= Solution
A <Kelvin-Voigt model> element places a <spring> and <dashpot> in parallel, giving
$$
\sigma=G\gamma+\eta\dot\gamma.
$$
Under a <stress> step from a relaxed state,
$$
\boxed{\gamma(t)=\frac{\sigma_0}{G}(1-e^{-t/\tau}),\qquad\tau=\eta/G,\quad t>0.}
$$
It captures delayed but bounded <material creep> and eventual <strain> recovery after unloading, rather than steady terminal flow.
For an ideal <strain> step,
$$
\sigma(t)=G\gamma_0H(t)+\eta\gamma_0\delta(t).
$$
After the loading impulse, the <stress> remains $G\gamma_0$. \b[The model does not produce a gradual stress-relaxation tail at held <strain>.] A real finite-rate ramp replaces the impulse by a finite <dashpot> <stress> during loading, but the subsequent held-strain <stress> is still constant. Thus this model describes <material creep> retardation but not a solid's genuine time-dependent relaxation toward another finite modulus.
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