Software: PFC: Pre-Release 4.0
PFC - Enhanced FishTank
The new material-genesis procedure employs the following steps:
1. The material vessel is filled with a dense packing of frictionless particles at an initial particle size distribution throughout the material vessel.
2. The particle sizes can be refined in regions that are defined by named ranges. The refinement regions and levels (0 = no refinement) to produce a graded assembly with smaller particles inside the refinement regions are shown in the image below. Each particle in a refinement region is replaced by two smaller particles that have the same total volume as the original particle, and this replacement process is repeated for each particle until the specified refinement level is reached.
Left: the refinement regions and the level of refinement (l) applied to the particles. Middle: the particles in each refinement region that will be affected are shown. Right: The resulting particles after refinement has been completed.
3. If the initial grain shape is a clump, then each particle in the system described in steps 1 and 2 above is replaced by a clump with the same volume as that particle. The clump is easily defined using a clump template that specifies radius and position of any number of particles composing the clump. The procedure automatically calculates the volume of the clump, computes the scale factor and scales the clump, and randomly orients and positions the clump centroid at the centroid of the original particle.
4. A specified isotropic stress is obtained (Potyondy and Cundall, 2004). If the grain shape is circular or spherical, then floating particles (those with less than a user-specified number of contacts, usually set to three) are removed. The final specimen is produced by adding contact and/or parallel bonds (if desired).
Left: the particles after the clump-insertion procedure. Middle: the material after frictionless packing. Right: clumps and contact forces after isotropic stress installation (bonds will be added at all contacts with a non-zero force).
Together, particle-size refinement and clumping can have significant impact on specimen properties, including friction angle, coordination number, porosity, stiffness, etc.
The Effect of Clumps
Defining complex clump shapes can be done by simply defining the disk/sphere locations and radii of a unit clump. An initial disk/sphere packing (as described above) is then replaced by an equivalent volume unit clump with a random orientation.
Two examples of a unit clump in PFC3D.
Studies have shown that the use of multi-particle clumps increases the effective friction angle of the PFC specimen as compared to spherical particles.* This, in turn, can yield a more realistic representation of the macroscopic material behavior.
Results of simulated triaxial testing on spherical and clumped particles.
After the clump is defined, multiple material micro-properties and size refinement is accomplished automatically by defining the desired ranges for modification and degree of size refinement, as seen in the figure below.
Examples of two specimens generated using the clump base units shown in the figure above containing a core zone of different micro-properties and two levels of refinement to produce a graded material (finer towards the middle).
Slope Stability describes a project that makes complete use of the enhanced material genesis techniques available in PFC 4.0.
*M.E. Pierce et al. "PFC3D modeling of caved rock under draw," in Numerical Modeling in Microemchanics via Particle Methods, Proceedings of the International PFC Symposium, Gelsenkirchen, Germany, 6-8 November 2002. Heinz Konietzky, Editor. Taylor & Francis, 2004.
