September 03, 2012
Credit: Photo Credit: Abaris
Jerome Greer Chandler
Toughened resins and simpler composite damage-detection technology certainly exist, but they have scant applicability to most current aircraft. The aerospace industry will have to wait for the Boeing 787 and other high-percentage composite aircraft to mature before the repair arena can accommodate new material and methods. And it will have to wait for built-in barriers to fall.
As the emerging fleet flies with ever-increasing percentages of composite structure, preventing and detecting delamination are becoming essential issues. Delamination's most typical trigger is denting, not environmental wear and tear. “It's impact,” says Michael J. Hoke, president of Abaris Training Resources Inc.
Abu Dhabi Aircraft Technologies' manager for aircraft structures, Shevantha Weerasekera, agrees: “Almost all the delamination that we see can be attributed to impact damage.” Contamination from hydraulic fuel spills can also play a role. But bumps by ground service vehicles, rock strikes on propellers or flaps, even tools dropped onto a composite wing structure can initiate the delamination sequence and open avenues of opportunity for moisture ingress.
Once water is inside the structure, be it salt or fresh, it freezes and expands as the aircraft ascends. “It cracks the resin and enlarges the delamination,” says Hoke. Chances are virtually “100 percent . . . that once you've created some sort of imperfection that's exposed to the environment . . . water ingression will start,” says Applied Composites Engineering President Leigh R. Sargent.
The prevalence of delamination remains unknown. “If we knew of all the delamination flying around, we would be backlogged for the next several decades,” says Sargent.
“Preventing impacts is a wonderful goal,” says Hoke. “But we know how difficult that is.” Training helps, but it can only go so far. Educating maintenance and especially ramp personnel about locations of key composite structures on specific aircraft is critical. Hoke says, “They generally can't tell [just] by looking” because the parts are painted over.
Impact mitigation is one thing, impact reporting another. Ground crew must report if they hit a structure, whether they think they damaged it or not, Hoke says. “[For] somebody trying to protect their job, that's a tough sell,” he concedes.