The St. Venant-Kirchhof, Neo-Hookean are nonlinear for finite deformation (finite strain).
The St. Venant-Kirchoff is only linear in Lagrangian frame, as the 2.PK S stress is "linear" w.r.t. Green-Lagrange strain G. But the relationship of true stress and true strain (Cauchy stress and Almansi strain) is highly nonlinear in the Eulerian frame.
Question 1:
Why do we always say that the material is linear? Is there any material model linear? What is about Biot stress or Biot strain energy?
Question 2:
If there is no linear material model, it is only linear because of small deformation and small strain, so that "linear" means more or less geometrically linear, is that right?
Thanks for reply.
Lei