Carbon nanotubes offer a crack cure

Composite materials

January 2, 2008

Carbon nanotubes randomly dispersed in an epoxy resin. (Credit: N. Koratkar, Rensselaer Polytechnic Institute.)

Researchers from Rensselaer Polytechnic Institute have developed a simple technique using carbon nanotubes for the detection and repair of cracks in polymer composites used for structural applications [Zhang et al., Appl. Phys. Lett. (2007) 91, 133102].

The catastrophic failure of structural components, such as aircraft wings, is usually the result of fatigue, arising from crack propagation under cyclic loading.

Carbon nanotubes have attracted interest in the pursuit of improved polymer composites, but scant attention has been paid to their potential for detecting and monitoring damage in such materials.

Nikhil A. Koratkar and colleagues have shown that adding multiwalled carbon nanotubes (MWNTs) to a polymer composite allows the extent and propagation of fatigue-induced defects, such as cracks, to be determined.

The addition of MWNTs to a standard thermosetting resin makes the composite electrically conductive because the nanotubes form a three-dimensional network. However, when a crack is initiated and propagates through the composite, the conductivity decreases or, in other words, the crack changes the conduction pathways and increases the resistance.

The same concept can be applied to detect delamination in a hybrid hierarchical composite, in this case one containing a graphite-fiber cloth.

The addition of carbon nanotubes provides an extra and even more exciting capability: self-healing. By introducing a heat-curing additive into the composite, and using the MWNTs for fast heating of the region of the composite in the vicinity of a crack, it is possible to repair the damage.

“What’s novel about this application is that we’re using carbon nanotubes not just to detect the crack, but also to heal the crack,” says Koratkar. “We use the nanotubes to create localized heat, which melts the healing agent, and that’s what cures the crack.”

The process takes a few seconds and can recover ~70% of the failure load.

Such a system could be integrated into the built-in computer system of an aircraft or other structure for real time monitoring of its structural integrity.

“The beauty of this method is that carbon nanotubes are everywhere. The sensors are actually an internal part of the structure, which allows you to monitor any part of the structure,” notes Koratkar.

Cordelia Sealy