In previous articles, Mass Flow Design Considerations, (Arching Parts 1 & 2), we discussed interlocking and cohesive arching. We realize that cohesive arching is more prevalent than interlocking arching and, as such, we need to be able to determine the cohesive strength of our bulk solid.

Cohesive strength is measured using a bench scale laboratory testing device such as a direct shear tester (Jenike Shear Tester). This device is used to determine a material's "Flow Function" (strength, pressure relationship).

The material's cohesive strength is measured as a function of applied consolidation pressure. In a laboratory, a sample of the material is placed in a shear cell on a direct shear tester and both compressive and shear loads are applied to simulate flow conditions in a bin.

Once the material in the shear cell is consolidated, its strength is measured by shearing it to failure. By repeating this procedure under different conditions the resulting value of strength vs. consolidating pressure can be developed. The process is fairly straightforward; however, it takes quite some time to simulate a range of pressures acting in a bin or silo.

Several test points are developed for each level of pressure. These test points form a yield locus that indicates the strength of the material at that particular pressure level. Typically, three pressure levels are used to simulate the range of pressures that are representative of the pressures the material will experience in storage.

This test procedure allows us to simulate several other conditions which effect material flowability. We can control the sample's moisture content and particle size while the direct shear tester allows us to simulate the effects of temperature and time of storage at rest. In our next article, we will discuss the effects of temperature and time of storage at rest on material flowability.