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Generation of Explosible Dust Clouds 209

referred to a specific powder sample for which olis the maximum principal consolida-
tion stress during preparation of the sample.The porosity (and bulk density) of the spe-
cific powder in question is a unique function of 0,. S is the tensile force, N the normal
force, and A is the area of the powder specimen in the shearing plane. The quantityfc is
the maximum principal stress at failure when the powder sample is in a situation where
the minor principal stress is 0. Here, o,is the tensile strength of the powder sample and
c is the cohesion, defined as the shear strength of the powder sample at zero normal load.
For a given type of cohesive powder, there exists a continuousrange of yield loci, each
locus characterized by a given porosity ~(o,).Further,f,, the cohesion c, and the tensile
strength o,increase systematically with decreasing E or increasing 0,. The straight line
z= oArx tan qeis called the efective yieZd locus. The angle qeis a measure of the inter-
nal friction in the powder during steady flow (plastic deformation).
For a noncohesive, free-flowing powder, the yield locus and the effective yield locus
coincide and pass through the origin, and both 0,and c are 0.

3.4.2.2
Shear Cells

Yield loci as illustrated in Figure 3.4 are determinedby means of shear cells.A cross sec-
tion of the well-known Jenike cell is shown in Figure 3.5.

Figure 3.5 Vertical cross section of the Jenike
shear cell for measuring the mechanical strength
of powders. All dimensions are in mm (From
Schwedes, 1976).

This cylindrical cell of 95 mm diameter is split, and the upper ring can be pushed hor-
izontally in relation to the lower, fixed part. The test procedure for obtaining a point on
a yield locus (Figure 3.4) consists of two steps. First, the powder is consolidated during
plastic flow to a given porosity E under the action of a major principal stress 0,. In the
second step, the sample is shear strained at a constant strain rate, while being com-
pressed by a constant normal stress o, =N/A, where N is the normal force and A is the
cross section of the cell (71 cm2).The shear force S, which is recorded continuously
during the process, increases with the strain to a maximum value, at which the powder
sample fails and S drops suddenly. This maximum value of S defines the z= S/A value
that, together with the corresponding o, =N/A, gives a point on the yield locus. By shear-
ing identical powder samples (the same ~(q)),at different o,, the entire yield locus is
determined.
In the context of dust dispersibility, the mechanical “strength” of a given powder,
consolidated to a given porosity E by a major principal stress o,,can be characterized