Nonlinear Buckling Imperfection from Linear Buckling - How to do?

[Anonymous]

Hello all,

I want to do nonlinear buckling analysis, I want to use mode shape from eigen buckling (image below) as initial geometric imperfection.

Please can someone show me how to do this?  I am using nastran 2019

Thanks

aiyubi

[Anonymous]

Hello all,

I want to do nonlinear buckling analysis, I want to use mode shape from eigen buckling (image below) as initial geometric imperfection.

Please can someone show me how to do this?  I am using nastran 2019

Thanks

aiyubi

[shigeaki.k · Accepted solution]

Hello @Anonymous ,

you can export the scaled displacement by setting the parameter "TRSLDFGMDATA" to "ON". This should create a *.bdf file in the folder "InCAD" in one of the subfolders where your model is located. Use a text editor to optn the *.bdf file and you should see someting similar to what is shown below.  You can use DISPGEOMSFACT to contro the scale of the results. 

Nastran In-CAD works with CAD files so you will need to use another 3rd party software to plot these results, convert it to a CAD model and import it back into Inventor. 

$
$ OUTPUT PRODUCED BY ADS NASTRAN VERSION 13.2.0.168 11:53 01/24/19
$
$ DEFORMED GEOMETRY OUTPUT FOR MODE = 1 SCALE FACTOR = 1.0
$
$-------2-------3-------4-------5-------6-------7-------8-------9-------0-------
GRID* 1 0 7.4390316169951 2111.9066855483
* -216.xxx-xxxxxxxx 0
GRID* 2 0 2.8265117685209 26.134434939553
* -19.664739377517 0
GRID* 3 0 0.7185908079235 8.2318667844749

Regards,

Shigeaki K.

-----------------

If my post answers your question, please click the "Accept as Solution" button. This helps everyone find answers more quickly!

[shigeaki.k · Accepted solution]

Hello @Anonymous ,

you can export the scaled displacement by setting the parameter "TRSLDFGMDATA" to "ON". This should create a *.bdf file in the folder "InCAD" in one of the subfolders where your model is located. Use a text editor to optn the *.bdf file and you should see someting similar to what is shown below.  You can use DISPGEOMSFACT to contro the scale of the results. 

Nastran In-CAD works with CAD files so you will need to use another 3rd party software to plot these results, convert it to a CAD model and import it back into Inventor. 

$
$ OUTPUT PRODUCED BY ADS NASTRAN VERSION 13.2.0.168 11:53 01/24/19
$
$ DEFORMED GEOMETRY OUTPUT FOR MODE = 1 SCALE FACTOR = 1.0
$
$-------2-------3-------4-------5-------6-------7-------8-------9-------0-------
GRID* 1 0 7.4390316169951 2111.9066855483
* -216.xxx-xxxxxxxx 0
GRID* 2 0 2.8265117685209 26.134434939553
* -19.664739377517 0
GRID* 3 0 0.7185908079235 8.2318667844749

Regards,

Shigeaki K.

-----------------

If my post answers your question, please click the "Accept as Solution" button. This helps everyone find answers more quickly!

[Anonymous]

Hello shigeaki

Thanks for the help

I was expecting more direct workflow in InCAD, but anyway with your info of "TRSLDFGMDATA", for now I am able to do this with NASTRAN Editor, but hopefully there is less "painful" way to do this in future InCAD version

Thanks

aiyubi

[Anonymous]

Hello shigeaki

Thanks for the help

I was expecting more direct workflow in InCAD, but anyway with your info of "TRSLDFGMDATA", for now I am able to do this with NASTRAN Editor, but hopefully there is less "painful" way to do this in future InCAD version

Thanks

aiyubi

[Anonymous]

Hi aiyubi

I'm looking to perform a similar process but I can't make the jump from the text in the .BDF file to a CAD object - how did you achieve this?

Many thanks, TEddy

[Anonymous]

Hi aiyubi

I'm looking to perform a similar process but I can't make the jump from the text in the .BDF file to a CAD object - how did you achieve this?

Many thanks, TEddy

[dido32511]

Hi, I am trying to do the same thing but I don't know what to do after modifying the BDF file since nastran doesn't accept this format.

Can you suggest a bit detailed procedure after modifying the BDF file ?

[dido32511]

Hi, I am trying to do the same thing but I don't know what to do after modifying the BDF file since nastran doesn't accept this format.

Can you suggest a bit detailed procedure after modifying the BDF file ?

[John_Holtz]

Hi @dido32511 

Just to clarify, Inventor cannot do anything with the BDF file. Nastran, as in the Nastran Editor Utility, can read the BDF file. See How to edit the input file in Nastran In-CAD | Inventor Nastran | Autodesk Knowledge Network where you would be using the Nastran Editor.

But working with a plain Nastran file to make it an analysis is not an easy task. You would need to understand the commands in the Nastran file to be able to modify or add commands to the BDF file. Generally, you would setup a "dummy" analysis with everything you want, see what commands are put in the Nastran file to do that, and then transfer those to the BDF file.  The typical user is not going to do that.

Unless extreme accuracy is required, it is better to modify the CAD model based on the results of the initial analysis. For example, the "bubble-like" displacements shown in the plate model in the original post would be used to stretch the original CAD model to reproduce the shape. The modified model would be sent into Inventor Nastran to setup the analysis.

By the way, I know that someone, somewhere has a program to convert a mesh (such as the BDF file) into a CAD solid model. I have searched for such a program in the past but have not found one. Autodesk Fusion 360 can read a "mesh" and create a model, but I think the mesh needs to be the outside surface only, not the complete volume mesh. If that is true, it still leaves the step of converting the solid mesh to a surface mesh, assuming you are working with solids. (The original post could be a solid or a shell/surface -- it is hard to know from the image.)

John

[John_Holtz]

Hi @dido32511 

Just to clarify, Inventor cannot do anything with the BDF file. Nastran, as in the Nastran Editor Utility, can read the BDF file. See How to edit the input file in Nastran In-CAD | Inventor Nastran | Autodesk Knowledge Network where you would be using the Nastran Editor.

But working with a plain Nastran file to make it an analysis is not an easy task. You would need to understand the commands in the Nastran file to be able to modify or add commands to the BDF file. Generally, you would setup a "dummy" analysis with everything you want, see what commands are put in the Nastran file to do that, and then transfer those to the BDF file.  The typical user is not going to do that.

Unless extreme accuracy is required, it is better to modify the CAD model based on the results of the initial analysis. For example, the "bubble-like" displacements shown in the plate model in the original post would be used to stretch the original CAD model to reproduce the shape. The modified model would be sent into Inventor Nastran to setup the analysis.

By the way, I know that someone, somewhere has a program to convert a mesh (such as the BDF file) into a CAD solid model. I have searched for such a program in the past but have not found one. Autodesk Fusion 360 can read a "mesh" and create a model, but I think the mesh needs to be the outside surface only, not the complete volume mesh. If that is true, it still leaves the step of converting the solid mesh to a surface mesh, assuming you are working with solids. (The original post could be a solid or a shell/surface -- it is hard to know from the image.)

John