Simulation of fold-line stiffness in deployable membranes

Abstract

New designs for space structures such as solar sails and star shades are based on architectures that follow folding and packaging ofthin membranes. By leveraging recent advances in origami science, it is possible to design structures in which folded thin membranes deploy following a predetermined and robust path. Design and product optimization of deployable space structures are limited by complex environmental conditions experienced by them. However virtual simulations can be the perfect solution provided proper idealization techniques are followed. Presence of fold-lines alter the geometrical and mechanical properties ofthin membranes which have not being accounted in previous virtual simulations. Two major characteristics identified was the self-opening of the membrane to an equilibrium angle (defined as neutral angle) and the rotational spring stiffness ofthe membrane at the fold-line. An experimental study was devised to investigate the variation of fold-line stiffness while varying the neutral angles and membrane thickness for Kapton HN polyimide. A linear empirical relationship between resistive moment and fold-angle is proposed for each thickness. Self-opening and subsequent unfolding of a single fold was modelled using commercial finite element package, Abaqus/Explicit. Fold-line characteristics were represented with rotational spring connector elements defined between two shell portions. Compared to common idealization approaches (perfect hinge and perfect weld), rotational spring connectors were able to accurately predict the deformation profile and unfolding forces. Finally, the developed fold idealization technique was applied in an experimental case study of a deploying solar sail. It was shown that neglecting foldline stiffness underestimate the deploying force ofthe sail.