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Handheld IR In The Hangar

Boeing uses spectroscopy to monitor airplanes made of carbon-based composites

by Celia Henry Arnaud
August 22, 2011 | A version of this story appeared in Volume 89, Issue 34

Taking Aim
Credit: Boeing
Vahey makes an infrared measurement on the outside of an airplane about 15 to 20 feet off the ground, where mechanical lifts and harnesses are required for safety.
Credit: Boeing
Vahey makes an infrared measurement on the outside of an airplane about 15 to 20 feet off the ground, where mechanical lifts and harnesses are required for safety.

Boeing expects to deliver its first 787 Dreamliner to All Nippon Airways in September. The Dreamliner is the first commercial airplane to be composed primarily of composite materials instead of a metal like aluminum.

Using new materials has forced Boeing to change the monitoring methods it uses in manufacturing planes. The organic materials—50% of the structural materials in the Dreamliner are carbon based—mean that the conventional electrical analyzers used to test mechanical strength and structural integrity in aluminum planes no longer apply.

To replace them, Boeing chemists Paul G. Vahey, Paul H. Shelley, and Gregory J. Werner have developed near- and mid-infrared spectroscopy methods using handheld devices that can be used to test planes and parts during manufacture and maintenance. They have worked with scientists at instrument companies A2 Technologies, now part of Agilent, and Polychromix, now part of Thermo Fisher Scientific, to adapt their instruments for measurements on airplanes.

The different regions of the IR spectrum are suitable for testing different properties of the airplanes. For example, the Boeing chemists use near-IR to measure the thickness of coatings on the plane. Those coatings include paints, primers, adhesives, and sealants. Mid-IR tells them about chemical changes in the composition of the resins and epoxies used in the underlying composite.

Navy Uses IR To Assess Heat Damage To Planes

Carbon-based composites may be new to commercial aircraft, but the military has been using composites in planes for years. The Navy is evaluating portable infrared spectroscopy for identifying heat damage resulting from incidents such as fire, engine overheating, or lightning strikes, says Ray Meilunas, a scientist at the Naval Air Warfare Center Aircraft Division Non-Destrutive Inspection Lab in Patuxent, Md.

Heat damage is a serious repair issue for Navy aircraft such as the F/A-18, AV-8B, and V-22, all of which incorporate a high percentage of composite parts, Meilunas says. Because they don’t have a portable technique for assessing damage, Navy engineers usually end up replacing rather than repairing a component, often at 10 times the cost of a repair. “A portable technique for assessing composite heat damage could save the Navy a lot of money in part replacement costs,” Meilunas says.

The external sign is usually discolored paint, but after that is sanded away, there’s no visual way to tell if the underlying layers are also damaged. IR measurements can reveal whether chemical changes have occurred. As each successive layer is ground away, repair engineers use IR measurements to check the integrity of the next layer. They remove only the damaged layers and then adhesively bond a patch to the remaining undamaged layers.

“The Navy has been steadily increasing the percentage of polymer matrix composite parts in the aircraft fuselage relative to metal components,” Meilunas says. “As more of these composite-based aircraft enter the fleet, the probability of heat damage incidents increases.”

“A wide variety of coatings protect our materials over ambient temperatures ranging from –60 °F to 140 °F, depending on what part of the plane you’re talking about,” Vahey says. “The coatings are getting thinner and thinner to save weight. As we’re launching this generation of 787, the coatings are thinner than they’ve ever been.”

Those thin coatings make near-IR light, which can penetrate several thousandths of an inch into a material, an ideal probe. Such thin layers are extremely difficult to measure accurately with conventional devices such as micrometers.

With the new handheld infrared methods, the thickness resolution Boeing scientists can achieve depends on the particular composite material and its ability to transmit light. Near-IR is generally restricted to the coatings. “You can look at the top layer of resin in a composite matrix, but as soon as you hit carbon fibers, the infrared light pretty much stops,” Vahey says.

In contrast, mid-IR spectra provide information about chemical changes in the structural composite. “Infrared can easily see chemical changes in the epoxy,” says John Seelenbinder, an applications and product manager at Agilent who has worked with the Boeing scientists. “The difficult part is that the epoxy is in a carbon-fiber matrix. The carbon fiber is a nonspecific absorber that absorbs much of the light.”

That means IR methods are “limited to looking at materials over the topmost layer of carbon fiber,” Vahey says. “That still leaves a great deal of chemistry and coatings to evaluate.” But they don’t really need to see more than the topmost layer at a time, because the materials are multilayer laminates. If the top layer is damaged, mechanics sand it away and test the next layer, until there is no more discernible damage. Mid-IR measurements can tell mechanics responsible for maintenance and repair whether heat damage has led to changes in the underlying resin. “If they’re going to repair an area where they have delamination due to heat damage, they don’t want to bond it to resin that is compromised,” Seelenbinder says. “They’re looking to map out all the area where the resin has been affected.”

“We’re providing information that has only recently become available outside the laboratory,” Vahey says. Engineers and technicians in quality control, manufacturing, and maintenance are evaluating how they can use information gleaned from handheld IR measurements.

To gain the confidence of customers and Boeing’s own measurement scientists, Vahey and his coworkers compare the IR methods head-to-head with the old methods. “One of our hardest challenges has been convincing ourselves that these methods can stand up,” Vahey continues. “We’re in a very conservative industry. There’s no room for error on an airplane.”

To provide that kind of confidence, Boeing scientists create standards that can be analyzed both ways. They also use traditional methods to verify that their IR results are in the right ballpark.

Meanwhile, Vahey and his colleagues have started training the engineers and technicians who will use the technology in the manufacturing plant. “Spectroscopy is not something you can just pick up like a hammer,” he says. “It takes a little time to get used to it.”

Plus, the instruments are heavier than a hammer. Agilent has two handheld spectrometers. The larger one weighs 7 lb, including the optics, the controller, and the battery. By moving the battery and controller to a belt and connecting them to the optics by a cable, they have pushed the handheld weight down to 3.5 lb.

Boeing hopes that its suppliers will adopt the technology as well. “Once parts get to us, it’s final assembly,” Vahey says. “The ideal is to make measurements upstream of the point of manufacture, where something can be changed.” Boeing has put enough faith in the handheld spectroscopic methods that the company has included them in the repair manual for the 787.  



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