Thin brittle coatings on polymers can be used to improve barrier properties. The interfacial adhesion is known to affect cracking of the coating under deformation. It is therefore desirable to experimentally quantify the adhesive properties. For thin coating systems, this can be done e.g. by tensile fragmentation tests or nanoindentation techniques. For crack opening, an alternative method presents itself in a secondary damage mechanism typically found in tensile fragmentation tests. At large strains, the transverse compressive strains can give rise to ridge cracks, which may be observed in high-resolution scanning electron microscopy. It is shown that such observations can be used to estimate the critical energy release rate in interfacial crack growth. The method is exemplified for 20 nm thick coatings of TiO₂ and coatings of mixed oxide TiO₂ and Al₂O₃ on polyethylene terephthalate films. The experiments gave values of 2.5 J/m² and 1.6 J/m² for these interfaces, respectively. Energy release rates from a micromechanical model based on beam theory were found to be comparable with non-linear finite element predictions for the present crack configurations. Direct measurement of the transverse strains from digital image correlation resulted in predictions closer to the more exact numerical results as compared with predictions based on an assumed linear elastic Poisson contraction. The effects of the non-linear stress-strain behavior of the polymer film could thus be dealt with by measuring the compressive strains giving rise to the ridge cracks. Overall, this work shows how the ridge cracking can provide insight in interfacial crack growth in predominantly opening mode, and possibly be used to roughly estimate fracture toughness even for ultrathin coatings in this investigation.