Dear Lorenzo,
Damping in the rotor comes from aerodynamics, dynamics (gyroscopic / coriolis, etc.), and structural (natural damping in a material due to friction). The first two are intrinsicly included in FAST. The last term–the structural damping–is specified as an input in FAST.
The blade structural damping matrix in FAST is defined as follows: C_ij = zeta_jK_ij/(pif_j), where zeta_j is the damping ratio (in fraction of critical) of mode j and f_j is the natural frequency of mode j. The ratio of zeta_j to f_j comes from the stiffness-proportional term in Rayleigh damping—that is, a given stiffness-proportional damping coefficient produces a damping ratio that scales linearly with natural frequency.
In the absence of more specific information, I typically assume structural damping ratios (zeta) of 2-3% for composite blades and 0.5-1.5% for steel towers.
I hope that helps.
Best regards,