From 052ff9237b82a7b2daee882da846411aae4ca670 Mon Sep 17 00:00:00 2001
From: FEA-eng <59876896+FEA-eng@users.noreply.github.com>
Date: Mon, 18 Aug 2025 17:33:49 +0200
Subject: [PATCH] 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/femobjects/solver_calculix.py      | 18 +++++++++++++++++-
 src/Mod/Fem/femsolver/calculix/solver.py       | 10 ++++++++++
 .../calculix/write_femelement_geometry.py      |  9 ++++++++-
 src/Mod/Fem/femsolver/calculix/write_mesh.py   |  5 ++++-
 4 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/src/Mod/Fem/femobjects/solver_calculix.py b/src/Mod/Fem/femobjects/solver_calculix.py
index 6405da800d..3e007dcca8 100644
--- a/src/Mod/Fem/femobjects/solver_calculix.py
+++ b/src/Mod/Fem/femobjects/solver_calculix.py
@@ -320,5 +320,21 @@ class SolverCalculiX(base_fempythonobject.BaseFemPythonObject):
                 value=["electrostatic"],
             )
         )
-
+        prop.append(
+            _PropHelper(
+                type="App::PropertyBool",
+                name="ExcludeBendingStiffness",
+                group="Solver",
+                doc="Exclude bending stiffness to replace shells with membranes",
+                value=False,
+            )
+        )
         return prop
+
+    def onDocumentRestored(self, obj):
+        # update old project with new properties
+        for prop in self._get_properties():
+            try:
+                obj.getPropertyByName(prop.name)
+            except Base.PropertyError:
+                prop.add_to_object(obj)
diff --git a/src/Mod/Fem/femsolver/calculix/solver.py b/src/Mod/Fem/femsolver/calculix/solver.py
index b589f206c1..64b3da0b9b 100644
--- a/src/Mod/Fem/femsolver/calculix/solver.py
+++ b/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"""
diff --git a/src/Mod/Fem/femsolver/calculix/write_femelement_geometry.py b/src/Mod/Fem/femsolver/calculix/write_femelement_geometry.py
index c08c65e546..a839093bab 100644
--- a/src/Mod/Fem/femsolver/calculix/write_femelement_geometry.py
+++ b/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
diff --git a/src/Mod/Fem/femsolver/calculix/write_mesh.py b/src/Mod/Fem/femsolver/calculix/write_mesh.py
index e67bf5e707..859c5b3cc9 100644
--- a/src/Mod/Fem/femsolver/calculix/write_mesh.py
+++ b/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":