Shape reconfiguring bistable structures using heat activated fibers

Engineering structures have increasingly begun to utilize mechanical instability in areas such as energy absorption, shape reconfiguration, and vibration attenuation. Most multi-stable mechanisms use 1D architectures such as pre-curved beams or inclined trusses to achieve multiple strain energy minima. These structures pose several limitations, (1) they generally require external support for functionality, (2) they often balance the competing demands of large stiffness and large amplitude in shape change, and (3) they are inherently less energetically dense compared to shell-based structures as there is negligible stretching in the material. In this work, we introduce a novel class of 4D printed bistable shell structures that morph from flat surfaces. We investigate both the formation of the structural geometry, and the mechanics of the shells once morphed. Following the precepts of 4D printing, the structural geometry is achieved in two stages. First 3D printing is used to fabricate flat polygon unit cells which are then morphed into symmetrical bistable shells. Utilizing Fused Filament Fabrication, a viscoelastic material is pre-strained as it is deposited on the print bed. By tuning the printing parameters, a predictable 2D to 3D shape morphing is triggered with heat, resulting in a height increase of up to 22.9 times the original size. Some resulting unit cells exhibit an elastic bistable response with a high peak force from a point indentation. Shape morphing and structural bistability is predicted using Finite-Element Analysis with prescribed pre-strain aligned with the fiber extrusion direction. The resulting geometries are analyzed for their mechanical behavior. Buckling simulations show good agreement with experimental results. To demonstrate an energy-dissipating metamaterial, we fabricate a planar meta-surface tiled using triangular unit cells to create an auxetic pattern. This sheet morphs to conform to a doubly curved surface globally, with each unit-cell transforming to a bistable shell locally. With this approach, we create a new generation of 4D printed bistable structures that can be applied to a variety of engineering applications.