Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

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8. FLOW FUNCTION
In the early 1960s the angle of repose was used as an index of flowability of
granular material. In 1961 [74] Jenike published his best known work in which he
proposed direct shear test for estimation of flowability, while his analytical method
provided a physical interpretation of test results. After Jenike’s recommendation,
shear testing to determine the flow function has been widely accepted by researchers
and practitioners and still remains in use. The concept of flow function may be
explained by a theoretical experiment as follows. Let us consider a cylindrical sample
of material compacted under major principal stress, 1 1 , in a container with frictionless
walls (see fig. 8.1). After completing the compaction, the container is removed and
the vertical compressive load required to just crush the sample is measured; that is
equal to the unconfined yield strength of the material, 1 c .
Contrary to the conditions of the shear test, steady state flow cannot be reached
during consolidation, thus the Mohr circle will be smaller. As a result, both density
! b and unconfined yield strength 1 c will be smaller compared to the yield locus
obtained in direct shear testing for strength. The experiment may be repeated for
several values of consolidating pressure and pairs of 1 1 1 c are obtained. A plot of 1 1
against 1 c is termed the flow function for the considered material. The slope of the
linearized flow function:
is termed flowability.
II 1 1 1 c (8.1)
Fig. 8.1. Unconfined yield strength 1 c
In reality the test as illustrated in figure 8.1 would be very difficult to perform for
materials of low cohesion, as a majority of industrial granular materials are. In
practice shear tests are used to determine the flow function. The characterization
of flowability of granular materials by shear testing began with the theory and
apparatus proposed by Jenike [74]. Since that time several other methods were
proposed, but analysis of results of shear test remained essentially unchanged.
Yield loci are determined as shown in figure 8.2 [151, 163].