Approximate coefficient of restitution for nonlinear viscoelastic contact with external load

We derive an approximate coefficient of restitution (CoR) for a single bead impacting the ground when a constant external load acts upon the colliding body. Some of the main applications concern classical nonlinear viscoelastic models such as the Kuwabara-Kono model, Simon-Hunt-Crossley model, and Hertzian stiffness with linear damping, however the proposed approach applies to a wide class of nonlinear viscoelastic contact models. It is shown that suitable expansions allow to derive a computable expression of the CoR which provides accurate predictions for a valuable range of external loads, viscosity coefficients and impact velocities.