Elastic Plates vs Linear Elasticity

[stoykov]

Hello,

I just started to use Elmer, and the first thing to do is to solve linear eigenvalue problem of elastic beams. I wish to compare the eigenvalues of thin beam which I obtained by Bernoulli-Euler or Timoshenko beam theories with Elmer. The problem is that if I use define the equation as Elastic Plates and define the beam as plate, with very small width, I obtain correctly the eigenvalues. But if I define the equation by using Linear Elasticity and the beam mesh is 3D, then I have much higher and obviously wrong eigenvalues. Does anyone have an idea why the problem occurs for Linear Elasticity models?

Thanks,
Stan

[raback]

Hi

I would guess that you might be using linear elements and are experiencing "locking". Try setting in Solver section

Code: Select all

Element = p:2


To use 2nd order elements.

-Peter

[stoykov]

Thank you Peter, now it works very good

Could you please tell how can I define higher order elements using ElmerGUI or do I have to define it in Gmsh (I am using Gmsh to define the mesh)?

Thank you in advance,
Stan

[raback]

Hi

You can do that in Gsmh by choosing quadratic elements. In ElmerGUI you can probably set the "Element = p:2" in free text input of the Solver specific options. This creates p-elements which are however, not the same as the Lagrange elements created by Gmsh. The p-elements are hierarchical which may result to easier matrix equations. Unfortunately the p-elements cannot increase the accuracy of the geometric presentation. If this is your wish the quadratic elements in Gmsh may be better choice.

-Peter