A novel electro-fragmentation method was developed as a fast alternative to the time consuming fragmentation test carried out in situ in a microscope, to investigate the failure of dielectric coatings on polymer substrates using an ultrathin conductive layer. Focus was put on SiNx coatings on polyimide substrates. A careful selection of the conductive layer was carried out to avoid artifacts resulting for instance from a change of the residual stress state of the investigated coating. Au layers were found to be too ductile and Al-Ti layers altered the stress state of the nitride, which invalidated their use to probe the failure of the nitride coatings. In contrast, a 20 nm thick graphite layer was found to accurately reproduce the failure of the nitride, which was analyzed in terms of residual strain and cohesive properties of the graphite layer. An electro-fatigue device was built and preliminary results suggest that damage already develops in ultrathin coatings at relatively low strain levels under long-term fatigue loading.