Applying the finite element method to real-life problems is not always a piece of cake. Especially achieving convergence for nonlinear applications (large deformation, nonlinear material behavior, contact) can be quite tricky. However, adhering to a couple of simple rules can make life a lot easier. According to my experience, the following guidelines are quite helpful:

- Check the quality of your mesh in CalculiX GraphiX or by using any other
good preprocessor.
- If you are dealing with a nonlinear problem, RUN A LINEARIZED VERSION FIRST: eliminate large deformations (drop NLGEOM), use a linear elastic
material and drop all other nonlinearities such as contact. If the linear
version doesn't run, the nonlinear problem won't run either. The linear
version allows you to check easily whether the boundary conditions are
correct (no unrestrained rigid body modes), the loading is the one you meant
to apply etc. Furthermore, you get a feeling what the solution should look
like.
- USE QUADRATIC ELEMENTS (C3D10, C3D15, C3D20(R), S8, CPE8, CPS8, CAX8,
B32). The standard shape functions for quadratic elements are very
good. Most finite element programs use these standard functions. For linear
elements this is not the case: linear elements exhibit all kind of weird behavior such
as shear locking and volumetric locking. Therefore, most finite element
programs modify the standard shape functions for linear elements to
alleviate these problems. However, there is no standard way of doing this,
so each vendor has created his own modifications without necessarily
publishing them. This leads to a larger variation in the results if you use
linear elements. Since CalculiX uses the standard shape functions for linear
elements too, the results must be considered with care.
- If you are using shell elements or beam elements,
use the option OUTPUT=3D on the *NODE FILE card in CalculiX (which is default). That way you
get the expanded form of these elements in the .frd file. You can easily
verify whether the thicknesses you specified are correct. Furthermore, you
get the 3D stress distribution. It is the basis for the 1D/2D stress
distribution and the internal beam forces. If the former is incorrect, so
will the latter be.
- If you include contact in your calculations and you are using quadratic
elements, use the face-to-face penalty contact method or the mortar method
(which is by default a face-to-face method). In general, for
contact between faces the face-to-face penalty method and the mortar method will converge
much better than the node-to-face method. The type of contact has to be
declared on the *CONTACT PAIR card. Notice that the
mortar method in CalculiX can only be used for static calculations.
- if you do not have enough space to run a problem, check the numbering. The memory needed to run a problem depends on the largest node and element numbers (the computational time, though, does not). So if you notice large gaps in the numbering, get rid of them and you will need less memory. In some problems you can save memory by choosing an iterative solution method. The iterative scaling method (cf. *STATIC) needs less memory than the iterative Cholesky method, the latter needs less memory than SPOOLES or PARDISO.

If you experience problems you can:

- look at the screen output. In particular, the convergence information
for nonlinear calculations may indicate the source of your problem.
- look at the .sta file. This file contains information on the number of
iterations needed in each increment to obtain convergence
- look at the .cvg file. This file is a synopsis of the screen output: it
gives you a very fast overview of the number of contact elements, the
residual force and the largest change in solution in each iteration (no
matter whether convergent or not).
- use the “last iterations” option on the *NODE FILE
or similar card. This generates a file with the name
ResultsForLastIterations.frd with the deformation (for mechanical
calculations) and the temperature (for thermal calculations) for all
non-converged iterations starting after the last convergent increment.
- if you have contact definitions in your input deck you may use the
“contact elements” option on the *NODE FILE
or similar card. This generates a file with the name jobname.cel
with all contact elements in all iterations of the increment in which this
option is active. By reading this file in CalculiX GraphiX you can visualize
all contact elements in each iteration and maybe find the source of your
problems.
- if you experience a segmentation fault, you may set the environment
variable CCX_LOG_ALLOC to 1 by typing “export CCX_LOG_ALLOC=1” in a
terminal window. Running CalculiX you will get information on which fields
are allocated, reallocated or freed at which line in the code (default is
0).
- this is for experts: if you experience problems with dependencies between different equations you can print the SPC's at the beginning of each step by removing the comment in front of the call to writeboun in ccx_2.19.c and recompile, and you can print the MPC's each time they are set up by decommenting the loop in which writempc is called at the beginning of cascade.c and recompile.