Preparation of laminate surface. With most OEMs, there are two aspects to this task. The mechanical abrasion of the repair area creates a heightened level of molecular activity on the laminate surface, ensuring a stronger chemical bond with the repair materials. The second aspect is the scarfing or stepping down of the damaged laminate to the full depth of damage. This not only effectively reduces the height of the repair, but ensures a better interface between the repair materials and the damaged laminate. Each OEM has their preference here, but both require high levels of technician tool skill and practice.
Preparation of repair materials. This task contains several processes that will vary among the OEMs.
1. The OEM will specify wet lay or pre-preg materials. With pre-preg, the resin has been incorporated into the fabric and dried and requires a bonding machine for curing. Consult the AMM/SRM for the list of approved materials.
2. The number of repair plies specified is generally a function of the number of damaged plies.
3. The repair ply overlap is how much larger or smaller each ply must be relative to the damage and the previous ply.
4. When the fabric is woven, the dominant yarns, called warp, run in one direction and are crossed by lesser fill yarns. The OEM designers specify which direction the warp fibers must run relative to the structure. This is referred to as ply orientation (0-, +45-, -45-, and 90-degree orientations are typical).
5. Some OEMs require plies be stacked from smallest to largest, while others require the opposite stack.
6. If the repair requires replacement of damaged core material, consult the AMM/SRM for approved core type and thickness (as well as ribbon direction if honeycomb).
Laminating. Most OEMs have restrictive laminating environmental conditions in regards to airborne contaminants and temperature/humidity limitations. OEM publications will include information on correct resin to fabric ratio and avoiding entrapped air (porosity) in the lamination. While these conditions are easiest to maintain by applying the lamination one ply at a time, the repair location may dictate that you pre-stack the plies and apply the lamination as one unit (to maintain ply overlap and orientation).
Curing. Pre-preg repairs require a schedule of vacuum bagging materials as well as a bonding machine. While you can use this equipment on wet lay repairs, due to their expensive nature, many OEMs allow less expensive and less complex curing options. Heat sources may include heat lamps, heat guns, or hair driers so long as cure temperature is monitored and controlled (recording cure data is also a good practice). The actual cure parameters are more a function of the resin manufacturer than the aircraft OEM and include minimum/maximum temperature and time requirements as well as temperature ramp rates and scheduled hold periods or dwells.
Inspection. Some of the criteria set forth by the OEMs include inspecting the repair for proper curing of the resin, complete bonding between the repair and the structure, voids or porosity in the repair, and evidence of correct resin content.
While organizing OEM procedures into a set of related tasks may help in developing a sense of a standard composite repair process, it must be emphasized that strict adherence to the individual OEM-approved data, materials, and processes is imperative. If, for example, you find you prefer the method of stacking plies that OEM A uses more than that of OEM B, you must use OEM B’s method when repairing an OEM B aircraft. There is no substitution of material or procedure allowed without OEM approval.
Like them or not, composites have earned a place in modern aircraft design and chances are you may end up in a position where you will be faced with the prospect of inspecting or repairing them. Will you be ready?
Tim Wright most recently developed and launched aerospace composite programs at Northland Community and Technical College in Thief River Falls, MN, and Wisconsin Indianhead Technical College in Superior, WI. He is currently on the faculty of Wisconsin Indianhead Technical College.
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