Fragmentation tests in the uniaxial mode were performed on poly(ethylene terephthalate) (PET) films coated with a silicon oxide layer of thickness ranging from 30 to 156 nm. The coating’s fragmentation process was investigated to reveal the crack onset strain and the crack density at fragmentation saturation. Adhesive strength was modeled from the Kelly-Tyson approach, including a Weibull distribution of the coating strength. The prediction was found to be independent of coating thickness, and equal to the substrate shear stress at saturation. The cohesive strength of the coating was characterized from the crack onset strain. The measured decrease in crack onset strain with coating thickness increase was modeled by means of Weibull and fracture mechanics theories, the latter providing the best predictions.