Inelastic deformation morphs elastic-plastic bilayers into helices, rolls, saddles, and tubes*

A strain mismatch can induce shape changes in elastic bilayers, for example, thermally induced strain mismatch causes bending of bimetallic strips. We show elastic-plastic bilayers undergo shape changes upon stretching and then releasing them due to inelastic deformation of the plastic layer. Our experiments show that when rectangular rubber plastic bilayers are uniaxially stretched to small deformation and then released, they from arches or rolls along the direction of the applied stretch with the plastic on the outside. As the uniaxial stretch is further increased, non-Euclidean saddle shapes first appear, followed by half-tube shapes that bend perpendicular to the direction of the applied deformation with the plastic on the inside. Thus, both the sign and direction of the curvature flips as the applied stretch increases. In contrast, uniaxial strain mismatch in elastic-elastic bilayers only creates arch or roll shape along the direction of the applied stretch; strain-dependent shape changes do not appear. We show that the remarkable behavior of rubber-plastic bilayer is attributable to (1) the formation of wrinkles at the rubber plastic interface, (2) the in-plane compressive yielding in the plastic during unloading. Both these phenomena allow the strain mismatch due to Poisson effects to decouple from the strain mismatch along the stretching direction. We quantify the development of curvature in such elastic-plastic bilayers, and its dependence on thickness and width of the rectangular specimens. Interestingly, thin samples or narrow width samples readily form helical shapes with perversions similar to those seen in plant tendrils. A simple energy-based model was developed to show how the modification of strain mismatch due to inelastic deformation determines the final shape.