Surface preparation holds an important consideration in bonded structures
Surface preparation holds an important consideration in bonded structures. The process to prepare the surface consists of paint removal, anodizing and priming. Liquid chemical paint strippers are not recommended, as they may become entrapped in cracked areas and faying surfaces of adjoining structures, thereby causing a corrosion problem.
Aluminum oxide abrasive cloth has been found to be suitable for small repair areas. Both silane and phosphoric acid non-tank anodize (PANTA) are also suitable. The silane process has the advantage over PANTA for being non-acid process. PANTA process might be desirable when look at the long term durability of repairs, as sufficient test data is available on this process.
To prevent contamination and improve long-term durability, primer is applied to the aluminum surface after anodizing with PANTA. BR-127 primer has been found to be suitable for FM-73 adhesive.
3.3.2 Adhesive Material Selection
The service temperature requirements is 180F (82C) in the majority of aircraft repair applications which make room temperature cure adhesives are not considered suitable. Room temperature cure adhesives are paste adhesives and generally do not result in uniform bond line thickness in the repair. Thus, affecting the load transfer to composite patch. Hence, high temperature film adhesives are preferred. Also, long term durability of room temperature adhesives is not well characterized. A 350F (177C) cure film adhesive is not considered desirable, as the curing at such a high temperature is likely to cause undesirable high thermal stresses. Also, an aluminum structure exposed to a 350F (177C) temperature will undergo degradation in mechanical properties. A 250F (121C) cure adhesive system is considered suitable for the composite patch repair of aluminum structure. Ductile adhesives such as FM-73 are preferred over brittle adhesives such as FM-400 due to the tendency of the brittle adhesives to disbond around the damage area, thereby reducing the load transfer to the repair patch.
3.3.3 Composite Repair Material Selection
Both boron/epoxy and graphite/epoxy composites are suitable for the repairs. The choice between boron or graphite fibers should be based on availability, handling, processing and the thickness of the material to be repaired. Boron has higher modulus than graphite and would result in thin repair patches. Thin patches are more efficient in taking load from damaged parts as compared to thick patches. For repairing relatively thick parts, boron may be preferred over graphite. It is considered desirable to use highly orthotropic patches, having high stiffness in the direction normal to the crack, but with some fibers in directions at 45 and 90 degrees to the primary direction to prevent matrix cracking under biaxial loading and inplane shear loads which exist for typical applications. This patch configuration can be best obtained with unidirectional tape. Woven material has greater formability and could also be used, although it would not make a very efficient patch.
The composite patches may be precured, prestaged or cured in place. For locations where vacuum bagging represents a problem, a precured patch may be prepared in an autoclave and then secondary bonded to the repair area. For relatively minor contours, a prestaged patch may be used. For curved surfaces the patch may be cured in place during the bonding operation.
3.3.4 Bonding Operation
Bonding of repair patches requires a proper temperature control within + 10F and -5F in the repair area. Thermal blankets are available to provide temperature in excess of 1000F (538C). A proper temperature control within tolerances is necessary for bondline to achieve desirable strength. A large aircraft structure compared to a small repair area may act as a heat sink and jeopardize maintaining desired temperature control for the required duration. Proper heat blankets for surrounding areas may be required for such cases.