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

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N=σ r
πD 2
4
T = τ πD 2
4
ro u g h
H
Fig. 7.1. Jenike shear cell
D
The up-to-date method of testing flow properties is based on the concept of
Jenike, first published in 1961. The apparatus consists of the lower ring, the upper
ring and the base. The chamber of the apparatus, comprised of the lower and
upper ring, is filled with a sample of granular material. The lid is loaded with
vertical force N and horizontal force is applied on a bracket attached to the lid.
Shear tests performed with identically consolidated samples under different
normal loads give maximum shear forces T for every normal force N. Ratios of
forces N and T to the shear cell cross-section area give normal stress σ and shear
stress τ. Characteristic of τ versus σ (see fig. 8.2) represents the maximum shear
stress that a sample can support under a certain normal stress and is called yield
locus. Material bulk density ρ is the parameter of the yield locus. With an increase
in normal consolidation stress, bulk density increases and the yield locus moves
upwards. Each yield locus terminates at a point E in the direction of increasing
normal stress σ. The conditions of point E are called steady state flow, that is the
flow with no change in bulk density and stresses.
The original procedure of the shear test was as follows:
1. A prescribed mass of material was placed into the compartment of the
apparatus;
2. Vertical consolidation reference pressure, σ r , was applied for a prescribed
period of time;
3. Sample was sheared until an asymptotic value of frictional force (steady
flow) was approached, thus values of σ r and τ a stresses at the terminus of
yield locus were determined;
4. Steps (1) and (2) were repeated;
5. Sample was sheared until 95% of an asymptotic value of frictional force
was achieved;