  We consider fractional generalizations of the ordinary differential equation
that governs the creep phenomenon. Precisely, two Caputo fractional Voigt
models are considered: a rheological linear model and a nonlinear one. In the
linear case, an explicit Volterra representation of the solution is found,
involving the generalized Mittag-Leffler function in the kernel. For the
nonlinear fractional Voigt model, an existence result is obtained through a
fixed point theorem. A nonlinear example, illustrating the obtained existence
result, is given.
