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[Jan Stránský <question225776@xxxxxxxxxxxxxxxxxxxxx>]

Question #225776 on Yade changed:
https://answers.launchpad.net/yade/+question/225776

    Status: Open => Answered

Jan Stránský proposed the following answer:
Hello Eugen,

if you look at [1] there is one thing I dont understand.
> I give you an example:
> in [1] you see a box which is placed on a plane. Mass of the box causes
> force N on the plane. Now I push the box with an increasing force T.
> using the maximal possible T for which the box wont start sliding gives me
> a frictionAngle. (static friction)
>
> If we assume that the box is already moving (with constant speed) there
> is an T for which acceleration will be zero - so velocity stays
> constant. This T gives an dynamic frictionAngle.
>
> This leads to my second question from above: Does yade use static or
> dynamic frictionAngle
>

it is similar case as "young" or "poisson" parameters - each contact law
may consider this parameter in different way. The friction angle itself is
related to internal friction angle of a material [1] and (in case of
FrictPhys_CundallStrack) is used for Mohr-Coulumb plasticity condition

shearStress <= normalStress * tan(frictionAngle)

But in general (maybe not all contact laws) Yade uses static friction angle
between two individual particles.

It depends on the simulation setup and what you really call "friction
angle" (like young parameter, it is parameter of one bond, but real Young's
modulus of particle assembly is different). If you have a cube made from
spherical particles placed on "rough plane" made of spherical particles, it
would probably be possilbe (I am not sure) to get different values for
static and dynamic friction angle between such cube and plane (although
using only one parameter of contact law).

But you are rihgt, that these parameters could be documented better :-)


>
> Concerning my first question - let us enhance this little example.
> may the box be made of material1 and the plane of material2.
> If you determine the frictionAngle it is valid for this pair of materials
> only.
>
> Think of a different plane made of material3.
> The experiment from above will give you a diffrent frictionAngle. But when
> you simulate this box on a plane in Yade both materials must be given a
> frictionAngle, from which the matchmaker [2] will use the lowest for
> calculation of interactions[3].
>

Acoording to your link to MatchMaker, you can define what law will be used
for "averaging" (min, max, average..)


>
> This leads to a problem: if frictionAngle of material1 is lower than
> material2 and material3 than the matchmaker will use its frictionAngle
> whatever plane it will be placed on. Hence force T would be equal for
> both planes. This does not go in hand with real experiments.
>
>
see above.


> As I mentioned earlier - in my eyes frictionAngle cant be defined for a
> single material but for a pair of two materials.
>

it can be defined for two bodies (possible with the same material) and if
the material is composed of many bodies, it has some its own angle of
repose [1], which is related to (internal) friction ange of such material.

HTH
Jan

[1] http://en.wikipedia.org/wiki/Angle_of_repose

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