Chapter 7 Aircraft Repair PDF

The repair can be stepped to reduce peel stresses at the edges. Step 4: Vacuum Bagging A film adhesive is placed over the damaged area and the repair layup is placed on top of the repair. The vacuum bagging materials are placed on top of the repair (see Prepreg Layup and Controlled Bleed Out) and a...

The repair can be stepped to reduce peel stresses at the edges. Step 4: Vacuum Bagging A film adhesive is placed over the damaged area and the repair layup is placed on top of the repair. The vacuum bagging materials are placed on top of the repair (see Prepreg Layup and Controlled Bleed Out) and a vacuum is applied. Figure 7-69. Lightning protection strips on a radome. strike. It is important that these lightning protection strips are in good condition to avoid damage to the radome structure. Typical failures of lightning protection strips that are found during inspection are high resistance caused by shorts in the strips or attaching hardware and disbonding of the strips from the radome surface. [Figures 7-69] External Bonded Patch Repairs Repairs to damaged composite structures can be made with an external patch. The external patch repair could be made with prepreg, a wet layup, or a precured patch. External patches are usually stepped to reduce the stress concentration at the edge of the patch. The disadvantages of the external patch are the eccentricity of the loading that causes peel stresses and the protrusion of the patch in the air stream. The advantage of the external patch is that it is easier to accomplish than a flush scarf-type repair. External Bonded Repair With Prepreg Plies The repair methods for carbon, fiberglass, and Kevlar® are similar. Fiberglass is sometimes used to repair Kevlar® material. The main steps in repairing damage with an external patch are investigating and mapping the damage, removal of the damage, layup of the repair plies, vacuum bagging, curing, and finish coating. Step 1: Investigating & Mapping the Damage Use the tap test or ultrasonic test to map out the damage. Step 2: Damage Removal Trim out the damage to a smooth round or oval shape. Use scotch or sand paper to rough up the parent surface at least 1 inch larger than the patch size. Clean the surface with an approved solvent and cheese cloth. Step 3: Layup of the Repair Plies Use the SRM to determine the number, size, and orientation of the repair plies. The repair ply material and orientation must be the same as the orientation of the parent structure. Step 5: Curing the Repair The prepreg patch can be cured with a heater blanket that is placed inside the vacuum bag, oven, or autoclave when the part can be removed from the aircraft. Most prepregs and film adhesives cure at either 250 °F or 350 °F. Consult the SRM for the correct cure cycle. Step 6: Applying Top Coat Remove the vacuum bag from the repair after the cure and inspect the repair, remove the patch if the repair is not satisfactory. Lightly sand the repair and apply a protective topcoating. External Repair Using Wet Layup & Double Vacuum Debulk Method (DVD) Generally, the properties of a wet layup repair are not as good as a repair with prepreg material; but by using a DVD method, the properties of the wet layup process can be improved. The DVD process is a technique to remove entrapped air that causes porosity in wet layup laminates. The DVD process is often used to make patches for solid laminate structures for complex contoured surfaces. The wet layup patch is prepared in a DVD tool and then secondary bonded to the aircraft structure. [Figure 7-70] The laminating process is similar to a standard wet layup process. The difference is how the patch is cured. Double Vacuum Debulk Principle The double vacuum bag process is used to fabricate wet layup or prepreg repair laminates. Place the impregnated fabric within the debulking assembly, shown in Figure 7-70. To begin the debulking process, evacuate the air within the inner flexible vacuum bag. Then, seal the rigid outer box onto the inner vacuum bag, and evacuate the volume of air between the rigid outer box and inner vacuum bag. Since the outer box is rigid, the second evacuation prevents atmospheric pressure from pressing down on the inner vacuum bag over the patch. This subsequently prevents air bubbles from being pinched off within the laminate and facilitates air removal by the inner vacuum. Next, heat the laminate to a predetermined debulking temperature in order to reduce the resin viscosity and further improve the removal of air and volatiles from the laminate. Apply the heat through a heat blanket that is controlled with thermocouples placed directly on the heat 7-41 Rigid outer box: manufacture sides from 2" x 4", top from 1" plywood, drill 1/4" air holes on each side. Rigid box with two layers of breather and vacuum bag Box top 1" thick plywood Air holes (approxmately 0.25" diameter) Nails Inner vacuum bag Inner vacuum bag extends past rigid box Side boards (wooden 2" x 4") Figure 7-70. DVD tool made from wood two by fours and plywood. blanket. Once the debulking cycle is complete, compact the laminate to consolidate the plies by venting the vacuum source attached to the outer rigid box, allowing atmospheric pressure to reenter the box and provide positive pressure against the inner vacuum bag. Upon completion of the compaction cycle, remove the laminate from the assembly and prepare for cure. DVD tools can be purchased commercially but can also be fabricated locally from wood two-by-fours and sheets of plywood, as illustrated in Figure 7-70. Patch Installation on the Aircraft After the patch comes out of the DVD tool, it is still possible to form it to the contour of the aircraft, but the time is typically limited to 10 minutes. Place a film adhesive, or paste adhesive, on the aircraft skin and place the patch on the aircraft. Use a vacuum bag and heater blanket to cure the adhesive. [Figures 7-71 and 7-72] Vacuum port Rigid outer box V2 8 7 5 6 5 4 V1 3 2 1. Insulation 2. Heat blanket 3. Caul plate 4. Nonporous film 5. Porous film 6. Patch laminate 7. Bagging materials 8. Bagging film V1: Inner vacuum V2: Outer vacuum 1 Bottom plate Vacuum port Figure 7-71. Double vacuum debulk schematic. External Repair Using Precured Laminate Patches Precured patches are not very flexible and cannot be used on highly curved or compound curved surfaces. The repair steps are similar as in External Bonded Repair With Prepreg Plies, except step 3 and 4 that follow. Step 3: A Precured Patch Consult the SRM for correct size, ply thickness, and orientation. You can laminate and cure the precured patch in the repair shop and secondary bond to the parent structure, or obtain standard precured patches. [Figure 7-73] Step 4: For a Precured Patch 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 adhesives 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 versus Bolted Repairs 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 7-42 Hold at 125 °F for 90 minutes ± 5 minutes 140 120 Ramp rate 1 °F to 5 °F per minute 100 Vent outer box after 60 minutes 80 60 Inner bag Full vacuum Outer box Full vacuum 40 Inner bag Full vacuum Outer box No vacuum 20 0 10 20 30 40 50 60 70 80 90 100 110 120 SMP-0029M1-9 Double Vacuum Debulk Cycle—Laminate Thickness