The evolution of the contact area during an Indentation process has be
en examined by an ac technique and also by finite element analysis on
five mechanically different materials, Constant contact area regimes w
ere observed during the initial unloading stage and the duration of th
at regime depends strongly on material properties, The consistency of
the results obtained by the two approaches not only proves the validit
y and advantage of the ac indentation technique but also confirms the
applicability of contact stiffness equation. The influence of a hardne
ss impression on unloading characteristics has also been clearly demon
strated by numerically simulating a reloading process.