Interaction of two different materials goes crazy

Hi Behzad,

is it possible to create two interactions, one
with CohBurgersMat-CohBurgersMat, the other "the same", but
CohBurgersMat-CohBurgersMat and check its phys and geom properties to see
if / how they are different? What about damping parameters? The system with
damping might be stable for larger time step than non-damped system..

If rotational speed goes unstable and decreasing time step helps (guessing
from the end of your email), than the stiffness related to rotational
motion is too high for original time step. How do you compute the time step
in your case?

cheers
Jan

2016-02-01 23:21 GMT+01:00 behzad <email address hidden>:

> New question #284341 on Yade:
> https://answers.launchpad.net/yade/+question/284341
>
> Hey guys,
>
> I have interactions of two different materials in my model.
> The first material is CohFrictMat. and the second one is CohBurgersMat
> which has viscoelastic properties and is a subclass of CohFrictMat.
>
> The contacts of these two materials are handled by
> Ip2_CohFrictMat_CohBurgersMat_CohBurgersPhys.
>
>
> But, the problem is whenever we have this interaction in the model, the
> bodies rotational velocities go unstable and I need to reduce the DOFs by
> blocking the rotational DOFs.
>
> Is there any other way to treat this problem without blocking the
> rotational DOFs or using small time-steps?
>
> Can I do something on the contact model?
>
> Thanks.
>
>
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- I didn't understand the question (or comment)

- I have damping=0.4 in the model.

- 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?

Thank you,
Behzad

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

Jan,

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

I see. I think the reason is that I have not defined any parameters to control or limit the rotation and twisting. I have only defined the rules to compute and normal and shear load-displacements.