Effect of Assigning Different Materials to Orphan Mesh Sections 1

[Emre SENGUL]

Hello everyone,

I'am trying to form a vertebrae model and I have a orphan mesh model of a vertebrae (C3D8R). I directly assigned different materials to outer and inner elements (separated with sections) and I could solve it without any error. Although the inner elements have a small young modulus value relatively, I see huge difference between results of this model and results of other model without inner elements (in stress/moment value manner). What might be the reason of it?

Best regards

 

[FEA way]

If I understand it correctly, you compare the results of two models - one with flexible core and one without core (empty space inside), right ? Make sure that all units are correct first. If you can, please attach some pictures showing these models (colored by material) and describe the way they are loaded (including magnitudes). Also what are the values of Young's moduli ?

 

[Emre SENGUL]

Yes you are right, I have two models. One model is that I assigned different materials to outer and core section. Other one is that I obtained from the first model by deleting the core elements. And outer elements have 12000 MPa young modulus while core elements have ~100 MPa young modulus. I obtained ~30 Nm in the first model and ~7 Nm in the second model during the same movement for each.

 

[FEA way]

The difference is not that large. Even though the core elements are very flexible, when compared with outer elements, they still provide additional resistance to applied load (torsion, apparently).

Maybe hyperelastic material model would be more appropriate for core elements.

 

[Emre SENGUL]

Actually, I thought that it might be. However, in the literature, effect of core elements doesn't create as ~5 times greater moment. In addition, there are some models used under bending loads without core parts. So, I think thickness of outer side might cause this results. (Usually, they use linear elastic material for core elements in the literature.)

On the other hand, I am not sure whether the calculation of the stress transfer between outer and core sections can be done correctly, when I directly assigned different materials to outer and inner elements. Could I trust the results?

 

[FEA way]

It's not a problem that you assigned different materials to these layers of elements in orphan mesh. In fact, that's a really common approach. Only stresses at the interface between these two layers may be affected, global response shouldn't change.