Neo-Hookean elastic VS viscoelastic

[tdetroux]

Hello,

I would like to investigate the effect of viscosity on my simulations (cylinder with prescribed harmonic rotation ~50Hz along its main axis). To this end, I am comparing two types of material:
(1) Elastic - neo-Hookean (Young = 6000 Pa, Poisson = 0.45 and density = 1100 kg/m3).
(2) Viscoelastic (g1 = 0.67 and tau1 = 0.1s) with the same neo-Hookean law as in (1).

Simulations using configuration (2) lead to a system which seems much stiffer than when I use configuration (1). I repeated the simulation in configuration (2) but this time I considered a relaxation parameter g1 = 0, which was supposed to cancel the viscous effect; however, the results were still different than for configuration (1) (much stiffer).

Could you please explain why? Is it wrong to suppose that g1 = 0 would cancel the viscous effect?

Thank you very much for your help!

[ateshian]

Hi,

In principle what you are doing should work, i.e., setting g1=0 should recover the elastic response. The fact that you get a very different response suggests that the issue is elsewhere. Can you clarify if your analysis is static or dynamic? If dynamic, you should move the <density> field from the <elastic> material up to the parent <viscoelastic> material.

Best,

Gerard

[tdetroux]

Hi Gerard,

Thank you very much for your help! My analysis is dynamic, and moving the <density> field to the parent <viscoelastic> material, as you suggested, could solve the issue and brought more consistency in my comparisons.

What is the effect of moving density from elastic field to parent viscoelastic field? Is there a way to make this shift automatically using FEBio Studio?

[ateshian]

Hi,

In FEBio2 the density is only taken from the parent material, that's why you need to move the <density> field to the <viscoelastic> material. In FEBio3 this issue will be fixed.

Regarding FEBioStudio, the density field needs to be added as a material parameter for the parent viscoelastic material, we will make this change for future releases.

Best,

Gerard