FEM: Add support for CalculiX membrane elements (#22912)

* FEM: Update solver.py

* FEM: Update write_femelement_geometry.py

* FEM: Update write_mesh.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* FEM: Update solver_calculix.py

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/Mod/Fem/femsolver/calculix/solver.py

@@ -423,6 +423,16 @@ class _BaseSolverCalculix:
             )
             obj.BucklingAccuracy = 0.01
 
+        if not hasattr(obj, "ExcludeBendingStiffness"):
+            obj.addProperty(
+                "App::PropertyBool",
+                "ExcludeBendingStiffness",
+                "Fem",
+                "Exclude bending stiffness to replace shells with membranes",
+                locked=True,
+            )
+            obj.ExcludeBendingStiffness = False
+
 
 class Proxy(solverbase.Proxy, _BaseSolverCalculix):
     """The Fem::FemSolver's Proxy python type, add solver specific properties"""

src/Mod/Fem/femsolver/calculix/write_femelement_geometry.py

@@ -115,7 +115,14 @@ def write_femelement_geometry(f, ccxwriter):
                 shellth_obj = matgeoset["shellthickness_obj"]
                 if ccxwriter.solver_obj.ModelSpace == "3D":
                     offset = shellth_obj.Offset
-                    section_def = f"*SHELL SECTION, {elsetdef}{material}, OFFSET={offset:.13G}\n"
+                    if ccxwriter.solver_obj.ExcludeBendingStiffness:
+                        section_def = (
+                            f"*MEMBRANE SECTION, {elsetdef}{material}, OFFSET={offset:.13G}\n"
+                        )
+                    else:
+                        section_def = (
+                            f"*SHELL SECTION, {elsetdef}{material}, OFFSET={offset:.13G}\n"
+                        )
                 else:
                     section_def = f"*SOLID SECTION, {elsetdef}{material}\n"
                 thickness = shellth_obj.Thickness.getValueAs("mm").Value

src/Mod/Fem/femsolver/calculix/write_mesh.py

@@ -48,7 +48,10 @@ def write_mesh(ccxwriter):
 
     # Use 2D elements if model space is not set to 3D
     if ccxwriter.solver_obj.ModelSpace == "3D":
-        face_variant = "shell"
+        if ccxwriter.solver_obj.ExcludeBendingStiffness:
+            face_variant = "membrane"
+        else:
+            face_variant = "shell"
     elif ccxwriter.solver_obj.ModelSpace == "plane stress":
         face_variant = "stress"
     elif ccxwriter.solver_obj.ModelSpace == "plane strain":