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

ABSTRACT 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 (&lt; 0.5). This optimized 2D‐DIC framework successfully captures localized strain zones as reliable structural precursors to electrical breakdown.