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Aging of U.S. Air Force Aircraft: Final Report
qualified suppliers, requiring the repair depot to stock a variety of repair materials. There is a pressing need to standardize repair materials and processes across the Air Force inventory. The Commercial Aircraft Composite Repair Committee (CACRC) has been formed to address composite service and repair concerns of the commercial aircraft industry. The Air Force should monitor the activities of the CACRC and evaluate the applicability of the recommendations of the CACRC to Air Force aircraft.
Recommendation 10-3. Develop tools and methods for bolted repairs of composite primary structures.
The thicker laminate construction used in composite primary structures, as well as the size and nature of discrete damage from typical aircraft service (e.g., impact damage, lightning attachment damage, delaminations), are not conducive to wet lay-up patch repair technologies. Thin facesheets on honeycomb panels are generally repaired using bonded scarf patches with a scarf taper of 20:1, which, if applied to thicker laminate constructions, would result in the removal of a large amount of undamaged material (Bodine et al., 1994). Much of the emphasis in the development of primary structure repairs has therefore been on fastened, precured composite or metallic splice plates, similar to current bolted repair techniques for metal structure. The issues that must be addressed in these types of repairs include (1) criteria for determining when repairs are required; (2) availability of standardized repair elements; (3) drilled hole quality; (4) ability to restore original strength, durability, and damage tolerance; and (4) ability to match existing contours.
Recommendation 10-4. Evaluate environmentally benign paint removal methods recommended in Chapter 7 for compatibility with polymeric composite substrates.
Composites must be protected by finishes with resistance to fluid penetration and UV degradation. Maintenance of protective finishes represents significant operational costs to the Air Force. The removal of finishes from composites is a slow and expensive process. Because chemical strippers attack the polymer matrix, finishes generally are removed using mechanical abrasion processes. New paint removal processes such as laser, heat, frozen carbon dioxide blasting, and wheat starch blasting are being evaluated. Rapid, low-cost, on-aircraft paint removal techniques are needed to reduce the cost of maintaining composite structures and to preclude damage to the structure.