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Damping in BondingV2

Matthias Vink

Hi, I am trying to simulate a very flexible complex 3D shape in EDEM using the bondingv2 model. But I noticed that my meta-particles are very bouncy, even when setting the coëfficicents of restitution to 0.0001 for the sub-particles. I think that there is a significant amount of energy stored in the bonds.

I tried setting the tangential stiffness to zero (or a very small value), this solved the problem when testing on a very small scale (8 subparticles). But when I am using my shape (a chicken fillet) again the meta-particle explodes after a few timesteps of wiggling when it it something with a bit of speed. Lowering the timestep by a factor of 100 did not help

here are some of my simulation settings: (the large bonded disk scale combined met the low stiffnesses ensures sufficient flexibility and ensures that the bonds do not break due to particle seperation.

Is there any other way to damp out the energy (or solve the exploding problem)?

regards,

Matthias

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Stephen Cole

Hi Matthias,

We are developing some new bond models which include damping however these haven't been released yet. You could consider this API version (BondedParticleCustom_3_5_0.zip in the link below) which introduces a simplistic 'rotational friction' to the bonds, this works in a similar way to the Standard Rolling Friction model in that it damps the rotational torque.

https://community.altair.com/community/en/edem-api-contact-model-example-updated-bond-model-example-of-customisation?id=kb_article&sysparm_article=KB0122075

Generally I would not recommend using Restitution lower than 0.1 or maybe 0.05, really low values require very small time-steps to resolve the forces correctly and can end up adding instabilities which give the effect of increasing the restitution.

It's an interesting set of inputs, although there are no specific limits the bond disk scale is a scale of the particle radius and this represents a disk of that radius between the two elements. A disk 1000 larger than the particle would typically resist rotation but physically I'd usually set the max to be around 1.2x the radius. That said the torque feedback is off so no rotational stiffness included on a particle-particle basis. The Strength values and the particle contact radius (different to the bond disk radius) are the inputs which influence when the material breaks.

Regards

Stephen

Stefan Pantaleev_21979

Hi Matthias,

One more thing to be mindful of - if the meta-particle bonds are soft, the disintegration at high velocity impact could also be due to particles coming out of contact. If meta-particles are deforming a lot you need to make sure the contact radius of the constituent particles is large enough to maintain the contact between them, otherwise the bond will disappear without breaking.

Best regards,

Stefan

Matthias Vink

Hi Matthias,

One more thing to be mindful of - if the meta-particle bonds are soft, the disintegration at high velocity impact could also be due to particles coming out of contact. If meta-particles are deforming a lot you need to make sure the contact radius of the constituent particles is large enough to maintain the contact between them, otherwise the bond will disappear without breaking.

Best regards,

Stefan

Hi Stefan,

Yes I know, that what the absurd bonded disk scale is for, I noticed that that allows for a good amount of flexibility while still keeping the meta-particle together. In the documentation, it says that the scale increases the 'radius of the glue' but I think it might also increase the stretch a bond can have before breaking.

regards,

Matthias

Stefan Pantaleev_21979

Hi Stefan,

Yes I know, that what the absurd bonded disk scale is for, I noticed that that allows for a good amount of flexibility while still keeping the meta-particle together. In the documentation, it says that the scale increases the 'radius of the glue' but I think it might also increase the stretch a bond can have before breaking.

regards,

Matthias

Hi Mathias,

The bonded disk scale and the contact radius are different. We model the bond as a cylinder with a radius equal to a fraction of the equivalent radius of the bonded particle pair. This fraction is the bonded disk scale. It controls the stiffness of the bond as a thicker cylinder is stiffer. Currently you have a very low shear modulus but massively thick bonds so the stiffness is still high and that is why the particles are not coming apart and the bonds stay intact but also why the meta-particle restitution is too high and the required time step is too low. The contact radius controls the distance from the sphere center within which contacts including bonds are resolved. You need to reduce the bond disk radius to soften the bonds but increase the contact radius of the constituent particles so that the bond is resolved even it is stretches more and the time step is more manageable. This should result in a more physical behaviour and practical solve times. The contact radius is controlled from the particle template settings in the creator. You should be careful not to increase the contact radius too much, however, because then you would be bonding distant particles in the meta-particle and these bonds might disappear easily at small deformations causing instability. There is a balance to be struck here.

Best regards,

Stefan

Matthias Vink

Hi Mathias,

The bonded disk scale and the contact radius are different. We model the bond as a cylinder with a radius equal to a fraction of the equivalent radius of the bonded particle pair. This fraction is the bonded disk scale. It controls the stiffness of the bond as a thicker cylinder is stiffer. Currently you have a very low shear modulus but massively thick bonds so the stiffness is still high and that is why the particles are not coming apart and the bonds stay intact but also why the meta-particle restitution is too high and the required time step is too low. The contact radius controls the distance from the sphere center within which contacts including bonds are resolved. You need to reduce the bond disk radius to soften the bonds but increase the contact radius of the constituent particles so that the bond is resolved even it is stretches more and the time step is more manageable. This should result in a more physical behaviour and practical solve times. The contact radius is controlled from the particle template settings in the creator. You should be careful not to increase the contact radius too much, however, because then you would be bonding distant particles in the meta-particle and these bonds might disappear easily at small deformations causing instability. There is a balance to be struck here.

Best regards,

Stefan

Yes, and increasing the contact radius beyond r * sqrt(2) will result in unwanted diagonal bonding, making the whole structure stiffer again.

Is there a way to increase this radius after the bonds are formed?

regards,

Matthias

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