Characterizing the mechanics of permanent creases in folded films

Folding a thin film can alter the overall response of the design, paving the path for many innovative engineering applications ranging from mechanical meta-materials to deployable and flexible structures. Creases and film panels are the building blocks for such designs, and the overall response of the design can be tuned by controlling the geometry and crease behavior. Our work focuses on characterizing the behavior of permanent creases and developing modelling techniques to predict the behavior of foldable structures. We perform experiments on the crease formation and the folding-unfolding process to characterize its properties, such as the equilibrium angle and the torsional stiffness. We rationalize the observed experimental trend with an elasto-plastic elastica beam model. Using the analytic tools, we investigate the localization of the curvature, which allows us to define a length scale of a crease. Non-dimensional analysis shows robust relationships that could extend the results to other geometries and materials.