FEM CalculiX

    From FreeCAD Documentation


    This page collects information on the CalculiX finite element solver, the default solver in the Workbench FEM.svg FEM Workbench for structural and thermo-mechanical analysis as of FreeCAD 0.17. Depending on the operating system you are working with, you'll need to install CalculiX before running your first simulation. Please see FEM Install.

    The solver is able to do linear and non-linear calculations, for static, dynamic, and thermal problems. The solver operates on an Abaqus input file (.inp), which means it can be used with different pre-processors that support this format. The program includes its own graphical preprocessor which, however, is not used by FreeCAD, only the solver itself.

    CalculiX is designed to run on Unix platforms like Linux and Irix computers but also on MS-Windows. CalculiX was developed by engineers from MTU Aero Engines, Munich, Germany, to assist them in designing machinery such as jet turbines. The software is currently released to the public on the terms of the GPL version 2.

    Integration with FreeCAD

    Interaction between the FEM Workbench and CalculiX is done through writing and reading text files. The sequence of operations is as follows:

    1. A CalculiX input file is created with details required to run the simulation
    2. The CalculiX solver is started with this input file
    3. The output from the solver is logged
    4. The output files from the solver are read, if they are available

    The FEM Control Solver tool manages the whole process. User interaction in the process is possible.

    Preprocessing interface

    The input file that CalculiX uses can be prepared and edited before the solver is started. The units used in the input file are independent of the units set in FreeCAD; they will always be millimeters (mm) and Newton (N).

    (ToDo: check this. What happens with the mesh if inch is used in FreeCAD? As density was introduced, with this we have kg and s and no longer N?! how about this?!)

    The CalculiX interface supports the following objects:

    FEM Elements


    • Linear static analysis
    • Frequency analysis
    • Coupled thermal-structural analysis


    • One linear elastic isotropic material (uniformity in all directions)
    • Multiple material use is in development

    Postprocessing interface

    The FEM Workbench loads CalculiX results into a result object which will contain:

    • Displacements
    • Stresses
    • Strains
    • Equivalent plastic strain (PEEQ) – if nonlinear material was used
    • Temperature – for thermomechanical analysis

    FreeCAD reads results from *.frd file which was created by CalculiX. If these results contain multiple time steps, each time step is imported to FreeCAD as a new result object. Same behavior applies for Frequency or Buckling analysis with multiple eigenvalues.

    Reaction forces can be found in ccx_dat_file which contains reaction force components (fx, fy, fz) for each Constraint fixed and for each Constraint displacement which constrains translation degrees of freedom.