topic Re: Nonlinear buckling vs LBA in Inventor Nastran Forum

Nonlinear buckling vs LBA

autodesk428JLG — Wed, 04 Jan 2023 14:33:56 GMT

Hi!

I did two analyses in buckling, linear and non linear and got 1500 load multiplier in LBA and 7000 in nonlinear which to me , after what i read about LBA is weird. Usually the LBA is overestimated right?

Is there certain things i need to click in the advance settings for the nonlinear settings? I realized i forgot to click the arc length option and running my analyse with that clicked as im writing this to see if thats the issues. Can someoe elaborate on the arc length option please?

Best regards, Daniel

Re: Nonlinear buckling vs LBA

Roelof.Feijen — Wed, 04 Jan 2023 15:35:03 GMT

Hi Daniel,

So you got an eigenvalue of 7000? If the eigenvalue is far above 1, then the applied load is below the buckling load. If the eigenvalue is not close to a value of 1, consider adjusting the applied loads by the calculated eigenvalue and re-running the analysis. Repeat the process until the buckling load multiplier is approximately 1.

As for the arc-length method, this will allow the solver to reach convergence with a lower applied load and find the equilibrium. The highest reachable Load Scale Factor can be used to calculate the buckling load. You can also use this to see the post-buckling response.

I would like to suggest that you do not use the arc-length option for now, but adjust the applied load first to a reasonable value.

Re: Nonlinear buckling vs LBA

autodesk428JLG — Wed, 04 Jan 2023 16:58:34 GMT

Well, i got a higher eigenvalue for the nonlinear analysis, 7 times higher, after what i read the linear analyses often over predicts the buckling loads(?). However, i can see that i get two different buckling scenarios where the LBA mode kind of looks like i would excpect, but the nonlinear show a more twisting thing, see below:

Re: Nonlinear buckling vs LBA

autodesk428JLG — Wed, 04 Jan 2023 17:05:55 GMT

So,

LBA, Mode 1, 100 in eigenvalue and nonlinear, mode 1, 700 in eigenvalue

Re: Nonlinear buckling vs LBA

John_Holtz — Wed, 04 Jan 2023 17:53:01 GMT

Hi @autodesk428JLG

The two buckling shapes look completely different, so they are finding different frequencies. The linear result looks like buckling in compression, and the nonlinear result looks like the beam is twisting. You should look at the stress result in the nonlinear analysis (probably identified as "subcase 3") to make sure the load is not causing the model to twist.

John

Re: Nonlinear buckling vs LBA

autodesk428JLG — Thu, 05 Jan 2023 07:40:28 GMT

Good morning!

So, i have contacts as below for my special beam: Offset bonded(Weld) and seperation.

So, you are saying that axial loads in my model, due to the setup of my welded beam, will make it twist before it buckle? I cant see from the result that those are super high:

Re: Nonlinear buckling vs LBA

Roelof.Feijen — Thu, 05 Jan 2023 10:09:58 GMT

Your non-linear buckling analysis contains a Beam idealization, which is not present in the linear buckling analysis.

What is the purpose of this Beam idealization?

Re: Nonlinear buckling vs LBA

autodesk428JLG — Thu, 05 Jan 2023 12:28:25 GMT

Its just my wire that isnt constrained or anything so its just there from the static analysis i did 😃 I think however i got the twisting buckling thing due to the fact that i forgot the friction coefficient.