MIT engineers are using carbon nanotubes only billionths of a meter thick to stitch together aerospace materials, in work that could make airplane skins some ten times stronger at a nominal increase in cost, according to a March 4 news release from the institute.
Advanced composites reinforced with nanotubes also are more than one million times more electrically conductive than their counterparts without nanotubes, meaning aircraft built with such materials would have greater protection against damage from lightning, said Brian L. Wardle, the Charles Stark Draper Assistant Professor in the Department of Aeronautics and Astronautics. Wardle is lead author of a theoretical paper on the new nanotube-reinforced composites that will appear in the Journal of Composite Materials.
The advanced materials currently used for many aerospace applications utilize layers of carbon fibers held together by a polymer glue. That glue can crack, resulting in the plies coming apart, Wardle wrote. As a result, engineers have explored a variety of ways to reinforce the interface between the layers by stitching, braiding, weaving or pinning them together. All of those processes are problematic because the relatively large stitches or pins penetrate and damage the carbon fiber plies themselves, Wardle wrote, “and those fiber plies are what make composites so strong.”
Wardle wondered whether it would make sense to reinforce the plies in advanced composites with nanotubes aligned perpendicular to the carbon-fiber plies. Using computer models of how such a material would fracture, “we convinced ourselves that reinforcing with nanotubes should work far better than all other approaches,” Wardle said. His team went on to develop processing techniques for creating the nanotubes and for incorporating them into existing aerospace composites.
How does nanostitching work? The polymer glue between two carbon fiber layers is heated, Wardle explained, becoming more liquid-like. Billions of nanotubes, positioned perpendicular to each carbon-fiber layer, then are sucked up into the glue on both sides of each layer. Because the nanotubes are 1,000 times smaller than the carbon fibers, they don’t affect the much larger carbon fibers detrimentally, instead filling the tiny spaces around them, stitching the layers together.
“We’re putting the strongest fibers known to humankind [nanotubes] in the place where the composite is weakest,” Wardle wrote, stating that dramatic improvements can be achieved with nanotubes comprising less than 1% of the mass of the over-all composite. In addition, he said, the nanotubes should add only a few percent to the cost of the composite “while providing substantial improvements in bulk multifunctional properties.”
The research was sponsored by MIT’s Nano-Engineered Composite Aerospace Structures Consortium.