Composite Repairs Revisited
COMPOSITE REPAIRS REVISITED
When Atlanta-based Delta Air Lines Inc. announced plans to purchase scores of new airplanes from Airbus and Bombardier, the carrier made clear its focus was on remaking its fleet with lighter, more fuel-efficient aircraft.
Aerospace manufacturers relied heavily on composite materials for this latest generation of passenger jets. While composite parts have been used for decades, today as much as half of all airplane components can be made of composites, including major structures such as wings and the fuselage.
Chuck Zhang, a professor in the Stewart School of Industrial and Systems Engineering, inspects a composite part at a Delta Air Lines repair facility.
For airlines, the shift to composites creates an opportunity to rethink the repair and maintenance operations needed to keep jets in top form. Although the first of Delta’s new jets won’t enter service until fall 2017, the airline is already searching for better ways to maintain and repair composite aircraft parts — which are very different from the metal parts it has been maintaining for years.
The airline is partnering with Georgia Tech to take a close look at current methods used to repair composite parts and identify ways to increase efficiency and bring down costs.
“Airlines want to create their own know-how on how to fix these structures because it’s cheaper and probably faster,” said Chuck Zhang, a professor in the Stewart School of Industrial and Systems Engineering. “But improved technologies are needed to help in the repair of composite parts. Much of it today is done by hand.”
Recently, inside Delta’s maintenance shop for composite parts, airplane nose cones stood in different stages of the process, with black markings identifying areas that needed further inspection or repair. Nearby, thrust reversers awaited sanding, finishing, and painting.
“We’ve certainly been doing composite repairs for many years,” said Todd Herrington, general manager of fleet projects at Delta. “However, what’s changed is that the type of structure now includes what we call principal structural elements — essentially the type of structure that is critical to the aircraft’s continued safe flight.”
Currently, when repairs are needed for composite components that are part of an aircraft’s principal structure, technicians can use metallic or pre-cured composite patches and secure them with metal fasteners. But that’s not ideal, Herrington said.
“The more weight we permanently add to an airplane the less range or more fuel burn we’re adding to that airplane,” he said. “External repair patches are also going to add drag, which will impact aerodynamics in certain places.”
Zhang’s team is researching ways to perfect bonded repairs so that metal fasteners can be replaced with adhesives, which would preserve the composite’s lightweight advantage. And the aerospace industry isn’t the only sector that could benefit from this effort. The automotive industry, for example, also uses advanced composite materials that need improved repair technology, Zhang said.
In an effort sponsored by the National Institute of Standards and Technology, Georgia Tech has led the Consortium for Accelerated Innovation and Insertion of Advanced Composites in creating a roadmap to chart the development of composite repair technologies over the next 15 years.
Todd Herrington, general manager of fleet projects at Delta Air Lines, and Chuck Zhang, a professor in the Stewart School of Industrial and Systems Engineering, peer into the underside of a jet at a Delta Air Lines repair facility.
One immediate challenge Zhang’s team is trying to overcome is how to test bonded repairs for strength after they have been completed. Current practices often call for performing a repair, testing its strength to the breaking point, and then making another repair using the destructive testing results as a guideline.
“There needs to be a way to check the integrity of that joint without actually destroying the joint in the process,” Herrington said.
Researchers at Georgia Tech are working on technology that could ensure uniformity in the thickness of the adhesive and pressure during the joining process.
“What we’d really like to do is eliminate any differences due to variability in the operator accomplishing a repair,” Herrington said.
One possibility would be to embed a miniature sensor array into the adhesive bond line itself without negatively affecting the strength of the repaired part using advanced nanomaterials and printed electronics techniques. Those sensors could enable technicians to verify uniformity.
“The idea is to embed some type of grid structure so that you can measure the difference in capacitance values between each node, and that might tell you where you have the highest thickness or pressure variation,” Herrington said.
Zhang’s team is also researching ways to test whether a composite surface has been prepared properly for the bonding process. Part of that prep process is carefully cleaning and sanding composites on both sides of the bond to get the surface just right to ensure a proper adhesion.
“When you’re preparing an original structure for bonding, what grit sandpaper you use, how worn is that sandpaper, how rough that has left the surface — that all matters,” Herrington said. “It would be nice to be able to determine up front all of those things, including whether there’s contamination of the surface.”
Zhang’s group is in the early stages of evaluating whether infrared technology could detect the presence of contamination as well as measure the characteristics and texture of the composite surface. For both projects with Delta, preliminary research will take place over the next two years to determine whether the technologies hold promise.
“If we can show that a bonded repair can be measured and inspected in a non-destructive way,” Herrington added, “that’s really kind of the holy grail.”
Josh Brown is a senior science writer at Georgia Tech. A journalist by training, he’s spent the past decade writing about economic development, medical research, and scientific innovation.