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Re: How to keep a constant stress boundary?


Thanks, Bruno,

On Wed, Sep 23, 2009 at 11:04 AM, Bruno Chareyre <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.

> 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?

Thanks .

> 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>> 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
> --
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