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[Jan Stránský <question206415@xxxxxxxxxxxxxxxxxxxxx>]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Jan Stránský proposed the following answer:
Hello Ning,

this equality is of course always valid :-) I had in my mind different
aspect: the derivation of the stress of one (I will call it "investigated")
particle is based on the equation [1]

stress = sum ( x_i * f_i )

where x_i is the contact point (with respect to the center of particle),
where some external force "f" is applied (external in the sense with
respect to the investigated particle). Here we do not care about the
opposite particles at all. For the special case of spheres, this "x" equals
half of the branch vector between investigated particle and the formula can
be accordingly simplified with real branch vectors.

But for the clump case, if you use some point on the line connecting
clumps' centers (which in general differs from real "x"), you are in fact
using different formula.

It is the same case as if you have force acting on rigid body. You can
shift the force to any point, but you have to add some moment to preserve
the equality of these two situations.

@Chiara: Exactly, "the contact point does not need to be located in line with
the branch vector", but at the same time you can't place it to the line
with the branch vector (what is done by the approach proposed by Ning) if
it is located in different place. Or of course you can, but then the result
would be different..

Jan

[1] https://yade-
dem.org/doc/yade.utils.html#yade._utils.bodyStressTensors


2012/12/18 Chiara Modenese <question206415@xxxxxxxxxxxxxxxxxxxxx>

> Question #206415 on Yade changed:
> https://answers.launchpad.net/yade/+question/206415
>
> Chiara Modenese proposed the following answer:
> Hi Jan,
>
> I agree with Ning, the contact point does not need to be located in line
> with the branch vector for the derivation to be valid. See Bagis' paper
> (or similar references).
>
> Chiara
>
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