial sectors. 
         [0080]    Although the invention has been described with reference to preferred embodiments, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms without departing from the scope of the invention.