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

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7.2.2. Bulk density
Stress-strain behaviour of granular material depends on the bulk density of
the sample. Dense samples dilate during shear test while loose samples decrease
in volume. In dense samples shear stress attains a peak value, and with continuing
shear displacement it drops back to a lower ultimate value and remains at that
constant level during further shear. In the loose state most granular materials tend
to decrease in volume when subjected to shear under constant normal load. For
such samples shear stress gradually increases until it reaches ultimate value.
Thereafter, with increasing displacement it remains stable. The density at which
material is sheared without change in volume is termed, after Cassagrande [30],
the critical density.
Fig. 7.5. Relationships of stress versus strain obtained in triaxial compression test for wheat grain
of moisture content of 13% and four levels of bulk density
Figure 7.5 illustrates relationships between axial total stress and axial strain in
triaxial compression of wheat grain of 13% in moisture content at four levels of bulk
density [101]. The graphs show that increase in density ρ resulted in a quantitative
change in material properties. For denser samples higher were both the maximum
value of σ 1 and the maximum value of axial strain ε 1 . For the sample of bulk density
of 822kg m -3 the axial stress reached maximum value σ 1max for ε 1 of 0.1. For the
sample of the lowest density (of 729 kg m -3 ) the stress σ 1max was attained at ε 1 of 0.22,
thus at the strain about four times higher than ε 1 of 0.06 recommended by Eurocode 1.