UD scientists use carbon nanotubes to detect defects in composites

The discovery has important implications both in the laboratory, where the scientists hope to better predict the life span of various composite materials, and in everyday applications, where it could become an important tool in monitoring the health of composite materials used in the construction of a variety of essential products, including commercial airliners.

The research is the work of Tsu-Wei Chou, Pierre S. du Pont Chair of Engineering, and Erik Thostenson, assistant professor of mechanical engineering, and is featured in an article published in the influential journal Advanced Materials.

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

“Carbon nanotubes are very small but have superb qualities,” Chou said. “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 are 30 times as strong as high-strength steel and as stiff as diamonds.”

Besides being very strong and very light, the carbon nanotubes have an incredible ability to conduct heat and electricity. In the latter case, they are 1,000 times more effective at carrying an electrical current when compared to copper.

“Carbon nanotubes have excellent properties and the challenge has been how best to utilize them, to translate those properties into applications,” Chou said.

Given the various properties, Chou and Thostenson set out to develop the carbon nanotubes as sensors embedded within composite materials.

Composite materials are generally laminates, sheets of high-performance fibers, such as carbon, glass or Kevlar, embedded in a polymer resin matrix. Chou said that the 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 composites in the interface areas in the matrix materials, particularly where there are pockets of resin,” Chou said.

As a result, defects, including tiny microcracks, can occur. Over time, those microcracks can threaten the integrity of the composite.

Thostenson said 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 said.

Because the carbon nanotubes conduct electricity, they create a nanoscale network of sensors that work “much like the nerves in a human body,” he said.

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

He added that the carbon nanotubes are minimally invasive and just 0.15 percent of the total composite volume.

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 said.