Researchers turned off the Massachusetts Institute of Technology wind tunnel last year, satisfied with the most recent tests of a tough, new, ultralight composite material: a flexible, meshlike substance that snaps together like Lego bricks or K'nex. These pieces can be assembled to form 3-D chain mail–like structures that are 10 times stiffer for a given weight than existing ultralight materials. The researchers were testing how prototype airfoils and wing structures made of these new materials flexed and deformed in the strong air currents. Their dream of lighter yet more durable airplanes, spacecraft, even bridges made from snap-together parts had taken a major step forward.
This fundamentally different approach to devising materials could lead to incredibly lightweight and strong structures for aerospace, industrial and consumer products. The design is inspired by lattices in spongy bone that give animal skeletons strength at low weight. The composite could also overcome limitations in how large ultralight structures can be, imposed by conventional fabrication techniques.
Composites are typically built up with layers of molded fiber sheets slathered in resin. A heat-curing process hardens the resin, setting the desired shape. Manufacturing large structures requires huge, cylindrical ovens that bake big parts, which are later riveted together into a larger whole. The M.I.T. material could be made into an entire large object—such as an airplane fuselage—without rivets. Each piece of material would be linked to the next to create a full assembly. The bits could also be unlinked, allowing a plane or bridge to be repaired or modified where it stands or disassembled for recycling.
The composite expands the definition of material, says Hod Lipson, a Cornell University associate professor of mechanical and aerospace engineering who was not involved in the M.I.T. work. Typical materials repeat their basic components at an atomic and molecular scale. This material repeats at the macroscale, making the entire assembly a strong, flexible solid, he says. Structures made with the composite would not have joints that can fail catastrophically, and the bit-by-bit construction could make custom fabrication easier.
The basic building block is a carbon-fiber-reinforced polymer shaped like a flat X about two inches across, with a rectangular node at the center and small loops at the end of each arm. One piece is linked to another by connecting a loop to a node. Each node holds four loops, secured in place by a stiff carbon-fiber clip, forming a cubic lattice of octahedral cells—a structure with eight plane surfaces called a cuboct. The M.I.T. laboratory has built and tested dozens of blocks that sport arm lengths from almost six inches down to tenths of an inch. Making the composite with different densities (determined by arm thickness) and different cell sizes (determined by arm length) affects the material's stiffness and weight, determining how well it resists bending or buckling.
How to best assemble the pieces remains a challenge. At this stage the researchers painstakingly link the pieces using tweezers. But M.I.T.'s Center for Bits and Atoms is devising a team of robots that could build objects out of the material or crawl over an existing structure to make repairs. Kenneth Cheung, a former postdoctoral M.I.T. researcher who helped to develop the composite and now works at the NASA Ames Research Center, says an assembly system “will work best with separate robots for individual tasks, such as inspection, installation or removal.”
Other researchers are also tackling the rapid assembly problem. Lipson leads a team that has designed robots that can identify where they are and know where they need to go. Such robots could construct materials involving billions of building blocks, he says. Robots could also quickly construct temporary levees during a flood or fabricate satellites in space.
The composite will have to pass qualification and safety certification standards before airplanes or cars made of it roll off robotic assembly lines. Completely novel structures could be built, including planes with morphing wings and robots with flexible yet jointless arms, Cheung says. Yet the material will probably appear in other applications sooner; unmanned satellites or space probes could launch in a few years, he says.