Hi all, and Ryan in particular,
I have started implementing large deformations into the code. Nothing is finished yet, but I have already some proof-of-concept.
I will commit relatively soon (i.e. after my holidays) a patch adding the hyperelastic neo-Hookean material.
Ultimately, I would like to have in SfePy all the functionality I had in its predecessor (a matlab code called mafest1), but it is a long term goal, as it was more a one-purpose tool than a general FE system.
All that the old code can do is nicely summarized in my thesis, that you can find at
http://ui505p06-mbs.ntc.zcu.cz/sfe/RobertCimrman
at the bottom (thesis.pdf).
Ryan, try to look in particular at Section 2.1.2 (TL formulation), Chapter 3 (FE discretization in TL context) and Chapter 6 (FE discretization in a linear problem context), as it may provide you some insight. Appendix A might be also interesting, as the usual constitutive (stress-strain) relations are summarized there, e.g. the neo-Hookean one.
r.