2. Experimental SRST
2.1 Material
Glass beads in size ranges of 30-50µm (Polysciences Inc. 18901-100, lot
685012) and 150-210µm (Polysciences Inc. 05483-250, lot A760996) were
acquired from Polysciences Inc. A particle density of
2500kg/m3 is used in both the DEM simulation and
experimental analysis, consistent with averaged reported values from
vendor specifications.
Prior to testing, samples were screened using a #100 US standard sieve
(150µm opening). Images of each bead type were taken at 200x
magnification using a Keyence VHX-2000 optical microscope. Binary blends
of small and large glass beads were prepared at ratios of 3:1, 1:1, and
1:3 on a mass basis, and prepared at a 30g scale. Weighed and screened
materials were transferred to a 135ml glass container and mixed using a
Turbula mixer at 32rpm for 5min.
2.2 Shear testing
Ring shear testing of each blend was performed on the Schulze RST-XS
using the XS-MV4 shear cell. The shear cell is annular in shape with an
inner diameter of 32mm and an outer diameter of 64mm. The initial gap
between the base and the lid is 4mm; the size of this gap decreases as
pressure is applied to the shear cell and particle mixtures are
compressed. The shearing bar of the XS-MV4 cell is pyramidal in shape
and covers the surface of both the base and the lid.
Shear measurements were performed using a shear rate of
0.004s-1 and a 2000 Pa pre-shear load, with subsequent
consolidation loads of 400, 700, 1000, 1300, and 1600 Pa. For this work,
the shear data and the solid fraction data taken at the pre-shear
condition were used to compare to the DEM simulation. Two replicates
were performed for each blend condition, with new particle mixtures
prepared to generate the replicate data.
The segregation of the binary blend is very apparent. After blending,
there are regions in the glass jar where the small and large beads are
separated. Given the large size ratio (approximately 4.5), it is very
difficult to mix the particle into a random mixture. Segregation is an
inherent factor to the shear test and it was not possible to completely
eliminate from our experiment. The shear cell was filled by taking
multiple scoops of blend from the bulk mixture, with each scoop taken
from a random location within the blending vessel. Segregation is
observed even when filling the shear cell, the area of segregated
particle can be seen when preparing the shear cell. Figure 1 presents an
image taken on the surface of the shear cell before testing. All shear
tests were performed on the material as is. Due to the method of fill
for the shear cell (scooping from a mixture prone to segregation), the
content of the shear cell is emptied out after the shear test. The
content is again sieved through a #100 US standard sieve, separating
them into smaller (<150µm) and larger (≥150µm) segments. The
mass of each sample is then determined to verify the mixture
composition.