C - Lublin

Some powder materials exhibit tendency to change volume during shear,
known as dilatancy. Strength and resistance to slow deformation can be profoundly
affected by dilation [1,2,4]. Sequences of compaction-dilation events were thought
to be the most probable source of the shear stress oscillations. Oscillation can be
considered as part of a sequence of compaction-dilation events occurring around
the area of shear zones developing in the material. Compaction of the particulate
material resulted in an increase in material strength and the ability to withstand
higher shear loads. When the maximum strength of the particulate material is exceeded
dilation in the shear zone occurs resulting in a sharp decrease in the shear
load. This leads to limiting mechanisms of slow dilatant plastic shear deformation.
Pitch of oscillation [mm]
0.6
0.5
0.4
0.3
0.2
0.1
σ n
agglomerated milk
fine milk
potato starch
0
0 50 100 150 200 250 300
Normal stress [kPa]
Fig. 4. Pitch of the tangent stress oscillation as influenced by the normal stress.
Based on the saw blades model of dilatancy proposed by Bolton [1] the two
following strength components were determined from the tangent stress – shear
displacement relationships: the physical friction strength and the extra component
of strength related to dilation. It was assumed that the minimum value of tangent
stress during oscillation corresponds to a critical state of the sample with zero dilation.
Contribution of the stress oscillation to the total strength was approximately
30% for the agglomerated milk, 35% for the fine milk and 45% for the potato
starch.
CONCLUSIONS
The effective angle of internal friction δ of all tested materials was found to be
in the range of 30-35 deg. There was no clear influence of the consolidation pressure.
The angle of internal friction ϕ was smaller than the effective angle of friction
δ of approximately 0.2-5 deg.
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