High-amplitude Ridge Structures Induced by Plastic Deformation

Ridging instability occurs in bilayer stiff film/compliant substrate systems where the elastomer substrate is subject to a large pre-stretch prior to attachment of the film. When the bilayer is then gradually compressed, sinusoidal wrinkles first form on the surface, and subsequently become unstable, giving way to localized ridges with high amplitudes. However, the large pre-stretch of the elastomer substrate during thin film attachment is not compatible with conventional thin film deposition methods. A new and simple method for constructing ridge structures is to deposit a plastic thin film on a stress-free elastomer, and stretch the resulting bilayer system. Upon the release of the stretching, the deformation of the elastomer is reversible, while the plastically deformed thin film stays elongated. The large length mismatch generates ridge structures. In this work, we study the mechanics of ridge instability formed by stretching and unloading a bilayer of plastic thin film on an elastomer substrate. The critical condition for the formation of ridges and the ridge morphology highly depend on the elastoplastic material properties, such as the yielding strain and hardening exponent of the film. A comparison between plastic and elastic ridges is also presented.