Laser-induced damage threshold decreases when an optic is exposed to multiple laser pulses. In the femtosecond regime, the fatigue of dielectric materials is attributed to incubation of laser-induced lattice states. The goal of this study is to investigate sub-critical damage in a quantitative manner so that the lifetime of single-layer dielectric coatings could be predicted for femtosecond applications. Sub-critical damage was found to consist of localized nanogrooves (100 nm wide, 1-2 um long) oriented perpendicularly to laser polarization. Simple fatigue model was used to simulate fatigue curves which were in good agreement with experimental S-on-1 data for the catastrophic damage.