Plasticity effects in thin film wrinkling: Wrinkling behavior of plastic films bonded to elastomers with large strain mismatch*

We examine the mechanics of composite films comprising a SEPS elastomer layer sandwiched between two thinner surface layers of plastic (polyethylene). Upon stretching such composite films to over twice their length and then releasing, the plastic surface films develop a highly wrinkled surface texture. The mechanism for this texturing is that during stretching, the plastic layers yield and stretch irreversibly whereas the elastomer stretches reversibly. Thus upon releasing, the plastic layers buckle due to compressive stress imposed by the elastomer. Although the wrinkling process appears somewhat similar to the wrinkling of a stiff elastic film bonded to a soft elastic substrate, our experiments and simulations show that plasticity plays a major role at all stages: (1) during stretching, the plastic layer yields in tension; (2) during recovery, the plastic layer first yields in-plane in compression and then buckles; (3) post-buckling, plastic hinges are formed at high-curvature regions. Homogeneous wrinkles are predicted only within a finite window of material properties: if the yield stress is too low, the plastic layers yield in-plane without wrinkling, whereas if the yield stress is too high, non-homogeneous wrinkles are predicted.