A technique enabling composite structures to initiate self-healing repairs to surface cracking has been demonstrated by researchers at the Rensselaer Polytechnic Institute (RPI) in St Troy, New York.

Embedded wires are criss-crossed in a matrix that forms an x-y grid over the surface of the structure, and over this is applied epoxy filled with carbon nanotubes (CNT).

Electrical pulses sent through the wires and nanotubes detect the tiny cracks where they cause breaks in the grid.

Once the cracked region is identified, a higher-current pulse is sent through the CNT. This heats up healing agents also mixed into the epoxy and these flow into the crack to stop its growth. The heat to make the agents flow is produced by the nanotube's ability to conduct the short, high-current pulses.

RPI researchers claim the healing process can recover 70% of the original strength of a structure's surface. "The really novel part of our technique is that nanotubes are used to both detect and repair cracks that otherwise might grow to affect the structural integrity of an aircraft wing or other polymer composites," says RPI assistant professor Nikhil Koratkar.

While commercially available healing agents can already seal and recover most of the strength lost due to nano- or micrometer-sized cracks in polymer composites, identifying the source and location of cracking remains a problem.

The RPI team is also developing software that can automatically scan structures for cracks, delamination and other defects and repair them in real time using controlled high-current pulses.

Source: Flight International