🐛 Bug report | [Bug Title] strain energy density return 0

[gabixz]

Description:
Hi,
I was trying to export the strain energy density and some other energy-related parameters in my simulation.
However all the energy-related parameters export 0 value. I checked your code and found out that it was set to 0 by default now.
Is it on purpose? Will this function/parameter restored in the future version?
Thanks a mill,
Gabi

Version:
4.13.0
Steps To Reproduce:
1.

[gateshian]

Hi Gabi,

Your bug report seems to be incomplete. You started by providing Steps To Reproduce, but your presentation is truncated.

As far as I know, the only material that returns 0 strain energy density is the 'viscoelastic material' (and the associated 'uncoupled viscoelastic material'). The reason for that is that the quasilinear viscoelasticity formulation employed in this material model is not derived from an energy function (it is an ad hoc formulation). Technically speaking, one can only provide a strain energy density for this viscoelastic material under infinitesimal strains in an isotropic material. Since this is a very constraining set of conditions, and since FEBio is a finite strain solver, we decided to remove the strain energy density, but only in this material. All other materials in FEBio should return a non-zero strain energy density (if they don't, please let us know and we will fix this bug).

Best,
Gerard