EVANSTON, Ill. -- As the country’s fleet of commercial aircraft gets older, so do the hundreds of miles of plastic-coated wiring responsible for delivering such critical systems as power and communications in each airplane.
The principal way to routinely monitor wiring is by visual inspection. Now, researchers at Northwestern University are developing a non-destructive test for potentially detecting problems that sometimes can be missed by visual inspection -- small flaws, latent flaws and general degradation that can lead to wire failure.
The Federal Aviation Administration (FAA) is funding the Northwestern research with a three-year, $450,000 grant -- the first grant from the FAA to a university for the study of commercial aircraft wiring.
In 1998, the agency acted to expand its aging aircraft inspection program to include non-structural components of airplanes, such as electrical wiring.
"Aging" refers to the health of the wire’s insulation, not the conductor, and is related to the various conditions the wiring is exposed to rather than its age in years. Aging factors include vibrations and mechanical stress, moisture, temperature variations and exposure to chemicals.
Wire bundles are difficult to inspect because they often run through inaccessible places. Moving or dismantling the bundles can damage wire if its insulation is brittle or close to cracking, further complicating traditional inspection techniques.
The goal of the Northwestern researchers is to develop a routine test, using a technique called impedance/dielectric spectroscopy, that could be used to detect critical degradation levels of aging wiring without requiring removal from its location.
"Our vision for the end result of our work is to be able to take an intact bundle of wire, plug it into our system, send a small amplitude current through the wire and then read and interpret the information it sends back," said Cate Brinson, associate professor of mechanical engineering, who is leading the project. "By reading the dielectric spectra, we should be able to detect any problems with the insulation, such as pinholes, cracks or general polymer degradation."
The research also will help define the impact of individual aging factors on wire insulation, determine which factors are most destructive and provide tools to predict insulation degradation.
To do this, the team of mechanical engineers and materials scientists will study properties of new wire, laboratory-aged wire and naturally aged wire from aircraft, initially focusing on Kapton insulation, an aromatic polyimide commonly used in the wiring of older aircraft. The results, however, will be applicable to other polymers used in aircraft wire, which will be targets of study later in the project.
Impedance/dielectric spectroscopy is a rapid, non-destructive method used to investigate a large variety of materials and, in theory, is an ideal technique to use on aging aircraft wiring. Brinson and her team will work to demonstrate that the technique has potential for meeting the needs of the commercial aircraft industry.
Using impedance/dielectric spectroscopy, the researchers’ goal is to apply a small alternating current to the wire or wire bundles over a broad frequency range and in a few minutes have readings for the whole frequency spectrum. This is important because the influence of different problems, such as pinholes, cracks and degradation, will show up in different frequency ranges. A key part of the research will be defining each defect’s signature.
A distinct advantage of the impedance/dielectric spectroscopy technique is that it can distinguish microstructure problems in the insulation, such as pinholes, cracks and pores, from changes in the molecular structure, which are associated with general polymer degradation. Ideally, the monitoring system will be able to indicate the type of problem and its location.
"The challenge will be applying the knowledge we obtain in the laboratory to the wiring in actual aircraft," said Thomas Mason, professor of materials science and engineering and an expert in impedance/dielectric spectroscopy.
Technology transfer lies at the heart of the research. Throughout the project, the Northwestern team will be working closely with the FAA, Boeing and the airline companies. Aircraft engineers will share wiring and wiring problems with the researchers who in turn will ask the engineers to try out tests developed at Northwestern.
The Northwestern team is optimistic about the work. "Our research has the potential for improving both materials and diagnostics for aircraft wiring," said Brinson. "The goal is to make airplanes safer to fly."