Dear Tetsuo,
The bilaplacian is a fourth order derivative term and the weak
formulation contains second order derivative terms of the unknown and
the test function. So this is normal that you obtain zero for a P1
Lagrange finite element. In fact, you have to use a C1-conformal finite
element to use the bilapalcian brick. For instance Argyris or HCT elements.
Best regards,
Yves
On 12/12/2020 06:52, Tetsuo Koyama wrote:
> Hello, GetFEM project.
>
> I have a problem using the add_bilaplacian_brick method of the Model
> object. My wish is to get the following stiffness matrix in the
> attached script file, but I got an empty stiffness matrix.
>
> Trace 2 in getfem_fourth_order.cc, line 88: Stiffness matrix assembly
> of a bilaplacian term
> Computed matrix is...
> matrix(2, 2)
> ( )
> ( )
>
> Expected matrix is...
> matrix(2, 2)
> ( (r0, 100) (r1, -100) )
> ( (r0, -100) (r1, 100) )
>
> I'm sorry, but could you please review my script to see if there is
> any problem?
> I used the developing version in the git repository.
--
Yves Renard (Yves.Renard@insa-lyon.fr) tel : (33) 04.72.43.87.08
INSA-Lyon
20, rue Albert Einstein
69621 Villeurbanne Cedex, FRANCE
http://math.univ-lyon1.fr/~renard
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