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[Eugen Kubowsky <question225776@xxxxxxxxxxxxxxxxxxxxx>]

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

    Status: Answered => Open

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.

> ...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.

> 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.

> 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.

> 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?

>> 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?

@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. 

@wasabi:
I agree that there is a difference between frictionAngle between two materials as shown in [2] and angle of internalFriction as mentioned in [3]. So if the frictionAngle was merely used for Mohr-Coulomb condition than it could be interpreted as angle of internal friction. 

@Bruno:
> The three bodies problem you refer to is the classical caveat of computing contact parameters from bodies properties. I had 
> the same problem years ago with PFC. 
Could you please give a brief summary of your PFC problem and your solution? maybe this helps.

> Note that your three bodies problem has in fact an easy solution without matchMakers.
> If you want friction f12 between b1 and plane 2, and friction f13 between b1 and plane 3, then why would you assign the 
> minimum friction paramater to b1? You are not looking for troubles are you? ;)
> Since b1's friction is a meaningless parameter with the only purpose of defining contacts friction, you will give it any value (say > 6e66), then you will assign f12 to plane 2 and f13 to plane 3.
Yes, this can be a great solution, providing that the dummy friction of b1 is used for that purpose only. I'm not sure about that

> Of course, if we must have at the same time friction f23 between planes 2 and 3, we are toasted already, then goto: 
> matchmakers.
Good news are: I can guarantee that planes 2 and 3 arent longing for contact ;-)

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...

---
[1] https://en.wikipedia.org/wiki/Polyurethane
[2] http://www.tribology-abc.com/abc/friction.htm
[3] http://en.wikipedia.org/wiki/Angle_of_repose
      it's interesting that the german version of this wikipedia page is relly diffrent vom the english one (any german speaking here?)
      http://de.wikipedia.org/wiki/Reibungswinkel

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