yade-dev team mailing list archive

[Jan Stránský <_honzik@xxxxxxxxxx>]

Hi Bruno,

thanks for comments :-)

in the case of PeriTriaxController the symetry is anyway irrelevant, because only diagonal components are used..

>>    This definition of stress is assuming static equilibrium. In this situation, there is a mathematical proof that the result will be symmetric, even if it there is no obvious symmetry in the formula. IIRC, the proof is based on (1) the decomposition of branch into O1C-O2C, and (2) the fact that the total torque of all forces on one grain is 0.
Ok, I have nothing to oppose with :-) do You have any literature or link about it? as my master thesis will probably deal with this problem, I would like not to omit some important facts, and this proof seems to be very powerful and useful.

>If we are not stricly at static equilibrium, this definition does not apply. DEM is never stricly at equlibrium, so this stress is never exactly symmetric, but it is usually very close.
I agree that all these formulas are only estimations for dynamic problems. From my point of view, the role of dynamics is only the fact, that the computed stress is estimated and inaccurate, not the antisymetric. But I have no proof about it, it is only my personal opinion (as influenced by continuum mechanics, antisymetric stress tensor will always be weird for me :-)

>you get something symmetric. Ok, it looks better, but you are in fact only hidding the initial mistake : abusing static equlibrium assumption, and the result in itself is not better.
Ok, my opinion is explained above.. let me show an example: the current definition is
stress=f*branch.transpose().
why it is not
stress=branch*f.transpose?

>I'd keep the non-symmetric definition, and the non symmetry of the result would just remind people to be carefull with that.
>For dynamic cases, there are other definitions of stress, considering kinetic energy of grains, probably not what you want at the moment.
Not yet, but it is on my plan list to learn something about  them :-)

Thank You ones more, it makes me thinking about it more detailed :-)
Cheers,

Jan