 Series spring and Voigt-branch creep

A spring $E_0$ in series with a parallel spring-dashpot branch $E_1,\eta$ has creep compliance $J(t)=1/E_0+(1/E_1)(1-e^{-E_{1}t/\eta })$. It includes immediate and delayed recoverable strain, but no unbounded viscous flow. Large strain, damage or anisotropy require additional constitutive structure.