CERIG Vs RBE3
CHETAN MAHAJAN
Mechanical System Technical Specialist - Analytical at CUMMINS TECHNOLOGIES INDIA LIMITED
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While applying constraint equation CAE Engineers have a lot of confusion over CERIG and RBE3.
Which is the correct method?? Which method gives appropriate results??
So, In today's article I will try to simplify some things as per my understandings.
First of all, what are the Applications of constraint equations:-
1. Representing parts of the system not modeled.
2. Tying together dissimilar meshes.
3. Distributing loads and applying constraints.
Now, following difference will give clear idea about the constraint equations:-
RBE3:- As specified in the first point, RBE3 is used to distributes the load (force/moment) from master to slave nodes.
Suppose we are not modelling any component but we need to consider its mass, so in order to consider its weight we will define mass element and will connect it using RBE3 to the supporting location.
Consider example of motor supported on frame, motor is not modelled as we need to do static analysis of support frame. So motor is modelled as mass element(master node) and connected with support frame wherever it is rested on it(slave nodes). It's just a rough representation of actual scenario:-
CERIG :- CERIG is used to form directional rigid zone between different components where connections are present by adding DOF.
The ‘master node’ in this case means that this is the node (and DOF) which control the behavior of the rigid region.
Consider a classical example of plate with hole, fixed at both ends and mass element (2 tonnes) at the center connected via constraint equation as shown in Figure 1.
This scenario is simulated considering: -
a) RBE3
b) CERIG
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1. Use of either of constraint equation has its own importance and limitations. Both the methods have their advantages.
2. It is not mentioned in any FEA book that which is the correct method, it depends on the application, basic behavior and the FE model.
3. Displacement result and comment is shown in Figure 2.
4. As the CERIG is stiffer, so stiffness (K) is more in natural frequency equation therefore frequency is higher as shown in Figure 3.
5. Mode shape behavior for RBE3 and CERIG is different as shown in Figure 4.
6. Hole will change shape in case of RBE3 as slave nodes contain independent DOF. Refer Figure 5
I guess everyone would have got some idea about the use of constraint equations.
Please feel free to discuss.
Will sign off with a inspiring quote I read today,
"A comfort zone is the beautiful place, but nothing ever grows there"
Graduate Computer Science (M.Sc.) Student at ?zU ???? ± Ex-Mechanical Engineer ± Multi-Agent Systems Enthusiastic ???? AL & ML
4 å¹´Additional question is, can one use both commands, by using CERIG make a rigid part, and by using REB3 apply force to that part?
Graduate Computer Science (M.Sc.) Student at ?zU ???? ± Ex-Mechanical Engineer ± Multi-Agent Systems Enthusiastic ???? AL & ML
4 å¹´Thanks for your information. I have a question. As I understand one cannot apply load to the master node in CERIG command in hope that effects all slave nodes, on the other hand in REB3 command one could apply load in hope that will be distributed on all slave nodes. Am I correct? I am working on modelling 3PB test of I-beam with ANSYS APDL. I want to apply force without modelling a roller, which command should I used to apply force on the top part of the I-beam which results in the same displacements where load applied?
Design Technical Advisor @ Cummins Inc. | Global Design Leader
6 å¹´Nicely explained.