“Carbon fiber composites consist of a polymer matrix, such as epoxy, into which reinforcing carbon fibers have been embedded,” says Amit Naskar, leader of ORNL’s Carbon and Composites Group. “Because of differences in the mechanical properties of these two materials, the fibers can detach from the matrix under excessive stresses or fatigue.

“That means damage in carbon-fiber composite structures can remain hidden below the surface, undetectable by visual inspection, potentially leading to catastrophic failure,” explains Naskar. “Carbon fiber composites fail catastrophically, so you won’t see damage until the entire structure has failed.

“By knowing what’s going on within the composite, you can better judge its health and know if there is damage that needs to be repaired,” claims Naskar.

When enough coated fiber is embedded in a polymer, the fibers create an electrical network and the bulk composite becomes electrically conductive. The semiconducting nanoparticles can disrupt this electrical conductivity in response to applied forces, adding an electromechanical functionality to the composite.

“If the composite is strained, the connectivity of the coated fibers is disrupted and the electrical resistance in the material changes,” says Naskar. “[For instance, if] storm turbulence causes a composite airplane wing to flex, an electrical signal may warn the plane’s computer that the wing has endured excessive stress and prompt a recommendation for an inspection.

“[Our] demonstration proved in principle that the method could be scaled up for high-volume production of coated fibers for next-generation composites,” says Naskar. “Self-sensing composites, perhaps made with a renewable polymer matrix and low-cost carbon fibers, could find themselves in ubiquitous products, even including 3D-printed vehicles and buildings.”

To fabricate nanoparticle-embedded fibers, Naskar and his colleagues loaded spools of high-performance carbon fiber onto rollers that dipped the fiber in epoxy loaded with commercially available nanoparticles. The fiber was then dried in an oven to set its coating.

To test the strength with which nanoparticle-embedded fibers adhered to the polymer matrix, the engineers made fiber-reinforced composite beams with the fibers aligned in one direction. They conducted stress tests in which both ends of this cantilever were fixed while a machine assessing mechanical performance pushed on the beam’s middle until it failed.

To investigate the sensing capabilities of the composite, the engineers affixed electrodes on both sides of the cantilever. In a machine called a dynamic mechanical analyzer, they clamped one end to hold the cantilever stationary. The machine applied force at the other end to flex the beam while engineers monitored the change in electrical resistance.

“[We] also tested composites made with different amounts of nanoparticles for the ability to dissipate energy—as measured by vibration-damping behavior—a capability that would benefit structural materials subjected to impacts, shakes and other sources of stress and strain,” says Naskar. “At every concentration, the nanoparticles enhanced energy dissipation by 65 percent to 257 percent.”