Fig. 5c and d also show the best-fit curves using the 3-parameter standard linear solid (SLS) model, whose configuration is
schematically shown in Fig. 6. Since the mathematically simple
and physically meaningful SLS model can predict both creep and
stress-relaxation behaviors, it has been preferred over Maxwell,
Kelvin-Voigt models and non-linear viscoelastic models [25,27].
For SLS model, the stress as function of time is given as follows