Question #403385 on Yade changed: https://answers.launchpad.net/yade/+question/403385
Jan Stránský proposed the following answer: Hi Mohsen, please really provide a MWE :-) it would be much easier to determine the source of problem.. First some explanation why the equivalence of micro and macro parameters is almost never the case. DEM is discrete, so what is really computed and important on contact is f=k*u f ... force [N] u ... penetration depth [m] k ... contat stiffness [N/m] For easier definition of material models (and to make the elasticity particle size independent, see Bruno's answer), it is possible to define a contact law as stress=E*strain For this, you need to convert the contact to **fictitious** truss, assigning its length L and crosssection area A. L can be easily defined as distance of particles' centers. A is much more tricky, it is basically arbitrary value. It is covnenient to make it proportional to particle size square. It is radius*radius in FrictPhys models, but pi*radius*radius in CPM. So by changing this, you change the explicit micro-macro relationship, preserving the model consistency. Another parameter is interaction ratio (aabbEnlargeFactor and interactionDetectionFactor). With higher value, there is more trusses and basically overall this more trusses makes higher stiffness. Computed E and nu are calculated after axial loading (z axis) up to special > strain (1e-2) as follows: > E=slope of sigma-epsilon graph > nu= - 0.5(epsx+epsy)/(epsz) 1e-2 is relatively high, there might be already some inelastic processes (with the MWE I could tell if yes or no) I could not set it till now. It may be because of high desired nu which is > near 0.35. Setting inter-particle nu as 0.35 causes large values of E which > seems correct; however it is not may case nu=0.35 is relatively high and difficult to achieve with cohesive particle models, this indeed might be the reason. For cylinder with height to diameter ratio of two, I have not found such geometry till now. Also it may be interesting for you to check [3]. They could not achieve a set of inter particle parameters (E, nu, phi) that matches the macro ones. I think it is also the case for [2]. What do you think am i right? > Can we say: It is very hard to find a geometry satisfying this hypothesis: > 'micro and macro parameters are the same'. If it is true then: yes, it is hard, but there is no reason for it (see above). 'Is there any reference that can interpret the difference between micro > and macro parameters theoretically '? Is it a usual case in DEM > modeling? It is a usual case in DEM modeling. The difference is intrinsic feature of DEM models. Anyway, my question finds still no answer: In references [1] and [2] > have they compared values of micro and macro E and nu? I could not find > any details about geometry in [2]. yes, macro E, nu and micro (young and poisson) parameters were compared. Cubic periodic cell was used, bud the elastic parameters should be independent on geometry. There is another point that i forgot: why in CPM material the ratio of > ks/kn is called Poisson ration? That is clear by increasing ks/kt (or > inter particle nu), macro nu decreases as the lateral deformations are > affected by shear relative movements. Hence such a physical definition > (ks/kn) and terminology (nominating it as nu) are not consistent. it is not only in CPM, but through almost all material models. I agree that the name is not the best chosen, but on the other hand, it is the only parameter influencing macroscopic Poisson's ratio :-) You can observe (and theoretically prove) that changing poisson changes both Poisson's ratio and Young's module, but changing young parameter does change only Young's modulus, but not Poisson's ratio. cheers Jan 2016-10-25 16:27 GMT+02:00 Bruno Chareyre < question403...@answers.launchpad.net>: > Question #403385 on Yade changed: > https://answers.launchpad.net/yade/+question/403385 > > Bruno Chareyre proposed the following answer: > Actually no, the micro-macro relations can always be put in a way such > that they do NOT depend on particle sizes. > In Yade, at least, it is the case (and it is obviously a good feature as > you may found out). If you agree that cylinder size/aspect ratio should not > play a role either, then interaction radius if any is the only remaining > geometrical parameter, but it is in the end a contact parameter more than a > geometrical parameter. > > What should bee kept the same in every geometry is porosity, > coordination number, and fabric (an)isotropy. I think that's it. > > Bruno > > -- > You received this question notification because your team yade-users is > an answer contact for Yade. > > _______________________________________________ > Mailing list: https://launchpad.net/~yade-users > Post to : yade-users@lists.launchpad.net > Unsubscribe : https://launchpad.net/~yade-users > More help : https://help.launchpad.net/ListHelp > -- You received this question notification because your team yade-users is an answer contact for Yade. _______________________________________________ Mailing list: https://launchpad.net/~yade-users Post to : yade-users@lists.launchpad.net Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
