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

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pistons move together. The outer piston receives all the drag effects on the cylinder
wall and this way eliminates any drag effects that act on the inner piston when the
compaction pressure is measured. The sample is consolidated up to a prescribed level
of major principal stress. Once the consolidation is complete, the vertical load is
removed. The lower piston drops to allow the sample to be unconfined. The outer,
upper piston is then raised relative to the inner piston and the shear failure induced by
the downward movement of the inner piston. This way the unconfined yield strength
1 c is directly measured. The author claims that his method gives more accurate results
than the Jenike method while the procedure of determination requires less time, and
states that it allows the researcher, expert, designer, engineer, purchasing agent or
equipment vendor “to access solids flow properties with ease and confidence”.
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Fig. 13.1. Johanson indicizer test cell [77]
The Johanson indicizer was tested by specialists of DuPont [16]. The authors point
out to the importance of flowability determination and inform that in industrial practice
the aim of 95% of examinations of powders is performed in regard to product design
and quality, not for silo design. The authors state that they were very interested in
Johanson’s tester because it was the first unit to be fast in use, relatively simple and
available at a moderate price. Examination of several common powders was
performed and compared with results of the Jenike method. It was found that the
equipment gave highly repeatable results for a relatively large group of materials.
Performing measurement does not require long time or special skills of the technician.
Calculation of unconfined yield strength 1 c using Johanson’s test parameters showed
that the indicizer gave lower values than the direct shear test. When the ratio of