When a thin, stiff film bound to a compliant foundation is compressed, it buckles into a variety of modes including sinusoidal wrinkles, tall well-spaced ridges, or deep folds. In purely-elastic “energy-conserving” systems, the buckling mechanics can be captured by finding the buckling mode that minimizes the total energy of the film and the compliant foundation. But not all systems conserve energy, and this talk will discuss two examples of non-elastic behavior. 

The first is an elastic film on a viscous foundation undergoing compression. Remarkably, buckles can appear either as uniform wrinkles, or as tall ridges separated by nearly-flat regions. Experiments, theory, and numerical simulations offer key insights on how three different mechanisms of energy release: relaxation from the film ends, wrinkle growth, or ridge growth compete. A key finding is that ridges initiate from a long-wavelength buckling mode appearing very close to the buckling threshold. Thus, unlike in elastic systems where ridge formation is a post-buckling transition, here ridge formation appears as a near-threshold phenomenon. We map the state of the film as flat, wrinkled, or ridged in the parameter space of compression rate vs strain.

The second case is compression of a plastic film on an elastic foundation. Simply stretching and releasing a plastic-elastic bilayer composite induces intense wrinkles on the plastic film. We show that plasticity affects the wrinkling process at all stages: inducing differential stress upon stretching, yielding in compression after release, and formation of plastic hinges. If the bilayer is permitted to bend, it can warp in strongly non-intuitive ways that simply cannot be anticipated from fully-elastic bilayers. We show how such warping can be exploited to create mechanoactivated origami structures.