Composite repairs

Delta Air Lines plans to purchase new airplanes from Airbus and Bombardier to remake its fleet with lighter, more fuel-efficient aircraft. Although the first of Delta’s new jets won’t enter service until this fall, the airline is already searching for better maintenance, repair, and overhaul (MRO) options – partnering with Georgia Tech to look at current methods used to repair composite parts and identify ways to increase efficiency and lower costs.

“Airlines want to create their own know-how to fix these structures because it’s cheaper and probably faster,” says 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.”

“We’ve certainly been doing composite repairs for many years,” says 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 technicians need to repair composite principal structural components on aircraft they can use metallic or pre-cured composite patches and secure them with metal fasteners. But that’s not ideal, Herrington notes.

“The more weight we permanently add to an airplane, the less range or more fuel burn we’re adding,” he says. “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, preserving the composite’s lightweight advantage.

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 says.

Researchers at Georgia Tech are working on technology that could ensure uniformity in the thickness of the adhesive and pressure during the joining process – eliminating differences due to variability in operators doing the repairs.

One possibility would be to embed a miniature sensor array into the adhesive bond line without negatively affecting the strength of the repaired part. Those sensors could enable technicians to verify uniformity.

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 notes.

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.

“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.”

Georgia Institute of Technology

www.gatech.edu