Is it possible to make bonded particles share the same velocity when using JCFpm?

Asked by jasperb on 2020-01-20

Hi there,

Recently I am using JCFpm to model a cluster of bonded spheres(bond strength is randomly assigned) impacting a layer of fixed spheres.
1. My first question is that after the impact, the member sphere of the cluster demonstrates a wide range of rebound velocity(from 1m/s to 8m/s) even when they are bonded together. My understanding is that, different from BPM where constraint is added so that large difference in two bonded particles' motion is prevented, JCFpm allows deformation and relative displacement of two bonded particles so long as the critical force is not reached. However, my idea is that as a rigid rock, its member particles should better have identical velocity once they are bonded. I am wondering whether it is feasible to make the bonded particles in the cluster have the same ( or not quite different) velocity?

2. My second question is that the bond break criterion seems to only consider the tensile and shear failure. Why torque is ignored and is such practice making sense?

Cheers,
Jasper

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jasperb
Solved:
2020-01-31
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2020-01-31
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2020-01-31
Jérôme Duriez (jduriez) said : #1

Hi,

1. I think you have to choose at some point whether you would like your simulation to involve rigid entities (all components of the entity share the same velocity, modulo angular velocity) or breakable entities (entity components have different velocities, and this is the source for relative displacements and possible breakages)

Breakage can not be described in YADE without prior relative displacements (hence without different velocities), and it is the same with Potyondy-Cundall model [*]. The magnitude of relative displacements / velocities dispersion is a matter of (stiffness) parameters but they exist anyway.

(As for 2., scientific journals/conferences might be more suitable than this technical forum to discuss the validity of the assumptions lying behind YADE code. Anyway, YADE open-source nature enables one to change everything !)

[*] https://www.sciencedirect.com/science/article/pii/S1365160904002874

Robert Caulk (rcaulk) said : #2

Hello,

>>My understanding is that, different from BPM where constraint is added so that large difference in two bonded particles' motion is prevented

Let me try to clarify the difference between the two models. In [1], the cement is modeled explicitly and contributes to the total force on incident bodies. Thus, the bodies can still move, the cement model helps reduce movements. In [1], they also briefly suggest a "simplified" version of BPM where no slip is allowed to occur for bonded particles. That sounds like what you are referring to, but I have not seen many DEM studies using that.

In JCFpm, the cement is not modeled explicitly, however, the normal force is allowed to become negative (pulling on both bodies) until the tensile strength is reached. Thus, the bond is also restricting body movement in a similar manner as the original BPM.

>> I am wondering whether it is feasible to make the bonded particles in the cluster have the same ( or not quite different) velocity?

Assign the same velocity to all the spheres. Am I missing something in the question?

>> However, my idea is that as a rigid rock,

If the particles are simply bonded in JCFpm, the cluster is not (and should not be) perfectly rigid. If you want a perfectly rigid clump, you can clump the bodies, but I dont think that is what you are looking for. If you want your cluster to be "more rigid", then you can increase the contact stiffnesses.

>>Why torque is ignored and is such practice making sense?

Depends what you are studying. JCFpm, as the name implies, is designed to study rock joint behavior in DEM. CohFrictPhys on the other hand, does consider rolling resistance.

Cheers,

Robert

[1]Potyondy, D. O., & Cundall, P. A. (2004). A bonded-particle model for rock. International Journal of Rock Mechanics and Mining Sciences, 41(8), 1329–1364. https://doi.org/10.1016/j.ijrmms.2004.09.011

Jan Stránský (honzik) said : #3

> Breakage can not be described in YADE without prior relative displacements

another possible approach is to have a rigid body / clump and estimate breakage according to external forces acting on the body.
Breaking of a single polyhedron into several smaller pieces was simulated using Yade this way.
The same could be done with clumps - dividing a clump into several smaller clumps or individual particles according to acting forces.
I want to emphasize the word "possible approach". It sounds to me like not very easy and quick task, but with very uncertain results :-)

cheers
Jan

jasperb (jasperb) said : #4

Hey
Thank you so much for your advice.

I also found in [Scholtes2012] that the interaction range coefficient is already implemented in YADE which achieves the high ratio of tensile strength to compression strength. But I searched in the document and found nothing related to this keyword"interaction range". And I also check engines connected with JCFPM but found no concerned attributes. Could you please hhelp me with it?
Thanks!

Jasper

*Scholtès, L., Donzé, F.V. (2012), Modelling progressive failure in fractured rock masses using a 3D Discrete Element Method. International Journal of Rock Mechanics and Mining Sciences (52), pages 18–30. DOI 10.1016/j.ijrmms.2012.02.009 (fulltext)

Robert Caulk (rcaulk) said : #5

I am happy to help you, but first please follow our posting guidelines [1]:

"Do not merge different questions in one single post, even if - in your mind - they are connected to one single project. If your question received satisfying answer(s), mark it answered immediately. Do not use the same thread to jump to another question, open a new question instead."

Thank you.

[1]https://www.yade-dem.org/wiki/Howtoask

jasperb (jasperb) said : #6

Thanks