= Solution
In a \b[<material creep> experiment], apply a small step <shear stress> $\sigma_0H(t)$ and measure the resulting <strain>. The <creep compliance> is $J(t)=\gamma(t)/\sigma_0$. An elastic solid reaches a bounded <strain>; a fluid can continue deforming.
In a \b[stress-relaxation experiment], impose a small <strain> step $\gamma_0H(t)$ and measure <stress> while holding the deformation. The <relaxation modulus> is $G_{\rm rel}(t)=\sigma(t)/\gamma_0$ after the loading transient. A fluid's stored <shear stress> can decay to zero, while an elastic solid retains a nonzero long-time contribution.
The original curves distinguish the controlled input from measured output. The <strain>/<stress> must be small enough for <linear response>, and real finite loading rise time, instrument compliance, wall slip and <temperature> must be characterized. Otherwise an instrumental transient can be mistaken for a material relaxation mode.
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