Wrinkle, fold, and ridge patterns of thin films attached to a compliant substrate: Direct numerical simulations involving plastic yielding

Abstract

Surface wrinkling is a frequently observed form of deformation instability when a thin film bonded to a compliant substrate is under in-plane compression. In this study, a recently developed computational approach is applied to study the formation and transformation of wrinkles involving plastic yielding of the thin film. The two-dimensional (2D) finite element models contain an embedded imperfection with perturbed material properties at the film-substrate interface, which serves to activate the bifurcation modes. The extension of this technique to allow for film plasticity is demonstrated, including the evolution of surface patterns and its correlation with the overall load-displacement and amplitude responses. More localized forms of wrinkles are revealed by the simulations, including the transition of periodic wrinkles to more isolated folds and how it can be influenced by the elastoplastic properties of the film material. A through-thickness variation of material properties is shown to significantly alter the wrinkle morphology. Plastic folding can be suppressed by only a very thin elastic sub-layer, and the waveform can become ridge-like. We also present a preliminary analysis by applying the same numerical methodology to three-dimensional (3D) models, which generates consistent results compared to the 2D counterpart.