“…For an elastic beam, there will be no such third-order time derivative in the equations of motion. Furthermore, if one adopts the Kelvin viscoelastic model (cf., [22][23][24]28,29]), there will be no higher-order time derivative for the stress; therefore, the third-order time derivative for the equations of motion will also disappear. Though Fung et al [21] and Hou and Zu [31] employed the SLS model, Fung et al [21] assumed the viscosity constant to be zero for simplicity, and Hou and Zu [31] applied the multiple scale method in a different fashion.…”