Software Use > FEM Coupling (HyperFEA)

ANSYS Eigenvalue Buckling + Modal Analysis - User defined analysis?

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mraedel:
Sure, the buckling shapes are just the result of the deformation for the eigenvalue inserted in the eigenvalue problem/shape functions. In general in ANSYS this does not seem to be normalized to the mass.

I did verify the deformations and they look appropriate.

The expansion pass in ANSYS seems to do the necessary calculation of the normalized displacements.

mraedel:
I was happy too soon.

ANSYS performs the eigenvalue buckling analysis for all 4 load cases. I can display the requested modes for all load cases in the FEMViewer with the corresponding eigenvalue in the title.

Lets say I have the following results:

--- Code: ---Load case 1: critical eigenvalue: 2.5
Load case 2: critical eigenvalue: 1.9
Load case 3: critical eigenvalue: 1.8
Load case 4: critical eigenvalue: 1.7

--- End code ---

But, now my problem: in the Iteration Report of the FEMViewer the MAXIMUM eigenvalue is shown as lambda_actual, here 2.5. Is that just a wrong display and internally  minimum eigenvalue is used for the calculation of required stiffnesses? Or is just the value of the first load case displayed? Or is the inverse of the eigenvalue actually needed here?

On the other hand: Everything works fine for the eigenfrequencies. The minimum of all 4 load cases is found. But that might just be a coincidence, because the smallest eigenfrequency is present in load case 1.


--- Code: ---Load case 1: first eigenfrequency: 60.0
Load case 2: first eigenfrequency: 70.5
Load case 3: first eigenfrequency: 70.2
Load case 4: first eigenfrequency: 71.1

--- End code ---

I tried the following: I switched the model, load case and result files of load case 1 and 3 (renamed the files) for the stability and modal analysis. Thus, now the maximum buckling eigenvalue and the minimum frequency are in load case 3. The minimum buckling eigenvalue is still in load case 4.

The result is the following:

* The wrong minimum modal eigenfrequency 70.2 from the current load case 1 is shown
* The wrong critical buckling eigenvalue 1.8 from the current load case 1 is shown
Thus, my conclusion: The buckling and modal eigenvalue and eigenfrequencies are always obtained from the first load case, no matter which one is critical.

Is this a bug or did I do something wrong?

Ryan:
Since eigenvalue 2.5 was reported in both scenarios, it sounds like the modes from load cases 2-4 are being filtered out due to the normalized deflections. You could confirm this by running with only a single buckling run deck.

-Ryan

mraedel:
I do not think so. The maximun normalized deflections for all load cases are >1 in the FEM Viewer.

Also, I guess it was misunderstanding what I wrote. When I switched load cases 1 and 3, the printed buckling eigenvalue was 1.8, so still not the minimum, but the eigenvalue of the current (after the switch) load case 1. If your assumption would be the case, the printed eigenvalue should still be 2.5 . I updated my previous answer.

Ryan:
I believe we found the issue. There is an issue with the reporting logic - for multiple buckling run decks. If all eigenvalues are greater than the limit, the first eigenvalue is reported as the minimum. If any eigenvalue is below the limit, the correct minimum eigenvalue is reported. This issue does not affect the optimization logic.

-Ryan

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