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

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Numerous granular materials when consolidated reveal cohesion that
allows to maintain shape enforced under load. Ratholes or channels may
be formed in granular materials but not in liquids.
No fundamental mechanical model is currently available to describe behaviour
of granular materials. The lack of precise description of material behaviour results
in serious practical problems. Unpredictability leads to clogged chutes and
catastrophic failures of industrial silos. Mechanical properties of materials stored
in silos influence pressures exerted on the walls and flow patterns developing
during discharge. Variation in raw material properties may result in a lack of
reliability and repeatability of final product properties that may cause high costs
in food industry, but may be disastrous in the case of pharmaceutical product.
In pharmaceutical industry uniform mixing of medicinal components may be
critical, as well. Considering that granular materials are so widespread and their
use in industry increasing further understanding of how these media behave can
have a profound impact on economy worldwide. Research in this area that has
been intensively conducted in last 40 years had important implications for
manufacturing and new processes.
Background for the recent development in granular mechanics and technology
has been given by results of investigations of Andrew Jenike presented
in „Gravity flow of bulk solids” published in 1961 [74]. Although at that time it
was clear that most of processing industries dealt with flow of granular materials
Jenike’s book was the first comprehensive study of the subject. The fact that the
work appeared at that time stemmed from the progress in theory of plasticity and
in techniques of numerical calculations that had taken place in former fifteen
years [74]. Jenike adopted testing technique and some concepts of soil mechanics
but his creative input was substantial in that he analyzed granular material under
100 to 1000 lower load. In such conditions some effects that were never observed in
soil mechanics got great importance. One example is curvature of the envelope of
Mohr circles in (σ,τ) coordinates with no meaning in soil mechanics and of crucial
importance for determination of flowability of granular material.
For Jenike silo technique was natural field of application of the theory where he
contributed significantly. After 40 years of work in numerous laboratories specialists
achieved agreement in some questions and a set of national and international codes of
practice as in the case of silo design: American ACI 313-91 [1] and ASAE EP433 [4];
Australian AS 3774 [10], Polish PN-B-03254 [135], and European Eurocode 1 [50].
Usually code of practice are contains standard procedures for determination of
mechanical properties of stored granular materials.
The earliest approach of scientists for predicting behaviours of bulk solids
was continuum theory that looks at a volume of material as a whole, as a solid