# yade-users team mailing list archive

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

```On Thu, Sep 24, 2009 at 4:50 AM, Bruno Chareyre
<bruno.chareyre@xxxxxxxxxxx>wrote:

> kan a écrit :
>
>> Thanks, Bruno,
>>
>> On Wed, Sep 23, 2009 at 11:04 AM, Bruno Chareyre <
>> bruno.chareyre@xxxxxxxxxxx <mailto:bruno.chareyre@xxxxxxxxxxx>> wrote:
>>
>>    > The algorithm is : displacement = (stress offset) / (total
>>    box-spheres stiffness)
>>
>> hmmmm..... for the unit...
>> stress offset unit is  N/m^2
>> stiffness unit is   N/m  ?am I right?
>> then (N/m^2) / (N/m) =(1/m), this is not the unit of displacement of m, I
>> think if we use force may be better.
>> then Force/stiffness=(N)/(N/m)=m.
>>
>>
> Yes. Or consider that the stifness is a stress/meter (increment of stress
> on the boundary associated to a unit displacement).
>
>
>    On one hand you compute the total stiffness on boundary N = the
>>    sum of all sphere-box(N) contact stiffnesses.
>>
>> ok, here the stiffness is due to the sphere-box are parallel connected
>> along the boundary.
>>
>>
>>    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).
>>
>> so from here, we can get the stress offset, if the applied force (stress)
>> is not the target force (stress)
>>
>>
>>    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)
>>
>> well, if the sphere move fast (such as compressing loose sand to a solid
>> condition), then this may not sufficient to be corrected in the next
>> timestep (I am not very sure , but I feel it may not sufficient to...) . but
>> for static condition, I believe it is correct.
>>  One more question:
>> Assume the boundary spheres will not change (always those spheres nomatter
>> they are moving or not), how about I just always apply a constant force on
>> each sphere (if the stress on this boundary is S, then assume the area of
>> the sphere is A, then we can get the force F=S*A, so that since the stress S
>> is a constant, so the force is also a constant ) and then nomatter it is
>> static or it is dynamic, can we say it is also under a constant stress
>> boundary? what do you think?
>>
>>
> It sounds complex. How will you define which spheres are on the boundary?
> What will happen if one sphere is going out of the packing even with this
> constant force on it? You need a stiffness here, not a force.
>
I have developed an algorithm to determine which spheres are on the
boundary.
Yes, sometimes I may meet the condition that a sphere may go out of
packing....

so with the boundary, it is a rigid boundary, right?
thanks.

>
> Bruno
>
>   Thanks .
>> Yongfeng
>>
>>    This algorithm has no name that I know.
>>
>>    Bruno
>>
>>    kan a écrit :
>>
>>        Thanks, Bruno,
>>
>>         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 ?
>>         Thanks a lot.
>>        yongfeng
>>
>>
>>         On Fri, Sep 11, 2009 at 10:22 AM, Bruno Chareyre
>>        <bruno.chareyre@xxxxxxxxxxx
>>        <mailto:bruno.chareyre@xxxxxxxxxxx>
>>        <mailto:bruno.chareyre@xxxxxxxxxxx
>>        <mailto:bruno.chareyre@xxxxxxxxxxx>>> wrote:
>>
>>           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
>>
>>
>>
>>
>>    --
>>    _______________
>>    Chareyre Bruno
>>    Maître de Conférences
>>
>>
>>    Grenoble INP
>>    Laboratoire 3SR - bureau E145
>>    BP 53 - 38041, Grenoble cedex 9 - France
>>    Tél : 33 4 56 52 86 21
>>    Fax : 33 4 76 82 70 43
>>    ________________
>>
>>
>>    _______________________________________________
>>
>>
>>
>
> --
>
> _______________
> Chareyre Bruno
> Maître de Conférences
>
> Grenoble INP
> Laboratoire 3SR - bureau E145
> BP 53 - 38041, Grenoble cedex 9 - France
> Tél : 33 4 56 52 86 21
> Fax : 33 4 76 82 70 43
> ________________
>
>
> _______________________________________________