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Re: Granular ratchetting explained


Hi Chiara,
allow me a question about the way we compute the shear part. Following the Cundall's model, no problem of ratcheting would arise since he defines the branch vectors as radius*normal.

I recently checked PFC manual, and it defines shear with OC vector, resulting in ratcheting. Maybe you'll find radius*normal in other Cundall's papers, not sure, but at least PFC3D gives ratcheting.

Now in Yade if we avoid granular ratcheting we follow exactly the same way. So, apart from ratcheting, why should the current length of the interaction be preferred? Why this was introduced in the code?

I think it is often introduced by DEM programmers because it is somehow intuitive, once you admit that particles can overlap each other, with a "contact point" somewhere in the overlap. Current length is also used in torque definition, whatever the definition of shear displacement. Personally, I try to never use the words "overlap" or "penetration", and refer to relative displacement of centers instead. The "overlap" concept is not needed to derive the equations and it misleads people (another example is the computation of void ratio : people sometimes tend to remove the "overlapping" volume, which not suitable in most cases IMHO).


Bruno Chareyre
Associate Professor
ENSE³ - Grenoble INP
Lab. 3SR
BP 53 - 38041, Grenoble cedex 9 - France
Tél : +33 4 56 52 86 21
Fax : +33 4 76 82 70 43

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