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## Re: [Getfem-users] Plasticity brick

 From: Yves Renard Subject: Re: [Getfem-users] Plasticity brick Date: Mon, 24 Sep 2012 15:16:02 +0200 User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120827 Thunderbird/15.0


Dear Kostas,

This behavior is not intentional. In fact the elastic threshold s in the
brick is taken to be |\sigma^{dev}| and thus it represent \sqrt{2/3}
\sigma_{vm} ...
I agree that it would be clearer if the elastic threshold would be
expressed in term of Von Mises stress. This can be changed.

Yves.

Le 21/09/2012 16:03, logari81 a écrit :
> Hi all,
>
> I have recently worked with the elastoplasticity brick and I have one
> relevant comment.
>
> As a scalar measure of stress one can use either the euclidean norm of
> deviatoric sigma or the Von Mises stress. The difference is an
> sqrt(3/2) factor:
>
> $\sigma_{vm} = \sqrt{3/2} |\sigma^{dev}|$
>
> In getfem, |\sigma^{dev}| is used through the whole brick
> implementation except one point. Only in the stresses calculated by
> compute_elastoplasticity_Von_Mises_or_Tresca the result is multiplied
> by sqrt(3/2).
>
> The stresses calculated by "compute_plastic_part" and the input
> parameter "datathreshold" do not include the sqrt(3/2) factor. So the
> user of the brick has to divide the tensile yield strength \sigma_y by
> sqrt(3/2) before assigning it to "datathreshold" and he also has to
> multiply the results of "compute_plastic_part" with the same factor in
> order to receive stresses comparable to the Von Mises definition.
>
> Is this behavior intentional? Wouldn't it be more consistent to either
> remove the factor from "compute_elastoplasticity_Von_Mises_or_Tresca"
> or add it to "compute_plastic_part" and "datathreshold"?
>
> Best regards
>
> Kostas
>
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--

Yves Renard (address@hidden)       tel : (33) 04.72.43.87.08
Pole de Mathematiques, INSA-Lyon             fax : (33) 04.72.43.85.29
20, rue Albert Einstein
69621 Villeurbanne Cedex, FRANCE
http://math.univ-lyon1.fr/~renard

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