FEM units

[smallpixel]

Hello,
I am try to use the FEM Module in Freecad with a simple shape to get an idea what the units could be.
What I did
Create a cube 2 x 2 x 100mm
Created a Mesh
Assigned the Material S235JR
Put the FermConstraintFixed on the upper end
Put the FermConstraintForce at the lower end with an Area load of 200,0
Trigger CalculiX (Write file and Run Calculix)
Viewing the result I could see a Z displacement of Min -237,892m, Avg -118,729m and Max 0mm after switching to Y displacement and back the values are Min -0,0237892mm, Avg -0,0118729mm and Max 0mm.

Could somebody please tell what Area load means ?
I try to create a bicycle frame and would like to know is the selected tubes would fit or not. This why I try to use the FEM Module

System Info:
OS: Windows 10 Word size of OS: 64-bit Word size of FreeCAD: 64-bit Version: 0.16.6704 (Git) Build type: Release Branch: releases/FreeCAD-0-16
Hash: 0c449d7e8f9b2b1fb93e3f8d1865e2f59d7ed253 Python version: 2.7.8 Qt version: 4.8.7 Coin version: 4.0.0aOCC version: 6.8.0.oce-0.17


Thank you very much.

[bernd]

constraint pressure --> distributed load in MPa

constraint force --> load in N distributed on all given shapes. If you have one face the distributed load on the face will be value in N / Area of the face

see also FEM_CalculiX_Cantilever_3D

hope that helps bernd

[smallpixel]

If i get you right that would mean in this case 200 N / 4mm². In every case you need to take care of the area face to which the force is applied.
That would make it pretty complicated using complicated shapes.

[bernd]

In every case you need to take care of the area face to which the force is applied.
That would make it pretty complicated using complicated shapes.

No, you do not need to take care of this ... If you would like to apply a force (N) you use the constraint force. If you would like to apply a pressure (force per area --> MPa) you would use the pressure constraint.

[smallpixel]

Sorry for my late replay.
Maybe I am the problem.

I created a tube
Created a "Mesh"
Set "MechanicalMaterial"
Created a "FemConstraintFixed" where the tube will be fixed later.
If I use the function "Create FEM constraint for a force acting on a geometric entity."
I could only select the area load.

From all items offered there this is the only one which makes sense to me.
Please correct me if I am wrong.

Windows 10 64-bit Word size of FreeCAD: 64-bit Version: 0.16.6704 (Git) Build type: Release Branch: releases/FreeCAD-0-16 Hash: 0c449d7e8f9b2b1fb93e3f8d1865e2f59d7ed253 Python version: 2.7.8 Qt version: 4.8.7 Coin version: 4.0.0a OCC version: 6.8.0.oce-0.17

[bernd]

use FEM --> Constraint pressure if your face is planar

but you are right partially ... Since Constraint pressure uses as direction the face normal it only works on planar faces. If the face is not planar it is not possible to apply a force in MPa on a face and all force is applied in a certain user direction. You woul need to calculate the force with the face area and use the Constraint force.

This should be added to the constraint force task panel.

[PrzemoF]

use FEM --> Constraint pressure if your face is planar

but you are right partially ... Since Constraint pressure uses as direction the face normal it only works on planar faces. If the face is not planar it is not possible to apply a force in MPa on a face and all force is applied in a certain user direction. You woul need to calculate the force with the face area and use the Constraint force.

This should be added to the constraint force task panel.

I might be missing something here, but I think Pressure Constraint uses _local_ normal - I just tested squashing a sphere and it seems to be OK. I was testing Pressure on a vessel when I was coding it, so unless something has changed it should be OK. See attached.

sphere_fem.jpg (77.44 KiB) Viewed 3499 times

[bernd]

Thats exact the problem the pressure works great.

Imagine a snow load on an half sphere in FreeCAD FEM. Snowload is in MPa and Snowload is in z-direction. We are not able to model this in FreeCAD FEM at the moment. Constraint Pressure is in MPa bu in normal direction of the sphere. Constraint Force can be applied to the half sphere in z-direction, but only the sum of the load and not in MPa.

One has to calculate the Constraint force force by using the area of the half sphere himself.

[PrzemoF]

Thanks for the explanation!

It might deserve a new constraint - something like DierctionalLoad (for wind load or snow load)?

[bernd]

Thanks for the explanation!

It might deserve a new constraint - something like DierctionalLoad (for wind load or snow load)?

It could be simply a tongle in contstraint force between load in N and load in N/area for faces as well as N/length for edges. This is not difficault but it is C++ which makes it difficault for me.

May be you are right and it is eassier to make a new constraint even in input file writer ...