FLAC is a two-dimensional explicit finite difference program for engineering mechanics computation. This program simulates the behavior of structures built of soil, rock or other materials that may undergo plastic flow when their yield limits are reached. Materials are represented by elements, or zones, which form a grid that is adjusted by the user to fit the shape of the object to be modeled. Each element behaves according to a prescribed linear or nonlinear stress/strain law in response to the applied forces or boundary restraints. The material can yield and flow, and the grid can deform (in large-strain mode) and move with the material that is represented. The explicit, Lagrangian calculation scheme and the mixed-discretization zoning technique used in FLAC ensure that plastic collapse and flow are modeled very accurately. Because no matrices are formed, large two-dimensional calculations can be made without excessive memory requirements. The drawbacks of the explicit formulation (i.e., small timestep limitation and the question of required damping) are overcome to some extent by automatic inertia scaling and automatic damping that do not influence the mode of failure.

Though FLAC was originally developed for geotechnical and mining engineers, the program offers a wide range of capabilities to solve complex problems in mechanics. Several built-in constitutive models that permit the simulation of highly nonlinear, irreversible response representative of geologic, or similar, materials are available.
FLAC also contains the powerful built-in programming language FISH (short for FLACish). With FISH, you can write your own functions to extend FLAC? usefulness, and even implement your own constitutive models if so desired. FISH offers a unique capability to FLAC users who wish to tailor analyses to suit specific needs.


- Large-strain simulation of continua, with optional interfaces that simulate distinct planes along which slip and/or separation can occur
- Explicit solution scheme, giving stable solutions to unstable physical processes
- Groundwater flow, with full coupling to mechanical calculation (including negative pore pressure, unsaturated flow, and phreatic surface calculation)
- Convenient specification of general boundary conditions
- Structural elements (including non-linear material behavior), with general coupling to the continuum
- Library of material models (e.g., elastic, Mohr-Coulomb plasticity, ubiquitous joint, double-yield, strain-softening, modified Cam-Clay, and Hoek-Brown)
- Automatic re-meshing during the solution process in large strain simulations
- Pre-defined database of material properties; users may add and save their own material properties specifications to the database
- A range of optional modules, including: thermal and creep calculations, dynamic analysis capability, two-phase fluid flow model, and user-defined constitutive models written in C++
- Coupling to infinite elastic boundary
- Statistical distribution of any property with extensive facility for generating plots of virtually any problem variable
- Built-in language (FISH) to add user-defined features (e.g., new constitutive models, new variables or new commands)