Dear Raquel,

In a manner of speaking, there is another method that can produce results simular to the Jenike shear cell tester and it has been computerized. I am speaking of the Discrete Element Method (DEM).

As I am sure you are aware, many factors control the apparent friction coefficient: rock shapes by size distribution, surface texture or roughness, moisture, elasticity and plasticity, pressure and time history of measurement related to elastic-plastic rheology, and so on.

The apparent friction, excepting relaxation, moisture, electrostatics, magnetics, Van der Wal and Einstein forces falls in the domain of surface asperity and fabric shear strength of the granular structure.

Many of these characteristics can be modelled with DEM and its off-shoots. DEM today can model complex non-round particles of large proportions. Its main advantage is in modeling larger rock size groups above 10 mm to 1000mm and larger. It can also model finer particles. Every thing is dimensionless. A DEM model size is limited to about one million simple spherical particles at present. More complex shapes utilize some of the degrees of freedom. There are other limits to the model size and size variations. Particles can be fractured and chipped.

There are a number of DEM codes that do various parts of the above described model domain. You can visit our website: www.conveyor-dynamics.com for a brief introduction on the shape complexities. Aside from our DEM code see Itasca (google search) or search for Prof. Peter Cundall, and CSIRO Australia Math and Information Sciences Dr. Paul Cleary.

Just a Thought,

Lawrence Nordell

Conveyor Dynamics, Inc.

Bellingham, Washington, USA 98225

ph 360-671-2200

fx 360-671-8450 ■

Mr. L.A. van Wijk,

I agree and disagree with your comment on the use of DEM vs the Jenike shear test. As with the shear test, the fundamentals need calibration. DEM captures the shear fabric dynamics and can separate out many variables at play. In time, a library of data will be avaiable to share.

The most obvious benefit is the treatment of larger particle groups. No such test machine is commercially available. Jenike's tester needs a large number of particles across the shear cell, through its depth, and in compaction treatment. Do we look at the particle shapes that yield differeing apparent friction values or the many other attributes that affect friction measurements?

A significant amount of lab measurements are being compiled to authenticate the DEM procedures. Certainly, actual lab measurements are the belt procedure, if the particle group fits within the test machines specs. However, often is not the case. The machine is then testing a psuedonym. By separating the differing influences, we can better determine which procedure will yield the superior result.

Rheology is another important area where constitutive equations can be experimented with to evaluate relaxation time dependent properties. As more of these tools reach commercial status, researchers will acknowledge the benefits and funding will be available to compile such a library.

With appropriate data and granular physical properties, DEM can capture arching, ratholing, wall boundary stresses, elastic foundation interaction with the granular charge stresses and their interdependences, more physical representation of silo granular flow/stresses with all types of reclaim devices.

The Jenike shear cell tester can not differentiate the influence of charge inertias, such as with many types of compaction. Pressure reducing beams and compaction stresses on the granular charge and walls can be simulated in large containers with DEM.

We can debate DEM's applications vs the shear cell tester. I believe both have their place. I also believe, in the near future, DEM will become the tool of choice when the material properties have been cataloged. The more fundamental problem of predicting the true physics in the infinite number of granular charge configurations and their dynamics, along with optimizing charge and withdrawl methods, DEM will provide the answer.

The 3rd International Conference on DEM being held in Santa Fe, New Mexico, September 23-25, 2002 at the La Fonda hotel. Check the website: www.sandia.gov/dem/ .

With appologies, my eyes are blinded by the glory of DEM.

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com ■