American GFM (AGFM, Chesapeake, Va., USA), part of the worldwide GFM machine tool organization, has been at the forefront of composite repair for aerostructures for several years, leading and coordinating the Inspection and Repair Preparation Cell (IRPC) consortium. This multi-company, multi-technology effort has focused on development of an automated system that allows for fast defect detection, scarfing and repair of composite structures of all types. The goal, as the Boeing 787 enters the field, and as the first Airbus A350 XWB enters assembly, is to establish a repair process that not only helps airlines maintain a healthy aerostructure, but do so with minimal aircraft downtime.
AGFM recently hosted an event at its Chesapeake facility to demonstrate the current state of the repair technology it has developed with the IRPC. Frank Elliott, advanced initiatives coordinator at AGFM, says the repair strategy developed relies on three primary functions: Defect detection, scarfing and repair. The demonstration focused primarily on detection and scarfing.
For the demonstration, AGFM had on hand the section of a radome panel from a C-130 aircraft. The panel, about 2 ft/0.6m wide and 3 ft/0.9m long, was about 40 years old and comprised a sandwich structure of honeycomb core surrounded front and back by a glass fiber laminate skin.
A traditional tap test of the panel indicated several potential locations of defects, which were marked. This was followed by another assessment using non-destructive inspection (NDI) via laser shearography provided by Steinbichler Vision Systems (Plymouth, Mich., USA). Defects detected during this inspection were also marked — most of which did not coincide with the tap test results.
With possible defect locations identified, the radome panel was placed on a 25,000-rpm router (Fig. 1), which was used to scarf potential defect locations. The first site, identified via laser shearography, was an area about 3 inches/76 mm in diameter. It was machined in three passes, based upon a 50:1 scarfing angle with a depth of 0.007 inch/0.18 mm on each pass. The scarfing revealed two delamination sites between the laminate and the core (see Fig. 2), confirming results of the laser shearography. One more pass was made over the site, increasing the depth of the scarfing by 0.007 inch and revealing even more damage from delamination (Fig. 3).
A second site, also identified via laser shearography, was machined next. This was an angled scarf, which involves routing out the damaged/defective material, down through both the outer skin and the honeycomb core to the inner skin; then angle scarfing the outer skin at the 50:1 angle in order to enable an acceptable repair. An obvious flaw was not revealed, but inside the hole created by the router, it was revealed that there had been a resin drip, which might have been flagged by NDI as a potential flaw.
Flaw detection is the biggest challenge the IRPC faces. Elliott notes that there is “no absolute in NDI,” and reports that a variety of technologies have been evaluated by the IRPC. Laser shearography was chosen as the global (primary) NDI technology because it provides a quick indication of areas that contain anamolies that require further inspection by a local (secondary) NDI process, to determine more precisely the location, boundary and depth of the defects. The secondary NDI technology chosen is Terahertz NDI, provided by Picometrix LLC (Ann Arbor, Mich., USA), because of its accuracy, resolution and speed of inspection.
Elliott, AGFM and the IRPC will continue to develop, fine-tune and integrate its processes, and in the meantime, the program has also attracted interest from other aerospace manufacturers looking for viable repair technologies.