ABSTRACT
In the static case, the tangential spring is incremented, ξ′ = ξ + vttMD, with the time step tMD of the DEM simulation, to be used in the next iteration, and the tangential force, Eq. (3), is used.
In the latter, sliding case, the tangential spring is adjusted to a length, which is consistent with Coulombs condition
with the tangential unit vector, tˆ = f t0 /| f t0 |, defined by the direction of the tangential test force above, and thus the magnitude of the sliding Coulomb force is used. Inserting the new spring length into Eq. (3) leads to f t0 ≈ f dC . Note that f t0 and vt are not necessarily parallel in three dimensions.
