RBE2_RBE3_common nodes technique
Altair EmployeeHello,
I have a request for everyone here. I need some documentation on the connection between 2 parts using RBE2, RBE3. I need first some general info on the 2 types of connections, and any other types of connection for Optistruct (without any contacts) used to tie 2 different parts, maybe 2 parts meshed with different kinds of elements.
Thank you very much !!
Answers
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Hello,
there are a significant difference between RBE2 and RBE3
RBE3 is a Interpolation Constraint Element
RBE2 is a Rigid Body Element
The RBE3 element is a powerful tool for distributing applied loads and mass in a model. Unlikethe RBAR and RBE2 elements, the RBE3 doesn’t add additional stiffness to your structure. Forces and moments applied to reference points are distributed to a set of independent degrees of freedom based on the RBE3 geometry and local weight factors.
If you wish to connect 2 different parts (like weldings), then use RBE2 instead.
For linear and NLSTAT solutions it is possible to use a CONTACT of type FREEZE (works very well).
Please look into the online help!!!
Regards
Mario
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Mario,
what's that about the local weight factor from the RBE3? I searched the help in HM but it's nothing there on that subject, can you point in the right direction on this part?
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Hello,
The use of weighting factors allows the uneven distribution of forces or torques.
Example: Forces
The force at the reference node is divided and applied to each interpolation node proportionally according to the node's weighting factor...
Please be careful with the use of RBE3 elements. Here a statement from the MSC.Nastran Common Question and Answer manual:
The theory used to generate the internal MPC equations caused by RBE3 elements is difficult to understand. The actual load redistribution is often surprising even to those who understand the theory, particularly for complicated geometry and weighting function specifications. It is good practice to check out a typical RBE3 element specification with a small test model to allow you to feel comfortable with its action. Expert users of the element do so every time they use a pattern of geometry and weighting functions that is new to them. They sometimes find it necessary to adjust the weighting factors to obtain the behavior needed to match the device that the element models.
The most common user error in RBE3 element specification results from placing 4, 5 or 6 in the Ci (independent DOF) field when also including some translation components. The action of the element is somewhat irrational for this specification. DO NOT USE 4, 5 or 6 FOR Ci VALUES UNLESS YOU HAVE A GOOD REASON TO DO SO !!!
Regards,
Mario
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Hello,
I tried today a small experiment. I meshed 2 tubes with different diametres, one entering the other, with hexahedral elements, 3 rows on the thickness, equal number of elements on the radius. One end was constrained. On the other end of this tube both tubes were connected using RBE2. I did 2 kinds of analysis.
1. The nodes in the contact area were tied to a calculated node on the tubes axis.
2. Each node, in the contact area, from 1 surface was tied to a node on the other surface.
On these to analyses i applied 5 Gs on an axis normal to the tubes axis.
In the first case, the results looked ok, both tubes connected move with the gravity force.
On the second case, i think i have some problems. I was expecting the results, or atleast the displacement to be the same (not the same value, but the same direction), but the inner tube rotates around his axis, instead of pulling on the other tube.
What could be the explanation ? I can tell you that i didn't create a cylindrical csys.
Regards,
Sergiu
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