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[Jan Stránský <question225776@xxxxxxxxxxxxxxxxxxxxx>]

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

Jan Stránský proposed the following answer:
Hi Eugen,

Eugen Kubowsky is still having a problem:
> Thanks again for all your help so far!
> let me start with Jan:
> @Jan
> frictionAngle <-> internalFrictionAngle <-> contact law:
> > 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]
> I'm using Law2_ScGeom_FrictPhys_CundallStrack and
>  Law2_ScGeom_MindlinPhys_Mindlin right now. For a better impression what
> I'm using Yade for see this screenshot.
> http://s7.directupload.net/images/130404/9x582i65.png
> As you see there are several spheres inside a (green) facet cylinder. The
> grey facet cylinder is rotating around the z-axis on a circular path inside
> the green cylinder.
> All the spheres in my application are bodies made of the same material
> (ceramic), I don't use clumps.
> The cylinders are made of a different material. In some scenarios the
> cylinders are made of the same material (namely steel) - in other scenarios
> the grey cylinder is made of steel and the green cylinder is made of
> polyurethane [1] coated steel.
> So I have three diffrent materials and need to specify material properties
> young, poisson and frictionAngle for each.
>

In my opinion, the MatchMaker is perfect solution for such scenario (see
below).


>
> > ...and (in case of FrictPhys_CundallStrack) is used for Mohr-Coulumb
> plasticity condition
> >
> > shearStress <= normalStress * tan(frictionAngle)
> So what happens if Mohr-Coulumb plasticity condition is hurt? Particles
> wont break, will they? Is it possible to apply a force on a single particle
> in a simulation? This way I could check what happens when Mohr-Coulumb
> plasticity condition is hurt.
>

This law takes place in interactions, i.e. between two particles. If two
particles overlap, they generate some normal force
(Fn=normalStiffness*normalDisplacement). If they further slide or rotate,
this mutual transverse displacement causes shear force
(Fs=shearStiffness*shearDisplacement). In continuum plasticity, this would
be trial stress. Now the plasticity condition is checked Fs.norm() <=
normalForce*tan(frictionAngle). If it is satisfied, nothing happens. If it
is not, then the force (while preserving its direction) is reduced such
that its magnitude satisfied condition Fs.norm() =
normalForce*tan(frictionAngle). In continuum plasticity, this would be
stress return.


>
> > But in general (maybe not all contact laws) Yade uses static friction
> angle
> > between two individual particles.
> Do you mean static friction angle between two particles (e.g. spheres)
> made of the same material? (at least if I use these 2 mentioned contact
> laws) Because this would give a hint of how to determine the static
> friction angle exactly: follow method 2 or 3 from [2] with plane and box
> being made of identical material.
>

Yes (also according to Bruno's answer), these contact laws does
not differentiate between static and dynamic, simply uses plastic condition
mentioned above.


>
> > 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
> forexperiment
> > static and dynamic friction angle between such cube and plane (although
> > using only one parameter of contact law).
> As mentioned above I dont use clumps, so I'm interested in assigning right
> material parameters to my (independent) spheres and my facet models.
> Following this frictionAngle of the material of the spheres means
> frictionAngle of a single sphere.
>

This my note probably does not fit to your scenario, sorry, just ignore it
:-)


>
> > Acoording to your link to MatchMaker, you can define what law will be
> used
> > for "averaging" (min, max, average..)
> Yes, I read about that. But I don't know of any appropriate mechanical
> theory that justifies choosing any of these laws. Considering the
> CundallStrack contactLaw (as you mentioned) this frictionAngle will be used
> for Mohr-coulomb plasticity condition only, right? So it is plausible to
> chose the minmal angle, because this material will fail first. But what
> about MindlinPhys_Mindlin?
>

Just an example (I have no idea what the parameters should be in reality):
for the materials of particles, you know frictionAngle = 45 deg,
particles-steel = 20 deg and particles-coated steel=10 deg. So you simply
define frictionAngle:
particles: 45 deg
steel: 20 deg
coated steel: 10 deg
and use MatchMaker to use minimal value. If the values are oposite, use
maximum, Therefore interpaticle interactions will use their own friction
angle and interactions between particles and other material will inherit
parameters from the other materials.


>
> >> 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.
> But how Yade uses frictionAngle is depending on contact law, right?
> According to Bruno - no contact law distinguishes between static and
> dynamic friction(angle). So again the question: is there any further use of
> frictionAngle than checking plasticity condition when using
> MindlinPhys_Mindlin?
>

According to quick look to source code, it has no other effect.


>
> @Bruno:
> > When body1 and body2 have different values of friction, the minimum is
> used for the contact by default. More sophisticated
> > combinations are possible with matchMakers (do you have something
> precise in mind?)
> As I said before, I dont know the reason why the minimal value of friction
> is used. It would make sense if the value of friction (frictionAngle) is
> used for checking plasticity condition only in Yade.
>

I will just repeat Bruno's answer. Some procedure how to deal with two
different material properties should be set as default. In this case, the
minimum is chosen, nothing else :-) for more sophisticated dealing, use
MatchMaker.


>
> What about this idea:
> using method 2 or 3 from [2] with a box (material1) and a plane
> (material2) gives certain values for friction (angle). Is it possible to
> simulate a single sphere (material1) which is placed on a horizontal plane
> (material2). Now the sphere has to be forced to NOT roll but slide. If I
> apply a increasing horizontal force on the sphere it should at some point
> start sliding. Consequently I could check whether this happens at the
> expected magnitude of horizontal force.
> A second way could be cluming two spheres together, both with material1,
> and again placed on a plane (material2). Now they there's no way than
> sliding if horizontal force is high enough. But I'm not sure if this will
> give the same result as the first setup...
>

Yes, you can play with such simple examples if you really want to
understand what is hapenning :-) in your second example, the force wuold
probably be double as there are two particles :-)

cheers
Jan

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