On 26 May 2010 07:51, Bruno Chareyre <[email protected]> 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 > ________________ > > > _______________________________________________ > Mailing list: > https://launchpad.net/~yade-users<https://launchpad.net/%7Eyade-users> > Post to : [email protected] > Unsubscribe : > https://launchpad.net/~yade-users<https://launchpad.net/%7Eyade-users> > More help : https://help.launchpad.net/ListHelp >
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