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

58
near the box wall and lifted along the wall generatrix so that the outlet was constantly
approximately 20 mm above the surface of gathered grains. The grain was allowed to
slide down the surface of natural repose to the front side of the box. Any change in
the angle of inclination of the shear box to the horizontal resulted in a change of
preferred inclination γ of long axes of grains to the bottom of the box.
Examination of an influence of the angle of preferred grain orientation on the
shear stress-strain characteristic was performed under the normal pressure of 100 kPa.
Samples were prepared so that the preferred orientation of grain formed angles, γ, of
inclination to the shearing direction of: 0, 10, 20, 30 and 40 degrees. To preserve the
packing structure of the bedding no additional consolidation was applied. The angle
of preferred orientation of grains was found to influence strongly the stress-strain
characteristics as shown in figure 7.7. The strongest was the sample with γ of 40°
showing maximum shear stress τ max of approximately 50 kPa, while the lowest τ max of
approximately 30 kPa was found in the case of γ = 0°. The probable reason for the
observed difference of behaviour is the distribution of contact normals.
Fig 7.7. Relationships of shear stress versus ratio of displacement to the sample diameter ∆L/D.
Samples of rye with an angle of preferred inclination of kernels long axes to horizontal ranging
from 0 to 40 degrees
Oda [126] suggested that, for a two dimensional system, the distribution of contact
normals may be approximated by an ellipse. The major axis of such an ellipse is
initially oriented normal to the bedding plane. The preferred orientation of the long
axes of the particles is parallel to the bedding plane. In the process of biaxial
deformation the distribution of the contact normals changes so that a greater number
of contact normals tend to orient themselves in the direction of contact force.
Testing with grain samples deposited in various ways and subsequently consolidated
by twisting (as recommended by Eurocode 1 [50]) did not show any
differences in stress-strain characteristics, and consequently in obtained values of