mix boundary control

Asked by Chiara Modenese on 2010-12-05

Hi,

question on the way the servo-control algorithm works in case we want to control strains and stresses on the boundaries at the same time (e.g. triaxial phase). I am referring here to the PeriTriaxController engine. I am not sure to understand how the algorithm works when you have the mix control. In particular, how is the user supposed to set the maxStrainRate in such a case? (Bruno?)

Hope the question is clear, otherwise let me know.
Thanks! Chiara

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Last query:
2010-12-05
Last reply:
2010-12-06

Hi Chiara,

You can't apply a strain rate directly, so you define a target strain and a max strainRate, and it will load at this rate on direction k, as soon as k is not stress-controlled.
This is how I set periTriax for a constant strain rate loading in direction 1, and constant stress on 0 and 2 (a few revisions ago, but I don't think it changed) :

triax.goal=[-1e4,triax.strain[1]-finalStrain,-1e4]
triax.maxStrainRate=[rate*10.0,rate,rate*10.0]
triax.stressMask=5

In goals "triax.strain[1]-finalStrain" means final strain with reference to current state.
Higher rates are allowed for axis 0 and 2, so that its not a limiting factor for stress regulation (maxima apply for all boundaries).
stressMask=5 set which directions are stress controled with bit mask (True*1+False*2+True*4).

I hope it helps.

Bruno

Hi Bruno,
thanks for answer. I was wondering if permitting a higher rate could not be
problematic. I mean, I see the need for such a limitation. However, I am not
sure whether the factor of 10 you propose is always enough or perhaps too
restrictive in some cases. In the end everything seems to require
calibration :-(
Btw, looking at the code, strain_rate does seems to be constantly applied
even if the control is of strain type (line 201?).
cheers. Chiara

On 5 December 2010 20:10, Chareyre <email address hidden>wrote:

> Your question #136704 on Yade changed:
> https://answers.launchpad.net/yade/+question/136704
>
> Status: Open => Answered
>
> Chareyre proposed the following answer:
> Hi Chiara,
>
> You can't apply a strain rate directly, so you define a target strain and a
> max strainRate, and it will load at this rate on direction k, as soon as k
> is not stress-controlled.
> This is how I set periTriax for a constant strain rate loading in direction
> 1, and constant stress on 0 and 2 (a few revisions ago, but I don't think it
> changed) :
>
> triax.goal=[-1e4,triax.strain[1]-finalStrain,-1e4]
> triax.maxStrainRate=[rate*10.0,rate,rate*10.0]
> triax.stressMask=5
>
> In goals "triax.strain[1]-finalStrain" means final strain with reference to
> current state.
> Higher rates are allowed for axis 0 and 2, so that its not a limiting
> factor for stress regulation (maxima apply for all boundaries).
> stressMask=5 set which directions are stress controled with bit mask
> (True*1+False*2+True*4).
>
> I hope it helps.
>
> Bruno
>
> --
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>

Nah, don't be afraid. :-)
If you consider Poisson's ratio (the real one) and usual dilatancy angles, you'll see that
x10 will not limit any realistic deformation.
If it was, you would see lateral stress increasing during the loading phase.

Bruno

> Btw, looking at the code, strain_rate does seems to be constantly applied
> even if the control is of strain type (line 201?).

I forgot to answer this one : it is constantly applied, see line 208.

B.

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