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Tuesday, 27 March 2012

Buckle in: Collapsible structure fabricated from a single piece of material represents a new class of 3-D structures

March 26, 2012

Video: Lucy Lindsey/Melanie Gonick; footage courtesy of Jongmin Shim, Katia Bertoldi and Pedro Reis

Motivated by the desire to determine the simplest 3-D structure that could take advantage of mechanical instability to collapse reversibly, a group of engineers at MIT and Harvard University were stymied — until one of them happened across a collapsible, spherical toy that resembled the structures they’d been exploring, but with a complex layout of 26 solid moving elements and 48 rotating hinges.

The toy inspired the engineers to create the “buckliball,” a hollow, spherical object made of soft rubber containing no moving parts, but fashioned with 24 carefully spaced dimples. When the air is sucked out of a buckliball with a syringe, the thin ligaments forming columns between lateral dimples collapse. This is the engineering equivalent of applying equal load on all beams in a structure simultaneously to induce buckling, a phenomenon first studied by mathematician Leonhard Euler in 1757.

When the buckliball’s thin ligaments buckle, the thicker ligaments forming rows between dimples undergo a series of movements the researchers refer to as a “cooperative buckling cascade.” Some of the thick ligaments rotate clockwise, others counterclockwise — but all move simultaneously and harmoniously, turning the original circular dimples into vertical and horizontal ellipses in alternating patterns before closing them entirely. As a result, the buckliball morphs into a rhombicuboctahedron about half the size (46 percent) of the original sphere.

The researchers named their new structure for its use of buckling and its resemblance to buckyballs, spherical all-carbon molecules whose name was inspired by the geodesic domes created by architect-inventor Buckminster Fuller. The buckliball is the first morphable structure to incorporate buckling as a desirable engineering design element. The buckling process induces folding in portions of the sphere — similar to the way paper folds in origami — so the researchers place their buckliball in a larger framework of buckling-induced origami they call “buckligami.”

Because their collapse is fully reversible and can be achieved without moving parts, morphable structures such as the buckliball have the potential for widespread applications, from the micro- to macroscale. They could be used to create large buildings with collapsible roofs or walls, tiny drug-delivery capsules or soft movable joints requiring no mechanical pieces. They also have the potential to transform Transformers and other kinds of toys. (The toy that provided the researchers’ epiphany is the Hoberman Twist-O.)
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