Toylike blocks make lightweight, strong structures

A carbon-fiber skeleton of Tinkertoy-like building blocks is 10 times as stiff as structures of similar densities. And because the framework is made of mostly identical pieces, broken parts can be easily swapped out for new ones, its inventors report in the Aug. 16 Science. The new design could one day form light, stiff, easy-to-repair bones of airframes, bicycles, bridges and even buildings.

“It’s fascinating,” says materials scientist Rainer Adelung of Kiel University in Germany. “When you read this article, you think, ‘Why hasn’t anyone done this before?’ It’s a simple idea, but it has such a large impact.”

For years, fancy bicycles and luxury cars have used glued-together carbon fibers, called composites, to trim weight from their frames. Now, manufacturers are starting to craft huge sections of airplanes in single swaths of the lightweight materials. Fewer parts means fewer joints, which tend to be heavy and tricky to fix.

So manufacturers want to make even bigger plane pieces. In 2008, Spirit AeroSystems, a manufacturer that makes parts for Boeing and Airbus, came to MIT materials engineer Kenneth Cheung and his lab leader, Neil Gershenfeld, with a wild idea: What if they could 3-D print an entire plane in one gigantic piece?

Cheung and Gershenfeld had doubts. Though a giant piece of composite has fewer joints, it can be hard to repair, Cheung says. When composites break, they break big-time. A wallop violent enough to crack a composite part in one place has a domino effect. The energy of the crash rebounds through the part, busting it in multiple places.

So Cheung and Gershenfeld came up with an idea to assemble a plane out of millions of identical pieces, rather than one enormous one.

Cheung played with several designs, slicing shapes out of cardboard and plywood before settling on the repeatable unit: a flat “X” of carbon-fiber composite, with a hole in the center and a loop at the end of each arm. The unit is 2 inches long, Cheung says, but could scale to virtually any size. “You can think of it as a really high-performing Lego,” he says.

Cheung hooked the Xs together to make cube-shaped lattices of repeating triangular pyramids and then crushed the structures to measure their strength and stiffness. An 8-inch cube about as heavy as an egg could hold more than 650 pounds before crumpling, Cheung says. And given the material’s very low weight, Adelung says, the lattice is “remarkably stiff.”

The geometry of the cube’s lattice is a key part of its stiffness and strength. “If you built a bunch of triangular pyramids out of toothpicks and marshmallows, you could probably rest a book on top,” Cheung says.

When Cheung and Gershenfeld’s composite structures hit their breaking point, individual pieces in the lattice snapped, confining breaks to discrete spots. This useful property would allow manufacturers using the design to gain structural damage control. “You could incrementally replace single parts, knowing that the structure is completely stable while going through that process,” Cheung says. He envisions using robots to build the structures and crawl through them, inspecting parts and switching out cracked pieces. 

The researchers also tinkered with their structures’ flexibility by plugging thinner, more bendable, pieces into the lattice. By fitting these pieces into specific parts of the lattice, the researchers could force certain areas to buckle while keeping other areas rigid. In an airplane, this type of design could let pilots maneuver their crafts by flexing the wings instead of lifting and lowering flaps.

The structures don’t come close to rivaling the strength and stiffness of denser materials, says James Tour, a materials chemist at Rice University in Houston. But they’re incredibly lightweight — and for cars, planes and spacecraft, he says, “weight is a huge, huge concern.”

In September, Cheung will join NASA to craft lattices for structures in space.