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Large deformations

Large deformations

Large deformations

(OP)
Hi,
  I need to use large deformations with Abaqus\standard so I activate the Nlgeom command in the step module but I receive a lot of error messages and my job is usually aborted. Where's my error?
Do you also know where I can find a tutorial about non linearities and large deformations?
Thanks a lot

RE: Large deformations

Ewoks,
There are a number of things that can go wrong with large deformation analyses, including buckling, plastic collapse and elements becoming too badly distorted. Let us know what you are trying to do and give some examples of the warning messages and the main error message in the .msg file. In particulary show us some warnings close to the final error message.
MRG

http://www.analysis.demon.co.uk

RE: Large deformations

Sighhh.... We need more information. Like, what's the error message for a start? Also "useful" (i.e. also absolutely necessary):

* a complete and full description of the problem and model
* the type of analysis
* physical description of the behavior in question you are trying to model

Try not to be so vague. This will do you no favours at all. Professionally it helps you as well if you're able to present a problem in a clear and concise way. Vague is what vague does. Writing out a clear, logical step-by-step description of your problem has two benefits. Most important is that it solidifies the situation in your own mind. Going through your work mentally, step-by-step, helps you understand exactly what you know and what you don't know. Second, with a clear statement of the problem, you're likely to get an answer quicker (or an answer at all for that matter).

Hope this is useful,

-- drej --

RE: Large deformations

(OP)
Ok, on monday I'll be again in my lab and i'll provide you more details about the problem.
Remember that i'm learning abaqus for two weeks and that i don't have any detailed manual...
Thanks a lot

Tommy

RE: Large deformations

(OP)
Ok, here me back.
Let's try to explain my problem. I have a cylinder modelled as a shell. Thickness is 0.075, radius is 4.76 and lenght is 15. Young module is 4.84e-6 and poisson 0.19. One end is encastred and the tube must be subjected to compression. I need to see large deformations (as a compressed Coke can), cause the tube should pack on itself. I've done a general/static analysis but the tube only decrease in lenght, without any large deformation. So, how should I set up my analysis?
The problem is the same as a compressed steel cylinder or a beam with large deformations, no matter which material properties I have and which dimensions....
Hope these new infos helps.
Thanks a lot
Tommy

RE: Large deformations

what are the error messages as seen from the message file (.msg)
harry

RE: Large deformations

(OP)
For harry123456

I had errors like:
the solving matrix diverges
there are x negative eigenvalues

Thanks
Tommy

RE: Large deformations

1.are you trying this without plasticity-related properties?
2.in addition, to get a "compressed Coke can"-like result I think you might need a very refined mesh and quadratic elements. You also might need to adjust the minimum allowed step increment.

RE: Large deformations

Ewoks,

the problem you're describing arises if you're working with a geometrical perfect cylinder. You need a kind of geometric imperfection or a perturbation load to let the structure run into a buckling mode. Use *Imperfection to add for example the first buckling mode (scaled) to your structure and then rerun your analysis.

BTW, I recommend to use an explicit code like PamCrash to simulate the crash of your cylinder

Best regards
Pam

RE: Large deformations

(OP)
Hi again,
        which kind of analysis should I set up to have a better behaviour of the structure in a compression case?
Is "linear, buckling" analysis the one that I should use? Or is better to use "static, Riks"?
By the way, which are the application fields of these two methods?
Thanks a lot

Tommy

RE: Large deformations

Hi,

a buckling analysis is used to determine the buckling mode shapes of a structure and the critical buckling loads. If the load you want to apply is higher than this critical buckling load your structure may fail due to buckling.

If you are interested in the behaviour (e.g. deformations) of your component under a given load (a load possibly above the critical load) resp. the postbuckling behaviour running a static, risks can be a solution

Pam

RE: Large deformations

Ewoks,

Pam is right, you need to perform an eigenvalue buckling analysis on the tube first to find the buckling modes.

You can then use these modes together with the *IMPERFECTION command in Abaqus explicit using the *DYNAMIC, EXPLICIT command  to allow the tube to collapse.

Matt

RE: Large deformations

(OP)
Hi there,
I'm pretty new in abaqus so I have a very basic question: how can I use the *IMPERFECTION command? I opened the Keywords file and I tried to write the command but I still have no results.
Can someone please write for me an "example file" with the *IMPERFECTION command or, simply, the command line?
For example, can you write the *IMPERFECTION command you would use in the analysis of a compressed cylinder?
Thanks a lot

Tommy

RE: Large deformations

Ewoks,

See abaqus 6.4 example problems manual section 1.2.6.

This seems to be pretty much what you're trying to do.

Matt

RE: Large deformations

(OP)
I've done a buckilg analysis, searching for 20 eigenvalues and this analysis worked well. Now I've my buckilng mode for my structure but I'm not able to use them in a riks analysis. I'm new to abaqus and i'm not so good in using it, so it's also hard to use the documentation....
Is it possible to use commands like *IMPERFECTION directly with the graphic interface of the program?
Another question: the *IMPERFECTION command requests a file with the buckling eigenvalues. How can I create it or where does abaqus create it?
Thanks a lot

Tommy

RE: Large deformations

(OP)
The *IMPERFETCION command requires a .fil file (result file) with the buckling mode obtained from a previous buckling analysis. I've done this analysis and I got my eigenvalues but I did not have any .fil file as output.
What I need to do?
What output I need?

Another question: I have to perform a postbuckling analysis like a STATIC, RIKS after a BUCKLING analysis. Do I need to use the same file with two different steps or can I simply do two separate file?

Thanks a lot

Tommy

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