Basic types of composite repair include the following:
A superficial, non-structural filler is used to restore a surface to keep fluids out until a more permanent repair is made. This type of repair will not regain any strength and is used only where strength is unimportant. Due to high shrinkage, cosmetic repairs may start to crack after a relatively short time in service.
This type of repair can be effective in limited instances, where the delamination is restricted to one ply. However, not much strength is regained, and the primary benefit is that it is quick and cheap. At best, this type of repair can hope to slow the spread of delamination and is generally considered a temporary measure.
This type of repair can regain some strength. The mechanically-fastened plug (i.e. core plug) and patch repair can be especially effective where thick solid laminates are used, since they take bolt loads well.
Structural Mechanically–fastened Doubler
Full structural repairs using bolted doublers can be used in heavily loaded solid laminates. This is often the only practical means of repairing such structures. However, such repairs are not aerodynamically smooth, and may cause “signature” problems in structures where low-observability by radar is required. They also leave the original damage and simply attempt to transfer loads around the damage. Finally, they can create stress concentrations at their corners and edges.
A doubler is a localized area of extra layers of reinforcement, usually to provide stiffness or strength for fastening, or other abrupt load transfers, such as repairs.
Structural Bonded External Doubler
Bonded external doublers are often used to perform repairs to lightly loaded thin laminate structures. This type of repair is especially common using wet lay-up materials. They may be room-temp or high-temperature cured, depending on the matrix resin system used. These repairs can regain a significant portion of the original strength of the structure—or even full strength—although with a significant stiffness and weight penalty in many cases. This type of repair is generally easy , relatively quick and does not require the highly developed skills of flush structural repairs.
Structural Flush Repair
This repair restores full structural properties by forming a joint between the prepared repair area and the repair patch. The repair patch is made by replacing each ply of the composite laminate that has been removed from the damage area. The size of the repair patch should fit exactly the area prepared for repair, except for a final cosmetic or sanding layer, which is often slightly larger to allow for sanding down to achieve a smooth and/or cosmetic surface.
Published courtesy of Abaris Training Resources, Inc