Re: Problem of bilaplacian brick.

[Yves Renard]

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
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  http://math.univ-lyon1.fr/~renard

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