Calculating Strain Energy Density in Hyper-Elastic Analysis

Hello,

I am trying to have CodeAster calculate the strain energy density of my model (which uses elastic and hyper-elastic) elements. Upon reading the document: (CALC_ELEM: U4.81.01 Revision: 3835) CALC_ELEM, and it gives an error:

[code:1]
!

---

!
! <EXCEPTION> <ELEMENTS_15> !
! !
! la nature du matériau ELAS_HYPER n'est pas traitée !
! seules sont considérées les natures : ELAS, ELAS_ISTR, ELAS_ORTH . !
!

---

!
[/code:1]
Is it possible to get the strain energy density with CodeAster and hyper-elastic materials and if so how can I command it? I have tried reading the docs and not found the correct answer so far. Could somebody point me in the right direction please? Thank you.

In all the reading of docs & the forum, and various trials with POST_ELEM, CREA_CHAM, I am still stuck. Would somebody kindly throw me a hint? Thanks.

Regards,
JMB