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

shearing. In the triaxial compression test, the angle φ increased with increasing
values of angle γ 1 – of granule long axis inclination to the slip plane. The trend was
observed for all the cereal species under study. The relation φ(γ 2 ) obtained in the
direct shear test had an approximately parabolic form, with a minimum for the angle
γ 2 ≈ 25 o . The divergence between the relations φ(γ 1 ) and φ(γ 2 ) results from
differences in the mechanisms of sample deformation in the tests compared. In the
triaxial compression test, the slip plane forms freely, conforming to the state of stress
and to the structure of the material, while in the direct shear test the slip direction is
forced. In the latter test, the process of shearing is additionally complicated by the
non-uniform state of deformation and by the anisotropy of the material; the
displacement and the rotation of individual grains are forced. Figure 3.7c presents
schematically the two components of the shearing process under macroscopic
observation. The minimum of the angle of friction φ obtained with the method of
direct shearing should be interpreted as the result of optimum spatial distribution of
grains under conditions of forces direction of sample shearing.
The results of the experiments described above permit the formulation of
several methodological remarks concerning the measurement techniques. Due to the
relatively large dimensions of the grains, it is recommended to increase the size of
samples with relation to those routinely used for soils. It appears that the sample
dimensions used in the studies presented here (triaxial sample: D = 150 mm,
H = 300 mm, direct shear test sample: D = 210 mm, H = 120 mm) are sufficient.
Especially worthy of recommendation for measurement of the angle of internal
friction is the triaxial compression method, though in the case of spherical granules
the less complex direct shear test yields similar results. The recommendation of the
triaxial compression method is also supported by the fact that the method has been
frequently used for the determination of theoretical and empirical parameters of
models describing the stress-strain relation. The method was used by Zhang et al.
[173] and by Li et al. [94] for the determination of parameters of the elasto-plastic
model adapted by the authors for the description of the stress-strain relation in wheat
grain. In that model, formulated by Lade [92] for cohesionless sand, the total
increase of strain caused by increase in stress equals the sum of three components:
increase in elastic strain, increase in plastic strain caused by normal stress, and
increase in plastic strain caused by stress deviator. The values of model parameters
determined in the triaxial compression test permit an accurate description of the
response of a medium to other loading conditions. For the description of anisotropy
and hysteresis in the stress-strain relation in the case of multiple loading of wheat
grain, Zhang et al. [173] used an elasto-plastic model that included the density and
kinematic hardening of the medium. The determination of the values of the
parameters of the model also involved the application of the triaxial compression
test. Likewise, the method of triaxial compression was applied by Zhang and Jofriet
27