Rigid body and Reference node

About the development of the FEM module/workbench.
sodelo
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Rigid body and Reference node

Postby sodelo » Fri Mar 17, 2017 7:55 pm

Hi,

I would be happy to have the possibility to create rigid bodies since this is useful in many situations (for example to simulate forming: the punch and the mould are most of the time considered as rigid) and lower the cost of the simulation (since mechanical equations are not solved for the rigid body).
I wonder whether this is possible in Freecad or what are the necessaru changes to be able to get the concept of a reference node (geometric point entity where BCs and forces can be defined) ? Ideally, the point should not be attached to any other entity.

The slave nodes would be related to faces or solids, which are available of course.

Thanks,

Sodelo
Jee-Bee
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Re: Rigid body and Reference node

Postby Jee-Bee » Sun Mar 19, 2017 8:37 am

See http://www.feacluster.com/CalculiX/ccx_2.11/doc/ccx/node279.html
and reference node i can't find that fast but probably there is an workaround for.
sodelo
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Re: Rigid body and Reference node

Postby sodelo » Sun Mar 19, 2017 12:20 pm

Thanks for your answer. My question was probably unclear however.
I know how to define a rigid body with Calculix. I was wondering what would be the necessary changes to be able to define a Rigid Body within Freecad.
It would be needed:
- to select faces or solid belonging to the rigid body. This is easy since similar to the interaction necessary for example to define a mesh.
- to be able to create a reference point (probably from coordinates) and to have a geometric representation for it,
- to have a representation for the rigid body (for example, lines between the slave nodes and the reference node),
- to be able to apply boundary conditions (forces or imposed displacements or rotations) on the reference node.

I have not seen any point entity so far so I was guessing this would be the most important change for Freecad. Maybe I am wrong ?

BR,

Sodelo
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bernd
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Re: Rigid body and Reference node

Postby bernd » Mon Mar 20, 2017 7:50 am

I still do not understand what the Rigid body is useful for. May be I need some visual example ...

@sodelo:
Do you wanne do it in python? For selecting vertices, edges, faces and solids see the selection parser of mesh region object. https://github.com/FreeCAD/FreeCAD/blob ... on.py#L138 On this object you can see how these objects are saved as reference shape for the document object.
ulrich1a
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Re: Rigid body and Reference node

Postby ulrich1a » Mon Mar 20, 2017 10:51 am

bernd wrote:I still do not understand what the Rigid body is useful for. May be I need some visual example ...
Have a look here: https://forum.freecadweb.org/viewtopic.php?f=18&t=20381
The rigid body is used in dynamic analysises in Calculix. Only the mass of the solid is taken into account. The solid moves relative to a reference-node, which can be any point in 3D-space. A fixed reference node let the rigid body only rotate around this reference node. The solid is rigid, means it does not deform due to forces. So all those equations regarding elasticity of the material are not solved for a rigid body.

The rotation axis of the two cubes in the example is defined via a rigid body. The rotation point is given as an extra node in space (reference node), which does not need to be within the solid. In this example I defined a node in the 3D-space for each cube, with the coordinates of the rotation point.
The cubes itself can deform, ony a few nodes are part of the rigid body.
In order to use this directly in FreeCAD, one would need an input dialog, which lets one either click a vertice in 3D-space or give the coordinates as data.

Ulrich
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bernd
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Re: Rigid body and Reference node

Postby bernd » Mon Mar 20, 2017 11:41 am

thanks Ullrich, I see light in the rigid darkness ... Could someone give an real world example?
fandaL
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Re: Rigid body and Reference node

Postby fandaL » Mon Mar 20, 2017 12:45 pm

bernd wrote: Could someone give an real world example?

A) as Sodelo pointed out: if some part is much more rigid then a next one, you can use rigid body element (RBE)– typically in forming processes as a die or a punch. See https://github.com/mkraska/CalculiX-Exa ... en/Biegung

B) for applying boundary conditions. It is possible to use a rotation node or a reference (translation) node so rigid body rotates or translates around 1 node. So you can apply force to one exact position and you know how it influences momentum equilibrium. Or you can apply moment to the rotation node ROT NODE and ccx will distribute it to 3D element nodes.
(This is a bit similar to how we define force now in FreeCAD).

C) e.g. hinges, flanges or bolts which connect 2 parts. If you are not interested in such details you can model them as RBEs (even without need to define reference point).
sodelo
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Re: Rigid body and Reference node

Postby sodelo » Mon Mar 20, 2017 12:55 pm

Imagine you want to simulate the mechanical testing of this specimen:
cct1.jpg
cct1.jpg (3.06 KiB) Viewed 75 times

And the testing configuration is this one:
images.jpg
images.jpg (6.05 KiB) Viewed 75 times


The specimen is loaded thanks to 2 axes. The distance between them is increased during the testing.
To simulate this, you can describe the specimen, the 2 axes and use contact between the different parts. However this is costly, especially for this configuration because the initiation of the contact will be very difficult to do, so you will get a lot of increments.

A simple way is to use rigid bodies like this:
cct3.png
cct3.png (17.12 KiB) Viewed 75 times

You define a first rigid body for the upper axis between reference node A and the nodes of half of the hole. The same for the lower axes. Locally the stresses will be wrong but at the region of interest the results will be good.
In terms of BC, you have to defined :
- Point B : DX,DY,DZ, RX, RY fixed (RZ is free),
- Point A : DX, DZ, RX, RY fixed (RZ is free and you impose DY at a given displacement) Alternatively DY can be free and you impose a force in the Y direction.
sodelo
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Re: Rigid body and Reference node

Postby sodelo » Mon Mar 20, 2017 1:04 pm

You want to simulate the impact of a car against a wall. Assume the wall is very rigid in comparison to the car. You can use the following model.

crash.png
crash.png (16.33 KiB) Viewed 72 times


In that case you define a rigid body between the wall nodes and the reference node. You give the car an initial velocity and the reference node is flly clamped (DX,DY,DZ, RX,RY, RZ fixed).

PS: To do this simulation, you need a transient dynamic procedure with an explicit scheme to be sure you get convergence (an implicit scheme will fail because non linearities are too severe). I will try to make an example.
sodelo
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Re: Rigid body and Reference node

Postby sodelo » Mon Mar 20, 2017 1:12 pm

You want to simulate the testing of a specimen with a fixed displacement. You wan to know the reaction force. One option is to sum all the reaction forces on the nodes where the displacement is applied but this is not convenient. Another option is to create a rigid body at the extremity and to impose the displacement only at the reference node. After the calculation, you only post process the reaction force on the reference node.
Attachments
tensile.png
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