MIT researchers have found a way to fuse together materials that gives a whole new meaning to the concept of sewing. “Nanostitching” uses carbon tubes only billionths of a meter thick to reinforce advanced materials for airplane skins and more.
The resulting composites could be more than 10 times as strong as their conventional counterparts. They are also more than a million times as conductive, meaning that they could better protect aircraft against damage from lightning strikes. And since only a tiny quantity of nanotubes produces the desired effects, they aren’t expected to add much to the overall cost of the materials.
The advanced materials currently used for many aerospace applications are composed of layers, or plies, of carbon fibers that are glued together. But that glue can crack. Though engineers have explored a variety of ways to reinforce the bond between the layers by stitching, braiding, weaving, or pinning them together, all these processes are problematic. The relatively large stitches or pins penetrate and damage the carbon-fiber plies themselves, defeating the purpose.
Enter Brian Wardle, SM ‘95, PhD ‘98, an assistant professor of aeronautics and astronautics, who got the idea of trying nanotubes. Using computer models, “we convinced ourselves that reinforcing with nanotubes should work far better than all other approaches,” he says. His team then developed techniques for creating the nanotubes and incorporating them into aerospace composites. They describe their work in the April issue of the Journal of Composite Materials.
In nanostitching, each carbon-fiber layer is impregnated with polymer glue. Forests of trillions of nanotubes aligned perpendicular to the layers are placed between them, and the resulting composite is heated. This converts the glue, which is semisolid at room temperature, into a resin that is sucked into the forests of nanotubes by capillary action. Because the nanotubes are just a thousandth the diameter of the carbon fibers, they don’t affect the fibers themselves. Instead, they fill the spaces around the fibers to “sew” the layers together, strengthening the bond between them as the polymer glue hardens.
Nanotubes are “the strongest fibers by weight known to humankind,” Wardle says. Nanostitching puts them “in the place where the composite is weakest, and where they’re needed most.”