A simple and reliable method to measure the adhesion of thin, hard coatings on polymer substrates is presented, based on the rupture mechanics of brittle films on ductile substrates. The regular fragmentation pattern of the coating obtained after straining specimens under uniaxial tension is analyzed through a classical shear-lag analysis at the coating/substrate interface. The model links the mean crack spacing measured on strained specimens to the interfacial shear strength and the reversible adhesion energy. Fragmentation tests were carried out on a PET film coated on both sides by SiO2 layers (24 nm on the thick side, and 6 nm on the thin side). The interfacial shear strength was found to be close to 100 MPa for both coatings and the adhesion energy of SiO2 on PET was found to be of the order of 230 mJ/m2, both values being slightly higher for the thin coating side.