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[Jan Stránský <question284341@xxxxxxxxxxxxxxxxxxxxx>]

Question #284341 on Yade changed:
https://answers.launchpad.net/yade/+question/284341

Jan Stránský proposed the following answer:
Hi Behzad,


> - I didn't understand the question (or comment)
>

see below, I tried to clarify it


>
> -  I have damping=0.4 in the model.
>

Sorry, my comment for damping is irrelevant in your case, since the
 "safer" material model with damping (CohBurgers) is problematic, not that
without damping (CohFrict)


>
> - I have O.dt=1e-6. The spheres are monosize with r= 2.5e-4 m. With this
> time-step it works well if the materials are all CohFrictMat. The problem
> is when two different materials are in contact.
>
> Is there any independent parameter for rotational stiffness for
> CohFrictMat for example?
>

>From documentation of CohFrictPhys, there are kr, ks and ktw stiffnesses
(rotational, shear, twist) involved for rotational DOFs. I would expect
some equvialent in CohBurgersPhys, right?

So you can check two spheres with some simple scenarios (tension, mutual
rotation, mutual twisting etc.), in one ("correct") case with
CohFrictMat-CohFrictMat-CohFrictPhys, in the other ("incorrect") case
CohFrictMat-CohBurgersMat-CohBurgersPhys. Just check
- the stiffnesses
- the resulting forces and torques on bodies. In the second case, forces or
torques (for instable regime) are higher then "they should be".

there could be two reasons:
- The stiffness is for some reason higher
- stiffness is OK, but forces/torques is not computed correctly

The answers should be in Ip2_CohFrictMat_CohBurgersMat_CohBurgersPhys
and/or Law2_...CohBurgersPhys...

cheers
Jan

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