# yade-users team mailing list archive

## Re: How to keep a constant stress boundary?

> The algorithm is : displacement = (stress offset) / (total box-spheres stiffness)
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On one hand you compute the total stiffness on boundary N = the sum of all sphere-box(N) contact stiffnesses.
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On the other hand you have the current stress (or, say, the current force if you prefer) applied on boundary N. From this you can define the "offset", which is (target stress - current stress).
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Then, the displacement computed in the equation above is the one "so that the new force will be exactly the one we want after the displacement". (assuming spheres are not moving in the meantime, which is not true, but it will be corrected again on the next time step, so it finally converge relatively well)
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This algorithm has no name that I know.

Bruno

kan a écrit :
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```Thanks, Bruno,
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I checked the code, but I still did not understand the physical meaning of the code. May I ask : what is the physical meaning behind the code? or what is the mathematic of the algorithm?
```how is it called ?
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Thanks a lot.
```yongfeng

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On Fri, Sep 11, 2009 at 10:22 AM, Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx <mailto:bruno.chareyre@xxxxxxxxxxx>> wrote:
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More precisely, it is in TriaxialStressController. This engine
needs to be assigned 6 boxes (one for each boundary). The boxes
are used to define the surfaces (since you need a surface to
define a stress).
If you don't have 6 boxes, you need to find a workaround.

The algorithm is : displacement = (stress offset) / (total
box-spheres stiffness)

Bruno

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```

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