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* Carbon nanotubes are randomly dispersed in an epoxy resin
In this image, carbon nanotubes are randomly dispersed in an epoxy resin, which can be molded into different structures. By infusing the polymer with electrically conductive carbon nanotubes and monitoring the electrical resistance at different points in the structure, he can pinpoint the location and length of even the tiniest stress-induced crack. Once a crack is located, Kotakar can then send a short electrical charge to the area in order to heat up the carbon nanotubes and in turn melt an embedded healing agent that will flow into and seal the crack.
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Using Nanotubes To Detect and Repair Cracks in Aircraft Wings and Other Structures

New Technology Enables Real-time Diagnostics and On-site Repair

Adding even a small amount of carbon nanotubes can go a long way toward enhancing the strength, integrity, and safety of plastic materials widely used in engineering applications, according to a new study. 

Researchers at Rensselaer Polytechnic Institute have developed a simple new technique for identifying and repairing small, potentially dangerous cracks in high-performance aircraft wings and many other structures made from polymer composites. 

By infusing a polymer with electrically conductive carbon nanotubes, and then monitoring the structure’s electrical resistance, researchers pinpointed the location and length of a stress-induced crack in a composite structure. Once they locate a crack, engineers can send a short electrical charge to the area to heat up the carbon nanotubes and melt an embedded healing agent that flows into and seals the crack with a 70 percent recovery in strength.

Real-time detection and repair of fatigue-induced damage will greatly enhance the performance, reliability, and safety of structural components in a variety of engineering systems, according to principal investigator Nikhil A. Koratkar, an associate professor in Rensselaer’s Department of Mechanical, Aerospace and Nuclear Engineering. 

Details of the project are outlined in the paper “In situ health monitoring and repair in composites using carbon nanotube additives,” which Applied Physics Letters published online. Rensselaer graduate students Wei Zhang and Varun Sakalkar were co-authors of the paper.

How it Works

The majority of failures in any engineered structure are generally due to fatigue-induced microcracks that spread to dangerous proportions and eventually jeopardize the structure’s integrity, Koratkar said. His research looks to solve this problem with an elegant solution that allows for real-time diagnostics and no additional or expensive equipment. 

Koratkar’s team made a structure from common epoxy, the kind used to make everything from the lightweight frames of fighter jet wings to countless devices and components used in manufacturing and industry, but added enough multi-walled carbon nanotubes to comprise 1 percent of the structure’s total weight. The team mechanically mixed the liquid epoxy to ensure the carbon nanotubes were properly dispersed throughout the structure as it dried in a mold.

The researchers also introduced a series of wires into the structure in the form of a grid, which can measure electrical resistance and also apply control voltages to the structure.

By sending a small amount of electricity through the carbon nanotubes, the research team could measure the electrical resistance between any two points on the wire grid. They then created a tiny crack in the structure, and measured the electrical resistance between the two nearest grid points. Because the electrical current now had to travel around the crack to get from one point to another, the electrical resistance – the difficulty electricity faces when moving from one place to the next – increased. The longer the crack grew, the higher the electrical resistance between the two points increased.

Photo credit: Nikhil Koratkar
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“Using Nanotubes To Detect and Repair Cracks in Aircraft Wings and Other Structures”
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