View Video Presentation: https://doi.org/10.2514/6.2021-1258.vid

Creases are regions of permanent deformation found in thin membranes after folding or crumpling, which makes it possible to transform the geometry of the membrane. This is a benefiting tool for the engineering community to design deployable/tunable structures. A crease can be idealized as a torsional spring which is characterized by the equilibrium angle and the spring stiffness. In this paper we present an experimental approach that estimates the moment-angle relation at the crease, which has been applied to 0.127 mm thick Kapton HN film samples. The setup is able to measure the crease behavior for both opening and closing of the fold with respect to the equilibrium angle. We observe a linear spring stiffness for the crease, as long as the applied forces are small enough to avoid new permanent deformation at the crease. We also derive an analytical modeling technique for the crease combining an elastica with a torsional spring. Using this tool, we perform a dimensional analysis on the crease behavior to predict the residual effects such as out-of-plane, in-plane deformation and non-flatness of the film. We define the crease parameter γ that combines the membrane properties, hinge stiffness and the unfolding force. Unfolded membrane profiles are only dependent on γ and equilibrium angle. Furthermore, the value γ dictates the relation between fold-opening and film bending during the unfolding of a film.