Composite repair: Lessons learned, challenges and opportunities, Part II

The aerospace industry is introducing automated methods (e.g., robotic CNC machining) and standardizing repair technician training, aimed at better quality, reduced cost and faster turn-around, and is extending bonded repairs to increasingly larger areas of damage and flight-critical primary structures. This effort has been largely the result of industry collaboration through the Commercial Aircraft Composite Repair Committee (CACRC), administered by SAE International (Warrendale, PA, US). SAE’s Performance Review Board will be the certifying body for general and, soon, aerospace composite repair technicians.

The above holds much promise for those in other industries willing to benefit from the aerospace industry’s pioneering experience in composite repair. The wind turbine industry, in the repair of composite wind blades, and the now weight-conscious, and therefore, composites-aware, automotive industry both stand to gain. In each case, a similar spirit of collaboration, in terms of training and standardization, has the potential to improve quality and cost, and establish an effective supply chain.

Opportunities and challenges. For wind energy and automotive repairs, one challenge will be the current lack of standards. Wind turbine OEMs today bear little responsibility for developing repair methods and standard materials because they typically cover maintenance only for the first few years as part of the wind blade warranty. Thus, the onus of blade inspection and repair falls on wind farm developers/operators and their subcontractors. For example, the industry has struggled with the step vs. taper debate, and with how to replace multiaxial reinforcements in a way that matches constituent fiber axial orientation to ensure sufficient load transfer. Repairs, then, have commonly been oversized, not aerodynamic and possibly lacking in load efficiency. But as training has increased and as aerospace industry knowledge has been adapted, blade repair techniques have improved. Further, significant development in epoxy and polyurethane resins and cure methods for blade repair have provided better bond performance, making systems more tolerant of environmental conditions during repair and the large temperature range blades experience in service.

The automotive situation is more nascent and critical. Three years ago at SPE’s Automotive Composites Conference & Exhibition, the prevailing opinion was that carbon fiber would never figure significantly in new car design and “we’ll just replace parts vs. repair them.” McLaren, Lamborghini and other high-end manufacturers, however, already have vehicles on the road with significant amounts of carbon composite structures and have teams of “flying doctors” who perform aerospace-type repairs. However, they have not yet developed a system of zones or limits for allowable damage and composite repair designs. (Such a system has been proposed for wind blades.) These OEMs have done some analysis of front and rear impact, identifying where fractures might occur, which is used to assist the “repair doctors” with NDI. But not much has been done with side impact, and right now it is common to see composite monocoque “tubs” scrapped, even though they are repairable. Damage analysis, repair design and substantiation methods could easily be adapted from the aerospace industry.