General/motivation; there are quite a few practical and interesting engineering problems that are hard to do or even impossible with the current FreeCAD/Calculix combination and I don’t think adding more external solvers or waiting for their developer(s) is the answer. For example: mixed meshes (sandwich panels, soil/structure interaction, pre-stressed concrete, bridges), analysis beyond limit points (plastic collapse of frames, plates and soil bodies, non-linear buckling of structures, etc.)
Specific: I built a macro that takes FC mesh and material data and performs the basic tasks of a finite element program. Now I am adding functionality that allows me to do the analyses that were previously out of reach. Zero-thickness elasto-plastic interface elements were high on the list for me as they are a simple and robust alternative for (small deformation) contact problems and can be used in mixed meshes to model the interaction (e.g. cohesive or frictional slip) between 1D/2D structural elements and the surrounding 3D matrix (eg concrete reinforcement, soil anchors, tunnel linings, foundation plates, piles).
Status: basic linear elastic solver framework and curved 2D interface elements work, although I am still running into trouble with more complex contact geometries. This is work in progress.
Next steps: 1) calculate and export stress results (note: interface elements are really handy to visualize contact stresses); 2) extend the solver framework to handle plasticity and collapse (simplest example being the slip and pull-out of structural elements along elasto-plastic interfaces); 3) include simple, traditional, beams and shell elements; 4) include general material and geometric non-linearity for 1D, 2D and 3D elements; 5) Add essential QT5 GUI functionality to provide specific input; 6) Make available for testing.