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

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(cereal grain, sand, soil), the stress-strain curves display qualitative similarity
[70, 94, 114]. The wide hysteresis loop during the first loading (curve 0AB) is
dominated by plastic deformation with less contribution of elastic strain. During
repeated loading of the sample, at a load value below the previous maximum, the
material is stiffer than before (curve BC), while after exceeding the maximum
value of the previous loading (section CD) the stiffness of the material decreases
and the material behaves as if never subjected to loading. The behaviour of the
material along the curve sections AB, BC, DE can be described by the modulus of
elasticity K and along the curve 0ADG by the modulus of compressibility E i . The
values of the moduluses are strain dependent.
Fig. 6.1. Stress-strain relation during loading-unloading cycles of wheat grain in uniaxial test
Zhang et al. [174] determined the modulus of volumetric elasticity K and the
modulus of plasticity H o of wheat grain in bulk in the triaxial compression test at
constant value of minor principal stress σ 3 from the following power relations:
K
= K
A
( σ ) ,
o
Pa
3/Pa
(6.2)
H
C
( σ ) ,
o
= BPa
3/Pa
(6.3)
where:
A – exponent of the modulus of elasticity,
B – constant of the modulus of plasticity,
C – exponent of the modulus of plasticity,
K o – constant of the modulus of elasticity,
P a – atmospheric pressure,
σ 3 – minor principal stress.