How to prevent distortion of TetrahedralFEM in collision?

[Wong]

Hi everyone,

I think some behaviors of the soft body using Tetrahedral-FEM-like force field are not realistic when collision occurs. Distortion can always be seen in arbitrary collision.

You can download the attachment and see more details in the video and the scene within:
https://www.dropbox.com/s/ev7z9g5hw959xel/distortion.rar?dl=0

So how the solve this problem?

Wong

[Hugo]

Dear @outtt

This is a complex question.
First of all, in FEM, tetrahedra are known to be prone to “distortion” or more commonly called “numerical locking”. Their efficiency in term of integration has this drawback. Hexahedra are more robust to this problem.

But looking into your scene, the first point would be to use a dense mass (MeshMatrixMass) and not a lumped mass (DiagonalMass). Second, you need to use appropriate values for your physical properties (mass density, Young’s modulus etc). Finally, when this is not sufficient, decreasing the mesh refinement and the time step is the only solution.

I hope I gave several point of investigation here. Do not hesitate to keep us posted.
Best wishes,

Hugo

PS: curious question, why are you not using boundary conditions (like using the FixedConstraint) and applying a RestShapeSpringsForceField instead ?

[Wong]

Hello @hugo,

Can you tell me how to use MeshMatrixMass because there is no existing example.

I use RestShapeSpringsForceField because I do not want to make the fixed part so hard sometimes.

Wong

[Hugo]

Hey @outtt

You’re correct. I will add this to my todolist.

Adding a MeshMatrixMass in a python scene would look like:

node.createObject("MeshMatrixMass", name="SparseMass", massDensity="0.1", topology="@TopologyContainerLink")

I hope this helps.
Best

Hugo

[Author]

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