Viscoelasticity

Viscoelasticity is modeled with $n$ parallel group_materi_maxwell_chains. Each of the chains contains a spring with stiffness $E^m$ in line with a dash pot with relaxation time $t^m$ ($m$ indicates the $m$-th maxwell chain). The viscoelastic stress rate is given by (with $C_{ijkl}^m$ is the elastic tensor modulus of the $m$-th maxwell chain (depending on $E^m$ and the poisson ratio))

$$\sum_{m=0}^{m=n-1} ( {C_{ijkl}}^m \dot{\epsilon_{kl}}^{\rm elas} - \frac {{\sigma_{ij}}^m}{t^m} )$$