Brookfield PFT Powder Flow Tester Operating Instructions Manual page 55

Note: Two types of hopper shape are considered: conical hoppers and wedge (or plane)
hoppers as shown in Figure A-3a & A-3b.
Key differences between powders and fluids
For any fluid the resistance to shear (viscosity) is independent of the normal pressure but
dependent on the shear rate. In powders the effect of these factors is reversed so that shear stress
of a powder is strongly dependent on the normal stress but independent of the shear rate. Hence
when characterizing powders, test are undertaken at a single speed but over a range of normal
stresses. The other key difference is that powders are anisotropic so the stresses are not equal in
all directions and are frictional so that they can generate shear stresses at wall boundaries (see
wall friction section).
Flow function test
The primary measure of powder flowability is the powder flow function – which gives a measure
of the amount of strength the material retains at a stress free surface following consolidation
to a given stress level. The simplest way of explaining the flow function is with the uniaxial
unconfined failure test shown in Figure A-4, which measures the strength of a free standing
column of powder. This condition is analogous to the condition of the powder arch across a
hopper outlet shown in Figure A-2b.
i) Consolidation of sample. Powder is placed in a cylindrical cell and compacted under
a known normal stress σ
ii) Unconfined sample. The mold is now carefully removed to reveal a compacted column
of powder.
iii) Unconfined failure of sample. The normal stress acting on the column of powder is
gradually increased until failure occurs, and the peak normal stress σ
The above uniaxial unconfined failure test is conducted over a range of consolidation stresses
and the flow function is constructed by plotting the unconfined failure strength versus the
consolidation stress as shown in Figure A-5.
AMETEK Brookfield
.
1
σ
1
Figure A-4: Uniaxial unconfined failure test
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σ
3
=0
σ
3
is recorded.
c
Manual No. M09-1200-F1016