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

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mechanical theory of friction suggesting a “dual nature of friction”. Later investigations
by numerous researchers did not lead to any general theory of friction.
Currently two types of coefficients of friction are used [19], one that represents
friction opposing the onset of relative motion, and one that represents friction
opposing the continuance of relative motion once that motion has started. The former
is called static coefficient of friction, and the latter – kinetic coefficient of friction. It is
currently widely accepted that friction is not an intrinsic material property of the two
contacting materials. The system approach has became a tool for the interpretation
and use of friction data in modelling friction, developing friction mitigating materials,
developing friction test methods, and designing machinery.
9.2. Experimental Methods
Testing the friction of granular materials requires an apparatus in which relative
motion of the material and a sample of construction material takes place. Relative
motion may be rectilinear or rotary. In the case of rectilinear motion the sliding surface
has the shape of a flat plate or band, while in the case of rotary motion it has the shape
of a disc or cylinder. In figure 9.1 apparati used by various authors are presented
as reported in literature [11, 24, 26, 46, 52, 89, 97, 117, 141, 142, 151, 153, 161, 164].
The choice of a specific shape of sliding surface decides on important features of
the measuring system. Apparatuses with the flat plate (see fig 9.1 a, b) assure
uniform distribution of sliding velocity, uniform distribution of normal pressure and
easy interchange of sliding surface. However, sliding velocity and sliding path are
limited. Use of continuous band (see fig. 9.1 e) allows for uniform distribution of
velocity and pressure with higher sliding speed and unlimited sliding path.
However, frictional element in this shape is susceptible to vibration and may be
produced only out of flexible materials. The shape of cylinder (see fig. 9.1 f)
assures uniform distribution of sliding speed, but distribution of pressure is uneven,
and obtaining interchangeable frictional surface poses a hard task to design and
machine. Disc rotating around vertical axis (see fig. 9.1 c, d) allows for easy change
of sliding surface, unlimited sliding path, high sliding velocity and uniform
distribution of pressure. However, sliding velocity varies along the disc radius.
Values of coefficient of wall friction presented in the Appendix of this study
have been determined following Eurocode 1 [50]. The test apparatus is a cylindrical
shear cell as shown in figure 9.2. The diameter of the cylindrical shear cell
should be at least 20 times of the maximum particle size and not less than 40
times the mean particle size. The compacted height H of the sample should be
between 0.15D and 0.2D.