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# Questions on RBE2

## Questions on RBE2

(OP)
I have a simple question about RBE, I need to know how they work:
let's say I have 2 surfaces on top of each other, both modelled with 2D quad element. The top has a very low thickness, 0.016 in (0.41 mm) and it's aluminum, the bottom is a slab of aluminum 1 in (25.4 mm) thick.

To model a contact between the two surfaces I connect 1 by 1 the nodes of the top surface (masters) to the nodes of the bottom surf.

I apply a pressure on the top surface. Max displacement is 16 mm (0.63 in)

Which of these answers is correct:
1. The nodes of the bottom surface will move EXACLTY of 0.63 in, forces by the RBE2;
2. Due to the very high stiffness of the bottom surface, it will counteract the displacement of the top surface.

### RE: Questions on RBE2

What are the boundary conditions in this scenario ? Without constraints both plates will move together due to pressure (assuming that it's a dynamic analysis because in static simulation the problem simply won't converge).

### RE: Questions on RBE2

The RBE is perfectly rigid.

### RE: Questions on RBE2

1) why not glue the two surfaces together ?

2) if the two surfaces ate tied together with a RIGID link, then you can answer your own question, if you understand what RIGID means.

3) If you modelled a finite stiffness connection then there'd be some relative displacement.

4) Try simple models yourself before you ask questions ? You could model a rigid link by using the same nodes for both surfaces.

### RE: Questions on RBE2

(OP)
FEW way: both surfaces are simply supported

rb1957: If I had found the answer myself I wouldn't have be here asking for clarification...... don't you think?

### RE: Questions on RBE2

sorry, the point to my "snipe" was anyone using finite elements should understand what a rigid element is, or perform tests to understand how they work, or read up about them. There's nothing in your original post to say you've done anything beyond thought of a question.

It is IMHO not a professional question.

### RE: Questions on RBE2

Dear Flyforever,
You don`t tell us the relationship between the two bodies, if the thick & thin layer are glue or can slide one in top of the other.
You have a few options here:
• Offset Distance in Contact Region: in FEMAP we can use the OFFSET field to account for a rigid layer between contact face regions. For example, a model which has two metal surfaces coming into contact, and one of these has a ceramic coating. If the ceramic material stiffness is not significant enough to be included in the analysis, it may not have been specifically modeled, but the thickness it adds to the face of the metal may be important when considering the contact problem. In this case the stiffness of ceramic material is ignores, but the small thickness is taking in account because is important in contact problems.

Coating Mesh: you can mesh the thicker part with solid elements, and use 2-D shell elements to mesh the skin of the solid to account for the thin layer of material, sharing nodes between the shell & solid component. If the thickness is very low, not need to apply OFFSET to the skin shell elements. In this meshing approach you account for mass & stiffness of the thin material.

• Using the PCOMP Method for Composite material: The PCOMP entry in Simcenter Nastran & FEMAP provides a convenient way of entering appropriate material properties for a plate consisting of layer-by-layer material definition. You can use PCOMP to define the thickness, orientation, and the material identification number of each of the individual lamina.

Alternatively, you can use the PCOMPS method to model laminates with 3-D CHEXA solid elements. Like the PCOMP method, the PCOMPS method allows you to define the laminate on a layer-by-layer basis

As you see, you have many options, the last resource is to use the RBE2 rigid elements, I only recommended the use of RBE2 elements when not other better option exist.
And I know pretty well the RBE2 & RBE3 elements, see my blog:
https://iberisa.wordpress.com/2015/10/13/rbe2-vs-r...

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog de FEMAP & NX Nastran: http://iberisa.wordpress.com/

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