Contact area evolution during an indentation process

The evolution of the contact area during an indentation process has been examined by an ac technique and also by finite element analysis on five mechanically different materials. Constant contact area regimes were observed during the initial unloading stage and the duration of that regime depends strongly on material properties. The consistency of the results obtained by the two approaches not only proves the validity and advantage of the ac indentation technique but also confirms the applicability of contact stiffness equation. The influence of a hardness impression on unloading characteristics has also been clearly demonstrated by numerically simulating a reloading process.