How to use only rolling but no twisting when using CohFricMat.

Asked by Leonard on 2020-11-22

I am using CohFricMat[1] and CohFrictPhys_CohesionMoment law(useIncrementalForm=True,always_use_moment_law=True) for my simulations. I referred to many good papers which seem to use the same contact law (I'm not sure). And I got some doubts as follows:

For example, in paper [2] and [3], they introduced the rolling resistance (rolling stiffness Kr and dimensionless rolling coefficien eta are defined in the paper), so I guess it is the CohFricMat that was used. But I didn't find any parameters about twisting (i.e., Ktw and etaTwist). Did they use another material? Or did they also use CohFricMat but just not active the twisting? If so, my question is how to introduce only rolling but no twisting when using CohFricMat.



[2]Tejchman, J., J. Kozicki, and D. Leśniewska. "Discrete simulations of shear zone patterning in sand in earth pressure problems of a retaining wall." International Journal of Solids and Structures 48.7-8 (2011): 1191-1209.

[3]Hosn, R. A., Sibille, L., Benahmed, N., & Chareyre, B. (2017). Discrete numerical modeling of loose soil with spherical particles and interparticle rolling friction. Granular matter, 19(1), 4.

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Jérôme Duriez
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Best Jérôme Duriez (jduriez) said : #1


I'm not the best one to speak about [3] but I bet they did use CohFrictMat. To do so without twisting, you just have to assign Ktw or (and, if you wanna wear "belt and suspender") etaTwist to 0, and twisting relative movements won't be restricted by any torque.

(I have in mind restricting those do not change much the results anyway)

Luc Sibille (luc-sibille) said : #2


What Jérôme explained is exatly what has been done in [3].


Leonard (z2521899293) said : #3

Hi Jérôme and Luc,

Thanks very much for the reply from both of you! This perfectly answered my question.

Just another curiosity: I didn't get the meaning of the "(belt and suspender)" in #1.
Is there a difference between the case"Ktw =0 or etaTwist=0" and the case "Ktw =0 and etaTwist=0"? if I understand the answer correctly, set any one of the two parameters to zero can make the twisting inactivated.


Jérôme Duriez (jduriez) said : #4

> I didn't get the meaning of the "(belt and suspender)"

It comes from a French expression.. ;-) Which you can use to speak about two measures to prevent a risk, when just one is enough.
I see some theoretical differences among all choices in "Ktw =0 or etaTwist=0" and "Ktw =0 and etaTwist=0" (the torque will be null either because of a zero elastic stiffness or because of a zero limit plastic threshold, or both) but the consequence is always the same.

Leonard (z2521899293) said : #5

Thanks Jérôme Duriez, that solved my question.

> I have in mind restricting those do not change much the results anyway

It depends very much on how strongly their are restricted, in fact. :)
There's no rolling without twisting with 3D spheres. So you could freeze everything with just repulsion and twisting resistance.