When Deborah Chung wanted to improve the way advanced aircraft take stock of their structural health, she found the answer in an unexpected place - the very material used to build such craft.
An electrical current that runs through the weave of carbon fibers stretched over wings and used in propeller rotors could teach smart structures some new tricks, Chung asserts.
"The beauty [of this discovery] is that you don�t have to embed anything," explained Chung, a professor of mechanical and aerospace engineering at the State University of New York at Buffalo. "The [carbon] fibers are metallic and electrical connections are built in."
Today�s smart-structure technologies, used to monitor the integrity and operation of materials, rely on sensors that, in turn, often require circuitry and interconnections of a scale common to computer motherboards, Chung said.
Carbon composites, commonly found in skis, bikes, and tennis rackets, are also favored in the aerospace industry - where the fiber weaves are valued for their light weight and durability.
Chung was studying how temperature changes in carbon composite material could be detected by an aircraft. In the process, she discovered a change in the material's electrical properties, a condition she interpreted as semiconducting behavior.
Chung found that the current inside the composite material flows in one direction, instead of multiple directions as in computer semiconductors. The metallic contents of the composite operate in the opposite direction of the flow of current, creating a contact bridge between layers of fibers. The semiconducting activity results when two or more layers of the fibers are perpendicular to each other, said Chung.
But if the composite material is to serve as a reliable conductor, Chung discovered, the gap over which the electrons travel needs to be strictly controlled. Too wide a gap means energy is lost; too narrow and it's more likely that heat, rather than electrical, energy will be generated. Chung claimed that by altering the manufacturing process, composite makers could fine-tune the gap between the metallic and semiconductor layer.
Designing an aircraft out of material that would be able to detect its own damage could save on weight, structural integrity, efficiency, and manufacturing.
Already, some sports equipment manufacturers, such as Active Control Experts use embedded sensors like Piezo devices to help adjust the shock-absorption of mountain bikes and skis. These sensors assess the stress to the overall structure and make adjustments in weight distribution during travel and turning to make a ride smoother and leave a driver in control.
With the help of research such as Chung's, the two distinct fields of structural health maintenance and active control are blurring. The elusive goal: Structures that not only support their own weight but also act as their own central nervous system.
This is no easy feat, said Jim Sirkis, associate professor and director of the Smart Materials and Structural Research Center at the University of Maryland. Sirkis said there are many possible tacks to take, including the notion of a structure or craft sensing a crack and automatically responding by reducing the stress in and around the weakened area.
Sirkis noted that researchers are also working to tackle the load-bearing sensor problem by distributing crushed magneto-optical materials throughout a structure that is embedded with fiber optics.
In the meantime, researchers are left with a less-than-perfect situation. "That's the moral to the story � there's no utopia," said Sirkis.
"Fortunately, sensors are very sophisticated now that we take them for granted."
