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[Alexander <question269393@xxxxxxxxxxxxxxxxxxxxx>]

New question #269393 on Yade:
https://answers.launchpad.net/yade/+question/269393

Hello Jan

>But to be scietifically correct, I would let the results as they are,
>commenting why the boundary is different and tell that in limit this
>anomally is a set of measure zero :-) Or you can postprocess the results a
>bit (e.g. taking the prescribed force into account for stress computation)

As i understand the best and most scientific way is the following:

1) for  boundary particles  without prescribed force use bodyStressTensor, but the values will be different in compare with inner particles because of  boundary particles has same interactions in parallel direction, but only half in perpendicular and
diagonal direction (by the way i plotted interactions - http://i11.pixs.ru/storage/0/9/8/pic3JPG_4835644_18081098.jpg :). Also i should tell in the final presentation of my research that in limit this anomally is a set of measure zero. 

2) for boundary spheres with  prescribed force compute stress manually, because of  they have only half interactions in parallel and diagonal direction. So i gotta take into account prescribe force on them, because of force F doesn't belong to any interaction and that's why bodyStressTensor doesn't see it.

3) for inner particles use bodyStressTensor. As i understood from Bruno's previous answer:

<Last thing, as far as per-body stress is a topic of concern: getBodyStress is exact.
<It should be exactly equal to applied stress as long as you account for the volume occupied each sphere consistently (in a <porous medium the local solid phase stress is larger than the macroscale stress).
<In your regular array, you can apply a simple conversion rule for each sphere:
<macroStress = sphereStress*sphereVolume/cubeVolume
<where the cube circumbscribes the sphere.
<The average of the above (in a row or in the full domain) must be exactly equal to the ansys stress - which is itself exactly <equal to applied stress.

The stress values for inner particles must be equal prescribed pressure (1,5e8 in my case) or not? 

Because i tried to check Y component of normal stress for inner particles and it's not 1,5e8 . For simplicity i set exact stress value 1,5e8 to all boundary spheres with prescribe force manually, so as u can see in the picture stress inside the model is lower (made a slice parallel XOY - http://i11.pixs.ru/storage/0/9/1/pic2JPG_2016907_18081091.jpg), at the same time displacements are ok (http://i11.pixs.ru/storage/0/8/4/pic1JPG_4520635_18081084.jpg). May be for inner particles stress become exact in the infinite case as well as for boundary particles  without prescribe force?

4) For displacements and stresses described above were computes with young=1.338e11, but the real value for steel is 2e11.   Should i also tell in my final presentation, that young and pysson in DEM are just some approximation of real physics parameters which is used in FEM (like it was describe in you article). Also u said that:

>For elastic material, you want to fit 2 macroscopic constants, e.g. Young's
>modulus and Poissons's ratio. The main conclusion from the paper:
>- The Young's modulus is directly proportional to 'young' CpmMat
>parameters, so it can be fit trivially.
>- Poisson's ratio does depend on 'poisson' CpmMat parameter, but
>nonlinearly, so you can run simulations with different values to see the
>relationship. As it is 1 variable, the optimization is not so difficult.

Considering your comments is my process of calibration correct?

- i started calibration  from Young's modulus because for the value young=2e11 the model stretched lesser than it should be, i mean the extreme values of Y component of displacement vector was lower.

- After young=1.338e11 value was found, i checked extreme values of X component of displacement vector, so for my model it was [-0.00152,-0.00152], in Ansys it was [-0,0018,0,0018].  So i was satisfied with this, that's why left pysson=0,3 like for the real steel.

I'm so sorry for reposting previous messages and asking same questions over and over again, i just want to make all things  clear.

So is it the best result that i can achieve for uniaxial tension except of increasing the number of particles?  I mean in generally, if it will be a little bit better with another settings then i don't see any point to check them.

with regards Alexander

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