As composite structures become larger, more integrated and more critical, guaranteeing the integrity of a major repair when the aircraft returns to service is a growing imperative. Research into methods of checking repairs in situ could pave the way for wider structural health monitoring.

France's GMI Aero has been involved in composite structural repairs almost from its creation, says founder Roland Chemama. “We began designing equipment for composites manufacture and process control, and quickly started designing solutions for composite repairs,” he says.

The Paris-based company has become heavily involved in collaborative projects under Europe's Clean Sky public-private research program. “Under Clean Sky we have won eight tenders related to composites repair and structural health monitoring,” Chemama says.

Under Clean Sky, research programs are funded by government and industry and performed by teams of industry and academic participants. “It is a good approach to developing technology in harmony with the requirements of the OEMs [original equipment manufacturers] and allows us to go beyond the constraints of our internal R&D,” he says.

Key efforts include the Advanced project, led by nacelle manufacturer Aircelle and involving GMI and the National Technical University of Athens (NTUA), which has developed a method for repairing large areas of damage to critical composite structures. One target application is the internal bifurcation surface in the thrust reverser for the Rolls-Royce Trent 900 powering the Airbus A380.

“The proposed solution can repair up to 2 square meters of structure with very stringent temperature control—±5 deg at 275 degrees Celsius,” says Chemama. The 48-kw prototype uses electric blankets with up to 18 heating zones that are individually controlled and 80 thermocouples that monitor the heat transfer. The technology also can be used out of the autoclave, to reduce repair costs.

Under the Inducer project led by Alenia Aermacchi and involving NTUA and the Welding Institute, GMI is developing a method of curing bonded composite patches using induction heating and adding sensors to monitor the health of the repair in service. “The challenge is qualifying a repair on a large structure. We have made advances in structural health monitoring to help qualify the bonding of a patch,” he says.

GMI has developed a sensing array using hair-thin magnetorestrictive wire sensors that are bonded to the patch and allow the repair to be monitored in service. “When the aircraft is on the ground, we can scan the surface with a transducer, without contact, and get an image that is compared automatically with the structure at fabrication of the patch to detect any delamination,” Chemama says.

With the transducer held above the sensing array, the wires pick up a voltage that is different if they have been strained by in-service damage versus their original condition. This produces an image of the repair. “We wouldn't use it in flight, but on the ground it accelerates checking of the patch,” he says, adding the technology has been qualified by Alenia in collaboration with regional aircraft manufacturer ATR and is planned to be flight tested on two parts.

With aircraft such as the Airbus A350 and Boeing 787 having thick carbon-fiber primary structures, GMI has developed a method for repairing fastener holes that are damaged during drilling on the assembly line. The technique uses hot bolts that are placed in delaminated holes and apply controlled pressure and heat to repair the damage.

For composite fuselage structures, such as the A350's, GMI is developing a positive overpressure system to enhance repairs. “Classically we bond a patch under vacuum pressure, but when it is necessary to improve the quality of the bond and reduce the porosity of the patch we need to apply positive pressure,” he says, noting the work has been accelerated by Clean Sky. The system will inflate to 2-3 atmospheres while applying counter-pressure to prevent deformation of the structure.

With the European Union planning to launch the follow-on Clean Sky 2 research program this year, GMI has proposed creating a composite repair cluster of up to 10 participants with complementary capabilities. This would provide repair solutions to the integrated aircraft demonstrator programs. Projects might include extending embedded sensing from repairs to following structures throughout their life.