Post-wrinkle bifurcations in elastic bilayers with modest contrast in modulus

Wrinkles, folds, creases and other elastic surface instabilities play a crucial role in many systems in nature and engineering. While surface instabilities of ideal bilayer structures with large contrasts in elastic stiffness are well understood, many natural and man-made structures are far from this ideal. To better understand the behavior of systems with modest stiffness contrast, in particular their secondary post-wrinkling bifurcations, we systematically vary the modulus contrast between the film and the substrate through a combination of experiments and finite element simulations. Above a modulus contrast of about 2, but below approximately 14, wrinkles represent the primary bifurcation mode, but can undergo two distinct types of secondary bifurcations upon further compression: (1) a direct transition from wrinkles to creases, and (2) wrinkles that first undergo period doubling, followed by a transition to creases.