I am in the process of converting an Abaqus UEL (user element) to a CalculiX user element in order to perform calculations on open source software. Unfortunately, I have run into issues during the conversion. Specifically, I receiving the following error “*ERROR in frd: cannot open substructure stiffness file for reading…” when I try running the compiled subroutine on a single element calculation. Does anyone know what might cause this? My initial debugging trial was to fully specify the matrix (assuming a plane-stress quadralateral linear element) so I could troubleshoot other aspects of the user element (it needs 27 constants and at least 8 state variables), but this resulted in the same error message. I’m using the 2D case for unit testing because that was how the original subroutine was implemented.
As I worked through some of the source files, additional questions came up. In ``matrix2userelem.f’', it is stated that it is assumed that there would be 3 (or 6) dofs, not 2 as I provided. Is this causing the issue?
Initially, I was only planning on running 2D calculations (they run significantly faster and I typically deal with planar geometries), but I have read through the documentation and the different forums that CalculiX automatically expands 2D elements into 3D and then uses constraints to enforce plane stress (or plane strain) conditions. Does this mean I need to do this myself in the user element? Note that I was already hoping to write a 3D element as well (in addition to quadratic order elements for 2D and 3D), but I obviously would prefer to start with the simplest case before moving onto the more challenging cases.
Does anyone have some suggestions as I fix the aforementioned issues?
James