Abstract:
A test apparatus for measuring the cohesive, adhesive and frictional properties of bulk solids has a bulk solid sample confined between two essentially parallel plates with a single load applied that produces both shear and normal stress and strain to the bulk solid in a manner that nearly uniformly distributes stress throughout the granular solid test region. The apparatus has a close proximity of the plates compared with the length of the plates in the direction of shear, the design of a load hanger to produce a resultant load near the shear plane, the roughening of any surface in contact with the solid that is not intended to have shear on it and involves one-directional movement of the parallel plates with respect to each other and the control of both the direction and magnitude of the applied load relative to the parallel plates. A test method uses the apparatus to measure the unconfined yield strength angle of internal friction and steady state deformation effective angle of internal friction with a single test. A test method uses replacement of the lower part of the test sample with a plate of material that the bulk solid may be required to slip on and measures the friction angle and adhesion of the bulk solid on the plate of material.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to bulk solids and more particularly to measuring cohesive, adhesive and frictional properties of bulk solids. 
   Measuring the cohesive, adhesive and frictional properties (sometimes called the Theological properties) of bulk solids has previously been accomplished by a split ring shear cell to which normal and shear forces are applied to a specimen of particulate solid confined between flat top and bottom disks and at the sides with top and bottom split rings that move with respect to each other. Examples of there testers include the Jenike tester and the Pechel tester. The magnitude of the normal and shear forces applied to the rings then determine the state of stress in the mass of particulates. Sometimes a tri-axial test cell is used in which a cylinder of bulk solids is subjected to a lateral pressure applied through a flexible membrane and an axial force from which the state of stress in determined. Most recently a uni-axial tester is used in which solids are consolidated uni-axially in a cylinder and then sheared along a conical surface that is coaxial with the cylinder. 
   Each of these testers and test methods share similar deficiencies. Each imposes a severely non-uniform stress and strain in the sample during the load application and each is difficult to interpret. For example, the Jenike type split ring shear cell develops a high stress concentration at the front of the ring being pushed by the shear load and at the back of the ring which remains stationary. This concentration is so severe that with cohesive bulk solids, a void often forms at both the front of the bottom ring and the back of the top ring of the test apparatus. This makes a proper analysis of the test results difficult at best and decreases both accuracy and precision of the test results. The split rotational ring tester causes a strain rate gradient from zero at the center to a maximum on the outer edge of the rings thus causing an ill-defined strain and stress state. The Tri-axial tester allows an uncontrolled shear plane to develop during sample failure. The uni-axial tester provides relatively uniform initial compaction. However, during failure a very non-uniform stress occurs at both the top and the bottom of the specimen. 
   All but the uni-axial tester are very difficult to interpret making them time consuming to run. For example, to measure the strength of a bulk solid at a single consolidation pressure with a split ring shear tester requires three to six tests to obtain enough data for proper interpretation. The tri-axial tester requires at least three cumbersome tests for a single strength value. While the uni-axial tester can produce one approximate strength value with a single test, an auxiliary test is required to obtain the internal frictional properties of the bulk solid. 
   There is thus a need in the art for a test apparatus and method for properties of bulk solids that provides uniform stress and strain in the sample during the load application and is uncomplicated to interpret. 
   SUMMARY OF THE INVENTION 
   The present invention advantageously addresses the needs above as well as other needs by providing a test apparatus and method for measuring cohesive, adhesive and frictional properties of bulk solids. 
   In one embodiment, the invention can be characterized as an apparatus for measuring properties of granular solids comprising an upper plate and a lower plate parallel to the upper plate. The upper plate is rotatably attached to a load application bracket such that the lower and upper plate may rotate around the bracket on an axis substantially parallel to a planar surface of the upper plate. 
   In another embodiment, the invention can be characterized as a method of testing bulk solids to determine an unconfined yield strength fc, an angle of internal friction phi and an effective angle of internal friction delta at a major principal consolidation pressure sigma 1 . A bulk solid is placed in a test apparatus as described above. Then, a known consolidation force CF is applied to the upper plate while the direction of applied force CF is substantially perpendicular to a planar surface of the upper plate. The test apparatus is rotated with respect to the direction of the applied force until a steady state movement of the bulk solid occurs between the parallel upper and lower plates of the apparatus. An angle of rotation alpha 1  between the force direction and an axis perpendicular to a planar surface of the upper plate of the apparatus necessary to just maintain steady state movement is recorded. The angle alpha 1  is reduced slightly to stop the steady state movement and maintain the force CF for a desired time of consolidation. The applied force CF is then reduced to near zero. The angle alpha 1  is then increased to a failure angle and the applied force CF is increased to a condition of failure of the specimen at which point the value of the applied force CF at the condition of failure of the specimen is recorded as the failure force FF. 
   Finally, phi, fc, delta and sigma 1  are calculated as follows:
 
phi=Arc tan(( CF  Sin(alpha1)− FF  Sin(alpha2))/( CF  Cos(alpha 1)− FF  Cos(alpha2)))
 
 fc= 2( FF/A )((Sin(alpha2)Cos(phi)−Cos(alpha2)Sin(phi))/(1−Sin(phi)
 
delta=Arc sin(Sin(alpha1)/Cos(alpha1−phi))
 
sigma1=( CF/A )Sin(alpha1)(1+Sin(delta))/Sin(delta))
 
   A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
       FIG. 1  is a side elevation view of a test apparatus in a tilted test position according to the present invention; 
       FIG. 2  is a side elevation view of the test apparatus of  FIG. 1  except a lower half of the bulk solid is replaced by a solid plate of material representing a surface that the bulk solid may be required to slide on; 
       FIG. 3  is a graph of normal stress and shear stress acting on a shear plane between parallel plates of the test apparatus of  FIG. 1  and the relation between these stresses during various stages of the test procedure according to the present invention; 
       FIG. 4  is a side elevation view of the test apparatus of  FIG. 1  showing a means of applying force to the test apparatus of  FIG. 1  using a spring mounted in a circular guide according to the present invention; 
       FIGS. 5A through 5C  are side elevation, front elevation and top planar views, respectively, of the test apparatus of FIG.  1  showing alternate means of applying force according to the present invention; 
       FIG. 5D  is a side elevation view of the test apparatus of  FIGS. 5A-5C  in a tilted position; 
       FIGS. 6A through 6C  are side elevation, front elevation and top planar views, respectively, of a test apparatus according to the present invention for use with bulk solids having a large cohesive strength; 
       FIG. 6D  is a side elevation view of the test apparatus of  FIGS. 6A-6C  in a tilted position and ready for the failure part of the test; 
       FIGS. 7A through 7C  are side elevation, front elevation and top planar views, respectively, of the test apparatus of  FIG. 1  incorporating a test stand capable of applying force, rotating the test apparatus and monitoring the motion of the test apparatus during testing according to the present invention; 
       FIG. 7D  is a side elevation view of the test apparatus of  FIGS. 7A-7C  in a tilted position; and 
       FIGS. 8A and 8B  are side and front elevation views, respectively, of the test apparatus of  FIG. 1  incorporating alternate means of support and rotation according to the present invention. 
   

   Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
   Referring to  FIG. 1 , shown is a side elevation view of a test apparatus  1  in a tilted test position according to the present invention. 
     FIG. 1  shows the basic test unit  1  comprised of a lower plate  2 , an upper plate  3 , and a load application bracket  5  securely attached to the upper plate  3  with a pin connection  6  to the applied force  7 . 
   The entire assembly  1  rotates to an angle  11  with respect to an axis perpendicular to the plates and the direction applied force  7 . The pin connection  6  allows this rotation with minimal frictional resistance and is located approximately at the center of the bulk solid specimen  4  between the plates  2  and  3 . When the applied force is small with respect to the sum of the weight of plate  3 , load bracket  5  and the material above the shear plane  8 , it is essential that the force  7  directions be essentially in the same direction as gravity. The total force  7  acting on the shear plane  8  is then the applied force plus the sum of the weights of plate  2 , bracket  5  and material  4  above the shear plane  8 . If the applied force were not in the direction of gravity, the angle used in the analysis would need to be adjusted to the angle between the axis perpendicular to the plates  2  and  3  and the resulting total force acting on the test specimen. The surface of plates  2  and  3  in contact with the test material  4  must be rough enough to insure that the shear plane remain in the material  4  and not at the surfaces  9  or  10 . This roughness can be achieved by cutting groves in the surfaces  9  and  10  of plates  2  and  3 , by protruding pins from the surfaces  9  and  10  or by coating the surfaces with a rough media such as course sand paper. 
   Referring next to  FIG. 2 , shown is a side elevation view of the test apparatus  1  of  FIG. 1  except a lower half of the bulk solid  4  is replaced by a solid plate of material  12  representing a surface that the bulk solid may be required to slide on. 
   The shear plane  8  now corresponds with the surface of the plate  12 . The roughness of surface  9  is still essential to prevent slippage on surface  9 . 
   Referring next to  FIG. 3 , shown is a graph of normal stress and shear stress acting on a shear plane between parallel plates of the test apparatus  1  of  FIG. 1  and the relation between these stresses during various stages of the test procedure according to the present invention. 
     FIG. 3  shows the relation between shear stress tau and normal stress sigma in the test specimen during the test. The Mohr stress circles such as  16  and  18  in the figure represent the stresses. Two stress states are presented in the diagram. The first is given by the effective yield locus line  29  passing through the axes of the diagram ( 0 ,O) at the effective friction angle  13  (delta). The circle  16  is tangential to the effective yield locus  29 . This circle represents a steady state deformation of a bulk solid at essentially a constant density. Such a state is approximated during flow of a bulk solid in a bin or feeder. The second stress state is given by circles tangent to the yield locus line  14 . In general, this yield locus, characterized by the angle of internal friction angle  15  (phi), does not pass through the axes of the diagram ( 0 ,O) unless the bulk solid is cohesionless (has no strength). Both circles  18  and  16  are tangent to the yield locus  14  because both are conditions of yield. For circle  16  the condition is continuous yield and consequently this circle is also tangent to the effective yield Locus  29 . For circle  18  the yield occurs suddenly as the strength is broken. Circle  18  passes through the axes ( 0 ,O) and consequently represents the unconfined yield stress condition of the bulk solid that has been consolidated by the steady state stress represented by circle  16 . This strength is characterized by the unconfined yield strength fc given by the normal stress point  30 . The consolidation causing fc is characterized by the major principal consolidation stress sigma 1 , the normal stress point  17 . The object of the test using the apparatus in  FIG. 1  is to determine phi, fc, delta and sigma 1 . The test procedure to determine phi, fc, delta and sigma 1  is as follows:
     1. With the angle  11  at zero, apply a known consolidation force CF ( 7 ).   2. Rotate the test apparatus with respect to the direction of the applied force until steady state flow of the bulk solid occurs between the parallel plates  2  and  3  of the apparatus.   3. Record the angle of rotation angle  11  between the force direction and the perpendicular to the plates of the apparatus necessary to just maintain the steady state movement as alpha 1 .   4. Reduce the angle  11  slightly to stop flow and maintain the force  7  for a desired time of consolidation.   5. Reduce the force  7  and angle  11  to near zero and then increase the angle  11  and force  7  to a failure conditions angle alpha 2  and failure force FF. In general there are unlimited alpha, FF combinations that will cause failure. The results will be more accurate if alpha and FF are substantially different from steady state conditions. Alpha 2  will generally be greater than the angle delta ( 13 ) unless the bulk solid is cohesionless and FF will be much less than CF unless the bulk solid is extremely cohesive.   6. The data collected can now be used in  FIG. 3  to determine phi, fc, delta and sigma 1  as follows:   1. Determine the length of line  20  as CF/A where A is the cross-sectional area of the test apparatus plate  3 .   2. Determine the location of point  22  by setting angle  21  to the recorded steady state angle alpha 1 .   3. Determine the length of line  23  as FF/A where A is the cross-sectional area of the test apparatus plate  3 .   4. Determine the location of point  24  by setting angle  25  to the recorded failure angle alpha 2 .   5. Draw line  14  between points  24  and  22  and measure the angle of internal fiction phi as angle  15 .   6. Draw circle  18  tangent to line  24  and through point ( 0 ,O) and determine fc as point  30 .   7. Draw circle  16  tangential to line  14  at point  22  and determine the Effective Yield Locus line  29  and delta angle  13 .   8. Determine sigma 1  at point  17 . The above procedure can be expressed mathematically as follows:
 
phi=Arc tan(( CF  Sin(alpha1)− FF  Sin(alpha2))/( CF  Cos(alpha1)− FF  Cos(alpha2)))
 
 fc= 2( FF/A )(Sin(alpha2)Cos(phi)−Cos(alpha2)Sin(phi))/(1−Sin(phi))
 
delta=Arc sin(Sin(alpha1)/Cos(alpha1−phi))
 
sigma1=( CF/A )Sin(alpha1)(1+Sin(delta))/(Cos(phi)Sin(delta))
   
   In some cases it may not be desirable to compact the bulk solid using a shearing action since this will not duplicate the desired stress conditions. Examples of this are when the strength required is that of a bulk solid inside of a bin without flow at the walls where the strength must represent the non-flowing material or the strength at a hopper outlet when a bin in initially loaded without flow or when the strength of a tablet that was compressed uni-axial is needed. The procedure in this case is to compact the sample with the angle  11  near zero at a load CFU equivalent to the desired major principal pressure sigma 1 =CFU/A for the required length of time consolidation. The load is reduced to near zero and the angle  11  increased as above to a failure angle alpha 1  and force FF 1 . The test is repeated with the same consolidation load, time, and a different failure angle  11  (alpha 2 ) greater than alpha 1  and a new failure load FF 2  less than FF 1  is then determined. In general there is an unlimited number of alpha, FF combinations that will cause failure. The test accuracy is increased when the two selected are substantially different soon each other with FF less than CFU. The calculation is performed the same as above producing phi and fc as follows:
 
phi=Arc tan(( FF 1 Sin(alpha1)− FF 2 Sin(alpha2))/( FF 1 Cos(alpha1)− FF 1 Cos(alpha2)))
 
 fc= 2( FF/A )(Sin(alpha2)Cos(phi)−Cos(alpha2)Sin(phi)/(1−Sin(phi)
 
   The test procedure to determine the adhesive and the kinematic friction angle or angle of slide between the bulk solid  4  and the plate  12  in  FIG. 2  is simply to apply force  7  (CFA) with angle  11  near zero. CFA/A represents the impact pressure sigma_i of material on the plate. The value of force  7  is then reduced to friction failure force FFF and the angle  11  is increased until failure occurs. This angle alpha_a is the adhesion angle for a bulk solid consolidated under impact pressure sigma_i with pressure FFF/A. The angle  11  is then reduced to that value just required to keep the bulk solid moving on the plate at a constant steady rate. The angle  11  (alpha_f) thus determined is the kinematic (moving) angle of slide or surface friction angle of the bulk solid on the plate tested. 
   Referring next to  FIG. 4 , shown is a side elevation view of the test apparatus  1  of  FIG. 1  showing a means of applying force to the test apparatus of  FIG. 1  using a spring  35  mounted in a circular guide  32  according to the present invention. 
     FIG. 4  shows a version of the apparatus  1  in which the force  7  is applied to the top of the plate  3  by means of a spring  35  that acts between the adjustable nut  33  and the non-attached washer and guide  32 . The shaft  30  runs through the spring  35  and acts to stabilize the compressed spring. The circular guide  36  with its circle centered at the shaft  30  pivot point  6  allows for the adjustment of the angle  11  without changing the spring length and consequently the magnitude of force  7 . Bracket  31  is attached securely to the top plate  3  so as to provide the pivot point  6  as close to the shear plane  8  as possible and thus minimize any misdistribution of the force  7  onto the material  4 . The force  7  is adjusted in magnitude by advancing the nut  33  on the threads  34  of the shaft  30 . 
   Referring next to  FIGS. 5A through 5C , shown are side elevation, front elevation and top planar views, respectively, of the test apparatus of  FIG. 1  showing alternate means of applying force according to the present invention. Also,  FIG. 5D  is a side elevation view of the test apparatus of  FIGS. 5A-5C  in a tilted position 
   This embodiment is used when the bulk solid has little cohesion and might otherwise spill from the edges of the apparatus  1 . The sides  37  are secured to the bottom plate  2  with an increasing distance between them in the direction of movement of top  2  so that end plate  38  is shorter than end plate  39 . The difference in length of plate  38  and  39  need only provide a frictional force relief for material  4  moving with respect to the side plates  37 . 
   It is essential that end plate  39  not protrude up to the shear plane  8  and thus causes a pressure concentration on the sample. The force  7  is applied to the center of the upper plate  3  by means of a yoke composed of pivoting side supports  40  and connecting cross support  41 . As with other versions of the apparatus, surfaces  9  and  10  must be rough. 
   Referring next to  FIGS. 6A through 6C , shown are side elevation, front elevation and top planar views, respectively, of a test apparatus according to the present invention for use with bulk solids having a large cohesive strength. Also,  FIG. 6D  is a side elevation view of the test apparatus of  FIGS. 6A-6C  in a tilted position and ready for the failure part of the test. 
   In this case, a solid hollow cylinder  37 , 38 , 39  that totally enshrouds the material  4  during compaction acts as the side and end containment plates  37 , 38  and  39 . The lower plate  2  fits inside of the shroud to allow slippage during compaction and thus create a more uniform compaction of the sample as it compresses significantly during the uni-axial compaction stage of testing.  FIG. 6D  shows the test apparatus in a tilted position ready for the failure mode of testing with the shroud  37 , 38 , 39  pushed down to expose the shear plane  8 . The test and analysis is preformed in the same way as with the uni-axial compression test described earlier. 
   Referring next to  FIGS. 7A through 7C , shown are side elevation, front elevation and top planar views, respectively, of the test apparatus of  FIG. 1  incorporating a test stand capable of applying force, rotating the test apparatus and monitoring the motion of the test apparatus during testing according to the present invention. Also,  FIG. 7D  is a side elevation view of the test apparatus of  FIGS. 7A-7C  in a tilted position. 
   The apparatus  1  sits on a tilting support plate  42  that is hung from cross-supports  43 . Supports  43  are secured to the end support plates  44  which are in turn supported by the pivot pins  45 . The pivot pins  45  are secured to the major support end plates  46  which are secured to the base plate  56 . A plate  48  holds in position a linear actuator  50  and is supported by cross plates  49  which tie securely to the major support plates  46 . 
   The linear actuator  50  pushes the load cell  51  onto the load yoke cross-support  56  to provide force  7 . The load cell  51  is zeroed to account for the weight of the load yoke supports  40 , the top plate  3  and the weight of the material  4  (shown in  FIG. 1 ) above the shear plane  8  so that the measured force  7  is the total applied force at the shear plane  8 . The purpose of this many faceted support system is to allow the vertical yoke supports  40  to penetrate the support system without interference even when the support plate  42  is tilted. 
   A rod  52  is attached securely to the apparatus top plate  3  so as to move whenever the top plate moves. The rod attaches to the core of the linear variable differential transformer (LVDT)  53  to provide a position of the apparatus top plate  3 . The LVDT  53  is secured to the tilting cross plate  43  by the bracket  54 . A rotational drive  55  provides the necessary control of the rotational motion. The rotational drive is preferably a stepper motor with a counter to sense the angle  11 , but may also be, for example, a hand crank with a protractor to indicate the angle. The actuated load cell could optionally be a stack of weights and the LVDT  53  could also optionally be eliminated and the motion of plate  3  simply observed visually. An actuated load cell that senses the pull on rod  52  and adjusts the allowable position accordingly could optionally be used instead of the LVDT  53 . 
   Referring next to  FIGS. 8A and 8B , shown are side and front elevation views, respectively, of the test apparatus of  FIG. 1  incorporating alternate means of support and rotation according to the present invention. 
   A portion of a cylinder  57  is attached to plate  42  such that when the test unit is rotated on supports  59 , which are mounted on shafts  58 , the rotation occurs essentially about the center of the shear plane  8  at the rotation axes  6  of the load hanger. The motion of rotation is provided by the actuator  55  which turns the shaft  58  that is attached to the supports  59 . 
   While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.