[SOLVED] Elastoplastic deformation

[Andres]

Hello everyone,

I’m trying to implement an elastoplastic behaviour on a simple 3d model of what would be a colon. I’m using the TetrahedronFEMForceField for doing so by tunning the parameters of the plasticYieldThreshold, plasticMaxThreshold and the plasticCreep. However, the deformation of the object always seems to be elastic no matter what parameter I set. I checked already the different scenes in the SOFA files such as the TetrahedronFEMForceField_plasticity or the strain example in plugins>flexible in an attempt to correct any possible errors in my scene but I fail to find out what’s wrong. I’ve seen other people having kind of the same problem in other topics in the forum, but I couldn’t find a solution with that either.

I attached here the link to a dropbox file containing the scene and the obj. file in case someone wants to have a look.
Scene and obj.

At the moment, the objective is to at least observe a deformation similar to the one in the TetrahedronFEMForceField_plasticity scene. I’m new to SOFA so maybe there’s something I’m missing out.

Thanks,
Andrés

[Hugo]

Hi Andrés

I deeply apologize for the delay of my reply.
I actually already came once on your post, but the behavior is so strange and I could not explain it. I need to take a bit more time to investigate it.

I poke @krewcunc who has experience with plastic codes in SOFA now 😉
I will also give another look to your point.

Best wishes,

Hugo

[Andres]

Hi Hugo,

No problem about the delay and I apologise for my late replay too!
In the end I decided to implement an hyperelastic behaviour with HyperelasticFEMForceField which is working just fine and is more backed up by literature.
However, I’m still interested in knowing what’s wrong with the plasticity parameters in the tetrahedronFEMforcefield hence, if it’s possible, I’ll keep this thread open just in case someone knows what’s the problem.
Thank you for the attention.

Kind regards,
Andrés

[CamilleK]

Hi Hugo, hi Andrés,

I’m also sorry for delaying this issue (which is indeed of interest to me, you poked right Hugo ;)). I have quiet a lot on my plate at the moment, but it should be better by the end of next week. I’ll gladly have a look on the issue then !
I’ll eventually update this tread if I find anything interesting.

Regards,

Camille

[CamilleK]

Hello,

Sorry once again for this late answer, but I managed to get plasticity deformation in your scene file @Andres.

I modified slightly the scene to get conditions close to the ones in TetrahedronFEMForceField_plasticity.scn, in order to understand what was going on. So I removed the FixedConstraint component, and added a PlaneForceField component so that the colon would just crash on the planeforcefield under it’s own weigth. I also multiplied by 10 the mass (from 0.7 to 7) in order to amplify this effect. The last thing I did was to apply a scale factor to the dimensions of the colon mesh : as the gravity you defined was expressed in SI units (9.81 m/s^2), the dimensions of the mesh are supposed to be in meters for coherency. The coordinate values I could see in the .obj were globally between 0 and 40, so I applied a 0.01 scale factor to stand for a colon with maximum dimensions around 40cm.

My guess is that there was nothing wrong with your scene, but that the values you used for the plasticity parameters did not allow plastic deformation, probably either because the deformation didn’t go above the plasticYieldThreshold you specified (although the one you used was pretty low), or (and more likely) because the plasticMaxThreshold was too low and therefore did not allow for much plastic deformation.
I used the following line for the TetrahedronFEMForceField parameters :
<TetrahedronFEMForceField name="FEM" youngModulus="4000" poissonRatio="0.4" method="large" computeGlobalMatrix="false" plasticYieldThreshold="2e-5" plasticMaxThreshold="2.5e-2" plasticCreep="0.1"/>
to see plastic deformation when the colon mesh hits the PlaneForceField. For instance if you switch the plasticYieldThreshold to “4e-5”, it will be too high for plastic deformation to occur.

Hope this answers the issue.

Cheers

Camille

PS : I used <PlaneForceField normal="0 1 0" d="-3e-1" stiffness="100000" showPlane="1" showPlaneSize="1"/> for the PlaneForceField and scale3d="1e-2 1e-2 1e-2" in the MeshObjLoader options.

[Andres]

Hello Camille,
Thank you for your answer and apologies for my late reply! I’m glad to see that there was no particular error in the scene other than the set values, it looks like I did not think them through enough. Your answer indeed fixed this issue, thank you. I will mark this topic as resolved.

Kind regards,
Andrés

[Author]

Posts