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

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7.2. Factors influencing angle of internal friction of cereals
7.2.1. Moisture content of the material
Grain moisture content has deep influence on the mechanical properties of
grain in bulk [67] as it modifies surface properties of seed-coat as well as the
properties of kernel endosperm. Changing moisture content of grain influences
shear stress-strain characteristics, and consequently the determination of strength
parameters: the angle of internal friction φ and the cohesion c. Figure 7.4 presents
data obtained in direct shear test for wheat of moisture contents of 12, 20 and 22%.
The results show that for wheat at 10% of m.c. (fig. 7.4 a) experimental
curves stabilized or attained maximum below the 0.05 ∆L/D level of strain
required by Eurocode 1. An increase in grain moisture content resulted in an increase
in shear path to attain a stable level. In the case of grain of 20% moisture content (fig.
7.4 b) and 100 kPa of normal pressure the test curve stabilized at a stress level clearly
above the 0.05 of the sample diameter. For grain moisture content of 22% (fig. 7.4c)
shear stress stabilized only for the two lowest levels of normal stress and for strain of
approximately equal to 0.05 ∆L/D that precluded determination of the strength
parameters. These results show that to determine strength parameters of wet grain
an extension of shearing path up to 0.10 of sample diameter is necessary.
Shear stress (kPa)
90
80
70
60
50
40
30
20
10
a
0
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0
5
4
3
2
1
Shear stress (kPa)
90
80
70
60
50
40
30
20
10
b
5
4
3
2
1
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
0 0
0
Shear stress (kPa)
90
80
70
60
50
40
30
20
10
0
c
4
3
2
1
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
5
Fig. 7.4. Shear stress τ versus ratio of displacement to the sample diameter ∆L/D for wheat of
moisture contents of : a) 12%, b) 20% and c) 22%, for normal stress σ n of: 1-20 kPa, 2-40 kPa,
3-60 kPa, 4-80 kPa and 5-100 kPa