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Negative eigenvalues

mahmoodseraji
Dear friend

I am using Abaqus for analyzing a composite plate under bending, but
unfortunately it does not complete and i got some warning like this:
The system matrix has 3 negative eigenvalues
i tried to find a proper solution for this warning from different forums. the
deformed shape seems to be OK, but when i check the vertical reaction forces, i
see some of them are negative. i think the negative support reactions are the
reason behind the negative eigenvalue. i am still wrestling with that in Step
module, but no result till now. do you have any recommend?
 Best Regards
-------------
Mahmood Seraji
H/P: +60-17-319-5820


     

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Re: Negative eigenvalues

Akhil Sharma
Hi Mahmud,

 Negative eigenvalue just means that the plate would buckle if the load is
applied in the opposite
direction, i.e. opposite to what you already have in the model. In such cases,
the first positive eigenvalue
usually corresponds to the fundamental buckling mode for the direction of load
applied.

 This is generally not a problem in the analysis. If your analysis stops
randomly, it may be due to something else.
Check the .msg and .dat files. The analysis stops randomly only due to errors
(or due to numerical singularity, which usually
occurs after significant non-linearity in the system). You seem to have some
other problem. It should be displayed in either of the

two files I mentioned.

Regards,

Akhil





________________________________
From: MAHMUD SERAJI <[hidden email]>
To: abaqus <[hidden email]>
Sent: Sun, 21 November, 2010 9:06:19 PM
Subject: [Abaqus] Negative eigenvalues

 
Dear friend

I am using Abaqus for analyzing a composite plate under bending, but
unfortunately it does not complete and i got some warning like this:
The system matrix has 3 negative eigenvalues
i tried to find a proper solution for this warning from different forums. the
deformed shape seems to be OK, but when i check the vertical reaction forces, i
see some of them are negative. i think the negative support reactions are the
reason behind the negative eigenvalue. i am still wrestling with that in Step
module, but no result till now. do you have any recommend?
 Best Regards
-------------
Mahmood Seraji
H/P: +60-17-319-5820

[Non-text portions of this message have been removed]


 



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Re: Negative eigenvalues

alireza mahdavi
In reply to this post by mahmoodseraji
Dear Mahmood
Nagitive eigenvalues may be preduced because of the elements, which goes
under distorting conditiones. Some of the reasones are:
1-material properties have not defined properly.
2-geometrical properties of the meshes are not applicable or not enough for
converging the solutions.
3-Boundry Conditions have not restrained enough, so the model goes to be act
as a mechanism.
I hope checking of these reasones can help you.
With best regards.
Alireza Mahdavi
On Mon, Nov 22, 2010 at 5:36 AM, MAHMUD SERAJI <[hidden email]>wrote:

>
>
> Dear friend
>
> I am using Abaqus for analyzing a composite plate under bending, but
> unfortunately it does not complete and i got some warning like this:
> The system matrix has 3 negative eigenvalues
> i tried to find a proper solution for this warning from different
> forums. the
> deformed shape seems to be OK, but when i check the vertical reaction
> forces, i
> see some of them are negative. i think the negative support reactions are
> the
> reason behind the negative eigenvalue. i am still wrestling with that
> in Step
> module, but no result till now. do you have any recommend?
>  Best Regards
> -------------
> Mahmood Seraji
> H/P: +60-17-319-5820
>
> [Non-text portions of this message have been removed]
>
>
>


[Non-text portions of this message have been removed]

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Re: Negative eigenvalues

mahmoodseraji
Thank you dear Akhil and Alireza for your reply.

the analysis stopped at the last increment.
what about the negative support reaction obtained from the analysis?  
Best Regards
-------------
Mahmood Seraji
H/P: +60-17-319-5820




________________________________
From: alireza mahdavi <[hidden email]>
To: [hidden email]
Sent: Mon, November 22, 2010 6:40:36 PM
Subject: Re: [Abaqus] Negative eigenvalues

 
Dear Mahmood
Nagitive eigenvalues may be preduced because of the elements, which goes
under distorting conditiones. Some of the reasones are:
1-material properties have not defined properly.
2-geometrical properties of the meshes are not applicable or not enough for
converging the solutions.
3-Boundry Conditions have not restrained enough, so the model goes to be act
as a mechanism.
I hope checking of these reasones can help you.
With best regards.
Alireza Mahdavi
On Mon, Nov 22, 2010 at 5:36 AM, MAHMUD SERAJI <[hidden email]>wrote:

>
>
> Dear friend
>
> I am using Abaqus for analyzing a composite plate under bending, but
> unfortunately it does not complete and i got some warning like this:
> The system matrix has 3 negative eigenvalues
> i tried to find a proper solution for this warning from different
> forums. the
> deformed shape seems to be OK, but when i check the vertical reaction
> forces, i
> see some of them are negative. i think the negative support reactions are
> the
> reason behind the negative eigenvalue. i am still wrestling with that
> in Step
> module, but no result till now. do you have any recommend?
> Best Regards
> -------------
> Mahmood Seraji
> H/P: +60-17-319-5820
>
> [Non-text portions of this message have been removed]
>
>
>

[Non-text portions of this message have been removed]





     

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Re: Negative eigenvalues

Akhil Sharma
Hi Mahmud,

 As I said before, negative eigenvalue means that the plate will buckle if the
load is reversed. With the reversed load,
you will get the opposite support reaction that expected. (The meaning of
negative eigenvalues is typically given in the dat file
of the analysis).

I'm not sure what you mean by "last increment". Is it the last eigenmode you
asked for or is it the last increment
of the first mode where the analysis stops? If it is the last eigenmode, then I
would suggest that your input have more

eigenvectors requested than the eigenvalues, i.e., if you are asking for the
first 25 eigenmodes, ask for , say 30 eigenvectors.
Else, ABAQUS usually has trouble with convergence for the last eigenmode.
If it is the first mode where the problem is, the error should be displayed in
either the msg or dat file.

Regards

Akhil






________________________________
From: MAHMUD SERAJI <[hidden email]>
To: [hidden email]
Sent: Mon, 22 November, 2010 8:02:57 PM
Subject: Re: [Abaqus] Negative eigenvalues

 
Thank you dear Akhil and Alireza for your reply.

the analysis stopped at the last increment.
what about the negative support reaction obtained from the analysis?  
Best Regards
-------------
Mahmood Seraji
H/P: +60-17-319-5820

________________________________
From: alireza mahdavi <[hidden email]>
To: [hidden email]
Sent: Mon, November 22, 2010 6:40:36 PM
Subject: Re: [Abaqus] Negative eigenvalues

 
Dear Mahmood
Nagitive eigenvalues may be preduced because of the elements, which goes
under distorting conditiones. Some of the reasones are:
1-material properties have not defined properly.
2-geometrical properties of the meshes are not applicable or not enough for
converging the solutions.
3-Boundry Conditions have not restrained enough, so the model goes to be act
as a mechanism.
I hope checking of these reasones can help you.
With best regards.
Alireza Mahdavi
On Mon, Nov 22, 2010 at 5:36 AM, MAHMUD SERAJI <[hidden email]>wrote:

>
>
> Dear friend
>
> I am using Abaqus for analyzing a composite plate under bending, but
> unfortunately it does not complete and i got some warning like this:
> The system matrix has 3 negative eigenvalues
> i tried to find a proper solution for this warning from different
> forums. the
> deformed shape seems to be OK, but when i check the vertical reaction
> forces, i
> see some of them are negative. i think the negative support reactions are
> the
> reason behind the negative eigenvalue. i am still wrestling with that
> in Step
> module, but no result till now. do you have any recommend?
> Best Regards
> -------------
> Mahmood Seraji
> H/P: +60-17-319-5820
>
> [Non-text portions of this message have been removed]
>
>
>

[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]


 



[Non-text portions of this message have been removed]

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