Negative Eigen Values

[aneesh-2]

Hello Everybody

I am a student and I recently started using Abaqus for my research project. I am trying to model bolted joint of 2 plates(1 plate is steel and another plate is orthotropic). I have modeled the plates and the bolt by using hexahedral elements. I get a warning saying that " The system matrix has 1 negative eigenvalues " and the analysis completes without any errors. While going through the data available online i found out that Eigen values are generally associated with buckling problems. In my case are the eigen values present because of the bolt buckling or is it because of the plates or is it some numerical instability due to low stiffness present in my model. Can someone please help me out here...

Thank You,
Aneesh

[arun sunny]

Hi
Sometimes, negative eigenvalues are reported in an eigenvalue buckling analysis. In most cases such negative eigenvalues indicate that the structure would buckle if the load were applied in the opposite direction. A classical example is a plate under shear loading; the plate will buckle at the same value for positive and negative applied shear load. Buckling under reverse loading can also occur in situations where it may not be expected. For example, a pressure vessel under external pressure may exhibit a negative eigenvalue (buckling under internal pressure) due to local buckling of a stiffener. Such "physical" negative buckling modes are usually readily understood once they are displayed and can usually be avoided by applying a preload before the buckling analysis.

Negative eigenvalues sometimes correspond to buckling modes that cannot be understood readily in terms of physical behavior, particularly if a preload is applied that causes significant geometric nonlinearity. In this case a geometrically nonlinear load-displacement analysis should be performed.

Refer:
http://ol.cadfamily.com/ABAQUS/Documentation/docs/v6.6/books/usb/default.htm?startat=book01.html#usb

--- In Abaqus@..., "aneesh" <aaneeshkr@...> wrote:

>
>
>
> Hello Everybody
>
> I am a student and I recently started using Abaqus for my research project. I am trying to model bolted joint of 2 plates(1 plate is steel and another plate is orthotropic). I have modeled the plates and the bolt by using hexahedral elements. I get a warning saying that " The system matrix has 1 negative eigenvalues " and the analysis completes without any errors. While going through the data available online i found out that Eigen values are generally associated with buckling problems. In my case are the eigen values present because of the bolt buckling or is it because of the plates or is it some numerical instability due to low stiffness present in my model. Can someone please help me out here...
>
> Thank You,
> Aneesh
>

[srinivas srinivas-2]

negative eigen values indicate the rigid body motion. means you have not defined any constraints to the model. i mean the tie constraints or the contacts. if you have defined check properly for the tolerances.

      study about the errors, you get others like neumerical singularity error also if you have over constrained.

         HOPE THIS HELPS YOU...




________________________________
From: aneesh <aaneeshkr@...>
To: Abaqus@...
Sent: Fri, 30 October, 2009 12:31:34 PM
Subject: [Abaqus] Negative Eigen Values

 


Hello Everybody

I am a student and I recently started using Abaqus for my research project. I am trying to model bolted joint of 2 plates(1 plate is steel and another plate is orthotropic) . I have modeled the plates and the bolt by using hexahedral elements. I get a warning saying that " The system matrix has 1 negative eigenvalues " and the analysis completes without any errors. While going through the data available online i found out that Eigen values are generally associated with buckling problems. In my case are the eigen values present because of the bolt buckling or is it because of the plates or is it some numerical instability due to low stiffness present in my model. Can someone please help me out here...

Thank You,
Aneesh


   


      Yahoo! India has a new look. Take a sneak peek http://in.yahoo.com/trynew

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