Quantitative Full‐Field Analysis of Local Electromechanical Strain in Semicrystalline Polyethylene Naphthalate Films Using 2D ‐Digital Image Correlation ( DIC ) Under DC Electrical Stress

Accurate characterization of electromechanical coupling in semicrystalline polymers is fundamental for high‐voltage insulation reliability. This study investigates the local strain kinetics of polyethylene naphthalate (PEN) films (17.42% crystallinity, 25‐μm thickness) under a DC voltage ramp until breakdown using 2D‐digital image correlation (2D‐DIC). Full‐field measurements reveal highly heterogeneous spatial strain kinematics. Under Maxwell stress, a transition occurs at 4 kV (160 kV/mm) from a stable baseline to a field‐induced planar microexpansion. Approaching the 6 kV (240 kV/mm) threshold, a major electromechanical instability develops, marked by progressive electro‐optical degradation (correlation coefficient declining from 0.85 to 0.80) and a final single‐frame collapse to 0.475 at breakdown, synchronized with local strain concentrations across a 98 × 71 grid. Furthermore, the regularized apparent Poisson's ratio peaks at +0.47 (4 kV) before shifting into a stable high‐field negative regime (−0.4 to −0.8), confirming a severe in‐plane biaxial contraction prior to failure while strictly adhering to continuum mechanics boundaries (< 0.5). This optimized 2D‐DIC framework successfully captures localized strain zones as reliable structural precursors to electrical breakdown.