Step 4: For a Precured Patch Bonded versus Bolted Repairs Bolted Repairs Repair Procedures Figure 7-74.  Bolted versus bonded repair.  Table 2 X X X X X X X X X X X Lightly loaded structures –  laminate thickness less than 0.1" Highly loaded structures –  laminate thickness between 0.125" – 0.5" Highly loaded structures –  laminate thickness larger than 0.5" High peeling stresses Honeycomb structure Dry surfaces Wet and/or contaminated surfaces Disassembly required Restore unnotched strength Bonded versus bolted repair Bonded Bolted Apply film adhesive or paste adhesive to the damaged area  and place the precured patch on top. Vacuum bag the repair  and cure at the correct temperature for the film adhesive or  paste adhesive. Most film adhesive cure at either 250 °F  or 350 °F. Some paste adhesives cure at room temperature  although an elevated temperature could be used to speed the  curing process. Bonded repair concepts have found applicability in both  types of manufacturing assembly methods. They have the  advantage of not introducing stress concentrations by drilling  fastener holes for patch installation and can be stronger than  original part material. The disadvantage of bonded repairs is  that most repair materials require special storage, handling,  and curing procedures.  Bolted repairs are quicker and easier to fabricate than  bonded repairs. They are normally used on composite skins  thicker than 0.125-inch to ensure sufficient fastener bearing  area is available for load transfer. They are prohibited in  honeycomb sandwich assemblies due to the potential for  moisture intrusion from the fastener holes and the resulting  core degradation. Bolted repairs are heavier than comparable  bonded repairs, limiting their use on weight-sensitive flight  control surfaces. Honeycomb sandwich parts often have thin face sheets  and are most effectively repaired by using a bonded scarf  type repair. A bonded external step patch can be used as an  alternative. Bolted repairs are not effective for thin laminates  because of the low bearing stress of the composite laminate.  Thicker solid laminates used on larger aircraft can be up  to an inch thick in highly loaded areas and these types of  laminates cannot be effectively repaired using a bonded scarf  type repair.  [Figure 7-74] holes. The advantage of a bolted repair is that you need  to select only patch material and fasteners, and the repair  method is similar to a sheet metal repair. There is no need  for curing the repair and storing the prepreg repair material  and film adhesives in a freezer. Patches may be made from  aluminum, titanium, steel, or precured composite material.  Composite patches are often made from carbon fiber with an  epoxy resin or fiberglass with an epoxy resin. You can repair a carbon fiber structure with an aluminum  patch, but you must place a layer of fiberglass cloth between  the carbon part and the aluminum patch to prevent galvanic  corrosion. Titanium and precured composite patches are  preferred for repair of highly loaded components. Precured  carbon/epoxy patches have the same strength and stiffness  as the parent material as they are usually cured similarly.  Aircraft designed in the 1970s used composite sandwich  honeycomb structure for lightly loaded secondary structure,  but new large aircraft use thick solid laminates for primary  structure instead of sandwich honeycomb. These thick solid  laminate structures are quite different from the traditional  sandwich honeycomb structures used for flight controls,  landing gear doors, flaps, and spoilers of today’s aircraft.  They present a challenge to repair and are difficult to repair  with a bonded repair method. Bolted repair methods have  been developed to repair thicker solid laminates. Titanium or stainless steel fasteners are used for bolted repairs  of a carbon fiber structure. Aluminum fasteners corrode if  used with carbon fiber. Rivets cannot be used because the  installation of rivets using a rivet gun introduce damage to  the hole and surrounding structure and rivets expand during  installation, which is undesirable for composite structures  because it could cause delimination of the composite material. Bolted repairs are not desirable for honeycomb sandwich  structure due to the limited bearing strength of the thin face  sheets and weakened honeycomb structure from drilling  The tap test is not effective to detect delamination in thick  laminates unless the damage is close to the surface. An  Step 1: Inspection of the Damage 7-44