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Can the plasticity ratio of EEPA determine the deformation of particles

Hello everyone, I would like to inquire whether overlapping is used instead of particle deformation in Edem software. Why did I set a high plasticity ratio when compressing particles using the EEPA model, and the rebound of the particles was still significant even after releasing the pressure. And many particles will separate after significant overlap and release of pressure, which is equivalent to no plastic deformation.And I also found that when a large constant pulling force is applied, the particle blocks will shrink significantly, sticking together and overlapping greatly I hope to receive everyone's answers, thank you.

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jpmorr

Two different issue here.

Plasticity ratio can be set between 0-1 where 0 is a perfectly elastic contact and 1 is perfectly elastic. If you are still seeing some significant rebound with a plasticity ratio greater than 0.9, there is some other issue with the simulation.

As for the large constant force causing shrinkage, this is to be expected as it is an adhesive force that increases the overlap. However, your adhesion force should not be so large that is it causing significant or even noticeable shrinking. This suggest ill-chosen parameter or an insufficiently small timestep.

yfzz

Thank you very much for your answer, but I recently found out that particles in EDEM do not undergo deformation, and overlap is only used to assist in calculating contact forces. I am wondering if the plastic deformation of EEPA is reflected macroscopically through overlap. If you have any insight on this, I would appreciate your response. Thank you.

PrasadAvilala

Hi yfzz,

If you set plasticity ratio 0.9 but still observe rebound might be due to adhesive force i.e Particle to geometry interaction, please check which physics model is defined.

Also, can you try with some larger values like 0.95 or 0.99.Thanks,

Thanks,

Prasad a

yfzz

Hi,

Changing the plasticity ratio can indeed reduce rebound. However, even when I set the plasticity ratio to 0.95, the rebound is still much larger than that in the real physical experiment, and the magnitude of the compression force also deviates significantly. When I set the adhesion force between particles to a constant value, the particles will pull apart, causing the particle block to shrink.

I've basically conducted simple single-factor experiments on each parameter of my physical model. The influence of each parameter on rebound is not that significant.

Another important question I have is whether overlapping in the software can be considered equivalent to particle deformation. Because in the macro - scale, relying solely on particle rearrangement, the compaction definitely cannot achieve the rebound amount of particle deformation in real physical tests.

If overlap is not completely equivalent to deformation, I'm thinking whether I should not consider the parameter of springback in my research.I hope to get your reply.Thanks.

Thanks,

jpmorr

Correct, particles in DEM do not deform and we ideally use high stiffness to prevent significant overlap, which is an assumption of the method. All DEM model parameters are numerical parameters, and although they often aim to represent real material parameters, this can not always be achieved accurately.

The plasticity parameter allows the EEPA model to capture plastic deformation of a cohesive material on a macro scale, although the parameters do need to be calibrated for your material. The other key purpose of the overlap parameter is to track the stress history of the material - higher stresses lead to larger overlaps and larger historical overlaps should lead to high cohesive forces. This is something the EEPA model will do that the JKR model wont.

jpmorr

Your rebound will depend on many things - particle stiffness, plasticity ratio, adhesive force, friction and bulk porosity. A highly porous, highly adhesive material can easily be perfectly plastic in behaviour. If you have a dense packing, then the major controlling parameter should be the plasticity ratio. Also, the unloading stiffness is a multiple of the loading stiffness - if you have the wrong initial loading stiffness, then you will likely not get the correct unloading behaviour.

How you consider your model will answer your question on whether your model is equivalent to reality. If you are simulating micron sized particles that only re-arrange in space in a loose packing with mm sized DEM spheres in a dense packing, then you need to ask is your source of deformation likely to be the same - clearly not. If this assumption is OK for your model, then you treat your large DEM particles as agglomerates, whose deformation can be captured by the plasticity parameters quite well.