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9/8/98 AC 43.13-1B SECTION 2. METALLIC SANDWICH SECONDARY STRUCTURE REPAIRS 3-10. REPAIRS TO METALLIC SAND- WICHSECONDARYSTRUCTURE. Mag nesium, titanium, or stainless steel facings re quire special procedures that are not included in the following methods of repair. Aluminum alloys such as 7075-T6, 2024-T3, and 2014-T6 are commonly used for the repair of facings for sandwich structural parts having aluminum facings. For maximum corrosion resistance, use only clad aluminum for repairs to clad aluminum alloy facings. a. Dents, scratches, or fractures, not ex ceeding 1/4 inch in largest dimension in alu minum facings, may be repaired with a suit able filler such as viscous epoxy resin. Dents that are delaminated shall not be filled but re paired. Thoroughly clean the repair area with fine sandpaper and acetone before applying the filler. After the resin has partially cured, re move any excess resin with a sharp plastic scraper. When the resin has completely cured, sand to the original contour. If the damage in cluded a fracture, reclean the area around the filled hole and apply a surface patch. b. Fractures or punctures in one facing and partial damage to the core of an alumi num-faced laminate may be repaired by several different methods. The technique used will depend upon the size of the damage, the strength required, and the aerodynamic loads of the area involved. If the repair requires aerodynamic smoothness, the facing sur rounding the repair core cavity may have to be step cut to one-half its thickness. This can be done by using a router with an end mill bit and a template. c. Damage that extends completely through the core and both facings may be re paired using the same general techniques as those used for repairing fiberglass laminates when both facings are accessible. d. After locating the extent of the total damaged area by tapping or other nondestruc tive test methods, remove the damaged facing and that portion of the core material that is also affected. The depth to which the core must be removed will depend upon the type of core material and the method of repair. The re placement core material must be the same material and core cell size as the original. Fab ricate core material to shape, keeping the same core ribbon or grain direction. When a sub stitution is permissible, wood or glass-fabric honey-comb cores are sometimes used in the repair of aluminum honeycomb cores, as they are generally easier to shape. Typical types of core replacements are shown in figure 3-10. Resin fills can be used to replace the core and facing where smaller core damage exists. Phenolic microballoons, low-density insulating materials, and/or other ingredients are added to lower the density and give greater flexibility. e. For the repair of larger holes in which it is inconvenient to use a face patch because of aerodynamic smoothness requirements in that area, both the core and facing are some times replaced with glass-fiber fabric discs and resin. Undercut the core, as shown in fig ure 3-11, in order to obtain a better bonding of the fill with the facing. Fill the core cavity with accurately shaped resin-saturated glass cloth discs, and press each ply down to remove any air bubbles. Special care should be taken that the final plies fit well against the under side of the top facing. When the core cavity is filled, close the cutout in the facing with resin- impregnated glass fiber fabric discs that have been precut to size. f. Overlap repairs, typically called scab patches, have a long history of use in repairing aircraft structures. These repairs simply cover the damaged area with patch material. Overlap repairs can be bonded and/or mechanically Par 3-10 Page 3-9PDF Image | AFS-640
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