 Kelvin-Voigt step-stress creep

For $E\epsilon +\eta \dot{\epsilon }={\sigma }_0\Theta (t)$ with initially zero strain, $\epsilon =({\sigma }_0/E)(1-e^{-t/\tau })$ and $\tau =\eta /E$. Strain is continuous at load application, approaches a bounded value and recovers exponentially after unloading.