Sealant tape (tacky tape) is used to provide an air-tight seal between the part and the vacuum bagging film, thus creating a vacuum chamber that allows atmospheric pressure (14.7 psi at sea level) to apply compacting pressure to the repair patch. (See Figure 1 above).
The repair process
The actual repair process involves many different steps. Any given repair can be very different from another depending on the type of structure, type of damage, and the materials used for construction. Since most aircraft structures are "sandwich" type construction, this is what we will discuss here.
When an ACM part is suspected of having damage, the first step would be to inspect the part to determine the type, size, and location of the damage. Small damage to the surface may be allowed to continue in-service with no repair or only minor repair action until a convenient time in which the repair can be accomplished. The more severe the damage, the more likely the part will require some lengthy repair prior to being flown again. Sometimes, the repair can be accomplished directly on the aircraft, or the part may have to be removed and repaired in the shop. The most common types of damage include:
Surface Damage: Cuts, gouges, scrapes, scratches, pits, etc. on the top ply of material or resin coating. Usually does not penetrate the top ply.
Delamination: Separation of two or more plies of material. Typically the result of impact or an inclusion during fabrication.
Disbond: Letting go at a bond line. Usually at the bond between the core and skin or between the skin and substructure.
Core Crush: Damage to the core usually caused by impact on the skin that depresses the skin into the core and causes it to be crushed. Skin may or may not have visible damage.
Puncture/Penetration: Damage through one or more plies to any depth or through both sides of the part.
Typical repair procedure
The following is a description of a "typical" repair procedure for one side skin penetration damage with core crush and disbond. Not all repairs will follow these exact steps. Always refer to the applicable SRM.
Where one side skin penetration damage with core crush and disbond is incurred, the first step is to perform an inspection to map the damaged area. This may be accomplished by an acoustic "tap test" or by ultrasonic A-scan pulse-echo Non-Destructive Inspection (NDI). The map is drawn to outline the damage. (See Figure 2).
Once all damage is mapped, the repair layout is drawn to enclose all of the damage in a geometrical pattern; either circle, rectangular, or oval, etc. The pattern should enclose all the damage, but take in as little of the undamaged structure as possible. Square repairs are discouraged due to high stresses at right angles. All corners should have a radius with no right angles.
Removing the damaged skin
Once the pattern has been laid out, a controlled-depth pneumatic router with router bit (material specific) can be used to remove the damaged skin. The preferred method is to use a router and router guide to steady the tool and prevent kickback that may cause more damage. The depth of the cut should be regulated to cut through the skin but no deeper into the core than is necessary. Typically, the router is set to approximately 1/8-inch below the skin's lower surface. After cutting around the layout pattern, the skin can be peeled off using a pair of duckbill pliers or pried off with a flat-tip screwdriver on a block of wood or other rigid material to act as a fulcrum. Depending on the extent of damage and the thickness of the core, a full core plug can be removed or a partial core plug can be routed out to a depth to just remove the damage. Full core plug is more common on thinner structures (up to 2-inches ±).
Before removing the damaged core, the honeycomb ribbon direction should be marked to ensure the replacement core is installed in the same direction for strength and stiffness. If full core plug is used, a core cutter or X-Acto™ knife can be used to cut around the outer edge of the core to be removed. Cut to the surface of the lower skin, using care to not damage the lower skin. Once the core has been cut all around, use a needle-nose or duckbill pliers to grasp the core cells and twist to fracture the cell node bond to the lower skin. A twisting motion is recommended to prevent delamination of the lower skin. Remove all old core. Use a sanding disc and arbor to lightly sand the core node adhesive bond on the lower skin surface. Take care to not sand into the fabric of the skin ply. It is not necessary to remove all adhesive as long as the surface does not have any high spots that would prevent a good, level seat for the new core plug.