An optical nondestructive strain measurement technique was performed to analyze the mechanical deformation induced by an electrical field within the insulating materials. Poly(ethylene naphthalene 2,6-dicarboxylate) (PEN) films were then subjected to constant electrical fields right up to their electrical breakdown. The experimental technique made it possible to follow the various stages of the mechanical behavior of PEN in real time. The final breakdown occurred in the observation zone and the related mechanical deformation was captured. A “margarita” structure was observed with a hole at the center. The experimental results indicated that the level of the induced-mechanical deformations depended on the local environment. We defined two different zones representing the inside and the outside of the damaged area. The induced-deformations were larger in the damaged zone. It was also observed that deformations increased when the sample had a lower degree of crystallinity