NEGATIVE EIGENVALUES 2

[nhekipa]

hello all

I'm doing an 3D airplane's tail using beam elements and when I run job a error occur, in the message file is written that i´ve negative eigenvalues, how to solve this problem?

thanks for the help

 

[xerf]

You should check if the boundary conditions suffice in precluding any potential rigid body motion of the structure. If you have multiple bodies (parts), you should check that their interactions are well defined.

 

[Clay1969]

Also, negative eigenvalue means that your stifness matrix lost its positive definiteness. This could be due to not only rigid motion as xerf said but could be to instability (material or numerical) or failure of your material

 

[Nagesh051]

As clay1969 said, it could be an instability problem. It could be investigated by introducing the nonlinearity in steps. ie material, geometry nonlinearities one at a time.