= Recovery after a finite-duration stress load
{title2=$\varepsilon(t)=\frac{\sigma_0}{E_1}(1-e^{-t_h/\tau})e^{-(t-t_h)/\tau},\quad t>t_h$}
Apply constant <stress> $\sigma_0$ at $t=0$ and remove it at $t=t_h$. For <series spring and Voigt-branch creep>, the series <spring> recovers its <strain> $\sigma_0/E_0$ immediately on unloading. The <Kelvin-Voigt model> branch has <retardation time> $\tau=\eta/E_1$ and must keep continuous <strain> because an instantaneous dashpot displacement would require an impulse. Its post-unloading equation is $\eta\dot\varepsilon+E_1\varepsilon=0$. Solving with the branch's strain at $t_h$ gives the title formula. Thus all remaining <strain> eventually recovers: this <standard linear solid model> has no permanent set.
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