Two College of Engineering researchers have discovered a means to detect and identify damage within advanced composite materials by using a network of tiny carbon nanotubes, which act in much the same manner as human nerves.

The discovery could become an important tool to track the wear and tear on materials used in such products as airplanes.

The research by Tsu-Wei Chou, Pierre S. du Pont Chair of Engineering, and Erik Thostenson, assistant professor of mechanical engineering, was featured in a recent article in the influential journal Advanced Materials.

Chou and Thostenson say their findings have important implications both in the laboratory, where the scientists hope to better predict the lifespan of various composite materials, and in everyday applications, where they could be used in monitoring the health of composite materials used in the construction of a variety of essential products.

Chou says the research team has been working in the field of fiber composites in conjunction with UD’s Center for Composite Materials and recently has taken an interest in the reinforcement of composites with minute nanomaterials—a nanometer is a bare one-billionth of a meter—and particularly with carbon nanotubes.

“Carbon nanotubes are very small but have superb qualities,” Chou says. “They are very light, with a density about one-half that of aluminum, which itself is considered exceptionally light in comparison to other metals, and yet they are 30 times as strong as high-strength steel and as stiff as diamonds.”

Besides being unusually strong and light, the carbon nanotubes are able to conduct heat and electricity extremely well. In the latter case, they are 1,000 times more effective than copper at carrying an electrical current.

“Carbon nanotubes have excellent properties, and the challenge has been how best to utilize them, to translate those properties into applications,” Chou says.
Given the various properties, Chou and Thostenson say they set out to develop the carbon nanotubes as sensors embedded within composite materials.
Composite materials are generally laminates, sheets of such high-performance fibers as carbon, glass or Kevlar embedded in a polymer resin matrix. Chou says traditional composite materials have inherent weaknesses because the matrix materials—plastics—surrounding the fibers are “strong, but far less strong than the fibers.”

This results in weak spots in the composites, particularly where there are pockets of resin, Chou says. As a result, tiny microcracks and other defects can occur and, over time, can threaten the integrity of the composite.

Thostenson says the carbon nanotubes can be used to detect defects at onset by embedding them uniformly throughout the composite material as a network capable of monitoring the health of the composite structures.

“Nanotubes are so small they can penetrate the areas in between the bundles of fiber and also between the layers of the composite, in the matrix-rich areas,” Thostenson says. Because the carbon nanotubes conduct electricity, they create a nanoscale network of sensors that work “much like the nerves in a human body,” he says.

When the researchers pass an electrical current through the network, Thostenson says, “If there is a microcrack, it breaks the pathway of the sensors and we can measure the response.”

At present, composite material engineers have limited means to either detect the initial onset of microcracks or identify the specific type of defect. This finding will change that because the method is simple, does not require expensive equipment and is remarkably sensitive to the initial stages of microcracking, Thostenson says.

For the technique to be successful, the carbon nanotubes must be scattered everywhere throughout the material, and Chou credits his colleague with developing a technique to disperse them uniformly.

The work provides a new tool for current research in the laboratory and has many potential applications in the future. By identifying and tracking defects in a laboratory setting, the researchers can begin to develop strategies for more accurate predictions of the lifespan of composite materials.

“This is a very practical ‘today’ project,” Thostenson says. “We can take advantage of this new scale now with wide applications in the future.”

Composite materials are increasingly used in everyday life, in such products as sporting goods, civil infrastructure including bridges and pipes and in the aircraft industry.

The research is supported by funding from the Air Force Office of Scientific Research and the National Science Foundation.

—Neil Thomas, AS ’76