  In this work, we investigate the use of pre-twisted metallic ribbons as
building blocks for shape-changing structures. We manufacture these elements by
twisting initially flat ribbons about their (lengthwise) centroidal axis into a
helicoidal geometry, then thermoforming them to make this configuration a
stress-free reference state. The helicoidal shape allows the ribbon to have
preferred bending directions that vary throughout its length. These bending
directions serve as compliant joints and enable several deployed and stowed
configurations that are unachievable without pre-twist, provided that
compaction does not induce material failure. We fabricate these ribbons using a
bulk metallic glass (BMG), for its exceptional elasticity and thermoforming
attributes. Combining numerical simulations, an analytical model based on shell
theory and torsional experiments, we analyze the finite-twisting mechanics of
various ribbon geometries. We find that, in ribbons with undulated edges, the
twisting deformations can be better localized onto desired regions prior to
thermoforming. Finally, we join together multiple ribbons to create deployable
systems. Our work proposes a framework for creating fully metallic, yet
compliant structures that may find application as elements for space structures
and compliant robots.
