Modal analysis for simple beam - possible program error?

[Jacob.1.nielsen]

Stumbled upon a strange result in modal analysis for an .ipt

If a simple retangular beam 200mm wide and 300mm high and 8mm thk. Length 3880mm.

Constrained on outer surface of two Ø700x10 circular plate.  See enclosed picture for better understanding. 

If both retangular beam and circular plate is constructed as one solid, then first modal frequency is 57,5 Hz

If retangular beam and circular plates is made as tree solids and automatic bonded, then first modal frequency is 47,3 Hz.

I can not see why result should be different and difference of 10 Hz seem alot?

[Jacob.1.nielsen]

Made a static analysis with vertical load of 5000 kg on beam... This also gave different result??

69 MPa when part is made of tree solids with automatic bonding. 

45 MPa when part is made of one solid.

[admaiora]

Just 2 question.

Have you done a FEm analysis in the past?

Can you attach your assembly?

[Jacob.1.nielsen]

Part is enclosed.

Yes I have made FEM analysis in Inventor for 10+ years. 

But if static analysis is blurring the question then please look past this. I know you have to make the model converge to use the result for anything. 

I just cannot see the why the part model have different result, depending on if it is one solid or bonded as three solids.

[admaiora · Accepted solution]

Good. So based on your experience you will know that there is a great difference between multibody and one body merged in FEM.

That's in Inventor and in another solvers.

In the past Inventor didn't support multibody analysis.

When you mesh a multibody or one body there is great difference in elements and nodes due to the closed meshing on every body.

Different meshing, different nodes, different elements, different stiffness >> different results

[Jacob.1.nielsen]

Yes I see now the difference in mesh for the to models. And therefore overlooked the convergence part of modal analysis.

Both model now give the same result of 46,4 Hz.... Thank you for clearing this for me.

[admaiora]

You are welcome Jacob!

[manuel.degracia3]

Hello, I am doing an analysis very similar to yours (a modal analysis of a simply supported beam) and they give me different results compared to the calculations done manually. How did you modify the meshing of the body?