Cohesive, frictional powders: contact models for tension

Abstract

The contacts between cohesive, frictional particles with sizes in the range 0.1-10 µm are the subject of this study. Discrete element model (DEM) simulations rely on realistic contact force models-however, too much details make both implementation and interpretation prohibitively difficult. A rather simple, objective contact model is presented, involving the physical properties of elastic-plastic repulsion, dissipation, adhesion, friction as well as rollingand torsion-resistance. This contact model allows to model bulk properties like friction, cohesion and yield-surfaces. Very loose packings and even fractal agglomerates have been reported in earlier work. The same model also allows for pressure-sintering and tensile strength tests as presented in this study.

Sound wave propagation in weakly polydisperse granular materials

“…Note that the above equation is only valid for isotropic bodies, like spheres as also used in the following [17,20].…”

“…For the simulations where friction is active, the force in the tangential direction, f t , is implemented in the spirit of Ref. [18], where a tangential spring (with stiffness k t = 0.2k) was introduced to account for static friction [17]. The tangential contact forces are related to the normal forces via Coulombs law, i.e., f t ≤ µf n , with µ = 0.5 for both sliding and sticking.…”

“…Relating the layer distance l z = z 1 − z 0 = √ 2a 0 and the "contact duration" (Ref. [17]) t c ≈ 2.03 10 −5 s, leads to a velocity v c = l z /t c ≈ 70.7 m/s. The chosen contact stiffness, k (=10 5 N/m), as mentioned earlier, correspond to the stiffness of a Plexiglas bead.…”

“…[17], the overall frequency spectrum extends to higher frequencies, around 54 kHz, as a consequence of the tangential spring at the contact, which induces a higher particle (or layer) "eigenfrequency". However, the higher frequencies of the wave vanish earlier than in the frictionless case (between layers 15-25), while the remaining low frequency band is broader and extends until about 12 kHz, see Fig.…”

“…The use of a three-dimensional discrete element method (DEM) [7,17,18,19,20,21] allows the study of the role of various micro-parameters in detail. Except for a regular crystal structure of exactly equal sized particles, geometrical (and hence contact) disorder is intrinsic to granular systems.…”