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

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13.1.1. Peschl rotational split level shear tester
Peschl [131] presented a promising proposal of a rotational shear apparatus with
associated method of determination of mechanical properties of powders [132], and a
method of quality control of powders for industrial application [133]. The author
analysed existing equipment and found that some apparatuses overestimate the values
of parameters as compared to values in a full-scale installation, while others underestimate
those values. Peschl, based on his examinations of industrial powders, stated
that rotational split level (RSL) shear testers give values similar to those encountered
in practice. In the RSL apparatus, shearing of the samples takes place through the
rotational movement of the lower part of the sample against the upper part of the
sample. The horizontal cross-section area of the sample equals 30 cm 2 while its
volume is 45 cm 3 . With rotational movement, no limit on shear displacement exists,
and the shear plane forms in the middle of sample height where the disturbances from
horizontal borders of the sample are the least. The author recommends choosing such
experimental procedure that would most closely reflect the conditions of real process
for which the material parameters are determined. Peschl preferred not to suggest
any standard procedure, but offered the possibility of programming of shear
conditions with automatic control of the apparatus. Procedure of data analysis
allows for linear or exponential approximation of yield condition. For majority of
easy flowing or low-cohesion materials, linear approximation is a sufficient solution.
In such a case material parameters have a clear interpretation.
Some materials are characterized by strongly non-linear yield condition in
a range of low normal pressures. For this group Peschl [132] suggested linear approximation
in two intervals. Out of straight line estimated for the range of higher pressure
the higher principal stress 1 1 should be determined, while cohesion c and unconfined
yield stress 1 c should be determined out of the course of a straight line estimated for
the lower range of normal pressure. If the mentioned above do not give satisfactory
accuracy of description of the yield condition, a non-linear approximation using
exponential function is necessary. The method of approximation cannot be settled at
advance, but must be an outcome of consideration of particular case. Peschl claimed
that his method gave the possibility of simulation of all conditions existing in practice,
proper determination of parameters and, consequently, reliable operation of industrial
installations. The author concluded that powder technology gained the level of
engineering science and allowed for design of industrial installations without beforehand
experimenting on equipment and installation.
Flowability is a material property of increasing importance. With passage of
time, this property is more extensively used as a measure of material quality.
Consumers expect constant flowability of washing powder, milk powder or sugar.
For mixing, dosing and packing, stable flowability is a crucial parameter for