1,Exterior restoration using wet paving and double vacuum debonding method
In general, wet-layer patches do not perform as well as patches made with prepregs; however, the performance of the wet-layer process can be improved by using the double-vacuum debonding method (DVD method), a technique that removes stagnant air that causes porosity in wet-laminated panels.The DVD process is commonly used to create patches for solid laminated structures with complex contoured surfaces. The wet-laminate patches are prepared in the DVD tool and then secondary bonded to the aircraft structure. As shown in Figure 70 below, the lamination process is similar to the standard wet lay-up process. The difference is in the way the patch is applied.
1.1 Principle of double vacuum compaction
The double vacuum bagging process is used in the manufacture of wet lay-up or pre-impregnated repair plywood. It is shown in Figure 70 below. To start the decompression process, the air is removed from the inner flexible vacuum bag. The rigid outer box is then sealed to the inner vacuum bag and the volume air between the rigid outer box and the inner vacuum bag is expelled. Since the outer box is rigid, a second venting prevents patching of the inner vacuum bag under atmospheric pressure. This subsequently prevents air bubbles from being pinched off within the laminate and promotes removal of air by the internal vacuum. Next, the laminate is heated to a predetermined de-expansion temperature to reduce resin viscosity and further enhance removal of air and volatiles from within the laminate. Heat is applied via a heat transfer blanket, which is controlled by a thermocouple placed directly on the heat transfer blanket. Once the decompression cycle is complete, the laminate is compressed and connected to an external rigid box via an exhaust vacuum source to consolidate the layer, allowing atmospheric pressure to re-enter the box and provide positive pressure on the internal vacuum bag. Upon completion of the compaction cycle, the laminate is removed from the assembly and prepared for curing.
DVD tools can be purchased commercially, but can also be made locally from 2-by-4 lumber and plywood, as shown in Figure 70.
Figure 70: DVD tool made of wood two-by-four and plywood
1.2 Aircraft Patch Installation
Once the patch is removed from the DVD tool, it can still be formed into the outline of the aircraft, but the time is usually limited to 10 minutes. Film adhesive, or paste adhesive, is placed on the aircraft skin and the patch is placed on the aircraft. A vacuum bag and heating blanket are used to cure the adhesive. This is shown in Figures 71 and 72.
Figure 71: Schematic diagram of double vacuum compaction
Figure 72: DVD Curing Cycle
2,External repairs using pre-cured lamination
Pre-cured repairs are not very flexible and cannot be used on highly curved or compound surfaces. The repair procedure is similar to an externally bonded repair using prepreg. Refer to SRM for correct size, thickness, and orientation. You can laminate and cure the pre-cured patch in a repair shop and secondary bond it to the primary substructure or obtain a standard pre-cured patch. As shown in Figure 73. Apply film adhesive or paste adhesive to the damaged area and place the pre-cured repair on top. Vacuum bags are repaired and cured at temperatures appropriate for the film adhesive or adhesive. Most film adhesives cure at 250°F (121°C) or 350°F (176.67°C). Some paste adhesives cure at room temperature, although elevated temperatures can be used to speed up the curing process.
Figure 73: Pre-cured repair
3,Bonding and bolt repair
The concept of bonded repairs has found applicability in two types of manufacturing assembly methods. They have the advantage of not introducing stress concentrations drilled fastener holes for patch installations and can be stronger than the original part material. The disadvantage of adhesive repairs is that most repair materials require special storage, handling and curing procedures.
Bolted repairs are faster and easier than adhesive repairs. They are typically used on composite housings greater than 0.125 inch thick to ensure adequate fastener bearing area for load transfer. They are prohibited for use in honeycomb sandwich assemblies because of the potential for moisture intrusion through the fastener holes and degradation of the core layer. Bolted repairs are heavier than similar adhesive repairs, limiting their use on weight-sensitive flight control surfaces.
Honeycomb sandwich components typically have thin surface panels where the use of a bonded round-the-horn type repair is most effective. A bound external step patch can be used as an alternative. Bolt-on repairs are ineffective for thin plywood because of the low bearing stresses of composite laminates. The thicker solid laminates used on larger aircraft can be up to an inch thick in high load areas, and these types of laminates cannot be effectively repaired using a bonded corner-around repair. As shown in Figure 74.
3.1 Repair of bolts
Aircraft designed in the 1970s used composite sandwich honeycomb structures as light-load secondary structures, but newer, larger aircraft use thick solid laminates as primary structures rather than sandwich honeycomb structures. These thick solid laminates are very different from traditional sandwich honeycomb structures and are used for flight controls, landing gear doors, flaps and spoilers on today's aircraft. They present a challenge to repair and are difficult to repair with bonded repair methods. Bolt repair methods have been developed for repairing thicker solid laminates.
Honeycomb sandwich structures do not require bolt repairs because of the limited load-bearing strength of the laminate and the drilled holes weaken the honeycomb structure. The advantage of bolted repairs is that only patch material and fasteners need to be selected and the repair method is similar to sheet metal repair. There is no need to cure the repair and store the prepreg repair material and film adhesive in a refrigerator. Patches can be made of aluminium, titanium, steel or pre-cured composite materials. Composite repairs are usually made from carbon fibre with epoxy or glass fibre with epoxy.
You can repair a carbon fibre structure with an aluminium plate, but you must place a layer of fibreglass cloth between the carbon fibre part and the aluminium plate to prevent galvanic corrosion. Titanium and pre-cured composite panels are preferred for repairing highly loaded components. Pre-cured carbon/epoxy patches have the same strength and stiffness as the substrate because they usually cure similarly.
Titanium or stainless steel fasteners are used for bolt repairs on carbon fibre structures. Aluminium fasteners will corrode if used with carbon fibre. Rivets cannot be used as installation of rivets using a rivet gun can cause damage to the hole and surrounding structure, rivets expand during installation, which is undesirable for composite structures as it can cause the composite material to expand its boundaries.
3.2 Repair procedure
Step 1: Damage Inspection
In thick plywood, the tap test is not effective in detecting delamination unless the damage is close to the surface.
Ultrasonic inspection is necessary to determine the area of damage. Consult SRM to find the applicable NDI procedure.
Step 2: Removal of Damage
To prevent stress concentrations, the damaged area needs to be trimmed into a round or rectangular hole with a smooth radius. Remove the damage with a sander, planer or similar tool.
Step 3: Preparing the Repair
Determine the size of the patch based on the repair information in the SRM. Cut, shape and set the patch before applying it to the damaged structure. It is easier to make the patch slightly larger than calculated and trim to size after drilling all fastener holes. In some cases, the patch is pre-shaped and pre-drilled. If cutting is to be done, standard shop procedures applicable to the patch material should be used. Titanium is difficult to work with and requires a strong slip roller to bend the material. Metal patches need to be filed flat to prevent cracking around the cut. When drilling pilot holes in composites, the holes used to repair fasteners must be at least 4 diameters from the existing fastener and have a minimum edge distance of 3 fastener diameters. This differs from standard practice for aluminium, which allows for a distance of two diameters. Specific guide hole sizes and the type of drill bit used should follow specific SRM instructions. As shown in Figure 75.
Figure 75: Material Repair Layout for Bolt Repair of Composite Structures
Step 4: Hole Layout
To locate the repair in the damaged area, draw two perpendicular centre lines on the primary substructure and the patch material that define the primary load or geometric direction. Then, draw the hole pattern on the patch and drill guide holes in the patch material. Align the two vertical centre lines of the patch with the lines on the primary substructure and transfer the guide holes to the primary substrate material. Secure the repair with a minimum difference. Mark the edges of the repair so that it can be easily placed back in its original position.
Step 5: Drill and Ream Holes in Patch and Main Substructure
The composite skin should be backed up to prevent splitting. Enlarge the pilot holes in the patch and main substrate with a drill bit smaller than 1/64 size, then ream all holes to the correct size. A tolerance of +0.0025/-0.000 inches is usually recommended for aircraft parts. For composites, this means no interference fasteners.
Step 6: Fastener Installation
Once the fastener holes are drilled full size and reamed, the permanent fasteners are installed. Before installation, measure the fixture length of each fastener with a fixture length gauge. Since different repairs require different fasteners, refer to the SRM for allowable fastener types and installation procedures; however, install all fasteners wetted with sealant, applying the proper torque to install screws and bolts.
Step 7: Sealing of Fasteners and Repairs
Sealants are applied to bolt repairs to prevent water or moisture intrusion, chemical damage, galvanic corrosion, and fuel leaks. They also provide contour smoothing. Sealants must be applied to a clean surface. Masking tape is usually placed on the periphery of the patch, parallel to the edge of the patch and leaving a small gap between the edge of the patch and the masking tape. Sealant is applied to this gap.
Step 8: Apply topcoat and lightning netting
The repair will need to be sanded, primed and painted using an approved paint system. If a composite repair is used in an area susceptible to lightning strikes, a lightning net will need to be added.
To be continued
Source "Composites Frontier" Public Website