Bistable Snapping Structures With Spatially Tailored Deformations via Distributed Pre-Strain
Materials capable of exhibiting inherent morphing are rare and typically reliant on chemical properties. The resulting diffusion-driven shape adaptability is slow and limited to specific environmental conditions. In contrast, natural composites, such as those found in carnivorous plants, have evolved hierarchical architectures displaying remarkably fast adaptation in response to environmental stimuli. Whereas pre-stress in synthetic structures has conventionally been limited to uniform straight lines and rectilinear geometries, here we present two strategies to achieve spatially distributed pre-stress fields. The first approach entails the use of magnetically responsive aluminum oxide platelets as reinforcements in an epoxy matrix to create bilayer composite shells. Using rotating magnetic fields, the reinforcements of each layer are aligned during curing at elevated temperatures. When cooled to room temperature, the result is spatially distributed pre-stress fields. The high degree of tailoring possible with this technique allows for the control of a structure's multistability and complex stable configurations. Using similar material parameters as explored in previous experimental studies, we employ nonlinear FEA to investigate the effects of introducing curvilinear spatially distributed micro-reinforcements on the deformation of a shell with an unusual bio-inspired geometry. The FEA model is subject to experimental validation with magnetically aligned specimens. Comparison to a traditional [90/0] composite layup demonstrates the advantages of magnetically aligned reinforcements to achieve complex, snapping morphing structures with tailored characteristics. Unlike purely chemically based approaches to multistability, distributed pre-stress is not limited to any specific material system or manufacturing method. The second approach examined here relies on fused deposition modeling (FDM) 3D printing. When thermoplastic filament is extruded during the printing process, the polymer chains are stretched and quickly cooled before they can relax. Thus, directional pre-strain is encoded in the structure along the path of extrusion. By leveraging the shape memory effect of polymer chains in combination with a bio-inspired bilayer architecture, we can create opposing pre-stress in printed polymer shells. At room temperature, the shells are too stiff to undergo the large strains necessary for snap-through. However, above their glass transition temperature, the polymer shells become rubbery elastic, and the opposing pre-strain fields can be designed such that the shells are multistable with fast morphing at this elevated temperature range. These switchable bistable structures (SBS) may be easily printed in complex, multi-domain layups, again achieving a high degree of spatial tailoring of curvatures and multistability. The shells are analyzed using nonlinear finite element analysis. By leveraging the vast array of geometries accessible with 3D printing, this method can be extended to complex, multi-domain shells, including bio-inspired designs.
Arrieta, Purdue University.
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