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

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J.R. Johanson, who has been working with Jenike since 1958, summarized
the deficiencies of shear cell technique in the article of 1992 [77]. One of the
major problems with using the shear cell is that during shear, shear force
concentrates at the front of the shear cell. Both shear force and vertical force are
applied non-uniformly to the sample. According to Johanson, at best the results
represent average stress conditions typically varying from a near zero stress up to
the maximum applied. One of early innovations was applying the shear stress
through both the top cover and the upper ring. This helps distribute the shear
stress but applies a torque to the top disc. This results in concentration of vertical
force at the front of the test cell. The non-uniform stresses in the shear cell also
cause the major principal stress to be undefined. This undefined direction of
principal stresses results in variable “steady state” consolidation and the frequent
scatter in the measured failure values of shear stresses. The Jenike method
indirectly measures the material unconfined yield strength and as such, requires
several test points to establish the yield locus and its accompanying Mohr circle
representing the unconfined yield strength. The variations in consolidation stress
state as well as physical differences from sample to sample cause scatter in the
data points. Objections as cited after Johanson and alike stimulated numerous
researchers to look for more simple methods of examination of flowability. These
methods will be treated in a wider extent in chapter 12.
9. COEFFICIENT OF FRICTION
9.1. Theories of dry friction
Friction is a set of phenomena taking place in the contact area between two
bodies in relative displacement that cause resistance to motion. The measure of
friction is the resultant tangential force acting during relative displacement of the
two bodies. The earliest researchers of friction explained this effect by the
necessity of rising of one of the bodies on the asperities of the other body (Perent,
1704; Euler, 1748 following Hebda and Wachal [62]). In this approach, the
coefficient of friction was equal to the angle of inclination of individual asperity.
The first wider examinations of friction were performed by Guillaume Amontons. In
the publication of 1699 this author formulated two laws of friction that had been
forgotten after being formulated by da Vinci in 15 th century. The relationship 7 1,
where N is normal force, T is tangential force, is coefficient of friction, is known as
the Amontons low of friction and in some applications is used up till now [25].
A different point of view was presented by Desaguliers in 1724 [62]. This
author indicated that smooth surfaces of metals or other substances may be polished