C - Lublin

METHOD AND MATERIALS
The shear strength parameters of food powders were determined according to
the Eurocode 1 [3] recommendations using the Jenike [5] shear tester. The effective
angle of internal friction δ, the angle of internal friction ϕ, the cohesion c and
the flow index i were determined from experiments performed in the tester of
60 mm in diameter. A displacement velocity was equal to 0.03 mm⋅s -1 . The sample
was consolidated under the reference normal stress σ n of 30, 60, 80 and 100 kPa. In
the case of powders which reveal the tangent stress oscillation the reference normal
stress was enlarged to about 200 kPa to verify the influence of the consolidation
pressure. The standardized twisting of the top plate of the shear box were applied
to consolidate the sample. Two shear tests were carried out for each variant of experiments.
One sample was sheared when loaded to the reference stress. The other
was sheared at half of the reference stress after pre-loading to the reference stress
(Fig. 1). Tests were performed in the controlled temperature (20 o C) and air relative
humidity (65%) laboratory. Tests were performed for 12 grocery food powders:
products of cereal grain, milk, sugar and kitchen salt.
N=σ πD 2
n
4
τ
τ 0
0
τ
τ 0
0
T
τ 1
1
τ 1
1
ϕ
∆L
c
δ
1/2σ n
σ n
σ
RESULTS
Fig. 1. Jenike shear tester and procedure of the strength testing [3].
The effective angle of internal friction δ of all tested materials was found to be
in the range of 30-35 deg. mainly (Table 1). There was no clear influence of the
consolidation pressure. The largest value of the effective angle of internal friction
(approx. 40 deg.) was obtained for potato starch and agglomerated milk for the
lowest value of the consolidation pressure. The smallest was for coarse wheat flour
(27 deg.) for the highest value of the consolidation pressure. About twice higher
standard deviation of the effective angle of internal friction for coarse sugar as
58