Fig. 2

Estimation of tensile and compressive strain induced by forming spherical surface from planar sheet. a Photograph of spherical PEN surface with square pattern of Au thin film. Histograms of longitudinal and lateral side lengths of square pattern b at the center of the spherical surface and c in the peripheral region. Strain distribution map for d longitudinal length, e lateral length, and f, g diagonal length. MD denotes the machine direction, which is the direction in which the PEN film was drawn into the roll during the manufacturing process, and TD denotes the transverse direction, which is the direction perpendicular to MD in the plane of the film. The thermal contraction ratios along MD and TD are generally different for commercial plastic films