Mechanical engineers at the Rensselaer Polytechnic Institute in New York have developed a simple technique for detecting and repairing small fatigue-induced cracks in aircraft wings and other structures made from polymer composites.
In contrast to the sensing skin for bridges and buildings reported in Nanomaterials News 3.13, the Rensselaer technology involves dispersing conductive carbon nanotubes into the bulk of an epoxy resin, and monitoring the resistance between any two points on a grid of electrical contacts.
If a small current is sent through the nanotubes, any crack present will mean that the electrons have to move around the crack to close the circuit, thereby increasing the resistance. The longer the crack, the higher the resistance, and with this approach one can pinpoint the location and length of stress-induced fractures in bulk materials.
“The sensors are actually an integral part of the structure, which allows you to monitor any part of the structure,” says research leader Nikhil Koratkar. “You can generate damage under static loading and correlate that to conductivity changes, but it’s much more meaningful to detect fatigue induced cracks (under dynamic loading) in real time as the cracks are growing.”
Once a crack is located, a short electrical charge is injected into the area to heat the carbon nanotubes. This in turn melts an embedded glue that flows into and seals the fissure with a 70% recovery in strength. It may not be a permanent repair, but real-time damage detection and remediation could prevent catastrophic failure of critical structures leading to loss of life.
Koratkar is confident that the method will work just as well with larger structures, and says that it should eventually be more cost effective and convenient than the ultrasonic detectors commonly used today.
If new research funding materialises, Koratkar and his colleagues expect to have a prototype of a working structure ready within the next couple of years.
Further reading: In situ health monitoring and repair in composites using carbon nanotube additives, Zhang et al., App. Phys. Lett. 91, 133102 (2007).
Figure: Multiwalled carbon nanotubes randomly dispersed in an epoxy resin can be used to detect and repair tiny cracks in structures such as aircraft wings (source: Nikhil Koratkar/Rensselaer Polytechnic Institute).
Article first published in Nanomaterials News.