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Re: Granular ratchetting explained

 

On 26 May 2010 07:51, Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx> wrote:

> For those interested, I elaborated the comment a little in ScGeom.cpp
> (possible wiki paragraph in the future), as this "granular ratchetting"
> needed explanation.
> We could put a simple py script to simulate the cycle explained below, and
> test any law in Yade to see if it generates ratchetting.
>
> Cheers
>
> Bruno
>
>
> "The following definition of c1x and c2x is to avoid "granular ratcheting".
> This phenomenon has been introduced to me by S. McNamara in a seminar help
> in Paris, 2004 (GDR MiDi). The concept is also mentionned in many McNamara,
> Hermann, Lüding, and co-workers papers (see online : "Discrete element
> methods for the micro-mechanical investigation of granular ratcheting", R.
> García-Rojo, S. McNamara, H. J. Herrmann, Proceedings ECCOMAS 2004, @
> http://www.ica1.uni-stuttgart.de/publications/2004/GMH04/), where it
> refers to an accumulation of strain in cyclic loadings.
>               Unfortunately, published papers tend to focus on the "good"
> ratcheting, i.e. irreversible deformations due to the intrinsic nature of
> frictional granular materials, while the talk of McNamara in Paris clearly
> mentioned a possible "bad" ratcheting, purely linked with the formulation of
> the contact laws in what he called "molecular dynamics" (i.e. Cundall's
> model, as opposed to "contact dynamics" from Moreau and Jean).
>             Giving a short explanation :
>       The bad ratcheting is best understood considering this small elastic
> cycle at a contact between two grains : assuming b1 is fixed, impose this
> displacement to b2 :
>       1. translation "dx" in the normal direction
>       2. rotation "a"
>       3. translation "-dx" (back to initial position)
>       4. rotation "-a" (back to initial orientation)
>             If the branch vector used to define the relative shear in
> rotation*branch is not constant (typically if it is defined from the vector
> center->contactPoint like in the "else" below), then the shear displacement
> at the end of this cycle is not null : rotations a and -a are multiplied by
> branches of different lengths.
>       It results in a finite contact force at the end of the cycle even
> though the positions and orientations are unchanged, in total contradiction
> with the elastic nature of the problem. It could also be seen as an
> inconsistent energy creation or loss. It is BAD! And given the fact that DEM
> simulations tend to generate oscillations around equilibrium (damped
> mass-spring), it can have a significant impact on the evolution of the
> packings, resulting for instance in slow creep in iterations under constant
> load.
>       The solution to avoid that is quite simple : use a constant branch
> vector, like here radius_i*normal."
>
>
Very good, thanks for explanation!
Chiara


> --
> _______________
> Bruno Chareyre
> Associate Professor
> 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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