[Dune] Help requested for my first test case in non linear mechanics

[HELFER Thomas 202608]

Hi Oliver,

thanks for this feedback. I will compare my stiffness matrix to the one get with another code,
that probably the best way to go. But I felt rather confident...

But it does seems like something nasty is happening in my code.

Something is itching me about the linear solver: the defect becomes very high but the solver
converges to the given criterion (a reduction to 1.e-5). However, my Dirichlet boundary
conditions are not met.

So either my handling of the Dirichlet boundaryd condtions is wrong, or my usage of the linear solver is
(or both).

Is there an example of how to replace the linear solver by a direct solver ?

Regards,

Thomas
________________________________________
De : Oliver Sander [oliver.sander at tu-dresden.de]
Envoyé : jeudi 16 avril 2020 09:07
À : dune at lists.dune-project.org; HELFER Thomas 202608
Objet : Re: [Dune] Help requested for my first test case in non linear mechanics

Hi Thomas,

> It almost works. I can simulate a tensile test for an elastic behaviour and a plastic behaviour.
> I probably can use any smal strain behaviour generated by MFront. The non-linearity is treated
> very crudely as we use an Newton algorithm with a fixed number of iterations.

that sounds pretty good already.

> The trouble is that it only works if the grid contains only a single element. This probably means
> that I made a mistake in the assembly of the stiffness matrix or the assembly of the inner
> forces. I reread my code several times without finding the trouble.

Did you check whether the matrix and forces are assembled correctly?
What I would do is choose a very small grid (say, 2x2), assemble everything,
and write the matrix and forces to a file (or print them to the screen).
Then compare with one of your working implementations.  This will allow you
to narrow down the cause of your problem.

Helpers for writing/printing vectors and sparse matrices can be found in dune/istl/io.hh.

Bonne chance,
Oliver



>
> Please find enclosed my project, modified to drop the dependency to the MGIS library and
> only integrate an isotropic elastic behaviour. The relevant file is
> dune-mgis/src/dune-mgis-tensile-test.cc.
>
> The purpose of my request is twofold:
>
> - I would be grateful if anyone could find what I have done wrong.
> - I would be grateful if anyone could review the code to tell me what could be improved.
>
> With kind regards,
>
> Thomas Helfer
>
>
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