Growing old can be just as debilitating for aircraft as it is for people. And Purdue University researchers, as "aero-gerontologists," are taking a holistic approach to understanding and treating aging in planes.
"Aging aircraft are a precious resource," says Alten F. Grandt, professor of aeronautics and astronautics. "Because of military cutbacks and the large replacement costs for commercial aircraft, it's become more common to operate planes beyond what used to be retirement age.
"Designers and manufacturers have been very successful in ensuring that planes are safe from damage caused by cracking and corrosion over a normal lifetime. But as we extend planes' operational lives, we have to re-examine these and other damage tolerance factors and how they apply to aging aircraft. It's a significant challenge to keep air operations reliable, safe and economic."
There are a number of basic research issues associated with the aging of aircraft, says Grandt, who heads a team of materials scientists, structural engineers and statisticians that has completed the first year of a four-year project established last year with a $3.3 million grant from the Air Force Office of Scientific Research. The goal of the Purdue Aging Aircraft Program is to improve safety and extend the operational life of planes.
"We're primarily concerned with the technical issues, but there are many other complicated matters that come into play, such as economics and issues of public safety, or equally importantly, perceptions of safety," says Grandt, who was a materials engineer in the Air Force for eight years.
Areas the Purdue researchers are studying include how cracks and corrosion form over time and how tiny cracks in an airplane's structure and skin grow and interact with larger cracks.
The researchers also are investigating possible treatments and preventatives for these airplane ailments, such as using composite materials to repair cracks, techniques to inhibit corrosion, and methods to help maintenance personnel more accurately assess structural problems.
The team presented research results from the first year of the project in December at the United States Air Force's annual Structural Integrity Program Conference in San Antonio, Texas.
Currently more than 7,500 jets are operated by U.S. passenger and cargo carriers. According to the Nov. 3, 1994, Wall Street Journal, 1,011 jetliners in use are more than 20 years old, and 549 are more than 25 years old. The paper says the average age of all jets in use is 12.67 years. Grandt says some tanker planes used in the military are more than 30 years old.
Commercial jets are designed to be used for approximately 20 years, Grandt says, but the actual safe operating life depends on several factors in addition to age, including a plane's number and type of flights, maintenance procedures and the environment in which it flies.
Structural damage and failure -- such as the 1988 Aloha Airlines flight in which a 19-year-old jet lost an 18-foot piece of its roof, landing with one fatality -- can be caused by many factors, Grandt says. In older aircraft, cracking is a major concern.
"New aircraft are designed to withstand large cracks and still fly, but as planes age, they develop tiny cracks from corrosion, service procedures and years of fatigue cracking," he says. "These small cracks are not necessarily harmful themselves, but over time they proliferate around the plane, decreasing the structure's strength and its ability to arrest large cracks."
One of the challenges, Grandt says, is determining how much cracking a plane can tolerate before it becomes unsafe. In several labs across campus, he and his colleagues crack, corrode and stress pieces of aluminum alloy and parts of old planes to simulate various environmental conditions, fatigue and procedures aircraft endure.
"We have developed a good numerical analysis that predicts how these small cracks grow and interact when they originate around open holes, such as the small pits that form from corrosion," Grandt says. "Now we're working to make the model more realistic by incorporating the rivets, fasteners and other components found on a plane."
Grandt says a variation of the model may be used someday by inspectors to more reliably estimate the remaining life of a plane based on the amount of cracking found on the craft.
As with human ailments, prevention may be the key to avoiding structural failure. Cutting down on corrosion and keeping small cracks in check with high-tech patches may help reduce maintenance costs and extend an airplane's life.
Corrosion is a chemical attack on the aluminum alloys used to make most modern aircraft. As a plane ages, it is partially protected from corrosion by an oxide layer that grows on its surface. When this layer is disturbed, corrosion quickly begins to eat away metal, particularly where pieces of the plane's skin overlap. This reduces its thickness and strength, which leads to cracking.
"Right now there's no good way to quantify corrosion and relate it to how much life the structure has left," Grandt says. "One of the things we're trying to do is develop a technique for evaluating the effects of corrosion, so that a maintenance person could inspect the plane and determine whether a panel needs to be replaced or whether it's safe to fly for another period of time."
Purdue materials engineer Eric P. Kvam and his students are testing the effectiveness of various corrosion inhibitors used to wash planes. The chemicals are supposed to slow down the effects of corrosion. In the lab, the researchers accelerate the effects of aging by exposing pieces of metal to inhibitors and simulated environmental conditions, such as salt water and dry air.
Cracks that are detected before they cause major problems often are patched by riveting an aluminum alloy sheet over them, says Chin-Teh Sun, professor of aeronautics and astronautics. One disadvantage of this repair technique is that it disturbs the protective oxide layer on the plane and increases the chance of corrosion.
Composite materials may offer a better solution. For more than 10 years, the U.S. and Australian air forces have repaired cracks in military aircraft using composite materials, which have outlasted aluminum alloy patches. Sun says the use of composites for the U.S. commercial fleet is catching on.
"Composites are new types of materials that are made from high-strength fibers, such as carbon or boron, embedded in other materials, such as polymers," he says. "Composites are stiffer than aluminum and are very light. Also, composite materials do not corrode in hostile environments."
Another advantage of composites is that sheets of the materials can conform to any shape.
"The procedure is quite simple," Sun says. "You don't need a lot of the material and you don't disturb the oxide layer. The material itself is very strong and very effective at arresting crack growth."
To repair a crack, a sheet of composite material is bonded over the damaged area with a high-performance adhesive. The only weakness in this type of repair is in the adhesive, Sun says. The patch can last forever if it remains undamaged, but it may break away from the plane along the glue line.
Sun and his students test the strength and effectiveness of composite patches in the lab by gluing them onto cracked aluminum plates and simulating various forms of damage, such as impacts from birds or debris. They also simulate repeated cycles of loading and unloading, applying force to the patch until it breaks.
Sun works primarily with carbon and boron composites because they are most widely used, but he says his work also may lead to the development of new combinations of materials for patches.
Other Purdue faculty members on the research team are Thomas N. Farris, professor of aeronautics and astronautics; Ben M. Hillberry, professor of mechanical engineering; and George P. McCabe, professor of statistics.
Sources: Alten F. Grandt, (765) 494-5141; Internet, email@example.com
Chin-Teh Sun, (765) 494-5130;; Internet, firstname.lastname@example.org
Writer: Amanda Siegfried, (765) 494-4709; Internet, email@example.com
Purdue News Service: (765) 494-2096; e-mail, firstname.lastname@example.org
NOTE TO JOURNALISTS: Two black-and-white or color photos of Alten Grandt inspecting a plane are available from Amanda Siegfried, Purdue News Service, (765) 494-4709. Electronic transmission is available. Ask for the photos called "Aging Aircraft/Grandt." To receive the text of this news release via e-mail, send an e-mail message with the text "send punews 9501fp3" to this address: email@example.com. Purdue News Service also posts recent news releases, experts lists and story tips on a gopher server at: newsgopher.uns.purdue.edu
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