Ring around a tetrahedon.

Ring around a tetrahedron 

The diamond crystal structure, in which each carbon atom is bonded to four other carbon atoms to create a tetrahedral framework, is one of the strongest and most rigid structures known in nature. A similar geometry lies at the heart of a novel method for constructing strong, stiff, lightweight frames that could be used for applications ranging from stable platforms for large telescopes and other optical systems to frameworks for large boats, storage tanks, buildings and space stations. The method, invented by John J. Gilman of the Lawrence Berkeley (Calif.) Laboratory, depends on the use of puckered, six-sided rings that can be easily joined together to create a tetrahedral framework (see illustration).

Four struts come together at each node in a tetrahedral structure. The problem that confronted Gilman was finding a way to join the struts without having to weld them together or having to use an elaborate coupling device into which the struts could be inserted. After building some models of tetrahedral frames, Gilman noticed taht his frames contained bent rings, each consisting of six struts attached together so that the angle between each pair of struts is the tetrahedral angle, or roughly 109.5 [deg.]. He discovered that he could assemble a complete tetrahedral framework just by fastening such rings together.

"This ring scheme solves several problems at once," says Gilman. His method is particularly useful when the framework is made from a composite material, such as a fiber-reinforced polymer, which can't be welded together at corners. Casting the rings as units and then fitting and gluing them together avoids the need for welding or special joints. The casting process works most easily when the rings have a triangular cross section. Gilman has also developed a way of creating his tetrahedral structures using sheet material.

"I have trouble trying to get people interested because they find it hard to understand the diagrams on a piece of paper," says Gilman. "They have to see the models to see how it works -- that it's not just a crazy jumble of materials. It's going to take a while to really get it into the practical sphere."