This is a special record to model the volumetric stress part of a nonlinear material, given the experimental results of a volumetric compression test (compression in one direction, fixed size in other two directions).

The table
specifies
the strain-stress results for the volumetric compression test.
Together with the poisson ratio as specified in **group_materi_elasti_volumetric_poisson**,
an isotropic law in a nonlinear Young's modulus and a constant poisson ratio
is fitted to this experiment.
The Young modulues in fact is taken as the polynomial
expansion
where
denotes the order of the polynomial expansion (as given
in **group_materi_elasti_volumetric_young_order**).

The poisson ratio should be taken very high, say 0.4999999 or so,
to ensure that the resulting law only models volumetric stresses.
Then afterwards a normal young-poisson isotropic law (**group_materi_elasti_young**
and **group_materi_elasti_poisson**) can be added to get an extra deviatoric part.

tochnog 2001-09-02