Viscoelasticity and plasticity in the formation of creases in thin sheets

Origami inspired folding, which enables the transformation of a flat sheet into different geometries, has recently become a topic of interest among the engineering community as it helps solving problems ranging from deployable structures for space engineering to micro scale grippers for bio-medical applications. However, the mechanics of a folded sheet are not only governed by the sheet properties and the folding pattern, but also by the properties of the creases. We study them by performing experiments exploring the influence of different control parameters, such as the applied force and the time the film was pressed. We rationalize the experimental results with numerical simulations: a 1D elastica beam model and a high-fidelity finite element analysis, both accounting for elasto-plastic behavior and non-linear geometry. By considering the curvature localization, we can define a crease length that depends on the properties of the film and the fold control parameters. We found that non-dimensionalized results provide a robust scaling that enables to extend the results to other geometries and material properties. We also explore the effect of viscoelasticity of the sheet material.