Figure 6. Flow function coefficient and packing density of the binary blends measured using the SRST.
While comparing FFc and packing density, indeed a correlation can be found as suggested by Vallejo12. This confirms that for a mixture of similar materials such as glass beads, packing has a significant influence on powder flowability. Increased packing is associated with decreased flowability and lower FFc. The contribution of adhesion and friction to FFc are further investigated.

4.1.2 Relationship between tensile strength and packing density

Figure 7 shows the key output from the shear test with its interrelationships. Figure 7 (a) shows that the FFc relationship mirrors that of the unconfined yield strength (σu) relationship. In theory, the total strength of the material should be the summation of the adhesion component and the frictional component of the system. The adhesive or tensile component (σt) is shown in Figure 7 (b). It is found that tensile strength (σt), which represents the stress required to fail the powder in tension, correlates with the degree of packing of the powder. As the degree of packing of the powder increases, greater forces are required to separate particles from their neighbors, increasing the overall tensile strength of the powder. The results are consistent with Rumpf’s theory of tensile strength 21.

4.1.3 Relationship between internal angle of friction φ and packing density

We further explore the relationship between the internal angle of friction φ and the packing density (Figure 7 (c)); φ shows the most deviation with the packing density. While φ shows a linear relationship with the packing density for 0% to 50% large glass beads, the results deviate from the packing density as the large glass bead fraction is increased from 50% to 75%. This seems counterintuitive, as φ still increases with decreasing packing density. On further increase of large glass beads beyond 75%, the φ decreases with decreasing packing density. This behavior is explored in sections 4.2-4.4 by simulating steady state shearing using DEM.
The steady state shear stress (τpre) correlates well with φ (see Fig 7(c)). This can be described mathematically using the Mohr Coulomb failure criterion:
\(\tau=\sigma tan\left(\varphi\right)+\tau_{c}\) (4)
Where τc is the cohesion term which is directly related to σt by a trigonometric relationship. For glass beads, the (σpre, τpre) point is colinear to the yield loci. Based on equation 4, the τ term is dominated by\(\text{σtan}\left(\varphi\right)\); therefore, the τpre is φ weighted by 2000 Pa.