**2. Experimental evidence**

The majority of the rigs developed to test underplatform dampers see a bladed system (equipped with dampers) excited at resonance [8, 35]. Often, the FRF of the system is used as the sole indicator of the damper performance. The FRF is certainly an important design indicator, however by itself, it is not capable of offering enough information on the damper working conditions. Furthermore, if FRFs are the only experimental evidence available it is likely that, as pointed out in Section 1.2, the contact parameter problem will remain underdetermined.
