Understanding the spring-like behavior of a creased sheet

Folding or crumpling can alter the structural response of a thin sheet due to the presence of plastic creases. This has paved the way for new engineering applications ranging from tunable meta-materials to deployable and flexible structures. To understand the behavior of a creased film, we study the mechanics of a single crease, which is the building unit cell. A crease is defined by its fold angle at rest and the stiffness of its spring-like behavior during further folding or unfolding. Our work focuses on understanding the role of geometry, material laws, and the parameters controlling the crease formation. We conduct experiments to characterize the crease behavior and compare the experimental data with analytical estimations using a 1-D elastica model and 3-D continuum finite element analysis. These tools help rationalize how the localization of plasticity results in the formation of the crease. We observe the crease length to be in the same length scale of the thickness of the sheet and provide a prediction based on curvature concentration along the arc-length. Our analysis has uncovered scaling laws across different geometries and materials that would be useful to determine the behavior of a creased sheet.