INTRODUCTION 1-1
1 INTRODUCTION
1.1 Background and Overview
PFC
3D
models the movement and interaction of spherical particles by the distinct element method
(DEM), as described by Cundall and Strack (1979). The original application of this method was as a
tool to perform research into the behavior of granular material; representative elements containing
several hundred particles were tested numerically. The particle model was used to understand
element behavior (in which conditions are “uniform”), and a continuum method was used to solve
real problems that involve complicated deformation patterns (with the element behavior derived
from the particle-model tests). Two factors have brought about a change in this approach. First, the
task of deriving general constitutive laws from test results on particle assemblies is very difficult.
Second, with the spectacular increase in powerofsmall computers, it is now possible to model entire
problems with particles; the constitutive behavior is built into the model automatically. PFC
3D
is
designed to be an efficient tool to model complicated problems in solid mechanics and granular
flow.
A physical problem that is concerned with the movement and interaction of spherical particles may
be modeled directly by PFC
3D
. It is also possible to create particles of arbitrary shape by attaching
two or more particles together, such that each group of particles acts as an autonomous object
(using the clump logic described in Section 4 in Theory and Background). PFC
3D
is also able
to model a brittle solid, by bonding every particle to its neighbor; the resulting assembly can be
regarded as a “solid” that has elastic properties and is capable of “fracturing” when bonds break in a
progressive manner. PFC
3D
contains extensive logic to facilitate the modeling of solids as closely-
packed assemblies of bonded particles (much of this logic has been implemented in the Augmented
FishTank — a set of functions written in FISH, the PFC
3D
embedded language); the solid may
be homogeneous, or it may be divided into a number of discrete regions or blocks. This type of
physical system may also be modeled by the distinct element programs UDEC (Itasca 2000) and
3DEC (Itasca 2003), which deal with angular blocks. However, PFC
3D
has three advantages. First,
it is potentially more efficient, since contact detection between circular objects is much simpler than
contact detection between angular objects; second, there is essentially no limit to the magnitude of
displacement that can be modeled; and third, it is possible for the blocks to break (since they are
composed of bonded particles), unlike blocks modeled with UDEC or 3DEC which cannot break.
The drawback to modeling a blocky system with PFC
3D
is that block boundaries are not planar; in
exchange for the advantages offered by PFC
3D
, the user must accept “bumpy” boundaries.
The specification of the geometry, properties and solution conditions is not so straightforward in
PFC
3D
as in programs such as FLAC (Itasca 2002) and UDEC. For example, with a continuum
program, a grid is created, initial stresses installed, and boundaries set as fixed or free. In a
particle code such as PFC
3D
, a compacted state cannot be pre-specified in general, since there
is no unique way to pack a number of particles within a given volume. A process analogous to
physical compaction must be followed until the required porosity is obtained. The initial stress
state cannot be specified independently of the initial packing since contact forces arise from the
relative positions of particles. Finally, the setting of boundary conditions is more complicated
than for a continuum program because the boundary does not consist of planar surfaces. These
issues are all addressed in this manual, and practical procedures are suggested for making realistic
PFC
3D
Version 3.0
