FEM and Blender

From wikiluntti

Introduction

Blender and CAD 

import bpy
import csv

def export_selected_mesh_dimensions_to_csv(filepath):
    """Export Parts
    Args:
        filepath (str): Le chemin du fichier CSV de sortie.
    """

    with open(filepath, 'w', newline='', encoding='utf-8') as csvfile:
        fieldnames = ['Part', 'Section X (mm)', 'Section Y (mm)', 'Length Z (mm)']
        writer = csv.DictWriter(csvfile, fieldnames=fieldnames)

        writer.writeheader()
        for obj in bpy.context.selected_objects:
            if obj.type == 'MESH':
                
                dimensions = obj.dimensions
                x = [0,0,0]
                x[0] = round(dimensions.x * 1000, 3)
                x[1] = round(dimensions.y * 1000, 3)
                x[2] = round(dimensions.z * 1000, 3)
                x.sort()

                writer.writerow({'Part': obj.name, 'X (mm)': x[0], 'Y (mm)': x[1], 'Z (mm)': x[2]})

filepath = "/home/user/Downloads/part_list.csv"  # Replace destination path

export_selected_mesh_dimensions_to_csv(filepath)

FreeCAD import

Steps:

  • Export from Blender. If exporting STL, scale on export 1000x. Problems with non-manifold geometry.
    • Use Boolean to make on mesh
    • Non-manifold: Select -> Select All by Trait -> Non Manifold, also the others from there. Remove the faces and then dissolve the edges (Limited Dissolve or X > Dissolve Edges).
      • (Select Similar). This is often useful to select vertices with only one connecting edge
      • (Select Linked)
      • W > Specials > Remove doubles Doubles,
    • Check the normals outward: Edit mode, Select All, Mesh -> Normals -> Outward (Shift + N)
    • Eliminate the bolts and some holes
  • FreeCAD
    • Part -> Create shape from mesh
    • Part -> MakeSolid


Some problems (see a Part check Geometry)

  • Geometry is a shell -- not a solid: a shell mesh on meshing
    • Check this by right clicking on the mesh and printing mesh informations (Tasks: Node count vs Triangle count vs Tetrahedron count)
  • Self intersections
    • Use Boolean in Blender for all parts (both, if two are overlapping)
    • Self intersecting meshes are considered dirty
    • Use Part -> View -> Transparency to see the intersection points, if they persist.
    • eg Netfabb cleans

FEM Calculations of the Door Canopy

Images of the displacement.

Results

Material: Wood-Generic

  • Density 700 kg/m3
  • Young's Modulus 12.00 GPa
  • Poisson Ratio 0.05

Roof is done with fine mesh, no roofed with even finer mesh.


Caption
Design Displacement magnitude Displacement X Displacement Y Displacement Z von Mises Stress Max Principal Stress Min Principal Stress Max Shear Stress
Min Max [mm] Min [µm] Max [µm] Min [µm] Max [µm] Min [mm] Max [nm] Min Max [kPa] Min [kPa] Max [kPa] Min [kPa] Max [kPa] Min Max [kPa]
No Rafters 0.00 2.01 -110 320 240 100 -2.01 800.04 0.00 5847.87 -191.14 6037.13 -5486.84 285.32 0.00 2969.41
22.5° rafter 0.00 0.22 -17.67 92.32 -46.55 14.54 -0.23 164.74 0.00 2020.39 -118.97 2101.76 -1742,35 119.10 0.00 1014.03
22.5° × 2 rafters 0.00 0.16 -13.67 63.63 -35.62 9.99 -0.15 148.73 0.00 1324.45 -108.65 1142.64 -1356.50 60.92 0 672.31
45° rafter 10 11 12
No Rafters + Roof 0.00 76.54 -15.85 8.37 -4.87 8.94 -0.076 121.03 0 2643.85 -391.25 618.96 -2727.37 33.93 0.0 1395.93
Roof + 22.5° rafter 4 5 6
Roof + 22.5° × 2 rafters 7 8 9
Roof + 45° rafter 10 11 12


Simulation parameters

  • Roof Sheet: 1150 x 900 x 22 and

Material: Wood-Generic

  • Density 700 kg/m3
  • Young's Modulus 12.00 GPa
  • Poisson Ratio 0.05

Force: 1000 N, downwards (about).

Caption text
Header text Nodes Triangles Tetrahedron Header text
No Rafters 16413 5424 7786 Example
22.5° rafter 20407 6684 9775 Example
22.5° × 2 rafters 20185 6650 9627 Example
45° rafter Example Example Example Example
No Rafters + roof Example Example Example Example
22.5° rafter + roof Example Example Example Example
22.5° × 2 rafters + roof Example Example Example Example
45° rafter + roof Example Example Example Example


Size

Name Width Height Length
sivuvaaka !! 40.0 !! 60.0 !! 1200.0
sivuvaaka2 40.0 60.0 1200.0
etulyhytpysty 40.0 60.0 400.0
lyhytvino2 40.0 60.0 1207.695
pystyseina 40.0 60.0 535.61
pitkavino 40.0 60.0 740.106
lujyvino 40.0 60.0 1207.695
pystyseina2 40.0 60.0 535.61
pitkavino2 40.0 60.0 740.106
vinavase 40.0 60.0 686.624
takapalkki 40.0 60.0 1270.0
vinooikea2 40.0 60.0 771.684
takapalkki.001 40.0 60.0 1270.0
vinavase.002 40.0 60.0 686.624
vinooikea1 40.0 60.0 771.684
vinavase.004 40.0 60.0 686.624
vinooikea 40.0 60.0 771.684

Convergence tests

To ensure that the FEM simulation results converge, we try a simple simulation with different size of triangulation.

Workflow and test examples

  1. Plan and sketch using Blender
  2. Create the FEM model using FreeCAD
  3. Do the simulations
  4. Make the plans.

Below is the non working idea.

  1. Make the Blender mesh and export as .obj file
  2. Make the mesh using GMSH or FreeCAD, and use FreeCAD to FEM calculations

Simple Cube

Works OK with the FreeCAD, not with the Gmsh

OBJ

  • The imported cube has double nodes, and the color is strange.

    The imported cube has double nodes, and the color is strange.

  • (If normals are fixed (shift+N in edit mode) the box will look better. This is not used in the following parts).

    (If normals are fixed (shift+N in edit mode) the box will look better. This is not used in the following parts).

  • It works flawless in the FreeCAD.

    It works flawless in the FreeCAD.

  • The FreeCAD FEM solver gives some results.

    The FreeCAD FEM solver gives some results.

  • However, when importing to GMSH, it will produce an error.

    However, when importing to GMSH, it will produce an error.

STL

  • Meshing works, if the normals are done.

    Meshing works, if the normals are done.

  • If the cube is exported as STL, then GMSH will work better (if not normals done).

    If the cube is exported as STL, then GMSH will work better (if not normals done).

  • The mesh generation does not work: "PLC Error: A segment and a facet intersect at point" and "Invalid boundary mesh (segment-facet intersection) on surface 1, intersection (0.333327,0.333327,1)". (if not normals done.)

    The mesh generation does not work: "PLC Error: A segment and a facet intersect at point" and "Invalid boundary mesh (segment-facet intersection) on surface 1, intersection (0.333327,0.333327,1)". (if not normals done.)

Two Cubes

  • The two cubes made in Blender. The mesh looks fine, and there are none self intersecting nodes.

    The two cubes made in Blender. The mesh looks fine, and there are none self intersecting nodes.

  • When importing the Obj file back to Blender, the seems to be missing some faces.

    When importing the Obj file back to Blender, the seems to be missing some faces.

  • Seems that some of the normals are faced wrong way. In edit mode use (Ctrl+A) shift+N to fix this.

    Seems that some of the normals are faced wrong way. In edit mode use (Ctrl+A) shift+N to fix this.

  • Tetrahedron count is more than zero! It seems to work.

    Tetrahedron count is more than zero! It seems to work.

  • The FEM results seems to be working. The force is 10 N.

    The FEM results seems to be working. The force is 10 N.

The Door Canopy

Couldn't do the model using Blender and Freecad/ Gmsh.

Make it step by step.

  • First part works, though the dimensions are totally wrong. In the inset is the Blender Edit mode image of the same part.

    First part works, though the dimensions are totally wrong. In the inset is the Blender Edit mode image of the same part.

  • Set the scale from Blender export (1000x). Now the stress is visible, and dimensions are correct. Nice to work with familiar units.

    Set the scale from Blender export (1000x). Now the stress is visible, and dimensions are correct. Nice to work with familiar units.

  • Already second part (or third) gives problems. The mesh cannot be generated. This is with STL export.

    Already second part (or third) gives problems. The mesh cannot be generated. This is with STL export.

  • If tested using OBJ format, there will be some places without faces, and no tetrahedron in the mesh.

    If tested using OBJ format, there will be some places without faces, and no tetrahedron in the mesh.

Gmsh

  • Gmsh makes the FEM mesh for the first part, and by refining the mesh, it will be clearer.

    Gmsh makes the FEM mesh for the first part, and by refining the mesh, it will be clearer.

  • Gmsh can use the second part, also. Need to check if FreeCAD can read that file.

    Gmsh can use the second part, also. Need to check if FreeCAD can read that file.

  • FreeCAD has also Gmsh mesh generator. That will generate the mesh, but while solving gives an error: "Nodes, but no results found in frd file. It means there only is a mesh but no results in frd file. Usually this happens if CalculiX returned no results (happens on nonpositive jacobian determinant in at least one element)"

    FreeCAD has also Gmsh mesh generator. That will generate the mesh, but while solving gives an error: "Nodes, but no results found in frd file. It means there only is a mesh but no results in frd file. Usually this happens if CalculiX returned no results (happens on nonpositive jacobian determinant in at least one element)"

  • By trying again, it works! Force is 10 N.

    By trying again, it works! Force is 10 N.

  • Changed the Force to 100 N, and the maximum size of mesh to 10. The later increased the computation time a lot, the time being about 9 s.

    Changed the Force to 100 N, and the maximum size of mesh to 10. The later increased the computation time a lot, the time being about 9 s.

FreeCAD Tutorials

Good tutorials for noobies

Theory

Frame formulas: https://structx.com/frames.html

References

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