Viscoelasticity is modeled with parallel **group_materi_maxwell_chains**.
Each of the chains contains a spring with stiffness in line with a dash pot
with relaxation time ( indicates the -th maxwell chain).
The viscoelastic stress rate is given by (with is the elastic
tensor modulus of the -th maxwell chain (depending on and the
poisson ratio))

tochnog 2001-09-02