Figure 2.

(A) For a narrow contact between a sphere and a plate, deformation occurs close to the contact, only. The contact may be depicted as a spring, or more generally, as a Voigt element, where the latter also accounts for viscous dissipation (B). In the case, where the sphere is heavy enough to be clamped in space by inertia, it can be depicted as a wall (right-hand side in Β). (C,D): Illustration of how Δf and ΔΓ depend on the shape of the force–displacement loop. (C): viscoelastic contact and (D): partial slip according to Cattaneo and Mindlin. The frequency shift is roughly proportional to the ratio of force and displacement at the turning point (full dots). ΔΓ is proportional to the area inside the loop divided by u₀² (hatched). (C) and (D) adapted with permission from [10], copyright 2013 the American Physical Society.