On the oscillatority of viscoelastic functions

We have investigated the possible oscillation of linear and non-aging viscoelastic functions of viscoelastic models whose governing differential equations representing the constitutive relation between stresses and strains are of second order. We have found that the Stieltjes inverse of the time-dependent Poisson ratio could be oscillatory for engineering materials whose bulk and shear relaxation moduli are monotonically decreasing.