  In a recent paper, Zhou et al. studied the time-dependent properties of Glass
Fiber Reinforced Polymers (GFRP) composites by using a new rheological model
with a time-variable viscosity coefficient. This rheology is essentially based
on a generalized Scott-Blair model with time-varying viscosity coefficient
involving Riemann-Liouville fractional derivatives. Motivated by this study, in
this note we suggest a different generalization of the Scott-Blair model based
on the application of Caputo fractional derivatives of a function with respect
to another function. This new mathematical approach can be useful in
viscoelasticity and diffusion processes in order to consider time-dependent
coefficients. We are able to find the exact analytic solution of the creep
experiment based on our new approach and we can compare it with the results
obtained by Zhou et al.
