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# Conatct analysis with a negative eigenvalue

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## Conatct analysis with a negative eigenvalue

(OP)
Dear Abaqus users,

I have a problem while doing a 3D contact analysis (nonlinear analysis) with Abaqus and thus need your help or suggestions. The model consists of a sandwich plate connected by rivets with an al-bracket. Both of them are deformable.
I meshed all with C3D8 elements. I applied vertical and horizontal force at the top the sandwich plate.

Now the problem is that negative eigenvalues always appear during analysis.
I noticed that if the model was coarsely meshed, then the analysis can be completed though there was still one negative eigenvalue. But if the model is finely meshed, much more negative eigenvalues appeared and the analysis can't be completed. Actually I don't know what the negative eigenvalue means.
This problems primary appeard by defining the contact pairs between the cylindrical rivet an the interior faces of the sandwich/bracket.

Could you please give me some explanations? And in this case how to solve the problems? Thank you very much!

JV

### RE: Conatct analysis with a negative eigenvalue

Negative eigenvalues might be because one part of the assembly isn't restrained and you're seeing rigid body motion. Try applying a first step of restraining the assemblies and displacing them to make contact to set up the contact forces. Then apply a second step where any unnecessary restraints are removed ,together with the applied contact displacements, and then apply your true forces.

### RE: Conatct analysis with a negative eigenvalue

(OP)
thanks corus,
you were right.. I found a rigid body motion and I restrained it.. now it works..

bye JV..

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